Kinetic Liquid Metal Synthesis of Flexible 2D Conductive Oxides for Multimodal Wearable Sensing

Kinetic Liquid Metal Synthesis of Flexible 2D Conductive Oxides for Multimodal Wearable Sensing | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Kinetic Liquid Metal Synthesis of Flexible 2D Conductive Oxides for Multimodal Wearable Sensing Md Saifur Rahman, Simon A. Agnew, Samuel Ong, William J. Scheideler This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4903114/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 23 Nov, 2024 Read the published version in npj Flexible Electronics → Version 1 posted 10 You are reading this latest preprint version Abstract Transparent conducting oxides (TCOs) are crucial for high-performance displays, solar cells, and wearable sensors. However, their high process temperatures and brittle nature have hindered their use in flexible electronics. We report an approach to overturn these limitations by harnessing the physics Cabrera Mott native oxidation to fabricate large-area, two-dimensional transparent electrodes via liquid metal printing. Our robotic, solvent-free and vacuum-free process deposits ultrathin (2–10 nm thick) 2D indium tin oxide (ITO) with exceptional flexibility, high transparency (> 95%) and superior conductivity (> 1300 S/cm) for wearable bioelectrodes. In a significant advance over previous work, we utilize hypoeutectic In-Sn alloys to print 2D ITO at < 140 ºC on flexible polymers. Our detailed materials characterization and microscopy reveal the efficacy of Sn-doping and high crystallinity with large, platelike grains formed by the liquid metal reaction environment. The ultrathin nature of 2D ITO yields significant enhancement to bending strain tolerance, scratch resistance exceeding durability of traditional PEDOT, and low contact impedance to skin comparable to Ag/AgCl. Finally, we utilize the conductivity and transparency of 2D ITO for synchronous, multimodal measurements via electrocardiography (ECG) and pulse plethysmography (PPG). This order-of-magnitude improvement to printed TCOs could enable new wearable biometrics and display-integrated sensors. Physical sciences/Materials science/Materials for devices Physical sciences/Materials science/Nanoscale materials/Two dimensional materials Physical sciences/Engineering/Electrical and electronic engineering Physical sciences/Nanoscience and technology/Nanoscale materials/Synthesis and processing liquid metal printing 2D oxides transparent electronics wearable sensors bioelectrodes Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction Transparent conducting oxides (TCOs) such as indium tin oxide (ITO) are critical materials for electronic and optoelectronic devices, providing the transparency necessary for displays, solar cells, sensors, and various user interface devices. However, state-of-the-art TCOs have two major drawbacks – they are comparatively expensive due to the need for vacuum deposition and they are traditionally considered to be brittle materials [ 1 ] with limited suitability for flexible, lightweight systems. The minimization of film stresses via strain-tolerant serpentine structures has shown the promise of transparent metal oxides as active materials for wearable sensors, but these approaches have the drawback of requiring complex patterning and yielding low areal density circuits [ 2 ] . Recent works have also demonstrated complementary approaches of fabricating on ultrathin substrates [ 3 ] to minimize the bending strain of metal oxide thin films and using multilayered organic-inorganic hybrid structures to relieve bending-induced mechanical stresses [ 4 ] . Studies of the mechanics of TCOs such as ITO have revealed that minimizing the TCO film thickness and reducing the process temperature both lead to enhanced mechanical reliability [ 5 ] . An emerging class of ultrathin liquid metal-derived [ 6 ] two-dimensional (2D) oxides [ 7 ] could overturn the longstanding limitations of transparent conducting oxides, offering a platform for high-performance wearable electronics based on inorganic, wide-bandgap materials. The field of 2D oxides has demonstrated, for example, the formation of ultrathin materials with layered polymorphs (e.g. β-TeO 2 ) just 5–40 Å thick [ 8 ] , the potential for assembling these materials into heterostructures with unique electronic properties [ 9 ] , a tendency to favor highly crystalline phases compared with oxides printed from solution [ 10 ] , and the ability to yield conductors with extremely high transparency (> 99%) and enhanced mechanical flexibility [ 11 ] . However, the 2D oxide field faces several ongoing challenges limiting its impact. For example, there is a demand for advancing large-area uniformity while precisely controlling the liquid metal meniscus – the key feature for modulating the printed film thickness and controlling the oxidation kinetics. Widely variable thicknesses are reported in the literature for a single material such as In 2 O 3 or Ga 2 O 3 due to the variable dynamics of manual methods based on blade coating [ 12 , 13 ] , dispensing from a syringe [ 14 ] , and squeeze printing between two flat substrates [ 15 ] . Additionally, there is a need to introduce extrinsic dopants to control and enhance electronic properties [ 7 ] since the precursor metal alloy constituents compete for representation in the surface oxide [ 16 ] . If these challenges can be addressed, 2D oxides could make a significant impact on the field of flexible electronics. Herein we report the first demonstration of high-speed automated liquid metal printing over large areas with Å-level precision control over Cabrera Mott oxidation kinetics. We apply this method to fabricate flexible, two-dimensional (2D) indium tin oxide (ITO) transparent electrodes from the surface oxides of liquid indium tin alloys at temperatures as low as 140 ºC on flexible polymer substrates. Our detailed materials characterization of these films reveals that by controlling crystallization and doping of 2D ITO it is possible to achieve superlative conductivity and high transparency at unprecedented process speeds as well as outstanding mechanical resilience, including high scratch resistance and enhanced bending strain. Finally, using these materials, we report the first demonstration of 2D oxide-based bioelectrical measurements, showing an efficient multimodal sensing approach for combining electrocardiography (ECG) and pulse plethysmography (PPG) utilizing the high transparency of liquid metal-printed ITO. 2. Results and Discussion 2.1 Liquid Metal Printing of 2D Indium Tin Oxide We have developed an automated platform for liquid metal printing (Fig. 1 a) that precisely controls the Cabrera-Mott oxidation kinetics of liquid metals. This is the first report of robotic, wafer-scale liquid metal printing of 2D oxides (Figure S1), replacing previous manual "touch printing" methods [ 11 ] . Our system can vary the printing speed over a 200-fold range, from 0.1 cm/s to 20 cm/s, allowing us to print uniform films over areas greater than 100 cm². The precursors for liquid metal printing in the present study are hypoeutectic, indium-rich In-Sn alloys, though we note that the automated method can be extended to a multitude of alloys for printing various 2D oxides. Indium and tin form a eutectic mixture at 52 − 48 wt.% Sn with a melting temperature of approximately 117°C (Figure S2). The concentration of Sn in the alloy determines the melting temperature, with hypoeutectic compositions (less than 48% Sn) lowering T m (Fig. 1 b) significantly below the T m of pure In. This allows us to print 2D ITO films at extremely low deposition temperatures, making the process compatible with thermally sensitive substrates such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyimide (PI), and glass (Fig. 1 c). The automated process gives us the ability to model the kinetics of liquid metal surface oxidation and explore the inherent limits to the thickness uniformity across large areas, assessing the fundamental repeatability of this synthetic method. Figure 1 d shows a printed 2D ITO film covering approximately 10 x 10 cm, with a thickness of 5.8 nm and a standard deviation of just 4 Å. The thickness is measured using spectroscopic UV reflectometry (see Materials and Methods and Figure S3) and confirmed with atomic force microscopy (AFM) (Figure S4). An essential feature of our method is that it offers the ability to print films of programmed thickness by modulating the printing speed. Figure 1 e shows the thickness of 2D oxide printed at 260 ºC at speeds ranging from 0.1 cm/s to 20 cm/s for both undoped In 2 O 3 and ITO films (7 at. % Sn). These results show that, at high speeds, the films asymptotically approach a thickness of approximately 2.4 nm in the case of ITO and 2.5 nm in the case of In 2 O 3 . Reducing print speed (< 1 cm/s) allows the printing of thicker single layers approaching 5–6 nm. To understand the physics determining the 2D oxide film thickness ( t ox ), we consider the Cabrera Mott oxidation kinetics model shown below (Eq. 1 ) where the speed of the roller ( v ) is treated as the inverse of time ( t ) for the oxide growth on the liquid metal meniscus. In this approximation, A and B are fitting constants, though we refer the reader to the literature showing their derivation from the oxygen diffusion length, Mott potential, and temperature [ 17 , 18 ] : $$\:\frac{1}{{t}_{ox}}=A-B\text{log}\left(\frac{1}{v}\right)$$ 1 While the field of 2D oxides has postulated that the growth is governed by Cabrera Mott kinetics, this is, to our knowledge, the first study to model the kinetic dependence of 2D oxide thickness on the rate of the liquid metal meniscus deformation. This is critical because the electronic and optical properties of 2D oxides are highly thickness-dependent [ 10 , 19 , 20 ] , for example, due to the quantum confinement-induced bandgap widening in 2 nm thick InO x close to the Bohr radius. To assemble thicker 2D oxide films and heterostructures, we can stack these layers to print films of 2–100 nm total thickness. Figure 1 f shows a histogram of thicknesses for single, double, and triple ITO layers, indicating the ability to freely control the thickness via multilayer assembly. To put the speed of Cabrera Mott oxidation in perspective, we highlight that each of these layers is printed in less than 2 s. We note that the equivalent growth rate of these films is orders of magnitude higher than what is currently achievable with nanoscale growth methods such as Atomic Layer Deposition (ALD) [ 21 , 22 ] . We also highlight the repeatability of our automated liquid metal printing process, which, over the course of ten prints executed on ten different substrates, varies by less than 6 Å, signifying a highly reproducible and reliable process for the growth of 2D ITO (Figure S5). 2.2 Materials Characterization of Liquid Metal Printed 2D ITO Materials characterization of the liquid metal printed 2D ITO films reveals their high crystallinity and large-grained nature. AFM image in Fig. 1 g shows 2D ITO exhibits grain sizes ranging from 24 to 67 nm, with an average of 40 ± 11 nm (histogram is shown in Figure S6). These large grains are notable, given that the film is just 6 nm thick. This is consistent with our previous reports of superlattices of 2D In 2 O 3 that were shown by HRTEM to exhibit large platelike grains [ 9 ] . For the 2D ITO in the present study, we have also observed that the grain size is substantially larger than that of sol-gel-derived ITO films [ 12 , 13 ] . Based on other similar reports in the 2D oxide field [ 7 ] , we expect that this could be a unique feature of the liquid metal interfacial reaction. One possible explanation for the high crystallinity of our 2D ITO is the total elimination of residual organic species through our solvent-free synthesis method. Moreover, the AFM images demonstrate smooth interfaces, with an average roughness of only 0.2 nm, as shown in Fig. 1 g. Such roughness number is comparable to or even smoother than some sputtered ITO films [ 25 , 26 ] . HRTEM images (Fig. 1 h) obtained via printing of these films onto TEM grids also show the high crystallinity of 2D ITO films. The appearance of Moiré Fringes in the 2D ITO HRTEM images indicates the overlap of stacked crystalline grains produced in a single print. This result could suggest that both the top and bottom interfaces of the liquid metal meniscus contribute to their surface oxide layers during the printing process. The ability to achieve such highly crystalline morphologies at low temperatures in such a rapid process is unique compared even with vacuum-deposited films that crystallize on longer time scales, require films above a critical thickness, and generally need post-annealing at higher temperatures [ 27 – 29 ] . The enlarged view in the inset of Fig. 1 h displays well-defined lattice fringes corresponding to the (400), (222), and (413) lattice planes of cubic In 2 O 3 . The selective area electron diffraction (SAED) data, taken from the (Fig. 1 i) show a diffraction pattern typical for the cubic phase of ITO, matching reference spectra [ 30 , 31 ] . We note that there is a lack of considerable amorphous signal at low angles, indicating the nearly full crystallization of the 2D ITO film printed at 260 ºC. The potential for extrinsic Sn doping of the 2D ITO films was investigated by varying the In-Sn alloy composition and measuring the resulting ITO film via XPS. Figure 2 a shows the Sn at. % extracted for films printed at 180 ºC and at 240 ºC. Sn doping concentrations in the 2D ITO match closely to the Sn concentration in the parent alloy. This corresponds well with previous literature [ 11 , 32 ] . Figure 2 c shows the oxygen O1s peaks and their decomposed constituent peaks, including stoichiometric M-O bonding, oxygen-deficient M-O bonding, and M-OH bonding for 2D ITO printed with varying Sn concentrations. The percentage of oxygen-deficient M-O bonding peaks at ~ 532 eV increases from 24.6% for 5 at% Sn doping to 31.6%. for 20 at. % Sn doping in the parent alloy. This matches previous literature [ 33 , 34 ] and could correspond to the higher free carrier concentration of these heavily doped 2D ITO films due to a higher oxygen deficiency concentration. We also observe that with high-temperature deposition, such as at 240 ºC, the XPS signal from M-OH peak bonds (~ 534 eV) weakens, while the stoichiometric M-O peak (~ 530 eV) signal increases by, compared to the film deposited at 180 ºC. (Figure S7). Figure 2 c shows the XRD spectra of 2D ITO films with varying tin doping levels (1 at. %, 7 at. %, and 20 at. % Sn) as deposited. These show peaks corresponding to a mixture of (222) and (400) orientations, indicating the presence of the cubic In 2 O 3 structure [ 29 ] . Notably, at 7 at. % Sn doping, the 2D ITO film exhibits preferential growth along the [100] axis, with the (400) peak showing higher intensity than the (222) peak. With high doping, such as 20 at. %, the crystalline peaks exhibit attenuation, likely due to the heavy Sn doping impeding In 2 O 3 grain growth. Figure 1 d displays XRD spectra for 2D films printed with 7 at. % Sn in a Sn-In alloy at various temperatures, again revealing the presence of (222) and (400) peaks, with the latter notably more intense. Remarkably, even at a modest processing temperature of 170 ºC, a distinct crystalline (400) peak is evident. These findings suggest that the typical range of electrically optimal Sn doping in liquid metal printed films consistently favor the growth of ITO in the (400) orientation, a parallel to reports of sputtered ITO [ 35 ] . 2.3 Electrical and Electromechanical Properties of Liquid Metal Printed 2D ITO The electrical properties of 2D ITO were extensively characterized as a function of Sn doping and print temperature, both of which have a large impact on the conductivity. Increasing Sn doping from 1 at. % to 7 at. % results in a substantial increase in the conductivity of 2D ITO. The electrical conductivity peaks at approximately 1340 S/cm at 7 at. % Sn for a deposition temperature of 260°C. For heavier Sn doping approaching 20%, the ITO films begin to exhibit slightly lower conductivity. Previous studies at lower temperatures [ 11 ] reported that Sn concentrations above 10% resulted in the amorphization of 2D ITO. The higher print temperatures and the automated printing process of our present study could contribute to maintaining the crystalline cubic ITO phase even at high Sn concentrations, leading to high conductivity. For a film with a thickness of ~ 6 nm and without any post-annealing, the conductivity is superior to that of previous reports of printed ITO and comparable to even post-annealed vacuum-deposited ITO [ 27 – 29 , 36 – 40 ] . Table 1 shows a comparison of the conductivity and post-annealing conditions for thin ITO deposited by printing (upper half of Table 1 ) as well as vacuum-based processing (lower half of the table). These literature results also reflect the general trend that ITO deposited by vacuum methods exhibits lower conductivity when the film thickness is scaled to allow for ultratransparency. The parity achieved here between our liquid metal printed 2D ITO and sputtered films speaks to the unique physics of Cabrera Mott oxidation and the process control facilitated by our automated methods. While a single ultrathin layer of ITO can achieve high conductivity, it is also possible to stack multiple layers to result in a lower sheet resistance, making the films suitable for various optoelectronic device applications. Figure S8 illustrates the relationship between the sheet resistance of ITO films and the number of printed layers, showing results for single, double, and triple layers. For example, stacking three layers under identical parameters—260°C, 0.5 cm/s, and 7 at. % Sn in the precursor In-Sn alloy—achieves a sheet resistance of 300 Ω/sq. Table 1 Summary of reported performance of printed and vacuum-deposited ultrathin indium tin oxide (ITO) as a function of thickness and annealing temperature. Reference Deposition System Thickness, nm Conductivity, S/cm Annealing Temperature, ºC This Work Liquid Metal Printing 6 1340 N/A Pan et al. [36] Inkjet Printing 60 440 500 Scheideler et al. [37] Gravure Printing 150 500 400 Serkov et al. [38] Gravure Printing 400 500 Laser Annealed Kwok et al. [27] Pulsed Laser Deposition 6 830 NA Gwamuri et al. [39] RF sputtering 22 62 300 Kim et al. [40] RF sputtering 10 1400 240 Qiao et al. [29] DC Sputtering 20 12 300 Chan et al. [28] DC Sputtering 20 1488 400 2.4 Mechanical and Functional Properties of Flexible 2D ITO for Bioelectrode Applications The utility of TCOs in user interfaces such as touch screens and displays could offer an opportunity for integrating new biosignal measurement functionality into wearable technology if the associated challenges of mechanical flexibility of TCOs can be overcome. We have characterized the mechanical properties of 2D ITO specifically to address the viability of wearable biosignal measurements. Wearable TCOs must pass rigorous mechanical testing, including abrasion, tape, and bending tests, to ensure they can withstand the physical demands of continuous use. These tests simulate conditions such as repeated friction against the skin and bending around curvilinear surfaces, ensuring the electrodes maintain functionality and integrity under mechanical stress. The clinical standard for biopotential and bioimpedance measurements involves using wet electrodes such as Ag/AgCl, though these gels can be uncomfortable, can dry out over time, and may cause irritation [ 41 ] , rendering them unsuitable for long-term ECG signal acquisition. Previous research has demonstrated thin Au films [ 42 ] , PEDOT: PSS [ 43 ] , and Ag NW for flexible bioelectrodes [ 44 ] due to their flexibility, but these materials have limitations to their mechanical and thermal stability. In this work, we established 2D ITO as a potential dry electrode because it is highly flexible, transparent, and abrasion-resistant compared to traditional dry electrodes. Traditional sputtered ITO films have been extensively examined and studied for use as biosensors for various purposes [ 45 ] but have not been shown as non-invasive epidermal bioelectrodes for measuring biopotential signals. Here we show that 2D ITO printed on polymer substrates is more strain-tolerant than sputtered flexible ITO. Figure 3 a shows the normalized resistance change after 100X bending cycles at 1% tensile strain for sputtered and liquid metal-deposited ITO films. The resistance change is 5X higher in the case of sputtered ITO compared with the liquid-metal printed film. One potential factor limiting the sputtered ITO film flexibility could be residual stresses from the high-energy growth method. Another potential hypothesis is that the mixed amorphous and crystalline phases in 2D ITO could provide a better balance of mechanical flexibility and electrical performance [ 46 , 47 ] . This makes 2D ITO films more suitable for applications where the electrodes need to endure bending and flexing, such as wearable electronics and flexible biomedical sensors. As shown in the SEM images in Fig. 3 b, the sputtered ITO films exhibit significant fractures transverse to the bending direction, whereas the 2D ITO films remain intact, demonstrating superior mechanical bendability. The mechanical stability of 2D ITO was also characterized via abrasion and adhesion tests, which effectively simulate real-world conditions by evaluating the response to potential debonding from substrates during skin contact and the capability to repeatedly apply the films to the skin [ 48 – 50 ] . We assessed the abrasion resistance according to the ASTM standard by using pencil leads of increasing hardness to determine the hardness at which the film could be visibly scratched and to measure changes to its resistance (details in the Materials and Methods section). As shown in Fig. 3 c, our 2D ITO films demonstrated abrasion resistance that is 4X higher than that of the control PEDOT: PSS film, a common dry bioelectrode material. The control PEDOT:PSS film was scratched by the softest lead (2B) available, whereas one of the hardest leads (2H, 4X stronger than 2B) did not scratch the 2D ITO film. This scratch resistance illustrates a unique advantage of ceramic bioelectrodes relative to softer alternative materials. Figure 3 d illustrates the adhesion capabilities of the 2D ITO film compared to a sputtered gold film. A peel test (details in the Methods section) was conducted to observe how resistance changed with repeated tape test applications at the same locations. After the tape tests, the gold film was nearly completely delaminated, indicated by a sharp increase in normalized resistance after five applications. In contrast, the ITO film maintained its adhesion and did not de-bond, showing only a small change in normalized resistance. Characterization of the electrode-skin impedance for 2D ITO was performed to assess its suitability for various biopotential measurements. Impedance testing ensures that dry bioelectrodes make consistent and adequate contact with the skin, which is crucial for accurately measuring bioelectric signals with a high signal-to-noise ratio [ 51 ] . We conducted electrode-skin impedance tests on a 2D ITO film and compared it to clinical standard Ag/AgCl gel control electrodes using a three-electrode setup (details in the Material and Methods section). As shown in Fig. 3 e, the 2D ITO electrode exhibits comparable skin-contact impedance to Ag/AgCl per unit area from 1 Hz to 100 kHz. For comparison, the 2D ITO electrodes also show better skin-contact impedance (∼95 kΩ/cm²) compared to PEDOT: PSS values reported in the literature (169–194 kΩ/cm² ) [ 52 , 53 ] , as well as gold electrodes (305 kΩ/cm²) [ 53 ] . This low contact impedance makes the 2D ITO electrodes promising candidates for dry bioelectrodes in biopotential measurements. Figure 3 f shows a spider chart of various dry bioelectrodes’ performance in terms of flexibility, transparency, conductivity, adhesion, bio-compatibility, and wear resistance. As highlighted in this chart, 2D ITO excels in its transparency and its mechanical resilience to abrasion while offering sufficient conductivity for accurate biopotential measurements. 2.5 Multimodal Biosignal Acquisition ECG measurements using flexible 2D ITO electrodes were performed by contacting the index fingers on both hands (as described in Materials and Methods ). The left inset of Fig. 4 a shows a zoomed-in view of the P wave, QRS complex, and T wave of a single period of the electrocardiogram signal. We performed a Pearson correlation analysis comparing the measured heart rates from both the standard gel electrodes and the 2D ITO electrode. As shown in Fig. 4 b, the analysis revealed a high positive correlation of 0.98 between measurements from both the 2D ITO and gel control electrodes under variable conditions (resting and light exercise). This strong correlation signifies the reliability and practical potential of the 2D ITO dry electrodes. Moreover, we employed our highly conductive 2D ITO films to perform electromyography (EMG) measurements for tracking two different hand gestures (open and closing a hand). Figure 4 c shows the placement of the electrodes on the forearm and the measured EMG signal overlaid with the corresponding time divisions for open and closed hand gestures. To leverage the multifunctionality of these transparent flexible bioelectrodes, we next demonstrated a set of multimodal heart rate measurements using both biopotential and optical methods. This is possible because 2D ITO is highly transmissive to the source wavelengths utilized in PPG (e.g. 530 nm and 940 nm), as shown in Fig. 4 d, with an average transmittance above 95%. This allows these electrodes to be co-located with light-based PPG sensors without hindering light transmission from the PPG LED or the collection of reflected light by the PPG photodetectors. The high transmittance of 2D ITO would also allow vertical integration of these bioelectrodes with displays in future wearable systems. Figure 4 e shows a schematic of the dual measurement of both ECG and PPG signals. Such integration enables simultaneous ECG and PPG measurements from the same site, combining the detailed electrical activity captured by ECG with the vascular blood volume changes detected by PPG. The advantage of performing these measurements simultaneously for cardiovascular monitoring is that ECG can be utilized to validate and correct motion artifacts commonly seen in PPG signals [ 54 , 55 ] . As shown in Fig. 4 f, heart rate measured using PPG (78–83 BPM) during light activity matches the precise measurements from electrical monitoring with the 2D ITO electrode (80–82 BPM). Additionally, as seen in Fig. 4 f, the peak of the PPG coinciding with the T peak of the ECG indicates temporally synchronized cardiac cycle phases. An additional advantage of these synchronous measurements could be the ability to detect anomalies, such as arrhythmias, that are impossible to perceive via PPG measurements alone. This dual measurement capability, facilitated by the transparency of ITO electrodes, provides a more robust and holistic approach to cardiovascular health monitoring while also minimizing the area needed for multimodal measurements in a miniaturized device form factor. 3. Conclusion In summary, we present flexible 2D TCOs fabricated at ultralow temperatures using vacuum-free Cabrera Mott oxidation of liquid hypoeutectic In-Sn alloys, demonstrating wafer-scale films with angstrom-level thickness control via an automated, kinetically driven approach. Our synthesis rapidly (at up to 20 cm/s) produces smooth, ultrathin 2D ITO with unprecedented conductivity (> 1300 S/cm) comparable to vacuum-deposited films. Surface morphology and structural characterization confirm the effectiveness of Sn-doping, revealing the high crystallinity of these 2D oxides and the large, plate-like grains formed in the liquid metal reaction environment. We observe that a significant result of the ultrathin nature of 2D ITO and the liquid metal printing process is enhanced bending strain tolerance, superior scratch resistance, and low contact impedance for 2D ITO when used as wearable bioelectrodes. Finally, leveraging the conductivity and transparency of 2D ITO, we enable simultaneous, multimodal measurements via electrocardiography (ECG) and photoplethysmography (PPG). These findings represent a significant improvement in the performance of printed metal oxides and introduce a promising new material for multimodal biometrics. 4. Experimental Methods Alloy Preparation The In-Sn alloys were prepared by melting In ( Luciteria , 99.995%) and Sn ( Luciteria , 99.995%) pellets in a graphite crucible at 300°C for 2 hours in an inert nitrogen atmosphere glovebox (< 10 ppm O 2 , < 10 ppm H 2 O) to minimize surface oxidation. Alloys containing 1, 2.5, 5, 7, 10, 20, and 30 at. % Sn was prepared by mixing the respective amounts of Sn with In. Automated 2D TCO Synthesis and Deposition 2D indium tin oxides (ITOs) were deposited by rolling molten In-Sn alloy droplets along a substrate (Si/SiO 2 , glass, PET, or PEN) on a hotplate using a silicone roller controlled by a 3-axis inline gantry robot ( Fisnar 5300N ). The deposition process was conducted at speeds from 0.1 to 20 cm/s and temperatures from 140 to 290°C (printing at 140 ºC was specifically enabled by high Sn-concentrations, as detailed in Fig. 1 b). Prior to deposition, the target substrates were treated with approximately 10 seconds of atmospheric plasma using a Plasma-Etch 1000W system supplied with 30 LPM compressed dry air to promote the 2D ITO film adhesion. Two dummy substrates placed before and after the target substrate were used to allow the deposition process to reach equilibrium and produce a uniform and continuous metal oxide film deposition. After deposition, the residual liquid metal on the surface of the oxide film was removed with a squeegee while still on the hotplate and again once the sample had cooled to room temperature. Thin Film Fabrication For the abrasion tests, the control PEDOT: PSS films were prepared by spin-coating a 1 wt. % PEDOT: PSS solution in H 2 O onto a SiO 2 (300nm)/Si substrate at 6000 RPM for 1 minute, followed by drying on a hotplate at 100°C. An Anatech LTD Hummer 6.2 sputtering system was utilized to deposit a 15 nm-thick gold film on a soda lime glass substrate for the peeling test. Materials Characterization : A Differential Scanning Calorimeter (DSC) (Discovery DSC 250, TA Instruments ) was used to measure the melting temperature of the Sn-In alloys with various Sn compositions with a ramp rate of 10°C /min under N 2 flow. X-ray photoelectron spectroscopy (XPS) was conducted using a Kratos Axis Supra XPS at approximately 10⁻⁹ Torr on three layers of 2D ITO films printed on 100 nm SiO₂ substrates. Elemental analysis of the 2D ITO films was performed by comparing the Sn 3d, In 3d, and O1s peaks. Optical microscope images were captured with a Keyence VHX-7100 microscope. UV-Vis spectroscopy was conducted using a DeNovix DS-11 FX + spectrophotometer to measure the printed ITO films' absorbance spectra (270–800 nm) on glass substrates. AFM was performed using an AIST-NT instrument in tapping mode to measure film thickness and grain morphology. High-resolution transmission electron microscopy (HRTEM) was carried out with a Thermo Scientific Talos F200i instrument. Samples for HRTEM imaging were prepared by liquid metal printing of 2D ITO (7 at. % Sn) directly onto TEM grids (Carbon Square Mesh, Cu, 300 Mesh, UL, EMS) at 260 ºC, with excess liquid metal removed using a silicone squeegee. X-ray diffraction (XRD) was performed using a Rigaku UltraX Cu-anode diffractometer (Cu Kα radiation at 40 kV, 300 mA, λ = 0.154 nm) with a scanning rate of 0.5° per minute on single printed layers of 2D ITO on 300 nm SiO₂ substrates. Grain size analysis was conducted via AFM phase imaging and confirmed through HRTEM images. Scanning electron microscopy (SEM) was performed using a Thermo Scientific Helios 5 CX tool. Electrical Characterization Sheet resistance was measured using a four-point probe at room temperature in air. Thickness Characterization : Films for measuring thickness were printed on SiO 2 /Si substrates (300 nm SiO 2 ). The exact SiO 2 thickness was measured to facilitate the modeling of the reflectance spectrum of the 2D ITO films on SiO 2 spectroscopic reflectometry (F3-sX, Filmetrics) from 380 nm – 1050 nm to extract the thickness of the films. These thicknesses measured by reflectometry were confirmed via AFM line scans of films patterned by wet etching. Multilayer films > 20 nm thick were also measured via stylus profilometry (KLA Tencor D-500) to confirm the reflectometry measured thickness. Mechanical Characterization of Flexible 2D ITO Films Bending resilience measurements were performed on 2D ITO films deposited onto 60 µm thick polyimide substrates at 260°C and sputtered ITO onto 175 µm PET substrate. The films were measured after the substrate was bent to 1%, 1.25%, and 1.5% of tensile strain until 100 bending cycles were achieved. The film hardness test was conducted using the ASTM D3363 standard, which entails scratching thin films, such as 2D ITO (printed with 7 at. % Sn doped at 260°C) on silicon and spin-coated PEDOT, using pencil leads of varying hardness. The films were imaged with optical microscopy, and changes in electrical resistance were observed after each iteration of abrasion with the specified pencil leads. Abrasion was applied with a force of 7 N, an angle of around 45°, and a speed of approximately 0.5 cm/s. An adhesion test for the 2D ITO film (printed with 7 at. % Sn doped at 260°C) on polyimide and sputtered Au on glass was performed using Kapton tape (Uline). The tape was removed at a speed of approximately 1 cm/s and at an angle of ~ 90° to the tested film. 2D ITO Bioelectrode Characterization ECG measurements were conducted by wrapping printed ITO electrodes that are printed with 7 at% Sn doped at 260°C on polyimide in a single lead setup (Fig. 4 a). As a control, a gel electrode (3M-2238 Electrode) was placed on the forearm. The heart rate was calculated using the Eq. 2, shown below: Heart rate (beats per minute) = 60/RR (2) Where RR is the time interval in seconds between two consecutive R peaks of the measured ECG signal. Both measurements were performed using a Vernier EKG Sensor (Go Direct). EMG measurements were performed by placing these ITO electrodes on polyimide on the forearm (Fig. 4 c) and using the Vernier Go Direct system for data collection. A Polar Verity sensor was used to measure the PPG from the wrist. The transparent ITO electrode on polyimide, which measures the ECG signal, was placed between the PPG LED and the wrist. This setup allows simultaneous measurements of both PPG and ECG. Declarations Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Conflict of Interest The authors declare no conflict of interest. Data Availability The data that support the findings of this study are available from the corresponding author upon reasonable request. Acknowledgments Saifur Rahman was supported by the Dartmouth PhD Innovation Fellowship. This research was supported by the National Science Foundation Electronic and Photonic Materials Program (Award #2202501) as well as the National Science Foundation Electronics, Photonics, and Magnetic Devices program (Award #2219991). We acknowledge Paul Defino at Dartmouth College for assistance in AFM scans as well as John Wilderman at the University of New Hampshire for performing XPS measurements. Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff)) References D. R. Cairns, D. C. Paine, G. P. Crawford, MRS Online Proceedings Library (OPL) 2001, 666 , F3.24. K. Sim, Z. Rao, Z. Zou, F. Ershad, J. Lei, A. Thukral, J. Chen, Q.-A. Huang, J. Xiao, C. Yu, Science Advances 2019, 5 , eaav9653. U. Kim, M. Han, J. Jang, J. Shin, M. Park, J. Byeon, M. Choi, Advanced Energy Materials 2023, 13 , 2203198. M. N. Le, K.-J. Baeg, K.-T. Kim, S.-H. Kang, B. D. Choi, C.-Y. Park, S.-P. Jeon, S. Lee, J.-W. Jo, S. Kim, J.-G. Park, D. Ho, J. Hong, M. Kim, H.-K. Kim, C. Kim, K. Kim, Y.-H. Kim, S. K. Park, M.-G. Kim, Advanced Functional Materials 2021, 31 , 2103285. H. S. Jung, K. Eun, Y. T. Kim, E. K. Lee, S.-H. Choa, Microsyst Technol 2017, 23 , 1961. A. Zavabeti, J. Z. Ou, B. J. Carey, N. Syed, R. Orrell-Trigg, E. L. H. Mayes, C. Xu, O. Kavehei, A. P. O’Mullane, R. B. Kaner, K. Kalantar-zadeh, T. Daeneke, Science 2017, 358 , 332. W. J. Scheideler, K. Nomura, Advanced Functional Materials n.d., n/a , 2403619. A. Zavabeti, P. Aukarasereenont, H. Tuohey, N. Syed, A. Jannat, A. Elbourne, K. A. Messalea, B. Y. Zhang, B. J. Murdoch, J. G. Partridge, M. Wurdack, D. L. Creedon, J. van Embden, K. Kalantar-Zadeh, S. P. Russo, C. F. McConville, T. Daeneke, Nat Electron 2021, 4 , 277. Y. Ye, A. B. Hamlin, J. E. Huddy, M. S. Rahman, W. J. Scheideler, Advanced Functional Materials 2022, 32 , 2204235. A. B. Hamlin, Y. Ye, J. E. Huddy, M. S. Rahman, W. J. Scheideler, npj 2D Mater Appl 2022, 6 , 1. R. S. Datta, N. Syed, A. Zavabeti, A. Jannat, M. Mohiuddin, M. Rokunuzzaman, B. Yue Zhang, M. A. Rahman, P. Atkin, K. A. Messalea, M. B. Ghasemian, E. D. Gaspera, S. Bhattacharyya, M. S. Fuhrer, S. P. Russo, C. F. McConville, D. Esrafilzadeh, K. Kalantar-Zadeh, T. Daeneke, Nat Electron 2020, 3 , 51. Q. Li, J. Lin, T.-Y. Liu, X.-Y. Zhu, W.-H. Yao, J. Liu, npj 2D Mater Appl 2021, 5 , 1. B. J. Carey, J. Z. Ou, R. M. Clark, K. J. Berean, A. Zavabeti, A. S. R. Chesman, S. P. Russo, D. W. M. Lau, Z.-Q. Xu, Q. Bao, O. Kavehei, B. C. Gibson, M. D. Dickey, R. B. Kaner, T. Daeneke, K. Kalantar-Zadeh, Nat Commun 2017, 8 , 14482. J. Lin, Q. Li, T.-Y. Liu, Y. Cui, H. Zheng, J. Liu, physica status solidi (RRL) – Rapid Research Letters 2019, 13 , 1900271. Y. Zhang, D. Venkatakrishnarao, M. Bosman, W. Fu, S. Das, F. Bussolotti, R. Lee, S. L. Teo, D. Huang, I. Verzhbitskiy, Z. Jiang, Z. Jiang, J. Chai, S. W. Tong, Z.-E. Ooi, C. P. Y. Wong, Y. S. Ang, K. E. J. Goh, C. S. Lau, ACS Nano 2023, 17 , 7929. Z. J. Farrell, A. R. Jacob, V. K. Truong, A. Elbourne, W. Kong, L. Hsiao, M. D. Dickey, C. Tabor, Chem. Mater. 2023, 35 , 964. N. Cabrera, N. F. Mott, Rep. Prog. Phys. 1949, 12 , 163. K. N. Goswami, R. W. Staehle, Electrochimica Acta 1971, 16 , 1895. I. Isakov, H. Faber, A. D. Mottram, S. Das, M. Grell, A. Regoutz, R. Kilmurray, M. A. McLachlan, D. J. Payne, T. D. Anthopoulos, Advanced Electronic Materials 2020, 6 , 2000682. J. Liu, Y. Nie, W. Xue, L. Wu, H. Jin, G. Jin, Z. Zhai, C. Fu, Journal of Materials Research and Technology 2020, 9 , 8020. J. W. Elam, D. A. Baker, A. B. F. Martinson, M. J. Pellin, J. T. Hupp, J. Phys. Chem. C 2008, 112 , 1938. H. Y. Kim, E. A. Jung, G. Mun, R. E. Agbenyeke, B. K. Park, J.-S. Park, S. U. Son, D. J. Jeon, S.-H. K. Park, T.-M. Chung, J. H. Han, ACS Appl. Mater. Interfaces 2016, 8 , 26924. R. Sarhaddi, N. Shahtahmasebi, M. Rezaee Rokn-Abadi, M. M. Bagheri-Mohagheghi, Physica E: Low-dimensional Systems and Nanostructures 2010, 43 , 452. S.-J. Hong, J.-I. Han, Current Applied Physics 2006, 6 , e206. M. Bragaglia, F. R. Lamastra, M. Tului, L. Di Gaspare, A. Notargiacomo, M. Valentini, F. Nanni, Surfaces and Interfaces 2019, 17 , 100365. A. P. Amalathas, M. M. Alkaisi, J Mater Sci: Mater Electron 2016, 27 , 11064. H. S. Kwok, X. W. Sun, D. H. Kim, Thin Solid Films 1998, 335 , 299. S.-H. Chan, M.-C. Li, H.-S. Wei, S.-H. Chen, C.-C. Kuo, Journal of Nanomaterials 2015, 2015 , 179804. Z. Qiao, R. Latz, D. Mergel, Thin Solid Films 2004, 466 , 250. J. Ba, D. Fattakhova Rohlfing, A. Feldhoff, T. Brezesinski, I. Djerdj, M. Wark, M. Niederberger, Chem. Mater. 2006, 18 , 2848. H.-W. Wang, C.-F. Ting, M.-K. Hung, C.-H. Chiou, Y.-L. Liu, Z. Liu, K. R. Ratinac, S. P. Ringer, Nanotechnology 2009, 20 , 055601. N. Nadaud, N. Lequeux, M. Nanot, J. Jové, T. Roisnel, Journal of Solid State Chemistry 1998, 135 , 140. S. Peng, X. Cao, J. Pan, X. Wang, X. Tan, A. E. Delahoy, K. K. Chin, J. Electron. Mater. 2017, 46 , 1405. A. J. Nelson, H. Aharoni, Journal of Vacuum Science & Technology A 1987, 5 , 231. K. Wang, P. Jiao, Y. Cheng, H. Xu, G. Zhu, Y. Zhao, K. Jiang, X. Zhang, Y. Su, Optical Materials 2022, 134 , 113040. Y. Pan, M. Liu, C. Lu, B. Liu, W. Shao, D. Pan, X. Shi, Langmuir 2023, 39 , 5107. W. J. Scheideler, J. Jang, M. A. U. Karim, R. Kitsomboonloha, A. Zeumault, V. Subramanian, ACS Appl. Mater. Interfaces 2015, 7 , 12679. A. A. Serkov, H. V. Snelling, S. Heusing, T. M. Amaral, Sci Rep 2019, 9 , 1773. J. Gwamuri, M. Marikkannan, J. Mayandi, P. K. Bowen, J. M. Pearce, Materials 2016, 9 , 63. Y. W. Kim, J. S. Goo, T. H. Lee, B. R. Lee, S.-C. Shin, H. Kim, J. W. Shim, T. G. Kim, Journal of Power Sources 2019, 424 , 165. A. A. Chlaihawi, B. B. Narakathu, S. Emamian, B. J. Bazuin, M. Z. Atashbar, Sensing and Bio-Sensing Research 2018, 20 , 9. Y. Zhao, M. Yu, Z. Liu, Z. Yu, Journal of Applied Physics 2019, 125 , 165305. B. Ji, M. Wang, C. Ge, Z. Xie, Z. Guo, W. Hong, X. Gu, L. Wang, Z. Yi, C. Jiang, B. Yang, X. Wang, X. Li, C. Li, J. Liu, Biosensors and Bioelectronics 2019, 135 , 181. A. C. Myers, H. Huang, Y. Zhu, RSC Adv. 2015, 5 , 11627. E. B. Aydın, M. K. Sezgintürk, TrAC Trends in Analytical Chemistry 2017, 97 , 309. S. Park, J. Yoon, S. Kim, P. Song, RSC Adv. 2021, 11 , 3439. H. Seo, B. Kim, K. H. Lee, S. Chae, J. Jung, ACS Appl. Mater. Interfaces 2022, 14 , 25620. D. Doci, M. Ademi, K. Tuvshinbayar, N. Richter, G. Ehrmann, T. Spahiu, A. Ehrmann, Coatings 2023, 13 , 1624. B. Vasconcelos, P. Fiedler, R. Machts, J. Haueisen, C. Fonseca, Front. Neurosci. 2021, 15 , DOI 10.3389/fnins.2021.748100 . I. Hutchings, M. Gee, E. Santner, in Springer Handbook of Materials Measurement Methods (Eds.: H. Czichos, T. Saito, L. Smith), Springer, Berlin, Heidelberg, 2006, pp. 685–710. K. Goyal, D. A. Borkholder, S. W. Day, Sensors 2022, 22 , 8510. L.-W. Lo, J. Zhao, K. Aono, W. Li, Z. Wen, S. Pizzella, Y. Wang, S. Chakrabartty, C. Wang, ACS Nano 2022, 16 , 11792. Y. Chen, W. Pei, S. Chen, X. Wu, S. Zhao, H. Wang, H. Chen, Sensors and Actuators B: Chemical 2013, 188 , 747. F. Sartor, J. Gelissen, R. van Dinther, D. Roovers, G. B. Papini, G. Coppola, BMC Sports Science, Medicine and Rehabilitation 2018, 10 , 10. H.-W. Chow, C.-C. Yang, JMIR mHealth and uHealth 2020, 8 , e14707. Additional Declarations No competing interests reported. Supplementary Files SupplementaryInformation.docx Cite Share Download PDF Status: Published Journal Publication published 23 Nov, 2024 Read the published version in npj Flexible Electronics → Version 1 posted Editorial decision: Revision requested 08 Oct, 2024 Reviews received at journal 02 Oct, 2024 Reviews received at journal 09 Sep, 2024 Reviewers agreed at journal 05 Sep, 2024 Reviewers agreed at journal 04 Sep, 2024 Reviewers agreed at journal 30 Aug, 2024 Reviewers invited by journal 30 Aug, 2024 Editor assigned by journal 20 Aug, 2024 Submission checks completed at journal 20 Aug, 2024 First submitted to journal 12 Aug, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4903114","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":353763992,"identity":"b24fc51f-2c5e-4a9e-afae-99a314abc271","order_by":0,"name":"Md Saifur Rahman","email":"","orcid":"","institution":"Dartmouth College","correspondingAuthor":false,"prefix":"","firstName":"Md","middleName":"Saifur","lastName":"Rahman","suffix":""},{"id":353763993,"identity":"1fc5042b-e936-4f66-bfcd-26183ff7c84f","order_by":1,"name":"Simon A. Agnew","email":"","orcid":"","institution":"Dartmouth College","correspondingAuthor":false,"prefix":"","firstName":"Simon","middleName":"A.","lastName":"Agnew","suffix":""},{"id":353763994,"identity":"1295fbf7-8421-48f1-ade8-13c5dd948324","order_by":2,"name":"Samuel Ong","email":"","orcid":"","institution":"Dartmouth College","correspondingAuthor":false,"prefix":"","firstName":"Samuel","middleName":"","lastName":"Ong","suffix":""},{"id":353763995,"identity":"c400bb4c-8bf1-46ef-bbc7-20bd59e398c8","order_by":3,"name":"William J. Scheideler","email":"data:image/png;base64,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","orcid":"","institution":"Dartmouth College","correspondingAuthor":true,"prefix":"","firstName":"William","middleName":"J.","lastName":"Scheideler","suffix":""}],"badges":[],"createdAt":"2024-08-13 00:07:19","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4903114/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4903114/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41528-024-00371-7","type":"published","date":"2024-11-23T15:56:52+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":65205036,"identity":"0533364c-9c7f-41ff-bb60-cf8610f27cec","added_by":"auto","created_at":"2024-09-24 18:04:04","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":80763,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eKinetically-controlled liquid metal printing, CM modeling, AFM, TEM\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(\u003cstrong\u003ea\u003c/strong\u003e) Schematic of automated liquid-metal printing of 2D ITO with controlled printing speed and film thickness. (\u003cstrong\u003eb\u003c/strong\u003e) Dynamic scanning calorimetry showing melting temperature vs. Sn concentration in the In-Sn alloy system. The inset shows the molten In-Sn alloy (20 at. % Sn). (\u003cstrong\u003ec\u003c/strong\u003e) Photographs of 2D ITO films printed on glass and flexible polymer substrates. The scale bar is 1 cm. (\u003cstrong\u003ed\u003c/strong\u003e) Large area (30 cm\u003csup\u003e2\u003c/sup\u003e) film of 2D ITO (left) with corresponding thickness map of 20 cm long 2D ITO strip (200 °C, 7 at. % Sn). (\u003cstrong\u003ee\u003c/strong\u003e) Measured thickness of 2D ITO and 2D In\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e films printed at 260 °C vs. printing speed with Cabrera-Mott growth kinetics equation shown. (\u003cstrong\u003ef\u003c/strong\u003e) Histogram of measured thicknesses for 1, 2, and 3-layers 2D ITO films shown in the inset. (\u003cstrong\u003eg\u003c/strong\u003e) AFM phase image of a single layer 2D ITO film. (\u003cstrong\u003eh\u003c/strong\u003e) HRTEM image of a single layer ITO film. Insets show crystallites of c-ITO in multiple orientations. (\u003cstrong\u003ei\u003c/strong\u003e) Selected area electron diffraction (SAED) pattern for single layer 2D ITO film of the same HRTEM area. All the films are as deposited, with no post annealing.\u003c/p\u003e","description":"","filename":"Picture1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4903114/v1/fdfde319ca5e2e6318d926aa.jpg"},{"id":65204761,"identity":"33c68fd0-6851-4dfa-bad9-19f8b7c77378","added_by":"auto","created_at":"2024-09-24 17:56:04","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":92220,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eXPS, XRD, and electrical characterization of 2D ITO film\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(\u003cstrong\u003ea\u003c/strong\u003e) Comparison of Sn concentration in oxide measured through XPS to Sn metal concentration (at. %) in the alloy, printed at 180°C and 240 °C with a print speed of ~1cm/s. (\u003cstrong\u003eb\u003c/strong\u003e) XPS O1s analysis of ITO films from 5, 10, and 20 at. % Sn doping, printed at 260 °C, with no post annealing and a print speed of ~1cm/s. (\u003cstrong\u003ec\u003c/strong\u003e) XRD spectra for a single layer ITO film printed with the alloys of 1, 7, and 20 at. % Sn at 260 °C with no post-annealing, a print speed of 0.5 cm/s. (\u003cstrong\u003ed\u003c/strong\u003e) XRD spectra for single layer ITO films vs. printing temperature, printed with the alloys of 7 at. % Sn, with no post-annealing and a print speed of 0.5 cm/s (\u003cstrong\u003ee\u003c/strong\u003e) Conductivity of a single layer ITO film vs. Sn concentration (at.%) in the alloy, printed at 260 °C with no post-annealing and a print speed of 0.5 cm/s (\u003cstrong\u003ef\u003c/strong\u003e) Conductivity of a single layer ITO film vs. deposition temperature, printed with 7 at.% Sn concentration in the alloy, no post-annealing, and a print speed of 0.5 cm/s\u003c/p\u003e","description":"","filename":"Picture2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4903114/v1/297020ea013fde38d5ead9b9.jpg"},{"id":65204759,"identity":"cbd78f1e-a656-4847-91fd-6fe964eae8ad","added_by":"auto","created_at":"2024-09-24 17:56:04","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":50144,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMechanical characterization of 2D ITO electrodes\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(\u003cstrong\u003ea\u003c/strong\u003e) Normalized resistance change vs. bending cycles for sputtered ITO (45 nm) and liquid metal printed 2D ITO (12 nm) at 1% tensile strain. Scale bars are 50 µm. (\u003cstrong\u003eb\u003c/strong\u003e) SEM images for 2D ITO film and sputtered ITO film after 100 cycles of tensile strain. (\u003cstrong\u003ec\u003c/strong\u003e) Normalized resistance change from scratches with various pencil leads with increasing hardness on PEDOT: PSS (100 nm) and 2D ITO film (23 nm). (\u003cstrong\u003ed\u003c/strong\u003e) Normalized resistance change vs number of tape test applications for adhesion test for 2D ITO (23 nm) and sputtered Au (15nm). (e\u003cstrong\u003e)\u003c/strong\u003eSkin-electrode impedance versus frequency for 2D ITO film and commercial Ag/AgCl gel electrodes, Inset shows the configuration of the ITO electrode impedance setup with ITO electrode as working electrode (WE) and commercial Ag/AgCl gel electrode as reference electrode (RE) and counter electrode (CE) (\u003cstrong\u003ef\u003c/strong\u003e) Spider chart on the relevant properties of various dry bioelectrodes.\u003c/p\u003e","description":"","filename":"Picture3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4903114/v1/c0ed7cc88362eb9f9b99a1cd.jpg"},{"id":65204758,"identity":"282610b9-6373-4130-a42f-eeb32e96248d","added_by":"auto","created_at":"2024-09-24 17:56:04","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":110275,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMultimodal measurement of biosignals with 2D ITO electrodes\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(\u003cstrong\u003ea\u003c/strong\u003e) ECG signals are recorded with a 2D ITO electrode; the left inset shows a representative peak showing the PQRST parts of the peak, right inset shows the single lead ECG setup at the fingertips. (\u003cstrong\u003eb\u003c/strong\u003e) Pearson correlation between heart rate measurements with 2D ITO and gel electrode for two different physiological conditions (c\u003cstrong\u003e)\u003c/strong\u003e EMG signal for two kinds of hand gestures are captured with 2D ITO electrodes; the inset schematic shows the placements of the electrodes. (\u003cstrong\u003ed\u003c/strong\u003e) Transmittance vs. wavelength for 1 layer of ITO on PI with 7 at. % Sn ITO, printed at 260 °C. (\u003cstrong\u003ee\u003c/strong\u003e) A schematic of the placement of the transparent ITO on the wrist area underneath the PPG. (\u003cstrong\u003ef\u003c/strong\u003e) Simultaneous measurement of PPG and ECG measurements.\u003c/p\u003e","description":"","filename":"Picture4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4903114/v1/eae443d855eb98a83ea65752.jpg"},{"id":69834794,"identity":"88ab9d3f-fb28-47fa-9650-209df3f54fba","added_by":"auto","created_at":"2024-11-25 16:08:57","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1058920,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4903114/v1/75c5b4bb-ab29-4e70-b8cd-aefa602895c3.pdf"},{"id":65205037,"identity":"7a50d282-10a1-4609-9bcf-4f15fd173aaa","added_by":"auto","created_at":"2024-09-24 18:04:04","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1727900,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-4903114/v1/4e97a7750be2acfcb0c0a3d1.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Kinetic Liquid Metal Synthesis of Flexible 2D Conductive Oxides for Multimodal Wearable Sensing","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eTransparent conducting oxides (TCOs) such as indium tin oxide (ITO) are critical materials for electronic and optoelectronic devices, providing the transparency necessary for displays, solar cells, sensors, and various user interface devices. However, state-of-the-art TCOs have two major drawbacks \u0026ndash; they are comparatively expensive due to the need for vacuum deposition and they are traditionally considered to be brittle materials\u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e with limited suitability for flexible, lightweight systems. The minimization of film stresses via strain-tolerant serpentine structures has shown the promise of transparent metal oxides as active materials for wearable sensors, but these approaches have the drawback of requiring complex patterning and yielding low areal density circuits\u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e. Recent works have also demonstrated complementary approaches of fabricating on ultrathin substrates\u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e to minimize the bending strain of metal oxide thin films and using multilayered organic-inorganic hybrid structures to relieve bending-induced mechanical stresses\u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e. Studies of the mechanics of TCOs such as ITO have revealed that minimizing the TCO film thickness and reducing the process temperature both lead to enhanced mechanical reliability\u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eAn emerging class of ultrathin liquid metal-derived\u003csup\u003e[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e two-dimensional (2D) oxides\u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e could overturn the longstanding limitations of transparent conducting oxides, offering a platform for high-performance wearable electronics based on inorganic, wide-bandgap materials. The field of 2D oxides has demonstrated, for example, the formation of ultrathin materials with layered polymorphs (e.g. β-TeO\u003csub\u003e2\u003c/sub\u003e) just 5\u0026ndash;40 \u0026Aring; thick\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e, the potential for assembling these materials into heterostructures with unique electronic properties\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e, a tendency to favor highly crystalline phases compared with oxides printed from solution\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e, and the ability to yield conductors with extremely high transparency (\u0026gt;\u0026thinsp;99%) and enhanced mechanical flexibility\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e. However, the 2D oxide field faces several ongoing challenges limiting its impact. For example, there is a demand for advancing large-area uniformity while precisely controlling the liquid metal meniscus \u0026ndash; the key feature for modulating the printed film thickness and controlling the oxidation kinetics. Widely variable thicknesses are reported in the literature for a single material such as In\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e or Ga\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e due to the variable dynamics of manual methods based on blade coating\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e, dispensing from a syringe\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e, and squeeze printing between two flat substrates\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e. Additionally, there is a need to introduce extrinsic dopants to control and enhance electronic properties\u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e since the precursor metal alloy constituents compete for representation in the surface oxide\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e. If these challenges can be addressed, 2D oxides could make a significant impact on the field of flexible electronics.\u003c/p\u003e \u003cp\u003eHerein we report the first demonstration of high-speed automated liquid metal printing over large areas with \u0026Aring;-level precision control over Cabrera Mott oxidation kinetics. We apply this method to fabricate flexible, two-dimensional (2D) indium tin oxide (ITO) transparent electrodes from the surface oxides of liquid indium tin alloys at temperatures as low as 140 \u0026ordm;C on flexible polymer substrates. Our detailed materials characterization of these films reveals that by controlling crystallization and doping of 2D ITO it is possible to achieve superlative conductivity and high transparency at unprecedented process speeds as well as outstanding mechanical resilience, including high scratch resistance and enhanced bending strain. Finally, using these materials, we report the first demonstration of 2D oxide-based bioelectrical measurements, showing an efficient multimodal sensing approach for combining electrocardiography (ECG) and pulse plethysmography (PPG) utilizing the high transparency of liquid metal-printed ITO.\u003c/p\u003e"},{"header":"2. Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Liquid Metal Printing of 2D Indium Tin Oxide\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eWe have developed an automated platform for liquid metal printing (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e1\u003c/span\u003ea) that precisely controls the Cabrera-Mott oxidation kinetics of liquid metals. This is the first report of robotic, wafer-scale liquid metal printing of 2D oxides (Figure S1), replacing previous manual \"touch printing\" methods\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e. Our system can vary the printing speed over a 200-fold range, from 0.1 cm/s to 20 cm/s, allowing us to print uniform films over areas greater than 100 cm\u0026sup2;. The precursors for liquid metal printing in the present study are hypoeutectic, indium-rich In-Sn alloys, though we note that the automated method can be extended to a multitude of alloys for printing various 2D oxides. Indium and tin form a eutectic mixture at 52\u0026thinsp;\u0026minus;\u0026thinsp;48 wt.% Sn with a melting temperature of approximately 117\u0026deg;C (Figure S2). The concentration of Sn in the alloy determines the melting temperature, with hypoeutectic compositions (less than 48% Sn) lowering \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003em\u003c/em\u003e\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) significantly below the T\u003csub\u003em\u003c/sub\u003e of pure In. This allows us to print 2D ITO films at extremely low deposition temperatures, making the process compatible with thermally sensitive substrates such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyimide (PI), and glass (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e1\u003c/span\u003ec).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe automated process gives us the ability to model the kinetics of liquid metal surface oxidation and explore the inherent limits to the thickness uniformity across large areas, assessing the fundamental repeatability of this synthetic method. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e1\u003c/span\u003ed shows a printed 2D ITO film covering approximately 10 x 10 cm, with a thickness of 5.8 nm and a standard deviation of just 4 \u0026Aring;. The thickness is measured using spectroscopic UV reflectometry (see \u003cem\u003eMaterials and Methods\u003c/em\u003e and Figure S3) and confirmed with atomic force microscopy (AFM) (Figure S4). An essential feature of our method is that it offers the ability to print films of programmed thickness by modulating the printing speed. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e1\u003c/span\u003ee shows the thickness of 2D oxide printed at 260 \u0026ordm;C at speeds ranging from 0.1 cm/s to 20 cm/s for both undoped In\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and ITO films (7 at. % Sn). These results show that, at high speeds, the films asymptotically approach a thickness of approximately 2.4 nm in the case of ITO and 2.5 nm in the case of In\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. Reducing print speed (\u0026lt;\u0026thinsp;1 cm/s) allows the printing of thicker single layers approaching 5\u0026ndash;6 nm. To understand the physics determining the 2D oxide film thickness (\u003cem\u003et\u003c/em\u003e\u003csub\u003e\u003cem\u003eox\u003c/em\u003e\u003c/sub\u003e), we consider the Cabrera Mott oxidation kinetics model shown below (Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) where the speed of the roller (\u003cem\u003ev\u003c/em\u003e) is treated as the inverse of time (\u003cem\u003et\u003c/em\u003e) for the oxide growth on the liquid metal meniscus. In this approximation, A and B are fitting constants, though we refer the reader to the literature showing their derivation from the oxygen diffusion length, Mott potential, and temperature \u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e:\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv id=\"Equ1\" class=\"Equation\"\u003e \u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:\\frac{1}{{t}_{ox}}=A-B\\text{log}\\left(\\frac{1}{v}\\right)$$\u003c/div\u003e \u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eWhile the field of 2D oxides has postulated that the growth is governed by Cabrera Mott kinetics, this is, to our knowledge, the first study to model the kinetic dependence of 2D oxide thickness on the rate of the liquid metal meniscus deformation. This is critical because the electronic and optical properties of 2D oxides are highly thickness-dependent \u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e, for example, due to the quantum confinement-induced bandgap widening in 2 nm thick InO\u003csub\u003ex\u003c/sub\u003e close to the Bohr radius. To assemble thicker 2D oxide films and heterostructures, we can stack these layers to print films of 2\u0026ndash;100 nm total thickness. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e1\u003c/span\u003ef shows a histogram of thicknesses for single, double, and triple ITO layers, indicating the ability to freely control the thickness via multilayer assembly. To put the speed of Cabrera Mott oxidation in perspective, we highlight that each of these layers is printed in less than 2 s. We note that the equivalent growth rate of these films is orders of magnitude higher than what is currently achievable with nanoscale growth methods such as Atomic Layer Deposition (ALD) \u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e. We also highlight the repeatability of our automated liquid metal printing process, which, over the course of ten prints executed on ten different substrates, varies by less than 6 \u0026Aring;, signifying a highly reproducible and reliable process for the growth of 2D ITO (Figure S5).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Materials Characterization of Liquid Metal Printed 2D ITO\u003c/h2\u003e \u003cp\u003eMaterials characterization of the liquid metal printed 2D ITO films reveals their high crystallinity and large-grained nature. AFM image in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e1\u003c/span\u003eg shows 2D ITO exhibits grain sizes ranging from 24 to 67 nm, with an average of 40\u0026thinsp;\u0026plusmn;\u0026thinsp;11 nm (histogram is shown in Figure S6). These large grains are notable, given that the film is just 6 nm thick. This is consistent with our previous reports of superlattices of 2D In\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e that were shown by HRTEM to exhibit large platelike grains\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e. For the 2D ITO in the present study, we have also observed that the grain size is substantially larger than that of sol-gel-derived ITO films \u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. Based on other similar reports in the 2D oxide field\u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e, we expect that this could be a unique feature of the liquid metal interfacial reaction. One possible explanation for the high crystallinity of our 2D ITO is the total elimination of residual organic species through our solvent-free synthesis method. Moreover, the AFM images demonstrate smooth interfaces, with an average roughness of only 0.2 nm, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e1\u003c/span\u003eg. Such roughness number is comparable to or even smoother than some sputtered ITO films \u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e. HRTEM images (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e1\u003c/span\u003eh) obtained via printing of these films onto TEM grids also show the high crystallinity of 2D ITO films. The appearance of Moir\u0026eacute; Fringes in the 2D ITO HRTEM images indicates the overlap of stacked crystalline grains produced in a single print. This result could suggest that both the top and bottom interfaces of the liquid metal meniscus contribute to their surface oxide layers during the printing process. The ability to achieve such highly crystalline morphologies at low temperatures in such a rapid process is unique compared even with vacuum-deposited films that crystallize on longer time scales, require films above a critical thickness, and generally need post-annealing at higher temperatures\u003csup\u003e[\u003cspan additionalcitationids=\"CR28\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. The enlarged view in the inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e1\u003c/span\u003eh displays well-defined lattice fringes corresponding to the (400), (222), and (413) lattice planes of cubic In\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. The selective area electron diffraction (SAED) data, taken from the (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e1\u003c/span\u003ei) show a diffraction pattern typical for the cubic phase of ITO, matching reference spectra \u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e. We note that there is a lack of considerable amorphous signal at low angles, indicating the nearly full crystallization of the 2D ITO film printed at 260 \u0026ordm;C.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe potential for extrinsic Sn doping of the 2D ITO films was investigated by varying the In-Sn alloy composition and measuring the resulting ITO film via XPS. Figure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e2\u003c/span\u003ea shows the Sn at. % extracted for films printed at 180 \u0026ordm;C and at 240 \u0026ordm;C. Sn doping concentrations in the 2D ITO match closely to the Sn concentration in the parent alloy. This corresponds well with previous literature \u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e2\u003c/span\u003ec shows the oxygen \u003cem\u003eO1s\u003c/em\u003e peaks and their decomposed constituent peaks, including stoichiometric M-O bonding, oxygen-deficient M-O bonding, and M-OH bonding for 2D ITO printed with varying Sn concentrations. The percentage of oxygen-deficient M-O bonding peaks at ~\u0026thinsp;532 eV increases from 24.6% for 5 at% Sn doping to 31.6%. for 20 at. % Sn doping in the parent alloy. This matches previous literature\u003csup\u003e[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/sup\u003e and could correspond to the higher free carrier concentration of these heavily doped 2D ITO films due to a higher oxygen deficiency concentration. We also observe that with high-temperature deposition, such as at 240 \u0026ordm;C, the XPS signal from M-OH peak bonds (~\u0026thinsp;534 eV) weakens, while the stoichiometric M-O peak (~\u0026thinsp;530 eV) signal increases by, compared to the film deposited at 180 \u0026ordm;C. (Figure S7).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e2\u003c/span\u003ec shows the XRD spectra of 2D ITO films with varying tin doping levels (1 at. %, 7 at. %, and 20 at. % Sn) as deposited. These show peaks corresponding to a mixture of (222) and (400) orientations, indicating the presence of the cubic In\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e structure\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. Notably, at 7 at. % Sn doping, the 2D ITO film exhibits preferential growth along the [100] axis, with the (400) peak showing higher intensity than the (222) peak. With high doping, such as 20 at. %, the crystalline peaks exhibit attenuation, likely due to the heavy Sn doping impeding In\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e grain growth. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e1\u003c/span\u003ed displays XRD spectra for 2D films printed with 7 at. % Sn in a Sn-In alloy at various temperatures, again revealing the presence of (222) and (400) peaks, with the latter notably more intense. Remarkably, even at a modest processing temperature of 170 \u0026ordm;C, a distinct crystalline (400) peak is evident. These findings suggest that the typical range of electrically optimal Sn doping in liquid metal printed films consistently favor the growth of ITO in the (400) orientation, a parallel to reports of sputtered ITO\u003csup\u003e[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Electrical and Electromechanical Properties of Liquid Metal Printed 2D ITO\u003c/h2\u003e \u003cp\u003eThe electrical properties of 2D ITO were extensively characterized as a function of Sn doping and print temperature, both of which have a large impact on the conductivity. Increasing Sn doping from 1 at. % to 7 at. % results in a substantial increase in the conductivity of 2D ITO. The electrical conductivity peaks at approximately 1340 S/cm at 7 at. % Sn for a deposition temperature of 260\u0026deg;C. For heavier Sn doping approaching 20%, the ITO films begin to exhibit slightly lower conductivity. Previous studies at lower temperatures\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e reported that Sn concentrations above 10% resulted in the amorphization of 2D ITO. The higher print temperatures and the automated printing process of our present study could contribute to maintaining the crystalline cubic ITO phase even at high Sn concentrations, leading to high conductivity. For a film with a thickness of ~\u0026thinsp;6 nm and without any post-annealing, the conductivity is superior to that of previous reports of printed ITO and comparable to even post-annealed vacuum-deposited ITO \u003csup\u003e[\u003cspan additionalcitationids=\"CR28\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan additionalcitationids=\"CR37 CR38 CR39\" citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows a comparison of the conductivity and post-annealing conditions for thin ITO deposited by printing (upper half of Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) as well as vacuum-based processing (lower half of the table). These literature results also reflect the general trend that ITO deposited by vacuum methods exhibits lower conductivity when the film thickness is scaled to allow for ultratransparency. The parity achieved here between our liquid metal printed 2D ITO and sputtered films speaks to the unique physics of Cabrera Mott oxidation and the process control facilitated by our automated methods. While a single ultrathin layer of ITO can achieve high conductivity, it is also possible to stack multiple layers to result in a lower sheet resistance, making the films suitable for various optoelectronic device applications. Figure S8 illustrates the relationship between the sheet resistance of ITO films and the number of printed layers, showing results for single, double, and triple layers. For example, stacking three layers under identical parameters\u0026mdash;260\u0026deg;C, 0.5 cm/s, and 7 at. % Sn in the precursor In-Sn alloy\u0026mdash;achieves a sheet resistance of 300 Ω/sq.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003e\u003cem\u003eSummary of reported performance of printed and vacuum-deposited ultrathin indium tin oxide (ITO) as a function of thickness and annealing temperature.\u003c/em\u003e\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eReference\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDeposition System\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e \u003cp\u003eThickness, nm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eConductivity, S/cm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAnnealing Temperature, \u0026ordm;C\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eThis Work\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eLiquid Metal\u003c/p\u003e \u003cp\u003ePrinting\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1340\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eN/A\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePan et al.\u003csup\u003e[36]\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eInkjet Printing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e440\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e500\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eScheideler et al.\u003csup\u003e[37]\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eGravure Printing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e150\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e500\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e400\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSerkov et al. \u003csup\u003e[38]\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eGravure Printing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e400\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e500\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eLaser Annealed\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKwok et al.\u003csup\u003e[27]\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003ePulsed Laser Deposition\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e830\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGwamuri et al.\u003csup\u003e[39]\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eRF sputtering\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKim et al. \u003csup\u003e[40]\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eRF sputtering\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1400\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e240\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eQiao et al.\u003csup\u003e[29]\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eDC Sputtering\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eChan et al. \u003csup\u003e[28]\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eDC Sputtering\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1488\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e400\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Mechanical and Functional Properties of Flexible 2D ITO for Bioelectrode Applications\u003c/h2\u003e \u003cp\u003eThe utility of TCOs in user interfaces such as touch screens and displays could offer an opportunity for integrating new biosignal measurement functionality into wearable technology if the associated challenges of mechanical flexibility of TCOs can be overcome. We have characterized the mechanical properties of 2D ITO specifically to address the viability of wearable biosignal measurements. Wearable TCOs must pass rigorous mechanical testing, including abrasion, tape, and bending tests, to ensure they can withstand the physical demands of continuous use. These tests simulate conditions such as repeated friction against the skin and bending around curvilinear surfaces, ensuring the electrodes maintain functionality and integrity under mechanical stress. The clinical standard for biopotential and bioimpedance measurements involves using wet electrodes such as Ag/AgCl, though these gels can be uncomfortable, can dry out over time, and may cause irritation \u003csup\u003e[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]\u003c/sup\u003e, rendering them unsuitable for long-term ECG signal acquisition. Previous research has demonstrated thin Au films\u003csup\u003e[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/sup\u003e, PEDOT: PSS\u003csup\u003e[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]\u003c/sup\u003e, and Ag NW for flexible bioelectrodes\u003csup\u003e[\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]\u003c/sup\u003e due to their flexibility, but these materials have limitations to their mechanical and thermal stability.\u003c/p\u003e \u003cp\u003eIn this work, we established 2D ITO as a potential dry electrode because it is highly flexible, transparent, and abrasion-resistant compared to traditional dry electrodes. Traditional sputtered ITO films have been extensively examined and studied for use as biosensors for various purposes\u003csup\u003e[\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]\u003c/sup\u003e but have not been shown as non-invasive epidermal bioelectrodes for measuring biopotential signals. Here we show that 2D ITO printed on polymer substrates is more strain-tolerant than sputtered flexible ITO. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e3\u003c/span\u003ea shows the normalized resistance change after 100X bending cycles at 1% tensile strain for sputtered and liquid metal-deposited ITO films. The resistance change is 5X higher in the case of sputtered ITO compared with the liquid-metal printed film. One potential factor limiting the sputtered ITO film flexibility could be residual stresses from the high-energy growth method. Another potential hypothesis is that the mixed amorphous and crystalline phases in 2D ITO could provide a better balance of mechanical flexibility and electrical performance\u003csup\u003e[\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]\u003c/sup\u003e. This makes 2D ITO films more suitable for applications where the electrodes need to endure bending and flexing, such as wearable electronics and flexible biomedical sensors. As shown in the SEM images in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, the sputtered ITO films exhibit significant fractures transverse to the bending direction, whereas the 2D ITO films remain intact, demonstrating superior mechanical bendability.\u003c/p\u003e \u003cp\u003eThe mechanical stability of 2D ITO was also characterized via abrasion and adhesion tests, which effectively simulate real-world conditions by evaluating the response to potential debonding from substrates during skin contact and the capability to repeatedly apply the films to the skin \u003csup\u003e[\u003cspan additionalcitationids=\"CR49\" citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]\u003c/sup\u003e. We assessed the abrasion resistance according to the ASTM standard by using pencil leads of increasing hardness to determine the hardness at which the film could be visibly scratched and to measure changes to its resistance (details in the \u003cem\u003eMaterials and Methods\u003c/em\u003e section). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, our 2D ITO films demonstrated abrasion resistance that is 4X higher than that of the control PEDOT: PSS film, a common dry bioelectrode material. The control PEDOT:PSS film was scratched by the softest lead (2B) available, whereas one of the hardest leads (2H, 4X stronger than 2B) did not scratch the 2D ITO film. This scratch resistance illustrates a unique advantage of ceramic bioelectrodes relative to softer alternative materials. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e3\u003c/span\u003ed illustrates the adhesion capabilities of the 2D ITO film compared to a sputtered gold film. A peel test (details in the Methods section) was conducted to observe how resistance changed with repeated tape test applications at the same locations. After the tape tests, the gold film was nearly completely delaminated, indicated by a sharp increase in normalized resistance after five applications. In contrast, the ITO film maintained its adhesion and did not de-bond, showing only a small change in normalized resistance.\u003c/p\u003e \u003cp\u003eCharacterization of the electrode-skin impedance for 2D ITO was performed to assess its suitability for various biopotential measurements. Impedance testing ensures that dry bioelectrodes make consistent and adequate contact with the skin, which is crucial for accurately measuring bioelectric signals with a high signal-to-noise ratio \u003csup\u003e[\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]\u003c/sup\u003e. We conducted electrode-skin impedance tests on a 2D ITO film and compared it to clinical standard Ag/AgCl gel control electrodes using a three-electrode setup (details in the \u003cem\u003eMaterial and Methods\u003c/em\u003e section). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e3\u003c/span\u003ee, the 2D ITO electrode exhibits comparable skin-contact impedance to Ag/AgCl per unit area from 1 Hz to 100 kHz. For comparison, the 2D ITO electrodes also show better skin-contact impedance (\u0026sim;95 kΩ/cm\u0026sup2;) compared to PEDOT: PSS values reported in the literature (169\u0026ndash;194 kΩ/cm\u0026sup2; )\u003csup\u003e[\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]\u003c/sup\u003e, as well as gold electrodes (305 kΩ/cm\u0026sup2;)\u003csup\u003e[\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]\u003c/sup\u003e. This low contact impedance makes the 2D ITO electrodes promising candidates for dry bioelectrodes in biopotential measurements. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e3\u003c/span\u003ef shows a spider chart of various dry bioelectrodes\u0026rsquo; performance in terms of flexibility, transparency, conductivity, adhesion, bio-compatibility, and wear resistance. As highlighted in this chart, 2D ITO excels in its transparency and its mechanical resilience to abrasion while offering sufficient conductivity for accurate biopotential measurements.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Multimodal Biosignal Acquisition\u003c/h2\u003e \u003cp\u003eECG measurements using flexible 2D ITO electrodes were performed by contacting the index fingers on both hands (as described in \u003cem\u003eMaterials and Methods\u003c/em\u003e). The left inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003ea shows a zoomed-in view of the P wave, QRS complex, and T wave of a single period of the electrocardiogram signal. We performed a Pearson correlation analysis comparing the measured heart rates from both the standard gel electrodes and the 2D ITO electrode. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, the analysis revealed a high positive correlation of 0.98 between measurements from both the 2D ITO and gel control electrodes under variable conditions (resting and light exercise). This strong correlation signifies the reliability and practical potential of the 2D ITO dry electrodes. Moreover, we employed our highly conductive 2D ITO films to perform electromyography (EMG) measurements for tracking two different hand gestures (open and closing a hand). Figure\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003ec shows the placement of the electrodes on the forearm and the measured EMG signal overlaid with the corresponding time divisions for open and closed hand gestures.\u003c/p\u003e \u003cp\u003eTo leverage the multifunctionality of these transparent flexible bioelectrodes, we next demonstrated a set of multimodal heart rate measurements using both biopotential and optical methods. This is possible because 2D ITO is highly transmissive to the source wavelengths utilized in PPG (e.g. 530 nm and 940 nm), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003ed, with an average transmittance above 95%. This allows these electrodes to be co-located with light-based PPG sensors without hindering light transmission from the PPG LED or the collection of reflected light by the PPG photodetectors. The high transmittance of 2D ITO would also allow vertical integration of these bioelectrodes with displays in future wearable systems. Figure\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003ee shows a schematic of the dual measurement of both ECG and PPG signals. Such integration enables simultaneous ECG and PPG measurements from the same site, combining the detailed electrical activity captured by ECG with the vascular blood volume changes detected by PPG. The advantage of performing these measurements simultaneously for cardiovascular monitoring is that ECG can be utilized to validate and correct motion artifacts commonly seen in PPG signals \u003csup\u003e[\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]\u003c/sup\u003e. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003ef, heart rate measured using PPG (78\u0026ndash;83 BPM) during light activity matches the precise measurements from electrical monitoring with the 2D ITO electrode (80\u0026ndash;82 BPM). Additionally, as seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003ef, the peak of the PPG coinciding with the T peak of the ECG indicates temporally synchronized cardiac cycle phases. An additional advantage of these synchronous measurements could be the ability to detect anomalies, such as arrhythmias, that are impossible to perceive via PPG measurements alone. This dual measurement capability, facilitated by the transparency of ITO electrodes, provides a more robust and holistic approach to cardiovascular health monitoring while also minimizing the area needed for multimodal measurements in a miniaturized device form factor.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"3. Conclusion","content":"\u003cp\u003eIn summary, we present flexible 2D TCOs fabricated at ultralow temperatures using vacuum-free Cabrera Mott oxidation of liquid hypoeutectic In-Sn alloys, demonstrating wafer-scale films with angstrom-level thickness control via an automated, kinetically driven approach. Our synthesis rapidly (at up to 20 cm/s) produces smooth, ultrathin 2D ITO with unprecedented conductivity (\u0026gt;\u0026thinsp;1300 S/cm) comparable to vacuum-deposited films. Surface morphology and structural characterization confirm the effectiveness of Sn-doping, revealing the high crystallinity of these 2D oxides and the large, plate-like grains formed in the liquid metal reaction environment. We observe that a significant result of the ultrathin nature of 2D ITO and the liquid metal printing process is enhanced bending strain tolerance, superior scratch resistance, and low contact impedance for 2D ITO when used as wearable bioelectrodes. Finally, leveraging the conductivity and transparency of 2D ITO, we enable simultaneous, multimodal measurements via electrocardiography (ECG) and photoplethysmography (PPG). These findings represent a significant improvement in the performance of printed metal oxides and introduce a promising new material for multimodal biometrics.\u003c/p\u003e"},{"header":"4. Experimental Methods","content":"\u003cp\u003e \u003cstrong\u003eAlloy Preparation\u003c/strong\u003e \u003cp\u003eThe In-Sn alloys were prepared by melting In (\u003cem\u003eLuciteria\u003c/em\u003e, 99.995%) and Sn (\u003cem\u003eLuciteria\u003c/em\u003e, 99.995%) pellets in a graphite crucible at 300\u0026deg;C for 2 hours in an inert nitrogen atmosphere glovebox (\u0026lt;\u0026thinsp;10 ppm O\u003csub\u003e2\u003c/sub\u003e, \u0026lt; 10 ppm H\u003csub\u003e2\u003c/sub\u003eO) to minimize surface oxidation. Alloys containing 1, 2.5, 5, 7, 10, 20, and 30 at. % Sn was prepared by mixing the respective amounts of Sn with In.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eAutomated 2D TCO Synthesis and Deposition\u003c/strong\u003e \u003cp\u003e2D indium tin oxides (ITOs) were deposited by rolling molten In-Sn alloy droplets along a substrate (Si/SiO\u003csub\u003e2\u003c/sub\u003e, glass, PET, or PEN) on a hotplate using a silicone roller controlled by a 3-axis inline gantry robot (\u003cem\u003eFisnar 5300N\u003c/em\u003e). The deposition process was conducted at speeds from 0.1 to 20 cm/s and temperatures from 140 to 290\u0026deg;C (printing at 140 \u0026ordm;C was specifically enabled by high Sn-concentrations, as detailed in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Prior to deposition, the target substrates were treated with approximately 10 seconds of atmospheric plasma using a Plasma-Etch 1000W system supplied with 30 LPM compressed dry air to promote the 2D ITO film adhesion. Two dummy substrates placed before and after the target substrate were used to allow the deposition process to reach equilibrium and produce a uniform and continuous metal oxide film deposition. After deposition, the residual liquid metal on the surface of the oxide film was removed with a squeegee while still on the hotplate and again once the sample had cooled to room temperature.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eThin Film Fabrication\u003c/b\u003e \u003c/p\u003e \u003cp\u003eFor the abrasion tests, the control PEDOT: PSS films were prepared by spin-coating a 1 wt. % PEDOT: PSS solution in H\u003csub\u003e2\u003c/sub\u003eO onto a SiO\u003csub\u003e2\u003c/sub\u003e (300nm)/Si substrate at 6000 RPM for 1 minute, followed by drying on a hotplate at 100\u0026deg;C. An Anatech LTD Hummer 6.2 sputtering system was utilized to deposit a 15 nm-thick gold film on a soda lime glass substrate for the peeling test.\u003c/p\u003e \u003cp\u003e \u003cb\u003eMaterials Characterization\u003c/b\u003e:\u003c/p\u003e \u003cp\u003eA Differential Scanning Calorimeter (DSC) (Discovery DSC 250, \u003cem\u003eTA Instruments\u003c/em\u003e) was used to measure the melting temperature of the Sn-In alloys with various Sn compositions with a ramp rate of 10\u0026deg;C /min under N\u003csub\u003e2\u003c/sub\u003e flow. X-ray photoelectron spectroscopy (XPS) was conducted using a Kratos Axis Supra XPS at approximately 10⁻⁹ Torr on three layers of 2D ITO films printed on 100 nm SiO₂ substrates. Elemental analysis of the 2D ITO films was performed by comparing the Sn 3d, In 3d, and O1s peaks. Optical microscope images were captured with a Keyence VHX-7100 microscope. UV-Vis spectroscopy was conducted using a DeNovix DS-11 FX\u0026thinsp;+\u0026thinsp;spectrophotometer to measure the printed ITO films' absorbance spectra (270\u0026ndash;800 nm) on glass substrates. AFM was performed using an AIST-NT instrument in tapping mode to measure film thickness and grain morphology. High-resolution transmission electron microscopy (HRTEM) was carried out with a Thermo Scientific Talos F200i instrument. Samples for HRTEM imaging were prepared by liquid metal printing of 2D ITO (7 at. % Sn) directly onto TEM grids (Carbon Square Mesh, Cu, 300 Mesh, UL, EMS) at 260 \u0026ordm;C, with excess liquid metal removed using a silicone squeegee. X-ray diffraction (XRD) was performed using a Rigaku UltraX Cu-anode diffractometer (Cu Kα radiation at 40 kV, 300 mA, λ\u0026thinsp;=\u0026thinsp;0.154 nm) with a scanning rate of 0.5\u0026deg; per minute on single printed layers of 2D ITO on 300 nm SiO₂ substrates. Grain size analysis was conducted via AFM phase imaging and confirmed through HRTEM images. Scanning electron microscopy (SEM) was performed using a Thermo Scientific Helios 5 CX tool.\u003c/p\u003e \u003cp\u003e \u003cb\u003eElectrical Characterization\u003c/b\u003e \u003c/p\u003e \u003cp\u003eSheet resistance was measured using a four-point probe at room temperature in air.\u003c/p\u003e \u003cp\u003e \u003cb\u003eThickness Characterization\u003c/b\u003e:\u003c/p\u003e \u003cp\u003eFilms for measuring thickness were printed on SiO\u003csub\u003e2\u003c/sub\u003e/Si substrates (300 nm SiO\u003csub\u003e2\u003c/sub\u003e). The exact SiO\u003csub\u003e2\u003c/sub\u003e thickness was measured to facilitate the modeling of the reflectance spectrum of the 2D ITO films on SiO\u003csub\u003e2\u003c/sub\u003e spectroscopic reflectometry (F3-sX, Filmetrics) from 380 nm \u0026ndash; 1050 nm to extract the thickness of the films. These thicknesses measured by reflectometry were confirmed via AFM line scans of films patterned by wet etching. Multilayer films\u0026thinsp;\u0026gt;\u0026thinsp;20 nm thick were also measured via stylus profilometry (KLA Tencor D-500) to confirm the reflectometry measured thickness.\u003c/p\u003e \u003cp\u003e \u003cb\u003eMechanical Characterization of Flexible 2D ITO Films\u003c/b\u003e \u003c/p\u003e \u003cp\u003eBending resilience measurements were performed on 2D ITO films deposited onto 60 \u0026micro;m thick polyimide substrates at 260\u0026deg;C and sputtered ITO onto 175 \u0026micro;m PET substrate. The films were measured after the substrate was bent to 1%, 1.25%, and 1.5% of tensile strain until 100 bending cycles were achieved. The film hardness test was conducted using the ASTM D3363 standard, which entails scratching thin films, such as 2D ITO (printed with 7 at. % Sn doped at 260\u0026deg;C) on silicon and spin-coated PEDOT, using pencil leads of varying hardness. The films were imaged with optical microscopy, and changes in electrical resistance were observed after each iteration of abrasion with the specified pencil leads. Abrasion was applied with a force of 7 N, an angle of around 45\u0026deg;, and a speed of approximately 0.5 cm/s. An adhesion test for the 2D ITO film (printed with 7 at. % Sn doped at 260\u0026deg;C) on polyimide and sputtered Au on glass was performed using Kapton tape (Uline). The tape was removed at a speed of approximately 1 cm/s and at an angle of ~\u0026thinsp;90\u0026deg; to the tested film.\u003c/p\u003e\n\u003ch3\u003e2D ITO Bioelectrode Characterization\u003c/h3\u003e\n\u003cp\u003eECG measurements were conducted by wrapping printed ITO electrodes that are printed with 7 at% Sn doped at 260\u0026deg;C on polyimide in a single lead setup (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). As a control, a gel electrode (3M-2238 Electrode) was placed on the forearm. The heart rate was calculated using the Eq.\u0026nbsp;2, shown below:\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Taba\" border=\"1\"\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHeart rate (beats per minute)\u0026thinsp;=\u0026thinsp;60/RR\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e(2)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eWhere RR is the time interval in seconds between two consecutive R peaks of the measured ECG signal. Both measurements were performed using a Vernier EKG Sensor (Go Direct). EMG measurements were performed by placing these ITO electrodes on polyimide on the forearm (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003ec) and using the Vernier Go Direct system for data collection. A Polar Verity sensor was used to measure the PPG from the wrist. The transparent ITO electrode on polyimide, which measures the ECG signal, was placed between the PPG LED and the wrist. This setup allows simultaneous measurements of both PPG and ECG.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eSupporting Information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSupporting Information is available from the Wiley Online Library or from the author.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no conflict of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003eAcknowledgments\u003c/p\u003e\n\u003cp\u003eSaifur Rahman was supported by the Dartmouth PhD Innovation Fellowship. This research was supported by the National Science Foundation Electronic and Photonic Materials Program (Award #2202501) as well as the National Science Foundation Electronics, Photonics, and Magnetic Devices program (Award #2219991). We acknowledge Paul Defino at Dartmouth College for assistance in AFM scans as well as John Wilderman at the University of New Hampshire for performing XPS measurements.\u003c/p\u003e\n\u003cp\u003eReceived: ((will be filled in by the editorial staff))\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Revised: ((will be filled in by the editorial staff))\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Published online: ((will be filled in by the editorial staff))\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eD. R. Cairns, D. C. Paine, G. P. Crawford, \u003cem\u003eMRS Online Proceedings Library (OPL)\u003c/em\u003e 2001, \u003cem\u003e666\u003c/em\u003e, F3.24.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eK. Sim, Z. Rao, Z. Zou, F. Ershad, J. Lei, A. Thukral, J. Chen, Q.-A. Huang, J. Xiao, C. Yu, \u003cem\u003eScience Advances\u003c/em\u003e 2019, \u003cem\u003e5\u003c/em\u003e, eaav9653.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eU. Kim, M. Han, J. Jang, J. Shin, M. Park, J. Byeon, M. Choi, \u003cem\u003eAdvanced Energy Materials\u003c/em\u003e 2023, \u003cem\u003e13\u003c/em\u003e, 2203198.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. N. Le, K.-J. Baeg, K.-T. Kim, S.-H. Kang, B. D. Choi, C.-Y. Park, S.-P. Jeon, S. Lee, J.-W. Jo, S. Kim, J.-G. Park, D. Ho, J. Hong, M. Kim, H.-K. Kim, C. Kim, K. Kim, Y.-H. Kim, S. K. Park, M.-G. Kim, \u003cem\u003eAdvanced Functional Materials\u003c/em\u003e 2021, \u003cem\u003e31\u003c/em\u003e, 2103285.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH. S. Jung, K. Eun, Y. T. Kim, E. K. Lee, S.-H. Choa, \u003cem\u003eMicrosyst Technol\u003c/em\u003e 2017, \u003cem\u003e23\u003c/em\u003e, 1961.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eA. Zavabeti, J. Z. Ou, B. J. Carey, N. Syed, R. Orrell-Trigg, E. L. H. Mayes, C. Xu, O. Kavehei, A. P. O\u0026rsquo;Mullane, R. B. Kaner, K. Kalantar-zadeh, T. Daeneke, \u003cem\u003eScience\u003c/em\u003e 2017, \u003cem\u003e358\u003c/em\u003e, 332.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eW. J. Scheideler, K. Nomura, \u003cem\u003eAdvanced Functional Materials\u003c/em\u003e n.d., \u003cem\u003en/a\u003c/em\u003e, 2403619.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eA. Zavabeti, P. Aukarasereenont, H. Tuohey, N. Syed, A. Jannat, A. Elbourne, K. A. Messalea, B. Y. Zhang, B. J. Murdoch, J. G. Partridge, M. Wurdack, D. L. Creedon, J. van Embden, K. Kalantar-Zadeh, S. P. Russo, C. F. McConville, T. Daeneke, \u003cem\u003eNat Electron\u003c/em\u003e 2021, \u003cem\u003e4\u003c/em\u003e, 277.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY. Ye, A. B. Hamlin, J. E. Huddy, M. S. Rahman, W. J. Scheideler, \u003cem\u003eAdvanced Functional Materials\u003c/em\u003e 2022, \u003cem\u003e32\u003c/em\u003e, 2204235.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eA. B. Hamlin, Y. Ye, J. E. Huddy, M. S. Rahman, W. J. Scheideler, \u003cem\u003enpj 2D Mater Appl\u003c/em\u003e 2022, \u003cem\u003e6\u003c/em\u003e, 1.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eR. S. Datta, N. Syed, A. Zavabeti, A. Jannat, M. Mohiuddin, M. Rokunuzzaman, B. Yue Zhang, M. A. Rahman, P. Atkin, K. A. Messalea, M. B. Ghasemian, E. D. Gaspera, S. Bhattacharyya, M. S. Fuhrer, S. P. Russo, C. F. McConville, D. Esrafilzadeh, K. Kalantar-Zadeh, T. Daeneke, \u003cem\u003eNat Electron\u003c/em\u003e 2020, \u003cem\u003e3\u003c/em\u003e, 51.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eQ. Li, J. Lin, T.-Y. Liu, X.-Y. Zhu, W.-H. Yao, J. Liu, \u003cem\u003enpj 2D Mater Appl\u003c/em\u003e 2021, \u003cem\u003e5\u003c/em\u003e, 1.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eB. J. Carey, J. Z. Ou, R. M. Clark, K. J. Berean, A. Zavabeti, A. S. R. Chesman, S. P. Russo, D. W. M. Lau, Z.-Q. Xu, Q. Bao, O. Kavehei, B. C. Gibson, M. D. Dickey, R. B. Kaner, T. Daeneke, K. Kalantar-Zadeh, \u003cem\u003eNat Commun\u003c/em\u003e 2017, \u003cem\u003e8\u003c/em\u003e, 14482.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ. Lin, Q. Li, T.-Y. Liu, Y. Cui, H. Zheng, J. Liu, \u003cem\u003ephysica status solidi (RRL)\u003c/em\u003e \u0026ndash; \u003cem\u003eRapid Research Letters\u003c/em\u003e 2019, \u003cem\u003e13\u003c/em\u003e, 1900271.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY. Zhang, D. Venkatakrishnarao, M. Bosman, W. Fu, S. Das, F. Bussolotti, R. Lee, S. L. Teo, D. Huang, I. Verzhbitskiy, Z. Jiang, Z. Jiang, J. Chai, S. W. Tong, Z.-E. Ooi, C. P. Y. Wong, Y. S. Ang, K. E. J. Goh, C. S. Lau, \u003cem\u003eACS Nano\u003c/em\u003e 2023, \u003cem\u003e17\u003c/em\u003e, 7929.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZ. J. Farrell, A. R. Jacob, V. K. Truong, A. Elbourne, W. Kong, L. Hsiao, M. D. Dickey, C. Tabor, \u003cem\u003eChem. Mater.\u003c/em\u003e 2023, \u003cem\u003e35\u003c/em\u003e, 964.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eN. Cabrera, N. F. Mott, \u003cem\u003eRep. Prog. Phys.\u003c/em\u003e 1949, \u003cem\u003e12\u003c/em\u003e, 163.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eK. N. Goswami, R. W. Staehle, \u003cem\u003eElectrochimica Acta\u003c/em\u003e 1971, \u003cem\u003e16\u003c/em\u003e, 1895.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eI. Isakov, H. Faber, A. D. Mottram, S. Das, M. Grell, A. Regoutz, R. Kilmurray, M. A. McLachlan, D. J. Payne, T. D. Anthopoulos, \u003cem\u003eAdvanced Electronic Materials\u003c/em\u003e 2020, \u003cem\u003e6\u003c/em\u003e, 2000682.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ. Liu, Y. Nie, W. Xue, L. Wu, H. Jin, G. Jin, Z. Zhai, C. Fu, \u003cem\u003eJournal of Materials Research and Technology\u003c/em\u003e 2020, \u003cem\u003e9\u003c/em\u003e, 8020.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ. W. Elam, D. A. Baker, A. B. F. Martinson, M. J. Pellin, J. T. Hupp, \u003cem\u003eJ. Phys. Chem. C\u003c/em\u003e 2008, \u003cem\u003e112\u003c/em\u003e, 1938.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH. Y. Kim, E. A. Jung, G. Mun, R. E. Agbenyeke, B. K. Park, J.-S. Park, S. U. Son, D. J. Jeon, S.-H. K. Park, T.-M. Chung, J. H. Han, \u003cem\u003eACS Appl. Mater. Interfaces\u003c/em\u003e 2016, \u003cem\u003e8\u003c/em\u003e, 26924.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eR. Sarhaddi, N. Shahtahmasebi, M. Rezaee Rokn-Abadi, M. M. Bagheri-Mohagheghi, \u003cem\u003ePhysica E: Low-dimensional Systems and Nanostructures\u003c/em\u003e 2010, \u003cem\u003e43\u003c/em\u003e, 452.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eS.-J. Hong, J.-I. Han, \u003cem\u003eCurrent Applied Physics\u003c/em\u003e 2006, \u003cem\u003e6\u003c/em\u003e, e206.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. Bragaglia, F. R. Lamastra, M. Tului, L. Di Gaspare, A. Notargiacomo, M. Valentini, F. Nanni, \u003cem\u003eSurfaces and Interfaces\u003c/em\u003e 2019, \u003cem\u003e17\u003c/em\u003e, 100365.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eA. P. Amalathas, M. M. Alkaisi, \u003cem\u003eJ Mater Sci: Mater Electron\u003c/em\u003e 2016, \u003cem\u003e27\u003c/em\u003e, 11064.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH. S. Kwok, X. W. Sun, D. H. Kim, \u003cem\u003eThin Solid Films\u003c/em\u003e 1998, \u003cem\u003e335\u003c/em\u003e, 299.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eS.-H. Chan, M.-C. Li, H.-S. Wei, S.-H. Chen, C.-C. Kuo, \u003cem\u003eJournal of Nanomaterials\u003c/em\u003e 2015, \u003cem\u003e2015\u003c/em\u003e, 179804.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZ. Qiao, R. Latz, D. Mergel, \u003cem\u003eThin Solid Films\u003c/em\u003e 2004, \u003cem\u003e466\u003c/em\u003e, 250.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ. Ba, D. Fattakhova Rohlfing, A. Feldhoff, T. Brezesinski, I. Djerdj, M. Wark, M. Niederberger, \u003cem\u003eChem. Mater.\u003c/em\u003e 2006, \u003cem\u003e18\u003c/em\u003e, 2848.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH.-W. Wang, C.-F. Ting, M.-K. Hung, C.-H. Chiou, Y.-L. Liu, Z. Liu, K. R. Ratinac, S. P. Ringer, \u003cem\u003eNanotechnology\u003c/em\u003e 2009, \u003cem\u003e20\u003c/em\u003e, 055601.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eN. Nadaud, N. Lequeux, M. Nanot, J. Jov\u0026eacute;, T. Roisnel, \u003cem\u003eJournal of Solid State Chemistry\u003c/em\u003e 1998, \u003cem\u003e135\u003c/em\u003e, 140.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eS. Peng, X. Cao, J. Pan, X. Wang, X. Tan, A. E. Delahoy, K. K. Chin, \u003cem\u003eJ. Electron. Mater.\u003c/em\u003e 2017, \u003cem\u003e46\u003c/em\u003e, 1405.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eA. J. Nelson, H. Aharoni, \u003cem\u003eJournal of Vacuum Science\u003c/em\u003e \u0026amp; \u003cem\u003eTechnology A\u003c/em\u003e 1987, \u003cem\u003e5\u003c/em\u003e, 231.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eK. Wang, P. Jiao, Y. Cheng, H. Xu, G. Zhu, Y. Zhao, K. Jiang, X. Zhang, Y. Su, \u003cem\u003eOptical Materials\u003c/em\u003e 2022, \u003cem\u003e134\u003c/em\u003e, 113040.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY. Pan, M. Liu, C. Lu, B. Liu, W. Shao, D. Pan, X. Shi, \u003cem\u003eLangmuir\u003c/em\u003e 2023, \u003cem\u003e39\u003c/em\u003e, 5107.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eW. J. Scheideler, J. Jang, M. A. U. Karim, R. Kitsomboonloha, A. Zeumault, V. Subramanian, \u003cem\u003eACS Appl. Mater. Interfaces\u003c/em\u003e 2015, \u003cem\u003e7\u003c/em\u003e, 12679.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eA. A. Serkov, H. V. Snelling, S. Heusing, T. M. Amaral, \u003cem\u003eSci Rep\u003c/em\u003e 2019, \u003cem\u003e9\u003c/em\u003e, 1773.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ. Gwamuri, M. Marikkannan, J. Mayandi, P. K. Bowen, J. M. Pearce, \u003cem\u003eMaterials\u003c/em\u003e 2016, \u003cem\u003e9\u003c/em\u003e, 63.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY. W. Kim, J. S. Goo, T. H. Lee, B. R. Lee, S.-C. Shin, H. Kim, J. W. Shim, T. G. Kim, \u003cem\u003eJournal of Power Sources\u003c/em\u003e 2019, \u003cem\u003e424\u003c/em\u003e, 165.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eA. A. Chlaihawi, B. B. Narakathu, S. Emamian, B. J. Bazuin, M. Z. Atashbar, \u003cem\u003eSensing and Bio-Sensing Research\u003c/em\u003e 2018, \u003cem\u003e20\u003c/em\u003e, 9.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY. Zhao, M. Yu, Z. Liu, Z. Yu, \u003cem\u003eJournal of Applied Physics\u003c/em\u003e 2019, \u003cem\u003e125\u003c/em\u003e, 165305.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eB. Ji, M. Wang, C. Ge, Z. Xie, Z. Guo, W. Hong, X. Gu, L. Wang, Z. Yi, C. Jiang, B. Yang, X. Wang, X. Li, C. Li, J. Liu, \u003cem\u003eBiosensors and Bioelectronics\u003c/em\u003e 2019, \u003cem\u003e135\u003c/em\u003e, 181.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eA. C. Myers, H. Huang, Y. Zhu, \u003cem\u003eRSC Adv.\u003c/em\u003e 2015, \u003cem\u003e5\u003c/em\u003e, 11627.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eE. B. Aydın, M. K. Sezgint\u0026uuml;rk, \u003cem\u003eTrAC Trends in Analytical Chemistry\u003c/em\u003e 2017, \u003cem\u003e97\u003c/em\u003e, 309.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eS. Park, J. Yoon, S. Kim, P. Song, \u003cem\u003eRSC Adv.\u003c/em\u003e 2021, \u003cem\u003e11\u003c/em\u003e, 3439.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH. Seo, B. Kim, K. H. Lee, S. Chae, J. Jung, \u003cem\u003eACS Appl. Mater. Interfaces\u003c/em\u003e 2022, \u003cem\u003e14\u003c/em\u003e, 25620.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eD. Doci, M. Ademi, K. Tuvshinbayar, N. Richter, G. Ehrmann, T. Spahiu, A. Ehrmann, \u003cem\u003eCoatings\u003c/em\u003e 2023, \u003cem\u003e13\u003c/em\u003e, 1624.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eB. Vasconcelos, P. Fiedler, R. Machts, J. Haueisen, C. Fonseca, \u003cem\u003eFront. Neurosci.\u003c/em\u003e 2021, \u003cem\u003e15\u003c/em\u003e, DOI \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3389/fnins.2021.748100\u003c/span\u003e\u003cspan address=\"10.3389/fnins.2021.748100\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eI. Hutchings, M. Gee, E. Santner, in \u003cem\u003eSpringer Handbook of Materials Measurement Methods\u003c/em\u003e (Eds.: H. Czichos, T. Saito, L. Smith), Springer, Berlin, Heidelberg, 2006, pp. 685\u0026ndash;710.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eK. Goyal, D. A. Borkholder, S. W. Day, \u003cem\u003eSensors\u003c/em\u003e 2022, \u003cem\u003e22\u003c/em\u003e, 8510.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eL.-W. Lo, J. Zhao, K. Aono, W. Li, Z. Wen, S. Pizzella, Y. Wang, S. Chakrabartty, C. Wang, \u003cem\u003eACS Nano\u003c/em\u003e 2022, \u003cem\u003e16\u003c/em\u003e, 11792.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY. Chen, W. Pei, S. Chen, X. Wu, S. Zhao, H. Wang, H. Chen, \u003cem\u003eSensors and Actuators B: Chemical\u003c/em\u003e 2013, \u003cem\u003e188\u003c/em\u003e, 747.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eF. Sartor, J. Gelissen, R. van Dinther, D. Roovers, G. B. Papini, G. Coppola, \u003cem\u003eBMC Sports Science, Medicine and Rehabilitation\u003c/em\u003e 2018, \u003cem\u003e10\u003c/em\u003e, 10.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH.-W. Chow, C.-C. Yang, \u003cem\u003eJMIR mHealth and uHealth\u003c/em\u003e 2020, \u003cem\u003e8\u003c/em\u003e, e14707.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"npj-flexible-electronics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjflexelectron","sideBox":"Learn more about [npj Flexible Electronics](http://www.nature.com/npjflexelectron/)","snPcode":"41528","submissionUrl":"https://submission.springernature.com/new-submission/41528/3","title":"npj Flexible Electronics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"liquid metal printing, 2D oxides, transparent electronics, wearable sensors, bioelectrodes","lastPublishedDoi":"10.21203/rs.3.rs-4903114/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4903114/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTransparent conducting oxides (TCOs) are crucial for high-performance displays, solar cells, and wearable sensors. However, their high process temperatures and brittle nature have hindered their use in flexible electronics. We report an approach to overturn these limitations by harnessing the physics Cabrera Mott native oxidation to fabricate large-area, two-dimensional transparent electrodes via liquid metal printing. Our robotic, solvent-free and vacuum-free process deposits ultrathin (2\u0026ndash;10 nm thick) 2D indium tin oxide (ITO) with exceptional flexibility, high transparency (\u0026gt;\u0026thinsp;95%) and superior conductivity (\u0026gt;\u0026thinsp;1300 S/cm) for wearable bioelectrodes. In a significant advance over previous work, we utilize hypoeutectic In-Sn alloys to print 2D ITO at \u0026lt;\u0026thinsp;140 \u0026ordm;C on flexible polymers. Our detailed materials characterization and microscopy reveal the efficacy of Sn-doping and high crystallinity with large, platelike grains formed by the liquid metal reaction environment. The ultrathin nature of 2D ITO yields significant enhancement to bending strain tolerance, scratch resistance exceeding durability of traditional PEDOT, and low contact impedance to skin comparable to Ag/AgCl. Finally, we utilize the conductivity and transparency of 2D ITO for synchronous, multimodal measurements via electrocardiography (ECG) and pulse plethysmography (PPG). This order-of-magnitude improvement to printed TCOs could enable new wearable biometrics and display-integrated sensors.\u003c/p\u003e","manuscriptTitle":"Kinetic Liquid Metal Synthesis of Flexible 2D Conductive Oxides for Multimodal Wearable Sensing","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-24 17:55:59","doi":"10.21203/rs.3.rs-4903114/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-10-08T20:00:42+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-02T16:08:55+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-09-09T07:39:59+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"29978375486654621636128112988038399840","date":"2024-09-05T16:38:13+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"304238949103024620287582280999068047458","date":"2024-09-05T03:08:57+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"283594875982485444562328956124685174518","date":"2024-08-31T01:41:21+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-08-31T01:32:04+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-08-20T06:35:37+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-08-20T06:34:25+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj Flexible Electronics","date":"2024-08-13T00:05:32+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-flexible-electronics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjflexelectron","sideBox":"Learn more about [npj Flexible Electronics](http://www.nature.com/npjflexelectron/)","snPcode":"41528","submissionUrl":"https://submission.springernature.com/new-submission/41528/3","title":"npj Flexible Electronics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"06a4c53b-c28d-4992-afcd-a380b8ab3342","owner":[],"postedDate":"September 24th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":37597421,"name":"Physical sciences/Materials science/Materials for devices"},{"id":37597422,"name":"Physical sciences/Materials science/Nanoscale materials/Two dimensional materials"},{"id":37597423,"name":"Physical sciences/Engineering/Electrical and electronic engineering"},{"id":37597424,"name":"Physical sciences/Nanoscience and technology/Nanoscale materials/Synthesis and processing"}],"tags":[],"updatedAt":"2024-11-25T16:00:06+00:00","versionOfRecord":{"articleIdentity":"rs-4903114","link":"https://doi.org/10.1038/s41528-024-00371-7","journal":{"identity":"npj-flexible-electronics","isVorOnly":false,"title":"npj Flexible Electronics"},"publishedOn":"2024-11-23 15:56:52","publishedOnDateReadable":"November 23rd, 2024"},"versionCreatedAt":"2024-09-24 17:55:59","video":"","vorDoi":"10.1038/s41528-024-00371-7","vorDoiUrl":"https://doi.org/10.1038/s41528-024-00371-7","workflowStages":[]},"version":"v1","identity":"rs-4903114","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4903114","identity":"rs-4903114","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}