Ultrasound Powered Wetting and Filling of Liquid Metal into Ultrafine Channels for Flexible Electronics

Ultrasound Powered Wetting and Filling of Liquid Metal into Ultrafine Channels for Flexible Electronics | 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 Ultrasound Powered Wetting and Filling of Liquid Metal into Ultrafine Channels for Flexible Electronics Xing Ma, Zirong Xu, Dongdong Jin, Sanhu Liu, Guoqiang Li, Zichang Guo, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3909065/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Liquid metals (LMs) that possess both metallic and fluidic properties at room temperature promise numerous benefits in flexible electronics. However, preparing fine LM circuits with complex architectures at the microscale and below poses great challenges due to large surface tension of LMs, hindering the development of flexible electronic devices with increased integration and/or complexity. Herein, we develop a facile strategy that employs ultrasonic stimulation to power the wetting and filling of LMs into non-wettable elastic microchannels for ultrafine LMs circuits fabrication. Taking advantage of ultrasound-induced asymmetrical acoustic pressure within LMs, rapid and complete filling of LMs into microchannels as fine as ~ 750 nm could be achieved within several seconds without destroying the soft substrate. Electromechanical investigations suggested that the LM-filled microchannels by ultrasound possessed excellent cyclic stability and robustness. Through ultrasonic filling and patterning ultrafine LM circuits, we created a miniaturized pressure sensor array capable of accurately sensing load pressure and spatial information. The sensitivity of the finger-wearable strain sensor was further enhanced by 22.6 times via introducing local microstructures and filling LM inside the microchannel. Finally, we demonstrated the efficient preparation of complex LM circuit patterns powered by ultrasound, e.g. , integrated circuits, spiderman, and snowflake patterns with multiple interlaced branches, planar structures, blind holes as well as vertical architectures, all of which could hardly be achieved by conventional injection methods. This work presented an ultrasound-based fabrication method for versatile LMs circuits, providing a platform technology for the advancement of next-generation compact and integrated flexible electronics. Physical sciences/Engineering/Electrical and electronic engineering Physical sciences/Materials science/Nanoscale materials/Electronic properties and materials Physical sciences/Materials science/Soft materials/Wetting Liquid metal Wetting behavior Ultrasound Microchannel Flexible circuits Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Room-temperature liquid metals (LMs), particularly gallium-based LMs, have been extensively investigated in recent years owing to their distinctive properties such as metallic conductivity, low toxicity, and intrinsic fluidity 1–3 . Serving as ideal soft and stretchable conductors, LMs are emerging as one of the most promising materials for various flexible electronics, including wearable sensors 4,5 , portable energy harvesting and storage devices 6–8 , implantable bioelectronics 9,10 , and so on 11–13 . Compared with traditional rigid circuits 14–17 , the leap in flexibility and adaptability endowed by LMs pushes the boundaries of electronic technologies, and contributes to bridging the gap between humans and devices 18,19 . Recent works have further pursued for LM circuits with finer patterning resolution and higher fabrication freedom, aiming to create miniaturized, compact yet versatile electronic devices with increased integration complexity 20,21 . However, it remains a grand challenge for current LM patterning methods to efficiently fabricate complex architected circuits with a resolution at microscale and below. Considerable approaches have been explored to prepare and pattern LM circuits on various soft substrates for flexible electronics, which can be broadly divided into two categories, i.e., direct and indirect strategies. The former includes direct ink writing 5,22 , lithography-enabled patterning 21,23 , 3D printing 24–26 and subtractive ablation 27,28 , which selectively deposit LMs at desired locations on the substrate, usually followed by sealed packaging with polymeric elastomers. Due to the intrinsically high surface tension, LMs are non-wettable to most soft substrate materials used in flexible electronics. Therefore, direct fabrication strategies are commonly accomplished with the aids of doping adhesives 29,30 , pre-patterning of wettable metal layers on substrates (Au 31 or Cu 32 ), or additionally oxidizing the LM surface 33,34 , aiming to enhance the adhesion between LMs and substrates. Nevertheless, such approaches may inadvertently deteriorate the electrical conductivity and fluidity of LM, as well as involving extra complex and labor-intensive processes, which would pose great difficulties in fabricating highly integrated flexible electronics. The indirect strategies of LM patterning are to fabricate soft substrates with encapsulated channels, followed by filling with LMs to constitute the desired circuits. Such indirect strategy is relatively simple, and more importantly, does not require any additional modification of the LMs, which can preserve the excellent conductive and fluidic properties of LMs. Manual injection is prevalently used to fill LMs into the channels of electronic devices as circuits 35,36 , which, however, poses challenges when meeting fine channels at microscale and below. The required injection pressure for filling channels is inversely proportional to channel dimension, and would become even tremendous if the injected objects are LMs with huge surface tension. It has been reported that filling microchannel with a diameter of ~ 40 µm needs a pressure of 89 kPa 37 , which increases 112-fold when channel dimension is reduced to hundreds of nanometers 38 . Such huge pressures can easily damage the structure of soft substrates and cause LMs leakage, thus are not applicable to flexible electronics. Although vacuum aspiration could assist to drive LMs into the microchannel as fine as 10 µm, it is actually a time-consuming procedure that takes approximately half an hour to fill a ~ 500 mm long channel 39,40 . In addition, current indirect methods are often accompanied by the presence of localized non-wetting and incomplete filling phenomena, leading to the generation of bubbles within channels and even interconnection failure of LM circuits. Therefore, it is urgently desired to develop a facile and rapid LMs filling strategy to efficiently fabricate miniaturized and fine flexible circuits. In this work, we reported an ultrasound powered strategy that could effectively power eutectic gallium indium (EGaIn) LM to actively wet and fill into ultrafine microchannels within seconds. It was found that ultrasound stimulation could generate an asymmetrical acoustic pressure field inside the LM, which could drive the spreading of the LM on soft substrates. Meanwhile, the oxidation film of LM naturally formed in the atmosphere could be simultaneously broken by the ultrasonic wave 41–43 , assisting the wetting and adhering of LM on non-wettable surfaces. The ultrasonic strategy could prevent the generation of defects such as cavities and exhibited excellent electromechanical performance. Therefore, it showcased clear advantages over conventional injection methods for LM flexible electronics. We successfully fabricated a compact pressure sensor array with a line width of 25 µm by ultrasound filling strategy, which could record both pressure and spatial information. It could even effectively fill a channel embedded with microscale structures, in which way the sensitivity of corresponding strain sensor was significantly enhanced. More importantly, complete filling of integrated circuits and complex patterns with multiple interlaced branches, blind holes and vertical structures were demonstrated. These results highlight the versatility and adaptability of the ultrasound powered strategy for the fabrication of LM circuits, offering promising prospects for the development of compact and integrated flexible devices. Results Ultrasound powered wetting and filling of LM in microchannel To demonstrate the LM wetting capability driven by ultrasound, flexible devices consisting of a LM chamber and microchannels with various diameters were fabricated at first. The detailed fabrication process is depicted in Fig. S1 , including 3D printing of the predesigned mold, reversing the mold with polydimethylsiloxane (PDMS) soft silicone, and finally bonding PDMS device to an uncured silicone film that was spin-coated on a rigid polymethyl methacrylate (PMMA) substrate. Afterwards, EGaIn was manually injected into the LM chamber, and the obtained device was fixed onto a titanium alloy clamp as shown in Fig. 1 a. Then an ultrasonic probe was placed on the upper surface of the clamp to generate high-frequency mechanical vibrations in the vertical direction. The ultrasonic wave was transmitted through the clamp to the fixed flexible device, and forced the wetting of LM from the chamber into microchannels. Finally, the flexible device was detached from the PMMA substrate for further mechanical and electronic testing. The LM wetting and filling results powered by ultrasound in the microchannels with a fixed length of 8 mm and various widths (ranging from 100 to 0.75 µm) were further presented. The smaller value of channel width and height was defined as the minimum characteristic dimension (CD m ), which characterized the fineness of microchannel. Through dividing the channel length (L) by minimum characteristic dimension, we could obtain the aspect ratio of channel (L/CD m ). For example, Fig. 1 b and Movie S1 showed the representative dynamic wetting process of LM in a microchannel with a CD m of 100 µm. When applying an ultrasonic signal with a power of 800 W and a frequency of 20 kHz, LM immediately started to enter the empty microchannel, and a complete filling of the 8 mm long microchannel could be achieved within ~ 0.16 s (aspect ratio L/CD m : 8 mm/100 µm = 80, filling speed in aspect ratio: 80/0.16 s = 500 s − 1 ). It was worth noting that the orientation of device did not affect the filling performance and further details were discussed and illustrated in Methods and Fig. S2. We then employed optical microscope and X-ray based microcomputed tomography (micro-CT) techniques to evaluate the LM filling outcomes in channels of different sizes, as illustrated in Fig. 1 c. The optical images presented in the upper row clearly indicated the complete filling of channels by LM, while the micro-CT results displayed in the lower row revealed that no notable defects were found inside the filled LM. Multi-angle observation of the LM lines with varying CD m captured by micro-CT was presented in Movie S2. Furthermore, we found that an ultrafine channel with a CD m as low as 750 nm could also be well filled (Fig. S3), indicating the excellent wetting capability of ultrasound powered LM in filling ultrafine channels. Moreover, the filling capability of the ultrasonic strategy was compared with the conventional injection method in Fig. 1 d. It was found that both methods were applicable for channels with CD m larger than 50 µm. When the fineness of microchannel was 25 µm or smaller, the injection method was no longer applicable because it required large injection pressure (> 140 kPa), which would easily damage the PDMS flexible device and lead to LM leakage. On the contrary, the ultrasonic method was still effective in powering LM to fill the microchannel without causing any damage, and could even achieve a higher filling speed. Furthermore, we had compiled a comprehensive overview of recent studies on filling LM into flexible micro/nano-channels in Fig. 1e 6,7,11,40,44–49 , which depicted both the minimum characteristic dimension of channels and filling speed achieved by different strategies. The injection method could only fill LM into channels wider than 50 µm. Although, the vacuum-assisted method could fill LM into microchannels as fine as 10 µm, it required much more time to complete the filling process. Our developed ultrasonic method outperformed the current studies by wetting and filling LM into much finer channels with higher efficiency, demonstrating its superiority in fabricating miniaturized flexible electronics. Mechanism of ultrasound powered wetting and channel-filling behavior of LM To investigate how ultrasound powered LM to fill microchannels, we first conducted theoretical simulation to understand the mechanism of LM wetting behavior altered by ultrasonic stimulation. In our model, a 4 µm diameter LM droplet was placed on a rigid substrate whose mechanical properties were set to be PMMA in material library. Then, a vertical vibration was applied to the substrate at 20 kHz to simulate ultrasound activation and further details could be found in Supplementary Note 1. Based on the simulation results in Fig. 2 a(i), when ultrasound was applied, it induced an asymmetrical acoustic pressure within the LM droplet. The maximum and minimum pressure points of the field were located at the center and bottom of LM droplet, respectively. Such difference in acoustic pressure would propel the flow of interior LM to move towards the substrate, thus overcoming the high surface tension of the LM. Consequently, as shown in Fig. 2 a(ii), the LM droplet would rapidly spread on the surface of the substrate within a duration of 12 ms, with the initial position demarcated by a black line. The LM droplet would continue to flatten over time, as indicated by the simulated flow direction. We then conducted experiments to verify the simulation results by placing a 20 µL LM droplet on a PDMS-coated PMMA plate, and a droplet of 1 M HCl solution was added to remove the oxidation film of LM. With a considerable high surface tension, LM did not exhibit effective wetting behavior on most nonmetallic materials 50 , so the initial contact angle between LM droplet and the plate was measured to be ~ 142°. Upon application of ultrasound, the LM droplet immediately spread on the plate surface with a remarkable decrease in contact angle to ~ 67°, which was consistent with our simulation. When removing ultrasonic stimulation, the LM droplet reverted to its original ellipsoidal shape owing to its high surface tension. The entire process was shown in Fig. 2 b and Movie S3, demonstrating the capacity of ultrasound to regulate the wetting behavior of LM on a non-wettable substrate. Given the practical application scenarios of LM filled microchannels, we further investigated the wetting behavior of bare LM droplets under ultrasonic stimulation in the atmosphere. At this time, the LM droplet would be quickly oxidized to form an oxidation film on its surface after being placed on the substrate. It was found that the applied ultrasound would flatten the LM droplet on the substrate. More importantly, with the assistance of adhesive oxidation film towards the PDMS substrate 37 , the spreading shape of LM droplet could be fixed on the substrate after ultrasonic stimulation, and the LM droplet could not bounce back after removing the ultrasound, leading to a significant increase in the wetting area of the LM droplet. Figure 2 c captured the spreading state of a 20 µL LM droplet on a PDMS surface after applying ultrasound for 0.1, 0.2, 0.3, 0.4, and 0.5 s, respectively. With the prolonging of ultrasonic time, the contact angle of the LM droplet decreased, meanwhile its spread area increased. We then calculated the ratio between the spread area (S) and the initial area (S 0 ), to characterize the spreading behavior of LM droplets in Fig. 2 d, which quantitatively depicted the improvement of the wettability of LM on PDMS substrate under ultrasound stimulation. With further increase of ultrasonic time from 0.3 to 0.5 s, the change in spread area ratio became less obvious, due to the limited amount of LM and the formation of excess oxidation film. Therefore, through ultrasonic treatment assisted with the adhesion of LM oxidation film, we could effectively promote and maintain the wetting and spreading of LM even on a non-wettable substrate. Subsequently, we focused on the scenario of ultrasound powered wetting and filling of LM into microchannels. Based on the finite element simulation (Fig. S4 and Fig. 2 e), when ultrasonic stimulation was applied to the LM confined within the device chamber, an asymmetric distribution of acoustic pressure would be generated within the LM. The maximum acoustic pressure point was found to locate at the LM chamber, while the minimum (zero) point was at the outlet of the microchannel. Such pressure gradient tended to drive LM to flow from the chamber into the microchannel, with the flow direction and velocity at the chamber/channel interface shown in Fig. 2 f. Detailed modeling parameters and the conversion method from ultrasonic power to pressure could be found in Supplementary Note 2. We investigated the flow velocity of LM at the liquid/air interface in Fig. 2 g, whose velocity distribution and streamline plot revealed a high filling speed of hundreds s − 1 for the microchannel with a CD m of 100 µm. To validate the simulation results, we conducted the ultrasound powered LM filling experiments in a flexible device with the same configurations as the simulation model. The dynamics of the filling process was captured using a high-speed camera at 20,000 frames/s (Movie S4). As shown in Fig. 2 h, the time of initially captured scene was defined as 0 µs, and the outline of the LM was illustrated by a blue dotted line. Once ultrasound was applied, the flow front of LM could be clearly observed to advance (red dotted line). According to the previous discussion, the oxidation film would attach to the deeper boundary of microchannel, which prevented LM from bouncing back and anchored the spreading front of LM when one pulse signal of ultrasound finished. If the next ultrasonic pulse continued to take effect, the LM would continue to fill inside microchannel. Therefore, ultrasonic stimulation constituted the key factor in powering LM to wet and fill into microchannels. We also quantified the filling speed of LM into microchannels with different CD m under various ultrasonic powers as shown in Fig. 2 i. The filling process powered by ultrasound was extremely fast, whose speed generally increased with increasing the ultrasonic power and channel size. Moreover, the LM filling speed of a 100 µm channel under 600 W power aligned with the simulated results in Fig. 2 g. However, a maximum wetting speed of 1760 s − 1 was obtained when the channel CD m was 25 µm and the reasons could be found in Supplementary Note 3 and Fig. S5. In summary, when ultrasonic stimulation was applied, the generated acoustic pressure gradient could drive the wetting and spreading of LM inside microchannels. The LM oxidation film assisted to anchor the LM spreading front to the elastic channel wall, and then the LM continuously advanced under the acoustic pressure gradient. Therefore, the ultrasonic stimulation and the anchoring effect of LM oxidation film synergistically explain the mechanism of ultrasound powered LM filling process. Electromechanical performance of ultrasound filled LM flexible circuits To validate the potential of ultrasound powered LM filling strategy for the fabrication of flexible electronic devices, we produced single-line strain sensor with a CD m of 100 µm and a length of 12 mm (Fig. 3 a) using ultrasonic method. We compared the electromechanical performance with a sensor of the same structure fabricated by injection method, which was one of the well-established methods for fabricating LM flexible circuits. The variations in relative resistance (R/R 0 , where R was the tested resistance under corresponding strain \(\varepsilon\) and R 0 was the initial resistance without strain) of the strain sensors were characterized (Fig. 3 b). The electrical performance of the strain sensors prepared by the ultrasonic method exhibited comparable performance to that by the injection method, demonstrating the effectiveness of the ultrasonic method in fabricating flexible electronics. Detailed electromechanical testing results of the strain sensor made by both methods were illustrated in Fig. S6. Subsequently, we tested the performance of the strain sensor by applying various strain levels (10%-40%) and meanwhile measuring the change in relative resistance (Fig. 3 c). With each 10% increase in strain, the relative resistance change of the sensor increased by approximately 6%, showing a linear relationship between the relative resistance change and strain. This demonstrated that the sensor was well suitable for strain sensing applications. The gauge factor (GF) of the fabricated strain sensor, which represented the sensitivity of the strain sensor, was calculated at various strain levels as shown in Fig. 3 d. The GF exhibited a linear correspondence with increasing strain, which was a normal phenomenon since the GF was defined by the slope of relative resistance curve versus strain, i.e. , \(\) (∆R/R 0 )/ \(\varepsilon\) . For bulk solid metal, GF can be derived from the following formula 51 : $$\frac{\varDelta R}{{R}_{0}\varepsilon }=(\frac{R}{{R}_{0}}-1)/\varepsilon = \left[{\left(\frac{l}{{l}_{0}}\right)}^{2}-1\right]/\varepsilon =\left[{\left(\varepsilon +1\right)}^{2}-1\right]/\varepsilon = \varepsilon +2$$ 1 where l is the stretching length and l 0 is the initial length of the metal. The GF of the LM strain sensor in our work was found to be notably lower than the theoretical value of (ɛ+2). Even at 40% strain, the average GF of our sensor was only ~ 0.61, which was much smaller than the theoretical value (ɛ+2 = 0.4 + 2 = 2.4). Such phenomenon could be attributed to several influencing factors, including the cross-sectional geometry, trace shape and regularity, and quantity of encapsulated LM 52 , which was broadly found in previous works 53–55 . Besides, to evaluate the reliability of the flexible electronics manufactured by our ultrasonic method, we further encapsulated LM strain sensor in a rectangular PDMS substrate and assessed its cyclic stability (Fig. S7). The strain sensor was subjected to 10,000 cycles of 40% strain in both horizontal (x) and vertical (y) directions, as depicted in Fig. 3 e and 3 f, respectively (the stretching direction was illustrated in the inset). Remarkably, even after 10,000 cycles, the LM sensor exhibited consistent electrical response performance. The relative resistance variation of the sensor was only approximately 8% in the x direction and less than 3% in the y direction. These findings indicated a robust cyclic stability of the strain sensor 10,28 and guaranteed the reliability of the ultrasound powered channel-filling method for fabricating flexible electronics. Moreover, we designed and prepared a pressure sensor based on a spiral LM pattern with a CD m of 25 µm, which could hardly be achieved by injection method. The pressure sensor had dimensions of 4.2 mm in length, 3 mm in width and 1.2 mm in height (Fig. 3 g). Figure 3 h characterized the electrical response performance of the designed pressure sensor by applying stress of 20, 40 and 80 kPa in sequence. With every increment of 20 kPa in applied stress, the sensor displayed an increase of approximately 1.1% in the change of relative resistance (R/R 0 ), revealing a linear relationship between the relative resistance variation and applied stress. We further evaluated the cyclical stability of the pressure sensor by imposing dynamic stress with sinusoidal variation (amplitude 40 kPa) for 10,000 cycles. The results shown in Fig. 3 i demonstrated only a small amount of relative resistance change (~ 4%) after the cyclical process, which had a minimal impact on the sensor function. Besides, no circuit failure caused by LM leakage was observed during all the testing process, indicating the robustness of pressure sensor. Subsequently, we assembled nine such pressure sensors together into a sensor array, which could still be conveniently placed on a finger (Fig. S8). The small size of the as-prepared sensor device indicated that the delicate fabrication method had great potential for further fabrication of complex flexible electronics with high integration degree. Each sensor was connected to an individual channel of a multimeter, and then mechanical loads with different shapes (“H”, “I” and “T”) were placed on the sensor array respectively. The schematic mold and results of relative resistance change of different channels were illustrated in Fig. 3 j and the detailed data were presented in Fig. S9-11. The signals from each channel would reveal whether there was a load above the sensor, as well as the magnitude of the pressure loaded. When the “H” load was applied to the sensor array, channels 1, 3, 4, 5, 6, 7 and 9 recorded changes in resistance. Upon switching to load “I”, channels 2, 5 and 8 exhibited signal variations, while channels 1, 3, 4, 6, 7 and 9 returned to the baseline. Similarly, with the “T” load, corresponding channels detected resistance signal changes. Notably, for applying “H” and “I” loads, channel 5 showed the most pronounced signal variation, and for applying “T” load, channels 2 and 5 exhibited relatively significant signal changes. This observation could be attributed to the fact that the center of mass for “H” and “I” loads was above sensor 5, whereas for the “T” load, the center of mass was between sensors 2 and 5. If there was no load on the sensor, R/R 0 would return to baseline after a transient disturbance. In summary, we validated the reliability of ultrasonic method for fabricating flexible electronic devices and demonstrated its applicability to strain and pressure sensing. Ultrasound powered LM filling into micro-structured ultrafine channels for strain sensor performance enhancement Based on the merits of the ultrasonic method, we then sought to further promote the performance of flexible electronic devices by filling LM into finer microchannel that usually cannot be accomplished by traditional injection methods. Following the aforementioned discussion, it was evident that the GF of LM single-line strain sensor, either in our work or in previous reports 53–55 , fell below the theoretical value. We then designed and fabricated a strain sensor by integrating five evenly distributed prismatic microstructures inside the single-line microchannel to enhance the GF, as depicted in Fig. 4 a. When the designed sensor was stretched to the same strain as the normal single-line sensor, the pressure exerted by the prismatic microstructures would induce a much larger local deformation of the LM in channel, which was expected to significantly enhance the relative resistance change. In this manner, the GF of sensor would be greatly boosted. We first conducted simulations to verify our hypothesis. The stress distribution in PDMS substrates and displacement in the z-axis direction of the upper surface of channels were firstly analyzed. It was worth noting that all LM channels in our study, either with or without microstructures, had a CD m of 100 µm and a length of 12 mm, while the prismatic microstructures were designed to be 75 µm in height and 100 µm in width. Thus, the size of the narrowest regions in the sensor located between the tips of microstructures and the channel boundary were only 25 µm. Compared with the normal single-line sensor, simulation results in Fig. 4 b indicated that the sensor with microstructure design possessed a significant increase in maximum stress at 40% strain (one order of magnitude). Examination of the z-axis displacement of the channel with microstructure further indicated that the prismatic microstructure had locally compressed the filled LM during the stretching process. Additionally, we extracted the stress distribution along channel for both types of sensors, as shown in Fig. 4 c. When subjected to 40% strain, the maximum stress in normal single-line sensor was found to exist at the two endpoints with a value of ~ 7×10 6 N/m 2 . In contrast, the introduction of microstructures led to the concentration of stress, with the maximum value at the tip of microstructure being ~ 15×10 6 N/m 2 . Therefore, the simulation results were consistent with our hypothesis and suggested that the introduction of prismatic microstructures could theoretically enhance the GF of strain sensor. Subsequently, we fabricated the micro-structured channel, and a local zoom-in image captured by scanning electron microscope (SEM) was shown in Fig. 4 d. Due to the narrow gap of ~ 25 µm between microstructure tips and channel boundary, the injection method could not drive LM across the microstructures inside channel, leading to the failure in fabricating flexible strain sensor. In contrast, ultrasound proved capable of powering LM to wet and pass through such narrow regions, and achieved rapid and complete filling of LM within 1 s (Fig. 4 e). Then, the LM-filled channel was further processed to serve as a finger-wearable strain sensor and its electromechanical performance was scrutinized. As shown in Fig. S12, the designed sensor exhibited a more pronounced change rate in resistance under various strains compared to the normal single-line strain sensor, which was consistent with our anticipated results. Figure 4 f compared the GF of sensors with and without microstructures at various strains from 10–40%, indicating an obvious enhancement of sensor sensitivity enabled by the microstructure design. It was found that the average GF at 40% strain increased from 0.61 to 13.54 after integrating prismatic microstructures, representing a remarkable 22.2-fold improvement. We further evaluated the performance of finger-wearable strain sensor at different bending angles (30°, 60°, and 90°) in Fig. 4 g. Similarly, the relative resistance change of the sensor with microstructures exhibited a significant increasement compared to the normal single-line sensor, particularly at 90° bending angle (~ 7.7 fold). This was due to the fact that a larger bending angle would result in greater squeezing of the LM by microstructures, leading to a more substantial change in resistance. Finally, we summarized the GF of various LM-based strain sensors at different strains from recent works in Fig. 4h 28,46,53–58 , indicating the majority of average GF values fell below the theoretical expectation. In contrast, we succeeded to design and fabricate an LM-based strain sensor by incorporating prismatic microstructures, which showed a much higher GF compared to previous works, as well as the theoretical value. Therefore, the developed ultrasonic method that powered filling of LM into finer microchannels could boost the electromechanical performance of flexible electronics. Ultrasound powered LM filling into complex and multi-branched channels with diverse circuit architectures Another unique advantage of the ultrasonic method was its applicability to complex and/or multiple channels with diverse circuit structures. For example, Fig. 5 a presented the ultrasound powered filling process of a tri-channel LED lamp array. Copper wires were connected to the LM droplets (serving as electrodes here) in the chambers, followed by the application of a 3 V voltage from a constant voltage source. After ultrasonic treatment, all six straight channels were immediately and completely filled with LM within 1 s, and the three LED lamps lit up almost simultaneously. The entire process was recorded by a digital microscope and could be found in Movie S5. To illustrate the advantages of the ultrasonic method, we also used the injection method to fill the multichannel devices with LM. However, conventional injection could only drive LM into the middle channel (Fig. S13), because the middle pathway required the lowest injection pressure according to the Hagen-Poiseuille Eq. 5 9 . Besides, we designed an integrated circuit pattern that was characterized by a microcontroller with 32 multichannels. The schematic diagram and experimental results of the filling process were presented in Fig. 5 b and Movie S6. Remarkably, within only 0.05 s, all channels connected to microcontroller pins were thoroughly filled with LM via ultrasonic stimulation, thus establishing complete electrical connections. Furthermore, the filling of LM into multi-branched and interlaced channels was performed by ultrasonic treatment. The fabrication of such LM circuits held nonnegligible value for various electronic applications (e.g., electromagnetic shielding 60,61 ), which, however, was easily accompanied by the generation of localized defects and bubbles using injection or vacuum methods. For example, we designed a spiderman pattern with multiple branches and interlaced architectures. Under ultrasonic stimulation, all the tiny channel branches of spiderman pattern were all filled with LM in 0.9 s (Fig. 5 c and Movie S7), highlighting the rapid and efficient features of our ultrasonic method. In sharp contrast, manual injection proved to be incapable of filling the multiple interlaced channels, posing a risk of circuit malfunction when fabricating flexible electronic devices. Additionally, Fig. 5 d and Movie S8 demonstrated that LM could be uniformly driven into a snowflake pattern, and all the planar structure and blind-hole corners were successfully filled, indicating our ultrasonic method could be applied even to LM circuits without outlets. This result illustrates that the ultrasonic strategy could be applied for circuit interconnection in flexible devices, especially when hermetic sealing or electronic packaging is required in advance. Besides, it was found that ultrasound could even overcome the gravity of LM and power LM into a comb-like channel with two vertical channels (Fig. S14), indicating the potential in three-dimensional multilayer flexible circuits. Therefore, in comparison to conventional injection methods, ultrasonic method demonstrated exceptional capabilities in wetting and filling of LM into various channels for the fabrication of flexible devices with diverse complex configurations, which promised substantial advancements in the preparation of compact and integrated electronics. Conclusion In this work, we presented an ultrasound powered LM channel-filling strategy for fabricating LM circuits in flexible electronics. Through finite element simulation and experimental validation, we demonstrated that ultrasound could alternate the wetting behavior of LM on non-wetting substrates by inducing an asymmetric acoustic pressure field inside LM. Combined with the adhesive LM oxidation film, LM could be powered by ultrasound to efficiently fill ultrafine microchannels, which could not be achieved by conventional injection methods. Besides, we validated the feasibility, reliability and stability of the flexible electronics produced by the ultrasonic strategy. A sensor array of 9 miniaturized pressure sub-sensors was successfully fabricated, which was capable of pressure and spatial location sensing. Besides, ultrasound stimulation was used to power LM filling into a straight channel containing five prismatic microstructures to yield a highly sensitive LM strain sensor, whose GF increased by more than 20 times compared to the single-line strain sensor. Furthermore, the developed ultrasonic strategy proved to be highly efficient and adaptable to diverse microchannels with intricate features, encompassing complex multiple interlaced structures, planar architectures, blind holes and vertical channels, all of which could hardly be achieved by conventional methods. These fabrication achievements, facilitated by ultrasonic stimulation, demonstrated exceptional versatility in the production of LM-based flexible circuits. This work holds great promise for the development of even more sophisticated and integrated flexible devices with high efficiency. Future endeavors will be contributed to the construction of three-dimensional flexible LM circuits incorporated with electronic components to build fully functional electronic devices, thereby exploring the potential of the ultrasound powered LM channel-filling strategy in fabricating flexible electronics with high degree of integration. Methods Materials EGaIn alloys were purchased from Dongguan Metal Technology. The flexible samples made of PDMS (Sylgard 184, Dow Corning Corporation) were prepared by mixing the base and the curing agent in a ratio of 10:1 by weight. The ultrasonic clamp, crafted by Dongguan Metal Technology, used TC4 material (80 mm × 30 mm × 4 mm). Preparation of diverse microchannels and flexible circuits The preparation process of microchannels and other patterns was detailed in Fig. S1 , involving five main steps: 3D printing of the mold, demolding of the PDMS sample, bonding, ultrasound powered filling of LM into microchannels, and peeling of the sample from the PMMA substrate. A BMF Nano Arch S130 was employed for projection microstereolithography to produce resin patterning molds with a minimum resolution of 2 µm and a thickness of 5 µm. Initially, the PDMS mixture was poured into the resin mold and subjected to vacuum drying for 30 minutes at room temperature to eliminate any bubbles. The sample was then cured at 60℃ for 2 hours, resulting in a cured PDMS sample demolded from the mold. Subsequently, a PMMA substrate (30 mm × 30 mm × 2.4 mm, Oudifu Company) was spin-coated with mixed PDMS solution at 750 rpm for 1 minute and cured at 60℃ for 30 minutes to obtain half-cured PDMS film. Then, the PDMS sample was bonded by placing it on half-cured PDMS on PMMA substrate, and the substrate was heated in a dryer at 60℃ for 2 hours to obtain a PDMS sample with diverse empty microchannels. EGaIn was then injected into the LM chamber. Following by ultrasonic treatment, the LM was completely filled into microchannels. Finally, the PDMS sample was peeled off the PMMA substrate to obtain the flexible circuits. Ultrasonic system The ultrasonic equipment used in this study (PTS-2000 series, Shenzhen Longxinda Ultrasonic Technology Company) had a maximum power of 2000W, and its ultrasonic probe had a diameter of 20 mm. The process of ultrasound powered LM spreading or filling into channels was recorded using a digital microscope (Bairuide Optical Instruments Company, up to 30 fps) and a high-speed camera (Phantom V12.1, up to 680 kfps). LM was injected into the LM chamber of the PDMS sample before the PMMA substrate was screwed to the TC4 clamp. To optimize ultrasound conduction, an air compressor was used to apply 3 MPa pressure to the clamp via the ultrasonic probe. Upon ultrasound application, LM was powered to fill into microchannels Characterization After exposing the LM to ultrasound for different durations, the contact angle of LM droplets was measured with a contact angle meter (JC2000D3B, Shanghai Zhongchen Digital Technology Equipment Co., Ltd.). The filling outcomes were detected using micro-computed tomography (Micro-CT, ZEISS, Xradia 515 Versa), and the microstructure of channel was examined using a Phenom Pro scanning electron microscope (SEM). The filling result of the 0.75 µm channel filled with LM was taken by confocal laser scanning microscope (CLSM, NiKon A1). Analysis of the impact of device orientation on ultrasonic strategy results Both longitudinal and transverse waves can propagate in solids, and we simulated the displacement of the clamp surface in X, Y and Z directions, respectively (Fig. S2a). The displacement in X and Y directions was found to be negligible compared to that in Z direction. The simulation results indicated that the displacement of the clamp surface in the Z direction within the PMMA substrate area exhibited a nearly uniform distribution and served as the primary acoustic source for the LM in chamber. Furthermore, the experimental results corroborated that the orientation of device did not impact the filling process (Fig. S2b). Preparation of flexible electronics To establish electrical contact with the LM within microchannels, LM chambers were severed, and connections were achieved using copper wires. Then, the samples were secondarily encapsulated in various molds, fabricated using a 3D printer (HORI 3D Printer Z300Plus), with mixed PDMS. Finally, the samples underwent vacuum curing at 60°C for 2 hours to fabricate flexible electronics for diverse applications. Performance tests We evaluated the electromechanical response of single-line strain sensors by conducting mechanical tests using a Shimadzu mechanical testing machine and a Keithley 2010 resistance meter. The resistance performance of the pressure sensor, pressure sensor array, and finger-wearable strain sensor was analyzed using a Keithley DAQ6510 multichannel universal meter. Fabrication and evaluation of pressure sensor array Nine pressure sensors were fabricated by ultrasonic method and LM chambers were severed to connect copper wires. To enhance the stability of the connection between the solid wires and the flexible LM, we performed secondary encapsulation of joints with mixed PDMS and placed the samples in a vacuum dryer at 60°C for 2 h. The PDMS samples were then peeled from PMMA substrates to obtain nine divided pressure sensors. These sensors were then placed on a new PMMA substrate coated with half-cured PDMS and PMMA substrate was set in a vacuum dryer at 60°C for 2 h to obtain the pressure sensor array. Compression loads with three letter shapes (H, I, T) were designed and printed by a 3D printer to distribute the 500 g load more evenly on specific pressure sensors. The resistance change of different sensors was monitored using a Keithley DAQ6510 multichannel universal meter. Fabrication and evaluation of finger-wearable strain sensor with microstructures The channel with microstructures was filled with LM using ultrasonic method and connected with copper wires. Subsequently, we utilized a 3D printer to create a hollow ring mold tailored to fit the researcher's index finger. Such mold was employed for the secondary encapsulation of the channel. Following a 2-hour curing process at 60°C in a vacuum dryer, we obtained a finger-wearable strain sensor by delicately demolding the sample from the mold. The sensor was positioned at the second joint of the index finger, and its resistance was recorded using a Keithley DAQ6510 multichannel universal meter with a 4-point probe (4PP) method to minimize parasitic resistance. To quantify the bending angles of the finger, we designed folding plates with 30°, 60°, and 90° angles, printed by a 3D printer. Recording the process of ultrasound powered filling of LM into multiple microchannels to lighten the LED array Utilizing the BMF Nano Arch P150 with a resolution of 25 µm, we 3D-printed a resin mold featuring three channels connected to the LM chamber. Following the demolding of PDMS samples from the mold, we positioned two samples and three LEDs on a PMMA substrate coated with half-cured PDMS. Subsequently, we subjected the assembly to vacuum curing at 60°C for 2 hours and clamped it to the TC4 clamp. The LM in both LM chambers was connected by copper wires and supplied with 3 V via a constant voltage source. Throughout the ultrasound application, we employed a digital microscope to record the entire process of ultrasound powered filling of LM into multiple microchannels to lighten the LED array. The fabrication and filling of other complex patterns were carried out using the same method. Declarations Data Availability The authors declare that the data supporting the findings of this study are available within the paper and its supplementary information files. Acknowledgments The work is financially supported by National Natural Science Foundation of China with a No. of 92163109 (X. M), National Key R&D Program of China with a No. of 2023YFE0208700 (X. M), Shenzhen Science and Technology Program with Nos. of JCYJ20200109113408066, and KQTD20170809110344233 (X. M), and the Fundamental Research Funds for the Central Universities with a Grant No. of HIT.OCEF.2021032 (X. M). Z. Xu thanks the support from National Natural Science Foundation of China with a No. of 51574099 (Z. X) and 51435004 (Z. X). D. Jin thanks the support from National Natural Science Foundation of China with a No. of 52202348 (D. J), Guangdong Basic and Applied Basic Research Foundation with a No. of 2023A1515011491 (D. J), and Shenzhen Science and Technology Program with a No. of GXWD20220818224716001 (D. J). Author Contributions Z.R. X, D. J, Z.W. X, X. M conceived the project, and D. J, Z.W. X and X. M supervised the studies. Z.R. X designed channel patterns and employed ultrasound powered filling of LM into microchannels. S. L helped to encapsulate flexible circuits. G. L and Z. 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S. & Kramer-Bottiglio, R. Are Liquid Metals Bulk Conductors? Adv. Mater. 34 , e2109427 (2022). Tavakoli, M. et al. EGaIn-Assisted Room-Temperature Sintering of Silver Nanoparticles for Stretchable, Inkjet-Printed, Thin-Film Electronics. Adv. Mater. 30 , e1801852 (2018). Chen, B. et al. Liquid metal-tailored gluten network for protein-based e-skin. Nat. Commun. 13 , 1206 (2022). Lopes, P. A., Santos, B. C., de Almeida, A. T. & Tavakoli, M. Reversible polymer-gel transition for ultra-stretchable chip-integrated circuits through self-soldering and self-coating and self-healing. Nat. Commun. 12 , 4666 (2021). Haake, A., Tutika, R., Schloer, G. M., Bartlett, M. D. & Markvicka, E. J. On‐Demand Programming of Liquid Metal‐Composite Microstructures through Direct Ink Write 3D Printing. Adv. Mater. 34 , 2200182 (2022). Matsuzaki, R. & Tabayashi, K. Highly stretchable, global, and distributed local strain sensing line using GaInSn electrodes for wearable electronics. Adv. Funct. Mater. 25 , 3806-3813 (2015). Ma, B. et al. A versatile approach for direct patterning of liquid metal using magnetic field. Adv. Funct. Mater. 29 , 1901370 (2019). Kirby, B. J. Micro-and nanoscale fluid mechanics: transport in microfluidic devices . (Cambridge university press, 2010). Wang, Z. et al. Rational Assembly of Liquid Metal/Elastomer Lattice Conductors for High‐Performance and Strain‐Invariant Stretchable Electronics. Adv. Funct. Mater. 32 , 2108336 (2022). Wang, C. et al. A general approach to composites containing nonmetallic fillers and liquid gallium. Sci. Adv. 7 , eabe3767 (2021). Additional Declarations There is NO Competing Interest. Supplementary Files SupportingInformationNE.docx Supporting Information MovieS1.UltrasoundpoweredfillingofLMintostraightmicrochannelwithaminimumcharacteristicdimensionof100XXm.mp4 Movie S1. Ultrasound powered filling of LM into straight microchannel with a minimum characteristic dimension of 100 μm MovieS2.Fillingresultsofmicrochannelswithvarioussizescapturedbymicrocomputedtomography.mp4 Movie S2. Filling results of microchannels with various sizes captured by micro-computed tomography MovieS3.ThewettingbehaviorofLMdropletwithoutoxidefilmonPDMSsubstrateregulatedbyultrasound.mp4 Movie S3. The wetting behavior of LM droplet without oxide film on PDMS substrate regulated by ultrasound MovieS4.HighspeedcameracapturedtheLMfillingprocessinmicrochannelXX0.0015speed.mp4 Movie S4. High-speed camera captured the LM filling process in microchannel (×0.0015 speed) MovieS5.UltrasoundpoweredfillingofLMintomultiplemicrochannelstolightenLEDarray.mp4 Movie S5. Ultrasound powered filling of LM into multiple microchannels to lighten LED array MovieS6.UltrasoundpoweredfillingofLMintomultiplechannelstoconnecta32pinmicrocontroller.mp4 Movie S6. Ultrasound powered filling of LM into multiple channels to connect a 32-pin microcontroller MovieS7.UltrasoundpoweredfillingofLMintoaspidermanpatternwithinterlacedchannels.mp4 Movie S7. Ultrasound powered filling of LM into a spiderman pattern with interlaced channels MovieS8.UltrasoundpoweredfillingofLMintoasnowflakepatternwithblindholes.mp4 Movie S8. Ultrasound powered filling of LM into a snowflake pattern with blind holes Cite Share Download PDF Status: Under Review Version 1 posted 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. 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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-3909065","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":270014072,"identity":"586cee77-8170-4570-836a-75f21b3180df","order_by":0,"name":"Xing Ma","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAwklEQVRIiWNgGAWjYJCCAwwMNglsIBYPCVrSEtjYSNECBIcTGIjWott+xvBwwa/zeXzyDYwP3rYxyJsT0mJ2Ji3h8My+28VAhzEbzm1jMNzZQEjLgeQDh3l7bie2sTGwSfO2MSQYHCCk5fzDBqCWcyAt7L+J03IDaAvPjwNgW5iJ1PIs4TBvQzJQS2Kz5JxzEoYbCDssx/gzzx+7xPnNhw9+eFNmI0/QFjBgbAOTDUBCghj1IPCHWIWjYBSMglEwIgEA7f1BZt1Ra/AAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-2248-4806","institution":"Harbin Institute of Technology (Shenzhen)","correspondingAuthor":true,"prefix":"","firstName":"Xing","middleName":"","lastName":"Ma","suffix":""},{"id":270014073,"identity":"40496a2d-2319-4198-bd82-39b9eea9f855","order_by":1,"name":"Zirong Xu","email":"","orcid":"","institution":"State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin150001, 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(Shenzhen)","correspondingAuthor":false,"prefix":"","firstName":"Chengqian","middleName":"","lastName":"Xiong","suffix":""},{"id":270014080,"identity":"9557a845-92dc-4f65-907c-55e77b9e5dfe","order_by":8,"name":"Zhiwu Xu","email":"","orcid":"","institution":"Harbin Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Zhiwu","middleName":"","lastName":"Xu","suffix":""}],"badges":[],"createdAt":"2024-01-29 13:55:34","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3909065/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3909065/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":52094577,"identity":"90185c6f-3b94-48a8-87c3-6a8886e82799","added_by":"auto","created_at":"2024-03-06 15:24:02","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":746050,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eUltrasound powered LM filling into microchannels diagram.\u003c/strong\u003e a) Schematic diagram of ultrasound powered LM filling process. b) The dynamic wetting process of LM in a microchannel with a minimum characteristic dimension of 100 μm. c) Different sizes of channels filled with LM by ultrasound taken by optical microscope and micro-CT. \u0026nbsp;d) Comparison between injection and ultrasonic methods for powering filling of LM into microchannels of varying sizes. e) Comprehensive overview of recent studies on filling LM into micro/nano-channels.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/8d3e7e9fd26be4c8961181ad.png"},{"id":52094580,"identity":"34915adc-69ef-4b39-90fc-5f5289b96c21","added_by":"auto","created_at":"2024-03-06 15:24:03","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":853810,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMechanism of ultrasound-altered wetting behavior of LM.\u003c/strong\u003e a) Simulation of acoustic pressure and velocity distributions within the LM droplet under ultrasonic treatment. b) Contact angle measurement of LM droplets without oxidation film under ultrasonic treatment. c) LM wetting behavior with oxidation film after different ultrasonic treatment times. d) Ratio of spreading area (S) to initial area (S\u003csub\u003e0\u003c/sub\u003e) of LM droplets varied with different ultrasonic treatment durations. e) Simulation of acoustic pressure distribution in LM chamber and along channel upon ultrasonic treatment. f) Simulation of velocity distribution within LM in the LM chamber upon ultrasonic treatment. g) Simulation of velocity distribution at LM spread front. h) High-speed camera-captured LM filling process. i) The average wetting speed in aspect ratio of LM to fill microchannels of different minimum characteristic dimension under various ultrasonic powers.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/2e7f39bc2ec4306f3f73dbbe.png"},{"id":52094575,"identity":"174a9d73-4b86-4ce4-8a82-fe8be76963a9","added_by":"auto","created_at":"2024-03-06 15:24:01","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":547173,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectromechanical performance of LM flexible electronics manufactured \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003evia\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e ultrasonic method.\u003c/strong\u003e a) Schematic and physical appearance of strain sensor. b) Electromechanical performance of strain sensor fabricated by ultrasonic and injection methods. c) Relative resistance change of strain sensor at various strains. d) Average GF of strain sensor at various strains from 10% to 40%. e) Relative resistance change of strain sensor during 10,000 cycles with 40% strain in horizontal (x) direction. f) Relative resistance change of strain sensor during 10,000 cycles with 40% strain in vertical (y) direction. g) Schematic and physical appearance of pressure sensor. h) Relative resistance response of pressure sensor under different stress. i) Relative resistance change of pressure sensor during 10,000 cycles with a maximum stress of 40 kPa. j) Schematic diagram of pressure sensor array and results of relative resistance variation from different channels.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/a4333cd515266f49da349373.png"},{"id":52094571,"identity":"82fe46a9-a2ce-43ac-902a-7efd3e783d8a","added_by":"auto","created_at":"2024-03-06 15:24:00","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":734080,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFinger-wearable strain sensor fabricated by ultrasonic method.\u003c/strong\u003e a) Schematic appearance of single-line strain sensor and strain sensor with microstructures. b) Simulation of stress distribution in PDMS substrate and displacement in z-axis direction of the channel upper surface at 40% strain without and with microstructure. c) The stress distribution along the channel upper edge of both simulation models. d) The physical and zoom-in image of finger-wearable strain sensor with microstructures. e) Filling results of finger-wearable strain sensor by injection and ultrasonic methods. f) Average GF of strain sensors at various strains from 10% to 40% with and without microstructures. g) Electromechanical performance of finger-wearable strain sensor with microstructures and single-line strain sensor at various bending angles. h) Average GF of strain sensors made by LM or LM composites at various strains in recent works.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/96c89827fbbf3d143d5a18c1.png"},{"id":52094576,"identity":"c821e220-faed-4e50-b23b-b9492b560582","added_by":"auto","created_at":"2024-03-06 15:24:02","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1925371,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eOther flexible demonstrations fabricated by ultrasonic method.\u003c/strong\u003e a) The schematic and physical images of the multichannels connecting LEDs array. b) Filling results by ultrasonic and injection methods of multiple channels to connect a 32-pin microcontroller. c) Filling results by ultrasonic and injection methods of spiderman pattern with diverse interlaced channels and multiple outlets filled with LM. d) Filling results by ultrasonic and injection methods of snow pattern with six outlets and branch-blind-hole structures filled with LM.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/ce042632c7c17520f25254f9.png"},{"id":55774136,"identity":"81f0b96b-3c76-4571-9d42-e268c34aa113","added_by":"auto","created_at":"2024-05-02 23:23:39","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5717683,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/e7247323-b00d-4560-b07b-431a0e5f5e80.pdf"},{"id":52094570,"identity":"32343f61-66f7-44b0-bd6e-c6b999c5e5b1","added_by":"auto","created_at":"2024-03-06 15:24:00","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":5354955,"visible":true,"origin":"","legend":"\u003cp\u003eSupporting Information\u003c/p\u003e","description":"","filename":"SupportingInformationNE.docx","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/52af204411d0ec809ff8a5ab.docx"},{"id":52094578,"identity":"bbafeaee-bc0a-43b4-8b03-45de12568425","added_by":"auto","created_at":"2024-03-06 15:24:02","extension":"mp4","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":5649028,"visible":true,"origin":"","legend":"Movie S1. Ultrasound powered filling of LM into straight microchannel with a minimum characteristic dimension of 100 \u0026#x03BC;m","description":"","filename":"MovieS1.UltrasoundpoweredfillingofLMintostraightmicrochannelwithaminimumcharacteristicdimensionof100XXm.mp4","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/09ac6b8155dfe790abd25cb7.mp4"},{"id":52094581,"identity":"cc7f5c56-8eb5-4bfb-b71f-a52752c6a88a","added_by":"auto","created_at":"2024-03-06 15:24:03","extension":"mp4","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":14364820,"visible":true,"origin":"","legend":"Movie S2. Filling results of microchannels with various sizes captured by micro-computed tomography","description":"","filename":"MovieS2.Fillingresultsofmicrochannelswithvarioussizescapturedbymicrocomputedtomography.mp4","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/19eb811d8c356fd68a3aba75.mp4"},{"id":52094573,"identity":"71df0785-6be7-48a0-b775-d69e14db25be","added_by":"auto","created_at":"2024-03-06 15:24:01","extension":"mp4","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":6368637,"visible":true,"origin":"","legend":"\u003cp\u003eMovie S3. The wetting behavior of LM droplet without oxide film on PDMS substrate regulated by ultrasound\u003c/p\u003e","description":"","filename":"MovieS3.ThewettingbehaviorofLMdropletwithoutoxidefilmonPDMSsubstrateregulatedbyultrasound.mp4","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/26f93f401ad92b49844c85ca.mp4"},{"id":52094574,"identity":"6bfb4eda-53ed-4841-b4bd-e97f7b936e2a","added_by":"auto","created_at":"2024-03-06 15:24:01","extension":"mp4","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":2944241,"visible":true,"origin":"","legend":"Movie S4. High-speed camera captured the LM filling process in microchannel (\u0026#x00D7;0.0015 speed)","description":"","filename":"MovieS4.HighspeedcameracapturedtheLMfillingprocessinmicrochannelXX0.0015speed.mp4","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/6e8a7358fa6a79b60f5da5eb.mp4"},{"id":52094579,"identity":"12520e1c-76e7-430d-8ff8-0fb7005adfb0","added_by":"auto","created_at":"2024-03-06 15:24:03","extension":"mp4","order_by":6,"title":"","display":"","copyAsset":false,"role":"supplement","size":17721671,"visible":true,"origin":"","legend":"Movie S5. Ultrasound powered filling of LM into multiple microchannels to lighten LED array","description":"","filename":"MovieS5.UltrasoundpoweredfillingofLMintomultiplemicrochannelstolightenLEDarray.mp4","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/a379c1b48cb628a2331a075c.mp4"},{"id":52094583,"identity":"5c115d6a-4351-44a0-8f80-ffa8dbdc979e","added_by":"auto","created_at":"2024-03-06 15:24:04","extension":"mp4","order_by":7,"title":"","display":"","copyAsset":false,"role":"supplement","size":5893971,"visible":true,"origin":"","legend":"Movie S6. Ultrasound powered filling of LM into multiple channels to connect a 32-pin microcontroller","description":"","filename":"MovieS6.UltrasoundpoweredfillingofLMintomultiplechannelstoconnecta32pinmicrocontroller.mp4","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/5d9da3387cc30dcd91441670.mp4"},{"id":52094582,"identity":"4de6f1d6-e63c-426f-a21f-ca25a8d88ace","added_by":"auto","created_at":"2024-03-06 15:24:03","extension":"mp4","order_by":8,"title":"","display":"","copyAsset":false,"role":"supplement","size":9079123,"visible":true,"origin":"","legend":"\u003cp\u003eMovie S7. Ultrasound powered filling of LM into a spiderman pattern with interlaced channels\u003c/p\u003e","description":"","filename":"MovieS7.UltrasoundpoweredfillingofLMintoaspidermanpatternwithinterlacedchannels.mp4","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/b49937b912b2425e0b75c87b.mp4"},{"id":52094584,"identity":"cfa8a790-95c0-4bff-8821-1e3fe8f58960","added_by":"auto","created_at":"2024-03-06 15:24:04","extension":"mp4","order_by":9,"title":"","display":"","copyAsset":false,"role":"supplement","size":12514178,"visible":true,"origin":"","legend":"\u003cp\u003eMovie S8. Ultrasound powered filling of LM into a snowflake pattern with blind holes\u003c/p\u003e","description":"","filename":"MovieS8.UltrasoundpoweredfillingofLMintoasnowflakepatternwithblindholes.mp4","url":"https://assets-eu.researchsquare.com/files/rs-3909065/v1/c9ddaaecb41d67f51e12bec1.mp4"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Ultrasound Powered Wetting and Filling of Liquid Metal into Ultrafine Channels for Flexible Electronics","fulltext":[{"header":"Introduction","content":"\u003cp\u003eRoom-temperature liquid metals (LMs), particularly gallium-based LMs, have been extensively investigated in recent years owing to their distinctive properties such as metallic conductivity, low toxicity, and intrinsic fluidity\u003csup\u003e1\u0026ndash;3\u003c/sup\u003e. Serving as ideal soft and stretchable conductors, LMs are emerging as one of the most promising materials for various flexible electronics, including wearable sensors\u003csup\u003e4,5\u003c/sup\u003e, portable energy harvesting and storage devices\u003csup\u003e6\u0026ndash;8\u003c/sup\u003e, implantable bioelectronics\u003csup\u003e9,10\u003c/sup\u003e, and so on\u003csup\u003e11\u0026ndash;13\u003c/sup\u003e. Compared with traditional rigid circuits\u003csup\u003e14\u0026ndash;17\u003c/sup\u003e, the leap in flexibility and adaptability endowed by LMs pushes the boundaries of electronic technologies, and contributes to bridging the gap between humans and devices\u003csup\u003e18,19\u003c/sup\u003e. Recent works have further pursued for LM circuits with finer patterning resolution and higher fabrication freedom, aiming to create miniaturized, compact yet versatile electronic devices with increased integration complexity\u003csup\u003e20,21\u003c/sup\u003e. However, it remains a grand challenge for current LM patterning methods to efficiently fabricate complex architected circuits with a resolution at microscale and below.\u003c/p\u003e \u003cp\u003eConsiderable approaches have been explored to prepare and pattern LM circuits on various soft substrates for flexible electronics, which can be broadly divided into two categories, i.e., direct and indirect strategies. The former includes direct ink writing\u003csup\u003e5,22\u003c/sup\u003e, lithography-enabled patterning\u003csup\u003e21,23\u003c/sup\u003e, 3D printing\u003csup\u003e24\u0026ndash;26\u003c/sup\u003e and subtractive ablation\u003csup\u003e27,28\u003c/sup\u003e, which selectively deposit LMs at desired locations on the substrate, usually followed by sealed packaging with polymeric elastomers. Due to the intrinsically high surface tension, LMs are non-wettable to most soft substrate materials used in flexible electronics. Therefore, direct fabrication strategies are commonly accomplished with the aids of doping adhesives\u003csup\u003e29,30\u003c/sup\u003e, pre-patterning of wettable metal layers on substrates (Au\u003csup\u003e31\u003c/sup\u003e or Cu\u003csup\u003e32\u003c/sup\u003e), or additionally oxidizing the LM surface\u003csup\u003e33,34\u003c/sup\u003e, aiming to enhance the adhesion between LMs and substrates. Nevertheless, such approaches may inadvertently deteriorate the electrical conductivity and fluidity of LM, as well as involving extra complex and labor-intensive processes, which would pose great difficulties in fabricating highly integrated flexible electronics.\u003c/p\u003e \u003cp\u003eThe indirect strategies of LM patterning are to fabricate soft substrates with encapsulated channels, followed by filling with LMs to constitute the desired circuits. Such indirect strategy is relatively simple, and more importantly, does not require any additional modification of the LMs, which can preserve the excellent conductive and fluidic properties of LMs. Manual injection is prevalently used to fill LMs into the channels of electronic devices as circuits\u003csup\u003e35,36\u003c/sup\u003e, which, however, poses challenges when meeting fine channels at microscale and below. The required injection pressure for filling channels is inversely proportional to channel dimension, and would become even tremendous if the injected objects are LMs with huge surface tension. It has been reported that filling microchannel with a diameter of ~\u0026thinsp;40 \u0026micro;m needs a pressure of 89 kPa\u003csup\u003e37\u003c/sup\u003e, which increases 112-fold when channel dimension is reduced to hundreds of nanometers\u003csup\u003e38\u003c/sup\u003e. Such huge pressures can easily damage the structure of soft substrates and cause LMs leakage, thus are not applicable to flexible electronics. Although vacuum aspiration could assist to drive LMs into the microchannel as fine as 10 \u0026micro;m, it is actually a time-consuming procedure that takes approximately half an hour to fill a\u0026thinsp;~\u0026thinsp;500 mm long channel\u003csup\u003e39,40\u003c/sup\u003e. In addition, current indirect methods are often accompanied by the presence of localized non-wetting and incomplete filling phenomena, leading to the generation of bubbles within channels and even interconnection failure of LM circuits. Therefore, it is urgently desired to develop a facile and rapid LMs filling strategy to efficiently fabricate miniaturized and fine flexible circuits.\u003c/p\u003e \u003cp\u003eIn this work, we reported an ultrasound powered strategy that could effectively power eutectic gallium indium (EGaIn) LM to actively wet and fill into ultrafine microchannels within seconds. It was found that ultrasound stimulation could generate an asymmetrical acoustic pressure field inside the LM, which could drive the spreading of the LM on soft substrates. Meanwhile, the oxidation film of LM naturally formed in the atmosphere could be simultaneously broken by the ultrasonic wave\u003csup\u003e41\u0026ndash;43\u003c/sup\u003e, assisting the wetting and adhering of LM on non-wettable surfaces. The ultrasonic strategy could prevent the generation of defects such as cavities and exhibited excellent electromechanical performance. Therefore, it showcased clear advantages over conventional injection methods for LM flexible electronics. We successfully fabricated a compact pressure sensor array with a line width of 25 \u0026micro;m by ultrasound filling strategy, which could record both pressure and spatial information. It could even effectively fill a channel embedded with microscale structures, in which way the sensitivity of corresponding strain sensor was significantly enhanced. More importantly, complete filling of integrated circuits and complex patterns with multiple interlaced branches, blind holes and vertical structures were demonstrated. These results highlight the versatility and adaptability of the ultrasound powered strategy for the fabrication of LM circuits, offering promising prospects for the development of compact and integrated flexible devices.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n\u003ch2\u003eUltrasound powered wetting and filling of LM in microchannel\u003c/h2\u003e\n\u003cp\u003eTo demonstrate the LM wetting capability driven by ultrasound, flexible devices consisting of a LM chamber and microchannels with various diameters were fabricated at first. The detailed fabrication process is depicted in Fig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e, including 3D printing of the predesigned mold, reversing the mold with polydimethylsiloxane (PDMS) soft silicone, and finally bonding PDMS device to an uncured silicone film that was spin-coated on a rigid polymethyl methacrylate (PMMA) substrate. Afterwards, EGaIn was manually injected into the LM chamber, and the obtained device was fixed onto a titanium alloy clamp as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea. Then an ultrasonic probe was placed on the upper surface of the clamp to generate high-frequency mechanical vibrations in the vertical direction. The ultrasonic wave was transmitted through the clamp to the fixed flexible device, and forced the wetting of LM from the chamber into microchannels. Finally, the flexible device was detached from the PMMA substrate for further mechanical and electronic testing.\u003c/p\u003e\n\u003cp\u003eThe LM wetting and filling results powered by ultrasound in the microchannels with a fixed length of 8 mm and various widths (ranging from 100 to 0.75 \u0026micro;m) were further presented. The smaller value of channel width and height was defined as the minimum characteristic dimension (CD\u003csub\u003em\u003c/sub\u003e), which characterized the fineness of microchannel. Through dividing the channel length (L) by minimum characteristic dimension, we could obtain the aspect ratio of channel (L/CD\u003csub\u003em\u003c/sub\u003e). For example, Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb and Movie S1 showed the representative dynamic wetting process of LM in a microchannel with a CD\u003csub\u003em\u003c/sub\u003e of 100 \u0026micro;m. When applying an ultrasonic signal with a power of 800 W and a frequency of 20 kHz, LM immediately started to enter the empty microchannel, and a complete filling of the 8 mm long microchannel could be achieved within ~\u0026thinsp;0.16 s (aspect ratio L/CD\u003csub\u003em\u003c/sub\u003e: 8 mm/100 \u0026micro;m\u0026thinsp;=\u0026thinsp;80, filling speed in aspect ratio: 80/0.16 s\u0026thinsp;=\u0026thinsp;500 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). It was worth noting that the orientation of device did not affect the filling performance and further details were discussed and illustrated in Methods and Fig. S2. We then employed optical microscope and X-ray based microcomputed tomography (micro-CT) techniques to evaluate the LM filling outcomes in channels of different sizes, as illustrated in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ec. The optical images presented in the upper row clearly indicated the complete filling of channels by LM, while the micro-CT results displayed in the lower row revealed that no notable defects were found inside the filled LM. Multi-angle observation of the LM lines with varying CD\u003csub\u003em\u003c/sub\u003e captured by micro-CT was presented in Movie S2. Furthermore, we found that an ultrafine channel with a CD\u003csub\u003em\u003c/sub\u003e as low as 750 nm could also be well filled (Fig. S3), indicating the excellent wetting capability of ultrasound powered LM in filling ultrafine channels.\u003c/p\u003e\n\u003cp\u003eMoreover, the filling capability of the ultrasonic strategy was compared with the conventional injection method in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed. It was found that both methods were applicable for channels with CD\u003csub\u003em\u003c/sub\u003e larger than 50 \u0026micro;m. When the fineness of microchannel was 25 \u0026micro;m or smaller, the injection method was no longer applicable because it required large injection pressure (\u0026gt;\u0026thinsp;140 kPa), which would easily damage the PDMS flexible device and lead to LM leakage. On the contrary, the ultrasonic method was still effective in powering LM to fill the microchannel without causing any damage, and could even achieve a higher filling speed. Furthermore, we had compiled a comprehensive overview of recent studies on filling LM into flexible micro/nano-channels in Fig.\u0026nbsp;1e\u003csup\u003e6,7,11,40,44\u0026ndash;49\u003c/sup\u003e, which depicted both the minimum characteristic dimension of channels and filling speed achieved by different strategies. The injection method could only fill LM into channels wider than 50 \u0026micro;m. Although, the vacuum-assisted method could fill LM into microchannels as fine as 10 \u0026micro;m, it required much more time to complete the filling process. Our developed ultrasonic method outperformed the current studies by wetting and filling LM into much finer channels with higher efficiency, demonstrating its superiority in fabricating miniaturized flexible electronics.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n\u003ch2\u003eMechanism of ultrasound powered wetting and channel-filling behavior of LM\u003c/h2\u003e\n\u003cp\u003eTo investigate how ultrasound powered LM to fill microchannels, we first conducted theoretical simulation to understand the mechanism of LM wetting behavior altered by ultrasonic stimulation. In our model, a 4 \u0026micro;m diameter LM droplet was placed on a rigid substrate whose mechanical properties were set to be PMMA in material library. Then, a vertical vibration was applied to the substrate at 20 kHz to simulate ultrasound activation and further details could be found in Supplementary Note 1. Based on the simulation results in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea(i), when ultrasound was applied, it induced an asymmetrical acoustic pressure within the LM droplet. The maximum and minimum pressure points of the field were located at the center and bottom of LM droplet, respectively. Such difference in acoustic pressure would propel the flow of interior LM to move towards the substrate, thus overcoming the high surface tension of the LM. Consequently, as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea(ii), the LM droplet would rapidly spread on the surface of the substrate within a duration of 12 ms, with the initial position demarcated by a black line. The LM droplet would continue to flatten over time, as indicated by the simulated flow direction. We then conducted experiments to verify the simulation results by placing a 20 \u0026micro;L LM droplet on a PDMS-coated PMMA plate, and a droplet of 1 M HCl solution was added to remove the oxidation film of LM. With a considerable high surface tension, LM did not exhibit effective wetting behavior on most nonmetallic materials\u003csup\u003e50\u003c/sup\u003e, so the initial contact angle between LM droplet and the plate was measured to be ~\u0026thinsp;142\u0026deg;. Upon application of ultrasound, the LM droplet immediately spread on the plate surface with a remarkable decrease in contact angle to ~\u0026thinsp;67\u0026deg;, which was consistent with our simulation. When removing ultrasonic stimulation, the LM droplet reverted to its original ellipsoidal shape owing to its high surface tension. The entire process was shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb and Movie S3, demonstrating the capacity of ultrasound to regulate the wetting behavior of LM on a non-wettable substrate.\u003c/p\u003e\n\u003cp\u003eGiven the practical application scenarios of LM filled microchannels, we further investigated the wetting behavior of bare LM droplets under ultrasonic stimulation in the atmosphere. At this time, the LM droplet would be quickly oxidized to form an oxidation film on its surface after being placed on the substrate. It was found that the applied ultrasound would flatten the LM droplet on the substrate. More importantly, with the assistance of adhesive oxidation film towards the PDMS substrate\u003csup\u003e37\u003c/sup\u003e, the spreading shape of LM droplet could be fixed on the substrate after ultrasonic stimulation, and the LM droplet could not bounce back after removing the ultrasound, leading to a significant increase in the wetting area of the LM droplet. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec captured the spreading state of a 20 \u0026micro;L LM droplet on a PDMS surface after applying ultrasound for 0.1, 0.2, 0.3, 0.4, and 0.5 s, respectively. With the prolonging of ultrasonic time, the contact angle of the LM droplet decreased, meanwhile its spread area increased. We then calculated the ratio between the spread area (S) and the initial area (S\u003csub\u003e0\u003c/sub\u003e), to characterize the spreading behavior of LM droplets in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed, which quantitatively depicted the improvement of the wettability of LM on PDMS substrate under ultrasound stimulation. With further increase of ultrasonic time from 0.3 to 0.5 s, the change in spread area ratio became less obvious, due to the limited amount of LM and the formation of excess oxidation film. Therefore, through ultrasonic treatment assisted with the adhesion of LM oxidation film, we could effectively promote and maintain the wetting and spreading of LM even on a non-wettable substrate.\u003c/p\u003e\n\u003cp\u003eSubsequently, we focused on the scenario of ultrasound powered wetting and filling of LM into microchannels. Based on the finite element simulation (Fig. S4 and Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ee), when ultrasonic stimulation was applied to the LM confined within the device chamber, an asymmetric distribution of acoustic pressure would be generated within the LM. The maximum acoustic pressure point was found to locate at the LM chamber, while the minimum (zero) point was at the outlet of the microchannel. Such pressure gradient tended to drive LM to flow from the chamber into the microchannel, with the flow direction and velocity at the chamber/channel interface shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ef. Detailed modeling parameters and the conversion method from ultrasonic power to pressure could be found in Supplementary Note 2. We investigated the flow velocity of LM at the liquid/air interface in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eg, whose velocity distribution and streamline plot revealed a high filling speed of hundreds s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for the microchannel with a CD\u003csub\u003em\u003c/sub\u003e of 100 \u0026micro;m. To validate the simulation results, we conducted the ultrasound powered LM filling experiments in a flexible device with the same configurations as the simulation model. The dynamics of the filling process was captured using a high-speed camera at 20,000 frames/s (Movie S4). As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eh, the time of initially captured scene was defined as 0 \u0026micro;s, and the outline of the LM was illustrated by a blue dotted line. Once ultrasound was applied, the flow front of LM could be clearly observed to advance (red dotted line). According to the previous discussion, the oxidation film would attach to the deeper boundary of microchannel, which prevented LM from bouncing back and anchored the spreading front of LM when one pulse signal of ultrasound finished. If the next ultrasonic pulse continued to take effect, the LM would continue to fill inside microchannel. Therefore, ultrasonic stimulation constituted the key factor in powering LM to wet and fill into microchannels. We also quantified the filling speed of LM into microchannels with different CD\u003csub\u003em\u003c/sub\u003e under various ultrasonic powers as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ei. The filling process powered by ultrasound was extremely fast, whose speed generally increased with increasing the ultrasonic power and channel size. Moreover, the LM filling speed of a 100 \u0026micro;m channel under 600 W power aligned with the simulated results in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eg. However, a maximum wetting speed of 1760 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was obtained when the channel CD\u003csub\u003em\u003c/sub\u003e was 25 \u0026micro;m and the reasons could be found in Supplementary Note 3 and Fig. S5. In summary, when ultrasonic stimulation was applied, the generated acoustic pressure gradient could drive the wetting and spreading of LM inside microchannels. The LM oxidation film assisted to anchor the LM spreading front to the elastic channel wall, and then the LM continuously advanced under the acoustic pressure gradient. Therefore, the ultrasonic stimulation and the anchoring effect of LM oxidation film synergistically explain the mechanism of ultrasound powered LM filling process.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n\u003ch2\u003eElectromechanical performance of ultrasound filled LM flexible circuits\u003c/h2\u003e\n\u003cp\u003eTo validate the potential of ultrasound powered LM filling strategy for the fabrication of flexible electronic devices, we produced single-line strain sensor with a CD\u003csub\u003em\u003c/sub\u003e of 100 \u0026micro;m and a length of 12 mm (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea) using ultrasonic method. We compared the electromechanical performance with a sensor of the same structure fabricated by injection method, which was one of the well-established methods for fabricating LM flexible circuits. The variations in relative resistance (R/R\u003csub\u003e0\u003c/sub\u003e, where R was the tested resistance under corresponding strain \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\varepsilon\\)\u003c/span\u003e\u003c/span\u003e and R\u003csub\u003e0\u003c/sub\u003e was the initial resistance without strain) of the strain sensors were characterized (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb). The electrical performance of the strain sensors prepared by the ultrasonic method exhibited comparable performance to that by the injection method, demonstrating the effectiveness of the ultrasonic method in fabricating flexible electronics. Detailed electromechanical testing results of the strain sensor made by both methods were illustrated in Fig. S6.\u003c/p\u003e\n\u003cp\u003eSubsequently, we tested the performance of the strain sensor by applying various strain levels (10%-40%) and meanwhile measuring the change in relative resistance (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec). With each 10% increase in strain, the relative resistance change of the sensor increased by approximately 6%, showing a linear relationship between the relative resistance change and strain. This demonstrated that the sensor was well suitable for strain sensing applications. The gauge factor (GF) of the fabricated strain sensor, which represented the sensitivity of the strain sensor, was calculated at various strain levels as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed. The GF exhibited a linear correspondence with increasing strain, which was a normal phenomenon since the GF was defined by the slope of relative resistance curve \u003cem\u003eversus\u003c/em\u003e strain, \u003cem\u003ei.e.\u003c/em\u003e,\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\)\u003c/span\u003e\u003c/span\u003e(∆R/R\u003csub\u003e0\u003c/sub\u003e)/\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\varepsilon\\)\u003c/span\u003e\u003c/span\u003e. For bulk solid metal, GF can be derived from the following formula\u003csup\u003e51\u003c/sup\u003e:\u003c/p\u003e\n\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equ1\" class=\"mathdisplay\"\u003e$$\\frac{\\varDelta R}{{R}_{0}\\varepsilon }=(\\frac{R}{{R}_{0}}-1)/\\varepsilon = \\left[{\\left(\\frac{l}{{l}_{0}}\\right)}^{2}-1\\right]/\\varepsilon =\\left[{\\left(\\varepsilon +1\\right)}^{2}-1\\right]/\\varepsilon = \\varepsilon +2$$\u003c/div\u003e\n\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003ewhere \u003cem\u003el\u003c/em\u003e is the stretching length and \u003cem\u003el\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e is the initial length of the metal. The GF of the LM strain sensor in our work was found to be notably lower than the theoretical value of (ɛ+2). Even at 40% strain, the average GF of our sensor was only\u0026thinsp;~\u0026thinsp;0.61, which was much smaller than the theoretical value (ɛ+2\u0026thinsp;=\u0026thinsp;0.4\u0026thinsp;+\u0026thinsp;2\u0026thinsp;=\u0026thinsp;2.4). Such phenomenon could be attributed to several influencing factors, including the cross-sectional geometry, trace shape and regularity, and quantity of encapsulated LM\u003csup\u003e52\u003c/sup\u003e, which was broadly found in previous works\u003csup\u003e53\u0026ndash;55\u003c/sup\u003e. Besides, to evaluate the reliability of the flexible electronics manufactured by our ultrasonic method, we further encapsulated LM strain sensor in a rectangular PDMS substrate and assessed its cyclic stability (Fig. S7). The strain sensor was subjected to 10,000 cycles of 40% strain in both horizontal (x) and vertical (y) directions, as depicted in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ee and \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ef, respectively (the stretching direction was illustrated in the inset). Remarkably, even after 10,000 cycles, the LM sensor exhibited consistent electrical response performance. The relative resistance variation of the sensor was only approximately 8% in the x direction and less than 3% in the y direction. These findings indicated a robust cyclic stability of the strain sensor\u003csup\u003e10,28\u003c/sup\u003e and guaranteed the reliability of the ultrasound powered channel-filling method for fabricating flexible electronics.\u003c/p\u003e\n\u003cp\u003eMoreover, we designed and prepared a pressure sensor based on a spiral LM pattern with a CD\u003csub\u003em\u003c/sub\u003e of 25 \u0026micro;m, which could hardly be achieved by injection method. The pressure sensor had dimensions of 4.2 mm in length, 3 mm in width and 1.2 mm in height (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eg). Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eh characterized the electrical response performance of the designed pressure sensor by applying stress of 20, 40 and 80 kPa in sequence. With every increment of 20 kPa in applied stress, the sensor displayed an increase of approximately 1.1% in the change of relative resistance (R/R\u003csub\u003e0\u003c/sub\u003e), revealing a linear relationship between the relative resistance variation and applied stress. We further evaluated the cyclical stability of the pressure sensor by imposing dynamic stress with sinusoidal variation (amplitude 40 kPa) for 10,000 cycles. The results shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ei demonstrated only a small amount of relative resistance change (~\u0026thinsp;4%) after the cyclical process, which had a minimal impact on the sensor function. Besides, no circuit failure caused by LM leakage was observed during all the testing process, indicating the robustness of pressure sensor.\u003c/p\u003e\n\u003cp\u003eSubsequently, we assembled nine such pressure sensors together into a sensor array, which could still be conveniently placed on a finger (Fig. S8). The small size of the as-prepared sensor device indicated that the delicate fabrication method had great potential for further fabrication of complex flexible electronics with high integration degree. Each sensor was connected to an individual channel of a multimeter, and then mechanical loads with different shapes (\u0026ldquo;H\u0026rdquo;, \u0026ldquo;I\u0026rdquo; and \u0026ldquo;T\u0026rdquo;) were placed on the sensor array respectively. The schematic mold and results of relative resistance change of different channels were illustrated in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ej and the detailed data were presented in Fig. S9-11. The signals from each channel would reveal whether there was a load above the sensor, as well as the magnitude of the pressure loaded. When the \u0026ldquo;H\u0026rdquo; load was applied to the sensor array, channels 1, 3, 4, 5, 6, 7 and 9 recorded changes in resistance. Upon switching to load \u0026ldquo;I\u0026rdquo;, channels 2, 5 and 8 exhibited signal variations, while channels 1, 3, 4, 6, 7 and 9 returned to the baseline. Similarly, with the \u0026ldquo;T\u0026rdquo; load, corresponding channels detected resistance signal changes. Notably, for applying \u0026ldquo;H\u0026rdquo; and \u0026ldquo;I\u0026rdquo; loads, channel 5 showed the most pronounced signal variation, and for applying \u0026ldquo;T\u0026rdquo; load, channels 2 and 5 exhibited relatively significant signal changes. This observation could be attributed to the fact that the center of mass for \u0026ldquo;H\u0026rdquo; and \u0026ldquo;I\u0026rdquo; loads was above sensor 5, whereas for the \u0026ldquo;T\u0026rdquo; load, the center of mass was between sensors 2 and 5. If there was no load on the sensor, R/R\u003csub\u003e0\u003c/sub\u003e would return to baseline after a transient disturbance. In summary, we validated the reliability of ultrasonic method for fabricating flexible electronic devices and demonstrated its applicability to strain and pressure sensing.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n\u003ch2\u003eUltrasound powered LM filling into micro-structured ultrafine channels for strain sensor performance enhancement\u003c/h2\u003e\n\u003cp\u003eBased on the merits of the ultrasonic method, we then sought to further promote the performance of flexible electronic devices by filling LM into finer microchannel that usually cannot be accomplished by traditional injection methods. Following the aforementioned discussion, it was evident that the GF of LM single-line strain sensor, either in our work or in previous reports\u003csup\u003e53\u0026ndash;55\u003c/sup\u003e, fell below the theoretical value. We then designed and fabricated a strain sensor by integrating five evenly distributed prismatic microstructures inside the single-line microchannel to enhance the GF, as depicted in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea. When the designed sensor was stretched to the same strain as the normal single-line sensor, the pressure exerted by the prismatic microstructures would induce a much larger local deformation of the LM in channel, which was expected to significantly enhance the relative resistance change. In this manner, the GF of sensor would be greatly boosted.\u003c/p\u003e\n\u003cp\u003eWe first conducted simulations to verify our hypothesis. The stress distribution in PDMS substrates and displacement in the z-axis direction of the upper surface of channels were firstly analyzed. It was worth noting that all LM channels in our study, either with or without microstructures, had a CD\u003csub\u003em\u003c/sub\u003e of 100 \u0026micro;m and a length of 12 mm, while the prismatic microstructures were designed to be 75 \u0026micro;m in height and 100 \u0026micro;m in width. Thus, the size of the narrowest regions in the sensor located between the tips of microstructures and the channel boundary were only 25 \u0026micro;m. Compared with the normal single-line sensor, simulation results in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb indicated that the sensor with microstructure design possessed a significant increase in maximum stress at 40% strain (one order of magnitude). Examination of the z-axis displacement of the channel with microstructure further indicated that the prismatic microstructure had locally compressed the filled LM during the stretching process. Additionally, we extracted the stress distribution along channel for both types of sensors, as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec. When subjected to 40% strain, the maximum stress in normal single-line sensor was found to exist at the two endpoints with a value of ~\u0026thinsp;7\u0026times;10\u003csup\u003e6\u003c/sup\u003e N/m\u003csup\u003e2\u003c/sup\u003e. In contrast, the introduction of microstructures led to the concentration of stress, with the maximum value at the tip of microstructure being ~\u0026thinsp;15\u0026times;10\u003csup\u003e6\u003c/sup\u003e N/m\u003csup\u003e2\u003c/sup\u003e. Therefore, the simulation results were consistent with our hypothesis and suggested that the introduction of prismatic microstructures could theoretically enhance the GF of strain sensor.\u003c/p\u003e\n\u003cp\u003eSubsequently, we fabricated the micro-structured channel, and a local zoom-in image captured by scanning electron microscope (SEM) was shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed. Due to the narrow gap of ~\u0026thinsp;25 \u0026micro;m between microstructure tips and channel boundary, the injection method could not drive LM across the microstructures inside channel, leading to the failure in fabricating flexible strain sensor. In contrast, ultrasound proved capable of powering LM to wet and pass through such narrow regions, and achieved rapid and complete filling of LM within 1 s (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ee). Then, the LM-filled channel was further processed to serve as a finger-wearable strain sensor and its electromechanical performance was scrutinized. As shown in Fig. S12, the designed sensor exhibited a more pronounced change rate in resistance under various strains compared to the normal single-line strain sensor, which was consistent with our anticipated results. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ef compared the GF of sensors with and without microstructures at various strains from 10\u0026ndash;40%, indicating an obvious enhancement of sensor sensitivity enabled by the microstructure design. It was found that the average GF at 40% strain increased from 0.61 to 13.54 after integrating prismatic microstructures, representing a remarkable 22.2-fold improvement. We further evaluated the performance of finger-wearable strain sensor at different bending angles (30\u0026deg;, 60\u0026deg;, and 90\u0026deg;) in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eg. Similarly, the relative resistance change of the sensor with microstructures exhibited a significant increasement compared to the normal single-line sensor, particularly at 90\u0026deg; bending angle (~\u0026thinsp;7.7 fold). This was due to the fact that a larger bending angle would result in greater squeezing of the LM by microstructures, leading to a more substantial change in resistance. Finally, we summarized the GF of various LM-based strain sensors at different strains from recent works in Fig.\u0026nbsp;4h\u003csup\u003e28,46,53\u0026ndash;58\u003c/sup\u003e, indicating the majority of average GF values fell below the theoretical expectation. In contrast, we succeeded to design and fabricate an LM-based strain sensor by incorporating prismatic microstructures, which showed a much higher GF compared to previous works, as well as the theoretical value. Therefore, the developed ultrasonic method that powered filling of LM into finer microchannels could boost the electromechanical performance of flexible electronics.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n\u003ch2\u003eUltrasound powered LM filling into complex and multi-branched channels with diverse circuit architectures\u003c/h2\u003e\n\u003cp\u003eAnother unique advantage of the ultrasonic method was its applicability to complex and/or multiple channels with diverse circuit structures. For example, Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea presented the ultrasound powered filling process of a tri-channel LED lamp array. Copper wires were connected to the LM droplets (serving as electrodes here) in the chambers, followed by the application of a 3 V voltage from a constant voltage source. After ultrasonic treatment, all six straight channels were immediately and completely filled with LM within 1 s, and the three LED lamps lit up almost simultaneously. The entire process was recorded by a digital microscope and could be found in Movie S5. To illustrate the advantages of the ultrasonic method, we also used the injection method to fill the multichannel devices with LM. However, conventional injection could only drive LM into the middle channel (Fig. S13), because the middle pathway required the lowest injection pressure according to the Hagen-Poiseuille Eq.\u0026nbsp;5\u003csup\u003e9\u003c/sup\u003e. Besides, we designed an integrated circuit pattern that was characterized by a microcontroller with 32 multichannels. The schematic diagram and experimental results of the filling process were presented in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb and Movie S6. Remarkably, within only 0.05 s, all channels connected to microcontroller pins were thoroughly filled with LM \u003cem\u003evia\u003c/em\u003e ultrasonic stimulation, thus establishing complete electrical connections.\u003c/p\u003e\n\u003cp\u003eFurthermore, the filling of LM into multi-branched and interlaced channels was performed by ultrasonic treatment. The fabrication of such LM circuits held nonnegligible value for various electronic applications (e.g., electromagnetic shielding\u003csup\u003e60,61\u003c/sup\u003e), which, however, was easily accompanied by the generation of localized defects and bubbles using injection or vacuum methods. For example, we designed a spiderman pattern with multiple branches and interlaced architectures. Under ultrasonic stimulation, all the tiny channel branches of spiderman pattern were all filled with LM in 0.9 s (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec and Movie S7), highlighting the rapid and efficient features of our ultrasonic method. In sharp contrast, manual injection proved to be incapable of filling the multiple interlaced channels, posing a risk of circuit malfunction when fabricating flexible electronic devices. Additionally, Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ed and Movie S8 demonstrated that LM could be uniformly driven into a snowflake pattern, and all the planar structure and blind-hole corners were successfully filled, indicating our ultrasonic method could be applied even to LM circuits without outlets. This result illustrates that the ultrasonic strategy could be applied for circuit interconnection in flexible devices, especially when hermetic sealing or electronic packaging is required in advance. Besides, it was found that ultrasound could even overcome the gravity of LM and power LM into a comb-like channel with two vertical channels (Fig. S14), indicating the potential in three-dimensional multilayer flexible circuits. Therefore, in comparison to conventional injection methods, ultrasonic method demonstrated exceptional capabilities in wetting and filling of LM into various channels for the fabrication of flexible devices with diverse complex configurations, which promised substantial advancements in the preparation of compact and integrated electronics.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this work, we presented an ultrasound powered LM channel-filling strategy for fabricating LM circuits in flexible electronics. Through finite element simulation and experimental validation, we demonstrated that ultrasound could alternate the wetting behavior of LM on non-wetting substrates by inducing an asymmetric acoustic pressure field inside LM. Combined with the adhesive LM oxidation film, LM could be powered by ultrasound to efficiently fill ultrafine microchannels, which could not be achieved by conventional injection methods. Besides, we validated the feasibility, reliability and stability of the flexible electronics produced by the ultrasonic strategy. A sensor array of 9 miniaturized pressure sub-sensors was successfully fabricated, which was capable of pressure and spatial location sensing. Besides, ultrasound stimulation was used to power LM filling into a straight channel containing five prismatic microstructures to yield a highly sensitive LM strain sensor, whose GF increased by more than 20 times compared to the single-line strain sensor. Furthermore, the developed ultrasonic strategy proved to be highly efficient and adaptable to diverse microchannels with intricate features, encompassing complex multiple interlaced structures, planar architectures, blind holes and vertical channels, all of which could hardly be achieved by conventional methods. These fabrication achievements, facilitated by ultrasonic stimulation, demonstrated exceptional versatility in the production of LM-based flexible circuits. This work holds great promise for the development of even more sophisticated and integrated flexible devices with high efficiency. Future endeavors will be contributed to the construction of three-dimensional flexible LM circuits incorporated with electronic components to build fully functional electronic devices, thereby exploring the potential of the ultrasound powered LM channel-filling strategy in fabricating flexible electronics with high degree of integration.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n\u003ch2\u003eMaterials\u003c/h2\u003e\n\u003cp\u003eEGaIn alloys were purchased from Dongguan Metal Technology. The flexible samples made of PDMS (Sylgard 184, Dow Corning Corporation) were prepared by mixing the base and the curing agent in a ratio of 10:1 by weight. The ultrasonic clamp, crafted by Dongguan Metal Technology, used TC4 material (80 mm \u0026times; 30 mm \u0026times; 4 mm).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n\u003ch2\u003ePreparation of diverse microchannels and flexible circuits\u003c/h2\u003e\n\u003cp\u003eThe preparation process of microchannels and other patterns was detailed in Fig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e, involving five main steps: 3D printing of the mold, demolding of the PDMS sample, bonding, ultrasound powered filling of LM into microchannels, and peeling of the sample from the PMMA substrate. A BMF Nano Arch S130 was employed for projection microstereolithography to produce resin patterning molds with a minimum resolution of 2 \u0026micro;m and a thickness of 5 \u0026micro;m. Initially, the PDMS mixture was poured into the resin mold and subjected to vacuum drying for 30 minutes at room temperature to eliminate any bubbles. The sample was then cured at 60℃ for 2 hours, resulting in a cured PDMS sample demolded from the mold. Subsequently, a PMMA substrate (30 mm \u0026times; 30 mm \u0026times; 2.4 mm, Oudifu Company) was spin-coated with mixed PDMS solution at 750 rpm for 1 minute and cured at 60℃ for 30 minutes to obtain half-cured PDMS film. Then, the PDMS sample was bonded by placing it on half-cured PDMS on PMMA substrate, and the substrate was heated in a dryer at 60℃ for 2 hours to obtain a PDMS sample with diverse empty microchannels. EGaIn was then injected into the LM chamber. Following by ultrasonic treatment, the LM was completely filled into microchannels. Finally, the PDMS sample was peeled off the PMMA substrate to obtain the flexible circuits.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n\u003ch2\u003eUltrasonic system\u003c/h2\u003e\n\u003cp\u003eThe ultrasonic equipment used in this study (PTS-2000 series, Shenzhen Longxinda Ultrasonic Technology Company) had a maximum power of 2000W, and its ultrasonic probe had a diameter of 20 mm. The process of ultrasound powered LM spreading or filling into channels was recorded using a digital microscope (Bairuide Optical Instruments Company, up to 30 fps) and a high-speed camera (Phantom V12.1, up to 680 kfps). LM was injected into the LM chamber of the PDMS sample before the PMMA substrate was screwed to the TC4 clamp. To optimize ultrasound conduction, an air compressor was used to apply 3 MPa pressure to the clamp \u003cem\u003evia\u003c/em\u003e the ultrasonic probe. Upon ultrasound application, LM was powered to fill into microchannels\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n\u003ch2\u003eCharacterization\u003c/h2\u003e\n\u003cp\u003eAfter exposing the LM to ultrasound for different durations, the contact angle of LM droplets was measured with a contact angle meter (JC2000D3B, Shanghai Zhongchen Digital Technology Equipment Co., Ltd.). The filling outcomes were detected using micro-computed tomography (Micro-CT, ZEISS, Xradia 515 Versa), and the microstructure of channel was examined using a Phenom Pro scanning electron microscope (SEM). The filling result of the 0.75 \u0026micro;m channel filled with LM was taken by confocal laser scanning microscope (CLSM, NiKon A1).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\n\u003ch2\u003eAnalysis of the impact of device orientation on ultrasonic strategy results\u003c/h2\u003e\n\u003cp\u003eBoth longitudinal and transverse waves can propagate in solids, and we simulated the displacement of the clamp surface in X, Y and Z directions, respectively (Fig. S2a). The displacement in X and Y directions was found to be negligible compared to that in Z direction. The simulation results indicated that the displacement of the clamp surface in the Z direction within the PMMA substrate area exhibited a nearly uniform distribution and served as the primary acoustic source for the LM in chamber. Furthermore, the experimental results corroborated that the orientation of device did not impact the filling process (Fig. S2b).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\n\u003ch2\u003ePreparation of flexible electronics\u003c/h2\u003e\n\u003cp\u003eTo establish electrical contact with the LM within microchannels, LM chambers were severed, and connections were achieved using copper wires. Then, the samples were secondarily encapsulated in various molds, fabricated using a 3D printer (HORI 3D Printer Z300Plus), with mixed PDMS. Finally, the samples underwent vacuum curing at 60\u0026deg;C for 2 hours to fabricate flexible electronics for diverse applications.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\n\u003ch2\u003ePerformance tests\u003c/h2\u003e\n\u003cp\u003eWe evaluated the electromechanical response of single-line strain sensors by conducting mechanical tests using a Shimadzu mechanical testing machine and a Keithley 2010 resistance meter. The resistance performance of the pressure sensor, pressure sensor array, and finger-wearable strain sensor was analyzed using a Keithley DAQ6510 multichannel universal meter.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\n\u003ch2\u003eFabrication and evaluation of pressure sensor array\u003c/h2\u003e\n\u003cp\u003eNine pressure sensors were fabricated by ultrasonic method and LM chambers were severed to connect copper wires. To enhance the stability of the connection between the solid wires and the flexible LM, we performed secondary encapsulation of joints with mixed PDMS and placed the samples in a vacuum dryer at 60\u0026deg;C for 2 h. The PDMS samples were then peeled from PMMA substrates to obtain nine divided pressure sensors. These sensors were then placed on a new PMMA substrate coated with half-cured PDMS and PMMA substrate was set in a vacuum dryer at 60\u0026deg;C for 2 h to obtain the pressure sensor array. Compression loads with three letter shapes (H, I, T) were designed and printed by a 3D printer to distribute the 500 g load more evenly on specific pressure sensors. The resistance change of different sensors was monitored using a Keithley DAQ6510 multichannel universal meter.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\n\u003ch2\u003eFabrication and evaluation of finger-wearable strain sensor with microstructures\u003c/h2\u003e\n\u003cp\u003eThe channel with microstructures was filled with LM using ultrasonic method and connected with copper wires. Subsequently, we utilized a 3D printer to create a hollow ring mold tailored to fit the researcher's index finger. Such mold was employed for the secondary encapsulation of the channel. Following a 2-hour curing process at 60\u0026deg;C in a vacuum dryer, we obtained a finger-wearable strain sensor by delicately demolding the sample from the mold. The sensor was positioned at the second joint of the index finger, and its resistance was recorded using a Keithley DAQ6510 multichannel universal meter with a 4-point probe (4PP) method to minimize parasitic resistance. To quantify the bending angles of the finger, we designed folding plates with 30\u0026deg;, 60\u0026deg;, and 90\u0026deg; angles, printed by a 3D printer.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eRecording the process of ultrasound powered filling of LM into multiple microchannels to lighten the LED array\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eUtilizing the BMF Nano Arch P150 with a resolution of 25 \u0026micro;m, we 3D-printed a resin mold featuring three channels connected to the LM chamber. Following the demolding of PDMS samples from the mold, we positioned two samples and three LEDs on a PMMA substrate coated with half-cured PDMS. Subsequently, we subjected the assembly to vacuum curing at 60\u0026deg;C for 2 hours and clamped it to the TC4 clamp. The LM in both LM chambers was connected by copper wires and supplied with 3 V \u003cem\u003evia\u003c/em\u003e a constant voltage source. Throughout the ultrasound application, we employed a digital microscope to record the entire process of ultrasound powered filling of LM into multiple microchannels to lighten the LED array. The fabrication and filling of other complex patterns were carried out using the same method.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Declarations","content":"\u003ch2\u003eData Availability\u003c/h2\u003e\n\u003cp\u003eThe authors declare that the data supporting the findings of this study are available within the paper and its supplementary information files.\u003c/p\u003e\n\u003ch2\u003eAcknowledgments\u003c/h2\u003e\n\u003cp\u003eThe work is financially supported by National Natural Science Foundation of China with a No. of 92163109 (X. M), National Key R\u0026amp;D Program of China with a No. of 2023YFE0208700 (X. M), Shenzhen Science and Technology Program with Nos. of JCYJ20200109113408066, and KQTD20170809110344233 (X. M), and the Fundamental Research Funds for the Central Universities with a Grant No. of HIT.OCEF.2021032 (X. M). Z. Xu thanks the support from National Natural Science Foundation of China with a No. of 51574099 (Z. X) and 51435004 (Z. X). D. Jin thanks the support from National Natural Science Foundation of China with a No. of 52202348 (D. J), Guangdong Basic and Applied Basic Research Foundation with a No. of 2023A1515011491 (D. J), and Shenzhen Science and Technology Program with a No. of GXWD20220818224716001 (D. J).\u003c/p\u003e\n\u003ch2\u003eAuthor Contributions\u003c/h2\u003e\n\u003cp\u003eZ.R. X, D. J, Z.W. X, X. M conceived the project, and D. J, Z.W. X and X. M supervised the studies. Z.R. X designed channel patterns and employed ultrasound powered filling of LM into microchannels. S. L helped to encapsulate flexible circuits. G. L and Z. G designed the prismatic microstructures in strain sensor. Z.R. X simulated physical processes and analyzed experimental data. Z. G, Z. L, C. X, S. L and G. L provided suggestions for the work. Z.R. X, D. J, Z.W. X and X. M co-wrote the manuscript. All authors have given approval to the final version of the manuscript.\u003c/p\u003e\n\u003ch2\u003eCompeting Interests\u003c/h2\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eMarkvicka, E. J., Bartlett, M. D., Huang, X. \u0026amp; Majidi, C. An autonomously electrically self-healing liquid metal-elastomer composite for robust soft-matter robotics and electronics. \u003cem\u003eNat. Mater.\u003c/em\u003e \u003cstrong\u003e17\u003c/strong\u003e, 618-624 (2018).\u003c/li\u003e\n\u003cli\u003eJiang, M.\u003cem\u003e et al.\u003c/em\u003e Liquid metal enabled plant injectable electronics. \u003cem\u003eMater. Today\u003c/em\u003e \u003cstrong\u003e66\u003c/strong\u003e, 50-61 (2023).\u003c/li\u003e\n\u003cli\u003eIdrus-Saidi, S. 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Adv.\u003c/em\u003e \u003cstrong\u003e7\u003c/strong\u003e, eabe3767 (2021).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":false,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Liquid metal, Wetting behavior, Ultrasound, Microchannel, Flexible circuits","lastPublishedDoi":"10.21203/rs.3.rs-3909065/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3909065/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eLiquid metals (LMs) that possess both metallic and fluidic properties at room temperature promise numerous benefits in flexible electronics. However, preparing fine LM circuits with complex architectures at the microscale and below poses great challenges due to large surface tension of LMs, hindering the development of flexible electronic devices with increased integration and/or complexity. Herein, we develop a facile strategy that employs ultrasonic stimulation to power the wetting and filling of LMs into non-wettable elastic microchannels for ultrafine LMs circuits fabrication. Taking advantage of ultrasound-induced asymmetrical acoustic pressure within LMs, rapid and complete filling of LMs into microchannels as fine as ~\u0026thinsp;750 nm could be achieved within several seconds without destroying the soft substrate. Electromechanical investigations suggested that the LM-filled microchannels by ultrasound possessed excellent cyclic stability and robustness. Through ultrasonic filling and patterning ultrafine LM circuits, we created a miniaturized pressure sensor array capable of accurately sensing load pressure and spatial information. The sensitivity of the finger-wearable strain sensor was further enhanced by 22.6 times \u003cem\u003evia\u003c/em\u003e introducing local microstructures and filling LM inside the microchannel. Finally, we demonstrated the efficient preparation of complex LM circuit patterns powered by ultrasound, \u003cem\u003ee.g.\u003c/em\u003e, integrated circuits, spiderman, and snowflake patterns with multiple interlaced branches, planar structures, blind holes as well as vertical architectures, all of which could hardly be achieved by conventional injection methods. This work presented an ultrasound-based fabrication method for versatile LMs circuits, providing a platform technology for the advancement of next-generation compact and integrated flexible electronics.\u003c/p\u003e","manuscriptTitle":"Ultrasound Powered Wetting and Filling of Liquid Metal into Ultrafine Channels for Flexible Electronics","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-03-06 15:23:51","doi":"10.21203/rs.3.rs-3909065/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-electronics","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"natelectron","sideBox":"Learn more about [Nature Electronics](http://www.nature.com/natelectron/)","snPcode":"","submissionUrl":"","title":"Nature Electronics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Research","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"87b75e83-acbe-43a7-8e9c-97abc073488f","owner":[],"postedDate":"March 6th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":28454743,"name":"Physical sciences/Engineering/Electrical and electronic engineering"},{"id":28454744,"name":"Physical sciences/Materials science/Nanoscale materials/Electronic properties and materials"},{"id":28454745,"name":"Physical sciences/Materials science/Soft materials/Wetting"}],"tags":[],"updatedAt":"2026-03-16T14:52:19+00:00","versionOfRecord":[],"versionCreatedAt":"2024-03-06 15:23:51","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-3909065","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3909065","identity":"rs-3909065","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}