A simple route of providing a soft interface for PEDOT:PSS film metallic electrodes without loss of their electrical interface parameters

A simple route of providing a soft interface for PEDOT:PSS film metallic electrodes without loss of their electrical interface parameters | 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 A simple route of providing a soft interface for PEDOT:PSS film metallic electrodes without loss of their electrical interface parameters Karolina Cysewska, Sylwia Pawłowska This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4382855/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 24 Sep, 2024 Read the published version in Electrochimica Acta → Version 1 posted You are reading this latest preprint version Abstract The work presents the procedure of developing a soft interface at PEDOT:PSS film without changing its electrical interface parameters. In the first step, PEDOT:PSS is electrodeposited on the commercial platinum electrode under the state-of-the-art conditions desirable for different electrochemical electrodes. Secondly, a pure hydrogel layer is deposited on the top of the electrodeposited PEDOT:PSS film under conditions, that provide desirable mechanical properties (Young’s modulus ~ 10–20 kPa) and high permeability to ions from the solution. As a result, a PEDOT:PSS electrode with a soft interface desirable for different electrode applications is fabricated. The electrode exhibits electrical parameters at the same level as the state-of-the-art PEDOT:PSS film applied already for electrode applications. Moreover, the hydrogel layer supports additionally the electrochemical stability of the polymeric film by inhibiting its oxidative degradation. The work shows that the specific choice of the hydrogel type and fabrication conditions allows to synthesis of the hydrogel interface on a stiff polymeric film, which does not block the ionic and electrical transfer. Moreover, the fabricated PEDOT:PSS electrode with hydrogel interface reveals interfacial impedance and potential window comparable or even better to the already published studies on PEDOT:PSS hydrogels. Physical sciences/Chemistry/Electrochemistry Physical sciences/Materials science/Materials for devices Physical sciences/Materials science/Soft materials conducting polymers electrical interface hydrogel PEDOT:PSS soft interface thin films Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction Recently, intensive research studies have been performed to fabricate conductive hydrogel-based electrodes for different electronic applications 1–5 . Especially, in the case of medical applications, where a soft electrode interface is desirable to avoid mechanical mismatch and consequent adverse immune responses at implant/cells interface 6,7 . This is especially observed in the case of electrodes for neural stimulation 8 . Most electrodes available on the market today are based on pure metals or their composites such as platinum (Pt), gold (Au), or iridium (Ir). Because of their inertness, limited reactivity, and relatively high corrosion resistance, they have become the materials of the first choice 9 . However, the metallic electrodes are characterized by unfeatured surface morphology and are around four orders of magnitude stiffer than neural tissue (Young modulus: 140 GPa) 10 . Interfacial impedance (Z) for typical bare metallic-based neural electrodes is determined to be around 2 MΩ (1 cm 2 ), while charge storage capacity (CSC) is around 0.05–0.3 mC·cm − 2 . Because of this, the electrodes integrate poorly with the tissue environment, causing high inflammation, which leads to degeneration of signal transmission and the effective lifetime of the electrode becomes limited. Since the overall chemical and electrochemical electrode properties depend strongly on the type of material present at the electrode/neural interface, coating the electrode surface with some nanostructured material has become an issue of recent studies 11 . The significant advantage of the presence of the coating at the tissue/electrode interface is its possibility to tailor the properties of electrode sites in a local manner. The most promising coating material for neural tissue/electrode interface has become poly(3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrene sulfonate anions (PSS) (PEDOT:PSS). PEDOT:PSS was reported to significantly reduce electrochemical impedance and electrical noise and improve neural adhesion compared to bare metallic electrodes 12,13 . Moreover, it was proved that PEDOT:PSS with specific topography can additionally promote neuronal development in terms of neurite outgrowth 14 . The mean value of Young’s modulus for state-of-the-art cast film of PEDOT:PSS was determined to be around 1.8 +/- 0.2 GPa, much lower compared to bare metallic electrodes 15 . Yet, it is still too high to provide an ideal mechanical match at the interface of the electrode and brain tissue, which is characterized by Young’s modulus in the range of 10–100 kPa. Thus, the platinum commercial electrode modified with PEDOT:PSS exhibited desirable electrical interface parameters, however, due to its still too high rigidity, it still reveals poor adhesion of the cell, chronic damage to the surrounding tissue, and limited electronic and chemical stability for the long term. The most desirable solution will be to provide an electrode with PEDOT:PSS coating in the form of hydrogel to both keep the electrical interface parameters of the PEDOT polymer and provide a soft interface for surrounding cells 16 . However, the huge challenge is to fabricate conductive-based hydrogel. In the literature, there are currently intensive studies to develop conducting polymer-based hydrogel suitable for bioelectronics. Kleber et al. 17 developed a conducting polymer hydrogel system consisting of synthetic hydrogel P(DMAA-co-5%MABP-co-2,5%SSNa) and conducting polymer (CP) PEDOT. CP was incorporated into the hydrogel by electrodeposition. However, the CSC of PEDOT-hydrogel was lower compared to the state-of-the-art PEDOT:PSS film. In another work, the hydrogel form of PEDOT was formulated from commercially available PEDOT:PSS by gelation with sulfuric acid and combined with reduced graphene oxide (rGO) to ensure electrochemical activity 18 . In this case CSC of PEDOT:rGO was significantly lower compared to the polymer film alone. A different possibility was to cross-link PEDOT hydrogel with an ionic liquid gelation agent 19 . Here, the electrical parameters were not compared to PEDOT film alone. Another electrode was based on the PEDOT:PSS particles dissolved in poly(vinyl alcohol) (PVA). The Young’s modulus of the electrode was determined to be 460 kPa 6 . In this work, PEDOT:PSS film with soft hydrogel interface characterizing with electrical interface parameters as state-of-the-art electrodeposited PEDOT:PSS electrode was fabricated. The electrode was prepared in two simple steps ( 1 ) electrodeposition of PEDOT:PSS under state-of-the-art conditions on the surface of commercial platinum electrode, ( 2 ) fabrication and drop casting of pure hydrogel layer on the top of the polymeric film electrode. The electrical interface parameters such as interfacial impedance, potential window, charge storage capacity, double-layer capacitance, and current injection limit of the electrodes were evaluated and discussed. 2. Results The electrode was prepared in two simple steps (Fig. 1 ) i.e. ( 1 ) PEDOT:PSS was electrodeposited on a platinum commercial electrode from an aqueous solution of 10 mM EDOT and 2.5 mg·ml − 1 NaPSS at 1 V vs. Ag/AgCl sat with deposition time limited to charge of 1 mC (Fig. 2 a); The selected conditions were the state-of-the-art parameters for PEDOT:PSS for the application of different electrode applications 20–22 ; ( 2 ) Second step included synthesis and deposition of hydrogel layer on the top of the PEDOT:PSS. The hydrogel precursor solution was a composition of 97.2 mg N-isopropylacrylamide, used as the main monomer (NIPAAm), 2.8 mg N,N′-methylenebisacrylamide (BIS-AAm) as a cross-linker in the proportion 35:1, dissolved in deionized water (90% wt.). The selected monomer and cross-linking agent lead to the formation of a biocompatible hydrogel, which has recently been used in many applications, including bioelectrodes 23,24 . In the end, 5 mg of photoinitiator 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone was added to the solution to trigger the hydrogel polymerization reaction upon UV irradiation. Ready hydrogel precursor solution in a volume of 2.5 ul was drop cast on the top of the PEDOT:PSS film and cross-linked by UV irradiation for 2 minutes in an ice bath. As a result, PEDOT:PSS-Hyd coating system was fabricated. The composition of the hydrogel solution was optimized to obtain a hydrogel layer with Young’s modulus of ~ 10–20 kPa 25 . The scanning electrode microscopy (SEM) with energy dispersive X-ray (EDX) mapping analysis proved the uniform distribution of the PEDOT layer at the surface of the platinum (Fig. 2 b-e). The thickness of the film was determined to be ~ 280 nm (+/- 30 nm). A highly porous hydrogel layer with a thickness of ~ 350 um was obtained on the top of the PEDOT:PSS electrode (Fig. 3 ). The electrical interface parameters such as interfacial impedance, potential window, double-layer capacitance (C dl ), charge storage capacity (CSC), and potential transient (stimulation) performance of the PEDOT:PSS, hydrogel (Hyd) and PEDOT:PSS-Hyd electrodes were determined electrochemically in an aqueous solution of phosphate-buffered saline (PBS). Electrochemical impedance spectroscopy (EIS) spectra of the electrodes are presented in Fig. 4 a. Coated the electrode with PEDOT:PSS and a combination of the polymer with hydrogel resulted in a significant drop of the impedance, especially for the frequency range between 1 kHz and 0.1 Hz. The latter was related to the lower double-layer capacitance (C dl ), and thus the much higher surface area of the PEDOT-coated electrode compared to bare or hydrogel-coated platinum (Fig. 5 ). The CV curves recorded within the narrow potential range revealed a pure capacitive behavior of the electrodes, thus allowing to determination of the C dl (Fig. 5 a). Modification of the platinum with PEDOT or PEDOT-Hyd increased the electroactive surface area of the electrode around ~ 16 times compared to the bare or hydrogel-based substrate (Fig. 5 b). This change influenced |Z|, especially in the high-frequency range. In theory, the recorded |Z| at high frequency i.e. 1000 − 100 Hz is mainly related to the solution resistance measured between working (WE) and counter electrode (CE) 26 . Any change in electrode geometry such as the increase in porosity, surface area, etc. will influence |Z| at high frequency due to the change in the distance between WE and CE 27 . Since the electrode modified with the polymer or polymer-hydrogel revealed higher C dl compared to the rest samples, it significantly reduced the |Z| at this frequency (Fig. 4 ). Further analysis of the EIS results showed that modification of the metallic electrode only with pure hydrogel resulted in a higher impedance of the electrode, especially observed for |Z| at 10 Hz (Fig. 4 b). It indicates that the hydrogel layer blocks the charge transfer resistance between the platinum electrode and the solution, thus decreasing the electroactive surface area of the electrode. Interestingly the fact that combination of PEDOT:PSS with hydrogel layer exhibited the same interfacial |Z| compared to PEDOT:PSS alone. The potential window of the electrode is another important parameter if the application is bioelectrodes. It should be specified firstly for further reliable evaluation of CSC and potential transient of the electrodes. The potential window corresponds to the safe potential range within which the electrode works efficiently avoiding the reactions of hydrogen and oxygen evolution. For this purpose, the CV of the electrodes was performed firstly within the wide potential range i.e. between − 1.2 to 1.2 V vs. Ag/AgCl sat to assess the possible undergoing reactions (Fig. 6 a). In general, similar redox behavior was obtained for all of the studied samples. In the case of bare platinum, higher intensity peaks of platinum oxide and hydrogen formation were determined at ~ 0.4 V and − 0.7 V vs. Ag/AgCl sat , respectively. Platinum-hydrogen desorption was observed at ~-0.8 V vs. Ag/AgCl sat , which is all in agreement with the literature 28 . Typical CV peak of PEDOT:PSS oxidation could be observed at around ~ 1.1 V Ag/AgCl sat for both PEDOT:PSS and PEDOT:PSS-Hyd electrodes 29 . The potential shift of the beginning of water oxidation and reduction was observed between samples with and without PEDOT:PSS layer. Therefore, based on the CV analysis, the potential window was determined to be -0.8 V to 1.1 V vs. Ag/AgCl sat for bare platinum and hydrogel-platinum electrodes, and − 0.8 V to 1 V Ag/AgCl sat for PEDOT:PSS and PEDOT:PSS-Hydrogel. Coating the bare metal with the polymer resulted in a slight reduction of the potential window, due to the appearance of the polymer oxidation peak, and as a result shift of the water oxidation peak to lower potentials. The potential window was independent of the presence of an additional hydrogel layer on the top of the PEDOT:PSS compared to the polymer film alone. CSC is the parameter, that revealed the ability to store the charge by the electrodes and was determined based on the CVs performed within the determined potential windows for two different scan rates (Fig. 6 b and 6 c). The evaluation of CSC at least two different rates is desirable if the application is bioelectrode such as a neural electrode, which can stimulate the neurons at different rates 30 . CSC was determined to be the lowest for bare platinum electrodes, which is in agreement with the literature 10 (Fig. 6 d). A slight increase of CSC could be observed for platinum modified with pure hydrogel (Hyd). Metallic electrode coated with PEDOT:PSS exhibited significantly higher CSC of 480 uC·cm − 2 / +/- 8 uC·cm − 2 (50 mV·s − 1 ) and 1550 uC·cm − 2 / +/-40 uC·cm − 2 (200 mV·s − 1 ) compared to Pt and Hyd electrodes. Moreover, PEDOT:PSS combined with hydrogel revealed virtually the same CSC as PEDOT:PSS alone. The higher ability to store a charge for the polymer-based electrode is due to their much higher electroactive surface area (porous structure) compared to the substrate or Hyd alone (Fig. 5 ). Additionally, voltage transient was evaluated for each of the studied electrodes (Fig. 7 ). The latter is especially required to assess in the case of the use of such electrodes for the application of stimulation. Figure 7 . Potential response of Hyd (a), PEDOT:PSS (b), and PEDOT:PSS-Hyd (c) to the biphasic current pulse of 0.5–12 mA. Comparison of potential response for all the studied sample to the biphasic pulse current of +/- 3 mA (d). For this purpose, a symmetric charge-balance biphasic current pulse with 1 ms pulse width was applied to the electrodes in PBS solution. The current injection limit is defined as a quantity of current, which polarizes the electrode interface to the potential corresponding to the potential window determined from CV. In this work, the current injection limit was determined to be 3 mA for Pt and Hyd (Fig. 7 a), and 12 mA for PEDOT:PSS (Fig. 7 b) and PEDOT:PSS-Hyd (Fig. 7 c). Therefore, when applying a current pulse of 3 mA to the electrodes, PEDOT:PSS and PEDOT:PSS-Hyd revealed the same and much smaller voltage rise across the interface compared to the bare or hydrogel-coated platinum (Fig. 7 d). The phenomenon was related to the much lower interfacial impedance (Fig. 4 ), higher surface area (Fig. 5 ), and CSC (Fig. 6 ) of these electrodes compared to the Pt or Hyd. The preliminary electrochemical stability of the electrodes was also evaluated. Figure 8 presents changes in |Z| (a) and CSC (b) before and after 200 cycles of CV carried out within the determined potential windows for each electrode. In each case, the stability of the electrode decreased. However, the change of the parameters before and after 200 cycles was lower for PEDOT:PSS-Hyd compared to PEDOT:PSS alone. CSC decreased by around ~ 1.5 times and ~ 1.3 times for PEDOT:PSS and PEDOT:PSS-Hyd, respectively. However, |Z| increased by around ~ 1.6 times and ~ 1 times for PEDOT:PSS and PEDOT:PSS-Hyd, respectively. The slower degradation of PEDOT:PSS-Hyd electrode can be due to the hydrogel layer, which partially limits the direct contact of the polymer with the solution, and thus it inhibits the process of conducting polymer overoxidation. 3. Discussion PEDOT:PSS electrode with hydrogel interface was fabricated via a simple method. The PEDOT-based soft electrode revealed electrical interface parameters including interfacial impedance, double layer capacitance, potential window, charge storage capacity, and injection current limit at the same level as PEDOT:PSS film. A hydrogel layer was deposited on the top of the PEDOT:PSS film under conditions, which provided specific mechanical properties and high permeability to ions from the solution. Thus, the work showed that the appropriate choice of the hydrogel type and fabrication conditions allowed to synthesis of the hydrogel interface on a rigid polymeric film, which did not block the ionic and electrical transfer. As a result, after immersion of PEDOT:PSS-Hyd electrode in the solution, there was immediately the transfer of the ions between the solution through the hydrogel to PEDOT film, i.e. hydrogel became saturated with the ions from the solution, due to its swelling properties 31 . The results of the work were additionally compared with the literature. The mechanical and electrical interface parameters of fabricated gere PEDOT:PSS-Hyd were compared with the most promising among already published PEDOT:PSS-based hydrogels electrodes for the application of bioelectrodes (Table 1 ). In most of the published works, the authors aimed to form a pure hydrogel of PEDOT:PSS to modify the elastic substrates such as dimethyl-polysiloxane (PDMS) 32,33 , polyethylene terephthalate (PET) 33,34 , carbon cloth 33 or dimethacrylate-functionalized perfluoropolyether (PFPE-DMA) 35 . It should be noted that the main aim of this work was different i.e. the studies were to test the hypothesis of whether it is possible to maintain the electrical interface parameters of a rigid commercial platinum electrode with state-of-the-art electrodeposited PEDOT:PSS film while ensuring a soft hydrogel interface on the outside. The synthesis of PEDOT:PSS film on a metallic substrate by electrodeposition was proved to reveal desirable electrical properties and high adhesion to the substrates, which could not be obtained by other techniques. Table 1 Comparison of mechanical and electrical interface parameters of the recently most promising PEDOT:PSS-hydrogel-based electrodes for bioelectrode application. Type of coating Fabrication Young’s modulus |Z| Potential window CSC References PEDOT:PSS film + NIPAAm hydrogel Electrodeposition of PEDOT:PSS on Pt, drop-casting of hydrogel 10–20 kPa 7.6 Ω·cm 2 (+/- 0.1) (1 kHz) 21.6 Ω·cm 2 (+/-0.6) (10 Hz) -0.8–1 V vs. Ag/AgCl sat 488 uC·cm − 2 (+/- 48) (50 mV·s − 1 ) 1420 uC·cm − 2 (+/-59) (200 mV·s − 1 ) This work PEDOT:P(SS-coNHMAA) + PEG200 Chemical polymerization / 3D printing (PDMS) 4–20 MPa 30.0 Ω (10 Hz) 28.0 Ω (1 kHz) -0.5–0.5 V vs. Ag/AgCl 18.9 mC·cm − 2 (100 mV·s − 1 ) 32 * surface area not defined PEDOT:PSS-13%DMSO Drop-casting/printing (PET) ~ 2 MPa - -0.5–0.5 V vs. Ag/AgCl (scan rate not defined) 60 mC·cm − 2 (scan rate not defined) 34 *surface area: 0.9 cm 2 (thickness not defined) PEDOT:PSS-PVA-PAA-NHS-CTS-GA 3D printing – direct ink writing (PET, PDMS, carbon cloth) ~ 1.5 MPa 20 Ω (10 Hz & 1kHz) -0.5–0.5 V vs. Ag/AgCl ~ 6 mC·cm − 2 (50 mV·s − 1 ) 33 *surface area: 0.75 cm 2 PEDOT:PSS in ionic liquid (4-(3-butyl-1-imidazolio)-1-butanesulfonic acid trifate) Spin-coating (PFPE-DMA) ~ 24–32 kPa 1 kΩ·cm 2 (10 Hz) 900 Ω·cm 2 (1 kHz) -0.6–0.8 V vs. Ag/AgCl (3M KCl) ~ 160 mC·cm − 2 35 In general, the use of fully elastic and soft electrodes is very desirable because of their biocompatibility and stability, however, such an electrode cannot penetrate inside all specific areas. Only a stiff electrode will be suitable for this purpose. The work showed that the formation of a hydrogel layer on the top of the PEDOT:PSS film did not affect the electrical interface parameters of the electrodes. Moreover, it supported additionally the stability of the film by inhibiting its oxidative degradation. The hydrogel interface of PEDOT:PSS-Hyd electrode formed in this work was characterized by a very low Young’s modulus of 10–20 kPa, which was not obtained by any of the already published works (Table 1 ). The interfacial impedance of the electrode was determined to be around 7.8 Ω·cm 2 (1 kHz) and 21.6 Ω·cm 2 (10 Hz), which is at the level of |Z| or even better revealed by already published pure hydrogel form of PEDOT:PSS. Moreover, the electrode could work in a potential range of -0.8 to 1 V vs. Ag/AgCl sat , which is much wider compared to the literature. However, because of this, the determined CSC was slightly smaller. 4. Conclusion To conclude, the work showed that it is possible to develop soft hydrogel interface PEDOT:PSS film metallic electrode and solution without changing its electrical performance. The formation of a hydrogel layer on the top of the PEDOT:PSS film did not affect the electrical interface parameters of the electrodes. Therefore, it can be easily applied for the application of already existing stiff electrodes without changing its electrical properties but providing a soft interface desirable for different applications including wearable and bioelectronics. 5. Methods Chemicals and materials : 3,4-Ethylenedioxythiophene (97 %, Sigma Aldrich), Poly(sodium 4-styrenesulfonate) (M w ~ 70 000, Sigma Aldrich), N-Isopropylacrylamide (97 %, Sigma Aldrich), N,N′-Methylenediacrylamide (≥ 98 %, Sigma Aldrich), 2-Hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (98 %, Sigma Aldrich), phosphate buffer saline pH 7.4 (PBS, Sigma Aldrich) containing 2.7 mM KCl, 140 mM NaCl and 10 mM phosphate. All solutions were made with Mili-Q water. The working electrode was platinum electrode (≥ 99.9 %, BioLogic). Before each deposition process, the electrode was polished with SiC 2000 grid abrasive paper (Struers), rinsed with ethanol, and dried in the air. Morphological and structure analysis : The morphology and structure of the electrodes were characterized by a scanning electron microscope (FEI QUANTA FEG 250) with energy dispersive X-ray (EDX). The Young modulus of the hydrogel was determined by atomic force microscopy nanoindentation and was described elsewhere 36 . The thickness of the electrodeposited PEDOT film on platinum was determined by profilometer (DektakXT Advanced, Bruker, Germany). Electrochemical studies : Experimental Electrodeposition and all the electrochemical studies were performed in a three-electrode, water-jacketed cell controlled by a potentiostat (VersaSTAT 4). The working electrode was a platinum commercial electrode (diameter 3 mm, BioLogic), platinum coated with hydrogel, PEDOT:PSS, or their combination. The reference and counter electrode were Ag/AgCl in KCl sat and Pt grid (BioLogic), respectively. The temperature of the measurements was controlled by a thermostat (Julabo F12). All the electrochemical studies were performed in PBS solution. After the deposition, a coated platinum electrode was rinsed with distilled water to remove any residuals from the synthesis solution. After the deposition of the hydrogel layer, the electrode stabilized in the PBS solution for 8 minutes before each electrochemical study. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 10 kHz to 0.1 Hz with an amplitude of 5 mV rms. The potential window (redox activity) of the electrodes was determined by performing cyclic voltammetry (CV) in the potential range of -1.2 to 1.2 V vs. Ag/AgCl sat with a scan rate of 50 mV·s -1 for 5 cycles. The double-layer capacitance (C dl ) was determined based on CV measurements, which were carried out within the potentials 0.3–0.35 V vs. Ag/AgCl sat with a scan rate of 10, 20, 40, 60, 80, and 100 mV·s -1 . C dl was determined based on the following equation: C dl = i dl ·(2ʋ) −1 = (i a − i c )·(2ʋ) −1 , where ʋ is the scan rate; i a and i c are the anodic and cathodic current densities, respectively; and i dl is the double-layer current density. Charge storage capacity (CSC) was determined as the surface area of the specific cycle of CVs performed within the determined potential window. To evaluate the potential transient, a biphasic current (I) pulse was injected across the working electrode and counter electrode while measuring the working electrode’s polarization potential vs. Ag/AgCl sat . The protocol was determined as follows: cathodic pulse (-I, 1 ms), break (I = 0 A, 20 us), and anodic pulse (+I, 1 ms). The detailed description of the measurement procedure is described elsewhere 37 . To assess the preliminary stability of the electrodes, a CV of the electrodes was performed within the selected potential window for 200 cycles in PBS solution. |Z| and CSC were determined after 200 cycles of CV. Each of the measurements was performed a minimum of three times to ensure reliable and reproducible data. Declarations Author Contribution KC: conception, design of the work, data acquisition – PEDOT:PSS synthesis, performing all electrochemical measurements, analysis and data interpretation, writing the manuscript. SP: data acquisition – fabrication and optimization of hydrogel layer. All authors reviewed the manuscript. Acknowledgement This work was supported by National Science Centre (NCN), Poland: Sonata grant based on the decision 2021/43/D/ST7/01362. Data Availability The datasets generated and/or analyzed during the current study are available in the BRIDGE OF KNOWLEDGE repository (https://mostwiedzy.pl/pl/open-research-data/soft-interface-pedot-pss-experimental-raw-data,507011053553723-0?_share=435c600211713337). References Fu, F., Wang, J., Zeng, H. & Yu, J. Functional Conductive Hydrogels for Bioelectronics. ACS Mater Lett 2 , 1287–1301 (2020). Dechiraju, H., Jia, M., Luo, L. & Rolandi, M. Ion-Conducting Hydrogels and Their Applications in Bioelectronics. Adv Sustain Syst 6 , 2100173 (2022). Sun, H. et al. Conductive and antibacterial dual-network hydrogel for soft bioelectronics. Mater Horiz 10 , 5805–5821 (2023). Chen, J. et al. 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Adv Funct Mater 2314471 (2023) doi:10.1002/ADFM.202314471. Lu, B. et al. Pure PEDOT:PSS hydrogels. Nature Communications 2019 10:1 10 , 1–10 (2019). Liu, Y. et al. Soft and elastic hydrogel-based microelectronics for localized low-voltage neuromodulation. Nature Biomedical Engineering 2019 3:1 3 , 58–68 (2019). Nakielski, P. et al. Hydrogel Nanofilaments via Core-Shell Electrospinning. PLoS One 10 , e0129816 (2015). Ganji, M., Tanaka, A., Gilja, V., Halgren, E. & Dayeh, S. A. Scaling Effects on the Electrochemical Stimulation Performance of Au, Pt, and PEDOT:PSS Electrocorticography Arrays. Adv Funct Mater 27 , 1703019 (2017). Additional Declarations No competing interests reported. Supplementary Files Graphicalabstract.tiff Cite Share Download PDF Status: Published Journal Publication published 24 Sep, 2024 Read the published version in Electrochimica Acta → 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4382855","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":302872643,"identity":"84793a52-b706-4fa9-9794-c941df506694","order_by":0,"name":"Karolina Cysewska","email":"data:image/png;base64,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","orcid":"","institution":"Gdansk University of Technology","correspondingAuthor":true,"prefix":"","firstName":"Karolina","middleName":"","lastName":"Cysewska","suffix":""},{"id":302872644,"identity":"49774541-b465-4071-abf1-bcaa37915c05","order_by":1,"name":"Sylwia Pawłowska","email":"","orcid":"","institution":"Gdansk University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Sylwia","middleName":"","lastName":"Pawłowska","suffix":""}],"badges":[],"createdAt":"2024-05-07 12:00:41","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4382855/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4382855/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1016/j.electacta.2024.145115","type":"published","date":"2024-09-25T00:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":56864250,"identity":"b9c209b6-bee6-424e-b3ca-85b13dd469d6","added_by":"auto","created_at":"2024-05-21 12:01:56","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":113220,"visible":true,"origin":"","legend":"\u003cp\u003eScheme of the PEDOT:PSS-Hyd fabrication.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4382855/v1/493840d0a4379fc5f2b8d7ee.png"},{"id":56863560,"identity":"f937983f-5848-4486-930f-3d45445f4017","added_by":"auto","created_at":"2024-05-21 11:53:56","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":853028,"visible":true,"origin":"","legend":"\u003cp\u003eChronoamperometric graph recorded during the electrodeposition of PEDOT:PSS on platinum electrode (1 mC) (a). SEM-EDX mapping analysis of deposited PEDOT:PSS (a), distribution of oxygen (c), platinum (d), and sulfur (e).\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4382855/v1/1a0ba7b94f6bf7ef69409b15.png"},{"id":56864251,"identity":"13986ce3-f425-4dc6-b4fb-0f4432426e4a","added_by":"auto","created_at":"2024-05-21 12:01:57","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":795870,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of dry-freeze hydrogel layer with the magnification of 1300x (a, b), 2000x (c), and 5000x (d).\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4382855/v1/36bc23f2c84f51d42a6a88df.png"},{"id":56864249,"identity":"e4c907bd-d8d7-4c9d-ac4f-8f1e2d66722d","added_by":"auto","created_at":"2024-05-21 12:01:56","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":265919,"visible":true,"origin":"","legend":"\u003cp\u003eEIS spectra (a) and evolution of interfacial impedance |Z| determined at specific frequencies (b) for the studied electrodes.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4382855/v1/9dc018796f1d5757f3e6694d.jpeg"},{"id":56863558,"identity":"8550abff-7ecc-433f-b5ee-f25688236434","added_by":"auto","created_at":"2024-05-21 11:53:56","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":57409,"visible":true,"origin":"","legend":"\u003cp\u003eCyclic voltammograms recorded within a narrow potential range (a), and evolution of C\u003csub\u003edl\u003c/sub\u003e determined from CVs (b) of the studied samples.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-4382855/v1/f0e718fbdb95d0afb28a55b8.png"},{"id":56863559,"identity":"6883516e-fd2a-4549-afc2-7342cbc30eab","added_by":"auto","created_at":"2024-05-21 11:53:56","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":199182,"visible":true,"origin":"","legend":"\u003cp\u003eCyclic voltammograms of the electrodes recorded within a wide potential range (a) and within the selected potential window for 50 mV·s\u003csup\u003e-1\u003c/sup\u003e (b), and 200 mV·s\u003csup\u003e-1\u003c/sup\u003e (c). Charge storage capacity (CSC) of the electrodes determined from CVs (d).\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-4382855/v1/2447fabac2c426d8e4545810.png"},{"id":56863562,"identity":"e84d22d2-3c81-4e3c-9b6b-60e97950676b","added_by":"auto","created_at":"2024-05-21 11:53:57","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":188182,"visible":true,"origin":"","legend":"\u003cp\u003ePotential response of Hyd (a), PEDOT:PSS (b), and PEDOT:PSS-Hyd (c) to the biphasic current pulse of 0.5-12 mA. Comparison of potential response for all the studied sample to the biphasic pulse current of +/- 3 mA (d).\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-4382855/v1/3fd0318631e1d29dfeb15d0c.png"},{"id":56863565,"identity":"a5ab5d2d-cd13-48d1-8db8-b1b5aa49c390","added_by":"auto","created_at":"2024-05-21 11:53:57","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":54330,"visible":true,"origin":"","legend":"\u003cp\u003e|Z|(a) and CSC (b) of the electrodes after 200 CV cycles recorded within the potential windows in PBS.\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-4382855/v1/880014e8f5d8e6cd61db0c3f.png"},{"id":65849490,"identity":"a7033a86-85f1-4277-afdf-6f912279655c","added_by":"auto","created_at":"2024-10-03 14:03:58","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3108998,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4382855/v1/ef9fb2f8-94f2-41ea-ace4-c849d9c849cc.pdf"},{"id":56863566,"identity":"fea76b23-cb02-4331-9628-5a33ef035096","added_by":"auto","created_at":"2024-05-21 11:53:57","extension":"tiff","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":41593934,"visible":true,"origin":"","legend":"","description":"","filename":"Graphicalabstract.tiff","url":"https://assets-eu.researchsquare.com/files/rs-4382855/v1/b54129c67528c227ca3fa092.tiff"}],"financialInterests":"No competing interests reported.","formattedTitle":"A simple route of providing a soft interface for PEDOT:PSS film metallic electrodes without loss of their electrical interface parameters","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eRecently, intensive research studies have been performed to fabricate conductive hydrogel-based electrodes for different electronic applications \u003csup\u003e1\u0026ndash;5\u003c/sup\u003e. Especially, in the case of medical applications, where a soft electrode interface is desirable to avoid mechanical mismatch and consequent adverse immune responses at implant/cells interface\u003csup\u003e6,7\u003c/sup\u003e. This is especially observed in the case of electrodes for neural stimulation\u003csup\u003e8\u003c/sup\u003e. Most electrodes available on the market today are based on pure metals or their composites such as platinum (Pt), gold (Au), or iridium (Ir). Because of their inertness, limited reactivity, and relatively high corrosion resistance, they have become the materials of the first choice\u003csup\u003e9\u003c/sup\u003e. However, the metallic electrodes are characterized by unfeatured surface morphology and are around four orders of magnitude stiffer than neural tissue (Young modulus: 140 GPa)\u003csup\u003e10\u003c/sup\u003e. Interfacial impedance (Z) for typical bare metallic-based neural electrodes is determined to be around 2 MΩ (1 cm\u003csup\u003e2\u003c/sup\u003e), while charge storage capacity (CSC) is around 0.05\u0026ndash;0.3 mC\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. Because of this, the electrodes integrate poorly with the tissue environment, causing high inflammation, which leads to degeneration of signal transmission and the effective lifetime of the electrode becomes limited. Since the overall chemical and electrochemical electrode properties depend strongly on the type of material present at the electrode/neural interface, coating the electrode surface with some nanostructured material has become an issue of recent studies\u003csup\u003e11\u003c/sup\u003e. The significant advantage of the presence of the coating at the tissue/electrode interface is its possibility to tailor the properties of electrode sites in a local manner. The most promising coating material for neural tissue/electrode interface has become poly(3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrene sulfonate anions (PSS) (PEDOT:PSS). PEDOT:PSS was reported to significantly reduce electrochemical impedance and electrical noise and improve neural adhesion compared to bare metallic electrodes\u003csup\u003e12,13\u003c/sup\u003e. Moreover, it was proved that PEDOT:PSS with specific topography can additionally promote neuronal development in terms of neurite outgrowth \u003csup\u003e14\u003c/sup\u003e. The mean value of Young\u0026rsquo;s modulus for state-of-the-art cast film of PEDOT:PSS was determined to be around 1.8 +/- 0.2 GPa, much lower compared to bare metallic electrodes\u003csup\u003e15\u003c/sup\u003e. Yet, it is still too high to provide an ideal mechanical match at the interface of the electrode and brain tissue, which is characterized by Young\u0026rsquo;s modulus in the range of 10\u0026ndash;100 kPa. Thus, the platinum commercial electrode modified with PEDOT:PSS exhibited desirable electrical interface parameters, however, due to its still too high rigidity, it still reveals poor adhesion of the cell, chronic damage to the surrounding tissue, and limited electronic and chemical stability for the long term.\u003c/p\u003e \u003cp\u003eThe most desirable solution will be to provide an electrode with PEDOT:PSS coating in the form of hydrogel to both keep the electrical interface parameters of the PEDOT polymer and provide a soft interface for surrounding cells\u003csup\u003e16\u003c/sup\u003e. However, the huge challenge is to fabricate conductive-based hydrogel. In the literature, there are currently intensive studies to develop conducting polymer-based hydrogel suitable for bioelectronics. Kleber et al.\u003csup\u003e17\u003c/sup\u003e developed a conducting polymer hydrogel system consisting of synthetic hydrogel P(DMAA-co-5%MABP-co-2,5%SSNa) and conducting polymer (CP) PEDOT. CP was incorporated into the hydrogel by electrodeposition. However, the CSC of PEDOT-hydrogel was lower compared to the state-of-the-art PEDOT:PSS film. In another work, the hydrogel form of PEDOT was formulated from commercially available PEDOT:PSS by gelation with sulfuric acid and combined with reduced graphene oxide (rGO) to ensure electrochemical activity \u003csup\u003e18\u003c/sup\u003e. In this case CSC of PEDOT:rGO was significantly lower compared to the polymer film alone. A different possibility was to cross-link PEDOT hydrogel with an ionic liquid gelation agent\u003csup\u003e19\u003c/sup\u003e. Here, the electrical parameters were not compared to PEDOT film alone. Another electrode was based on the PEDOT:PSS particles dissolved in poly(vinyl alcohol) (PVA). The Young\u0026rsquo;s modulus of the electrode was determined to be 460 kPa\u003csup\u003e6\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn this work, PEDOT:PSS film with soft hydrogel interface characterizing with electrical interface parameters as state-of-the-art electrodeposited PEDOT:PSS electrode was fabricated. The electrode was prepared in two simple steps (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e) electrodeposition of PEDOT:PSS under state-of-the-art conditions on the surface of commercial platinum electrode, (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e) fabrication and drop casting of pure hydrogel layer on the top of the polymeric film electrode. The electrical interface parameters such as interfacial impedance, potential window, charge storage capacity, double-layer capacitance, and current injection limit of the electrodes were evaluated and discussed.\u003c/p\u003e"},{"header":"2. Results","content":"\u003cp\u003eThe electrode was prepared in two simple steps (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) i.e. (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e) PEDOT:PSS was electrodeposited on a platinum commercial electrode from an aqueous solution of 10 mM EDOT and 2.5 mg\u0026middot;ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e NaPSS at 1 V vs. Ag/AgCl\u003csub\u003esat\u003c/sub\u003e with deposition time limited to charge of 1 mC (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea); The selected conditions were the state-of-the-art parameters for PEDOT:PSS for the application of different electrode applications\u003csup\u003e20\u0026ndash;22\u003c/sup\u003e; (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e) Second step included synthesis and deposition of hydrogel layer on the top of the PEDOT:PSS.\u003c/p\u003e \u003cp\u003eThe hydrogel precursor solution was a composition of 97.2 mg N-isopropylacrylamide, used as the main monomer (NIPAAm), 2.8 mg N,N\u0026prime;-methylenebisacrylamide (BIS-AAm) as a cross-linker in the proportion 35:1, dissolved in deionized water (90% wt.). The selected monomer and cross-linking agent lead to the formation of a biocompatible hydrogel, which has recently been used in many applications, including bioelectrodes\u003csup\u003e23,24\u003c/sup\u003e. In the end, 5 mg of photoinitiator 2-hydroxy-4\u0026prime;-(2-hydroxyethoxy)-2-methylpropiophenone was added to the solution to trigger the hydrogel polymerization reaction upon UV irradiation. Ready hydrogel precursor solution in a volume of 2.5 ul was drop cast on the top of the PEDOT:PSS film and cross-linked by UV irradiation for 2 minutes in an ice bath. As a result, PEDOT:PSS-Hyd coating system was fabricated. The composition of the hydrogel solution was optimized to obtain a hydrogel layer with Young\u0026rsquo;s modulus of ~\u0026thinsp;10\u0026ndash;20 kPa \u003csup\u003e25\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe scanning electrode microscopy (SEM) with energy dispersive X-ray (EDX) mapping analysis proved the uniform distribution of the PEDOT layer at the surface of the platinum (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb-e). The thickness of the film was determined to be ~\u0026thinsp;280 nm (+/- 30 nm). A highly porous hydrogel layer with a thickness of ~\u0026thinsp;350 um was obtained on the top of the PEDOT:PSS electrode (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe electrical interface parameters such as interfacial impedance, potential window, double-layer capacitance (C\u003csub\u003edl\u003c/sub\u003e), charge storage capacity (CSC), and potential transient (stimulation) performance of the PEDOT:PSS, hydrogel (Hyd) and PEDOT:PSS-Hyd electrodes were determined electrochemically in an aqueous solution of phosphate-buffered saline (PBS). Electrochemical impedance spectroscopy (EIS) spectra of the electrodes are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea.\u003c/p\u003e \u003cp\u003eCoated the electrode with PEDOT:PSS and a combination of the polymer with hydrogel resulted in a significant drop of the impedance, especially for the frequency range between 1 kHz and 0.1 Hz. The latter was related to the lower double-layer capacitance (C\u003csub\u003edl\u003c/sub\u003e), and thus the much higher surface area of the PEDOT-coated electrode compared to bare or hydrogel-coated platinum (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The CV curves recorded within the narrow potential range revealed a pure capacitive behavior of the electrodes, thus allowing to determination of the C\u003csub\u003edl\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). Modification of the platinum with PEDOT or PEDOT-Hyd increased the electroactive surface area of the electrode around ~\u0026thinsp;16 times compared to the bare or hydrogel-based substrate (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). This change influenced |Z|, especially in the high-frequency range. In theory, the recorded |Z| at high frequency i.e. 1000\u0026thinsp;\u0026minus;\u0026thinsp;100 Hz is mainly related to the solution resistance measured between working (WE) and counter electrode (CE) \u003csup\u003e26\u003c/sup\u003e. Any change in electrode geometry such as the increase in porosity, surface area, etc. will influence |Z| at high frequency due to the change in the distance between WE and CE \u003csup\u003e27\u003c/sup\u003e. Since the electrode modified with the polymer or polymer-hydrogel revealed higher C\u003csub\u003edl\u003c/sub\u003e compared to the rest samples, it significantly reduced the |Z| at this frequency (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eFurther analysis of the EIS results showed that modification of the metallic electrode only with pure hydrogel resulted in a higher impedance of the electrode, especially observed for |Z| at 10 Hz (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). It indicates that the hydrogel layer blocks the charge transfer resistance between the platinum electrode and the solution, thus decreasing the electroactive surface area of the electrode. Interestingly the fact that combination of PEDOT:PSS with hydrogel layer exhibited the same interfacial |Z| compared to PEDOT:PSS alone.\u003c/p\u003e \u003cp\u003eThe potential window of the electrode is another important parameter if the application is bioelectrodes. It should be specified firstly for further reliable evaluation of CSC and potential transient of the electrodes. The potential window corresponds to the safe potential range within which the electrode works efficiently avoiding the reactions of hydrogen and oxygen evolution. For this purpose, the CV of the electrodes was performed firstly within the wide potential range i.e. between \u0026minus;\u0026thinsp;1.2 to 1.2 V vs. Ag/AgCl\u003csub\u003esat\u003c/sub\u003e to assess the possible undergoing reactions (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea). In general, similar redox behavior was obtained for all of the studied samples. In the case of bare platinum, higher intensity peaks of platinum oxide and hydrogen formation were determined at ~\u0026thinsp;0.4 V and \u0026minus;\u0026thinsp;0.7 V vs. Ag/AgCl\u003csub\u003esat\u003c/sub\u003e, respectively. Platinum-hydrogen desorption was observed at ~-0.8 V vs. Ag/AgCl\u003csub\u003esat\u003c/sub\u003e, which is all in agreement with the literature \u003csup\u003e28\u003c/sup\u003e. Typical CV peak of PEDOT:PSS oxidation could be observed at around ~\u0026thinsp;1.1 V Ag/AgCl\u003csub\u003esat\u003c/sub\u003e for both PEDOT:PSS and PEDOT:PSS-Hyd electrodes \u003csup\u003e29\u003c/sup\u003e. The potential shift of the beginning of water oxidation and reduction was observed between samples with and without PEDOT:PSS layer. Therefore, based on the CV analysis, the potential window was determined to be -0.8 V to 1.1 V vs. Ag/AgCl\u003csub\u003esat\u003c/sub\u003e for bare platinum and hydrogel-platinum electrodes, and \u0026minus;\u0026thinsp;0.8 V to 1 V Ag/AgCl\u003csub\u003esat\u003c/sub\u003e for PEDOT:PSS and PEDOT:PSS-Hydrogel. Coating the bare metal with the polymer resulted in a slight reduction of the potential window, due to the appearance of the polymer oxidation peak, and as a result shift of the water oxidation peak to lower potentials. The potential window was independent of the presence of an additional hydrogel layer on the top of the PEDOT:PSS compared to the polymer film alone.\u003c/p\u003e \u003cp\u003eCSC is the parameter, that revealed the ability to store the charge by the electrodes and was determined based on the CVs performed within the determined potential windows for two different scan rates (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec). The evaluation of CSC at least two different rates is desirable if the application is bioelectrode such as a neural electrode, which can stimulate the neurons at different rates \u003csup\u003e30\u003c/sup\u003e. CSC was determined to be the lowest for bare platinum electrodes, which is in agreement with the literature\u003csup\u003e10\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed). A slight increase of CSC could be observed for platinum modified with pure hydrogel (Hyd). Metallic electrode coated with PEDOT:PSS exhibited significantly higher CSC of 480 uC\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e / +/- 8 uC\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (50 mV\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and 1550 uC\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e/ +/-40 uC\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (200 mV\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) compared to Pt and Hyd electrodes. Moreover, PEDOT:PSS combined with hydrogel revealed virtually the same CSC as PEDOT:PSS alone. The higher ability to store a charge for the polymer-based electrode is due to their much higher electroactive surface area (porous structure) compared to the substrate or Hyd alone (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAdditionally, voltage transient was evaluated for each of the studied electrodes (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). The latter is especially required to assess in the case of the use of such electrodes for the application of stimulation.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. Potential response of Hyd (a), PEDOT:PSS (b), and PEDOT:PSS-Hyd (c) to the biphasic current pulse of 0.5\u0026ndash;12 mA. Comparison of potential response for all the studied sample to the biphasic pulse current of +/- 3 mA (d).\u003c/p\u003e \u003cp\u003eFor this purpose, a symmetric charge-balance biphasic current pulse with 1 ms pulse width was applied to the electrodes in PBS solution. The current injection limit is defined as a quantity of current, which polarizes the electrode interface to the potential corresponding to the potential window determined from CV. In this work, the current injection limit was determined to be 3 mA for Pt and Hyd (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea), and 12 mA for PEDOT:PSS (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb) and PEDOT:PSS-Hyd (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec). Therefore, when applying a current pulse of 3 mA to the electrodes, PEDOT:PSS and PEDOT:PSS-Hyd revealed the same and much smaller voltage rise across the interface compared to the bare or hydrogel-coated platinum (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed). The phenomenon was related to the much lower interfacial impedance (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), higher surface area (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), and CSC (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e) of these electrodes compared to the Pt or Hyd.\u003c/p\u003e \u003cp\u003eThe preliminary electrochemical stability of the electrodes was also evaluated. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e presents changes in |Z| (a) and CSC (b) before and after 200 cycles of CV carried out within the determined potential windows for each electrode. In each case, the stability of the electrode decreased. However, the change of the parameters before and after 200 cycles was lower for PEDOT:PSS-Hyd compared to PEDOT:PSS alone. CSC decreased by around ~\u0026thinsp;1.5 times and ~\u0026thinsp;1.3 times for PEDOT:PSS and PEDOT:PSS-Hyd, respectively. However, |Z| increased by around ~\u0026thinsp;1.6 times and ~\u0026thinsp;1 times for PEDOT:PSS and PEDOT:PSS-Hyd, respectively. The slower degradation of PEDOT:PSS-Hyd electrode can be due to the hydrogel layer, which partially limits the direct contact of the polymer with the solution, and thus it inhibits the process of conducting polymer overoxidation.\u003c/p\u003e "},{"header":"3. Discussion","content":"\u003cp\u003ePEDOT:PSS electrode with hydrogel interface was fabricated via a simple method. The PEDOT-based soft electrode revealed electrical interface parameters including interfacial impedance, double layer capacitance, potential window, charge storage capacity, and injection current limit at the same level as PEDOT:PSS film. A hydrogel layer was deposited on the top of the PEDOT:PSS film under conditions, which provided specific mechanical properties and high permeability to ions from the solution. Thus, the work showed that the appropriate choice of the hydrogel type and fabrication conditions allowed to synthesis of the hydrogel interface on a rigid polymeric film, which did not block the ionic and electrical transfer. As a result, after immersion of PEDOT:PSS-Hyd electrode in the solution, there was immediately the transfer of the ions between the solution through the hydrogel to PEDOT film, i.e. hydrogel became saturated with the ions from the solution, due to its swelling properties \u003csup\u003e31\u003c/sup\u003e. The results of the work were additionally compared with the literature. The mechanical and electrical interface parameters of fabricated gere PEDOT:PSS-Hyd were compared with the most promising among already published PEDOT:PSS-based hydrogels electrodes for the application of bioelectrodes (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). In most of the published works, the authors aimed to form a pure hydrogel of PEDOT:PSS to modify the elastic substrates such as dimethyl-polysiloxane (PDMS)\u003csup\u003e32,33\u003c/sup\u003e, polyethylene terephthalate (PET)\u003csup\u003e33,34\u003c/sup\u003e, carbon cloth\u003csup\u003e33\u003c/sup\u003e or dimethacrylate-functionalized perfluoropolyether (PFPE-DMA)\u003csup\u003e35\u003c/sup\u003e. It should be noted that the main aim of this work was different i.e. the studies were to test the hypothesis of whether it is possible to maintain the electrical interface parameters of a rigid commercial platinum electrode with state-of-the-art electrodeposited PEDOT:PSS film while ensuring a soft hydrogel interface on the outside. The synthesis of PEDOT:PSS film on a metallic substrate by electrodeposition was proved to reveal desirable electrical properties and high adhesion to the substrates, which could not be obtained by other techniques.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComparison of mechanical and electrical interface parameters of the recently most promising PEDOT:PSS-hydrogel-based electrodes for bioelectrode application.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eType of coating\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFabrication\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eYoung\u0026rsquo;s modulus\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e|Z|\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ePotential\u003c/p\u003e \u003cp\u003ewindow\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCSC\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eReferences\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePEDOT:PSS film\u0026thinsp;+\u0026thinsp;NIPAAm hydrogel\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eElectrodeposition of PEDOT:PSS on Pt, drop-casting of hydrogel\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10\u0026ndash;20 kPa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.6 Ω\u0026middot;cm\u003csup\u003e2\u003c/sup\u003e (+/- 0.1) (1 kHz)\u003c/p\u003e \u003cp\u003e21.6 Ω\u0026middot;cm\u003csup\u003e2\u003c/sup\u003e (+/-0.6) (10 Hz)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-0.8\u0026ndash;1 V vs. Ag/AgCl\u003csub\u003esat\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e488 uC\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (+/- 48) (50 mV\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003cp\u003e1420 uC\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (+/-59) (200 mV\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003eThis work\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePEDOT:P(SS-coNHMAA)\u0026thinsp;+\u0026thinsp;PEG200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eChemical polymerization / 3D printing (PDMS)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4\u0026ndash;20 MPa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e30.0 Ω (10 Hz)\u003c/p\u003e \u003cp\u003e28.0 Ω (1 kHz)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-0.5\u0026ndash;0.5 V vs. Ag/AgCl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e18.9 mC\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (100 mV\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003csup\u003e32\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e* surface area not defined\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePEDOT:PSS-13%DMSO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDrop-casting/printing (PET)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e~\u0026thinsp;2 MPa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-0.5\u0026ndash;0.5 V vs. Ag/AgCl (scan rate not defined)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e60 mC\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (scan rate not defined)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003csup\u003e34\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e*surface area: 0.9 cm\u003csup\u003e2\u003c/sup\u003e (thickness not defined)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePEDOT:PSS-PVA-PAA-NHS-CTS-GA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3D printing \u0026ndash; direct ink writing (PET, PDMS, carbon cloth)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e~\u0026thinsp;1.5 MPa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e20 Ω (10 Hz \u0026amp; 1kHz)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-0.5\u0026ndash;0.5 V vs. Ag/AgCl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e~\u0026thinsp;6 mC\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (50 mV\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003csup\u003e33\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e*surface area: 0.75 cm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePEDOT:PSS in ionic liquid (4-(3-butyl-1-imidazolio)-1-butanesulfonic acid trifate)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSpin-coating (PFPE-DMA)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e~\u0026thinsp;24\u0026ndash;32 kPa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1 kΩ\u0026middot;cm\u003csup\u003e2\u003c/sup\u003e (10 Hz)\u003c/p\u003e \u003cp\u003e900 Ω\u0026middot;cm\u003csup\u003e2\u003c/sup\u003e (1 kHz)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-0.6\u0026ndash;0.8 V vs. Ag/AgCl (3M KCl)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e~\u0026thinsp;160 mC\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003csup\u003e35\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eIn general, the use of fully elastic and soft electrodes is very desirable because of their biocompatibility and stability, however, such an electrode cannot penetrate inside all specific areas. Only a stiff electrode will be suitable for this purpose. The work showed that the formation of a hydrogel layer on the top of the PEDOT:PSS film did not affect the electrical interface parameters of the electrodes. Moreover, it supported additionally the stability of the film by inhibiting its oxidative degradation. The hydrogel interface of PEDOT:PSS-Hyd electrode formed in this work was characterized by a very low Young\u0026rsquo;s modulus of 10\u0026ndash;20 kPa, which was not obtained by any of the already published works (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The interfacial impedance of the electrode was determined to be around 7.8 Ω\u0026middot;cm\u003csup\u003e2\u003c/sup\u003e (1 kHz) and 21.6 Ω\u0026middot;cm\u003csup\u003e2\u003c/sup\u003e (10 Hz), which is at the level of |Z| or even better revealed by already published pure hydrogel form of PEDOT:PSS. Moreover, the electrode could work in a potential range of -0.8 to 1 V vs. Ag/AgCl\u003csub\u003esat\u003c/sub\u003e, which is much wider compared to the literature. However, because of this, the determined CSC was slightly smaller.\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eTo conclude, the work showed that it is possible to develop soft hydrogel interface PEDOT:PSS film metallic electrode and solution without changing its electrical performance. The formation of a hydrogel layer on the top of the PEDOT:PSS film did not affect the electrical interface parameters of the electrodes. Therefore, it can be easily applied for the application of already existing stiff electrodes without changing its electrical properties but providing a soft interface desirable for different applications including wearable and bioelectronics.\u003c/p\u003e"},{"header":"5. Methods","content":"\u003cp\u003e\u003cem\u003eChemicals and materials\u003c/em\u003e: 3,4-Ethylenedioxythiophene (97 %, Sigma Aldrich), Poly(sodium 4-styrenesulfonate) (M\u003csub\u003ew\u003c/sub\u003e ~ 70 000, Sigma Aldrich), N-Isopropylacrylamide (97 %, Sigma Aldrich), N,N\u0026prime;-Methylenediacrylamide (\u0026ge; 98 %, Sigma Aldrich), 2-Hydroxy-4\u0026prime;-(2-hydroxyethoxy)-2-methylpropiophenone (98 %, Sigma Aldrich), phosphate buffer saline pH 7.4 (PBS, Sigma Aldrich) containing 2.7 mM KCl, 140 mM NaCl and 10 mM phosphate. All solutions were made with Mili-Q water. The working electrode was platinum electrode (\u0026ge; 99.9 %, BioLogic). Before each deposition process, the electrode was polished with SiC 2000 grid abrasive paper (Struers), rinsed with ethanol, and dried in the air. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eMorphological and structure analysis\u003c/em\u003e: The morphology and structure of the electrodes were characterized by a scanning electron microscope (FEI QUANTA FEG 250) with energy dispersive X-ray (EDX). The Young modulus of the hydrogel was determined by atomic force microscopy nanoindentation and was described elsewhere\u003csup\u003e36\u003c/sup\u003e. The thickness of the electrodeposited PEDOT film on platinum was determined by profilometer (DektakXT Advanced, Bruker, Germany).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eElectrochemical studies\u003c/em\u003e: Experimental Electrodeposition and all the electrochemical studies were performed in a three-electrode, water-jacketed cell controlled by a \u0026nbsp;potentiostat (VersaSTAT 4). The working electrode was a platinum commercial electrode (diameter 3 mm, BioLogic), platinum coated with hydrogel, PEDOT:PSS, or their combination. The reference and counter electrode were Ag/AgCl in KCl\u003csub\u003esat\u003c/sub\u003e and Pt grid (BioLogic), respectively. The temperature of the measurements was controlled by a thermostat (Julabo F12). All the electrochemical studies were performed in PBS solution. After the deposition, a coated platinum electrode was rinsed with distilled water to remove any residuals from the synthesis solution. After the deposition of the hydrogel layer, the electrode stabilized in the PBS solution for 8 minutes before each electrochemical study.\u003c/p\u003e\n\u003cp\u003eElectrochemical impedance spectroscopy (EIS) was performed in the frequency range of 10 kHz to 0.1 Hz with an amplitude of 5 mV rms. The potential window (redox activity) of the electrodes was determined by performing cyclic voltammetry (CV) in the potential range of -1.2 to 1.2 V vs. Ag/AgCl\u003csub\u003esat\u003c/sub\u003e with a scan rate of 50 mV\u0026middot;s\u003csup\u003e-1\u003c/sup\u003e for 5 cycles. The double-layer capacitance (C\u003csub\u003edl\u003c/sub\u003e) was determined based on CV measurements, which were carried out within the potentials 0.3\u0026ndash;0.35 V vs. Ag/AgCl\u003csub\u003esat\u003c/sub\u003e with a scan rate of 10, 20, 40, 60, 80, and 100 mV\u0026middot;s\u003csup\u003e-1\u003c/sup\u003e. C\u003csub\u003edl\u003c/sub\u003e was determined based on the following equation: C\u003csub\u003edl\u003c/sub\u003e = i\u003csub\u003edl\u003c/sub\u003e\u0026middot;(2ʋ)\u003csup\u003e\u0026minus;1\u003c/sup\u003e = (i\u003csub\u003ea\u003c/sub\u003e \u0026minus; i\u003csub\u003ec\u003c/sub\u003e)\u0026middot;(2ʋ)\u003csup\u003e\u0026minus;1\u003c/sup\u003e, where ʋ is the scan rate; i\u003csub\u003ea\u003c/sub\u003e and i\u003csub\u003ec\u003c/sub\u003e are the anodic and cathodic current densities, respectively; and i\u003csub\u003edl\u003c/sub\u003e is the double-layer current density. Charge storage capacity (CSC) was determined as the surface area of the specific cycle of CVs performed within the determined potential window. To evaluate the potential transient, a biphasic current (I) pulse was injected across the working electrode and counter electrode while measuring the working electrode\u0026rsquo;s polarization potential vs. Ag/AgCl\u003csub\u003esat\u003c/sub\u003e. The protocol was determined as follows: cathodic pulse (-I, 1 ms), break (I = 0 A, 20 us), and anodic pulse (+I, 1 ms). The detailed description of the measurement procedure is described elsewhere \u003csup\u003e37\u003c/sup\u003e. To assess the preliminary stability of the electrodes, a CV of the electrodes was performed within the selected potential window for 200 cycles in PBS solution. |Z| and CSC were determined after 200 cycles of CV.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eEach of the measurements was performed a minimum of three times to ensure reliable and reproducible data.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eKC: conception, design of the work, data acquisition \u0026ndash; PEDOT:PSS synthesis, performing all electrochemical measurements, analysis and data interpretation, writing the manuscript. SP: data acquisition \u0026ndash; fabrication and optimization of hydrogel layer. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work was supported by National Science Centre (NCN), Poland: Sonata grant based on the decision 2021/43/D/ST7/01362.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe datasets generated and/or analyzed during the current study are available in the BRIDGE OF KNOWLEDGE repository (https://mostwiedzy.pl/pl/open-research-data/soft-interface-pedot-pss-experimental-raw-data,507011053553723-0?_share=435c600211713337).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eFu, F., Wang, J., Zeng, H. \u0026amp; Yu, J. Functional Conductive Hydrogels for Bioelectronics. \u003cem\u003eACS Mater Lett\u003c/em\u003e \u003cstrong\u003e2\u003c/strong\u003e, 1287\u0026ndash;1301 (2020).\u003c/li\u003e\n\u003cli\u003eDechiraju, H., Jia, M., Luo, L. \u0026amp; Rolandi, M. 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A. Scaling Effects on the Electrochemical Stimulation Performance of Au, Pt, and PEDOT:PSS Electrocorticography Arrays. \u003cem\u003eAdv Funct Mater\u003c/em\u003e \u003cstrong\u003e27\u003c/strong\u003e, 1703019 (2017).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"conducting polymers, electrical interface, hydrogel, PEDOT:PSS, soft interface, thin films","lastPublishedDoi":"10.21203/rs.3.rs-4382855/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4382855/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe work presents the procedure of developing a soft interface at PEDOT:PSS film without changing its electrical interface parameters. In the first step, PEDOT:PSS is electrodeposited on the commercial platinum electrode under the state-of-the-art conditions desirable for different electrochemical electrodes. Secondly, a pure hydrogel layer is deposited on the top of the electrodeposited PEDOT:PSS film under conditions, that provide desirable mechanical properties (Young\u0026rsquo;s modulus\u0026thinsp;~\u0026thinsp;10\u0026ndash;20 kPa) and high permeability to ions from the solution. As a result, a PEDOT:PSS electrode with a soft interface desirable for different electrode applications is fabricated. The electrode exhibits electrical parameters at the same level as the state-of-the-art PEDOT:PSS film applied already for electrode applications. Moreover, the hydrogel layer supports additionally the electrochemical stability of the polymeric film by inhibiting its oxidative degradation. The work shows that the specific choice of the hydrogel type and fabrication conditions allows to synthesis of the hydrogel interface on a stiff polymeric film, which does not block the ionic and electrical transfer. Moreover, the fabricated PEDOT:PSS electrode with hydrogel interface reveals interfacial impedance and potential window comparable or even better to the already published studies on PEDOT:PSS hydrogels.\u003c/p\u003e","manuscriptTitle":"A simple route of providing a soft interface for PEDOT:PSS film metallic electrodes without loss of their electrical interface parameters","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-05-21 11:53:51","doi":"10.21203/rs.3.rs-4382855/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"0b1fccee-f1f9-4d5c-98ae-97c35a440ece","owner":[],"postedDate":"May 21st, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":31962847,"name":"Physical sciences/Chemistry/Electrochemistry"},{"id":31962848,"name":"Physical sciences/Materials science/Materials for devices"},{"id":31962849,"name":"Physical sciences/Materials science/Soft materials"}],"tags":[],"updatedAt":"2024-10-03T14:03:48+00:00","versionOfRecord":{"articleIdentity":"rs-4382855","link":"https://doi.org/10.1016/j.electacta.2024.145115","journal":{"identity":"electrochimica-acta","isVorOnly":true,"title":"Electrochimica Acta"},"publishedOn":"2024-09-25 00:00:00","publishedOnDateReadable":"September 25th, 2024"},"versionCreatedAt":"2024-05-21 11:53:51","video":"","vorDoi":"10.1016/j.electacta.2024.145115","vorDoiUrl":"https://doi.org/10.1016/j.electacta.2024.145115","workflowStages":[]},"version":"v1","identity":"rs-4382855","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4382855","identity":"rs-4382855","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}