Conducting Polymer Based Flexible Epidermal Patch Electrodes for Human Healthcare

The demand for wearable electronics and bioelectronics has sparked a need for stretchable, flexible, and conductive materials. Soft, adhesive, and self-healable materials with moduli similar to human tissues (Young’s modulus ranging from 0.5 to 500 kPa) have become a research hotspot, owing to their improved contact interface between electronics and biological tissues . To date, various conductive materials, in form of films, bio-inspired adhesives, fabrics, and hydrogels, ha ve been utilized for on-skin electronics. Conducting polymers, owing to their inherent flexibilit y, biocompatible property, easy processability, and combination of electronic and ionic conductivity, are considered ideal candidates for wearable electronics and bioelectronics. However, issues such as adhesion, mechanical properties, conductivity, self-healing performance, and insuffic ient understanding of self-healing types and their capabilities still remain. Thus, t here is a need for further investigation to enhance the use of conducting polymers in bioelectronics and w earable electronics. This thesis is devoted to the preparation conductive and soft materials and fabrication soft epidermal patch electrodes for electrophysiological signals monitoring. Therefore, this thesis utilizes a conductive polymer, poly(3,4 -ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), to prepare hydrogels, films, and organohydrogels. We investigate the mec hanical properties, self-healing ability, conductivity, adhesion, swelling behavior, anti-freezi ng and water retention ability, and biocompatibility of these materials. Furthermore, epiderma l electrodes fabricated from these materials were used to record bio-signals, including ele ctromyography (EMG), electrocardiography (ECG), and human-machine interface. Due to their biocompatibility, tunable electrical and mechanical properties, and high-water content, conductive hydrogels are highly promising in the field of epidermal electronics. The fi rst part of this thesis demonstrates an approach to fabricate highly adhesive, stretchable , and self-healable hydrogels by blending polyvinyl alcohol (PVA), borax, and a commercial screen-printing paste (CleviosTM S V3 STAB) containing PEDOT:PSS, diethylene glycol and propylene glycol. The addition of the paste to the PVA/borax network led to hydrogels with improved softness and plastic stretchability, shorter self-healing times, and enhanced adhesion. The resulting hydrogels demonstrate a high adhesion of 1.96 N/cm 2 on porcine skin, remarkable plastic stretchability (>10000% strain), low compressive modulus (range from 0.3 to 3.7 kPa), and outstanding self- healing performance. Epidermal electrodes fabricated from the hydrogels exhibited high ECG and EMG recording quality. These results were published in the journal of Acta Biomaterialia. Adhesive and self-healable films have emerged with promising potential for use in epidermal electrodes owing to their ability to form conformal contacts to the skin without adhesive tapes and recover their functionality after damage. In the second part of this thesis, we prepared stretchable, self-healable, and adhesive films with a low Young's modulus (~ 450 kPa) by mixing PEDOT:PSS, ethylene glycol (EG), and tannic acid (TA). We investigated the impact of TA on the mechanical, electrical, self-healing and adhesion properties of the films. Increasing TA concentration decreased the Young’s modulus of the films and increased the elongation at break. The presence of catechol and pyrogallol aromatic ring structures in TA imparted to the films high adhesion, with a value of ~ 6 N/cm on glass and 0.7 N/cm on porcine skin, when the TA concentration was increased to ~ 10 wt%. Additionally, the PEDOT/EG/TA films showed outstanding autonomic and c ut-stick healing properties because of their softness and large amount of hydrogen bonds. We evaluated the biocompatibility of the films by culturing C2C12 muscle cells on their surface. The cell viability on the films was 88%, which is higher than the non-cytotoxic criterion (70% of the control group), according to ISO 10993-5. The results indicate our films could endow a safe medium for cellular proliferation, which ensures their use towards bioelectronics. Lastly, we fabrica ted the adhesive film-based skin patch electrodes. These electrodes showed low skin-elect rode impedance at low frequency and high-quality signals for ECG and EMG recording compared to Ag/AgCl gel electrodes. We have submitted an article presenting these results to the Nano-Micro Letters journal. Conventional hydrogels typically contain high water content (70 – 99%), which may not be suitable for long-term use in dry and cold environments due to water evaporation or freezing, resulti ng in changes in both mechanical and electrical performance. Therefore, in the third part of the thesis, we developed anti-freezing and long-lasting water-retaining organohydrogels using a concentrated PEDOT:PSS suspension, EG, and TA. The formation of the gel is due to the physi cal network formed between PEDOT polymer chains, which is facilitated by the addition of TA. The PEDOT/EG/TA gels exhibited anti-freezing properties, remaining unfrozen down to -24°C, due to the formation of molecular clusters between EG and H2O molecules, leading to a reduced freezing point and water vapor pressure. Additionally, the gels demonstrated long-lasting wate r retention capabilities because of the formation of hydrogen bonds between water and EG, while also exhibiting a low compressive Young’s modulus (~20 kPa) and a high conductivity (15 S/cm). Furthermore, we investigated the volumetric swelling behavior of the gel and the dehydrated gel showed a volumetric swelling of ~135 % of its initial value. Finally, we utilized the gels to fabricate epidermal electrodes, which exhibited low skin-electrode impedance and high-quality for ECG and EMG recordings compared to commercial Ag/AgCl gel electrodes. Moreover, the dehydra ted gel also displayed excellent ECG recording quality. We submitted these results to the Advanced Sensor Research journal. The fabrication of epidermal electrodes that contain a plethora of attributes , such as being conductive, self-healing, stretchable, adhesive, and soft has become a research hotspot and remains a challenge in the field of wearable electronics and bioelectronics. This research paves the way to the design and preparation of many different forms of soft materials (hydrogels, films, and organohydrogels) with Young’s modulus matching biological tissues. In addition, this study al so provides some fundamental guidelines to the healing behavior of conductive polymers-based hydrogels and adhesive films. Furthermore, this research demonstrates the fabricati on of high- performance epidermal electrodes, which contributes to the growing market of bioele ctronics for personal healthcare systems.