Conducting Polymer-Based Bioelectrodes with Enhanced Wet Adhesion and Stretchability

Bioelectronic devices are mounted on human skin and tissue to get physiological information, enable electrical stimulation, and treat diseases. To fulfill these challenging roles, bioelectronic devices must possess the same mechanical properties as tissues that are soft, stretchable, and conform to the tissue surface. Also, it requires high conductivity and tough adhesion for high-quality electrical communication with devices and organizations. However, traditional bioelectronic devices are typically constructed using rigid metals, carbon nanotubes (CNT), or graphene, which offer excellent electrical conductivity but suffer a significant mechanical mismatch when interfacing with biological tissues. This disadvantage leads to non-conforming electrode-tissue interfaces and can trigger pronounced inflammation. To address these issues, recently, soft bioelectronics have been developed that aim at improving compatibility between the devices and tissues and reducing inflammations by replacing rigid components with soft materials that closely mimic the characteristics of natural tissues. Therefore, bioelectronic devices composed of conducting polymers within a soft matrix have been thoughtas one of the ideal choices of materials. This combination offers excellent biocompatibility, mechanical properties resembling natural tissues, and desirable electrical characteristics. Poly(3,4- ethylenedioxythiophene): poly(styrene sulfonate)(PEDOT: PSS) has been studied extensively in soft bioelectronics. However, pure PEDOT:PSS is brittle because PEDOT:PSS network cannot effectively dissipate strain energy. The methods for preparing stretchable electrodes using PEDOT:PSS can be broadly categorized into two main approaches: one involves the design of geometric structures, while the other entails blending with plasticizers, ionic liquids (IL), or polymers. Forming a second network within the existing PEDOT:PSS network to create an interpenetrating polymer network, can enhance the stretchability of the composite electrode. Polyurethane is a hydrophilic polymer that can be easily mixed with PEDOT:PSS. Blending with polyurethane can make low modulus and high stretchability, but it reduces electrical conductivity with increasing amounts of insulating polymer. A higher proportion of PEDOT: PSS is required to achieve higher electrical conductivity while blending with PEDOT: PSS. However, this may result in a more rigid and less stretchable blend due to the inherent properties of PEDOT:PSS. Here, we report a double-network conducting polymer electrode consisting of PEDOT: PSS and polyurethane with high electrical conductivity and large stretchability by using an additive method that effectively reduces the modulus of the matrix and increases conductivity. We use triton-x as an additive material. Triton-X effectively infiltrates the space between PEDOT:PSS chains and the polymer matrix, thereby increasing the free volume within both PEDOT:PSS and the polymer. This augmentation in free volume contributes to enhanced stretchability. Additionally, serving as a secondary dopant, Triton-X plays a crucial role in elevating the conductivity of PEDOT:PSS. When compared to DMSO, it has shown a more substantial improvement in conductivity. However, excessive Triton-X content can weaken the property of reverting to its original length, making it essential to incorporate an appropriate amount to maintain elastic characteristics. Subsequently, for effective attachment to tissues, wet adhesive properties are essential for signal measurement and stimulation. Polyacrylic acid (PAA) and N-hydroxy succinimide (NHS) have been employed in this regard. PAA-NHS forms covalent bonds on the tissue surface, resulting in robust adhesion. Using this approach, we demonstrate the electrodes that exhibit a low modulus, excellent stretchability, and effective wet adhesion properties, which can be used for implantable bio electrodes for nerve stimulation and implantable sensor for ECG.