Soft electronics are crucial for applications in wearable healthcare monitoring and medical implants because of their ability to conform to the body's contours and withstand mechanical deformations. The reliability of these devices heavily depends on electrodes and interconnects that bridge active components and biological tissues. These interfaces must form conformal attachments on irregular surfaces and maintain stability under deformation. While various soft materials like hydrogels, carbon-based composites, liquid metal composites, metal composites, and conducting polymers have been explored for creating electrodes to monitor physiological signals in real-time, challenges remain. Biological tissues (skin and muscles) are soft (60–850 kPa), and the contact area with electrodes is irregular and dynamic. This significant mechanical mismatch and weak interfacial adhesion lead to increased noise, reduced sensitivity, and device failure. Therefore, reliable interfacial connections between electrodes and biological tissues are critical for advancing soft electronics. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is a promising conductive polymer due to its solution processability, tunable electrical properties, and biocompatibility. However, it suffers from high rigidity (>500 MPa), low stretchability (about 5%), and poor interfacial adhesion, causing discomfort and potential immune responses. Methods to reduce modulus and enhance stretchability, such as small molecule doping, polymer blending, and hydrogel network construction, have been explored. However, the rigidity and modulus of PEDOT:PSS-based composites often remain higher than that of human tissues. Moreover, plastic deformation and potential leakage of dopants (ionic liquids) are concerns. Hydrogels have been used to create ultrasoft PEDOT:PSS composites, but their limited conductivity (<10 S cm−1) presents a trade-off between softness and conductivity. Enhancing interface adhesion, for example, through electrogelation, electrografting, or adhesion layers, usually requires complex modification or specific electrochemical deposition processes, resulting in disposable and non-recyclable interfaces. This research aims to overcome these limitations by developing a novel self-adhesive conductive polymer composite for robust and reliable soft electronics.

Existing literature highlights the need for soft, flexible, and highly adhesive conductive materials for bioelectronic applications. Studies have explored various materials, including hydrogels, carbon-based materials, and liquid metals, each presenting trade-offs between conductivity, mechanical properties, and biocompatibility. PEDOT:PSS, while attractive due to its biocompatibility and solution processability, suffers from inherent rigidity and poor adhesion. Several strategies, such as doping with ionic liquids, polymer blending, and hydrogel incorporation, have attempted to address these drawbacks, but often compromise conductivity or introduce issues like plastic deformation and dopant leakage. The challenge of achieving a material with simultaneously high conductivity, low modulus, high stretchability, and strong adhesion has motivated the current work. Existing methods for enhancing adhesion, such as electrogelation and electrografting, require complex processing and produce non-reusable interfaces. The development of a self-adhesive material offers a significant advantage in simplifying fabrication and enhancing device reliability.

This study developed a self-adhesive conductive polymer (SACP) composite by doping rigid and non-stick PEDOT:PSS with a biocompatible supramolecular solvent (SMS) consisting of citric acid and β-cyclodextrin. The SACP also incorporates an elastic polymer network of poly(vinyl alcohol) (PVA) covalently crosslinked with glutaraldehyde (GA). The fabrication process involved mixing citric acid, cyclodextrin, PVA, and GA into an aqueous solution of PEDOT:PSS, creating a homogeneous, “coffee-ring” free ink. The resultant SACP films were characterized using various techniques. Mechanical properties, including Young’s modulus, fracture stress, and fracture strain, were determined through tensile testing. The effect of varying PEDOT:PSS mass ratios on the mechanical properties was investigated. Electrical conductivity was measured using the four-point probe method, and the influence of PEDOT:PSS mass ratios and film thickness was assessed. The adhesion strength was evaluated using a 180° peeling test and lap shear testing on various substrates (PI, PEEK, Al, Cu, PET, PTFE, PDMS), comparing the adhesion performance with commercial tapes. The reusability and long-term stability of adhesion were also examined. Solution processability was demonstrated through techniques including drop-casting, spin-coating, and microfluid molding, along with transfer printing. The fabrication of ACEL devices using SACP as electrodes was conducted to demonstrate the material's applicability in soft electronics. Electromyography (EMG) monitoring was performed using SACP electrodes attached to human skin during various exercises, and the signals were compared with those obtained using commercial Ag/AgCl electrodes. Finally, an integrated system combining EMG sensors and ACEL arrays was developed to visualize muscle training in real-time.

The SACP composite demonstrated exceptional properties, successfully addressing the limitations of previous PEDOT:PSS-based materials. The incorporation of SMS and PVA networks resulted in a significant reduction in Young’s modulus, achieving values compatible with skin tissue (56.1–401.9 kPa). Simultaneously, high stretchability (up to 700%) and conductivity (up to 37 S/cm) were maintained, showcasing a successful trade-off between mechanical flexibility and electrical conductivity. The SMS doping effectively prevented PEDOT chain aggregation, increasing chain flexibility and improving stretchability. The PVA networks ensured reversible stretchability and minimized plastic deformation. The SACP films exhibited remarkably strong adhesion, exceeding 1.2 MPa in lap shear strength on PI substrates and demonstrating consistent adhesion across multiple substrate types. This adhesion strength surpassed that of commercial tapes. The self-adhesive nature allows for quick and easy bonding (within 30 seconds) to various substrates, showing high stability even after repeated use (over 100 cycles) and prolonged storage. The SACP ink’s solution processability enabled facile fabrication of thin films and patterns using diverse methods, including drop-casting, spin-coating, microfluid molding, and transfer printing, resulting in flexible and transparent conductive films suitable for applications such as ACEL devices. In applications, the SACP film acted as a reliable and robust interface for both EMG and ACEL device development. The EMG signals detected using SACP electrodes were comparable to those from commercial Ag/AgCl electrodes, exhibiting consistent performance across different gripping forces and repeated usage. The integrated system successfully visualized muscle activity during exercise by controlling the ACEL arrays based on EMG signal strength. The ACEL devices themselves were thin (<150 µm), flexible, and water-resistant.

This research successfully developed a novel SACP composite that overcomes the limitations of existing conductive polymer materials used in soft electronics. The combination of low modulus, high stretchability, strong adhesion, and good conductivity makes SACP an ideal candidate for creating reliable and comfortable bioelectronic devices. The results demonstrate that the strategic use of a supramolecular solvent and elastic polymer networks can effectively tune the mechanical and electrical properties of PEDOT:PSS, leading to a material that closely matches the mechanical characteristics of human skin while retaining sufficient conductivity for biomedical sensing applications. The superior adhesion strength and reusability are key advantages, eliminating the need for complex interfacial modifications or disposable layers often required with other materials. The successful fabrication and testing of ACEL devices and EMG sensors highlight the practical applicability of SACP in creating functional soft electronics. The ability to visualize muscle activity in real-time through the integrated system showcases the potential for advanced wearable and implantable technologies.

This study successfully demonstrated the fabrication and characterization of a novel self-adhesive conductive polymer (SACP) composite. SACP exhibits a unique combination of desirable properties: low modulus, high stretchability, high interfacial adhesion, and high conductivity. Its solution processability enables versatile fabrication techniques, including various printing methods. The successful applications in ACEL devices, EMG sensors, and an integrated EMG-ACEL system illustrate its significant potential for advanced bioelectronic devices. Future work could explore different SMS compositions, investigate biocompatibility and long-term stability in vivo, and expand its application to other wearable and implantable electronics.

While this study demonstrated the promising properties of SACP, several limitations should be acknowledged. The long-term stability and biocompatibility of SACP in vivo require further investigation. The current integrated system's complexity might limit its immediate practical application. A more streamlined and miniaturized system would enhance its feasibility for widespread use. The scaling up of the fabrication process for mass production also requires further optimization.