This study investigates the electrical behavior of conductive polypyrrole (PPy) thin films designed for flexible electrode applications in wearable sensing systems. A reference p-toluenesulfonic acid-doped PPy (PPy-TSA) film was compared with graphene-modified PPy composites developed to enhance mechanical robustness and electrical stability under deformation.
Morphological analysis (SEM) shows that the reference PPy–TSA exhibits a typical cauliflower-like structure, while graphene incorporation induces elongated, layered features associated with well-dispersed nanosheets, without disrupting the polymer matrix integrity. XPS confirms that the chemical structure and doping state of PPy are preserved after graphene incorporation.
Mechanical characterization reveals that the reference PPy-TSA film exhibits a low Young’s modulus (~0.008 GPa), while graphene increases stiffness (~0.016 GPa) and improves mechanical robustness. In contrast, the PPy-PEG graphene system maintains a low elastic modulus (~0.009 GPa), indicating an optimal balance between flexibility and reinforcement.
Electrical measurements show that PPy-TSA exhibits a conductivity of ~208 S/cm, while graphene-modified films show reduced conductivity (~62 S/cm) due to incomplete percolation. However, these films retain conductivity within the range required for electrocardiographic (ECG) sensing, and temperature-dependent measurements (25-85 °C) confirm stable semiconducting behavior. Optical analysis reveals a redshift in the π–π* transition and enhanced VIS–NIR absorption, indicating a modified electronic structure without proportional improvement in macroscopic conductivity.
Overall, graphene incorporation improves mechanical robustness and signal stability while maintaining adequate conductivity for ECG applications. The PPy-PEG–graphene composite has the best balance between flexibility, conductivity, and signal quality, making it a promising candidate for flexible, gel-free wearable electrodes.