Multi-Material 3D Printed Conductive/ Neat TPU Dry Electrodes for ECG Monitoring Wearable Applications

The limitations of traditional gel-based Ag/AgCl electrodes, such as skin irritation, unsuitability for long-term monitoring, etc., have spurred the development of next-generation "dry" electrodes for bioelectricity monitoring applications. This study reports the fabrication of high-performance and low-cost thermoplastic elastomer multi-material 3D printed (3DP) dry electrodes, utilizing Fused Filament Fabrication (FFF). The electrodes consist of neat and conductive thermoplastic polyurethane (cTPU) simultaneously printed to incorporate the conductive material properties into the electrodes' bulk structure, as well as "tune" the compliance, stretchability, and flexibility with respect to the human skin stiffness. Different electrode designs are proposed to optimize skin conformity, employing nature-inspired triply periodic minimal surface (TPMS) geometries, namely, gyroid (G) and square honeycomb (SH), with a 3 mm unit cell size. The final electrodes are characterized through thermogravimetric analyses, tensile testing, cyclic bending, and electrode-skin impedance, while different ECG signal acquisition scenarios are demonstrated, i.e., stationary, 24 h monitoring, standing, and walking. The multi-material gyroid electrode exhibited the lowest electrode-skin impedance (298 kΩ at 20 Hz) and the highest ECG signal quality. The proposed multi-material structure and TPMS design strategy enable a scalable route to comfortable, reusable, and high-performance electrodes by spatially combining conductivity and skin-compliant mechanics, supporting continuous bioelectricity monitoring in wearable applications.