An Improved Fabrication Method for Flexible COC-Based Implantable Neural Probes with Reliability Validation

Implantable neural interfaces require mechanically flexible and electrochemically stable materials to ensure long-term reliability, but conventional polymer substrates suffer from high moisture permeability and weak adhesion, leading to limited long-term stability. Cyclic olefin copolymer (COC) provides improved barrier properties, however, further optimization in flexibility, process scalability, and electrochemical performance remains important. Building on prior COC-based electrode developments, this study presents process updates and a comprehensive functional evaluation of COC-based neural probes. T-die-cast COC films show reduced thickness variation and increased flexibility, and femtosecond laser micromachining enables precise site opening with dimensional scalability. IrO$_{\mathrm{x}}$ electroplating reduces impedance to 681.04 $\Omega$ at 1 kHz and increases CSC$_{\mathrm{c}}$ to 23.51 mC/cm$^{2}$, while the probe maintained stable performance under repeated bending and folding. Accelerated soaking confirms insulation reliability, with leakage remaining below 10$^{-9}$ A for 320 days at 75$^\circ$C. Ex vivo retinal stimulation experiments showed evidence of evoked neural activity, supporting functional feasibility. Collectively, these findings indicate that refined COC processing and fabrication improvements address practical constraints of compression molded COC electrodes, enabling flexible, reliable, and electrochemically stable neural probes and broadening the practical applicability of COC.