Effects of Long-Term In vivo Stimulation on the Electrochemical Properties of a Porous Stimulation Electrode for a Suprachoroidal-Transretinal Stimulation (STS) Retinal Prosthesis

A high-performance porous stimulation electrode [femtosecond laser-induced porosity (FLiP) electrode] was designed for the safe and effective delivery of high stimulation currents. Herein, we evaluated the electrochemical properties of the FLiP electrode interface under prolonged in vivo stimulation. The FLiP electrodes were surgically implanted in rabbits and subjected to stimulation currents over a six-month period. The impedance of these electrodes was examined through equivalent circuit analysis, specifically employing the Randles circuit model. Comparative analysis with the results of previous in vitro/in vivo studies was conducted. Electrical stimulation served to purify the electrode interface, enhancing mass diffusion and promoting charge-transfer reactions at the electrode interface. As a result, the electric double layer capacitance (Cdl) could be enhanced by adjusting the ionic concentration at the electrode. The high charge injection capacity of the FLiP electrode is facilitated by not only capacitive currents resulting from Cdl but also pseudocapacitive currents stemming from the reversible redox reaction of H-2 molecules. Our findings suggest that the initial evaluation outcomes can extrapolate the electrode properties during long-term stimulation, and the combined method of electrochemical impedance spectroscopy and equivalent circuit analysis clarifies intricate in vivo interactions, allowing accurate analysis of the properties of implantable electrodes over extended periods.