Comparison of electrode materials for the use of retinal prosthesis.

Retinal implants may provide vision for people suffering from photoreceptor degeneration caused by different eye diseases. Electrode size in retinal implant should be decreased in order to increase the resolution provided by the implant. We defined electric properties of five different electrode materials (Au, Ir-b, Ti, TiN, Pt-b) widely used in retinal prostheses. The comparison of different electrode materials requires that the electrical properties of different materials are defined using exactly the same measurement conditions and devices. Existing studies about electrode material properties are often made using slightly different measurement parameters or electrode processing conditions making the comparison between different materials difficult. Here, the electrochemical characterization included cyclic voltammetry and electrochemical impedance spectroscopy. Ir-b and Pt-b had greater charge injection capacity than other materials. The fabricated material samples showed that in this experiment the electrode diameter larger than 200 μm should be used to suppress irreversible reaction of stimulus electrodes with the needed stimulus currents. Thus, either we have to find novel electrode materials or surface treatment methods to decrease the electrode area providing increased electrode and pixel number of the prosthesis or we have to show that stimulus currents smaller than 40 μA are enough to induce phosphenes.