Fabrication and in-vitro characterization of novel hydrogel-based electrodes for visual prostheses

Abstract

Purpose : Charge injection capacity (CIC), the maximum injectable charge without causing irreversible electrochemical reactions, is important characteristic of stimulating electrode for visual prostheses. Introducing hydrogel onto electrode surface is expected to enhance in-vivo CIC, but fabrication of such electrode is difficult because of poor mechanical properties of hydrogel. The purpose of this study is to fabricate and characterize the novel hydrogel-based electrodes.

Methods : Electrode arrays were fabricated using polydimethylsiloxane (PDMS) and platinum foil. After forming 0.4mm-thick PDMS base layer, 0.02mm-thick platinum foil was stuck on the base layer. The platinum foil was then cut to form electrodes and conductive lines by femtosecond laser. Next, the electordes were coated with polyethylene glycol. After applying PDMS top layer, electrode surfaces were exposed by dissolving the polyethylene glycol with water. Nanocomposite hydrogel, composed of poly (N,N-dimethylacrylamide) and synthetic hectorite, was injected into the electrode openings to fill 0.5mm-diameter 0.4mm-height spaces above the platinum foil electrodes. CICs were measured in room-temperature PBS.

Results : Fabrication of hydrogel-coated electrodes was successfully performed by applying 90°C heat to the hydrogel in DI water after hydrogel injection into the electrode openings (see Fig.1).
The average CIC of non-hydrogel electrodes and hydrogel electrodes were 182µC/cm² and 160µC/cm² respectively. No statistically significant difference was observed between non-hydrogel electrodes and hydrogel electrodes (paired t-test, N=3).

Conclusions : The abovementioned results indicated that hydrogel had enough permeability of water and ions to pass the current of electrical stimulation. This suggested that the hydrogel-based electrode had a potential to enhance the in-vivo CIC because the application of hydrogel onto metal electrode intervened to deteriorate in-vivo CIC of conventional electrodes such as protein absorption and low counterion availability. We will study in-vivo performance of the hydrogel electrodes in the next step.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

 

Fig.1 Electrode with hydrogel (right) and without hydrogel (left).