Evolution of the Electrode Impedance on Subretinal Implants in Dystrophic P23H Rats

Abstract

Purpose: : After photoreceptors degeneration, electrical stimulations of the retina can induce the perception of visual images. We recently showed that a 600 pixel visual window can allow text reading (Sommerhalder et al., 2004). Based on this information, implant prototypes were produced and introduced in the subretinal space of dystrophic P23H rats. This study was designed to monitor the electrode impedance over time as an indication of the tissue–implant interface.

Methods: : Implant prototypes were produced in polyimide with platinium electrode. The implant head supported 5 stimulating electrodes and the implant tail was linked to an external connector. The implant head was inserted into the subretinal space through a sclerotomy while the implant external connector was fixed on the head cranium. Impedance measurements were carried out using a commercial LCR–meter (Agilent 4284A) with peak–to–peak voltage below 50 mV to avoid tissue damage and frequency scan between 100 Hz–1MHz. The retina was examined in vivo on a regular basis and subsequently on histological araldite semi–thin sections.

Results: : Impedance measurements could be obtained without difficulties on more than 50 animals, the day of implantation, the following days, and every week during 3 months. The impedance increased significantly in the range 10–50 kHz during the first 24 hours and stabilized after 15–30 days. Great variations were observed in the impedance measurements between electrodes, not only from one rat to another, but also between neighbouring electrodes from the same implant. A correlation was also apparent between the impedance measurement and the eventual tissue reaction.

Conclusions: : These results demonstrate the feasibility of recording the electrode impedance and therefore stimulate with a subretinal implant in P23H rats. The impedance stabilization provides the temporal window to start testing the functional efficacity of the retinal implant. This study therefore opens the way to the in vivo validation of electrical stimulation paradigms in an animal model of retinitis pigmentosa.

Keywords: retina • degenerations/dystrophies • retinal connections, networks, circuitry