Electrical Behavior of Nominally Inactive ASR Devices

Abstract

Purpose:: Several studies involving subretinal Artifical Silicon Retina (ASR) devices have used nominally inactive implants as controls. We have recently discovered that the nominally inactive devices produce some electrical current in response to light. The composition, fabrication, and photovoltaic charge-injection characteristics of active and inactive devices have been investigated to interpret the implications of using nominally inactive devices in past studies.

Methods:: The photovoltaic charge-injection characteristics of active and inactive ASR devices were measured in a PBS electrolyte under a wide range of illumination conditions using an infrared source. Dopant profiles were determined by SRP analyses, and oxide thickness by ellipsometry. Data from RCS rats implanted with inactive devices and exposed to various intensities of light was analyzed to determine dose response relationships between current from the inactive device and preservation of photoreceptors.

Results:: Silicon-based nominally-inactive ASR devices are capable of producing low-level charge injection, however, the output of the inactive devices reaches a saturation limit at less than 0.1% of the saturation level of the active devices. Thus, the difference between active and inactive devices is more pronounced at high illumination levels. RCS rats implanted with inactive devices and exposed to two levels of IR stimulation suggest a possible dose response relationship in preservation of retinal function.

Conclusions:: Previous studies have shown some benefit derived from inactive devices (Pardue et al., 2005) which has previously been attributed to mechanical effects. The observation of a dose response relationship in RCS rats implanted with inactive devices suggests that electrical stimulation provided by nominally-inactive devices may account for some of the observed protection. Future studies using solid glass discs as genuinely inactive devices may clarify to what degree the very-low-level currents provided by nominally-inactive devices are relevant to retinal function and preservation.

Keywords: retina • neuroprotection • photoreceptors