Temperature Change in the Subretinal Space During Infrared Irradiation

Abstract

Abstract: : Purpose: Infrared (IR) irradiation is a possible source for external energy supply to subretinal implants ("active implant"). We investigated the thermal stress for ocular tissue around a subretinal IR receiver and the amount of scattering irradiation near the functional part of the implant. Methods: Polyimide foil strips (20.0 x 3.0 x 0.05 mm) carrying a thermal sensor (upper part consisting of a silicium -silicium oxide plate, 3 x 3 x 0.07 mm, and lower part of a nickel nickel-chrome wire, d=25µm, thermo-element Typ K) at 7 mm distance from an optical sensor (silicium solar cell, 1.7 x 1.85 x 0.07 mm) were implanted into the subretinal space via a novel ab externo procedure in animal eyes (2 pig eyes ex vivo and 2 rabbit eyes in vivo). The location of each sensor corresponded to the intended locations of the IR receiver and stimulation electrodes as designed for the active implant respectively, allowing the measurement of the absolute temperature at the IR receiver and of the scattering irradiation intensity at the intended site of the stimulation electrodes. The area of the thermal sensor was irradiated with IR laser (830 nm) with a corneal steady irradiation power ranging from 8.7 to 51 mW. Additionally, measurements were done before and after sacrificing the animal to examine the influence of ocular blood flow on local temperature in rabbits (cooling effect). Results: The temperature depended linearly on the IR power irradiated onto the IR-receiver with a factor of 1.3 Kelvin (K) (in the dead pig eye) and 0.73 K (in the living rabbit eye) per 10 mW laser power. A 0.82 K/10mW temperature increase in the rabbit eye was measured after sacrificing the animal, with circulation disrupted. The relative cooling effect due to ocular blood flow in the rabbit eye was calculated to be approximately 11 %. The irradiance of the scattering radiation at the position of the stimulation electrodes showed a linear correlation: 0.053% of the IR irradiation flux density measured at the site of the IR receiver (ex vivo). Conclusions: A temperature increase of approximately 1.78 K in the living rabbit eye is expected when powering a subretinal implant by 24.4 mW IR laser, which is the necessary external power supply for the active implant. This would cause a tolerable increase for eye tissues in the living rabbit. The scattering radiation may restrict the light stimulation dynamics of active implant, that is sensitive to IR irradiation as well as visible light.

Keywords: retina • vitreoretinal surgery • choroid