Background and objective: To study the heat and power dissipation effect of anintraocular electronic heater on the retina. The determination of thermal parameters that are nonharmful to the retina will aid in the development of an implantable intraocular electronic retinal prosthesis.
Materials and methods: In dogs, five different retinal areas were touched with a custom intraocular heater probe (1.4 x 1.4 x 1.0 mm) for 1 second while the heater dissipated 0 (control), 10, 20, 50, or 100 mW. In a second protocol, the heater was mechanically held in the vitreous cavity while dissipating 500 mW for 2 hours while monitoring intraocular temperature. The animals were observed for 4 weeks with serial fundus photography and electroretinography. The procedure was then repeated in the fellow eye. The dogs were killed and both eyes were enucleated and submitted for histology.
Results: In experiments using protocol 1, heater settings of 50 mW or higher caused an immediate visible whitening of the retinal tissue. Histologically, this damage was evident only if the eyeswere immediately enucleated. Permanent damage was caused by heater settings of 100 mW or higher. Under protocol 2, no ophthalmologic, electroretinography, or histologic differences were noted between the groups. Temperature increases of 5 degrees C in the vitreous and 2 degrees C near the retina were noted.
Conclusions: The liquid environment of the eye acts as a heat sink that is capable of dissipating a significant amount of power. An electronic chip positioned away from the retina can run at considerably higher powers than a chip positioned on the retinal surface.