Purchase this article with an account.
J. Loudin, D. Simanovskii, K. Vijayraghavan, C. Sramek, A. Butterwick, P. Huie, G. McLean, D. Palanker; Optoelectronic Prosthesis: System Design and Performance. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2552.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
We have previously described the design of a high-resolution retinal prosthetic system based upon a subretinal photodiode array which converts images projected by IR goggles into pulsed electric current. We now present the details and characterization of the system.
Optoelectronic performance of a single pixel was evaluated in several modes of operation by using a photodiode and IrOx electrode. The electric field in front of an electrode array with local and distant returns was calculated numerically. Prototypes of the IR projection system and RF power delivery system have been developed.
Actively biasing the photodiodes increases electrode charge injection (versus a passive system) by a factor of 4 for anodic- and by a factor of 40 for cathodic-first pulses. Stimulation pulse polarity can be changed by operating either with continuous illumination and a pulsed "readout" bias, or with pulsed illumination. Photodiode conversion efficiency was 0.3 A/W, providing stimulation currents of up to 20 µA (corresponding to a charge injection of about 0.8 mC/cm2) on a 40 µm IrOx electrode. With 0.5 ms pulses at 50 Hz, this requires 70 µW maximum peak light power and ~0.6 µW average light power per pixel. Thus, a 3 mm implant containing 640 pixels, each a square that is 100 µm in size, requires up to 45 mW of maximum peak power and ~0.4 mW of average light power, with ~1 mW average electrical power. The electrical system has greater than 30 dB linear dynamic range. However, the system’s dynamic range is limited by the IR projection system, which has a dynamic range of about 20 dB. Significant interference from neighboring electrodes was observed in a system with a return electrode at infinity, leading to decreased resolution and charge injection. This can be eliminated by introducing local returns; however, stronger confinement of electric field requires better proximity to the target cells. This may be achieved by using pillar electrodes.
A photodiode-array-based prosthesis that produces stimulation pulses controlled by local light intensity and enhanced by a pulsed bias can provide sufficient current and dynamic range for retinal stimulation, while remaining within safe electrochemical and thermal limits. Since visual information is processed externally and projected to all pixels simultaneously, the retinal prosthesis is compact, versatile, consumes low power, and maintains the natural link between image perception and eye movements.
This PDF is available to Subscribers Only