Development of a Telemetry System for the Boston Retinal Implant

Abstract

Purpose: : The prototype Boston Retinal Implant receives power and image data via an inductive telemetry link. Our original system used a class E amplifier for power transmission and a class A amplifier for data. These worked in bench testing, but created interference when tested with our implant chip. Revised transmitters allow more robust power and data transmission to the implant. With further testing, we will show that this link is reliable enough to allow implant movement with respect to the transmitter coils.

Methods: : Circuits were modeled in Spice and built on protoboards, then on printed circuit boards. An operational amplifier follower was used for the data transmitter, with coil inductance tuned by a series capacitor, and a resistor added to reduce the resonant quality factor (Q). This low Q circuit reduces overshoot and ringing on the modulation edges, which had been misinterpreted by the sensitive data receiver. A class D amplifier was used to transmit power, with the coil inductance tuned by a low resistance series capacitor for high Q and efficiency. This circuit is more robust than the previously used class E, and switching filters reduce noise which might interfere with transmission to the data receiver. The two transmitters were tested independently, with a bench version of our chip, and with the assembled implant.

Results: : Tests of our revised transmitters showed improved stability and reduced interference from the power transmitter. This transmitter drives over 500 mA through our power primary coil at 125 KHz, supplying power at a separation of 15 mm, consistent with our implant requirements in the human eye. The revised data transmitter eliminated ringing, increased thermal stability, and transmitted amplitude–shift–keyed (ASK) data at up to 100 Kbits per second on a 5 MHz carrier at a separation of 15 mm. Tests using a bench version of our chip achieved wireless power and data on spatially separate links, driving electrodes in buffered saline solution. We are now testing the transmitters with assembled implants, achieving wireless power and data transmission in separate tests. Testing continues as we receive more assembled implants.

Conclusions: : Our power and data telemetry circuits are robust and stable, providing improved data reception. Having achieved wireless power and data transmission in our bench stimulator, we are now progressing toward a functional prototype retinal implant tested in vitro and in vivo.

Keywords: retina • electrophysiology: non-clinical • perception