Closed-loop inductive link for wireless powering of a high density electrode array retinal prosthesis

Abstract

A retinal prosthesis intended for rehabilitation of vision impaired patients will require continuous power supply in order to achieve real-time moving images. In this work, we explore the use of inductive coils fabricated using flexible circuit technologies for inductive powering of the implanted prosthetic device. We found that manufacturing technologies dictate the optimum operating frequency of the coil. For a minimum track width and spacing of 4 mils, the optimum frequency was found to be 2.9 MHz. We also looked at the distribution of electric and magnetic fields generated by the inductive coils in and surrounding the eye. These simulation results show that there are electric field concentrations around the conductive coils. Apart from the coils, we need to design an efficient circuit to drive the transmit coil and recover the transmitted power. In order to maintain optimal operation of the link, a closed-loop load modulation feedback operation is proposed. Adaptive control using back-telemetry of the induced voltage on the secondary side can close the power supply loop and result in optimum power transfer by boosting the supply voltage on the primary side when load is high and reducing this voltage when load is small.

Physics-Based Modeling of Ferroelectric Hysteresis for Ceramic Capacitors in Inductively Coupled Microstimulators

“…In the target application, the microstimulator circuit will be inductively powered by a battery-operated wearable. Since the supply voltage is limited, a series resonant circuit is advantageous because it requires smaller voltage swings at its input as the phase of the inductor and capacitor voltages cancel at resonance [50]. A parallel resonant circuit on the microstimulator side amplifies the induced voltage and helps to overcome the turnon voltage of the rectifier diode, which is particularly advantageous at low inductive coupling factors [50].…”

Adaptive Transcutaneous Power Transfer to Implantable Devices: A State of the Art Review

“…Ng et al [ 14 , 41 ] proposed a system to compensate for eye movements and fibrous growth in retinal implants, regulating the supply voltage at the transmitter based on feedback on the voltage at the secondary, communicated through back telemetry. Adjusting the transmitter resonant frequency for maximum efficiency by tuning the transmitter capacitance was also proposed, based on voltage at the receiver.…”

“…Some systems employ flexible antennas on the tissue surface, where movement can change the antenna geometry as well as the alignment [ 40 ]. In retinal prostheses, the implant antenna is in near constant movement due to its location in the eye [ 41 ]. Migration of an implant can occur after implantation, resulting in uncertainty of the implant location and potential misalignment of the antennas [ 42 ].…”

“…For uncontrolled exposures of the general population, the U.S. Federal Communications Commission (FCC) limits SAR to 1.6 W/kg averaged over any 1 g cube of tissue [ 61 ]. In addition to dielectric losses, heating can also occur due to heat dissipation of the receiver circuitry, sometimes exceeding the one-degree Celsius temperature increase that is a basis for SAR limits [ 41 , 62 , 63 ].…”

“…Designing antennas with inherent resonance can avoid losses associated with an added matching network. Operating transmit and receive antennas at the same resonant frequency increases the voltage gain between the antennas [ 13 , 41 ].…”