A variable range bi-phasic current stimulus driver circuitry for an implantable retinal prosthetic device

Abstract

This paper reports a driver circuitry to generate bi-phasic (anodic and cathodic) current pulses for stimulating the retinal layer through electrodes which is part of a retinal prosthetic device for implants in blind patients affected by retinitis pigmentosa (RP) and age-related macular degeneration (AMD). Dual voltage architecture is used to halve the number of interface leads from the chip to the stimulation sites compared to a single voltage supply. The driver circuitry is designed to deliver currents with six bit resolution for a wide range of full scale currents up to 600 A. To cater to the varying stimulus requirements among patients and different regions of the retina, variable gain architecture is used to achieve fine resolution even for a narrow range of stimulus. 1:8 demultiplexing feature is embedded within the output stage thus allowing one DAC for eight outputs. A novel charge cancellation circuitry with current limiting capability is implemented to discharge the electrodes for medical safety.Measurement results of a prototype chip fabricated in 1.5-m CMOS technology are presented.

Silicon-Integrated High-Density Electrocortical Interfaces

“…Instead of external capacitors, capacitive electrodes made with high-k dielectric coatings have been investigated for safe neural interfaces [88], [100], [101]. Several other techniques for better charge balancing have been demonstrated: 1) shorting electrodes to ground [102]; and 2) utilizing a discharging resistor [94], active current balancing by feedback control [103], [104], generating additional balancing current pulses by monitoring electrode voltages [105], and embedded DAC calibration [93], [106].…”

On the cause and control of residual voltage generated by electrical stimulation of neural tissue

“…Charge build-up on the electrode (which results in a net residual voltage) is often attributed to mismatch errors in charge-balanced waveforms due to process variations in integrated circuit manufacturing technology [12], [13], [8], but is not the only cause for residual voltage. The transient output voltage for a typical biphasic current stimulus has been derived to illustrate the theoretical existence of a non-zero residual voltage due to R w .…”

Implantable CMOS neuro-stimulus chip for visual prosthesis

“…A number of implantable visual prostheses for outer retinal degeneration diseases are reported [5][6][7][8][9][10][11][12]. According to their implant locations, these prostheses are named sub-retinal, epi-retinal or optic nerve implants.…”