Feasibility Assessment of an Optically Powered Digital Retinal Prosthesis Architecture for Retinal Ganglion Cell Stimulation

TL;DR

This work demonstrates a wireless, optically powered retinal prosthesis architecture that transmits both power and data through the pupil to a digital stimulation controller, enabling high-rate sequential stimulation via a 288-electrode diamond array. Using a near-infrared multi-junction photovoltaic cell and a CMOS stimulator ASIC, the system delivers charge-balanced pulses while monitoring electrode impedance, all powered from an $850\text{ nm}$ laser within the pupil safety limit of $4.06\ \,\mathrm{mW/mm^2}$. Calcium imaging in degenerate rat retinas shows retinal ganglion cells respond to infrared-powered stimulation, with the capability to generate up to $35\,000$ pulses per second at the average threshold, validating the feasibility of the digital, optically powered approach. The results highlight potential improvements in safety and reliability from a hermetically encapsulated, wireless implant, while identifying practical limitations and future directions toward closed-loop, high-density epiretinal prostheses.

Abstract

Clinical trials previously demonstrated the notable capacity to elicit visual percepts in blind patients affected with retinal diseases by electrically stimulating the remaining neurons on the retina. However, these implants restored very limited visual acuity and required transcutaneous cables traversing the eyeball, leading to reduced reliability and complex surgery with high postoperative infection risks. To overcome the limitations imposed by cables, a retinal implant architecture in which near-infrared illumination carries both power and data through the pupil to a digital stimulation controller is presented. A high efficiency multi-junction photovoltaic cell transduces the optical power to a CMOS stimulator capable of delivering flexible interleaved sequential stimulation through a diamond microelectrode array. To demonstrate the capacity to elicit a neural response with this approach while complying with the optical irradiance limit at the pupil, fluorescence imaging with a calcium indicator is used on a degenerate rat retina. The power delivered by the laser at the permissible irradiance of 4 mW/mm2 at 850 nm is shown to be sufficient to both power the stimulator ASIC and elicit a response in retinal ganglion cells (RGCs), with the ability to generate of up to 35 000 pulses per second at the average stimulation threshold. This confirms the feasibility of generating a response in RGCs with an infrared-powered digital architecture capable of delivering complex sequential stimulation patterns at high repetition rates, albeit with some limitations.

Figures (13)

• Figure 1: Implant power and data delivery architecture. A near-infrared beam is sent through the pupil. A multi-junction photovoltaic cell captures the infrared light to power a CMOS stimulator ASIC and a photodiode recovers the data from the modulated laser beam. The ASIC delivers the stimulation through an ultrananocrystalline diamond substrate with conductive diamond electrodes.
• Figure 2: The photovoltaic cell connects to the power recovery block to capacitor C1 to ensure stability. The power recovery module linearly regulates the voltage to 3 V for the electrode drivers and 1.2 V for digital circuits. The clock recovery circuits provides a 935 kHz clock to the digital stimulation controller and a 37.4 MHz clock to the data recovery to oversample the Manchester-encoded data. From this recovered data, the digital stimulator ASIC sequences the pulse train for the electrode driver. The electrode monitor records the voltage at the output of any electrode driver and sends it out through the custom 2.4 GHz RF transmitter.
• Figure 3: Conceptual representation of the lower layer of the implant. The stimulator ASIC is assembled on the diamond substrate with solder bumps to connect to each of the 288 electrodes. The ASIC-diamond substrate assembly process is still under development.
• Figure 4: Conceptual representation of the implant stack. The photovoltaic cell, photodiode, crystal oscillator and RF transmitter are assembled on a 2-layer FR4 printed circuit board. A copper trace antenna surrounds the components. The printed circuit board is assembled on the diamond substrate (Figure \ref{['fig:Diamond_and_ASIC']}).
• Figure 5: To validate the implant powering method using laser illumination, an apparatus was designed to facilitate calcium imaging where the implant components are assembled on a printed circuit board. A 35 mW, 850 nm laser powers the implant. A cable connects the implant to a 5 $\times$ 5 electrode array. A degenerate rat retina stained with a calcium indicator is placed on the electrode array with retinal ganglion cells facing up. The RGCs's response is evaluated by measuring rapid fluorescence variations with a confocal microscope.