Amorphous silicon resistors enable smaller pixels in photovoltaic retinal prosthesis

Area of Science:

• Biomedical Engineering
• Materials Science
• Ophthalmology

Background:

• The PRIMA subretinal prosthesis demonstrated feasibility for prosthetic central vision.
• Improving prosthetic visual acuity requires decreasing pixel size below 100µm.
• Standard resistor materials are incompatible with small photovoltaic pixels, hindering miniaturization.

Purpose of the Study:

• To develop and integrate novel shunt resistors into photovoltaic arrays for smaller pixel sizes.
• To evaluate the impact of these shunt resistors on electrode discharge time and electric field contrast.
• To assess the compatibility of the fabrication process with existing photovoltaic array manufacturing.

Main Methods:

• Fabricated miniature shunt resistors using doped amorphous silicon (a-Si) with controlled resistivity (Ω*cm).
• Deposited a-Si via low-pressure chemical vapor deposition and encapsulated resistors with SiO2 and SiC for in-vivo stability.
• Integrated resistors into photovoltaic arrays with pixel sizes down to 20µm and tested photoresponsivity and discharge characteristics.

Main Results:

• Successfully fabricated high-resolution retinal implants with integrated shunt resistors (0.75–4 MΩ) in pixels as small as 20µm.
• Maintained high photoresponsivity (0.53 A/W) comparable to arrays without shunts.
• Shunt resistors significantly reduced electrode discharge time and increased spatial contrast (Landolt C pattern) at video frame rates.

Conclusions:

• Doped a-Si enables fabrication of miniature MΩ shunt resistors compatible with photovoltaic array manufacturing.
• Integrated shunt resistors enhance current injection and spatial contrast, crucial for improving visual acuity in prosthetic vision.
• These advancements are critical for scaling down pixel sizes below 100µm for next-generation retinal implants.