SPLTRAK Abstract Submission
GaAs-Based Photovoltaic Infrared Energy Harvesting for Microscale Biomedical Implants
Yi Sun1, Joseph Letner2, Jungho Lee1, Nuha Ahmed3, Cynthia Chestek2, David Blaauw1, Jamie Phillips1,3
1Electrical Engineering and Computer Science Department, University of Michigan, Ann Arbor, MI, United States
/2Biomedical Engineering Department, University of Michigan, Ann Arbor, MI, United States
/3Electrical and Computer Engineering Department, University of Delaware, Newark, DE, United States

Photovoltaics offer a pathway for efficient energy harvesting and wireless power transfer for microscale biomedical implantable devices. In comparison to techniques such as radiofrequency and ultrasound, optical devices are highly scalable to the sub-millimeter scale, since photovoltaic devices only have a weak dependence of power conversion efficiency with decreasing cell area. Further, the near-infrared region provides a key optical transparency window for tissue that enables wireless links that can provide sufficient energy to power bio-implantable devices at tissue depths ranging from millimeters to a centimeter. GaAs-based photovoltaic cells provide high power conversion efficiency that is well matched to a high transparency window for biological tissue. The high optical absorption and conversion efficiency can provide high power density at small form factor for scaled bio-implantable devices, and in combination with appropriate biocompatible packaging, provides a key technology for future biomedical applications. In this work, near-infrared power conversion in tandem microscale GaAs photovoltaic cells is explored to establish near-surface bio-implantable energy harvesting capabilities. The tandem photovoltaic devices consist of two current-matched GaAs junctions optimized for 850 nm monochromatic illumination with a cell area of 0.04 mm2. Cell characteristics are measured over a range of infrared (850 nm) irradiance through biospecimens of mouse skins at 0.6- and 0.8-mm thickness. The chosen biospecimens incorporate the epidermis, where the highest degree of optical absorption occurs that would limit subcutaneous energy harvesting. Based on an approximate minimum power density requirement of 50 nW/mm2 to power bio-implantable devices, sufficient energy harvesting is achieved at infrared irradiance on the order of 1 µW/mm2, which is orders of magnitude below the 3.99 mW/mm2 maximum permissible exposure established by ANSI Z136.1. The tandem cell devices generate operating voltages of 0.7-1.2 V under these low irradiance conditions, which can be directly applied for low-power CMOS circuitry without need for voltage upconversion.