An implantable photovoltaic energy harvesting system with skin optical analysis

Medical implantable devices can use photovoltaic (PV) energy harvesting to extend battery life span and increase their performance. The power conditioning and management circuitry is essential not only to regulate the voltage requirements of the load but also optimize the output power of PV cells. However, the optical losses due to the skin and the device characteristics of the PV cells are rarely analyzed before chip fabrication. This inevitably leads to sub-optimal system performance in in-vitro or in-vivo tests owing to the varying PV output characteristics. To address this problem, we use the finite-element-method (FEM) to analyze the optical and physical performance of the PV cell under the skin, and then export the model into the p-spice simulator for circuit-level implementation. We further demonstrate a 1:2 cross-coupled DC-DC converter using pulse density modulation for load regulation control to meet the loading requirement. In this work, the PV cell can achieve an 18% of efficiency, and the power conditioning circuit can provide an 84% of end-to-end efficiency.