Investigation of optical wireless power transmission across biological tissue using NIR light considering safe irradiance level

Abstract

Presently, implantable electronic devices (IEDs) are becoming integral choices in medical applications, still their reliance on nonrechargeable batteries limits their longevity. This challenge has driven worldwide interest in developing rechargeable IEDs. While energy transfer based on radio-frequency (RF) has been clinically employed to address the challenge of IEDs running out of battery power, research into alternative methods, such as near-infrared (NIR) light-based energy transfer schemes, is advancing rapidly. The key advantages of optical systems are security, privacy, safety, interference-free operation, etc. The properties of NIR light are attractive for optical wireless power transmission (OWPT) across biological tissue due to minimal scattering and absorption effect. NIR light has been widely employed for various therapeutic applications, such as photobiomodulation (PBM) therapy. When using power transfer, ensuring that do not exceed safety regulations to avoid damage to the tissue is important. In this study, we investigate OWPT under safe level irradiance referring to the typical PBM applications, i.e., 20 mW/cm², 25 mW/cm², and 200 mW/cm². The OWPT system is tested on an experimental testbed, consisting of an 810 nm NIR LED and a photovoltaic (PV) cell as transmitter and receiver, respectively. A tissue-mimicking optical phantom was employed as a propagation media that has long-lasting stability at a room temperature around the testbed, thus suitable for prolonged experiments. We measure the time it takes to charge an energy-storing supercapacitor under three different safe irradiance levels to assess the feasibility of safely charging an IED’s storage. In addition, we compare the results with the maximum irradiance of NIR LED, i.e., 527 mW/cm². Comparing 527 mW/cm² to PBM irradiance revealed a trade-off between supercapacitor charging time and tissue safety. Charging limitations and other related issues are also elaborated in this paper.