Comparative study of broadband and narrowband near-infrared LEDs for optical wireless power transmission to implantable electronic devices across biological tissue

Most current implantable electronic devices (IEDs) depend on finite-lifetime batteries whose depletion frequently requires surgical replacement. Near-infrared (NIR)-based optical wireless power transfer (OWPT) exploiting the “optical window” offers a promising method for wirelessly recharging IEDs. Photovoltaic (PV) cells are typically employed at the OWPT’s receiver end and can produce direct DC output at high power density. Most silicon-based PV cells, however, are optimized for the broad-spectrum AM1.5 solar reference rather than for narrowband NIR illumination. A broadband NIR LED is a controlled light source whose emission spans a wider portion of the NIR spectrum, which is expected to improve the PV cell’s spectral response and thereby enhance OWPT performance. This study aims to assess the performance of an OWPT on a tissue-mimicking optical phantom using broadband versus narrowband NIR LED sources (810 nm and 850 nm) for IEDs in two scenarios. First, each NIR LED source was calibrated to its wavelength-specific safety-limited irradiance, i.e., ∼418 mW/cm 2 , 332 mW/cm 2 , and 399 mW/cm 2 for broadband (at 860 nm reference), 810 nm, and 850 nm, respectively. Second, the irradiance of each NIR LED is matched (∼250 mW/cm 2 ) to isolate the spectral coupling factor. The experiments were performed using a vertical 3D-printed black-box testbed that housed the mounted NIR LED transmitter, a 40 mm-thick soft-tissue-mimicking optical phantom, and a commercial silicon-based PV energy harvester, all aligned consistently. Specifically, we repurposed small, rigid, non-implantable PV cells optimized for the solar spectrum in our system for test purposes. Performance was evaluated based on supercapacitor charging duration and energy-harvesting rate (the stored energy in the supercapacitor divided by the charging duration). In the experiment, a 0.16 F-rated supercapacitor was charged from 3 V to 4 V under the control of a power management integrated circuit. Although broadband NIR illumination was expected to improve spectral coupling to the silicon-based PV receiver, the optical-phantom results show that distributing optical power across the 770, 860, and 940 nm emission bands did not improve end-to-end wireless-charging performance compared with a well-selected narrowband source.