SAR-Constrained Wireless Power Transfer Modeling for an Implantable Optical Neurostimulator Sensors

This study investigates the optimal operating conditions for an implantable photonic stimulation device, focusing on energy delivery efficiency and electromagnetic safety in biological tissue. COMSOL Multiphysics simulations were conducted to evaluate key light source parameters, including wavelength, output power, and incident angle. A transmitting RF coil was designed at a 1.35 MHz resonance frequency for wireless power transfer (WPT), and its resonant characteristics were analyzed using inductance and capacitance values. Specific Absorption Rate (SAR) simulations were performed with a 10 g hemispherical averaging region following international safety standards. Results showed that light absorption was maximized in the cerebellum and cerebrospinal fluid at a wavelength of 660 nm, with a 20° incident angle enabling the deepest tissue penetration. In vascular reflectance analysis, 660 nm wavelength produced the largest reflectance variation (∆R) across cardiac cycles and the lowest overall reflectance, indicating its suitability for optical biosignal detection and neural stimulation. SAR analysis demonstrated an average value of 0.0074 W/kg and a peak value of 0.82 W/kg, both substantially below the 2 W/kg safety threshold. These findings confirm that the proposed device design meets optical performance and biocompatibility requirements, highlighting its potential as a next-generation platform for precision phototherapy and future neurotherapeutic applications.