Analysis of electromagnetic fields induced in operation of a wireless fully passive backscattering neurorecording microsystem in emulated human head tissue

This paper reports on a fully passive microsystem that wirelessly records and transmits neuropotentials exclusively by means of electromagnetic backscattering techniques, affording substantially simpler circuitry and potentially safer and more reliable approach for implantable wireless neurorecording. A fundamental practical barrier for wireless brain-implantable microsystems includes heat dissipation by on-chip circuitry, which may cause permanent brain damage. Hence, measurement of thermal profiles of surrounding tissue induced by operation of wireless implants is imperative in assessing the safety of these devices. Evaluation of specific absorption rate (SAR) is especially relevant for wireless electromagnetic transmission schemes operating at microwave frequencies and directly relates to the heat generated within biological tissue media. In this study, computational and empirical methods are used to measure SAR within a human-head-equivalent phantom during operation of the embedded fully passive wireless neurorecording microsystem. The maximum average SAR, coinciding with the worst case scenario, measured within 1 g of brain tissue is < 0.45 ± 0.11 W/kg, complying with the U.S. FCC threshold (1.6 W/kg).