Feasibility Study on Active Back Telemetry and Power Transmission Through an Inductive Link for Millimeter-Sized Biomedical Implants

This paper presents a feasibility study of wireless power and data transmission through an inductive link to a 1-mm 2 implant, to be used as a free-floating neural probe, distributed across a brain area of interest. The proposed structure utilizes a four-coil inductive link for back telemetry, shared with a three-coil link for wireless power transmission. We propose a design procedure for geometrical optimization of the inductive link in terms of power transmission efficiency (PTE) considering specific absorption rate and data rate. We have designed a low-power pulse-based active data transmission circuit and characterized performance of the proposed inductive link in terms of its data rate and bit error rate (BER). The 1-mm 2 data-Tx/power-Rx coil is implemented using insulated bonding wire with $\text{25}{\text{-}}\mu {\text{m}}$ diameter, resulting in measured PTE in tissue media of 2.01% at 131 MHz and 1.8-cm coil separation distance when the resonator coil inner radius is 1 cm. The measured BER at 1-Mbps data rate was $\text{2.7}\times \text{10}^{-6}$ and $\text{5}\times \text{10}^{-6}$ in the air and tissue environments, respectively.