Wireless Powering of Implanted Sensors using RF Inductive Coupling

Radio frequency inductive links are becoming extensively used for telemetry and wireless powering of implanted devices such as biomedical sensors and RFID implants. The design of RF coils for such links is often empirical and non-optimal. Based on loop antenna theory, this paper presents an analytical model for near-field magnetic coupling incorporating misalignment of the RF coil system. Formulae are derived for the magnetic field at the receiver coil when it is laterally and angularly misaligned from the transmitter. For the first time a near-field power transfer formula is suggested incorporating coil characteristics and misalignments. This novel power transfer function allows a comparison between different coil structures such short solenoids, with air or ferromagnetic core, and spirals with respect to the power delivered at the implant. It is also possible to define the maximum degree of misalignment permissible in a given application. This analysis allows a formal design procedure to be established in order to optimise wireless power transfer for a given application.