Self-tuning inductive powering for implantable telemetric monitoring systems

Inductive powering of implantable monitoring devices is a widely accepted solution for replacing implanted batteries. However, this method demands technically skilled operators to tune constantly the power link parameters. This becomes particularly important when the external system must also be portable. However, acceptable lifetimes for the external batteries can be obtained by a precise optimization of the power efficiency of the inductive link. In this paper, a technique is presented to render such a portable system fully compatible with a hospital environment. It is implemented in an independent, automatic and user-friendly powering set-up to avoid the need for engineers or additional electronic equipment for optimization of the link parameters. Multiple automatic tuning loops are incorporated to handle component tolerances and coil movements, and to optimize the power efficiency on a continuous basis. A visual and an acoustic indicator give the operator an additional feedback on the quality of the established link. As an example, the problem of mechanical strain sensing in femoral nailplates is given. It is representative for the envisaged range of applications. These are characterized by a low coil coupling (k less than 1%) and power transfers of the order of 20 mW.