This interesting paper (click for the PDF version, available online), published in 1999 by Townsend et al., presents an instrumented tibial baseplate for in-vivo measurements of knee tibiofemoral forces. A multichannel telemetry system is combined with strain gauges; the whole system is alimented wirelessly by means of magnetic induction.
Basically, electrical energy is
transferred from a primary external coil, properly positioned around the patient’s leg during the medical check, to a secondary coil, smaller and embedded in the tibial tray stem. Approximately 40 mW of power are transferred and supply the whole telemetry system, which can transmit data to an external receiver within a range of 3-5 meters.
Complete reference:
Townsend, C.P., and Arms, S.W., Hamel, M.J., ”Remotely Powered, Multichannel, Microprocessor Based Telemetry Systems for a Smart Implantable Total Knee Implant”, SPIE’s 6th Annual Int’l conference, Date: Mar. 1999.
andreacollo 
So basically the power transfer is done by electrical induction??? If so, any chance you know how the much interference.is caused by the skinand tissue??
actually it is like this: current in the external coil -> magnetic field is created around it ))–knee–(( the internal coil “feels” the magnetic field and produces one itself -> current flows in the internal coil and is harvested by the electronics.
As far as I understood, with proper frequencies there is no risk of having a barbecue with patient’s leg. And there are so many studies like that, it’s amazing even if the efficiency of the power transmission is pretty ridicolous. In fact, you can power low-power consumption devices… but (unfortunately) not a micromotor, for example…
Powelectrics supply equipment and telemetry solutions for monitoring, control and automation to a wide range of industries.