Transcutaneous devices: design for operation in alternating magnetic fields

Many implanted cochlear and other stimulators receive power and signals by magnetic induction and are called transcutaneous devices. The transmitter produces an alternating magnetic field around the primary coil which is placed against the skin opposite the implanted receiver or secondary coil. The magnetic field will induce currents into any other conductive loop in the vicinity and clearly it is important that no such tertiary loop significantly impairs the performance of the transcutaneous link. It has been a casual observation, when developing transcutaneous links, that metal sealing rings, even when tightly-coupled to the receiver coil, have surprisingly little effect. The author presents work to understand how this is, in order to be able to allow for the tertiary in the design of the link