Practical Problems of Controlling Medical Implants via Radio Links

An increasing number of stimulators implanted for medical purposes are powered and controlled via transdermal radio links. The implanted electronics may be kept relatively simple and hence reliable, while the more complex control circuits, being external, can remain accessible for servicing or alteration. The external circuitry needs little protection from moisture, unlike implants. However, some of the problems involved in implants are not entirely removed when the bulk of the electronics is external. The power supply remains a problem principally because the power transfer efficiency between two RF coils separated by 1–2 cm of flesh is generally in the range 10–30 percent. Pulse generators and other control circuits must consume current sparingly. With careful circuit design and NiCd rechargeable batteries it has been possible to produce stimulation controllers that are quite acceptably compact for patients to carry around. The devices must still be reliable – indeed – rugged. As it is impracticable to have transmitters and control circuits in the same box, highly durable cabling must connect between them. Some solutions to these problems are illustrated by the equipment designed to control implants for cerebeller stimulation for the treatment of some epileptics and also implants for the stimulation of the cauda equina of paraplegics, enabling them to stand on their own feet.