Implanted medical devices are rapidly becoming ubiquitous. They are used in a wide variety of medical conditions such as pacemakers for cardiac arrhythmia, cochlear implants for deafness, deep-brain stimulators for Parkinson's disease, spinal-cord stimulators for the control of pain, and preliminary retinal implants for blindness. They are being actively researched in brain-machine interfaces for paralysis, epilepsy, stroke, and blindness. In the future, there will undoubtedly be electronically controlled drug-releasing implants for a wide variety of hormonal, autoimmune, and carcinogenic disorders. All such devices need to be small and operate with low power to make chronic and portable medical implants possible. They are most often powered by inductive radio-frequency (RF) links to avoid the need for implanted batteries, which can potentially lose all their charge or necessitate re-surgery if they need to be replaced. Even when such devices have implanted batteries or ultra-capacitors, an increasing trend in upcoming fully implanted systems, wireless recharging of the battery or ultra-capacitor through RF links is periodically necessary.

Figure 16.1 shows the basic structure of an inductive power link system for an example implant. An RF power amplifier drives a primary RF coil which sends power inductively across the skin of the patient to a secondary RF coil.