Inductive wireless power transfer

Inductive power transfer supplies energy to an implant through near-field magnetic coupling between an external coil worn on the skin and an implanted coil, forming a weakly-coupled resonant transformer. It lets a fully sealed implant run without transcutaneous power wires and either without a battery or with a rechargeable cell, at the cost of requiring the external coil to be present and roughly aligned during operation.

Link efficiency is governed by the coupling coefficient between the coils and their quality factors, summarised by a figure of merit proportional to coupling squared times the product of the two quality factors; typical carriers run in the low-megahertz to roughly ten-megahertz range. The same link usually carries a downlink to the implant by load-shift keying and an uplink from it by backscatter, so power and data share the coil.

The delivered power is not free to increase: it is capped by the specific-absorption-rate and heating limits on tissue exposed to the field, and by coil size, which competes with the goal of a small implant. Alternatives that address depth or alignment include midfield RF, ultrasonic power for deep small motes, and photovoltaic powering of subcutaneous devices.