Power budget and tissue heating

The single hardest ceiling on an implant is that all the power it dissipates ends up as heat in the brain, whose temperature must not rise more than a small fraction of a degree — a widely cited working guideline is on the order of one to two degrees Celsius, with many arguing for keeping the rise below one degree. Heat spreads and clears according to the Pennes bioheat equation, in which blood perfusion and thermal conduction remove the deposited power, so a given power raises temperature more where perfusion is poor.

Practical design therefore also respects surface power-density guidelines — a historically cited figure is on the order of 40 mW per square centimetre of implant surface — and, for wirelessly powered devices, the regulatory specific-absorption-rate limits on the RF field itself. The total to be dissipated is the sum over channels of amplifier plus ADC plus digital power, plus any on-chip compute for detection and compression, plus the telemetry transmitter, plus any stimulation.

This budget couples nearly every other hardware choice: front-end noise-efficiency, whether to compress on-chip, and how much data to transmit are all ultimately decided against the thermal ceiling. In high-channel-count implants it is heat, not silicon area or transistor count, that caps how many channels can run at once.