Thermal and power-budget limits
Neural tissue tolerates only small temperature rises, with guidelines commonly citing on the order of one degree Celsius, and power-dissipation limits on the order of tens of milliwatts per square centimeter, before risking damage. Every recorded channel, amplifier, on-implant computation, and wireless transmission generates heat that the brain's perfusion must carry away. This makes the thermal budget, not silicon area, the true constraint on scaling channel count and on-device intelligence, and it is why neuromorphic, event-driven, in-memory computing is pursued for implants.
Powering the device compounds the problem, since batteries are bulky and finite, and wireless power, whether radio-frequency, ultrasonic, or inductive, deposits energy as heat with modest efficiency. There is also a thermodynamic floor: the Landauer limit places an absolute, if astronomically small, energy cost on erasing each bit. In practice real circuits run many orders of magnitude above it, so the binding limit for the foreseeable future is heat removal, not fundamental thermodynamics.
The commonly cited limits, about a degree of heating and tens of milliwatts per square centimeter, are conservative safety guidelines, not sharp physical thresholds; the true damage boundary is duration- and location-dependent and remains an active research question.