Human tissue temperatures achieved during recharging of new-generation neuromodulation devices

The advent of rechargeable biomedical implants for neuromodulation has introduced the practice of recharging implantable batteries through a patient's skin. Long-term operation of such implants is achieved by periodically recharging the implant's battery by means of a magnetic field produced by an antenna situated on the surface of the skin. During recharging periods, heat is generated within both the implant and the antenna. The heat flowing from these components into adjacent tissue creates the possibility of tissue temperature elevations that may be unsafe. This issue was investigated by means of a synergistic combination of experimentation and numerical simulation. The experiments measured the rates at which heat generated within the components flowed into their respective surroundings. This information was utilized as input information to the numerical simulations. The simulation model consisted of four tissue layers plus the skin-surface-mounted antenna. Two realistic external thermal environments were considered for the simulations. Both the experimentation and the simulations were performed for three leading neuromodulation devices: Precision Plus, Eon Mini, and Restore Ultra. In the presence of convective/radiative heat losses in a 20 °C environment, the maximum tissue temperature during recharging never exceeded 39 °C for the Restore Ultra, but exceeded 41 °C for the Precision Plus and the Eon Mini. In an adiabatic environment, similar findings were observed; the temperatures associated with the Precision Plus and Eon Mini exceeded 41 °C, while the tissue temperatures near the Restore Ultra maintained values less than 41 °C. The work reported here deals with devices that have not previously been investigated.