Potential tissue damage from transcutaneous recharge of neuromodulation implants

The magnetic-field-driven heat generation in neuromodulation systems consisting of implanted and skin-surface-mounted components gives rise to the potential of discomfort, cell damage, and possible necrosis. The skin-surface-mounted component, commonly termed the antenna, serves the function of the primary of a transformer, and the implant is the secondary. Heating occurs in both of these components during the recharging of a battery situated in the implant. Previously reported experimental data for the heat generation characteristics of three commercially available neurostimulation systems has been enhanced by further experiments carried out as part of this investigation. A numerical simulation of the temperature distribution in tissue beds adjacent to the implant and the antenna has been performed here. The aggregated data have been used as input information to a bio-heat-transfer model which yields both the spatial and temporal variations of the temperature field. It was found that during long-duration recharging periods, the temperature of the tissue rises in response to the heat generation. This information enables the identification of the magnitude and location in the tissue of the hot-spot temperature. The temporal temperature variation at the hot spot was employed in conjunction with a tissue-damage integral to identify the possibility of cell damage and/or necrosis. It was found that two of the three investigated neuromodulation systems did not give rise to temperature levels that may cause tissue damage. However, the third of the systems caused temperatures of sufficient elevation so that for recharging periods on the order of 2 h, necrosis was found to be likely in situations where heat transfer is suppressed at the surface of the skin.