Transcutaneous Recharge: A Comparison of Numerical Simulation to In Vivo Experiments

Numerical simulation and animal experiments quantified tissue temperatures during the transcutaneous recharge of neuromodulation implants. The temperature results were used to determine the likelihood of tissue injury in humans.

Experiments were completed using sheep with implants at different depths ranging from 0.7 to 2.15 cm. The calculations were replicates of the experiments. Additional calculations were completed for laterally offset implants (up to 2 cm). Benchtop tests were performed to determine the power dissipation in the components. These power dissipation rates were inputs to the simulation. The now-verified model was next applied to a human situation with a core temperature of 37°C.

There was excellent agreement between the simulations and the animal-model for all depths; the experimental and simulated temperatures near the implant were almost identical. The results were negligibly affected by a misalignment of the implant. The maximum experimental temperatures in the sheep were 41.8, 43.2, and 41.8°C while the calculated maxima were 41.9, 43.3, and 41.2°C for the shallow, medium, and deep cases, respectively. The experimental values are 3.1, 4.5, and 3.1°C above the body core temperature. The simulation results are 3.2, 4.6, and 2.5°C above the core temperature. The model was then applied to a human situation with a body core temperature of 37°C. The maximum values of the simulated temperatures are 39.9, 41.2, and 39.1°C. The cumulative equivalent exposure at 43°C (CEM43) for these three implant depths are 0.30, 0.88, and 0.12 min. These thermal exposures are below those known to cause thermal injury in human skin tissue.

The numerical simulation predicts tissue temperatures during transcutaneous recharge of implants. Results show that the implant depth does not have a large impact on the tissue temperatures and thermal exposures are sufficiently low so that they are unlikely to have any physiologic consequence.