Bioheat transfer model of transcutaneous spinal cord stimulation-induced temperature changes

Transcutaneous spinal cord stimulation (tSCS) has been extensively studied due to its promising application in motor function restoration. Many previous studies have explored both the essential mechanism of action and the methods for determining optimal stimulation parameters. In contrast, the bioheat transfer analysis of tSCS therapy has not been investigated to the same extent, despite widely existing, and being of great significance in assuring a stable and thermally safe treatment. In this paper, we concentrated on the thermal effects of tSCS using a finite element-based method. By coupling the electric field and bioheat field, systematic finite element simulations were performed on a human spinal cord model to survey the influence of anatomical structures, blood perfusion, and stimulation parameters on temperature changes for the first time. The results show that tSCS-induced temperature rise mainly occurs in the skin and fat layers and varies due to individual differences. The current density distribution along with the interactions of multiple biothermal effects synthetically determines the thermal status of the whole spinal cord model. Smaller stimulation electrodes have a higher risk of thermal damage when compared with larger electrodes. Increasing the stimulation intensity will result in more joule heat accumulation, hence an increase in the temperature. Among all configurations in this study that simulated the clinical tSCS protocols, the temperature rise could reach up to 9.4 °C on the skin surface depending on the stimulation parameters and tissue blood perfusion.