Case Western Reserve University

Heat Dissipation from Implantable Devices

For tetraplegic individuals, regaining upper extremity movement can significantly impact and improve quality of life. Functional electrical stimulation (FES), which can be used to restore voluntary movement in these patients, can lead to some functional recovery. For example, a recently developed fully implantable neuroprosthesis, developed at the FES center at Case Western Reserve University, electrically activates paralyzed muscles, enabling manipulation of objects, thereby enhancing one’s daily independence. This system is externally rechargeable and can accept an external wireless signal for control. Power is supplied using radio-frequency induction when the transmitter coil is place over the implanted device during the battery recharge period. While the minimization of external components is an attractive feature for the user, there is reason for concern that the implant may generate considerable heating during recharge, causing pain and/or cell necrosis. Moreover, the Food and Drug Administration (FDA) regulates that to maintain safe conditions, an implant cannot cause greater than a 2°C temperature rise in the body, and therefore knowledge of the temperatures in the vicinity of the implant during a battery recharge cycle as a function of power input is critical. For this work, computational and analytical investigations have been conducted to help determine to what degree the heat dissipation from the implant during battery recharge leads to a potential temperature rise in the surrounding skin/tissue layers. Since the implant will likely be placed either in the abdominal or thoracic areas, two separate cases were analyzed representative of two different people exhibiting drastically dissimilar subcutaneous (fat) layer thicknesses. A commercially available computational code, Fluent, was used to simulate, in two-dimensions, the heat dissipation from the implant in a multi-layered system comprised of epidermis, dermis, subcutaneous layer and inner tissue. A one-dimensional analytical model was also used to validate the computational results and provide a simple tool to investigate the effects of various parameters on temperature.

Steady-state temperature profile of 1.6 W of heat dissipating from the implant to the surrounding tissue; the plot is zoomed in on an area including and surrounding the implant (outline shown in light green). When the implant battery is being recharged, the interior heats up significantly. The maximum temperature outside the implant is ~57°C. Average implant temperature is ~54°C.