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Features

In order to accurately predict the effects of the heat flux emitted by a device, one must know or characterize the thermal properties for tissue. Hansen used COMSOL software to simulate a famous Cleveland Clinic experiment that characterized heat transfer from an implanted device through tissue.1 He used a parametric sweep in COMSOL to evaluate temperature throughout the tissue (Figure 4) for a range of thermal conductivity values.

He then compared the temperature distributions to experimental data from the Cleveland Clinic experiment to identify the thermal conductivity constant value that provided the best fit for the tissue. With this information, he could more accurately predict heat effects from the wireless transfer of power to an LVAD and use this information to ensure that the device would not cause an unsafe rise in body temperature.

Fig. 6 – Simulation of a steel ball impacting an LVAD controller to evaluate resiliency of the controller (left). Visualization of displacement along the vertical axis (right).

The wireless power transfer system induces small currents in the body tissue near the coils. Hansen modeled the heat generated in the tissue as a result of the induced currents (see Figure 5), combined this with models of heat generated inside the implant (in magnetic wires, electronics, and batteries), and then used the thermal conductivity coefficient that was determined from the simulated Cleveland Clinic study to determine the temperature in body tissue near the implant.

Protecting Life-Sustaining Batteries

Hansen also used numerical simulation to develop the external components of an LVAD. Patients must live with their LVADs every single day, which inevitably means that the external LVAD controller must be able to withstand the wear and tear of life, as well as the occasional dropping of the controller to the floor. To ensure that the controller (which contains crucial life-saving batteries) will continue to function even if the patient tosses it around, Hansen developed a simulation in which a steel ball is dropped on the controller in order to assess its resilience.

Hansen compared the amount of mechanical energy necessary to deform the device with the known amount of kinetic energy in the steel ball at the moment of impact to determine whether the controller is sufficiently resilient. He also checked edges and surfaces of the deformed structural shell and frame for twisting that would imply that the controller would break. The analysis proved that the controller would continue to provide life-sustaining power to the LVAD even after a substantial impact.

New Technology Shows Improved Options for Patients in the Future

In designing devices to assist and replace the function of the heart, numerical simulation has proven to be essential. Hansen combines experimental characterization and mathematical modeling to thoroughly understand the physics of ventricle assist devices and improve the biocompatibility of the device as well as the overall patient experience.

The latest innovations to mechanical pumping systems — including a smaller device size, a more hemocompatible pump, the introduction of pulsatile flow, and now the possibility of wireless power transfer — hold much promise for better treatment in the future.

Reference

1. C. R. Davies et al., “Adaptation of Tissue to a Chronic Heat Load,” ASAIO Journal 40(3), p. M514-M517, 1994.

This article was written by Sarah Fields, technical marketing engineer for COMSOL, Burlington, MA. Freddy Hansen, Ph.D., is senior R&D engineer for St. Jude Medical, St. Paul, MN. For more information, click here.

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