(494g) Computational and Experimental Modeling of Hyperthermia in Physiological Systems

In situthermal sterilization is an attractive means of treating bacterial infections on implanted medical devices but poses a significant risk of damaging surrounding tissue. Implementing an effective thermal dose requires understanding how hyperthermia will influence physiological systems while minimizing destructive effects. This work models the physiological conditions surrounding medical implants and solves the device-surface power output and resulting transient temperature profiles for a given time and temperature protocol necessary for bacterial sterilization. Specifically, temperature boundary conditions are imposed on the device surface and the resulting heat transfer is solved numerically via discretization of the governing transport phenomena expressions.  This computational model is validated experimentally for physiological conditions ranging from non-perfused tissue to fast blood flow using a custom-built thermal flow-cell system. 

The power requirements for a thermal dose depend on the surrounding heat-sink conditions for a given physiological system. For example, the amount of energy required to impose a temperature boundary condition is significantly less on the surface of an orthopedic device surrounded by bone tissue than the surface of a heart-stent in the aorta that is subject to large convective heat losses into the bloodstream. Subsequently, different power requirements are necessary to induce the same temperature boundary condition on a surface that is surrounded by both tissue and fluid flow or tissues with varying thermal diffusivities. Thus, quantifying the transient heat flux from the device/tissue interface and resulting hyperthermia will guide physical spatial design and control of the surface heat source for a specific device and location in the body.

Where computations enabled solutions for complex geometries and flow conditions, experimental measurements were used to validate numerical results via heat-sinks that physically mimic the thermal properties of systems inside the body. Static hydrogels were used to mimic tissues for the sole purpose of matching the thermal conductivities of tissues and organs. Zinc oxide nanoparticles could be loaded to tune the thermal conductivity of the hydrogel to a desired value. Hydrogels were prepared from glutaraldehyde-crosslinked poly(vinyl alcohol) to produce volume and temperature stable hydrogels containing 92wt% water. Water circulating at 37°C was used for all fluid scenarios and could be controlled to a desired Reynolds number. Temperature boundary conditions were implemented via feedback control of the surface temperature of an electrical resistance, nichrome-foil heating element. Temperature profiles were acquired via thermistor arrays embedded in the tissue mimic using a custom-built flow-cell.