IMECE2012 89706 Lubner

Accurate knowledge of the thermal conductivities of biological tissues is important for thermal bioengineering, including applications in cryopreservation, cryosurgery, and other thermal therapies. The thermal conductivity of biomaterials is traditionally measured with macroscale techniques such as the steady longitudinal heat flow method or embedded thermistor method. These techniques typically require relatively large, centimeter-scale samples, limiting their applicability to finer biological structures. They are also vulnerable to errors caused by thermal contact resistances and parasitic heat losses. In contrast, the thermal conductivity of inorganic solids such as semiconductor wafers and thin films is commonly measured using the “3 omega method” [1-3]. This frequency domain technique is robust against thermal contact resistances and parasitic heat losses. It routinely has sub-millimeter spatial resolution, with theoretical limits down to tens of microns. Here we adapt the 3 omega method for measurements of biological tissues. Thermal conductivity measurements are made on both frozen and un-frozen samples including agar gel, water, and mouse liver, including samples with sub-millimeter thicknesses. The measurement results compare favorably with literature values and span the range from around 0.5 to 2.5 W/m-K. This study demonstrates the promise that this technique holds for thermal measurements of bulk tissues as well as fine sub-millimeter samples. INTRODUCTION Thermal therapies are used in several areas of medical treatments. For example, thermal ablation has been used to treat hepatic cancer and has been shown to be superior to other treatment methods [4,5], while cryoablation has been used to treat prostate, breast, and renal cancer [6-8]. In cardiology, cryopreservation of heart valves and blood vessels is used to maintain tissue function for grafts and transplants [9-12], and controlled heating and cooling of blood vessels is used to treat atrial fibrillation, peripheral artery disease, and renal hypertension [13-16]. All of these techniques rely on repeatable and predictable cooling and heating of biological tissues. In the case of ablation, the thermal necrosis volume depends critically on the thermal properties of the tissue. Under-estimating the thermal conductivity of local tissues risks creating a larger thermal necrosis volume than intended and damaging nearby healthy and potentially vital tissue, while over-estimation risks failing to kill all cancerous cells leading to a future relapse after treatment. In the case of cryopreservation, controlling the rate of cooling also depends on the thermal properties of the tissue. Cooling too slowly risks dehydrating cells, while cooling too rapidly risks the formation of intracellular ice, both of which can cause undesirable damage or even death to the tissue being preserved [17,18].