55. Microsensors for Determination of Thermal Conductivity of Biomaterials and Solutions

Obtaining accurate thermal properties of biomaterials plays an important role in modern bioscience and medicine. This chapter discusses different thermal conductivity measurement techniques and describes a featured development of a BioMEMS-based thermal probe to accurately and readily determine the thermal conductivities for soft biological tissues and aqueous solutions. By employing a closely packed microfabricated serpentine gold wire to function as the heater and thermistor, the featured thermal sensor utilizes transient hot wire (THW) principle to measure thermal conductivities of various fluids and soft biomaterials. To ensure system accuracy and data reliability, the microscopic thermal conductivity measuring device has been comprehensively characterized with distilled water as thermal standard material at temperatures ranging from 0 °C to 40 °C. Despite the previous belief, the system calibration constant was observed to be favorably stable with respect to different sensors, but highly dependent to ambient temperatures. Distinctive thermal conductivity dynamic response during cooling (40 °C to −40 °C) was observed using the miniaturized single-tip sensor for various concentrated cryoprotective agents (CPAs), i.e., glycerol, ethylene glycol, and dimethyl sulfoxide. Experimental data suggests thermal conductivity of aqueous solutions can be significantly altered at different temperatures. Chicken breast, chicken skin, porcine limb, and bovine liver were also successfully assayed to study the effect of anatomical heterogeneity on thermal conductivity using the arrayed multi-tip sensor at 20 °C. Unique differences in localized thermal conductivity were also observed, which suggests the use of approximated or constant property values is expected to bring about results with largely inflated uncertainties when investigating bio-heat transfer mechanisms and/or performing sophisticated thermal modeling with complex bio-tissues. Overall, the presented microthermal sensor with automated data analysis algorithm has shown superior performance characteristics and proven to be a promising approach for direct thermal conductivity measurement of aqueous solutions and soft biomaterials with various shapes and sizes. It is of great value to cryopreservation of tissues, hyperthermia, or cryogenic and other thermal-based clinical diagnostics and treatments.