Micro and Macroscale Measurement of Thermal Properties for Cryobiological Applications

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Two important biomedical applications of freezing are cryopreservation, the preservation of material in the frozen state, and cryosurgery, the use of freezing to destroy tissue. These both are part of the larger field of cryobiology or low temperature biology. In both of these applications heat transfer prediction of biomaterial thermal response (i.e. cooling rate, end-temperature, dwell time and thawing rate) is important for the effective planning of protocols where thermally mediated events within the biomaterial such as intracellular ice formation, cellular dehydration, eutectic formation, (de)vitrification, and/or lipid and protein phase changes are important to the determination of outcome (i.e. survival or death). Unfortunately the database of biomaterial thermal properties necessary to make accurate heat transfer prediction of thermal response in the cryogenic regime is quite limited. Specifically, latent heat, thermal conductivity and specific heat properties are available only for a few biomaterials and little if any data is available below -40 C or in the presence of chemical additives. The thermal diffusivity of ice, an abundant component of frozen biomaterials, changes by essentially an order of magnitude between 0 and -150 C. Preliminary results show more modest thermal property changes in tissues, but even greater thermal property changes in frozen salt solutions than ice. The addition of other chemicals (i.e. cryoprotective agents or cryoadjuvants) in order to more effectively preserve or destroy the biomaterial may further alter the thermal properties in a manner which remains to be determined. This proposal aims to make both bulk and microscale measurements of thermal properties for a variety of solutions and native/artificial tissues with and without chemical additives down to a temperature of -150 C. Bulk measurements will be carried out with a pulse-decay thermistor apparatus for thermal conductivity measurement and a Differential Scanning Calorimetry (DSC) for specific heat and latent heat measurements.In addition, directional thermal conductivity (anisotropy), which depends on microscale ice crystal structure and orientation, will be measured with a new apparatus which allows visualization of ice crystal growth during freezing on a directional stage followed by thermal conductivity measurements directly on the sample visualized. It is anticipated that the presence of microstructure at the level of the individual crystals, eutectics and glass (vitrified material) may have a strong anisotropic nature which can be measured and incorporated into predictive models. Ultimately, the properties measured will contribute significantly to the expansion of a cryogenic biomaterial thermal property database thereby allowing improved predictive modeling for a variety of cryobiological applications.