016 Integration of transient hot-wire method into scanning cryomacroscopy in the study of thermal conductivity of dimethyl sulfoxide

A pilot study was conducted to investigate the dependency of thermal conductivity on temperature, phase of state, and solution concentration in the range of 2 M (classical cryopreservation) and up to 10 M Me 2 SO (simulative of highly concentrated cocktails used for vitrification). Cryoprotective agents (CPAs), such as dimethyl sulfoxide (Me 2 SO) are used to control ice formation—the cornerstone of cryoinjury. When cooled, the CPA may crystallize in low concentrations and relatively low cooling rates, or vitrify (form glass—be trapped in an amorphous state) in high concentrations and relatively high cooling rates. Analysis of cryopreservation protocols and explanation of related physical events may be assisted by computer simulations of the thermal process. Unfortunately, the physical properties of CPAs necessary for thermal analysis represent a largely uncharted area. Furthermore, the difference in thermal conductivity between an amorphous and crystalline material of the same composition can vary by orders of magnitude. A transient hot-wire technique was used to measure thermal conductivity of Me 2 SO solutions in a controlled-rate cooler. Physical events in the sample along the cryopreservation protocol were recorded with the application of the scanning cryomacroscope to confirm whether the sample was vitrified (transparent) or crystallized (opaque). Selected cooling and rewarming rates were chosen to promote either vitrification or crystallization, based on literature data. Thermal conductivity measurements were made continuously during the rewarming phase of the protocol, in the temperature range of −100 to +20 °C. Thermal conductivity of crystalline Me 2 SO was found to be five-fold higher than that of amorphous Me 2 SO, with relatively little dependency on the concentration. In the amorphous state, thermal conductivity decreases with the increasing concentration: at 15°C ranging from 0.24 to 0.31 W/m K for 10 M and 7.05 M Me 2 SO, respectively, and at −100 °C ranging from 0.22 to 0.26 W/m K for 10 M and 7.05 M Me 2 SO, respectively. The decrease in thermal conductivity with decreasing temperature is consistent with literature data for other amorphous materials such as SiO 2 , Se, and PMMA. The measured thermal conductivity of 2 M Me 2 SO above −4 °C is consistent with the dependency of thermal conductivity on concentration for the other solutions in the liquid phase. The onset of crystallization based on a water–Me 2 SO phase diagram, −4 °C, correlates well with the measured increase in thermal conductivity of the mixture around this temperature. The increased thermal conductivity upon phase transition is consistent with literature data on pure water, although thermal conductivity of pure water increases by a factor of four (from 0.566 to 2.25 W/m K), whereas the thermal conductivity of 2 M Me 2 SO increases by a factor of three, from 0.47 W/m K to higher than 1.4 W/m K. The integration of the transient hot-wire method into scanning cryomacroscopy developed in this study represents the only available method for measuring thermal conductivity in cryogenic temperature while visually verifying the phase of state. Source of funding: This study is supported, in part by, Award Number R21RR026210, National Center for Research Resources (NCRR) and R21GM103407, National Institute of General Medical Sciences (NIGMS). Conflict of interest: None declared. rabin@cmu.edu