114 Thermal expansion of a carotid artery model permeated with the cryoprotective cocktail DP6 and synthetic ice modulators at cryogenic temperatures

Abstract

Thermal expansion of tissues permeated with various combinations of the cryoprotective agent (CPA) cocktail DP6 and synthetic ice modulators (SIMs) was measured in this study. The ensuing data augments a database on the physical properties of biomaterials with applications to thermo-mechanical stress analysis, in an effort to improve the outcome of cryopreservation. All materials tend to change volume with temperature, which is known as the physical property of thermal expansion. If a material is constrained, or if adjacent regions in the material expand at different rates, thermo-mechanical stress may develop. When this stress exceeds the strength of the material, structural damage follows, which is a limiting factor to the widespread use of cryopreservation for large-scale specimens. During cryopreservation, thermo-mechanical stress may form as a result of three distinct mechanisms: (i) inhomogeneity of the material, (ii) temperature gradients, and (iii) phase transition. The effects associated with thermo-mechanical stress are exacerbated when preserving larger tissue samples since the reduced ratio of surface area to volume leads to steeper temperature gradients. Measurements were performed on a goat artery model permeated with the CPA cocktail DP6 when mixed with one of the following synthetic ice modulators (SIMs): 1,3 cyclohexanediol (1,3CHD), 2,3-butanediol (2,3BD), and polyethylene glycol 400 (PEG400). The general classification of SIMs includes molecules that modulate ice nucleation and growth, or possess properties of stabilizing the amorphous state, by virtue of their chemical structure and at concentrations that are not explained on a purely colligative basis. DP6 is a cocktail of 234.4 g/L Me 2 SO (3M), 228.3 g/L propylene glycol (3M), and 2.4 g/L HEPES in EuroCollins solution. Two different previously developed devices were utilized to measure thermal expansion: in the upper part of the cryogenic temperature range—where the material behaves as a fluid, and in the lower part of the cryogenic temperature range—where the material behaves as a solid for all practical applications. Data from the upper and lower parts of the cryogenic temperature range were combined to form thermal strain curves that encompass the entire cryogenic temperature range (0 °C to −160 °C). It was found that the addition of synthetic ice modulators increased thermal strain by about 50% and significantly reduced the cooling rate necessary to suppress crystallization in the upper part of the cryogenic temperature range. While the thermal strain was found to vary by as much as 19% between different combinations of DP6 and SIM mixtures at upper cryogenic temperatures, thermal expansion was equivalent between the three solutions at the lower temperatures. This study represents an important step in developing a database for the application of computational models, in order to analyze mechanical stresses during cryopreservation, with the long-term goal of optimizing cryopreservation protocols. Source of funding: This project has been supported by Award Number R21EB009370 from the National Institute of Biomedical Imaging and Bioengineering. Conflict of interest: None declared. rabin@cmu.edu