Solid Mechanics Effects in Cryopreservation Via Vitrification
Cryopreservation via vitrification is the most promising means for long-term preservation of bulky tissues and organs. While the road to successful cryopreservation is paved with obstacles, of paramount importance is thermo-mechanical stress, which can lead to fracture. In order to model the mechanical stress within a vitrifying domain during cryopreservation, the relevant thermal and mechanical properties must be explored. Toward that end, mechanical properties, such as coefficient of thermal expansion and Young’s modulus were measured in the cryogenic temperature range for a new class of cryoprotective cocktail involving DP6 (a less concentrated variant of VS55 without formamide) combined with synthetic ice modulators. A synthesis of material models and properties, some measured in the current research work and others available from the literature, were incorporated into finite element models in order to model thermo-mechanical stress. An array of representative cases has been selected to investigate the effects of a spectrum of thermal histories on the resultant mechanical stress. In particular, a temperature hold near the glass transition temperature (i.e. annealing) has shown to significantly decrease the amount of mechanical stress. Other representative cases have been used to investigate the effects of volumetric warming on mechanical stress. It was shown that internal heating using a volumetric rewarming method reduces the stress to tolerable levels, even in cases involving high cooling and rewarming rates. Additionally, photoelastic experiments were performed using an in-house proprietary device known as the cryomacroscope. These photoelastic experiments allowed for the in-situ visualization of the stress field in real time. Photoelasticity experiments were used to validate a key finding from finite element analysis presented in this thesis. Finally, a new phenomenon associated with the stress history was discovered, where stresses are seen to increase during rewarming around the glass transition temperature and an explanation was proposed based on the Narayanaswamy model of glasses.