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Thermo-Mechanical Stress in Cryopreservation

Cryopreservation technologies represent a potential long term and minimally damaging method to preserve both native and engineered tissues. Conventional cryopreservation of allogeneic veins involving freezing is currently being used clinically, but in vivo studies using these grafts in both animal models and patients have demonstrated poor long-term patency rates. An alternative approach to cryopreservation involving vitrification that avoids the hazards of ice formation leads to a markedly improved vascular product in terms of both structure and function. Vitrification (vitreous means glassy in Latin) is essentially the solidification of a super-cooled liquid by adjusting the chemical composition and cooling rate such that the crystal phase is avoided. This new preservation technology is now being scaled up for application to clinical specimens and ultimately engineered blood vessels. Nevertheless, additional hazards related to thermo-mechanical stresses in bulky vitrified specimens must be avoided for successful cryopreservation of tissues.

Our long-term goal is to reduce the destructive mechanical stresses induced during cryopreservation of tissues in general, and of blood vessels in particular. The purpose of this research is to develop engineering tools and to characterize the level of thermo-mechanical stresses in bulky cryopreserved tissues and thereby devise techniques to reduce, or circumvent these stresses and develop improved methods of long-term storage of both native and engineered vascular grafts.

This project includes a systematic study of thermo-mechanical stresses by measuring the thermal expansion and the stress-strain relationship of relevant cryoprotectants and cryopreserved blood vessels. The measured parameters, together with appropriate mathematical modeling and computers simulations, are incorporated to provide guidelines for minimizing the thermo-mechanical stresses and reduce the potential of fracture formation during cryopreservation. Although the experimental work in this project is targeted to blood vessels, the results of this study could be expanded and become useful for a wide variety of cryopreserved natural tissues and engineered constructs. 

· Example of cracking in vitrified heart valves (images)

· Cryomacroscopy of vitrification: selected experiments on DP6 and VS55

· Cryomacroscopy of fracture formation in vitrified thin films of cryoprotectants (experimental results summary)

This research has been supported, in part, by the National Heart Lung and Blood Institute, NIH Grant # 1R01HL069944

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