Effects of freezing and cryopreservation on the mechanical properties of arteries

Cryoplasty, a freezing therapy, is being used for the treatment of restenosis in peripheral arteries. In addition, cryo-preserved arteries are increasingly used in vascular grafts. While studies are being performed to establish the efficacy of such treatments, very little is known about the postcryosurgical or postcryo-preservation changes in mechanical properties of the arteries. Few studies have examined the effect of freezing in the absence of cryoprotective agents (CPAs), and the several studies done in the presence of CPAs have given mixed results. To examine this issue further, we froze pig femoral arteries in a controlled rate freezer, using an aluminum probe, both in the presence at (-80 degrees C to 1 degrees C/min) and absence (at -20 degrees C for 2 or 5 mins) of CPA and Fetal bovine serum (FBS). Following freezing, artery samples were subjected to uniaxial tensile testing. The weights of the tissue were measured before and after freezing. Our results suggest that freezing does have an effect on stress-strain properties, particularly in the low stress region corresponding to physiological conditions. The mechanisms of this change in mechanical properties may include the loss of smooth muscle cell viability, damage to extra cellular matrix (ECM), bulk redistribution of water, or changes in alignment caused by ice crystal growth. In the case of samples frozen in the absence of CPA or FBS, the results indicated a drastic reduction in weight of the tissue suggesting the importance of bulk water redistribution as one underlying mechanism. To further examine potential mechanisms, we subjected cryopreserved vessels to the same uniaxial tests. The extent of changes in mechanical properties and bulk water redistribution was greatly attenuated; reinforcing that water movement might play a role in the changes observed with freezing.