The impact of long term freezing on the mechanical properties of porcine aortic tissue. 
Preservation of the native artery׳s functionality can be important in both clinical and experimental applications.  Although, simple cryopreservation techniques offer an attractive solution to this problem, the extent to which freezing affects the tissue׳s properties is widely debated.  Earlier assessments of the mechanical properties post-freezing have been limited by one or more of the following: small sample numbers, uncontrolled inter-specimen/animal variability, failure to account for the impact of potential errors in thickness measurements, short storage times and uniaxial test methods.  Biaxial mechanical tests were performed on porcine aortic samples (n=89) extracted from superior, middle and inferior regions of five aortas, stored in isotonic saline at -20°C for 1 day, 1 week, 1, 6 and 12 months, thawed and retested.  The sample׳s weight and thickness were also measured pre and post-freezing.  A total of 178 tests were performed and elastic modulus was assessed by calculating the slope of the Cauchy stress-stretch curve at the low and high stretch regions in both the circumferential (θ) and longitudinal (L) directions.  The weight of the samples increased post-freezing.  However, in general, no significant difference was found between the elastic modulus of porcine aortic tissue before and after freezing at -20°C and was unaffected by storage time.  Although more accurate measuring instruments are warranted to confirm this finding, minor changes to the elastic modulus as a result of freezing were negatively correlated with regional variances i.e. changes in the elastic modulus decreased from the superior to the inferior region.  These results indicate that for applications which require preservation of the gross mechanical properties, storing the tissue at -20°C in isotonic saline, for an extended period of time, is acceptable.