Assessment of the Non-linear Stress-strain Characteristics of Poly (vinyl alcohol) Cryogel

Creation of tissue-mimicking constructs is of great importance in the field of biomedical engineering. Poly (vinyl Alcohol) (PVA) is a biomaterial capable of simulating a wide range of geometries and mechanical properties of biological tissues. It is nontoxic, biocompatible, and easy to produce. PVA can be physically crosslinked by repeated cycles of freezing and thawing. The final product of this process is called PVA cryogel (PVA-C). The mechanical properties of PVA-C can be accurately controlled by changing PVA molecular weight, PVA concentration, and number and duration of freeze/thaw cycles (FTC). In this study, the stress-strain behavior of PVA cryogel was studied for different strain ranges. Unconfined compression and rigid indentation tests were conducted to study the behavior of the material at large strains. Additionally, viscoelastic and poroelastic behavior of the PVA-C were investigated by conducting indentation force relaxation tests on different samples. Furthermore, mechanical behavior of the PVA-C at different strain rates was investigated by conducting unconfined compression tests. A piezoelectric ring actuator was used to estimate the velocity of shear wave propagation in the samples and therefore, to obtain reference shear moduli of the samples at strains of approximately 10-‑5. Finally, the mechanical behavior of the PVA-C for small strains was investigated by using the resonant column apparatus. Young’s modulus, shear wave velocity and shear modulus at different strains were obtained. Unconfined compression and indentation testing showed shear moduli that ranged between about 0.008 and 0.3 MPa at strains of 3%. It was found that there was some sample size dependency of the shear moduli for certain formulations. The cooperative diffusion coefficient of the PVA-C obtained from force relaxation test was found to decrease from 1.9 10-8 m2/s for 5% PVA, 6FTC to 6.1 10-10 m2/s for 20% PVA, 6FTC. Results of the tests conducted with piezoelectric ring actuator on samples with different PVA concentrations showed that shear wave velocity varies from 45 to 65 m/s in different samples, and has a positive correlation with PVA concentration and number of freeze/thaw cycles. Reference shear moduli of different samples were also found to range between 2.5 and 4 MPa. The results of a damping test on 20% PVA with 6 FTC showed that the damping ratio varies between 6 to 14% over strain magnitudes of 10-4 to 10-2. Finally, comparison between the results of the different mechanical tests shows that the shear moduli of PVA-C samples are generally constant for very low strains (less than 0.001). Shear moduli are found to decrease when the strain ranges between 0.001 and 0.05 (medium strain), and finally increase again for strains beyond 0.05 (large strain). Comparison of the mechanical behavior of PVA-C with tests reported in the literature for a range of biological tissues suggests that a number of formulations of PVA-C investigated in this study would be possible tissue mimics for these materials