Study of the Effects of Stress-State and Strain-Rate on Constitutive Response of Polymer Gels via Experiments and Continuum Mechanics Modeling
High - throughput manufacturing of tissues and organs is a recent emphasis to meet the needs of patients who are on the organ donation waiting list. For successful transplant of organs, the manufactured body parts must possess biocompatibility, tailored mechanical strength, toughness, and stiffness. In this regard, polymer gels have attracted tremendous attention as candidate materials for surrogates for cartilages, brain and corneal tissue, subcutaneous tissue etc. These tissues and organs are subjected to various types of mechanical stresses and loadingrates during physical activities, invasive surgeries, and accidents. Therefore, correlating the mechanical response of polymer gels with stress - states and strain rates is essential to determine their suitability for tissue applications. The intent of this study is to develop a composition - property - performance map for a hydrogel and an elastomer gel, which will correlate the gel compositions and loading parameters with mechanical and physical properties. Such map will serve as a guideline for choosing gel compositions with customizable properties for a given application. Mechanistic models will be developed for predicting the evolution of properties of the gels with compositions and loading parameters. In this study, an integrated experimental and continuum modeling - based approach will be used to characterize the stress-state and strain-rate dependent nonlinear mechanical response of soft polymer gels. Polyacrylamide (PAAM) hydrogels and Polydimethylsiloxane (PDMS) elastomer gels are chosen as model material systems for the ability (i) to widely vary the elastic modulus of the gels, (ii) to undergo large deformation under mechanical stress, and (iii) to exhibit time - dependent mechanical response. The composition of the materials will be varied to obtain a wide range of physical (e.g. mesh size, osmotic pressure etc.) and mechanical properties (e.g. modulus, strength, etc.). These gels will then be subjected to (i) compression, (ii) tension, and (iii) simple shear, loading to virtually cover all stress-states required to fully characterize the stress-strain responseof a material. The strain - rates during mechanical tests will be varied over a wide range between 10 - 4 - 101/s. Based on the experimental data, material specific hyperelastic and viscoelastic constitutive models rooted in continuum mechanics fundamentals will be developed. The model parameters, determined from simplistic tension, compression, and simple shear loading conditions will be used to predict and verify a realistic multiaxial response of the gels through analytical and finite element modeling. Finally, the effect of composition, stress-state and strain - rate on gel properties such as, elastic moduli, strain rate sensitivity of modulii, flow stress, permeability etc. will be presented in theform of a “composition-property - performance” map to guide selection of materials and for customized tissue engineering applications.