Mechanical characterization and simulation of biphasic viscoelastic gels

Approximately 40 million Americans suffer from degenerative joint disease such as osteoarthritis and damaged cartilage. Hydrogels are crosslinked polymer networks swollen with water, and are being considered as potential replacements for deceased load bearing tissues such as cartilage. For such applications mechanical characterization is very important. Hydrogels show nonlinear time dependent behavior experimentally which is a challenge to describe and simulate. Hydrogel constitutive model development is of great interest and importance.;Poly(vinyl alcohol) or PVA has been extensively studied for medical and other applications. Injection molded PVA hydrogels have shown good mechanical strength compared to traditional freeze/thaw processing. In this study, pure PVA hydrogels were selected as a baseline since they have been highly studied. The PVA hydrogels were manufactured with different polymer concentration to demonstrate that an increase in polymer concentration can improve mechanical properties. In addition, novel hydrogels were developed to incorporate both a hydrophilic and a hydrophobic segment. It is proposed that an increase in the hydrophilic segment ratio can improve the mechanical properties of the hydrogel. Poly(ethylene-co-vinyl alcohol) or EVAL, and poly(vinyl pyrrolidone) or PVP blend hydrogels were selected to demonstrate that the addition of hydrophobic segments (EVAL) can improve mechanical properties.;This dissertation developed a methodology to characterize the hydrogels. A biphasic viscoelastic constitutive model was developed based on the structure and nature of the hydrogel and implemented in a UMAT user subroutine in the ABAQUS(TM) finite element analysis (FEA) package. The material parameters of the PVA and EVAL-PVP hydrogel were characterized using an inverse finite element (FE) technique that combines experimental results with numerical simulations via an optimization method using a Matlab(TM)-based optimizer and the ABAQUS(TM) solver. Two types of mechanical testing methods have been explored: unconfined creep and indentation.;Results of the creep experiments showed for PVA hydrogels, an increase in polymer concentration decreases the equilibrium water content (EWC) as well as the creep strain. In EVAL-PVP hydrogels, an increase in the hydrophobic segments (EVAL) decreases the EWC as well as the creep strain. Results from the indentation experiments showed that the modulus of the PVA hydrogels estimated using the Oliver-Pharr approach is in the range of 0.5 to 2 MPa, and the modulus of the EVAL-PVP hydrogels is in the range of 2 to 6 MPa. The modulus of the PVA hydrogel increases with increase in polymer concentration. The modulus of the EVAL-PVP hydrogels increases with increase in the hydrophobic segments (EVAL). It demonstrates that for PVA hydrogels, an increase in polymer concentration can improve the mechanical properties, and for EVAL-PVP hydrogels an increase in hydrophobic segments (EVAL) can improve mechanical properties.;The results showed that simulation with the biphasic viscoelastic constitutive model was able to achieve good agreement with the experimental results in both creep and indentation tests. The moduli measured via indentation are one order lower than those from creep tests. The relaxation time of the PVA hydrogels is in the range of 17 to 28 seconds from the indentation tests and 1700 to 22000 seconds from the creep tests. The relaxation time of the EVAL-PVP hydrogels is in the range of 4 to 15 seconds from the indentation tests and 1500 to 4200 seconds from the creep tests.