PEGDA Hydrogel Network Model Evaluation: Correlating Mechanical Properties to Molecular Structure

Poly(ethylene glycol) diacrylate (PEGDA) hydrogels are important biomedical materials with particular use in drug delivery and tissue engineering. Modelling the underlying polymer network of these gels is of key importance in understanding the mechanical, diffusive, and swelling properties that dictate hydrogel behavior and performance in application. The gels are often assumed to have a network structure that lead them to follow neo-Hookean elastic models, from which network parameters such as crosslink density and network mesh size are calculated. However, several groups have used scattering techniques to demonstrate significant heterogeneity of PEGDA hydrogel networks. Because such specialized techniques are not feasible to be applied to biomedical hydrogel networks generally, it is still common to calculate network parameters assuming the validity of the neo-Hookean model despite the underlying heterogeneity. Thus a systematic study of PEGDA hydrogel mechanical properties was performed to evaluate the validity of the neo-Hookean model on PEGDA hydrogels and the ability to predict network structure from macroscopic mechanical tests using both the neo-Hookean and Mooney-Rivlin models. Mechanical properties of 700 Da and 5000 Da PEGDA hydrogels (PEGDA700, PEGDA5000) were determined. Hydrogels of PEGDA concentrations from 7 - 30 wt. % were prepared and synthesized via photoinitiation at a wavelength of 312 nm with the photoinitiator lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP). Hydrogels were molded into disks, swollen to equilibrium in deionized water, and evaluated using uniaxial compression tests to determine stress versus strain relationships as well as the elastic modulus (E) and shear modulus (G) for each gel under neo-Hookean assumptions. The swelling degree (Q) was also determined from the ratio of wet to dry weights of each gel. Gel properties were shown to vary widely across polymer concentration with neo-Hookean elastic moduli increasing with concentration and ranging from 81.0 kPa to 2620 kPa and 51.6 kPa to 1030 kPa for PEGDA700 and PEGDA5000, respectively. Swelling degree decreased with composition and ranged from 9.20 to 3.00 and 14.7 to 5.46 for PEGDA700 and PEGDA5000, respectively. Further evaluation using the Mooney-Rivlin network model highlighted the heterogeneity of these network structures, and suggests that these gels are deviating from neo-Hookean behavior. Gels with low polymer content approached neo-Hookean behavior and behaved as apparent Mooney-Rivlin solids, while high polymer content gels behaved similarly to networks that follow constrained polymer chain models with notable transition behavior for hydrogel networks with compositions near the polymer overlap volume fraction. These results suggest that low molecular weight PEGDA hydrogels deviate significantly from the neo-Hookean model and correlation of moduli to PEGDA network parameters is limited.