(161t) Modeling the Elastic Properties of Poly(ethylene glycol)-Based Hydrogels

Hydrogels, cross-linked polymeric networks, are of interest due to their widespread applications in tissue engineering and as drug delivery vehicles. Therefore it is significant to understand the functionalities between hydrogel design parameters and desired hydrogel mechanical, biocompatibility, and permeability properties. The shear modulus is most commonly determined from stress-strain data with the assumption of the Neo-Hookean model which is in turn used to calculate crosslink density and estimate a mesh size. However, for many practical systems the assumption of hydrogel ideality is not valid. This study investigates the efficacy of the Neo-Hookean and Mooney-Rivlin hyperelastic models for predicting hydrogel stress-strain behavior. Furthermore, Mooney-Rivlin curves are generated to evaluate the assumption of ideal elastomeric behavior. Poly(ethylene glycol) hydrogels are the investigated hydrogels as they are well characterized in the literature and have many commercial applications. Poly(ethylene glycol) hydrogels were synthesized via a variety of cross-linking chemistries, polymer volume fractions, and with the addition of reinforcing fillers. The shear modulus, compressive modulus, and fracture properties were determined by uniaxial compression studies on the RSA3 mechanical analyzer. Poly(ethylene glycol) diacrylate (PEGDA) hydrogels were chain cross-linked via irradiation in the presence of lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) photoinitiator. 4-arm Poly(ethylene glycol) (Tetra-PEG) macromers with amine terminated and succinimidyl glutarate terminated ends were cross-linked via an end-coupling reaction. PEGDA hydrogels were reinforced with the addition of montmorillonite nanoclay sheets to the pregelation solution. All hydrogels were investigated over a variety of polymer volume fractions, below and above the polymer overlap concentration. Shear modulus and consequently mesh sizes were determined from stress v. strain data with the assumption of either the Neo-Hookean or Mooney-Rivlin models for the appropriate systems. Additionally, the regressed model constants were used to recreate the stress v. strain curve to validate the efficacy of the model. For PEGDA systems, all models fit the stress v. strain behavior up to 20% strain; however, the Neo-Hookean model deviated significantly beyond 20 strain %. The Mooney-Rivlin model accurately predicted the strain-strain behavior until the gel fracture point. These results indicate that the PEGDA hydrogels do not fit the Neo-Hookean model, whereas the Tetra-PEG and PEGDA-MMT hydrogels system do. Furthermore, the model behavior of the PEGDA significantly varies below, near, and above the overlap concentration.