Due to the biocompatibility of poly(ethylene glycol) (PEG), PEG-based hydrogels have attracted considerable interest for use as biomaterials in tissue engineering applications. In this work, we show that PEG-based hydrogels prepared by photopolymerization of PEG macromonomers functionalized with either acrylate or acrylamide end-groups generate networks with crosslink junctions of high functionality. Although the crosslink functionality is not well controlled, the resultant networks are sufficiently well ordered to generate a distinct correlation peak in the small angle x-ray scattering (SAXS) related to the distance between crosslink junctions within the PEG network. The crosslink spacing is a useful probe of the PEG chain conformation within the hydrogel and ranges from approximately 6 to 16 nm, dependent upon both the volume fraction of polymer and the molecular weight of the PEG macromonomers. The presence of a peak in the scattering of photopolymerized PEG networks is also correlated with an enhanced compressive modulus in comparison to PEG networks reported in the literature with much lower crosslink functionality that exhibit no scattering peak. This comparison demonstrates that the method used to link together PEG macromonomers has a critical impact on both the nanoscale structure and the macroscopic properties of the resultant hydrogel network.

Hydrogels with high modulus and fracture strength are obtained by interpenetrating a tightly cross-linked poly(ethylene glycol) (PEG) network with a loosely crosslinked poly(acrylic acid) (PAA) network. Small-angle X-ray and neutron scattering (SAXS/SANS) are used in conjunction with swelling measurements to determine the structure of PEG/PAA interpenetrating polymer networks (IPNs) and to measure the average PEG chain extension within the IPN. At pH 7.4, the PEG chains within the IPN are extended to 45À70% of their maximum achievable length as a result of expansion of the ionized PAA network within the IPN. Near these high extension ratios, the force required to further strain the PEG chains is increased due to the entropic effects of finite chain extensibility. This leads to PEG/PAA IPN hydrogels with a 3-fold increase in both compressive modulus and fracture strength compared to PEG single networks with the same polymer volume fraction. The structure, mechanical properties, and mechanisms of strength enhancement for PEG/PAA IPN hydrogels are notably different than for the high toughness double-network hydrogels previously described by Gong et al.

Three-dimensional scaffolds based on inverted colloidal crystals (ICCs) were fabricated from sequentially polymerized interpenetrating polymer network (IPN) hydrogels of poly(ethyleneglycol) and poly(acrylic acid). This high-strength, high-water-content IPN hydrogel may be suitable for use in an artificial cornea application. Development of a highly porous, biointegrable region at the periphery of the artificial cornea device is critical to long-term retention of the implant. The ICC fabrication technique produced scaffolds with well-controlled, tunable pore and channel dimensions. When surface functionalized with extracellular matrix proteins, corneal fibroblasts were successfully cultured on IPN hydrogel scaffolds, demonstrating the feasibility of these gels as materials for the artificial cornea porous periphery. Porous hydrogels with and without cells were visualized non-invasively in the hydrated state using variable-pressure scanning electron microscopy.

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Cholesterol self-assembles into weakly ordered aggregates when tethered to a crosslinked hydrogel network of poly(ethylene glycol) (PEG). PEG-diacrylate and cholesterol-PEGacrylamide (PEG-chol) were co-polymerized in organic solvent and transferred to water for equilibrium swelling. Small-angle x-ray scattering revealed self-assembled cholesterol structures not present during network synthesis. At lower ratios of PEG-tethered cholesterol to PEG (<12% cholesterol based on total solid content), cholesterol aggregates into the dense, weakly ordered crosslink junctions of the PEG network. The hydrogel networks exhibited classic affine behavior during compressive mechanical testing, and cholesterol aggregation enhanced the elastic modulus. At high PEG-chol to PEG ratios (12-20% cholesterol based on total solid content), cholesterol self-assembles into domains with lamellar-like meso-ordering. The structural transition causes network deswelling and significantly reduces material brittleness upon deformation.