Reinforcement of Mono- and Bi-layer Poly(Ethylene Glycol) Hydrogels with a Fibrous Collagen Scaffold.

Biomaterial-based tissue engineering strategies hold great promise for osteochondral tissue repair. Yet significant challenges remain in joining highly dissimilar materials to achieve a biomimetic, mechanically robust design for repairing interfaces between soft tissue and bone. This study sought to improve interfacial properties and function in a bi-layer hydrogel interpenetrated with a fibrous collagen scaffold. 'Soft' 10% (w/w) and 'stiff' 30% (w/w) PEGDM was formed into mono- or bi-layer hydrogels possessing a sharp diffusional interface. Hydrogels were evaluated as single-(hydrogel only) or multi-phase (hydrogel + fibrous scaffold penetrating throughout the stiff layer and extending >500 μm into the soft layer). Including a fibrous scaffold into both soft and stiff mono-layer hydrogels significantly increased tangent modulus and toughness and decreased lateral expansion under compressive loading. Finite element simulations predicted substantially reduced stress and strain gradients across the soft-stiff hydrogel interface in multi-phase, bilayer hydrogels. When combining two low moduli constituent materials, composites theory poorly predicts the observed, large modulus increases. These results suggest material structure associated with the fibrous scaffold penetrating within the PEG hydrogel as the major contributor to improved properties and function-the hydrogel bore compressive loads and the 3D fibrous scaffold was loaded in tension thus resisting lateral expansion.