Abstract

Fracture toughness has occasionally been neglected in the development of tissue engineering scaffolds. In fact, almost all recent corneal scaffolds’ developments aim to achieve transparent scaffolds with the tensile strength and elastic modulus closely-matched to those of native cornea despite the fact that cornea is normally subjected to below-ultimate-strength cyclic tensile loadings due to intraocular pressure, ocular muscle contractions and eye blink. Similarly to other soft collagenous tissues, toughening mechanisms in cornea are not well understood, but the lamellar structure of orthogonally aligned collagen fibrils in corneal stroma is thought to account for its toughness. To examine this, transparent laminates of gelatin nanofibers in alginate gel, mimicking the corneal lamellar structure, were created in a three-step process. First, stacks of orthogonally aligned gelatin nanofibers were created by electrospinning followed by chemical cross-linking. Next, dehydrated cross-linked gelatin fibers were swollen in alginate solution, forming fiber-reinforced hydrogel composites. Finally, the resulting structures were subjected to cycles of dehydration and chemical cross-linking to increase their mechanical properties and optical transparency. Fracture toughness and time-independent tensile behaviors of the orthogonally-aligned fiber-reinforced hydrogels were characterized using trouser tearing and uniaxial tensile tests. Their behaviors were compared to those of pure hydrogels and hydrogels reinforced with uniaxially-aligned gelatin fibers. Relative orientation of fibers in adjacent layers was found to significantly affect the overall fracture toughness and time-independent tensile behaviors of the fiber-reinforced hydrogels and is therefore a key to achieve tough biomimetic scaffolds for corneal tissue engineering.