ABSTRACT
The recent development of interpenetrating network hydrogels with excellent mechanical properties has led to new opportunities for development of biocompatible devices suitable for ophthalmic or orthopedic applications. This chapter begins with a literature review of component polymers, single-component cross-linked networks, and dual-component, interpenetrating cross-linked networks so as to establish that these hydrogel materials have physical properties that are comparable to several classes of human tissues. In addition, a review of photochemical surface modification schemes suitable for functionalization of hydrogel surfaces in spatially patterned arrays is presented. Following these reviews, we present mechanical, permeability, and friction measurements on an interpenetrating
network composed of terminally linked poly(ethylene glycol) (PEG) as the neutral first network and poly(acrylic acid) (PAA) as the polyelectrolyte second network. Subsequently, we describe a surface modification protocol that tethers collagen I to the PEG/ PAA hydrogel surface, thus providing an intervening overcoat that mimics the extracellular matrix (ECM) in natural tissue. We show preliminary experiments demonstrating that this is a suitable substrate for growth of corneal epithelial cells. 10.1 Introduction Hydrogels are water-swollen polymer networks that are useful in a variety of biomedical device applications because of their biocompatibility, high water content, and, in some cases, responsiveness to stimuli. Unfortunately, the mechanical fragility of most hydrogels poses a formidable obstacle to their application as substitutes for natural tissues. Although a number of strategies, such as the use of high cross-linking density, fiber reinforcement, and copolymerization, can improve the strength of hydrogels, the enhancement afforded by these approaches often involves some compromise in other properties like hydrophilicity, transparency, and permeability. Articular cartilage and the cornea represent two classes of tissues for which replacement by a suitable hydrogel might be feasible. Articular cartilage is the tough, slippery material lining our joints. Trauma-related defects or degenerative joint disease (osteoarthritis) can initiate an irreversible cascade of tissue deterioration that ultimately requires total joint replacement. The cornea is the transparent, outermost part of the eye that serves as the primary refractive element in the visual pathway. Mechanical trauma, infectious disease, or inherited conditions lead to edema, disruption of the collagen lamellae, and, in turn, clouding or opacification of the stroma. Articular cartilage and corneal tissue share much in common: both are naturally occurring hydrogels made from a blend of collagen and proteoglycans and highly swollen with fluid, both are inherently strong, and both are completely avascular. However, because of their avascularity, both are poor at wound healing; once damaged, they are virtually never the same again and eventually need to be replaced.
