We evaluated the biocompatibility of a poly(ethylene glycol) and poly(acrylic acid) (PEG/PAA) interpenetrating network hydrogel designed for artificial cornea in a rabbit model. PEG/PAA hydrogel measuring 6 mm in diameter was implanted in the corneal stroma of twelve rabbits. Stromal flaps were created with a microkeratome. Randomly, six rabbits were assigned to bear the implant for 2 months, two rabbits for 6 months, two rabbits for 9 months, one rabbit for 12 months, and one rabbit for 16 months. Rabbits were evaluated monthly. After the assigned period, eyes were enucleated, and corneas were processed for histology and immunohistochemistry. There were clear corneas in three of six rabbits that had implantation of hydrogel for 2 months. In the six rabbits with implant for 6 months or longer, the corneas remained clear in four. There was a high rate of epithelial defect and corneal thinning in these six rabbits. One planned 9-month rabbit developed extrusion of implant at 4 months. The cornea remained clear in the 16-month rabbit but histology revealed epithelial in-growth. Intrastromal implantation of PEG/PAA resulted in a high rate of long-term complications.

This study compared the effects of implanting two interpenetrating polymer networks (IPNs) into rabbit corneas. The first (Implant 1) was based on PEG-diacrylate, the second (Implant 2) was based on PEG-diacrylamide. There were inserted into deep stromal pockets created using a manual surgical technique for either 3 or 6 months. The implanted corneas were compared with normal and sham-operated corneas through slit lamp observation, anterior segment optical coherence tomography, in vivo confocal scanning and histological examination. Corneas with Implant 1 (based on PEG-diacrylate) developed diffuse haze, ulcers and opacities within 3 months, while corneas with Implant 2 (based on PEG-diacrylamide) remained clear at 6 months. They also exhibited normal numbers of epithelial cell layers, without any immune cell infiltration, inflammation, oedema or neovascularisation at post-operative 6 month. Morphological studies showed transient epithelial layer thinning over the hydrogel inserted area and elevated keratocyte activity at 3 months; however, the epithelium thickness and keratocyte morphology were improved at 6 months. Implant 2 exhibited superior in vivo biocompatibility and higher optical clarity than Implant 1. PEG-diacrylamide-based IPN hydrogel is therefore a potential candidate for corneal inlays to correct refractive error.

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.

Aims: To determine the rate of diffusion of myoglobin and bovine serum albumin (BSA) through the human cornea. These small proteins have hydrodynamic diameters of approximately 4.4 and 7.2 nm, and molecular weights of 16.7 and 66 kDa, for myoglobin and BSA, respectively. Meth ods: Diffusion coefficients were measured using a diffusion chamber where the protein of interest and balanced salt solution were in different chambers separated by an ex vivo human cornea. Protein concentrations in the balanced salt solution chamber were measured over time. Diffusion coefficients were calculated using equations derived from Fick’s law and conservation of mass in a closed system. Results: Our experiments demonstrate that the diffusion coefficient of myoglobin is 5.5 ± 0.9 × 10–8 cm2/s (n = 8; SD = 1.3 × 10–8 cm2/s; 95% CI: 4.6 × 10–8 to 6.4 × 10–8 cm2/s) and the diffusion coefficient of BSA is 3.1 ± 1.0 × 10–8 cm2/s (n = 8; SD = 1.4 × 10–8 cm2/s; 95% CI: 2.1 × 10–8 to 4.1 × 10–8 cm2/s). Conclusions: Our study suggests that molecules as large as 7.2 nm may be able to passively diffuse through the human cornea. With applications in pharmacotherapy and the development of an artificial cornea, further experiments are warranted to fully understand the limits of human corneal diffusion and its clinical relevance.

A novel interpenetrating network (IPN) based on poly(ethylene glycol) (PEG) and poly(acrylic acid) was developed and its use as an artificial cornea was evaluated in vivo. The in vivo results of a first set of corneal inlays based on PEG-diacrylate precursor showed inflammation of the treated eyes and haze in the corneas. The insufficient biocompatibility could be correlated to poor long-term stability of the implant caused by hydrolytic degradation over time. Adapting the hydrogel chemistry by replacing hydrolysable acrylate functionalities with stable acrylamide functionalities was shown to increase the long-term stability of the resulting IPNs under hydrolytic conditions. This new set of hydrogel implants now shows increased biocompatibility in vivo. Rabbits with corneal inlay implants are healthy and have clear cornea and non-inflamed eyes for up to 6 months after implantation.

The development and characterization of collagen-coupled poly(ethylene glycol)/poly(acrylic acid) (PEG/PAA) interpenetrating polymer network hydrogels is described. Quantitative amino acid analysis and FITC-labeling of collagen were used to determine the amount and distribution of collagen on the surface of the hydrogels. The bioactivity of the coupled collagen was detected by a conformation-specific antibody and was found to vary with the concentration of collagen reacted to the photochemically functionalized hydrogel surfaces. A wound healing assay based on an organ culture model demonstrated that this bioactive surface supports epithelial wound closure over the hydrogel but at a decreased rate relative to sham wounds. Implantation of the hydrogel into the corneas of live rabbits demonstrated that epithelial cell migration is supported by the material, although the rate of migration and morphology of the epithelium were not normal. The results from the study will be used as a guide toward the optimization of bioactive hydrogels with promise in corneal implant applications such as a corneal onlay and an artificial cornea.

Epithelialization of a keratoprosthesis requires that the implant material be sufficiently permeable to glucose. We have developed a poly(ethylene glycol)/poly(acrylic acid) (PEG/PAA) interpenetrating polymer network (IPN) hydrogel that can provide adequate passage of glucose from the aqueous humor to the epithelium in vivo. A series of PEG/PAA IPNs with varying PEG macromonomer molecular weights were synthesized and evaluated through swelling studies to determine their water content and diffusion experiments to assess their permeability to glucose. One of the PEG/PAA hydrogels prepared in this study had a glucose diffusion coefficient nearly identical to that of the human cornea (∼ 2.5 × 10− 6 cm2/sec). When implanted intrastromally in rabbit corneas, this hydrogel was retained and well-tolerated in 9 out of 10 cases for a period of 14 days. The retained hydrogels stayed optically clear and the epithelium remained intact and multilayered, indicating that the material facilitated glucose transport from the aqueous humor to the anterior part of the eye. The results from these experiments indicate that PEG/PAA hydrogels are promising candidates for corneal implant applications such as keratoprostheses and intracorneal lenses, and that the PEG/PAA IPN system in general is useful for creating permeable substrates for ophthalmic and other biomedical applications.

Interpenetrating polymer networks (IPNs) have been the subject of extensive study since their advent in the 1960s. Hydrogel IPN systems have garnered significant attention in the last two decades due to their usefulness in biomedical applications. Of particular interest are the mechanical enhancements observed in "double network" IPN systems which exhibit nonlinear increases in fracture properties despite being composed of otherwise weak polymers. We have built upon pioneering work in this field as well as in responsive IPN systems to develop an IPN system based on end-linked poly-(ethylene glycol) (PEG) and loosely crosslinked poly(acrylic acid) (PAA) with hydrogen bond-reinforced strain-hardening behavior in water and high initial Young's moduli under physiologic buffer conditions through osmotically induced pre-stress. Uniaxial tensile tests and equilibrium swelling measurements were used to study PEG/PAA IPN hydrogels having second networks prepared with varying crosslinking and photoinitiator content, pH, solids content, and comonomers. Studies involving the addition of non-ionic comonomers and neutralization of the second network showed that template polymerization appears to be important in the formation of mechanically enhanced IPNs.

Research and development of artificial corneas (keratoprostheses) in recent years have evolved from the use of rigid hydrophobic materials such as plastics and rubbers to hydrophilic, water-swollen hydrogels engineered to support not only peripheral tissue integration but also glucose diffusion and surface epithelialization. The advent of the AlphaCor core-and-skirt hydrogel keratoprosthesis has paved the way for a host of new approaches based on hydrogels and other soft materials that encompass a variety of materials preparation strategies, from synthetic homopolymers and copolymers to collagen-based bio-copolymers and, finally, interpenetrating polymer networks. Each approach represents a unique strategy toward the same goal: to develop a new hydrogel that mimics the important properties of natural donor corneas. We provide a critical review of these approaches from a materials perspective and discuss recent experimental results. While formidable technical hurdles still need to be overcome, the rapid progress that has been made by investigators with these approaches is indicative that a synthetic donor cornea capable of surface epithelialization is now closer to becoming a clinical reality.

Hydrogels are hydrophilic, crosslinked polymers that are being used with increasing frequency in a variety of biomedical applications. The ability to obtain high-quality histological sections, without artifacts, after in vivo implantation is critical to their evaluation. Unfortunately, because of their high water content and environmental sensitivities, many hydrogels are unable to be successfully processed for histology using typical tissue processing methods. In this work, we present a method for Successfully processing pH-sensitive hydrogels for histological analysis. A modified glycol methacrylate (GMA) processing and embedding protocol is described. One cornea each from 10 New Zealand Red rabbits was implanted with a poly(ethylene glycol) and poly(acrylic acid) interpenetrating double network hydrogel disc for 2 weeks and processed for histology. Maintaining a neutral pH during fixation with glutaraldehyde is a critical initial step for processing. In addition, typical tissue processing methods require the use of alcohol and/or xylene for specimen dehydration, causing severe distortion of hydrogel morphology. The substitution of these chemicals with deionized water during the processing steps also proved to be vital for maintaining hydrogel structure and morphology. Slides stained with cresyl violet showed excellent tissue and hydrogel morphology with minimal histological artifacts or distortion