Histological processing of pH-sensitive hydrogels used in corneal implant applications

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.