Poly-N-isopropylacrylamide-based Thermoresponsive Hydrogels for Retinal Pigment Epithelial Cell Delivery

Abstract

Despite being the most prevalent presentation of Age-Related Macular Degeneration (AMD), dry AMD (dAMD) lacks a therapeutic treatment. Retinal pigment epithelium (RPE) dysfunction preceding the onset of dAMD has inspired interest in regenerative medicine approaches seeking to replenish the RPE and preserve visual acuity. Cell delivery to the subretinal space however has been met with challenges surrounding ease of access and invasive surgical implantation. Two-dimensional scaffolds have made use of natural and polymeric materials to act as carriers for RPE cells and various progenitor lines. These substrates mitigate issues surrounding the handling of delicate cell sheets harvested for transplant. As well, they are often successful in preserving RPE phenotype, supporting growth, and can be fine tuned to possess morphologies comparable to native extracellular matrix (ECM). Despite aiming to act as replacement Bruch’s membrane on which RPE resides, two-dimensional substrates are often notably bulky and require traumatic surgery for implantation. As a result, the use of injectable methods of cell delivery has gained appeal. Bolus injections, despite improved methods of administration, are correlated with issues of inadequate cell localization. In response, three-dimensional hydrogel carriers for retinal applications aim to encapsulate cells, allowing for better cell distribution as these materials spread throughout the subretinal space. Increased viscosity of hydrogels as compared to saline injections, is hypothesized to improve cell loss and reduce aggregation. Of particular interest are in situ gelling systems, which undergo physical changes upon injection. Gelation upon delivery works to further assist in maintaining the cells within their target site. Purity and reproducibility concerns associated with the use of natural materials in the development of hydrogel cell carriers, have inspired the use of synthetic thermoresponsive poly-N-isopropylacrylamide (pNIPAAm). pNIPAAm undergoes a liquid to gel transition at a lower critical solution temperature (LCST) of 32°C. Copolymerization with various hydrophobic and hydrophilic groups can be used to adjust gel properties such as increasing or decreasing LCST, allowing for degradation, and improving water retention. In the work described herein, two NIPAAm-based thermoresponsive hydrogels intended for use as subretinal cell carriers are proposed.