Ex-vivo evaluation of thermosensitive hydrogels as vitreous substitutes

Abstract

Purpose: Currently used vitreous substitutes such as gases or perfluorocarbon liquids are not compatible to stay intraocular for a longer time and silicone oils can lead to numerous complications. For this reasons recurrence rates of retinal detachment are still high. Therefore, it was envisioned to use injectable thermosensitive hydrogels as substitutes of the vitreous body, which gelate upon heating to body temperature. For this purpose, blends of hyaluronic acid and Poly(ethylene glycol-b-propylene glycol-b-ethylene glycol) (PEPE) were used in concentrations of 20 wt.% aq. solutions.

Methods: During the development phase of the hydrogels, tests on the operative handling had been conducted to examine thermosensitivity, its reversibility and injectivity through cannulas of different size. According to ISO 10993-5 tests for in vitro toxicity have been done to evaluate cell proliferation and cytotoxicity of mouse fibroblast cells L929 both qualitatively, by bringing cells and gel into direct contact, and quantitatively, through a test with dilutions of the hydrogels. To investigate gel effects on the retina in an organotypical model of the eye, perfusion tissue culture has been performed followed by a histological evaluation.

Results: Tests showed repeatedly reversible gelation of the at room temperature liquid gel after 2 minutes at 37°C, no spontaneous miscibility with an aqueous phase, but after 3 days with an aqueous phase at 37°C the gel became liquid through water absorption and lost its gelation ability. The hydrogel is injectable trough cannulas of 27g without losing its characteristics. In cell culture no migration of cells into the gel could be observed but cytotoxicity of the tested substances was found. Histological findings after perfusion tissue culture showed that the gel layer was adjacent to the retina but there were some toxic reactions of the retina, in terms of a disintegration of the internal layers, in those areas where there was direct contact between the hydrogel and the retina.

Conclusions: The hydrogels' physical properties suggest them as promising candidates for future eye surgery, but the biocompatibility has to be improved. One key point is to keep osmolarity and pH constant and close to physiological values.

Keywords: 764 vitreous substitutes • 762 vitreoretinal surgery • 688 retina