Rheological and physicochemical optimization of hydrogels as potential vitreous body substitutes for in vitro release testing

Due to an aging society and the associated increase in age-related eye diseases of the posterior segment of the eye, an optimized in vitro vitreous model would be beneficial to assess drug release and distribution in preclinical dosage form development. A key component in such a test system is a compartment simulating the vitreous body. Several hydrogels have been proposed for this purpose. In this work the rheological properties of several vitreous body substitutes based on hyaluronic acid, hypromellose, polyacrylamide, gellan gum in combination with hyaluronic acid, and hyaluronic acid in combination with agar were investigated. By systematically comparing these potential in vitro vitreous body substitutes with porcine vitreous bodies within one study employing a series of rheological characterizations, a direct comparison was achieved, allowing opportunities for optimization to be identified. The main characterization focused on the viscosity and loss factor in the linear viscoelastic region and, for the most promising gels, also on the behavior in frequency sweeps. Additionally, the recovery times, pH values, and osmolalities of the gels were determined. Initially, phosphate-buffered saline, which served as the basis for the hydrogels, was successfully adjusted to match the pH and osmolality of porcine and human vitreous bodies. Gels made from gellan gum-hyaluronic acid and hyaluronic acid-agar proved most promising. These could be adjusted in concentrations of 0.034 % gellan gum & 0.264 % hyaluronic acid or 0.22 % hyaluronic acid & 0.09 % agar to match both the viscosity of the vitreous body and the loss factor in the linear viscoelastic region. Additionally, the pH values, osmolalities, and behavior in frequency sweeps of the gels were also comparable to the vitreous body, as these exhibit physicochemical gel formation mechanisms and reversibly linked frameworks probably similar to those of the vitreous body. Under the measurement conditions used here, the gels can therefore be considered as good in vitro vitreous body substitutes. Further diffusion studies, which will likely be influenced by the adjusted rheological properties, should be conducted in the future to further investigate the suitability of the optimized gels presented here.