Histopathological evaluation of viscoelastic POSS administered subcutaneously using a rat model.

Introduction: Polyhedral oligomeric silsesquioxanes (POSS), have a structure resembling a cage in which the corners are silicon atoms bound with oxygen bridges. POSS cages may be dissolved in a polymer matrix or used as a cross-linker. When added into a polymer matrix, they remain non-toxic, improve mechanical properties, and resist biodegradation, which is important in tissue engineering. Limited studies evaluate long-term tissue responses to POSS gels. Methods: Adult male Sprague Dawley rat were used as a model to determine the tissue response towards multiple dosing of POSS gel compared to air injections for up to 180 days. A total of twenty-four rats was used for the study (Six rats were injected with the fluorescently labeled UVPOSS:POSS for imaging and eighteen additional rats were injected either with POSS SO1455 or air as a control group). Each group received three injections, and tissues were harvested 7, 14, and 21 days for early tissue responses, and 90- and 180-days post injection for late tissue reactions. Results: POSS vesicles formed after injection and remained pliable over 180 days. POSS complexed with Perylene dye could be imaged over 4 weeks showing lack of systemic mobility. Over the course of 180 days, POSS triggered a foreign body response with low- grade inflammation. The high viscosity nature of POSS resulted in the formation of a dense capsule preventing the material migration.