Physical Aspects of Photodynamic Corneal Collagen Crosslinking

Healthy vision depends on the stability of the shape of the cornea, which provides most of the~lens power of the optical system of the eye. ~Diseases in which the cornea progressively undergoes irregular deformation over time (e.g., keratoconus) can be treated clinically by inducing additional protein-protein crosslinks using a photosensitizing drug and a tailored dose of light. Unfortunately, the treatment moving through clinical trials is toxic to cells in and on the cornea. A path to a safer treatment is offered by the nanostructure of the corneal stroma---reminiscent of a HEX phase in block copolymers with 30nm~diameter collagen cylinders~spaced 60nm center-to-center in a hydrogel matrix of~proteoglycans and water. ~We show that using a photosensitizing drug~that sequesters itself in~the collagen fibrils can minimize the toxicity of therapeutic protein-protein cross-linking. Photorheology and transport measurements are used to quantify the parameters of a simple physical model that is useful for optimizing clinical protocols.