Eye Gel Ages Like We Do — And It Changes How Drugs Diffuse Inside

Millions of people receive intravitreal injections — shots directly into the eye — to treat conditions like age-related macular degeneration and diabetic retinopathy. These injections rely on protein-based drugs, such as IgG antibodies, diffusing through the vitreous humor to reach the retina. But the vitreous is not static: it changes significantly with age, and this study investigates what those changes mean for drug delivery.

The vitreous humor is a gel composed largely of hyaluronic acid (HA). Starting around age 40, a process called phase separation begins, creating liquid-filled pockets within the gel. By ages 80–90, more than half of the vitreous has become liquid. This structural shift reduces viscosity from approximately 1 Pa·s in younger individuals to about 0.1 Pa·s in those aged 65–80.

Researchers at Purdue University created HA solutions calibrated to these two viscosity levels to simulate the vitreous environment of younger versus older patients. They then tracked bovine IgG diffusion through these matrices using a novel label-free imaging technique based on tryptophan autofluorescence — meaning no chemical dyes or labels were required, preserving the natural behavior of the protein.

The key result: a 37% increase in mean diffusion coefficient as viscosity decreased from 1 to 0.1 Pa·s. This means protein drugs injected into older eyes diffuse substantially faster than in younger eyes. If drug formulations are designed without accounting for this difference, therapeutic concentrations at the retina could vary significantly by patient age.

The study introduces a practical preclinical screening tool for testing how protein formulations behave across age-representative vitreous environments. Caveats include the use of bovine IgG and simplified HA matrices rather than actual human vitreous, and the in vitro nature of the model limits direct clinical translation.