Drug delivery to the eye can be challenging for many reasons. Barriers such as pre-corneal drug loss from tear drainage through the nasolacrimal duct, poor corneal permeability, or absorption of drug into vessels of the conjunctiva prevent many drugs from effectively reaching their targets. Often the target, like the retina, is in the posterior portion of the eye. Conventional methods such as topical application or systemic administration are not very effective in overcoming these barriers and intravitreal injections pose safety concerns, because of the invasive nature of the procedure. We propose here the use of microneedles, i.e. micron scale needles, to administer drugs in a minimally invasive way into ocular tissues such as the sclera and cornea. We assess the capabilities of hollow microneedles to infuse a drug solution into ocular tissues. We also asses the ability of solid micorneedles coated with a drug formulation to release their drug coating within ocular tissues.

Hollow microneedles were used to asses the ability of infusing a sulforhodamine solution, nanoparticle suspension and microparticle suspension into human cadaver sclera in an in-vitro experiment. The hollow micorneedles delivered tens of microliteres of sulforhodamine solution and nanoparticle suspension at depths of hundreds of microns into the sclera without penetrating the tissue. Micorparticle delivery was also accomplished at this scale using either collagenase or hyaluronidase to break up densely packed collagen or glycosaminoglycans fibers within the sclera.

Solid microneedles were coated with sodium fluorescein and inserted into rabbit cornea in an in-vivo experiment. The sodium fluorescein concentration was monitored in the anterior portion of the eye over a 24 hour period. Concentrations were achieved in the eye at 70 times greater using our solid microneedles as compared to the conventional route of topical delivery of the same dose. A similar experiment using micorneedles coated with pilocaprine, which constricts pupil size and is used in the treatment glaucoma, was performed and provided similar results. A safety exam was performed by a clinical ophthalmologist and showed the eye had no adverse events or inflammatory response to the microneedle procedure.

These results show for the first time that microneedles are capable of delivering drugs into ocular tissues. The results also show that micorneedles, in a minimally invasive manner, can deliver drugs to provide higher bioavailability than conventional methods, such as topical application. Microneedles have the potential to deliver formulations that can provide controlled delivery for the treatment of diseases of the eye.