eye the majority of ocular therapeutic agents are administered to the eye via the topical route. cornea is one of the main barriers for drug diffusion because of its highly impermeable nature. its continuous irrigation with a tear fluid also results in poor retention of the therapeutic agents on the ocular surface. poor permeability of the cornea and quick wash out of therapeutic agents from ocular surface result in very low bioavailability of the drugs administered via topical route (typically less than 5%). poor retention of formulations on ocular surfaces could potentially be improved with the use of mucoadhesive polymers. drug permeability through the cornea could be facilitated with addition of penetration enhancers into topical formulations. transplantation main article: corneal transplantation if the corneal stroma develops visually significant opacity, irregularity, or edema, a cornea of a deceased donor can be transplanted. because there are no blood vessels in the cornea, there are also few problems with rejection of the new cornea. when a cornea is needed for transplant, as from an eye bank, the best procedure is to remove the cornea from the eyeball, preventing the cornea from absorbing the aqueous humor. there is a global shortage of corneal donations, severely limiting the availability of corneal transplants across most of the world. a 2016 study found that 12.7 million visually impaired people were in need of a corneal transplant, with only 1 cornea available for every 70 needed. many countries have years-long waitlists for corneal transplant surgery due to the shortage of donated corneas. only a handful of countries consistently have a large enough supply of donated corneas to meet local demand without a waitlist, including the united states, italy, and sri lanka.