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Ocular barriers for drug delivery.
a, Tear Film Barrier; b, Blood-Aqueous Barrier; c, Lens Barrier; d, Corneal Barrier; e, Conjunctival Barrier; f, Blood-Retinal Barrier; g, Scleral Barrier; h, Mucus Barrier; i, Drug Efflux Transport Protein Barrier; j, Metabolic Barrier, k, Drug Binding to Melanin.
Abbreviations：P-gp, P-glycoprotein.
