Fig. 3

Schematic diagram of the human eye with barriers [tear film (precorneal), corneal, conjunctival, blood aqueous, vitreoretinal interface to blood-retinal barrier] and the common route of administration. Primary methods of drug delivery to the eye are topical (1), local ocular [e.g., intravitreal (2) and subconjunctival (3)] and systemic [i.e., oral (4) and intravenous (5)]. Topical instillation is the most widely preferred non-invasive route of drug administration to treat diseases affecting the anterior segment and, potentially, the posterior segment. The ocular barriers block the entry of the most active molecules; hence, effective drug delivery systems are required to facilitate the passage of the drug across these barriers and transport the given pharmaceutical compound to its target site to achieve an optimal therapeutic effect [151]