In situ gels: The next new frontier in ophthalmic drug delivery system

Recently, the eye drug delivery system has received increasing attention. The in situ ophthalmic gel is a semisolid ophthalmic preparation that can be changed in the eyes immediately after the solution is administered, showing unique advantages as a new drug delivery system. Although there are still some problems to be solved, the in situ ophthalmic gel is a promising drug delivery system to treat ocular diseases, due to its properties in improving the bioavailability, prolonging the retention time of the drug, producing a sustained-release effect, and possessing little toxicity and irritation. In this review, the characteristics, classification, ocular barrier, and route of administration of in situ ophthalmic gel have been introduced in detail for expanding the horizon of nanoscale technologies in the treatment of ocular diseases in the foreseeable future.