**3.2 Ocular route**

The thin layer provides a variety of functions as the interface between the ocular surface and the external environment. It generates a refracting thin coating on the corneal surface that smooth's out the uneven topography. It maintains a somewhat constant extracellular environment for corneal and conjunctival epithelial cells in terms of pH, oxygen and carbon dioxide levels, nutrition, and growth factor concentrations. Microorganisms, which are also combated by a complex and powerful antibacterial system, are diluted and washed away by tears. The tear film changes its composition in response to physiological events. The human tear film appears to be quite stable and attached to the cornea while the eyes are maintained open, taking up to 60 seconds to disclose the initial break. When a contact lens is implanted, a stable tear film is not maintained throughout the lens, and the surface dries up between blinks. The tear film on the surface of a contact lens begins to break up in 4–6 seconds (hard) or 7–9 seconds (soft) when a human blinks once every 10 seconds on average. The single most critical element in deposit formation and long-term discomfort during contact lens usage is lens drying. Several theories have been presented to explain this phenomenon. A thin film containing surface-active molecules like those found in tears should not rupture as quickly from a curved surface, according to physical chemistry. Experiments with lenses in the lab have shown that certain lens surfaces may hold a water coating for up to 2 minutes. On-eye surface dryness is caused by many reasons. With a stiff lens, drainage is unavoidable due to the tear meniscus, which is evident around the lens periphery. With the lens on the eye, the structured layers of the tear film are unable to form up as they do on the cornea. Another explanation is that the lipid layer is thin or disturbed, leaving the tear film vulnerable to evaporation. Early tear components binding to lenses might also impair wettability [20].

It has various advantages for ocular distribution, including sanitation, ease of eye drop formulation, less irritation, increased precorneal residence duration, and improved ocular bioavailability of medications that are insoluble in tear fluid. In 2015, Mahajan HS and Deshmukh SR investigated the use of xyloglucan, a polysaccharide polymer, as a new film-forming agent for ciprofloxacin ocular administration. Ciprofloxacin ocular films were made utilising xyloglucan and the solvent casting process (2%). The formulas were made following the unusual transport release mechanism. The formulation is safe for the ocular mucosa, according to an ocular irritancy study. As a result, this research proposes that xyloglucan could be used as a film-forming polymer for ciprofloxacin ocular administration [21].
