Tears: artificial, 96 natural

Advances in chemical synthesis and in an understanding of the tear film of the eye have resulted in the development of compounds with two or more regions that vary in both their lipophilic nature and binding. The first of these to be tested in the eye was poloxamer 407, a block polymer vehicle with a hydrophobic nucleus of poly-oxypropylene,and hydrophilic end groups of polyoxyethylene. One advantage of poloxamers is their ability to produce an artificial microenvironment in the tear film, which can greatly enhance the bioavailability of lipophilic drugs such as steroids. 

The ideal artificial tear would reproduce the metabolic, optical, and physical characteristics of natmal tears. Additionally, it would have a long ocular residence time and would contain therapeutic additives to treat primary and secondary damage to the eye. Supplementation of natural tears with a substance that prolongs residence time generally improves tear film breakup time (TBUT) and is superior to tear replacement fluids of low retention time. 

Because they are not produced constantly, as are natural tears, tear substitutes should have properties to enhance their retention time in the tear film. The addition of various types of polymers to artificial tears not only improves retention of added fluid but also increases corneal surface wettability, decreases blink friction, and minimizes surfece tension. Natural tears contain glycoproteins and other surfectant macromolecules to decrease surface tension. Although polymers may enhance tear film stability without enhancing viscosity, there appears to be no correlation between retention time and viscosity. Polysaccharides, including mucilages, dextrans, and... 

Elettrolytes, The addition of electrolytes is designed to maintain or lower the osmolarity of artificial tears as compared with natural tears. Some electrolytes are important for corneal epithelial metabolism and as part of a buffer system. Sodium chloride contains the most important electrolytes in tears, but potassium is another necessary nutrient for corneal epithelial metaboUsm. 

PU is a strong, hard-wearing, tear-resistant, flexible, oil-resistant, and blood-compatible polymer. The functional properties of natural macromolecules can be merged with those of synthetic polymers having controllable structures and properties for the production of polymer/protein hybrids. In tissue engineering, silk fibroin/PU blend film can be used as scaffold material for artificial blood vessels [466] (Figure 2.62). Bacterial synthesized cellulose, which was designed... 

Protein deposition on materials surface is dependent on chemical nature of the surface (hydrophilicity, hydrophobicity, charge density) and also on the size and chemical composition of the proteins exposed to the synthetic surface (2). Since different biological media are composed of rather different types of proteins, materials designed for specific in lant applications must be tested using those proteins most likely encoimtered in vivo. Materials used in contact leiis applications are generally tested for in vitro protein adsorption using an artificial tear fluid (ATF) containing mixtures of proteins and lipids that are commonly found in hximan tear fluid and in contact lens deposits (15, 16). A... 

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