Tear film surface tension

The surface tension of tear fluid at the eye temperature has been measured as 43.6 to 46.6 rriNm-1 for normal eyes and 49.6 mNm"1 for patients with dry eye. The instillation of a solution containing drugs or adjuvants that lower the surface tension may dismpt the outmost lipid of the tear film into numerous oily droplets, which become solubilized. The protective effect of the oily film against evaporation of the tear film aqueous layer disappears and diy spots will be formed. The dry spots are painful and irritant and elicit reflex blinks to eliminate the material. This irritation does not always occur immediately after the instillation. In many cases it appears 30 minutes to 1 hour following the application and is dependent on the substance and on its concentration. The tear film is destabilized when the surface tension of the instilled solution is much lower than the surface tension of the lacrimal fluid. 

PVA can lower the surface tension of water, reduce interfacial tension at an oil/water interface and enhance tear film stability. These together with ease of sterilization, compatibility with a range of ophthalmic dmgs and an apparent lack of epithelial toxicity have led to the widespread use of PVA as a drag delivery vehicle and a component of artificial tear preparations. 

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... 

Figure 9.32 is a diagrammatic representation of those parts of the eye involved in dmg absorption. The cornea and the conjuctiva are covered with a thin fluid film, the tear film, which protects the cornea from dehydration and infection. Cleansed comeal epithelium is hydrophobic, physiological saline forming a contact angle of about 50° with it, and it has, in this clean condition, a critical surface tension of 28 mN m b The aqueous phase of tear fluid is spread by blinking. 

If the decision is taken to administer very small volumes (1-10 microlitres) of concentrated active substance solutions in order to compensate for dilution in the tear film, irritation becomes a problem. Consequently excessive lachrymation, drainage and wash out will occur, resulting in low active substance availability. To avoid irritation the surface tension, osmolality, pH and buffer capacity should be within certain limits (see Sects. 10.4.5 and 10.4.6). 

Tiffany JM, Winter N, Bliss G (1989) Tear film stability and tear surface tension. Curr Eye Res 8 507-15... 

Many novel high-performance soft contact lenses contain siloxane backbones because of the high oxygen transmissibility DJL) provided by such a chemistry [12]. The ocular environment remains healthier with siloxane-based lens materials. However, siloxane molecules are not inherently water wettable because of their low critical surface tensions [llj. In addition, siloxane surfaces attract lipid-soluble material, which can penetrate the lens [16]. Siloxane-based lenses can also attract and attach proteins and mucus on the surface of the lens [16]. The physical and chemical changes in the lens surface and bulk associated with deposit formation include tear film disruption, decreased vision, discomfort, decreased lens life, conjunctival hyperemia, and bacterial adhesion [17,18]. 

It may not be conunonly known that the tank wall above flie liquid level in a LNG tank is wetted by surface tension driven film flows up flie tank wall. This effect, an example of the so-called Marangoni effect, was first studied at Southampton in 1974 [2]. Have a look to see for yourself Then compare what you see with the tear-drops round the edge of a glass of sherry or port (see Fig. 6.1). 

Tearings reqnire relative movements of the molecnles, and the intrinsic viscosity of the snrface layers, viscosity which makes these movements difficult, also obstructs those which lead to tearings as well as those which bring interior molecules in the increased interstices of the external ones. Thus, while passing from soap to albumin, the tension, i.e. the force which tends to tear the films, becomes double, but the resistance to this tearing increases at the same time by the increase in the viscosity of the surface layers, and the albumin films extend in bnbbles like those of soap, only to a lesser degree. 

Thus, on the one hand, because of the continuous absorption of water vapor, the film can, in no phase of its existence, arrive at being very thin, and, on the other hand, the ratio between the surface viscosity and the tension remains large enough to make tearings difficult until the film has assimilated a very great proportion of water. These two circumstances, it is seen, fully explain the long persistence of the bubble. 

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