Tear film, proteins

Clinical experience has shown that certain types of lens materials are more prone to deposit problems. In general, lenses with negatively charged moieties at the surface accumulate greater amounts of lysozyme, the principal tear film protein (10). The introduction and use of disposable lenses make these deposits and their clinical problems less significant. 

VHL Lee, DJ Schanzlin, RE Smith. (1986). Interaction of rabbit conjunctival mucin with tear protein and peptide analogs. In FJ Holly, ed. The Preocular Tear Film in Health, Disease, and Contact Lens Wear. Lubbock, TX Dry Eye Institute, pp 341-355. 

Clinical analysis of the tear film has become increasingly more developed. Examples include tear osmolarity, tear function index, and tear protein analysis, including lactoferrin, lysozyme, albumin, and immunoglobulin.Tear osmolarity and lactoferrin concentration measurements, in particular, appear to have a reliable positive predictive value among dry eye patients. At this time these techniques have more application in research than in clinical practice. 

The osmolality of the tear film equals 310-350 mOsm/kg in normal eyes and is adjusted by the principal inorganic ions Na K+, CD, HC03 , and proteins. The mean pH value of normal tears is about 7.4. Depending on age and diseases, values between 5.2 and 9.3 have been measured. 

The pH value of the lachrymal fluid is about 7.4. Due to evaporation of CO2 from the tear film when the eyes are open, the pH value increases to 8 and even higher values [52]. Three buffer systems are present in tear fluid bicarbonate-carbonate, mono-dibasic phosphate and amphoteric proteins the buffer capacity is low [53]. The acid-neutralising capacity of the tear fluid of one eye is equal to about 8-10 microlitres 0.01 M NaOH. 

Green-Church KB, Butovich I, Willcox M et al (2011) The international workshop on Meibomian gland dysfunction report of the subcommittee on tear film lipids and lipid-protein interactions in health and disease. Invest Ophthalmol Vis Sci 52 1979-93... 

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]. 

Any instilled drug which has not been swept away from the precorneal area by the drainage apparatus, and has further escaped the effects of protein binding, is subject to metabolic degradation in the tear film (Cf. Fig. 6). Drug metabolism in the precorneal area (which, clearly, will be a function of the susceptibility of individual drugs), is a potential mechanism which can contribute to the loss of biological activity of topically applied medications. 

Proteins dissolved in the lacrimal fluid influence the viscosity of human tears, which ranges from 1.3 to 5.9 cps with a mean value of 2.92 cps. The tears have a pseudoplastic character with a yield value of about 32 cps at 33 °C. During a blink the lid moves at a high velocity and the film is submitted to a high rate of shear of about 10,000-40,000... 

The FHT measured by the Wilhelmy balance method can be effectively used to compare the liquid holding capabilities of different surfaces. The value of FHT depends on the experimental parameters and cannot be used in an absolute sense. In general, the aqueous film stability is obtained when spontaneous wetting occurs on imperturbable surfaces. However, moderately hydrophilic and possibly even some hydrophobic surfaces that are perturbable by water were found to be capable of holding continuous films of water. Multicomponent fluid, such as a dilute solution of protein used as a simulated tear fluid, may yield misleading liquid holding characteristics of surfaces due to preferential adsorption of components on a surface. 

During yuba production, protein and lipid contents in the film successively formed decrease, while the carbohydrate and ash contents gradually increase. Therefore, the first several pieces of yuba to be lifted off the heated soymilk are considered premium products. They have creamy white color, mild flavor, and less sweet taste. They stay relatively soft and flexible even when dried. The later formed products are regarded as second rate, because they become sweeter with a faintly reddish tinge, lack internal cohesiveness, tear more easily, and become brittle upon drying. 

The human cornea is extremely hydrophobic. Our tears treat the surface of the cornea by depositing hydrophilic proteins that stabilize the lachrymal film. Another interesting example is that of mushroom spores that can play havoc in rice plantations. Their destructive effect can be traced to their ability to alter the surface of the rice plant—normally very hydrophobic—by turning it hydrophilic, enabling the spores to readily attach themselves to it. 

The first studies on the topic of dewetting were concerned with the instability of the lachrymal film. When the protein composition of the tears becomes abnormal, the cornea turns hydrophobic and the eye undergoes dewetting, leading to a condition known as dry eye syndrome. Fatty cosmetic creams irritate the eyes because they, too, render the cornea hydrophobic. Those who wear soft contact lenses must take great care to prevent dewetting of the lachrymal film. Otherwise, the lens sticks to the eye, and prying it loose can be an extremely painful experience. 

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