Tear film composition

Anatomical characteristics and physiological mechanisms protect the eye against toxic external effects. These mechanisms include the specific structure of the cornea, blinking, baseline and reflex lachiymation, drainage, tear film composition and the corneal sensitivity. The combination of aU mechanistic, anatomical and physiological characteristics maintains the integrity of the eye, together... 

A number of ocular surface disorders collectively termed as Dry Eye Syndromes have also been associated with the conjunctiva. For example, a deficiency and/or imbalance in compositions of the tear film is often found on the ocular surface during keratoconjunctivitis sicca. Since the conjunctiva plays a direct role in the maintenance of the tear fluid stability via secretion of mucin [1] by its resident goblet cells [4] and basal fluid secretion driven by electro-osmotic gradients across the tissue [3], the conjunctiva is a well deserved, but not intensively studied, target of interest in research efforts aimed against combating Dry Eye Syndromes. 

McCulley, J.P., W.E. Shine. 1977. A compositional based model for the tear film lipid layer. Trans Am Ophthalmol Soc 95 79. 

A newer development in ocular drug delivery systems is the use of large molecules that exhibit reversible phase transitions whereby an aqueous drop delivered to the eye reversibly gels on contact with the precorneal tear film. Such changes in viscous properties can be induced by alterations in temperature, pH, and electrolyte composition. Gelrite, a polysaccharide low-acetyl gellan gum, forms clear gels in the presence of mono- or divalent... 

Figure 14-1 Structure and composition of tear film as previously proposed. (Modified from Holly FJ, Lemp MA.Tear physiology and dry eyes. Surv Ophthalmol 1977 22 70.)...

Murube J, Paterson A, Murube E. Classification of artificial tears. I. Composition and properties. In SuUivan DA, Dartt DA, Meneray MA, eds. Lacrimal gland, tear film, and dry eye syndromes 2 basic science and clinical relevance. New York Plenum Press, 1S>98 693-704. 

Etiology. Meibomian gland secretions are responsible for the lipid component of the precorneal tear film. The chemical composition of meibum and/or the lipase action of the normal lid bacteria is thought to contribute to or cause blepharitis and in many cases the dry eye that accompanies it. The composition of meibum has been found to be different in normal and blepharitis patients, and there is a distinct difference between the types of MGD as well. Most studies of lid flora in bacterial blepharitis cases did not find any appreciable isolates from meibum that were not found as normal flora on the lids, therefore disproving the theory that the meibomian glands act as a bacterial reservoir. 

Various film compositions of COC added to PP, HOPE, and LLDPE were tested by the company under retort conditions. Researchers found that the COC improved pouch seal strength and vapor transmission rate, with only slight losses in tear resistance. And after retort treatments, the COC content reportedly improved film stiffness, without causing other negative changes to properties or appearance... 

The lipid composition is kept within physiological limits by androgens [17]. The decrease of their secretion in elderly people is one of the reasons for development of dry eye syndrome. Patients with Meibomian gland dysfunction show a high tear film evaporation rate and a high tear osmolality [18, 19]. 

In general, LDPE provides film with better optical properties e.g., clarity and haze) and is easier to process. However, films made from LLDPE or HDPE display better mechanical properties (puncture resistance, tear strength, etc.), though they are more difficult to process. For this reason, LDPE is sometimes used as blend-stock with LLDPE and HDPE (18). The blended composition becomes easier to process while retaining good mechanicals. 

Tear tests are applicable to flexible materials and thin plastic films. For rigid plastics and composites, a parallel can be drawn with the various crack growth tests, which are covered in Chapter 22. In a normal tensile test, taken to break, the force to produce failure in a nominally flawless test piece is measured. In a tear test, the force is not applied evenly but concentrated on a deliberate flaw or sharp discontinuity, and the force to produce continuously new surface is measured. This force to start or maintain tearing will depend in rather a complex manner on the geometry of the test piece and the nature of the discontinuity. 

A copolymer composition of 80 parts of a methacrylate end capped 65 mole % trifluoropromrl siloxane, 20 parts of octmuoropentylmethacrylate and 5 parts VDMO gave a transparent, wettable film possessing a modulus of 120 g/rnm a tear stren h of 5.8 g/mm, an oxygen permeability of 230 Barrers and an oleic acid uptake of only 1. r%. 

Calcium carbonate is also used to manufacture NR composites because of its low cost and performance at high loading level. Cai et al. prepared NR latex films reinforced with ultrafine calcium carbonate by solution mixing. They found that calcium carbonate was well dispersed in the NR latex. It was found that the tensile strength, modulus and tear strength of NR composite films were improved by a relatively low loading of calcium carbonate (15 wt%). 

Reactor blends are prepared during polymerization. The strategy has been often explored in syntheses of multimodal PE compositions for the blown film with MD/TD tear balance and good processability. In bimodal blend, the optimum was usually found between 35 and 75 wt% of the higher-MW component ... 

PE blends were prepared during polymerization. This strategy has been widely implemented in syntheses of multimodal PE compositions. The blown film showed MD/TD tear balance. The bimodal blend eontained 0.35-0.75 weight fraction of a higher-MW component Ali A.H. et al. to Mobil Oil Co. Dammert et al. to Borealis. Catalyst TiCLj ntanol deposited on MgO/acetic acid and that treated with tri-n-hexyl-alummum solution... 

Some compositions give rise to films with good masking property [85], Synthetic leather with good abrasion and chemical and wear and tear resistance was prepared [86] using a dimer polyester elastic sheet and a stretched plastic sheet. The nontoxic property of dimer acid helped create personal care products, such as conditioners, remoisturisers, etc. [87]. 

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