Interaction with Tears

Several characteristics of polymers affect their interaction with tear contaminants, particularly the electrostatic 

There are other types of proteins that adhered to the CL surface such as albumin or lactoferrin, whose mechanism of adhesion seems to be different from lysozyme. However, because of the large molecular weight of albumin compared to lysozyme, this protein only forms deposits on the lens surface without penetration into the lens matrix [108]. Quantities of proteins recovered from worn CL are on the order of 50 pg per lens for non-ionic materials to more than 500 pg per lens for ionic group IV materials [101,109]. 

Previous in vitro studies have confirmed the cdfin-ity of certain deposits for certcun materials [102, 103]. Furthermore, ui in vivo study hcis shown that despite significant interaction between lens matericd cuid Ccffe system, the main contributor to the amount of cholesterol and lysozyme deposited on silicone hydrogel materials is lens material [112]. This effect was particularly remarkable for lotrafilcon B that deposited between 0.1 to 0.5 mg of cholesterol irrespective of the care system used. Polar and non-polar lipids show differences in their pattern of adhesion to different Si-Hy CL materials [103,113]. 

Clinical evaluation of surface wettability and front surface deposits was not significantly different between Si-Hy lenses worn continuously for 6 nights or 30 nights [114]. Maziarz et al. [115] found that the amount of cholesterol recovered from balafilcon A lenses worn overnight or under a daily wear schedule were similar. However, while lenses worn overnight presented values of cholesterol between 10 to 30 pg/lens, lenses worn on a daily wear schedule presented values that in the majority of patients were between 10 to 20 pg/lens. This study concluded that cholesterol was the lipid most frequently adhered to CLs of different types including Si-Hy and conventional hydrogel materials, while other authors obtained opposite results [106]. 

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The cellulose fiber in paper is attacked and weakened by sulfur dioxide. Paper made before about 1750 is not significantly affected by sulfur dioxide (11). At about that time, the manufacture of paper changed to a chemical treatment process that broke down the wood fiber more rapidly. It is thought that this process introduces trace quantities of metals, which catalyze the conversion of sulfur dioxide to sulfuric add. Sulfuric acid causes the paper to become brittle and more subject to cracking and tearing. New papers have become available to minimize the interaction with SO2. 

Acrylic acid AA Flexibility Elydrophilicity pEI sensitivity —acidic Reactivity—ionically interacts with positively charged tear components Wettability... 

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. 

The other major class of extracellular LBPs of mammals is the lipocalins (Flower, 1996). These are approximately 20 kDa, P-sheet-rich proteins, performing functions such as the transport of retinol in plasma or milk, the capture of odorants in olfaction, invertebrate coloration, dispersal of pheromones, and solubilizing the lipids in tears (Flower, 1996). The retinol-binding protein (RBP) of human plasma is found in association with a larger protein, transthyretin, the complex being larger than the kidney threshold and thus not excreted, although the RBP itself may dissociate from the complex to interact with cell surface receptors in the delivery of retinol (Papiz et al., 1986 Sundaram et al., 1998). 

A further possibility is the formation of liquid crystals on contact with body fluids at the site of application. The initially applied drug solution interacts with body fluids such as plasma, tears, or skin lipids and undergoes a phase transition into a mono-or multiphasic system of liquid crystals (Fig. 15). For example, oily solutions of reverse micellar solutions of phospholipids, which solubilize additional drug, trans-... 

The great majority of the available work has focused on blood/plasma/serum applications. In recent years the interaction of tear components with contact lenses have become very important. 

Other Vinyl Derivatives. PVP is a nonionic surfactant used in 3% to 5% concentrations to increase viscosity of solutions. Although it exhibits surface-active properties similar to the cellulose ethers, PVP appears to have less abiUty to lower the interfacial tension at a water-oil interface. Nevertheless, in contrast to the cellulose ethers, PVP appears capable of forming hydrophilic coatings in the form of adsorbed layers. Because conjunctival mucin is believed to interact with the ocular surfece to form an adsorbing surface for aqueous tears, the formation by artificial means of a hydrophilic layer that would mimic conjimctival mucin (mucomimetic) appears to be clinically desirable. Both mucin- and aqueous-deficient dry eyes would benefit, because the wetting ability of the corneal surfece would be enhanced. 

The tear film is a dynamic fluid layer with lipid, aqueous, and mucin components that interact with each other... 

With each blink the antigen-coated contact lens mechanically traumatizes the tarsal conjunctiva. This process causes the release of mediators, such as neutrophil chemotactic factor and eosinophil chemotactic factor, which attract inflammatory cells (e.g., neutrophils, eosinophils, mast cells, and basophils). The immunologic sequence of events results in an increase in tear immunoglobulins IgE and IgG and C3 anaphylatoxin. The tear immunoglobulins and C3 anaphylatoxin then interact with the inflammatory cells produced from the mechanical trauma. This interaction causes the release of vasoactive amines, resulting in subsequent clinical manifestations. Papillae formation is related to structural changes in the conjunctival epithelium and stroma associated with increased eosinophils and inflammatory cells. 

A filler with a high surface area increases the interaction with the matrix and thus increases tear strength (Figure 8.23) When rubber is filled with silica the large surface area of the silica interacts with the rubber and adheres to it. This adhesive interaction allows energy to be stored or dissipated. 

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