Oxygen permeability material

The number of contact lens wearers has grown to an estimated 24 million in the United States and 50 million worldwide. Concurrendy, there has been a proliferation of contact lens manufacturers and products. The 1980s saw the widespread introduction of lens products made of more oxygen-permeable materials, ie, rigid gas-permeable (RGP) materials that made PMMA lenses virtually obsolete and high water content hydrogels that competed with HEMA-based lenses. 

Hard lenses can be defined as plastic lenses that contain no water, have moduli in excess of 5 MPa (500 g/mm ), and have T well above the temperature of the ocular environment. Poly(methyl methacrylate) (PMMA) has excellent optical and mechanical properties and scratch resistance and was the first and only plastic used as a hard lens material before higher oxygen-permeable materials were developed. PMMA lenses also show excellent wetting in the ocular environment even though they are hydrophobic, eg, the contact angle is 66°. 

Eresh fmit and vegetable packaging is often in bulk in a variety of traditional wooden boxes and crates, and cormgated fiberboard cases. At or near the retail level, bulk produce may be repackaged in oxygen-permeable flexible materials such as PVC, with or without a tray of foamed polystyrene. 

Numerous variations and other appHcations are common for flexible packaging materials, eg, oxygen-permeable wraps for fresh red meat and produce shrinkable, low oxygen permeabiHty bags for meat and rigid tray closures. 

Certain copolymers of this type have been found to have excellent gas barrier properties, with the dry polymer having an oxygen permeability only about 1/lOth that of polyvinylidene chloride. Unsurprisingly, the copolymer has a high moisture absorption and a high moisture vapour transmission rate. Where the material is swollen by water, gas permeability is also higher. 

Commerical grades of EVOH typically have vinyl alcohol contents in the range 56-71%, but in contrast to the corresponding EVA materials these copolymers are crystalline. Furthermore, an increase in the vinyl alcohol content results in an increase in such properties as crystalline melting point, tensile strength and tensile modulus together with a decrease in oxygen permeability. This is a reflection of the fact that the ethylene and vinyl alcohol units in the chain are essentially isomorphous (see Sections 4.4 and 14.3.1). 

A large membrane pack of this type will act like an artificial gill, permitting a swimmer to breathe like a fish and remain submerged for much longer periods of time than are possible with scuba equipment. Speculative fiction has man returning to live in the seas, and this type of application may make it possible. Their application in spacecraft is obvious as a part of a continuously recycled air support system. The oxygen permeability of silicone materials is just one example of the selective permeability of plastics. 

Siloxane-(methyl methacrylate) networks showing high oxygen permeabilities were prepared by gamma-irradiation of tx,(o-methacryloyl terminated tetramethyl-disiloxanes and methyl methacrylate 325). Due to the utilization of very low molecular weight disiloxane modifier, no phase separation was observed in the networks produced. DSC studies showed only one Tg around 120 °C. These materials were evaluated... 

Grouping for rigid gas-permeable lenses was published by FDA in 1989. The generic names and oxygen permeabilities of rigid gas-permeable (RGP) lens materials are provided in Table 7 [310]. 

Moyssiadi T, Badeka A, Kondyli E, Vakirtzi T, Savvaidis I and Kontominas M G (2004), Effect of light transmittance and oxygen permeability of various packaging materials on keeping quality of low fat pasteurized milk chemical and sensorial aspects , International Dairy Journal, 14, 429 -36. 

In the early 1990s, Balachandran et al. (51,64,65) of the Argonne National Laboratory, in collaboration with Amoco (now part of BP), investigated the partial oxidation of methane using membrane materials consisting of Sr-Fe-Co-O mixed oxides with the perovskite structure, which have high oxygen permeabilities. In their experiments (51,66), the membrane tubes, which were... 

Examples of silicones in ophthalmic application have been described.5 1 However, some problems have been reported with the use of silicones in retinal tamponade.542 Silicones are widely used in contact lenses because of their unusually good oxygen permeability and bioinertness, but the need for novel materials continues. Low-modulus fluorosiloxane-based hydrogel lenses with a wide range of water content and high-oxygen permeability have been described in a number of papers.543-... 

Polysiloxanes are widely employed as biomaterials. Artificial skin can be fabricated from a bilayer fabricated from a cross-linked mixture of bovine hide, collagen, and chondroitin sulfate derived from shark cartilage with a thin top layer of polysiloxane. The polysiloxane acts as a moisture and oxygen-permeable support and protects the lower layer from the outer world. A number of drug delivery systems use polysiloxanes because of the flexibility and porous nature of the material. 

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