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Oxygen Permeability
Oxygen permeability (OP) is a parameter of a contact lens that expresses the ability of the lens to let oxygen reach the eye by diffusion. In soft contact lenses, it is dependent on the thickness of the lens and the material of the lens, especially concerning the water content. Because of this dependence on thickness, transmissibility level (abbreviated ''Dk''/''t''), the ''Dk'' per thickness of the lens, is more commonly used. The earliest models of soft contact lenses, based on hydrogel material, had a level of oxygen permeability of around 6–8 ''Dk''/''t''. Polymacon, the material used in the first hydrogel contact lenses in some countries in the 1960s and approved by the FDA in the U.S. in 1971, has a ''Dk'' of 9 . These days, typical values of oxygen permeability for hydrogel contact lenses range from 25 to 50. For example, Nelfilcon A has a ''Dk'' value of 26, and the Omafilcon A has a ''Dk'' of 25. While those numbers are typical of hydrogel contact lenses, many co ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Contact Lens
Contact lenses, or simply contacts, are thin lenses placed directly on the surface of the eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to correct vision or for cosmetic or therapeutic reasons. In 2010, the worldwide market for contact lenses was estimated at $6.1 billion, while the US soft lens market was estimated at $2.1 billion.Nichols, Jason J., et a"ANNUAL REPORT: Contact Lenses 2010" January 2011. Multiple analysts estimated that the global market for contact lenses would reach $11.7 billion by 2015. the average age of contact lens wearers globally was 31 years old, and two-thirds of wearers were female.Morgan, Philip B., et al"International Contact Lens Prescribing in 2010" ''Contact Lens Spectrum''. October 2011. People choose to wear contact lenses for many reasons. Aesthetics and cosmetics are main motivating factors for people who want to avoid wearing glasses or to change the appearance or ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), nonmetal, and a potent oxidizing agent that readily forms oxides with most elements as well as with other chemical compound, compounds. Oxygen is abundance of elements in Earth's crust, the most abundant element in Earth's crust, making up almost half of the Earth's crust in the form of various oxides such as water, carbon dioxide, iron oxides and silicates.Atkins, P.; Jones, L.; Laverman, L. (2016).''Chemical Principles'', 7th edition. Freeman. It is abundance of chemical elements, the third-most abundant element in the universe after hydrogen and helium. At standard temperature and pressure, two oxygen atoms will chemical bond, bind covalent bond, covalently to form dioxygen, a colorless and odorless diatomic gas with the chemical formula ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical potential. It is possible to diffuse "uphill" from a region of lower concentration to a region of higher concentration, as in spinodal decomposition. Diffusion is a stochastic process due to the inherent randomness of the diffusing entity and can be used to model many real-life stochastic scenarios. Therefore, diffusion and the corresponding mathematical models are used in several fields beyond physics, such as statistics, probability theory, information theory, neural networks, finance, and marketing. The concept of diffusion is widely used in many fields, including physics (Molecular diffusion, particle diffusion), chemistry, biology, sociology, economics, statistics, data science, and finance (diffusion of people, ideas, data and price v ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hydrogel
A hydrogel is a Phase (matter), biphasic material, a mixture of Porosity, porous and Permeation, permeable solids and at least 10% of water or other interstitial fluid. The solid phase is a water Solubility, insoluble three dimensional network of polymers, having absorbed a large amount of water or biological fluids. Hydrogels have several applications, especially in the biomedical area, such as in hydrogel dressing. Many hydrogels are synthetic, but some are derived from natural materials. The term "hydrogel" was coined in 1894. Chemistry Classification The crosslinks which bond the polymers of a hydrogel fall under two general categories: physical hydrogels and chemical hydrogels. Chemical hydrogels have Covalent bond, covalent cross-linking bonds, whereas physical hydrogels have non-covalent bonds. Chemical hydrogels can result in strong reversible or irreversible gels due to the covalent bonding. Chemical hydrogels that contain reversible covalent cross-linking bonds, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Polymacon
Polymacon is a non-proprietary (i.e., generic) term for a hydrophilic polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ... of 2- hydroxyethylmethacrylate (HEMA) cross-linked with ethylene glycol dimethacrylate (62%) and water (38%). It is used in the manufacture of soft contact lenses, and is considered a low-hydration hydrogel of nonionic polymer. References Organic polymers {{polymer-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Contact Lenses
Contact lenses, or simply contacts, are thin lens (optics), lenses placed directly on the surface of the Human eye, eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to corrective lens, correct vision or for cosmetics, cosmetic or therapeutic reasons. In 2010, the worldwide market for contact lenses was estimated at $6.1 billion, while the US soft lens market was estimated at $2.1 billion.Nichols, Jason J., et a"ANNUAL REPORT: Contact Lenses 2010" January 2011. Multiple analysts estimated that the global market for contact lenses would reach $11.7 billion by 2015. the average age of contact lens wearers globally was 31 years old, and two-thirds of wearers were female.Morgan, Philip B., et al"International Contact Lens Prescribing in 2010" ''Contact Lens Spectrum''. October 2011. People choose to wear contact lenses for many reasons. Aesthetics and cosmetics are main motivating factors for people who want to avo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Silicone Hydrogel
In organosilicon and polymer chemistry, a silicone or polysiloxane is a polymer composed of repeating units of siloxane (, where R = organic group). They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cooking utensils, thermal insulation, and electrical insulation. Some common forms include silicone oil, grease, rubber, resin, and caulk. Silicone is often confused with one of its constituent elements, silicon, but they are distinct substances. Silicon is a chemical element, a hard dark-grey semiconducting metalloid, which in its crystalline form is used to make integrated circuits ("electronic chips") and solar cells. Silicones are compounds that contain silicon, carbon, hydrogen, oxygen, and perhaps other kinds of atoms as well, and have many very different physical and chemical properties. History F. S. Kipping coined the word ''silicone'' in 1901 to describe the formula of polydiphenylsiloxa ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mass Diffusivity
Diffusivity, mass diffusivity or diffusion coefficient is usually written as the proportionality constant between the molar flux due to molecular diffusion and the negative value of the gradient in the concentration of the species. More accurately, the diffusion coefficient times the local concentration is the proportionality constant between the negative value of the mole fraction gradient and the molar flux. This distinction is especially significant in gaseous systems with strong temperature gradients. Diffusivity derives its definition from Fick's law and plays a role in numerous other equations of physical chemistry. The diffusivity is generally prescribed for a given pair of species and pairwise for a multi-species system. The higher the diffusivity (of one substance with respect to another), the faster they diffuse into each other. Typically, a compound's diffusion coefficient is ~10,000× as great in air as in water. Carbon dioxide in air has a diffusion coefficient of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Barrer
The barrer is a non- SI unit of permeability of gases used in the membrane technology and contact lens industry. It is named after the New Zealand-born chemist Richard Barrer. Definition The barrer is defined as follows: :\rm \ barrer = \frac Confusingly, the centimetre notation is used in four different ways. * To denote an amount of substance, the 'cm3STP' is standard cubic centimeter, which is a unit of amount of substance rather than a unit of volume. It represents the number of gas molecules or moles that ''would'' occupy one cubic centimeter at standard temperature and pressure, as calculated via the ideal gas law. * To denote a pressure differential, the notation 'cmHg' is used; a 'centimetre of mercury', which is ten times the more familiar 'millimetre of mercury'. * And finally, the centimetre and square centimetre are used in the normal way to measure thickness and area. The cm corresponds in the permeability equations to the thickness of the material whose per ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Oxygen Transmission Rate
Oxygen transmission rate (OTR) is the measurement of the amount of oxygen gas that passes through a substance over a given period. It is mostly carried out on non-porous materials, where the mode of transport is diffusion, but there are a growing number of applications where the transmission rate also depends on flow through apertures of some description. It relates to the permeation of oxygen through packaging to sensitive foods and pharmaceuticals. Measurement Standard test methods are available for measuring the oxygen transmission rate of packaging materials. Completed packages, however, involve heat seals, creases, joints, and closures which often reduce the effective barrier of the package. For example, the glass of a glass bottle may have an effective total barrier but the screw cap closure and the closure liner might not. ASTM standard test methods include: * F3136 Standard Test Method for Oxygen Gas Transmission Rate through Plastic Film and Sheeting using a Dynamic A ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
