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Material properties for contact lenses

19 Sep 2010
Nowadays there are many materials for soft contact lenses. Each is composed of different chemical components, and different physical properties and clinical characteristics. There are several basic physical properties, are taken into account when developing materials for soft contact lenses.
These include mechanical properties such as:
- Oxygen permeability;
- Tensile strength;
- Water content;
- Refractive index (refraction);
- Biocompatibility.
Included are also the following:
- The ability to precipitate proteins;
- Stability parameters (ability of a material not change the characteristics of the time);
- Easy to handle;
- Sverhisparyaemost.
In developing new material for a lens main task is to combine full-time characterization of these indicators, without compromising any of the important clinical indicators.
Oxygen permeability
Over the past few years has been much discussion about the characteristics oxygen contact lenses. Oxygen permeability and propuskaemost processes are complex and depend to a considerable extent on the water content in the material, design elements, temperature and type of monomer. Chemists and engineers work together to ensure the best combination of permeability of lens design to achieve the best clinical characteristics.
Oxygen characteristic is important when the lens is placed on the cornea, lens since in this case interrupts the air access to the cornea. When the eye of a large volume of lacks oxygen in the cornea violated the physiological processes that can lead to short-term or long-term impairment.
Short violation:
- Blurred central part of the cornea;
- Vertical grooves;
- Microcysts;
- Thinning of the epithelium and stroma.
Long-term abuse:
- Deformation of the cornea;
- Endothelial gap;
- Changes in refractive errors.
The four most common method for determining oxygen characteristics of the lens. These include:
- Measurement of the oxygen permeability;
- Measurement of oxygen propuskaemosty;
- Determination of oxygen equivalent characteristics;
- Measurement Night swelling of the cornea.
Measurement of oxygen permeability
The most common method for calculating the oxygen of a material for contact lenses is the measurement index or index Dk oxygen permeability. Size Dk material meets the inherent ability to pass through a oxygen. This method allows to determine permeability "in vitro," ie, in the laboratory, without using live tissue. The material is tested in polyarohrafycheskoy cell where oxygen is measured passing through the lens material layer in a given period of time.
The method is based on the formula P = D × k where:
P - permeability,
D - dyffuzyonnaya component
K - oxygen permeability of the material.
This method does not account for the thickness of the lens, so from a clinical point of view, its usefulness is limited. For example, this method of lens thickness of 0.1 mm and a thickness of 1.0 mm lens have the same Dk. But a thicker lens allows only one-tenth of the amount of oxygen that must be because the thicker lens, the greater the resistance to flow of oxygen regardless of what indicators Dk same.
Dk method also depends on temperature. If the laboratory tests are carried out at a higher temperature, the value of Dk increases. As the temperature and other conditions under which the investigated materials vary widely in different laboratories, there are differences in the values ​​Dk.
The method of determining oxygen propuskaemosty
A more useful measure of the oxygen laboratory characteristics are Dk / L, which affects propuskaemost and which is obtained by dividing the Dk (permeability) in thickness at the center of the lens (L). It is to measure the thickness at the center of the lens in millimeters (also sometimes in the center of the lens thickness denoted by the letter t). Since in this case taken into account the thickness of the central part of the lens, this figure is more convenient for the doctor than Dk.
Measuring oxygen equivalent characteristics (ESM)
This method is a measurement of the degree of corneal oxygen deficiency while wearing lenses. Measurement method ESM held on to the living eye by placing the sensor on the cornea with oxygen chamber. Oxygen is absorbed by the cornea, with the measured time during which oxygen from the camera goes into the cornea.
All contact lenses cause a reduction in the amount of oxygen that enters the eye, the cornea and therefore experiencing "oxygen starvation" because the oxygen required to restore normal eyes.
If the cornea is not enough oxygen, the camera sensor quickly spustoshuyetsya.
Tests to determine the ESM is usually carried out with materials of varying thickness. Based on the results of tests are graphs showing ESM percentage. This allows the doctor to compare the characteristics of lenses of different materials and with different central thick.
Corneal Swelling
Swelling of the cornea - is the most accurate indicator of real clinical characteristics. Corneal thickness is measured with great accuracy by computerized devices (optical or digital pahometra). For example, if the swelling of the cornea is 0.5 mm to 0.55 mm, the corneal swelling rate is 10%. Reduced oxygen supply leads to swelling of the eye, which causes swelling of the cornea.
Corneal thickness increases when the eye feels physiological stress, such as maintaining lenses in his eyes throughout the night. Available now extended wear lenses cause corneal swelling within 9-12%, if not to shoot at night. Closing the eyes during sleep, even without the lenses, causing a physiological response in the form of swelling of the cornea - usually within 2-4%. Night corneal swelling often disappears after 30-60 minutes after the man opened his eyes.
Tensile strength
Breaking strength lens affects its characteristics such as ease of handling, durability and pryhonka on the eye.
There are four types of parameters to assess the physical strength. These include:
- Tensile strength;
- Modulus of elasticity;
- Coefficient of elongation;
- Tearing strength.
Tensile strength
Tensile strength shows that efforts must be made to the material to tear it up. The greater the effort, the greater the strength and durability of the material.
Modulus shows the flexibility of the material. Materials with low elastic modulus is more flexible, and those in which the module is high - more resistant to bending.
Tougher lenses (less flexible) are considered easier to handle than those made of flexible materials. The rigidity and flexibility of materials play an essential role for the landing characteristics of the lens. Soft lenses obtained by casting in a centrifuge, very flexible, and therefore the modulus of elasticity of the lowest among all the materials for contact lenses. As a result, they tend to take the shape of the cornea.
The coefficient of elongation
Elongation ratio shows how much should stretch like to break it. This value is measured as a percentage of the size of the sample lens. For example, soft lens obtained by casting in a centrifuge, having a coefficient of elongation of 200%, which means that the lens can be stretched to 200% of their normal size before they will be damaged. Typically, the higher the score, the more resistant to damage lenses when dealing with them.
The water content of a contact lens
The water content of a contact lens is defined as the ratio of the weight of water in contact lens (Pw) until the weight of the saturated water contact lens (Pl) percentage.
SW = (RW / Pl × 100%)
The water content of a contact lens is one of the main parameters of soft contact lenses. The high water content provides the comfort of wearing lenses and corneal oxygen supply.
The water provides oxygen through promotion hydrogel lens material. Molecules of oxygen dissolved in the water and move through the material of the lens to the cornea.
However, the water content of hydrogel contact lenses is limited - if a lot of water, the bad contact lens keeps its shape, it is difficult to apply, it tends to severe dehydration (dehydration) at the end of the day, resulting in markedly worse comfort of wearing contact lenses. Maximum water content in existing to date hydrogel contact lenses not more than 80%.
The refractive index (refraction)
This indicator changes undergoes light beam passing through the material of the lens. Index of refraction is important to design lenses for obtaining an optical effect. While this has no effect on physiological characteristics of lenses, but changes in water content leads to a change in index of refraction. As the water content in the lens, the refractive index decreases. Therefore, the use of materials with low this type of lens to be thicker to provide the necessary optical effect or force. The refractive index of polymers used in contact vision correction is usually in the range of 1.35 to 1.52, and close to the refractive index of the cornea - 1.37 at 34 ° C almost identical to the cornea and refractive index of the tear fluid contribute to correct refractive errors (refraction light rays) eyes.
Due to the thin lacrimal layer obrazuyuschemusya between the lens and cornea, creating a unique optical system, resulting in reduced or eliminated various sizes of images on the retina of both eyes due to refraction difference ?? right and left eyes. Contact lenses also provide correction of astigmatism (violation of refraction of rays in which the rays coming out from one point, after refraction are not going to focus in one).
Since contact lenses interact with the tissue of the cornea, it is important to assess the physiological effect of material at eye Wednesday. This is done in the laboratory to the premises of the lens on the eye to ensure its security. Biocompatible contact lens - a lens that does not cause significant changes in the physiology of the eye. Materials for biocompatible contact lens - a group of polymers, specially designed to improve biocompatibility by simulating the natural membrane structure of cells. Clinical studies have shown that these materials have significant advantages, providing excellent comfort and resistance to dehydration and deposition.
There are a number of different tests to assess the biocompatibility.
Lens materials are tested for cytotoxicity to be sure that the material does not kill cells.
Dvadtsatiodnodnevnyy eye irritation test is conducted on rabbits who dress up lens on the eye for 21 days. Then eyes examined under a microscope for cellular changes.
The ability to precipitate proteins
Protein deposits in the form of films usually are the result of adsorption and / or absorption of proteins. When absorption of proteins in the molecular structure of the contact lens the more protein accumulates, the less is the contact lens dry. The degree of absorption depends on the size of molecules and protein matrix pore size of the contact lens. The process absorbatsyy protein starts immediately after putting contact lens in the eye. Usually squirrels are attracted to ionic contact lenses - positively charged amino acids are attracted to the negatively charged surface of the contact lens. Absorption of proteins - a one-way process, and over time the situation is only getting worse. Key proteins, components of deposits - is albumin, lysozyme, immunoglobulins. The presence of proteins on the surface of the contact lens can result in an immune response from the palpebralnoy conjunctiva (conjunctiva of the eyelids).
Lenses made of materials with high water content often have to make thick, as these materials tend dehydrated if the lens is thin. Moreover, dehydration can be so strong that they start to suck water out of the cornea that causes spots on the cornea epithelium or rupture. Degree sverhisparyaemosti material is measured during testing, whose main task - to make sure that the water content is stable, and the erosion of the cornea with superficial dehydration not cause clinical problems.
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