Oxygen permeability of the hottest contact lens ma

Oxygen permeability of contact lens materials

Abstract: the oxygen permeability of lenses is an important indicator of contact lenses, and its level directly affects the wearer's comfort when wearing lenses. This paper describes in detail the importance, influencing factors and testing methods of oxygen permeability of contact lens

key words: contact lenses, oxygen permeability, water content, oxygen permeability test

when we mention contact lenses, we most often think of its "stealth" effect, because the contact lens is directly attached to the corneal surface of the human eye to change the refractive ability of the eye to achieve the purpose of correcting myopia or hyperopia, Therefore, contact lenses have many advantages that Chinese scientists and industry insiders are focusing on them, which frame glasses do not have, such as no pressure on the wearer's nose and ears, and are especially suitable for some special professions (such as actors, athletes, etc.). In fact, the role of contact lenses is not only limited to correcting vision problems such as myopia, hyperopia, astigmatism, but also has many other functions, such as therapeutic effects (color blindness, adsorption of drugs to treat some eye diseases), etc

1. Material requirements for contact lenses

since contact lenses are directly attached to the surface of the cornea of the human eye, the biggest highlight of the wearer's choice of contact lenses is the new golden color model lens, which pays special attention to the choice of lens materials. Due to the influence of advertising, many wearers tend to unilaterally pursue lenses with high water content when purchasing contact lenses, believing that the higher the water content, the better the quality of lenses. In fact, one-sided pursuit of high water content of lenses is a misunderstanding in the choice of contact lenses. The wearer's choice of water content of lenses should be related to the secretion of tears, working environment and living environment. In fact, the oxygen permeability of lenses is the most important indicator of contact lenses. The various discomfort reactions of wearing contact lenses and the length of wearing time mostly depend on the oxygen permeability of lenses. Now the advertised lenses with high water content and ultra-thin are also to improve the oxygen permeability of lenses

corneal hypoxia is the most common discomfort reaction of contact lens wearers, which is caused by the inability of the cornea to get enough oxygen. The cornea used to breathe by taking oxygen directly from the air. Since the contact lens is pasted on the surface of the eye, although the oxygen can still pass through the lens, the transmission amount is significantly reduced. Therefore, the corneal neovascularization caused by corneal hypoxia increases, resulting in "red filaments" and increasing the chance of eye infection. Therefore, when selecting contact lens lenses, the lenses with high oxygen permeability should be selected. Because oxygen is transmitted by water, the oxygen permeability of the lens is also related to the water content of the lens and the thickness of the lens. However, whether the contact lens matches the wearer's cornea (whether the matching state is perfect) is the decisive factor affecting the wearing effect

2. Oxygen permeability test

because the oxygen permeability of contact lenses directly affects the wearing comfort of consumers, and then affects the sales of lenses, the detection of this index is of special concern to lens manufacturers

is10. Push the lower pressing plate into the experimental machine. O has formulated the test standard for the oxygen permeability test of contact lenses: ISO optics and optical instruments - contact lenses - Part 1: determination of oxygen permeability and transmission with the fat method (Optics and optical instruments - contact lenses - Part 1: Determination of oxygen permeability and transmission by Polarography), And ISO optics and optical instruments - contact lenses - Part 2: determination of oxygen permeability and transmission by the coulometric method. Among them, the testing principle and testing equipment of ISO are very close to the Coulomb sensor method used in the oxygen permeability testing of flexible packaging materials. Many contact lens manufacturers have entrusted Labthink blue light laboratory to test the oxygen permeability of lens materials, and the test results are very ideal

iso is applicable to hard lens materials and non hydrogel flexible contact lens materials. The test quantities include oxygen flux - J, oxygen permeability - DK, oxygen transmission - dk/t, and the thickness of the sample 0 Thickness - t, generally DK value is used to evaluate the oxygen permeability index of contact lenses. The detection principle is shown in Figure 1: after the sample is loaded into the test chamber, the test chamber is divided into upper and lower parts. The upper chamber is filled with test gas with a certain oxygen content. The carrier gas flow machine runs through the lower chamber and flows into the Coulomb sensor. The pressure on both sides of the sample is equal, but the oxygen partial pressure is different. Under the effect of concentration difference, oxygen enters the carrier gas flow through the film and is sent to the Coulomb sensor, The Coulomb sensor accurately measures the amount of oxygen carried in the nitrogen gas flow, so as to calculate the oxygen transmittance of the material. Before the formal test, the oxygen permeability testing equipment needs to use the reference film to calibrate the equipment, determine the correction factor of the equipment, and use it in the calculation of the formal test. The standard ISO clearly indicates that the sample used for calibration is SRM 1470, which is the same as the reference film used in the film test standard ASTM D 3985

Figure 1 Test schematic diagram

note: 1 Air source; 2. Carrier gas source; 3. Gas dryer; 4. Humidity regulator; 5. Temperature controller; 6. Coulomb sensor; 7. The test process of voltage recording device

mainly includes the following steps: purging the zero point of the system, introducing oxygen flow into the upper chamber of the test, stabilizing the output value of the sensor (judging that the penetration has reached equilibrium), and obtaining the test results. There are two main differences between the testing of contact lens and the testing of soft packaging materials: first, the sample clamping mechanism is different, and the sample shape and area vary greatly; The second is that the test environment is different. The standard test temperature of film oxygen permeability is 23 ℃. In order to simulate the actual use of contact lenses, the test temperature is selected as 35 ℃. Since the temperature change will significantly affect the oxygen permeability test results, the stability of temperature during the test is very important, so the equipment should preferably have a certain temperature control function, such as Labthink toy-c2 oxygen permeability instrument

3. The influence of lens fitting state on oxygen permeability

whether contact lenses are suitable or not mainly depends on whether the fitting state is perfect. A good contact lens should match the lens with the cornea, and there is a layer of tears between them, and this layer of tears is constantly flowing, bringing oxygen to the cornea and taking away waste. In this way, the wearing contact lens will not change the normal physiological condition of the eye and will not damage the wearer's cornea. If the contact lenses you wear do not match the cornea, even if you wear high water content and ultra-thin glasses, the lens will stick closely to the cornea, so that the flow of oxygen can only depend on the water content and the thickness of the lens. Of course, it is beneficial to choose lenses with high oxygen permeability regardless of the adaptation state

4. Summary

the oxygen permeability of lenses is an important indicator of contact lenses. Its level directly affects the wearer's comfort when wearing lenses. Therefore, all contact lens manufacturers are trying to improve the oxygen permeability of lenses. Now, the promotion of oxygen permeable rigid contact lenses (RGP) has significantly affected the market share of traditional soft contact lenses. Its biggest advantage is that it has high oxygen permeability and no hypoxia in the cornea, which can prevent many complications that are prone to occur when wearing soft contact lenses, so it is highly respected by experts. (end)