Oxygen is the basis of normal physiological metabolism of the cornea. Wearing contact lenses will cause the oxygen intake of the cornea to be damaged to varying degrees, resulting in corneal edema, angiogenesis and a series of other diseases. Therefore, the higher the oxygen permeability of contact lenses, the less damage to the cornea, more suitable for long-term wear. Dk oxygen transmission coefficient is the unity of the contact lens oxygen permeability index, using the coulomb method can quickly and accurately determine the hard and the hydrogel oxygen transmission coefficient of elastic contact lenses materials and oxygen permeability, to the enterprise new lens material selection, the design of the water content and the thickness and provides effective reference for the choose and buy of consumer.

Contact lenses, or contact lenses, are worn on the cornea of the eye to change the refractive power of the eye, to correct nearsightedness, farsightedness, astigmatism, and to treat certain eye diseases. Wearing contact lenses is not only beautiful and convenient, but also broad vision, so that more and more people abandon the frames into the contact lens family. When choosing contact lenses, the wearer should pay more attention to the material, thickness and moisture content of the lenses. In fact, these designs are mostly designed to improve the oxygen permeability of the lenses. Oxygen permeability is an important indicator of corneal health and wear comfort.

Contact lens oxygen permeability test method

Test method:

The oxygen permeability indexes of contact lenses are mainly tested by polarography and coulomb method, both of which are convenient, practical and reproducible. There are many literatures about polarography, so this paper will focus on the coulomb method. This method is suitable for hard and non elastic hydrogel contact lens material of oxygen transmission coefficient and the oxygen permeability and its principle is: the lens into the test chamber, the test cavity is divided into fluctuation two parts, using contact lens under 35, 100% RH before and after surface area (i.e., cavity fluctuation two parts) were exposed to oxygen and nitrogen as carrier gas. The pressure on both sides of the contact lens is the same, but the partial pressure of oxygen is different. Under the action of concentration difference, oxygen enters the airflow through the lens and is sent to the coulomb sensor, which generates a current proportional to the oxygen concentration flowing through the detector, so as to calculate the oxygen flow rate of the material and then calculate the oxygen permeability coefficient. The specific test methods are as follows:

1. Preparation of samples and instruments. Prepare 99.9% oxygen and 2% hydrogen nitrogen as test gas and carrier gas respectively. The apparatus adopts an oxygen permeability testing system, which includes an oxygen transmission analysis part and a testing chamber part. The former is composed of coulomb sensor, recording and analysis system, temperature and humidity control system and related pipelines, which is used for oxygen and carrier gas circulation, current conversion and recording, temperature and humidity control and other functions. The test chamber is oxygen-impermeable and divided into two halves. The outer side surrounds a groove slightly below the water surface of the tank wall, providing 100%RH humidity for the lens to simulate the service environment.

Before testing, all oxygen should be removed from the test sample material and the system. For materials with high oxygen permeability, oxygen can be removed from the test chamber before measurement. However, for non-hydrogel materials with low oxygen permeability, such as PMMA, lens samples should be placed under calcium chloride desiccant for at least 48h before the test chamber is closed and subsequently purified for oxygen removal.

2. Install samples. The rear surface of the lens sample is placed on a smooth circular surface coated with vacuum grease in the lower test chamber to ensure that the sample is free from wrinkles. The upper part of the test chamber is buckled on the lower part of the test chamber containing the sample, clamped, and a certain amount of distilled water is injected into the surrounding groove to close the test chamber cover.

3. Oxygen removal. The carrier gas was set to flow into the upper and lower parts of the test chamber at a rate of 50mL/min~60mL/min. After 3min~4min, the flow rate decreased to 5mL/min~15mL/min and was maintained for 10min~30min. The test cavity residual oxygen from straight out of the sample lenses, carry oxygen airflow to be delivered to the load of the coulomb sensors, sensor output rise quickly to peak at first, after gradually stable at a low value, namely the zero voltage output (where V0), according to different oxygen permeability test material, the deaerating process usually lasts for 30 minutes to ten hours.

Test. Oxygen and carrier gas are maintained to flow into the upper and lower sides of the test chamber at the final rate of oxygen removal. Oxygen passes through the lens sample and enters the coulomb sensor output voltage. When the output value is stable to a value, the voltage value at this time is the final value (VE).

5. Calculation of test results. First, the oxygen flow rate (LO2/s) is calculated according to equation 1. Then, the oxygen permeability coefficient of the lens is calculated according to equation 2.

qo2=K×(VE-V0)/RL--------------------------------------(1)

Where, K -- correction constant; VE -- final voltage output; V0 -- zero voltage output; RL -- load impedance;

Dk=t×qo2×0.001/(PA×A)-------------------------------(2)

Where, Dk -- oxygen permeability coefficient of the test sample,10-11（cm2/s）·[mLO2/(mL·hPa)]; PA -- (atmospheric pressure - vapor pressure), hPa; T -- radial thickness or mean harmonic center thickness, cm; A -- exposure area of test sample, cm2; Qo2 -- oxygen flow rate through the detector, LO2/s; 1/103 -- change L to mL coefficient;

Formula 1 involves a correction constant K, which leads to the concept of system calibration. Experience has shown that under certain conditions coulomb sensors can be depleted or damaged to the point where their efficiency and response time are impaired. Therefore, the use of coulomb sensors to test oxygen permeability requires a combination of periodic sensor calibration, which relies on a standard material with a known oxygen permeability (Dk/t). The standard material commonly used is SRM1470 with a Dk/t unit of 0.072±0.0045Dk/t. By testing the standard material with the above method, the oxygen flow rate qo2 is derived from equation 2, and then the correction constant K value is obtained according to equation 1. The calibration constant can be used to achieve the initial calibration of the test system and the results.

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