Ophthalmic Optics—Contact Lenses—
Part 7: Physicochemical Properties
Test Methods
眼科光学 接触镜
第7部分：理化性能试验方法
1 Scope
GB/T 11417.7 specifies the test methods of physicochemical properties for contact lenses materials, including extraction, oxygen permeability, refractivity, moisture content and contact angle.
This part is applicable to the test of physicochemical properties for contact lenses.
2 Normative References
The following referenced documents are indispensable for the application of this document. For dated reference, only the edition cited applies. For undated reference, the latest edition (including any amendment) applies.
GB/T 6682-2008 Water for Analytical Laboratory Use—Specification and Test Methods
GB/T 11417.1-2012 Ophthalmic Optics—Contact Lenses—Part 1: Vocabulary, Classification System and Recommendations for Labeling Specifications
GB 11417.2-2012 Ophthalmic Optics—Contact Lenses—Part 2: Rigid Contact Lenses
GB 11417.3-2012 Ophthalmic Optics—Contact Lenses—Part 3: Soft Contact Lenses
GB/T 11417.4-2012 Ophthalmic Optics—Contact Lenses—Part 4: Standard Saline Solution for Testing
GB/T 11417.6-2012 Ophthalmic Optics—Contact Lenses—Part 6: Testing Methods for Mechanical Properties
3 Terms and Definitions
For the purposes of this part, the terms and definitions given in GB/T 11417.1-2012 apply.
4 Physicochemical Properties of Contact Lenses
4.1 Test Methods, Unit and Repeatability
The physicochemical properties of hydrogel and non-hydrogel materials for contact lenses, as listed in Table 1, are measurable. The appropriate test methods, unit and repeatability are also specified in Table 1. If other test method is applied, the following requirements shall also be met.
Table 1 Test Methods, Unit and Repeatability of Physicochemical Properties
Item Repeatability Measuring unit Test method
Extraction b Mass fraction 4.2
Oxygen permeability 10% Dk unit a 4.3
Refractivity 0.01 Dimensionless 4.4
Moisture content 2% (Absolute value) % 4.5
Contact angle 2° (°) 4.6
a The unit of Dk is 10-11(cm2/s)[mLO2/(mL·hPa)].
b According to the terms and definitions given in GB/T 11417.1-2012, the repeatability of such test results shall be set by independent laboratory.
4.2 Extraction
4.2.1 Overview
It is a standard method that the extractable substance of contact lenses material is measured quantitatively by Soxhlet extraction method with different solvents. With the lenses dried to be constant, the mass difference of lenses before and after extraction at the dry state is the mass of extractable substance.
The quantitative and qualitative analysis of extractable substance contributes to assessing the new materials of contact lenses and the determination of preclinical inspection procedure. The extracted substance of lenses is measured by appropriate chromatography, spectrophotometry and wet analysis, thus to determine the content of residual monomer, crosslinking agent and initiating agent in the polymerization process.
4.2.2 Principle
The ordinary Soxhlet extractor is applied to this method. Water and an appropriate organic solvent at least shall be adopted in extraction. The solvent's effect on the lenses material substrate shall be considered for selection of organic solvent. Theoretically, the solvent shall not make the lenses material swelling or degraded. However, for the newly developed contact lenses materials, a solvent that can lead to reversible swelling may provide valuable information for overtime extraction.
4.2.3 Apparatus
The standard borosilicate glass Soxhlet extractor (See Figure 1) consists of round-bottom flask (100mL as recommended), Soxhlet extraction tube (30mL as recommended), condenser and heating jacket. Porous stainless steel sheet, sintered glass, filter paper or similar material with glass cotton plug or tube sleeve composed of other appropriate hermetic materials, as well as a vacuum drying oven or similar drying apparatus and an analytical balance of 0.1mg in precision are also needed.
Figure 1 Extraction Apparatus
4.2.4 Reagents
Distilled or deionized water that conforms to Level III requirements of GB/T 6682-2008 shall be adopted. Analytically pure or superior organic solvents shall be adopted (see Table 2). Laboratory-grade zeolite or anti-bumping particles shall be adopted. Appropriate drying agent shall be selected according to the properties of the tested material.
Table 2 Guidelines for Selection of Solvent for Lenses Extraction
Material Solvent Extraction
Hydrogel material Water (distilled or deionized)
N-hexane
Ethanol or methanol
Methylene chloride or chloroform Mild extraction (simulating intraocular extraction)
Slight extraction (nonpolar solvent)
Extraction of most non-crosslinking materials (may lead to swelling and degradation of lenses material)
Extraction of all non-crosslinking materials (may lead to swelling and slight degradation of lenses material)
Rigid lenses and polysiloxane elastic materials Water (distilled or deionized)
N-hexane
Methylene chloride or chloroform Mild extraction (simulating intraocular extraction)
Slight extraction (nonpolar solvent)
Extraction of all non-crosslinking materials (may lead to swelling and slight degradation of lenses material)
4.2.5 Test sample
The test sample shall be able to represent the finished product. It shall be packed in accordance with the finished contact lenses. The specimen shall be processed as per the routine manufacturing process of the product. The sample for lenses to be subject to sterilization treatment shall be treated likewise during preparation. Sufficient lenses shall be taken to ensure that the total dry mass of lenses sample is not less than 200mg before extraction.
The hydrogel lenses are generally placed in the solution containing inorganic salt. If water is used as extraction solvent, to reduce the impact of inorganic salt on the test result, appropriate adjustment shall be made in calculation and the moisture content of lenses shall also be determined, so as to accurately figure out the impact of inorganic salt on the extractable substance. Otherwise, immerse the specimen in water at room temperature before the test and change the water twice at least at an interval of 24h to reach equilibrium.
4.2.6 Test procedure
It is preferred to dry the specimen to be constant in the vacuum state at 60℃±5℃. Before weighing, cool the specimen to room temperature in the vacuum state or in a closed container containing active drying agent and then weigh to the nearest of ±0.1mg (m1). Put the specimen in the extraction sleeve and add appropriate solvent into the flask about 70% of its capacity (the solvent is selected in Table 2). Zeolite may be added into the flask if necessary. Put the round-bottom flask on the heating jacket, insert the extraction sleeve in the Soxhlet extractor, then connect the extractor to the flask and insert the condenser at the top. If extremely volatile or inflammable solvent is used, the complete device shall be placed in the ventilated chamber.
Connect with the water source and start heating and extract for 4h at least. After the solvent is cooled to room temperature, take out the specimen from the extraction sleeve. Dry the specimen according to the above method and weigh to the nearest of ±0.1mg (m2).
Note: the extraction rate is recommended to be 4~6 times/h.
4.2.7 Calculation of result
The amount of extracted material shall be expressed in mass fraction (%), see Formula (1):
(1)
Where,
m1—the specimen mass before extraction;
m2—the specimen mass after extraction.
4.2.8 Test report
It shall cover Chapter 5 and the following items:
a) the composition of the original hydration solution;
b) statement that whether the obtained percentage of extractable substance is adjusted according to the salt content in hydration solution;
c) whether the specimen is equilibrated in water before the test.
4.3 Oxygen Permeability
4.3.1 Overview
The oxygen permeability of contact lenses materials may be determined by two standard methods, of which the common elements are listed in 4.3.2. 4.3.3 specifies the requirements of polarography which are applicable to all materials with the measuring range of 0~145. 4.3.4 specifies the requirements of coulometry which is applicable to non-hydrogel materials. The calibration of both methods is detailed in 4.3.5 and the report requirements in 4.3.6. If other methods for measuring the oxygen permeability are deviated from the standard method, they may be applied after such deviation is calibrated.
The oxygen permeation rate of several sample materials for contact lenses is measured initially, so as to determine the oxygen permeability. The errors, although possibly induced in measuring the oxygen permeation rate, may be effectively reduced (corrected) by taking a derivative of oxygen permeability. Therefore, after such errors are corrected, the oxygen permeability derived from the initial (uncorrected) oxygen permeation rate is very practical and convenient. Then the corrected oxygen permeation rate is calibrated.
4.3.2 Common elements of both methods
4.3.2.1 Parameters
According to the terms and definitions given in GB/T 11417.1-2012, the key parameters for measurement and derivation of oxygen permeation rate are oxygen flow (j), oxygen permeation rate, oxygen permeability, thickness (e.g. radial thickness) and harmonic mean thickness.
If coulometry is applied, j is obtained by the oxygen rate flowing through the Coulomb probe (μLO2/s) divided by the area (A). If polarography is applied, j is obtained by the difference between the measured current and dark current multiplied by the coefficient in Formula (2) and then divided by cathode area.
Thickness t is the radial thickness at the measuring point or the harmonic mean thickness in the measurement area. According to the measured central thickness and the known refractivity, back vertex center curvature radius and back vertex power, the harmonic mean thickness is calculated. Unless otherwise specified, the unit of t should be in centimeters.
For coulometry, Dk is obtained by the measured oxygen permeation rate Dk/t multiplied by the sample thickness t. For polarography, the oxygen permeation rate is corrected by adjusting the oxygen coverage and the gradient 1/Dk is obtained from the curve of the measured oxygen resistance t/Dk versus thickness t. The oxygen permeability is a physical property of material rather than a function of the material sample from or thickness.
For coulometry, Dk/t is obtained by the oxygen flow j divided by the oxygen pressure difference of sample at both sides (local oxygen pressure). For polarography, the oxygen permeability is obtained by the oxygen permeation rate corrected by the fringe and interlayer multiplied by the central thickness t. The oxygen permeation rate is a characteristic value of lenses material and lenses thickness, which depends on the design of lenses.
4.3.2.2 Test sample
This part can be used to determine the oxygen permeability of finished lenses made from hydrogel and non-hydrogel elastic materials, and also to determine that of standard lenses made from hydrogel and non-hydrogel elastic materials.
If the study is to determine the oxygen permeability of finished lenses by measuring the initial oxygen permeation rate, the harmonic mean thickness of the central plane where the contact lenses are exposed to oxygen flow (see 4.3.2.1) shall be indicated in the test report (see 4.3.6). This thickness is independent of the initial oxygen permeation rate [see Formula (2) or Formula (10)].
The radius of back optical area of the test sample is not required, which may be flat sample or range from 7.00mm~9.00mm. The diameter of back/front optical area shall be greater than the chord diameter of the tested central area for gas exchange (2h in diameter at cathode). The test sample surface shall be clean and the polishing quality shall be consistent with the lenses normally used for human.
For hydrogel materials, the sample shall be kept in the standard saline solution (refer to GB/T 11417.4-2012) at room temperature (20℃±2.0℃) for at least 24h prior to the test and shall be equilibrated for at least 2h at the equivalent eye temperature (35℃±0.5℃).
4.3.3 Polarography
4.3.3.1 General
This part introduces the determination of oxygen permeability for hydrogel and non-hydrogel, as well as rigid and elastic contact lenses materials by polarographic oxygen probe. The measuring method and testing conditions are given in the procedure.
Polarography is applicable to the determination of the corrected oxygen permeability (Dk) of rotational symmetric geometrical lenses with various focal powers and that of finished lenses made from rigid and non-hydrogel elastic materials, as well as that of standard lenses made from hydrogel and non-hydrogel, rigid elastic contact lenses materials.
4.3.3.2 Principle
After oxygen molecules pass through the material, the polarography can remove them immediately from the solution by electrochemical process, thus to directly measure the oxygen molecular diffusivity of a single test material. If an oxygen molecule separated out from the sample contacts with the central electrode (cathode) of oxygen sensor covering the back surface of sample, it will immediately convert into four hydrogen ions by electrochemical process. The generated ions, which are in proportion to the moving oxygen molecule, form a current that can be measured with apparatus. The initial (uncorrected) oxygen permeation rate is calculated by substituting the measured current into Formula (2), in mLO2/(A·s).
Foreword i
1 Scope
2 Normative References
3 Terms and Definitions
4 Physicochemical Properties of Contact Lenses
4.1 Test Methods, Unit and Repeatability
4.2 Extraction
4.3 Oxygen Permeability
4.5 Moisture Content
4.6 Contact Angle
5 Test Report
