Microsoft Word C034480e doc Reference number ISO 18369 4 2006(E) © ISO 2006 INTERNATIONAL STANDARD ISO 18369 4 First edition 2006 08 15 Ophthalmic optics — Contact lenses — Part 4 Physicochemical prop[.]
Repeatability, test methods and units of measure
Table 1 presents the measurable physicochemical properties of hydrogel and nonhydrogel materials used in contact lens manufacturing, along with details on repeatability, testing methods, and measurement units It is important to specify any alternative methods employed in these assessments.
Table 1 — Physiochemical properties: Repeatability, test methods and units of measure
Property Repeatability Units of measure Test method
The water content is measured at 2% absolute and 4.6% in Dk units, which are reported in units of 10^{-11} (cm²/s) ml O₂/(ml × hPa) Additionally, the repeatability of these test results must be determined by individual laboratories following the specified terms and definitions.
Extractables
Soxhlet extraction using various solvents is a widely accepted technique for quantitatively assessing the substances that can be extracted from contact lenses By drying the lenses to a constant mass, the difference between the initial dry mass and the mass after extraction reveals the amount of extractable substances present.
Understanding the quantity and identity of extractable substances is crucial for assessing new contact lens materials and shaping the pre-clinical examination program The extracted material from contact lenses can be analyzed using chromatographic, spectrophotometric, and wet analytical methods to identify residual monomers, cross-linking agents, catalysts, and other components involved in the polymerization process.
The Soxhlet extraction method employs a standard apparatus utilizing water and at least one appropriate organic solvent for extraction When choosing the organic solvent(s), it is crucial to consider their impact on the material matrix, ensuring that the solvent does not swell or degrade the contact lens material However, in the development of new contact lens materials, a solvent that induces reversible swelling can provide valuable insights into the potential for extraction over prolonged periods.
A standard borosilicate glass Soxhlet extraction apparatus (see Figure 1) consisting of the Soxhlet extractor
For the extraction process, it is recommended to use a 30 ml solvent, a 100 ml round bottom flask, and a heating mantle An extraction thimble made of perforated stainless steel, sintered glass, or paper, along with a glass wool plug or suitable closure, is essential Additionally, a vacuum oven or similar drying apparatus and an analytical balance with a precision of 0.1 mg are necessary for accurate measurements.
For optimal results, use distilled or deionized water that meets Grade 3 of ISO 3696:1987, along with an appropriate analytical grade organic solvent as specified in Table 2 Additionally, laboratory-grade boiling stones or anti-bumping granules are essential, paired with a suitable active desiccant tailored to the specific characteristics of the test material.
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Table 2 — Guide to the selection of solvents for use in extraction of contact lenses
Water (distilled or deionized) Mild extraction (simulates in-eye extraction) n-Hexane Mild extraction (non-polar solvent)
Ethanol or methanol Extraction of majority of uncrosslinked material (but swells and may degrade material) Hydrogels
Dichloromethane, also known as chloroform, is effective for extracting uncrosslinked materials, although it may cause swelling and degradation In contrast, distilled or deionized water provides a mild extraction method that simulates in-eye extraction Additionally, n-hexane serves as a non-polar solvent for gentle extraction processes.
Hard and RGP and silicone elastomers
Dichloromethane or chloroform Extraction of all uncrosslinked material (but swells and is likely to degrade material)
Test samples must accurately represent the final product and be in the form of finished contact lenses The preparation and finishing methods should closely mimic standard production processes, including sterilization Additionally, a minimum of 200 mg of total dry mass is required before extraction, ensuring a sufficient number of lenses are utilized.
Hydrophilic lenses are typically stored in a solution with inorganic salts, which must be accounted for when using water as the extracting solution To accurately assess the contribution of these salts to the extractables, the water content of the lenses is essential As an alternative, the lenses can be equilibrated in two separate water changes, each lasting 24 hours at room temperature, before testing begins.
Dry the lenses under vacuum at a temperature of 60 °C ± 5 °C until they reach a constant mass Allow the lenses to cool to room temperature in a vacuum or a closed container with active desiccant before weighing Weigh the dry lenses to an accuracy of ± 0.1 mg (m₁) Next, place the lenses into the extraction thimble, add boiling stones to the flask if needed, and fill the flask to about 70% of its capacity with the appropriate solvent.
To conduct the extraction process, first, position the round-bottom flask within the heating mantle Next, insert the extraction thimble into the Soxhlet apparatus and securely attach it to the flask A condenser should be placed atop the extraction apparatus It is crucial to perform this procedure in a fume hood when utilizing volatile or flammable solvents for safety.
To extract the lenses, heat and water should be activated for a minimum of 4 hours After the extraction, let the solvent cool to room temperature before taking out the lenses from the extraction thimble Finally, dry the lenses until they reach a constant mass and weigh them to the nearest 0.1 mg.
The quantity of extracted material shall be expressed as a mass fraction, w extracted , in percent of the initial dry mass [Equation (1)]:
= − × (1) where m 1 is the mass of lenses prior to extraction; m 2 is the mass of extracted lenses
The extractables test report for hydrophilic materials must adhere to Clause 5 and include essential details such as the composition of the initial hydrating solution, whether the percentage of extractable substances has been adjusted for the salt content of the solution, and confirmation of whether the contact lenses were equilibrated in water prior to testing.
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Rigid lens flexural deformation and rupture
The destructive test applies an increasing load to the edge of a rigid lens until it fractures, with continuous monitoring of load and flexural deformation This test determines the flexural deformation strength, flexural deformation at rupture, and flexural deformation strength at 30% deformation, derived from the load-deformation curve It is applicable to both standard and specially constructed rigid contact lenses.
It should be noted that variability in the test results may also result from inconsistencies in lens manufacturing method and may not necessarily be indicative of the material itself
To assess the material's breakage resistance, standard commercially available rigid single vision contact lenses will be used for testing, ensuring they are unmodified and untreated.
Contact lenses which have toroidal zones or truncations shall not be used
The specified back vertex power (F′ v ) shall be the same for all samples and shall be between + 0,50 D and
The specified back optic zone radius (r 0 ), or radius of the vertex sphere, shall be the same for all samples and shall be between 7,75 mm and 7,85 mm
When special samples are prepared in order to compare materials, the contact lenses shall have the following specifications:
⎯ front surface: single cut, radius of curvature 8,00 mm ± 0,025 mm;
⎯ back surface: single cut, radius of curvature 7,80 mm ± 0,025 mm;
The method of manufacture shall be stated in the test report
Three contact lenses from each of three different material lots (total of nine contact lenses) shall be tested where a claim is made regarding flexure or strength
Samples shall be stored in standard saline solution conforming to ISO 18369-3:2006, 4.7, for at least 48 h prior to testing The temperature of this saline solution shall be 20 °C ± 5 °C
4.3.4.1 Testing machine (see Figure 2), applying a load to the sample at a fixed rate in either the horizontal or vertical plane, and composed of the units described in 4.3.4.2 to 4.3.4.4
2 recorder a See Figure 3 for detail X
4.3.4.2 Sample holding jig (see Figure 3), applying the load to the edge of the sample
The sample is set at the centre of the upper and lower contact faces so that the whole load is applied in the plane containing the edge
NOTE The contact faces are constructed so that the load is the only force applied to the sample
4.3.4.3 Load indicator, capable of indicating the total load applied to the sample
The data recorder, connected to the testing machine, records the total load applied to the sample over time once the load application begins.
Although it is conventional to use a paper-strip (chart) recorder, other devices may be utilized If a paper-strip recorder is used a minimum paper speed of 1 cm/s is recommended
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2 test specimen a Detail of Figure 2
Figure 3 — Test specimen setting jig 4.3.5 Procedure
Confirm the correct operation and calibration of the apparatus
Carry out the test at an ambient temperature of 20 °C ± 5 °C
Remove the conditioned sample from the saline solution and dry it carefully
To measure the back optic zone radius, total diameter, center thickness, and back vertex power, follow the guidelines outlined in ISO 18369-3 Ensure the sample is positioned in the jig with its upper and lower edges aligned along the centerline of the upper contact face Adjust the velocity of the moving contact face to 20 cm/min (3.33 mm/s) with a tolerance of ± 10%.
The sample and jig can be positioned either horizontally or vertically When employing a horizontal setup, it is essential to verify beforehand that the test results align with those obtained from a vertical configuration.
Begin by activating the data recorder before applying the load to the sample Cease load application once the lens breaks, and document the load in grams at the point of rupture Conduct this test for each of the test samples.
The test results include the calculation of arithmetic mean values and standard deviations for flexural deformation strength at rupture, flexural deformation at rupture, and flexural deformation strength at 30% deformation.
NOTE The estimated standard deviation (σ) is given by the expression
2 / 1 x x n σ = ⎛ ⎜ ⎝ ∑ ⎡ ⎣ − ⎤ ⎦ ⎞ ⎟ ⎠ − where x is the value of a single result; x is the arithmetic mean (Σx/n); n is the number of measurements/samples in the data set
4.3.6.2 Flexural deformation strength at rupture
The flexural deformation strength at rupture is the load, in grams, indicated at the moment of rupture during the test
To determine the distance (d) between the contact faces at the moment of rupture, it is essential to know the rupture time and the loading rate Additionally, the flexural deformation should be expressed as a percentage of the total initial diameter (∅ T) of the sample.
4.3.6.4 Flexural deformation strength at 30 % deformation
To determine the time at which the total diameter of the sample has decreased by 30%, it is essential to know the loading rate Additionally, the load applied at that specific time can be calculated, measured in grams, and can also be obtained from the flexural load-deformation curve.
⎯ Total diameter of the contact lens is 9,6 mm
⎯ Velocity of the moving contact face is 20 cm/min (3,33 mm/s)
⎯ Time taken for the moving contact face to cover 2,9 mm is 0,865 s
The value needed is the load applied 0,865 s from the start of deformation
The test report shall conform to that in Clause 5.
Oxygen permeability
Two standardized methods exist for determining the oxygen permeability of contact lens materials: the polarographic method, suitable for materials with oxygen permeability ranging from 0 to 145, and the coulometric method, which is limited to non-hydrogel materials Both methods share common elements as outlined in section 4.4.2, and calibration is essential for accurate results.
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The article discusses various methods for measuring oxygen permeability, as outlined in section 4.4.5, with reporting of results detailed in section 4.4.6 It also mentions alternative techniques and variations of standardized methods that can be utilized, provided they yield calibrated results comparable to those obtained from standardized methods.
Oxygen permeability in contact lens materials is derived from initial measurements of oxygen transmissibility While there are measurement errors in transmissibility, these can be minimized when calculating oxygen permeability Thus, it is efficient to first derive corrected oxygen permeability values from uncorrected transmissibility measurements These corrected values are then calibrated, allowing for the computation of corrected and calibrated oxygen transmissibility values, as outlined in ISO 18369-3.
4.4.2 Common elements of the methods
Key parameters for measuring and deriving oxygen permeability include oxygen flux, oxygen permeability, oxygen transmissibility, thickness (specifically radial thickness), and harmonic mean central thickness For detailed definitions of these terms, please consult ISO 18369-1.
In the coulometric method, the measurement of j is determined by the rate of oxygen flow (in àl O 2 / s) divided by the sample area, A Conversely, in the polarographic method, j is calculated as the difference between the measured and dark currents, multiplied by a constant from Equation (2) and divided by the area of the central cathode.
The thickness, denoted as \( t \), refers to the radial thickness at the measurement point or the harmonic mean central thickness across the measurement area By measuring the center thickness and knowing the refractive index, back surface curvature, and refractive power of a specific lens, one can also calculate the harmonic mean central thickness Unless specified otherwise, \( t \) should be expressed in centimeters (cm).
In terms of measurement using the coulometric method, Dk is equal to the measured oxygen transmissibility,
Oxygen permeability, a fundamental physical property of materials, is independent of the sample's shape or thickness In the polarographic method, it is adjusted by considering the area exposed to oxygen flow and analyzing the slope of the line derived from plotting the measured oxygen resistance (t/Dk) against the thickness (t) This approach involves multiplying the diffusion coefficient (Dk) by the sample thickness (t) to accurately assess oxygen permeability.
The coulometric method measures Dk/t as the oxygen flux, j, divided by the difference in oxygen tension between the two exposed surfaces of a contact lens In contrast, the polarographic method defines oxygen transmissibility as the oxygen permeability, Dk, adjusted for edge and barrier layer effects, divided by the lens thickness, t This property of oxygen transmissibility is influenced by both the lens material and its thickness, making it dependent on the contact lens design.
The oxygen permeability of both hydrogel and non-hydrogel flexible materials, including various designs and powers of finished contact lenses, can be assessed according to ISO 18369 Additionally, standardized test samples of hydrogel and non-hydrogel materials can also be evaluated for their oxygen permeability.
To assess oxygen permeability in finished contact lenses, it is essential to report the harmonic mean thickness of the central area exposed to oxygen flow However, this thickness does not influence the calculation of preliminary oxygen transmissibility (Dk/t) derived from oxygen flux measurements.
The back optic zone radii of test samples can be infinite, such as in flat samples, or range from 7.00 mm to 9.00 mm for contact lenses Additionally, the diameters of the back and front optic zones must exceed the chord diameter (2h) of the central lens area evaluated for gas exchange.
10 © ISO 2006 – All rights reserved test samples shall be clean and polished to the quality acceptable in normal contact lens production for human use
In the case of hydrogel materials, the test specimens shall be stored in standard saline solution (see
ISO 18369-3:2006, 4.7) for at least 24 h at room temperture (20 °C ± 2,0 °C) prior to testing and shall be equilibrated at eye temperature (35 °C ± 0,5 °C) for at least 2 h prior to measurement
This article outlines the process for measuring the oxygen permeability of both hydrogel and non-hydrogel contact lens materials, including rigid and flexible types, utilizing a polarographic oxygen sensor It details the measurement procedure and the specific conditions required for accurate results.
The polarographic method is used to measure the corrected oxygen permeability (Dk) of both rigid and non-hydrogel flexible contact lens materials, which include various refractive powers and rotationally symmetric geometries Additionally, it applies to hydrogel and non-hydrogel contact lens materials in standardized test samples.
The polarographic method measures the diffusion of oxygen molecules through a test material by electrochemically removing them from the solution upon contact When an oxygen molecule passes through the material and reaches the cathode of the oxygen sensor, it is converted into four hydroxyl ions, generating an electric current This current, which is proportional to the number of molecules removed, is quantified by the apparatus The resulting measurement is used to calculate the preliminary oxygen transmissibility, Dk/t (preliminary), expressed in ml O\(_2\)/(A⋅s), as shown in Equation (2).
= × × × (2) where p A is the (barometric pressure less the vapour pressure ), expressed in hPa, multiplied by 0,209 which is the oxygen fraction in oxygenated gas;
A is the area, in cm 2 , of cathode face in the oxygen sensor;
I is the current, in amperes, from oxygen sensor;
I d is the “dark current”, in amperes, of the oxygen sensor (i.e the current that flows in the absence of oxygen flux);
The value of 5,804 × 10^{-2} represents the volume of one kilogram mole at standard temperature and pressure (STP), divided by Faraday's constant and the number of charges per molecule of oxygen reduced, which is assumed to be four charges per molecule.
Refractive index
The refractive index is measured by determining the critical angle of incidence for total internal reflection of light at wavelengths of 546.1 nm (mercury e-line) or 587.6 nm (helium d-line) using a calibrated Abbe refractometer at room temperature During this process, light transitions from the prism surface of the refractometer into the contact lens material According to Snell's Law, the critical angle is related to the refractive index of both the tested specimen and the transparent reference surface of the refractometer, with calculations based on Equation (12).
= ° (12) where n is the refractive index of the test specimen; n′ is the refractive index of the reference surface; α is the critical angle of incidence upon the reference surface
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For accurate measurements with a refractometer, a contacting fluid is essential between the contact lens material and the prism, except for hydrogels This fluid must have a refractive index higher than the test specimen and should not damage either the test material or the refractometer assembly The use of the mercury e-line is standard, while the helium d-line must be specified The refractometer should achieve a precision of at least 0.0005, with a repeatability of ± 0.001 units of refractive index.
The refractive index of contact lens materials typically does not exceed 1.55, making a contacting fluid with a refractive index of at least 1.55 adequate It is important to avoid using organic solvents; instead, aqueous solutions like saturated zinc bromide, which has a refractive index of 1.564 and a density of 2.510 g/ml, are considered suitable.
Test samples must be flat or easily flattenable against the refractometer's reference surface during measurement The contact surface should be polished to a smooth finish Hydrogel specimens need to be equilibrated in standard saline solution (refer to ISO 18369-3) at a temperature of 20 °C ± 0.5 °C for a minimum of 2 hours before measurement, followed by lightly blotting or shaking off excess water from their surfaces just prior to testing.
4.5.3 Removal of excess surface saline
The accuracy of this method can be affected by the challenge of effectively blotting or shaking off the test specimen to eliminate excess saline from its surfaces prior to measuring the hydrated refractive index It is crucial to ensure that all surface water is removed without overblotting, which could extract moisture from within the material Additionally, blotting or shaking should be conducted swiftly to prevent water loss from evaporation.
When using tinted materials, the transmitted light may be inadequate for producing a clear image In such cases, if the material was not tinted prior to polymerization, the assessment should be conducted using a specimen of the same material that remains untinted.
4.5.4.1.1 Rigid and non-hydrogel materials
To prepare a test specimen for the refractometer, ensure it is sized to fit on the fixed half of the prism, with one surface being optically flat and polished for measurement If light must enter through an edge, that edge should also be optically flat and polished perpendicular to the measurement face.
NOTE 1 Specimens of soft non-hydrogel materials, such as silicones, can consist of actual contact lenses
NOTE 2 A convenient shape for the test specimen is a rectangle of thickness 0,5 mm to 3,0 mm, which is slightly smaller than the face of the refractometer prism
Ensure the measurement face of the test specimen in contact with the prism is smooth and has a consistent thickness Prior to testing, stabilize the specimen's hydration by keeping it in a saline solution at a temperature of 20 °C ± 0.5 °C for a minimum of 2 hours, as outlined in ISO 18369-3.
NOTE Contact lenses of approximately constant thickness provide the most convenient form of test specimen for hydrogel materials
4.5.4.2.1 Rigid and non-hydrogel materials
To obtain accurate measurements, ensure that both the specimen and refractometer are maintained at an ambient temperature of 20 °C ± 2 °C Apply a drop of contacting fluid onto the fixed refractometer prism and press the specimen firmly against the prism, positioning the polished edge towards the light source.
Satisfactory contact between the test specimen and the prism is evidenced by a distinct and straight dividing line that appears between the light and dark areas in the field of view.
To accurately read the refractive index on direct-reading, non-compensated refractometers, observe the dividing line between the light and dark fields For refractometers featuring an external scale, ensure the index arm is adjusted so that this dividing line aligns with the eyepiece hairline before taking the reading If the refractometer includes chromatic compensating prisms, adjust them to eliminate any color from the field prior to measuring the refractive index.
To measure the refractive index using a refractometer at an ambient temperature of 20 °C ± 2 °C, first remove the specimen from its hydrating solution and gently blot or shake off any excess fluid Next, firmly press the measurement face against the refractometer's fixed prism, compensate for color, and then proceed to measure the refractive index as outlined in section 4.5.4.2.1.
Satisfactory contact between the test specimen and the prism is indicated by a clear, sharp dividing line between the light and dark areas in the field of view Insufficient pressure on the specimen may result in a faint secondary line, caused by the hydrating fluid, which corresponds to a refractive index of 1.336.
To accurately measure the refractive index of a specimen with varying indices in different regions, it is essential to mask the measurement surface This approach allows for targeted assessment of the refractive index in specific areas of interest.
Each test result for a sample must be reported as the average of at least three independent measurements If additional measurements are included in the average, this should be noted in the test report Additionally, if the refractometer scale indicates percent solids or is calibrated at a different wavelength than the reference, the reading must be adjusted to reflect the refractive index at the reference wavelength.
The test report shall conform to that in Clause 5.
Water content
ISO 18369 outlines a gravimetric method for determining water content, as detailed in section 4.6.2, alongside a refractive index method found in section 4.6.3 The water content, represented as a mass fraction in percent (w H₂O), indicates the amount of water in a hydrated material that has reached equilibrium in standard saline at room temperature.
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H O hydrated material m 100 w = m × (13) where m H 2 O,hydrated material is the mass of water in the hydrated material; m hydrated material is the mass of the hydrated material
In this context dissolved solutes such as sodium chloride and buffers contribute to the mass of the hydrated material
4.6.2 Gravimetric determination of water content/absorption by loss on drying using an oven
Using gravimetric method, water content, w H 2 O, is calculated after measurement of the dry mass and hydrated mass of hydrogel and non-hydrogel test specimens at room temperature:
= − × (14) where m hydrated is the mass of the hydrated hydrogel and non-hydrogel test specimens; m dry is the mass of the dry hydrogel and non-hydrogel test specimens
The method involves a simple process where hydrated test specimens, after removing excess surface water, are weighed using an analytical balance These specimens are then dried in either a conventional or microwave oven and weighed again The difference in weight indicates the mass of water lost through evaporation.
4.6.2.2 Removal of excess surface saline
The method's accuracy is constrained by the challenge of effectively blotting the test specimen to eliminate excess saline from its surfaces prior to measuring the hydrated mass It is essential to ensure that all surface water is removed, but care must be taken not to overblot, as this could extract water from within the material itself.
Blotting the test specimen shall be performed as quickly as possible to avoid loss of water from the test specimen by evaporation
To reduce errors from variations in excess surface saline removal, hydrated test specimens should weigh between 100 mg and 300 mg A ring test demonstrated that thick polymer discs achieved a water content reproducibility of ± 0.4%, while single lenses had a reproducibility of ± 1.0% For lenses weighing less than 100 mg, additional samples are necessary to achieve the same level of reproducibility.
To accurately assess the water content of a specimen, place it on a dry, clean, lint-free cloth made of cotton or linen Gently fold the cloth over the specimen and lightly blot it three times with a fingertip Be cautious, as the dry blotting technique may increase the risk of overblotting, potentially resulting in an underestimation of the specimen's water content.
To prepare the specimen, place it on a clean, lint-free section of Whatman #1 filter paper that has been slightly dampened with saline Fold the filter paper over the specimen and gently blot it three times with a fingertip Be cautious, as using a wet blotting technique may increase the likelihood of leaving surface water on the lens, potentially resulting in an over-estimation of the water content.
4.6.2.3 Measurement of hydrated test specimen mass
Place the hydrated test specimen on a previously dried and weighed glass slide immediately after blotting
Weigh the slide and specimen to the nearest 0.1 mg, then subtract the slide mass to determine the hydrated specimen mass It is essential to conduct the weighing promptly to prevent water loss from the test specimen due to evaporation.
4.6.2.4 Oven drying of test specimens
Dry the glass slide and hydrated test specimen at 100 °C to 110 °C in a conventional oven to a constant mass
After drying the specimen for 16 to 18 hours, place the slide and specimen in a jar filled halfway with active desiccant, such as anhydrous calcium sulfate (CaSO₄) Allow the jar to cool to room temperature for 30 minutes.
NOTE If the test material is found to degrade at the temperature of 100 °C to 110 °C, the material can be dried according to 4.2.6
To prepare the hydrated test specimen, use contact lens tweezers to place it on a weighed Teflon disc Next, position the Teflon disc and specimen in a specimen jar that is half-filled with active desiccant Seal the jar and place it in a microwave oven with a volume of 0.015 m³, irradiating at maximum power for 10 minutes.
0,05 m 3 and 500 W to 650 W maximum power output Remove the jar from the oven and allow to cool to room temperature for 30 min
Repeat drying and weighing steps with new supplies of active desiccant until the sample reaches constant mass
4.6.2.5 Measurement of dry test specimen mass
Weigh the dry test specimen along with the pre-weighed glass slide or Teflon disc to the nearest 0.1 mg, then subtract the weight of the slide or disc to determine the mass of the dry specimen It is essential to conduct the weighing promptly to prevent the material from absorbing moisture from the air.
Having determined the dry and hydrated masses, Equation (14) is used to calculate water content in percent
Whatman filter paper is a commercially available product that is suitable for use, as noted in ISO 18369 However, it is important to clarify that this mention does not imply an endorsement of the product by ISO.
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4.6.3 Determination of water content by refractive index
The refractive index of hydrogel contact lens material depends on the refractive indices of its water and solid components, as well as their respective proportions, in accordance with the Principle of Gladstone and Dale The volume fraction of water, represented as \$\phi_{H_2O}\$ in percent, can be determined using the refractive indices of the hydrated material (\$n_{wet}\$), the dry material (\$n_{dry}\$), and the standard saline solution (\$n_{std}\$) used for equilibration, as outlined in ISO 18369-3.
To measure the refractive index of the hydrated hydrogel specimen, refer to section 4.5 If the dry material's refractive index is known and the standard saline solution is assumed to have an index of 1.333, the water content can be calculated using Equation (15) If not, both the dry material's refractive index and that of the standard saline solution must be measured as outlined in section 4.5 before substituting into the equations The volume fraction of water, \$\phi_{H_2O}\$, can be converted to mass fraction, \$w_{H_2O}\$, using the specific gravity of the test material.
When estimating the refractive index of a material, the water content is also an estimate, particularly if calculated by mass using an assumed specific gravity for the test material.
The test report shall conform to that in Clause 5
A test report must be prepared following the specifications of ISO 18369, containing essential information such as the laboratory's name, identification details of the tested contact lens, a reference to ISO 18369-4:2006 and its relevant subclause, any deviations from the specified method, the test result with an error estimation if possible, the test date, and the name of the responsible person.
[1] B ENJAMIN W.J., C APPELLI Q.A Oxygen Permeability (Dk) of Thirty-Seven Rigid Contact Lens Materials
[2] GALAS S.L., ENNS J.B Humidity-Conditioned Gravimetric Method to Measure the Water Content of Hydrogel Contact Lens Materials Optometry & Vision Science, 70(7), 1993, pp 577-586
[3] W INTERTON L.C., W HITE J.C., S U K.C Coulometric Method for Measuring Oxygen Flux and Dk of Contact Lens Materials International Contact Lens Clinic, 14(11), 1987, pp 441-449
[4] YOUNG M.D., BENJAMIN W.J Oxygen Permeability of the Hypertransmissible Contact Lenses Eye &
[5] YOUNG M.D., BENJAMIN W.J Calibrated Oxygen Permeability of 35 Conventional Hydrogel Contact Lens Materials and Correlation with Water Content Eye & Contact Lens, 29(2), 2003, pp 126-133
[6] ISO 18369-2, Ophthalmic optics — Contact lenses — Part 2: Tolerances
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