EN ISO 18369-4:2017
(Main)Ophthalmic optics - Contact lenses - Part 4: Physicochemical properties of contact lens materials (ISO 18369-4:2017, Corrected version 2017-10-01)
Ophthalmic optics - Contact lenses - Part 4: Physicochemical properties of contact lens materials (ISO 18369-4:2017, Corrected version 2017-10-01)
ISO 18369-4:2017 specifies the methods of testing the physicochemical properties of contact lens materials. These are extraction, rigid lens flexure and breakage, oxygen permeability, refractive index and water content.
Augenoptik - Kontaktlinsen - Teil 4: Physikalisch-chemische Eigenschaften von Kontaktlinsenmaterialien (ISO 18369-4:2017, korrigierte Fassung 2017-10-01)
Optique ophtalmique - Lentilles de contact - Partie 4: Propriétés physicochimiques des matériaux des lentilles de contact (ISO 18369-4:2017, Version corrigée 2017-10-01)
ISO 18369-4:2017 vise à spécifier les méthodes d'essai des propriétés physicochimiques des matériaux des lentilles de contact. Il s'agit en l'occurrence de spécifier l'extraction, la flexion et la rupture des lentilles de contact rigides, la perméabilité à l'oxygène, l'indice de réfraction et la teneur en eau.
Očesna optika - Kontaktne leče - 4. del: Fizikalnokemijske lastnosti materialov za kontaktne leče (ISO 18369-4:2017)
Ta dokument določa metode za preskušanje fizikalno-kemijskih lastnosti materialov za kontaktne leče. Te lastnosti so ekstrakcija, upogibanje in lomljivost trde leče, prepustnost kisika, lomni količnik in vsebnost vode.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-november-2017
1DGRPHãþD
SIST EN ISO 18369-4:2006
2þHVQDRSWLND.RQWDNWQHOHþHGHO )L]LNDOQRNHPLMVNHODVWQRVWLPDWHULDORY]D
NRQWDNWQHOHþH ,62
Ophthalmic optics - Contact lenses - Part 4: Physicochemical properties of contact lens
materials (ISO 18369-4:2017)
Augenoptik - Kontaktlinsen - Teil 4: Physikalisch-chemische Eigenschaften von
Kontaktlinsenmaterialien (ISO 18369-4:2017)
Optique ophtalmique - Lentilles de contact - Partie 4: Propriétés physicochimiques des
matériaux des lentilles de contact (ISO 18369-4:2017)
Ta slovenski standard je istoveten z: EN ISO 18369-4:2017
ICS:
11.040.70 Oftalmološka oprema Ophthalmic equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
EN ISO 18369-4
EUROPEAN STANDARD
NORME EUROPÉENNE
September 2017
EUROPÄISCHE NORM
ICS 11.040.70 Supersedes EN ISO 18369-4:2006
English Version
Ophthalmic optics - Contact lenses - Part 4:
Physicochemical properties of contact lens materials (ISO
18369-4:2017)
Optique ophtalmique - Lentilles de contact - Partie 4: Augenoptik - Kontaktlinsen - Teil 4: Physikalisch-
Propriétés physicochimiques des matériaux des chemische Eigenschaften von
lentilles de contact (ISO 18369-4:2017) Kontaktlinsenmaterialien (ISO 18369-4:2017)
This European Standard was approved by CEN on 1 July 2017.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
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Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
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CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2017 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 18369-4:2017 E
worldwide for CEN national Members.
Contents Page
European foreword . 3
European foreword
This document (EN ISO 18369-4:2017) has been prepared by Technical Committee ISO/TC 172 “Optics
and photonics” in collaboration with Technical Committee CEN/TC 170 “Ophthalmic optics” the
secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by March 2018, and conflicting national standards shall
be withdrawn at the latest by March 2018.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 18369-4:2017.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 18369-4:2017 has been approved by CEN as EN ISO 18369-4:2017 without any
modification.
INTERNATIONAL ISO
STANDARD 18369-4
Second edition
2017-08
Ophthalmic optics — Contact lenses —
Part 4:
Physicochemical properties of contact
lens materials
Optique ophtalmique — Lentilles de contact —
Partie 4: Propriétés physicochimiques des matériaux des lentilles
de contact
Reference number
ISO 18369-4:2017(E)
©
ISO 2017
ISO 18369-4:2017(E)
© ISO 2017, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
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Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
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ii © ISO 2017 – All rights reserved
ISO 18369-4:2017(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Physicochemical properties of contact lenses . 1
4.1 Repeatability, test methods and units of measure . 1
4.2 Extractables . 2
4.2.1 General. 2
4.2.2 Principle . 2
4.2.3 Apparatus . 2
4.2.4 Reagents . 3
4.2.5 Test samples. 4
4.2.6 Test procedure . 4
4.2.7 Calculation of results . 5
4.2.8 Test report . 5
4.3 Rigid lens flexural deformation and rupture . 5
4.3.1 Principle . 5
4.3.2 Sampling. 5
4.3.3 Preparation of samples . 6
4.3.4 Apparatus . 6
4.3.5 Procedure . 8
4.3.6 Test result . . 8
4.4 Oxygen permeability . 9
4.4.1 General. 9
4.4.2 Common elements of the methods . 9
4.4.3 Polarographic method . .10
4.4.4 Normalization of the corrected oxygen permeability using reference lenses .18
4.4.5 Test report .19
4.5 Refractive index .19
4.5.1 General.19
4.5.2 Abbe refractometer .19
4.5.3 Test samples.20
4.5.4 Procedure .20
4.5.5 Expression of test results .21
4.5.6 Test report .21
4.6 Water content .22
4.6.1 General.22
4.6.2 Gravimetric determination of water content/absorption by loss on drying
using an oven .22
4.6.3 Test report .24
5 Test report .24
Annex A (informative) Determination of oxygen permeability using the coulometric method .25
Annex B (informative) Determination of water content by refractive index .32
Annex C (informative) Calculation of oxygen permeability of hydrogel lenses based on
water content .33
Annex D (informative) Measurement of refractive index using a prism coupling device .34
Bibliography .36
ISO 18369-4:2017(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: w w w . i s o .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee
SC 7, Ophthalmic optics and instruments.
This second edition cancels and replaces the first edition (ISO 18369-4:2006), which has been
technically revised.
A list of all parts in the ISO 18369 series can be found on the ISO website.
iv © ISO 2017 – All rights reserved
INTERNATIONAL STANDARD ISO 18369-4:2017(E)
Ophthalmic optics — Contact lenses —
Part 4:
Physicochemical properties of contact lens materials
1 Scope
This document specifies the methods of testing the physicochemical properties of contact lens
materials. These are extraction, rigid lens flexure and breakage, oxygen permeability, refractive index
and water content.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 3696:1987, Water for analytical laboratory use — Specification and test methods
ISO 18369-1:2017, Ophthalmic optics — Contact lenses — Part 1: Vocabulary, classification system and
recommendations for labelling specifications
ISO 18369-3:2017, Ophthalmic optics — Contact lenses — Part 3: Measurement methods
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18369-1 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
4 Physicochemical properties of contact lenses
4.1 Repeatability, test methods and units of measure
The physicochemical properties or conditions listed in Table 1 are measurable characteristics of hydrogel
and non-hydrogel materials that have been used to produce commercially available contact lenses.
ISO 18369-4:2017(E)
Table 1 — Physicochemical properties: Test methods and units of measure
Property Units of Test method Repeatability
b
Extractables mass % 4.2
b
Flexural deformation g 4.3
a
Oxygen permeability Dk units 4.4 10 %
Refractive index dimensionless 4.5 0,01
Water content weight % 4.6 2 % absolute
a −11 2
Dk is reported in units of 10 (cm /s) ml O /(ml × mmHg) and called “Dk units” or barrer.
b
Repeatability of these test results shall be established in individual laboratories according to ISO 18369-1:2017,
3.1.12.8, 3.1.12.9, 3.1.12.9.1, 3.1.12.9.2 and 3.1.12.9.3.
Clause 4 is applicable to testing laboratories, suppliers and users of contact lens products or services in
which measurement results are used to demonstrate compliance to specified requirements.
Alternative test methods and equipment may be used provided the accuracy and precision are
equivalent to or more capable than the test methods described.
In developing new test methods, these should be capable of measuring the various parameters with a
precision (R&R) of ≤30 % of the allowed tolerance. Resolution greater than 10 % of the tolerance can
be used but will affect determination of accuracy, precision, process capability and gauge capability.
The number of independent measurements should be chosen for each method to ensure appropriate
precision and accuracy.
4.2 Extractables
4.2.1 General
Soxhlet extraction with different solvents is a standard method for quantitative determination of
substances extractable from contact lenses. The contact lenses are dried to constant mass and the
difference between the original dry mass of the lenses and the extracted dry mass determines the
quantity of extractable substances (extractables).
Knowledge of the quantity and identity of extractable substances is helpful in evaluating new
contact lens materials and in determining the subsequent pre-clinical examination programme.
The material extracted from the contact lenses may be examined by appropriate chromatographic,
spectrophotometric and wet analytical methods to identify residual monomers, cross-linking agents,
catalysts, etc. that were employed in the polymerization process.
4.2.2 Principle
This method uses a normal Soxhlet extraction apparatus. Water and at least one suitable organic
solvent are used for extraction. In selecting the organic solvent(s) to be used, consideration should be
given to the effect of the solvent upon the matrix of the material. Ideally, a solvent should not swell or
degrade the contact lens material. However, in the development of new contact lens materials, a solvent
that causes reversible swelling may give valuable information relating to the possibility for extraction
over extended periods of time. Choice of a solvent that degrades the polymer network during extraction
is not recommended, as it will remove both uncrosslinked and crosslinked material, resulting in
inaccurate measurement of extractables.
4.2.3 Apparatus
4.2.3.1 Standard borosilicate glass Soxhlet extraction apparatus (see Figure 1), consisting of
the Soxhlet extractor (30 ml suggested), condenser, round bottom flask (100 ml suggested) and a
heating mantle.
2 © ISO 2017 – All rights reserved
ISO 18369-4:2017(E)
4.2.3.2 Perforated stainless steel, sintered glass, paper or equivalent extraction thimble fitted
with a glass wool plug or other suitable closure.
4.2.3.3 Vacuum oven or equivalent drying apparatus and an analytical balance capable of
weighing to 0,1 mg.
Figure 1 — Extraction apparatus
4.2.4 Reagents
4.2.4.1 Distilled or deionized water complying with ISO 3696:1987, Grade 3.
4.2.4.2 Appropriate organic solvent (see Table 2) of analytical grade or better.
4.2.4.3 Laboratory-grade boiling stones or anti-bumping granules, along with a suitable active
desiccant. Selection of the desiccant will depend upon the characteristics of the test material.
ISO 18369-4:2017(E)
Table 2 — Guide to the selection of solvents for use in extraction of contact lenses
Material Suggested solvents Corresponds to
Water (distilled or deionized) Mild extraction (simulates in-eye extraction)
n-Hexane, Mild extraction (non-polar solvent)
Hydrogels (including
or
silicone hydrogels)
Organic alcohol (e.g. ethanol, iso- Extraction of majority of uncrosslinked material
propanol or methanol) (but swells and may degrade material)
Water (distilled or deionized) Mild extraction (simulates in-eye extraction)
n-Hexane, Mild extraction (non-polar solvent)
Rigid gas permeable
or
and silicone elastomers
Dichloromethane or chloroform Extraction of all uncrosslinked material
(but swells and is likely to degrade material)
4.2.5 Test samples
Test samples shall be representative of the finished product and shall be in finished contact lens form.
The method of preparing and finishing the lenses shall reflect, as far as possible, the normal production
processes including sterilization. A sufficient number of lenses shall be used so that the total dry mass
before extraction shall be no less than 200 mg.
Hydrophilic lenses are usually packaged in a solution containing inorganic salts. When using water as
the extracting solution, an adjustment in the calculation should be made for the contribution of the
inorganic salt of the packaging solution. The water content of the lenses will be required in order to
accurately calculate the contribution of the inorganic salt to the extractables. Alternatively, the lenses
may be equilibrated in at least two changes of water each for 24 h at room temperature prior to
beginning the test.
4.2.6 Test procedure
Dry the lenses, preferably under vacuum, at 60 °C ± 5 °C or other appropriate temperature to
constant mass.
NOTE 1 Drying to constant mass is achieved when two consecutive weighings between drying do not differ by
more than 0,5 mg per gram of lens weight.
Allow the lenses to cool to room temperature under vacuum or in a closed container over active desiccant
before weighing. Then, weigh the dry lenses to ±0,1 mg (m ). Next, place the lenses into the extraction
thimble, place boiling stones in the flask, if necessary, and fill the flask to approximately 70 % of its
capacity with the appropriate solvent (see Table 2). Place the round-bottom flask in the heating mantle.
Place the extraction thimble into the Soxhlet apparatus. Then, attach the Soxhlet apparatus to the flask.
Place a condenser on top of the extraction apparatus. When using a volatile or flammable solvent, the
extraction apparatus should be placed in a fume hood.
Turn on heat and water and extract the lenses for at least 4 h. Allow the solvent to cool to room
temperature before removing the lenses from the extraction thimble. Dry the lenses to constant mass
as described above and weigh to the nearest 0,1 mg (m ). Calculate results as per Formula (1).
NOTE 2 If the dried lenses are fragile and fragmentation may have occurred leading to inaccuracies in
measurement, the extraction solvent can be quantitatively dried down to constant mass and the resultant
extractables residue weighed to the nearest 0,1 mg (m ). In this case, calculate results as per Formula (2).
4 © ISO 2017 – All rights reserved
ISO 18369-4:2017(E)
4.2.7 Calculation of results
The quantity of extracted material shall be expressed as a mass fraction (m ) in percent of the
extracted
initial dry mass as shown in Formula (1):
mm−
()
% extracted= ×100 (1)
m
where
m is the mass of lenses prior to extraction;
m is the mass of extracted lenses.
Alternatively, the extraction solvent can be quantitatively dried down to constant mass and the
resultant extractables residue weighed to the nearest 0,1 mg (m ) and used to calculate the quantity of
extracted material as shown in Formula (2):
m
% extracted= ×100 (2)
m
4.2.8 Test report
The test report for extractables shall conform to that in Clause 5 and contain the following information
for hydrophilic material:
a) the composition of the initial hydrating solution;
b) a statement as to whether the percentage of extractable substances has been adjusted for the salt
content of the hydrating solution;
c) if the contact lenses were equilibrated in water before the beginning of the test;
d) the method used to calculate quantity of extracted material, e.g. whether Formula (1) or Formula (2)
was used for the calculation.
4.3 Rigid lens flexural deformation and rupture
4.3.1 Principle
The test, which is a destructive test, applies an increasing load at the edge of a rigid lens across the
total diameter until, ultimately, the test sample fractures. The test is carried out in an apparatus which
allows the load and flexural deformation to be monitored continuously. Both the flexural deformation
strength and flexural deformation at rupture are determined, as well as flexural deformation strength
at 30 % deformation. The latter is derived from the flexural load-deformation curve. Either normal
production or specially constructed rigid contact lenses can be tested.
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.
4.3.2 Sampling
4.3.2.1 General samples
In order to demonstrate the degree of resistance to breakage by the material, general samples for
testing shall be normal, commercially available rigid, single vision contact lenses and shall not have
been specially treated or adjusted.
Contact lenses which have toroidal zones or truncations shall not be used.
ISO 18369-4:2017(E)
The specified label back vertex power (F′ ) shall be the same for all samples and shall be between
L
+0,50 D and −0,50 D.
The specified back optic zone radius (r ), or radius of the vertex sphere, shall be the same for all samples
and shall be between 7,75 mm and 7,85 mm.
4.3.2.2 Samples for material comparison
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 8,000 mm ± 0,025 mm;
— back surface: single cut, radius of curvature 7,800 mm ± 0,025 mm;
— total diameter: 9,5 mm ± 0,1 mm;
— centre thickness: 0,20 mm ± 0,01 mm;
— edge thickness: 0,24 mm ± 0,01 mm;
— edge form: rounded;
— maximum prismatic error: 0,5 cm/m.
The method of manufacture shall be stated in the test report.
4.3.2.3 Quantity
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.
4.3.3 Preparation of samples
Samples shall be stored in standard saline solution conforming to ISO 18369-3:2017, 4.9, for at least
48 h prior to testing. The temperature of this saline solution shall be 20 °C to 25 °C.
4.3.4 Apparatus
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 4.3.4.1 to 4.3.4.3.
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.
4.3.4.2 Load indicator, capable of indicating the total load applied to the sample.
4.3.4.3 Data recorder, to which the testing machine is connected, and which, after commencement
of application of the load to the sample, provides a recording of the total load applied to the sample as a
function of time.
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.
6 © ISO 2017 – All rights reserved
ISO 18369-4:2017(E)
Key
1 load indicator
2 recorder
a
See Figure 3 for detail X.
Figure 2 — Testing machine
Key
1 test specimen setting jig
2 test specimen
a
Detail of Figure 2.
Figure 3 — Test specimen setting jig
ISO 18369-4:2017(E)
4.3.5 Procedure
Confirm the correct operation and calibration of the apparatus.
Carry out the test at a temperature of 20 °C to 25 °C.
Remove the conditioned sample from the saline solution and dry it carefully.
Measure the back optic zone radius, total diameter, centre thickness and label back vertex power
as described in ISO 18369-3. Position the sample in the jig so that the upper and lower edges of the
sample lie along the centre line of the upper contact face. Set the velocity of the moving contact face to
20 cm/min (3,33 mm/s) ± 10 %.
The sample and jig may be set horizontally or vertically. If a horizontal system is used, it is necessary
to confirm in advance of the test that results do not differ from those obtained using a vertical system.
Start the data recorder and then commence applying the load to the sample. Stop applying the load
when the lens ruptures. Record the load in grams at which rupture occurred. Repeat the test with each
of the test samples.
4.3.6 Test result
4.3.6.1 General
Use the test results to calculate the arithmetic mean values together with the standard deviation
(see note) for flexural deformation strength at rupture (see 4.3.6.2), flexural deformation at rupture
(see 4.3.6.3) and flexural deformation strength at 30 % deformation (see 4.3.6.4).
NOTE The estimated standard deviation (σ) is given by Formula (3):
()xx−
∑
σ = (3)
n−1
()
where
x is the value of a single result;
is the arithmetic mean ( xn );
x
∑
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.
4.3.6.3 Flexural deformation at rupture
Knowing the time of rupture and the rate of loading at this time, calculate the distance (d) between
the contact faces when rupture occurred. Express the flexural deformation as a percentage of the total
initial diameter (D ) of the sample as shown in Formula (4):
T
d
100× 1− (4)
D
T
8 © ISO 2017 – All rights reserved
ISO 18369-4:2017(E)
4.3.6.4 Flexural deformation strength at 30 % deformation
Knowing the rate of loading, calculate the time when the total diameter of the sample has decreased by
30 % (see example) and determine the load, in grams, that was being applied at that time. The load can
also be derived from the flexural load-deformation curve.
EXAMPLE
Total diameter of the contact lens is 9,6 mm.
Velocity of the moving contact face is 20 cm/min (3,33 mm/s).
30 % deformation = 2,9 mm.
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.
4.3.6.5 Test report
The test report shall conform to that in Clause 5.
4.4 Oxygen permeability
4.4.1 General
Two standardized methods are used for the determination of oxygen permeability of contact lens
materials. They are the polarographic method specified in 4.4.3 and the coulometric method is specified
in Annex A. Common elements of both methods are detailed in 4.4.2. Calibration of both methods is
given in 4.4.4 and the reporting of results is covered in 4.4.5.
There are other techniques of measuring oxygen permeability and variations upon the standardized
methods that may be used if shown to give results after calibration equivalent to those derived from a
standardized method.
Oxygen permeability of a material is determined from preliminary measurements of the oxygen
transmissibility of several samples of the material in the form of contact lenses. However, there are
certain errors in the measurement of transmissibility that can be effectively reduced to insignificance
(corrected) when oxygen permeability of the material is derived. It is, therefore, practical and
convenient to first cover the derivation of oxygen permeability values corrected for these errors, from
preliminary (uncorrected) oxygen transmissibility measurements. The corrected permeability values
are then calibrated. Corrected and calibrated oxygen transmissibility values may then be computed
from the corrected and normalized permeability values.
Annex C provides information on the calculation of oxygen permeability of hydrogel lenses based on
water content.
4.4.2 Common elements of the methods
4.4.2.1 Parameters
Important parameters relevant to the measurement and derivation of oxygen permeability are oxygen
flux, oxygen permeability, oxygen transmissibility, thickness (i.e. radial thickness) and mean central
thickness. Refer to ISO 18369-1 for the definitions of these terms.
In terms of measurement using the coulometric method, j is equal to the rate of oxygen flow past the
coulometric oxygen sensor (µl O /s) divided by the area of sample (A) through which the oxygen has
passed. With the polarographic method, j is the difference between the measured and dark currents,
multiplied by the constant cited in Formula (5) and divided by the central cathode area.
ISO 18369-4:2017(E)
The thickness (t) is the local radial thickness at the point of measurement or the mean central thickness
over the measurement area. Having measured the centre thickness and by knowing the refractive
index, back surface curvature and refractive power of a particular lens, the mean central thickness can
also be calculated. Unless otherwise indicated, t should be stated in centimetres (cm).
In terms of measurement using the coulometric method, Dk is equal to the measured oxygen
transmissibility (Dk/t) multiplied by the mean central thickness (t). With the polarographic method,
oxygen permeability is corrected by adjustment of the value taken for the area exposed to oxygen flow,
and by taking the slope of a line 1/Dk derived from plotting measured oxygen resistance t/Dk against
thickness (t). Oxygen permeability is a physical property of the material and is not a function of the
shape or thickness of the material sample.
In terms of measurement using the coulometric method, Dk/t is equal to the oxygen flux ( j) divided by
the difference in oxygen tension (partial pressure of oxygen) between atmospheres at the two exposed
surfaces of the sample contact lens. With the polarographic method, oxygen transmissibility is the
oxygen permeability corrected for edge and barrier layer effects (Dk) divided by thickness (t). Oxygen
transmissibility is a property of the lens material and lens thickness and, therefore, depends on the
design of the contact lens.
4.4.2.2 Test samples
The oxygen permeability of hydrogel and non-hydrogel flexible materials in the form of finished contact
lenses incorporating various powers and designs can be determined using this document. The oxygen
permeability of hydrogel or non-hydrogel materials in the form of standardized test samples can also be
determined. Test samples with opposing surfaces that are nearly parallel are preferred (see 4.4.3.7.1).
If the aim of investigation is to determine oxygen permeability through the measurement of preliminary
transmissibilities of finished contact lenses, the mean thickness within the central area of a contact lens
exposed to oxygen flow (see 4.4.2.1) should be included in the test report (see 4.4.5). This thickness,
however, is not a factor in the derivation of preliminary oxygen transmissibility (Dk/t) from oxygen flux
measurements [see Formula (5) and Formula (10)].
The back optic zone radii shall be known. The back and front optic zone diameters shall be greater than
the chord diameter (2h) of the central lens area tested for gas exchange. 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 should be stored in standard saline solution (see
ISO 18369-3:2017, 4.9) and should be equilibrated (see ISO 18369-1:2017, 3.1.1.21, for definition) at eye
temperature (35 °C ± 1,0 °C). The conditions of equilibration are described in the test report.
4.4.3 Polarographic method
4.4.3.1 General
The determination of oxygen permeability of hydrogel and non-hydrogel, rigid and flexible contact
lens materials, using a polarographic oxygen sensor is described. The procedure specifies how
measurements are taken and establishes the conditions under which measurements are made.
The polarographic method is applicable to the determination of the corrected oxygen permeability
(Dk) of rigid, hydrogel and non-hydrogel flexible materials in the form of contact lenses, incorporating
various refractive powers and rotationally symmetric lens geometries, and corrected oxygen
permeability (Dk) of hydrogel, non-hydrogel flexible materials, and rigid contact lens materials in the
form of standardized test samples.
4.4.3.2 Principle
The polarographic method directly measures the number of oxygen molecules diffusing through a
test material by electrochemically removing the molecules from solution after they pass through the
material. After a molecule of oxygen emerges from the sample material, it contacts the centre electrode
10 © ISO 2017 – All rights reserved
ISO 18369-4:2017(E)
(cathode) of the oxygen sensor, placed against the back surface of the sample, and is instantaneously
involved in a chemical reaction that yields four hydroxyl ions. This production of ions constitutes
the electric current which is quantified by the apparatus, and which is proportional to the number of
molecules removed. The measured current is used to calculate the preliminary (uncorrected) oxygen
transmissibility, (Dk/t) , expressed as ml O /(A⋅s), through the material as in Formula (5):
preliminary 2
II−
()
Dk
d
−2
= ×5,804×10 (5)
t pA×
preliminary A
where
p is the barometric pressure less the water vapour pressure, expressed in mmHg,
A
multiplied by the oxygen fraction in the oxygenated gas (e.g. 0,209 for air);
I is the steady state current, in amperes, from the oxygen sensor (100 % conversion
efficiency is assumed);
A is the area, in cm , of the cathode face in the oxygen sensor;
I is the “dark current”, in amperes, of the oxygen sensor (i.e. the current that flows in
d
the absence of oxygen flux);
−2
5,804 × 10 is the product of the volume of one kilogram mole at standard conditions of temper-
ature and pressure (STP) divided by Faraday’s constant divided by the number of
charges per molecule of oxygen reduced, assuming four charges per molecule.
In order to derive the oxygen permeability of lenses made of a particular material, correction shall be
made for edge effects (see 4.4.3.3) and boundary-layer effects (see 4.4.3.4). Only then may the corrected
oxygen transmissibility of a contact lens made from the material be calculated.
4.4.3.3 Correction for edge effects
An artifact common to diffusion-type methods, termed the “edge effect,” shall be accounted for. This
effect occurs whenever the front and back diffusion areas through which oxygen passes are not equal
and aligned. In the polarographic method, the oxygen that eventually contacts the cathode at the back
of the lens, funnels to the cathode from a larger frontal area of the lens sample than represented by
the cathode area at the back of the sample. In effect, A in Formula (5) has been underestimated and
oxygen flux is no longer a simple linear function of cathode area as noted in the formula. However, if
the cathode diameter is at least ×10 larger than the sample thickness, a comparatively simple numerical
procedure applied to the preliminary Dk/t values will correct for the edge effect, within the accuracy
necessary for this document.
As it will be necessary to have the reciprocal of transmissibility in 4.4.3.4, the edge effect correction
will be applied to the preliminary reciprocal (t/Dk, or resistance) values. Each preliminary t/Dk value
shall be corrected by using the appropriate formula given below. In Formula (6) to Formula (9), t and
the cathode diameter (D ) are expressed in millimetres, and for convenience, Dk values are stated
cathode
−11 11
in “Dk units”, i.e. in units of 10 multiplied by 10 before use in the formulae.
ISO 18369-4:2017(E)
For hydrogels, tested with a spherical cathode, the corrected t/Dk, (t/Dk) , is given by
corrected
Formula (6):
23, 5×t
()
t t
= × 1+ (6)
Dk Dk D
cathode
corrected preliminary
For hydrogels, tested with a flat cathode, the corrected t/Dk, (t/Dk) , is given by Formula (7):
corrected
18, 9×t
t t ()
= × 1+ (7)
Dk Dk D
cathode
corrected preliminary
For non-hydrogels, tested with spherical cathode the corrected t/Dk, (t/Dk) , is given by
corrected
Formula (8):
0,,587−00011934Dk × t
()
t t
...
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