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copyright. By accessing this material, you agree to the following terms and conditions:
You may only access and use the IACLE Contact Lens Course for personal or
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You may print materials for personal or educational purposes only. All copyright
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including text, images, &/or illustrations.
COPYRIGHT NOTICE
3. SPONSORS
Development and delivery of contact lens education by IACLE is supported
through educational grants and in-kind contributions
Major In-Kind
Supporters
Industry
Supporters
4. Published in Australia by
The International Association of Contact Lens Educators
First Edition 1997
The International Association of Contact Lens Educators 1996
All rights reserved. No part of this publication may be reproduced, stored in a
retrieval system, or transmitted, in any form or by any means, without the prior
permission, in writing, of:
The International Association of Contact Lens Educators
IACLE Secretariat,
PO Box 656
Kensington NSW 1465
Australia
Email: iacle@iacle.org
5. The IACLE Curriculum Project is the result of a desire to raise
the general standard of eyecare education, to make contact
lens wear safer and more successful, and to develop the
contact lens business further by creating the educational
infrastructure that will produce the teachers, students, and
practitioners of the future.
Full acknowledgements, along with the educator’s guide to the
IACLE Contact Lens Course (ICLC), can be found on the
IACLE website at www.iacle.org
ACKNOWLEDGEMENTS
7. TEMPERATURE EFFECTS OF
CONTACT LENSES
Open eye:
• With SCLs:
–anterior surface 0.5 °C cooler
• With GP lenses (lower conductivity):
–anterior surface >0.5 °C cooler
Closed eye (cornea warms 3 °C):
• No effect (GPs and SCLs)
• No differences between surfaces
8. TEAR FILM EFFECTS OF
CONTACT LENSES
• Evaporation rates: SCLs GPs
• CLs reduce BUT
• BUT:
–GPs - 4 to 6 s
–SCLs - 4 to 10 s (H2O content Dk/t)
9. OXYGEN PERMEABILITY
• Intrinsic material property
(resistance to gas flow)
• Permeability P = Dk
where:
D is the diffusion coefficient
k is the solubility coefficient
of oxygen in a given material
15. Dk UNIT
Lens material
1
cm
1 cm
after Refojo et al., 1984
1 sec
1 cm
1
cm
O2
1 cm
1
cm
O2 Hypothetical
contact lens
(very much larger
than a
real contact lens)
Conditions: STP (0 °C,
760 mm Hg) unless stated to be
otherwise, e.g. 21 or 34 °C @ 760 mm Hg
16. OXYGEN PERMEABILITY
• Contact lens range: 0 - 300 x 10-11
• Units:
–(cm2 x mLO2) / (s x mLLens x mm Hg)
or
–(cm2/s) x (mLO2 / [mLLens x mm Hg])
–commonly, the Lens subscript is omitted
17. OXYGEN TRANSMISSIBILITY
• Based on material permeability (Dk)
• Related to material thickness (t)
–transmissibility = Dk/t
• Relevant clinically
18. TRANSMISSIBILITY (Dk/t )
UNIT DERIVATION
Dk =
mmHg
x
mL
x
s
mL
x
cm
Lens
O2
2
Dk/t =
cm
x
mmHg
x
mL
x
s
mL
x
cm
Lens
O2
2
Dk/t =
mmHg
x
mL
x
s
mL
x
cm
Lens
O2
19. OXYGEN TRANSMISSIBILITY
• Contact lens range: 0 - 200 x 10-9
• Units:
–(cm x mLO2) / (s x mLLens x mm Hg)
or
–(cm/s) x (mLO2 / [mLLens x mm Hg])
–commonly, the Lens subscript is omitted
22. MEASURING OXYGEN
TRANSMISSIBILITY
• Sensor in measuring cell contains:
–anode (+)
–cathode (–)
–electrolyte
• Contact lens becomes the ‘membrane’
• Controlled humidity and temperature
POLAROGRAPHIC CELL TECHNIQUE
23. 'O' Ring
Oxygen flow
Rigid contact lens
sample
Tip of oxygen
sensor
Oxygen sensor cathode (–)
'O' Ring
Oxygen-free area
Saline-saturated
filter paper
155 mmHg
O2
POLAROGRAPHIC CELL
+
24. POLAROGRAPHIC CELL TECHNIQUE
• Oxygen passes through the lens into the
sensor’s electrolyte
• Sensor current is proportional to the
amount of oxygen available at the cathode
• Oxygen flux j from Fick’s & Henry’s laws:
j = Dk/t x D(pO2)
25. • Permeability calculated from:
–thickness of the lens (t)
–current required to reduce O2 (i)
–partial pressure of O2 (pO2)
–cell constant (C)
C x t x i
pO2
Dk =
POLAROGRAPHIC CELL TECHNIQUE
28. POLAROGRAPHIC CELL TECHNIQUE
• Not suited to highly permeable, non
hydrogel materials
• Over-estimates values for GP lenses
• Variability among investigators
DISADVANTAGES
29. MEASURING OXGEN
TRANSMISSIBILITY
• Two environmental chambers
–pure oxygen
–differential pressures
• Constant temperature (35oC)
• Pressure sensor in each chamber
GAS-TO-GAS
31. MEASURING OXYGEN
TRANSMISSIBILITY
• Constant pressure in anterior chamber
• Gas flow through the lens alters the
pressure in the posterior chamber
• No boundary layer or edge effects
• Can be used for any gas
GAS-TO-GAS TECHNIQUE
32. GAS-TO-GAS TECHNIQUE
• Not suitable for hydrogels
–pressure differential too great
–hydrogels too elastic
–hydrogels have low burst strength
DISADVANTAGES
33. MEASURING OXYGEN
TRANSMISSIBILITY
• Two 'environmental chambers'
–oxygen
–inert gas
• Coulometric sensor
• Oxygen flow through lens is measured
• Water saturated oxygen or liquid
reservoir required for hydrogels
COULOMETRIC TECHNIQUE
34. COULOMETRIC TECHNIQUE
Anterior
Environment
Chamber
Inflow of Oxygenated Gas
Outflow
Oxygen Flow
Rigid Contact
Lens Sample
Posterior
Environmental
Chamber
159
mm Hg
O2
'O' Rings
Outflow to coulometric
oxygen sensor
Inflow of Oxygen-
Free Gas
35. COULOMETRIC OXYGEN
SENSORS CHEMISTRY
4 e– + O2 + 2H2O 4OH–
Cathode (– ve, Carbon [graphite]):
2 Cd + 4OH– 2Cd(OH)2 + 4e–
Anode (+ ve, Cadmium [nickel-cadmium]):
2Cd + O2 + 2H2O + 4OH– 2Cd(OH)2 + 4OH–
Overview:
e = electron
36. COULOMETRIC TECHNIQUE
• Posterior lens surface is exposed
• Little or no boundary layer effects
• No edge effects
• More accurate than polarographic
techniques with GP contact lenses
ADVANTAGES
39. Low < 25
Mod25 - 50
High > 50
GP LENSES
TRANSMISSIBILITY CLASSIFICATION
40. THANK YOU
Table of Contents
Feedback on errors, omissions, or suggestions for
improvement are invited. Please contact us at:
iacle@iacle.org
See the following slides explaining the symbols,
abbreviations, and acronyms used in the IACLE Contact Lens
Course
