Properties of hydrogel and silicone hydrogel contact lens
1. R E S O U R C E P E R S O N
M R . G A U R I S H R E S T H A
Properties of hydrogel and
silicone hydrogel contact lens
Prativa Devkota
2nd yr
B.optom
2. Contact lens
A curved shell of glass or plastic applied directly
over the globe or cornea to correct refractive errors
/ to change cosmetic look / to protect diseased or
injured cornea
3. Properties of contact lens
Provide sufficient oxygen
Be physiologically inert
Be wettable on the eye
Resist lens spoilage
Maintain stable dimensions
4. Contd…
Be durable
Be transparent
Be optically regular
Have physical properties which allow creation and
retention of high quality surfaces
Require minimal maintenance
Be easy to fabricate lenses from
6. HISTORY
1940s,PMMA came into introduction
Properties
Maintained optical clarity
Readily machined and cleaned
Fairly wettable
Ease of sterilization
Major disadvantage
Highly rigid
impermeability
7. Synthetic elastomers
Came with advantageous feature of high oxygen
permeability,100 times more than PMMA
Due to backbone of alternate silicon and oxygen atom
Major drawback
Hydrophobic surface
8. Structure of PMMA can be made more hydrophilic
by incorporation of hydroxyl groups
Poly(2-hydroxyethyl methacrylate)
In absence of water ,polyHEMA is hard glassy
material which upon hydration is transformed into
familiar contact lens material
9. In 1960s, wichterle and his coworkers invented
hydrogel contact lenses made of polyHEMA
crosslinked with ethylene glycol dimethacrylate
(EDMA)
11. HYDROGELS
Water-swollen, crosslinked polymeric structure produced
by reactions of monomers or by hydrogen bonding
Hydrophilic polymers that can absorb up to thousands of
times their dry weight in H2O
Three-dimensional insoluble polymer networks
12. Acts as “washing line” polymers having long
backbone from which variety of chemicals may be
suspended.
The function of chemical groups in hydrogel is primarily to attract
and bind water within structure
Greater physical stability is achieved by fastening the washing line
together at interval by the use of cross-links
Oxygen dissolves in and is transported through
aqueous phase
13. FDA Classification of hydrogel contact lens
materials
group Water content Ionic content
I Low water content, non-ionic <50 < 1
II High water content ,non-ionic >50 < 1
III Low water content ,ionic <50 >1
IV High water content ,ionic >50 >1
15. Disadvantage of high water content
Greater fragility
More deposit prone
More susceptible to environment
Less stable parameters, lower reproducibility
Should not be thermally disinfected
Optical quality is more difficult to
16. Advantages of ionic materials
Polar functional groups at the lens surface increase
wettability
Ionic materials denature tear proteins less than non-
ionic materials, even they contain more protein.
17. Disadvantage of ionic materials
Ionic materials are more susceptible to pH changes,
especially their water content
Deposits may be bound and difficult to remove
Accumulate deposits more readily
18. Advantage
1. "Initial" Comfort
2. Visual acuity or comfort affected little by lens
movement
3. Simple to fit
4. Little adaptation
5. Occasional wear possible
19. Advantage
6. Little chance of dislodging the lens
7. Chance of trapped foreign body or abrasion small
8. Rarely causes photophobia or excessive tearing
9. Cosmesis
10. Minimal spectacle blur
20. Disadvantage
1. Bacterial contamination and infection
2. Increased solutions/complexity and solution cost
3. Increased care time
4. Less Durable
5. Greater risks with noncompliance
21. Disadvantage
6. Prone to deposits/GPC
7. Reduced oxygen permeability (corneal edema)
8. Quality of vision may be reduced from a couple of
letters to a line on Snellen chart & reduced CSF
9. Difficult to verify
10. Limitations of correction
23. In conventional hydrogel, the O2 dissolves in and is
tranported through aqeous phase and the carbon
backbone bahaves rather like PMMA
Oxygen is more soluble in water than it is in PMMA,
but far more in silicon rubber than it is in water
This is property or function of silicon-oxygen and
silicon –carbon bonds found in silicon rubber
24. Approaches to the development
Two Different Approaches:
o First by Bausch & Lomb: development of silicone monomers
with enhanced compatibility in hydrophilic hydrogel-forming
monomers.
o Second by Ciba Vision: development of siloxy macromers
containing hydrophilic polyethylene oxide (PEGS) segments
and oxygen permeable polysiloxane units.
25. introduction
Two major components
Silicone components
Provides extremely high
oxygen permeability
Hydrogel components
Facilitates fluid
transport and thus
lens movement
26. Contd…
“Continous wear” has become synonymous with
their use, as intended for continous wear for 30 days
27. Unique properties of silicone hydrogels
OXYGEN TRANSMISSIBILITY
Holden-Mertz criterion
Contact lenses must have o2
permeability of at least Dk of 87
In conventional hydrogels, Dk is
directly related to water content of
lens materials but water borne
transport is little affected in silicon
hydrogels
28.
29. Unique properties of silicone hydrogels
SURFACE PROPERTIES
silicone is inherently highly hydrophobic
In order to make the surfaces of silicone hydrogel
lens materials hydrophilic and more wettable,
different techniques are developed.
Incorporation of plasma into the lens surface
Incorporation of internal wetting agent, polyvinyl
pyrrolidone(PVP)
Transforms the silicone components into hydrophilic silicate
compounds
Create permanent ,ultrathin (25nm),high refractive index
,continuous hydrophilic surface
30.
31. Unique properties of silicone hydrogels
MECHANICAL PROPERTIES AND LENS
STIFFNESS
Stiffer than conventional hydrogels due to
incorporation of silicon
Newer silicon hydrogel materials have reduced
degree of stiffness due to less silicon within the base
material.
33. ADVANTAGES OF INCREASED MODULUS
Perfect choice for people who exihibit poor handling
capabilities
DISADVANTAGE
Lesser ability to confirm to the shape of eye
Donot result in an optimum fitting thus exhibiting
edge lift or slight fluting
Mechanical complication like CLPC,superior
epithelial splits
35. Clinical performance of silicon hydrogel
Dumbeston et al(1998) reported reduced limbal hyperaemia
for silicon hydrogels worn on EW basis over 6 month period
compared with conventional hydrogels.
Mc Monnies et al (1982) and Holden et al(1986) have shown
that more limbal vessels penetration occurs with hydrogel
EW than DW.
In comparison with disposable hydrogel materials,
biochemical analysis has shown that silicone hydrogel
materials have significantly reduced level of protein
deposition and higher level of lipids.
36. In CCLRU studies a greater percentage of patients
wearing silicone hydrogels lenses display deposits
than those wearing daily disposables.
Higher number of mucin balls are observed with
higher modulus silicone hydrogel materials
Corneal changes during hydrogel lens wear include
initial flattening followed by gradual steepening as
lens wear continues.
37. references
The IACLE Contact lens Course
Silicone Hydrogels : the rebirth of continuous wear
contact lenses
Optometry: science, Techniques , And Clinical
Management
Optics of eye, A .k khurana
