How different are Silicone Hydrogels?

How different are
Silicone Hydrogels?
Ralph P. Stone, Ph.D1., Rebecca Frederick, MS2, Jean
Jacob, Ph.D2.
1 RP Stone Consulting, Inc. 2. LSUHSC
Disclosure
—  Unrestricted funds provided for purchase of
radiolabelled lipids by Alcon Laboratories
—  No author has a proprietary interest in any
product in these investigations.
—  Dr. Stone is a consultant to Alcon Laboratories.
—  Dr. Stone is a member of ANSI and is a US expert
to ISO T172/SC7?WG-9, and is the project leader
for the classification of silicone hydrogels (ISO
18369-1)
Prior to 1999
—  All soft contact lenses were simple hydrogels
composed of hydrophilic monomers such as
HEMA, PVP, Methacrylic acid and glyceryl
methacrylate ranging in water content from 38%
to 78% water.
—  They were a continuous single phase material with
the surface and bulk essentially uniform in
composition.
—  In 1984, a classification system was developed for
testing these materials which remains in place
today.
The “Conventional” Soft
contact lens Classifications
Group 1 (or I)
Low water non-ionic lenses with
water content below 50% water
and containing 1 mole% or less of
an ionic monomer.
e.g. Polymacon 38% water
Group 2 (or II)
High water nonionic lenses with
water content 50% or greater of
water, and containing 1 mole% or
less of an ionic monomer at pH
7.2.
e.g. Lidofilcon B 70% water
Group 3 (or III)
Low water ionic lenses containing
less than 50% water and
containing greater than 1 mole
percent of an ionic monomer at
pH 7.2. e.g., Phemfilcon A
Group 4 (or IV)
High water ionic lenses containing
50% or greater or more of water
and containing 50 % of more an
ionic monomer at pH 7.2. e.g.
Etafilcon A
Post 1999
The Era of Silicone
Hydrogels
—  Lenses were introduced that combined the
silicone/fluorosilicone chemistry of rigid gas
permeable contact lenses with the hydrogels of
the soft contact lenses.
—  They achieved the enhanced oxygen permeabiity
lacking in conventional hydrogel but had
properties of both RGP materials and conventional
hydrogels.
—  For testing purposes, these materials provided
new challenges.
Changing Criteria
—  Our first approach was to include them in the soft
lens classification approach.
—  These materials became members of groups 1
and 3 based on their water content.
—  This provided different characteristics between the
traditional members of these group and the new
entries.
Jones Study 2002
—  In a randomized, crossover design clinical study
for one-month on each product, a PHMB based
multi-purpose solution was compared to a
Polyquad/Aldox solution using Balafilcon contact
lenses.
—  31% patients using PHMB based product showed
significant corneal stainging compared to 2%
using the Polyquad/Aldox product (p<.0001)
—  Only correlation in symptims was stinging on
insertion favoring the Poyquad/Aldox (p<.008)
Jones LW, MacDougall, Sorbara LG. OVS2002; 79(12):
753-761
The First Revision
Group 5 (V)
Materials having greater than 10%
water having an oxygen
permeability (Dk) greater than 40
and a Dk greater than that
expected on water alone.
Are There Differences in
Group 5?
—  We quickly found that the lenses quickly showed
differences in their interactions with the
environment and care systems.
—  It has become evident that the silicone hydrogel is
not simple
—  They are biphasic materials
—  They contain a high water hydrophilic phase
—  They contain a silicone hydrophobic phase
—  And we sometimes cover up cover up the
hydrophobic phase that can come to the surface with
a surface modification to keep the surface wettable.
The Current Approach
The hydrophilic phase
Group 5A
A group 5 materials that contains monomers or oligomers which are
ionic at pH 6-8.
Group 5B
Materials that contain 50% or more of water and no ionic monomers
an oligomers at pH 6-8
Group 5C
Materials containing less than 50% and no ionic monomers or
oligomers at pH 6-8
The Current Approach
Surface Modification Codes
No Code
The surface has not been modified e.g., comfilcon A
“c” Code
The surface has been chemically modified e.g. balafilcon A,
lotrafilcon B
“w” Code
Materials having releasing or internal wetting agents using
interpenetrating networks networks e.g., galyfilcon A
The Current Approach
The hydrophobic phase
At this time insufficient data is available to subclassify the material based on a description of the
hydrophobic phase. Until such time as further
classification of the hydrophobic phase can be made,
users should carry out a risk assessment to define
the appropriate materials to include in care product
evaluation.
What do we know about
the hydrophobic phase?
In-Vitro Uptake
20
18
16
14
ug/lens
12
Cholesterol
10
Phospholipid
8
6
4
2
0
Balafilcon
A
Senofilcon
A
Galyfilcon
A
Lotrafilcon Lotrafilon B
A
F.Carney, W Nash, K Sentell IOVS 2008; 49(1):120-124
Etafilcon
In Vitro Uptake
45
40
35
ug/lens
30
Galyfilcon A
25
Senofilcon A
Asmofilcon A
20
15
10
5
0
Total Lipids
Choleserol
Phospholipids
S Hitou, M Fukui, K Yatsui et al. J Optometry 2010; 3(3): 164-168
Ex Vivo Cholesterol Uptake
4
3.5
3
ug/lens
2.5
2
Cholesterol
1.5
1
0.5
0
Lotrafilcon
B
Galyfilcon
A
Enfilcon A
Senofilcon Confilcon A
A
W Nash, M Gabriel, M Mowrey-McKee. OVS 2010; e-abstract 105110
In-Vitro
Radiolabel Lipid Exposure
0.10
30min PC
0.09
30min Cholesterol
0.08
30min Triolein
μg Lipid
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
Acuvue
Oasys
Air Optix
Aqua
PureVision 2 Biofinity B
Flat A
Flat B
Flat C
In-Vitro
Radiolabeling Exposure
0.10
12hr PC
0.09
12hr Cholesterol
12hr Triolein
0.08
0.07
μg Lipid/lens
0.06
0.05
0.04
0.03
0.02
0.01
0.00
Acuvue Oasys
Air Optix
Aqua
PureVision 2
Biofinity B
Flat A
Flat B
Flat C
Summary
—  Silicone Hydrogel that now make up about 2/3 of
fit for soft contact lenses are very different from
our conventional soft contact lenses
—  Although we understand much of the differences
on these materials we still are working to
understand the lipophilic character of the silicone
phase.
—  Understanding the hydrophobic phase is critical to
the development of new approaches to disinfection
for more resistant organisms such as
Acanthamoeba and fungi.