water properties of soft cl materials
TRANSCRIPT
-
ft
is,
rsity
cont
manc
xamine the state of water in eight soft contact lenses manufactured from
retentional water. A polymer usually has both hydrophilic
dimensional network structure, its physical properties are
governed by the state of the water within the polymer [1].
This classification has been used by Pedley and Tighe [3]
and others and is the most widely used system. Tightly
hydrogen-bonding characteristic of pure water (freezing at
273 K). Loosely bound water is more vaguely defined and
Contact Lens & Anterior Eye 27covers diverse classes of water that remain in liquid state
below the normal freezing temperature. Melting tempera-
tures vary depending on the amount of water present and the* Corresponding author. Tel.: +44 161 306 3886; fax: +44 870 831 6625.
E-mail address: [email protected] (N. Efron).
1367-0484/$ see front matter # 2004 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.clae.2004.08.003and hydrophobic groups. The hydrated state is formed as a
result of the interaction between hydrophilic and hydro-
phobic groups and water. A water molecule is liable to be
bound to a polymer or trapped in a small space formed
within the polymer. An hydrated soft contact lens is a typical
example of a material carrying bound water, trapped in
molecular spaces. In an hydrated soft contact lens of three
bound water is usually associated with water molecules,
which have direct hydrogen bonding with the polar groups of
the polymer matrix, or water molecules that strongly interact
with ionic residues of the polymer matrix. As a result, it is
non-freezable under normal characterisation conditions.
Free or bulk water refers to the water molecules, which do
not interact at all with the polymer matrix and haveThe soft contact lens has a flexible property mainly due toHEMA/VP 55%, HEMA/VP 70%, VP/MMA 55%, VP/MMA 70%, HEMA 40%, HEMA/MAA 55% and HEMA/MAA 70% [HEMA = 2-
hydroxy-ethyl methacrylate, VP = vinyl pyrrolidone, MMA = methyl methacrylate, MAA = methacrylic acid]. Differential scanning
calorimetry (DSC) was used for measuring the free water content in the materials listed above. Some noticeable differences in water properties
among soft contact lens materials having approximately the same water contents were revealed. Low water content materials exhibited a
simple endotherm and all water melted around 0 8C. On the other hand, medium and high water content materials exhibited multiple meltingendotherms, representing a broad range of interactions between water and the polymer. Low water content soft contact lenses have
approximately the same amount of bound water as those with much higher water contents. Six subjects were then fitted with the same lenses
for one day. In vitro measurements of water content and oxygen transmissibility were taken at 35 8C, both before lens fitting and after 6 h oflens wear. Water content and oxygen transmissibility were correlated with the water properties of the soft contact lens materials. The relative
change in lens water content (%DWC) and relative change in lens oxygen transmissibility (%DDk/t) were calculated and correlated with thewater properties of the eight soft contact lens materials. It was concluded that (a) oxygen transmissibility, free water content and free-to-bound
water ratio are increased when the water content of a contact lens is increased and (b) water content, free water content and free-to-bound
water ratio cannot be used for the prediction of soft contact lens dehydration in vivo.
# 2004 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.
Keywords: Water properties; Hydrogel contact lens materials; Differential scanning calorimetry; Free water content; Bound water content; Free-to-bound
water ratio; Lens dehydration
1. Introduction Alternative approaches to classifying and describing the
state of water in hydrogels have been described (Table 1) [2].clinical and laboratory experiments that were conducted in order to e
different materials. Specifically, lenses made from the following eight materials (and nominal water contents) were used: HEMA/VP 40%,Water properties of so
Ioannis Tranoud
Eurolens Research, Department of Optometry, The Unive
Abstract
The properties of water in soft contact lenses such as the water
dehydrate during wear, are key determinants of their in eye perforcontact lens materials
Nathan Efron*
of Manchester, P.O. Box 88, Manchester M60 1QD, UK
ent, free-to-bound water ratio, and the extent to which soft lenses
e and oxygen transmissibility characteristics. This study describes
www.elsevier.com/locate/clae
(2004) 193208
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water compared with those polymers containing functional
groups of higher water-binding capability, such as the
pyrrolidone group [2].
The presence of a cross-linking agent is the key to
holding a hydrogel system together. Many hydrogel
properties are affected by the number of crosslinkages (or
cross-linking density). It is well documented that increasing
the density of crosslinkages causes a decrease in water
content [4]. At the same time, the absolute amount of bound
ns & Anterior Eye 27 (2004) 193208state of bonding with the polymer matrix. Loosely bound
water usually refers to water molecules in water-swollen
polymer systems which are only loosely associated with
polar groups through hydrogen bonding, but have higher
hydrogen-bonding energies than that of pure water.
Descriptions of bound water include tightly bound water
and loosely bound water.
There exists a great deal of confusion concerning non-
freezable and freezable bound water due to the limitation of
measurements by some analytical techniques. The existence
and quantification of free water, bound water, freezable
water and non-freezable water have been determined
through the use of differential scanning calorimetry (DSC),
thermogravimetric analysis (TGA) and nuclear magnetic
resonance (NMR) spectroscopy. Infra-red spectroscopy and
chromatography have also been used to study the state of
water.
The water absorbance characteristics of hydrophilic
polymers vary widely depending on many factors. The most
widely recognised factors are: (1) nature of hydrophilic
groups; (2) cross linking density of the polymer system; (3)
degree of water saturation; (4) the medium of the water-
swollen polymer system; (5) hydrophobic character and (6)
temperature. These factors are further elaborated below.
It is well known that polar groups in a polymer molecule
interact more strongly than non-polar groups with water
molecules, in the form of iondipole, dipoledipole or
hydrogen bonding. Functional groups of different polarity
bind with water in varying degrees, thus affecting the
distribution of bound to free water. Ionic functional groups/
residues and strong acids bind water more strongly than
other non-ionic polar groups, when compared on the basis of
number of water molecules per functional group. It is also
I. Tranoudis, N. Efron / Contact Le194
Table 1
Classification of the state of water in water-swollen polymer systems
Bound water Free water,
freezableNon-freezable Freezable
Freezable at/or
below 180 K
Freezing
between 180 and 273 K
Freeze at 273 K
Bound Interfacial Bulk
Primary bound Secondary bound Free bulk
Tightly bound Loosely boundclear that amide groups, particularly lactam groups such as
pyrrolidone, have stronger binding power than either
hydroxy or ether groups. However, when comparing the
binding capability of hydrophilic polymersexcept for
HEMA and polyvinyl pyrrolidone (PVP)there is little
information in the existing literature defining the amount of
free water, or the ratio of bound to free water. The limited
information that is available suggests that a functional group
with stronger water binding capacity does not necessarily
translate into a higher bound to free water ratio. Some
polymers with functional groups of average water binding
capacity, such as HEMA, have higher ratios of bound to freewater increases as the amount of free water decreases, due to
the reduced mobility of water molecules in the more rigid
structure [46]. Normally, at low levels of water content, all
water in hydrogels exists in a tightly bound form. This is
obvious in view of the available sites for hydrogen bonding
and strong iondipole interaction. At a medium level of
water content, loosely bound water exists in addition to
tightly bound water, which already occupies the available
sites for strong interaction. Free water begins to appear at
higher levels of water content.
The relationship between water content and tightly
bound, loosely bound and free water is illustrated in Fig. 1.
This relationship implies that for hydrogels derived from the
same polymeric system, a low water content hydrogel with
x% of water would have the same amount of tightly bound
water as those with much higher water contents, such as
those with y%, z% orv% of water. Similarly, a hydrogel withwater content z%, would have the same amount of tightly
and loosely bound water as a hydrogel with a much higher
water content, such as that with v% of water.Most free and bound water measurements have been
performed on polymers swollen in distilled water. Adding
ionic salts or water-soluble polar organic compounds
changes the distribution of bound and free water, favouring
bound water by creating additional water binding sites.
However, it does not necessarily change the saturated water
content of the swollen polymer system. This is due to the
reduced mobility of water molecules caused by the solvation
of ions and polar species. For example, HEMA lenses have
higher levels of tightly bound water in aqueous sodium
chloride solution than when the lenses are placed in distilled
water [7]. As hydrogen bonding is responsible to a large
degree for the amount of tightly bound water, inter-chain and
intermolecular hydrogen bonding reduce the total available
sites for hydrogen bonding with water molecules, thus
reducing the amount of bound water.
Fig. 1. The relationship between water content and tightly-bound, looselybound and free water for hydrogels.
-
ns &The amount of bound water is also affected by
temperature. Increasing temperature reduces the amount
of bound water due to many factors [2]. This phenomenon
has been observed for many water-swollen systems,
including cellulose (Ogiwara et al., 1969, cited in Bausch
and Lomb) [2].
Lee et al. [8] obtained bulk gel conductivity data for
HEMA. The activation energy for specific conduction was
calculated. A plot of the activation energy versus percent of
water in the gel, clearly indicated three different zones,
showing three possible classes of water in the gels. These
results were confirmed by thermal expansion measurements.
The high water content gels (50%) demonstrated an
extremely sharp volume change at 0 8C, indicating thepresence of normal free water. Lower water content gels
(20%) showed no anomalous change in thermal expansion,
indicating that the water is bound. The medium water
content gels exhibited intermediate behaviour. A semi-
quantitative analysis of the three classes of water using DSC
studies demonstrated that the low water content gel (20%)
consisted mainly of bound water, which exhibited no phase
transitions over the range 15 8C to 24 8C. The high watercontent gels showed phase transitions near 0 8C. Themedium water content gels show gradual changes in phase
transition at temperatures near 0 8C.DSC was also used by Pedley and Tighe [3] in a study of a
series of hydrogels, in an attempt to correlate water binding
and transport properties. DSC and oxygen transport studies
were carried out on a series of styrene-2-hydroxyethyl
methacrylate copolymers. The transport of dissolved oxygen
through those copolymers, which contained no free water,
was found to be negligible in comparison to those in which
both free and bound water was present. The free and bound
water contents in perfluorosulphonated membranes and
sulphonated hydrocarbon membranes using DSC were
estimated by Tasaka et al. [9]. They found that in the
sulphonated hydrocarbon membranes the amount of free
water decreased with increasing divinylbenzene content and
decreasing water content.
Mirejovsky et al. [10] examined the effect of absorbed
substances on the properties of the water in various soft
contact lens materials by exposing contact lenses [Hydron
04 (Allergan Optical, Irvine, CA, USA), B&L 70 (Bausch
and Lomb, Rochester, NY, USA), Durasoft 3 (Wesley
Jessen Inc., Chicago, IL, USA), Vistamarc and Acuvue
(Vistakon, Jacksonville, FL, USA)] to an artificial tear
solution for various periods up to 14 days. They found that
the only materials affected were the high water ionic lenses,
which absorbed a large amount of protein, predominantly
lysozyme. They evaluated the free water of contact lenses
using DSC. In the Durasoft 3 lenses, the equilibrium water
content dropped from 49% to 46% and the free water from
28% to 21%. Similar changes were seen in the Vistamarc
lenses. After a 10-day exposure of the Acuvue lens to
artificial tears, the water content decreased from 53% to 47%
I. Tranoudis, N. Efron / Contact Leand the amount of free water from 33% to 23%. The decreasein the permeability of water seen with these materials was
consistent with the decrease of the free water, i.e., the water
able to participate in diffusion. Since the free water content
determines the transport through hydrogels, it is anticipated
that lens characteristics dependant upon this would be
affected by the presence of proteins within the polymer
matrix. Mirejovsky et al. [10]using Dk data obtained from
manufacturersestimated that an absolute change of 10%
in the amount of free water could lead to a decrease in
oxygen permeability of as much as 7 Dk units.
Mirejovsky et al. [11] tried to determine a set of
properties for the water contained within soft contact lens
materials with the aim of developing a model, which would
predict their propensity to induce corneal desiccation
staining. They postulated that materials containing a larger
proportion free water would tend to induce corneal
desiccation more readily than materials containing a larger
proportion bound water. The water structure (as measured by
DSC) and the permeabilities of water and glucose were
determined for a series of commercial soft lenses. They
noted lower levels of staining for a material with a lower
glucose permeability and a larger amount of water melting
below 0 8C than for a control lens, even though bothmaterials were similar in water content and water
permeability. These authors used only two contact lens
materials to test their hypothesis, thus precluding the
drawing of general conclusions. DSC has been also used in
order to examine the state of water in human crystalline
lenses [12,13].
Isothermal weight loss studies have been performed on
soft contact lenses by Kwok [14]. He examined 35 lenses,
with nominal water contents of 37.5% to 74%. Isothermal
dehydration at 34 8C demonstrated two major phases, eachwith a different rate constant. The first phase was assumed to
be evaporation of a free water fraction. The second phase,
with a higher activation energy, was apparently due to water
ligand binding in the lens matrix. The estimated bound
fraction at zero time was found to reach 20% of total lens
water in low water content lenses. Kwoks results indicated
that the effective amount of free water may be lower than
previously assumed, especially in low water content soft
contact lenses.
The water structure in polymers [1517] and the
dehydration process in soft contact lens materials [18,19]
have been examined using NMR. For example, NMR
relaxation data has been used as a predictor for on-eye soft
contact lens dehydration [18]. Proton NMR relaxation times
(T1 and T2), were determined for a series of contact lenses for
which on eye dehydration data were also available. It was
demonstrated that NMR relaxation times are dependant
upon lens water content, but the dependence is not
monotonic. T1 values varied between 100 and 800 ms,
and T2 values varied between 6 and 85 ms for the lenses
studied. The mobility of these protons varies by more than a
factor of 10 for the lenses studied. A test for linear
Anterior Eye 27 (2004) 193208 195correlation between NMR relaxation rate, 1/T1 and relative
-
change in lens water mass, %Dmw gave r = 0.830 for alldata, and r = 0.904 if one lens was excluded.
Masters et al. [20] investigated the suitability of water
proton spinlattice relaxation time T1 as a measure of
corneal dysfunction of rabbit corneas. Proton NMR
measurements were performed on excised rabbit corneas
using the saturation recovery method to determine the
proton spinlattice relaxation time T1. Both freshly excised
and progressively swollen rabbit corneas were studied. The
experimental results were consistent with a two-compart-
ment model of bound water and free water. This model
resulted in a linear correlation between 1/T1 (measured) and
materials using the DSC technique and to examine the
unit contains a vinyl group that is capable of being
polymerized. The following are a few examples of
monomers that are commonly used in hydrogel contact
lens materials:
2-Hydroxy-ethyl methacrylate(HEMA) monomer and itsnon-cross linked low molecular weight polymer are
water-soluble. It is the primary monomer from which the
first commercial soft contact lens was made.
Methacrylic acid(MAA) is used to boost the water contentin the hydrogel.
Methyl methacrylate(MMA) is the monomer unit that
ex
an
fo
I. Tranoudis, N. Efron / Contact Lens & Anterior Eye 27 (2004) 193208196
FDA c
Group
Gro
Gro
Group II 70/30 AMAb
Gro
Gro
Gro
Grorelationships that exist among the various water properties of
soft contact lens materials.
2. Methods
2.1. Lenses
Notwithstanding the recent advent of silicone hydrogel
materials, the majority of soft contact lenses available at the
present time are manufactured from hydrogels. These are
cross-linked hydrophilic (water-loving) polymers and can be
made by polymerising suitable monomers with a cross-
linking agent, or less commonly, by the post-treatment of
non-cross linked hydrophilic polymers. The monomer is the
building block for these polymers. In general, the monomer
Table 2
Soft contact lens materials used in this study
Copolymer type (WC) Trade name UK classification
HEMA/VP (40%) Vistagel 42 HC Filcon 3a
HEMA/VP (55%) Vistagel 55 H Filcon 3a
HEMA/VP (70%) Vistagel 75 H Filcon 3a
VP/MMA (55%) Vistagel 60 Filcon 4a
VP/MMA (70%) Vistagel 75 Filcon 4a
HEMA (40%) Vistagel 38 R Filcon 1a
HEMA/MAA (55%) Vistagel 55 MA Filcon 1b
HEMA/MAA (70%) Vistagel 70 MA Filcon 1b
a Ethylene glycol dimethacrylate.1/corneal hydration. This correlation suggests that water
proton T1 values can be used as an index of corneal
dysfunction.
Quinn et al. [21] and Smyth et al. [22] also conducted
DSC and NMR studies on hydrated copolymers VP/MMA
and HEMA. They concluded that although HEMA is less
hydrophilic than VP/MMA, the relative fraction of bound
water is significantly higher. Castoro and Bettelheim [23]
and Wang and Bettelheim [24] investigated the cortical and
nuclear samples from rat lenses by DSC (free water content)
and TGA (total water content).
The aim of this experiment was to evaluate the free-to-
bound water ratio of eight different types of soft contact lensb Alkyl methacrylate.up II 75/25 AMA
up I 100 EGDMA
up IV 98/2 EGDMA
up IV 96/4 EGDMAup II
up IIlassification Material composition weight (%) Cross linker
I 90/10 EGDMAa
70/30 EGDMA
45/55 EGDMAbricated from each of these materials were used in these
periments. The measured parameters (BVP, BOZR, TD, tcd WC) of the lenses used in these experiments can be
und in Table 3.valu
fa(8.7 mm) and tc (0.12 mm).
The fabricated lenses covered a wide range of contact
lens groups according to UK and FDA classifications. The
water contents of the contact lenses are only approximate
es. These materials are described in Table 2. Two lensesmakes up the PMMA (polymethyl methacrylate) lens. It is
sometimes used to lower water content or in order to
improve hardness and strength in some soft contact
lenses.
Vinyl pyrrolidone(VP) is an important monomer inaddition to the methacrylates. Due to its hydrophilicity,
it is commonly used to increase water content.
In order to be able to compare lenses made from different
materials, non-commercial contact lenses were used thro-
ughout this study. The purpose was to examine lenses with
material composition of HEMA/VP, VP/MMA and HEMA/
MAA of low (40%), medium (55%) and high (70%) water
content.
Hence, the contact lens materials were formulated
specifically for these experiments by the same contact lens
material company (Vista Optics, Cheshire, England). Some
of the lenses are not commercially available. All lenses were
manufactured (lathe-cut) by the same contact lens laboratory
and had the same bicurve design with similar nominal
parameters: TD (14.00 mm), BVP (3.00 D), BOZR
-
ns & Anterior Eye 27 (2004) 193208 197
tc (mm) TD (mm) BOZR (mm) WC (%)
0.096 13.80 9.30 37
0.099 13.90 9.30 37
0.093 13.80 9.70 47
0.101 13.90 9.60 52
0.085 13.70 8.70 72
0.077 13.90 8.30 69
0.159 14.30 9.10 60
0.088 14.00 9.40 60
0.086 13.90 8.70 69
0.161 14.10 8.70 68
0.104 14.00 9.20 38
0.108 13.90 9.10 38
0.129 14.50 9.20 56
0.148 14.10 9.20 56
0.12
0.14
0.11
0.022.2. Differential scanning calorimetry
The 910 DSC System (Du Pont Company, Wilmington,
DE, USA) was used in this work; this is a plug-in module
that can be used with any of the Du Pont thermal analysers.
The system measures temperature and heat flow associated
with material transitions, providing quantitative and
qualitative data on endothermic (heat absorption) and
exothermic (heat evolution) processes.
2.3. Experimental procedure
I. Tranoudis, N. Efron / Contact Le
Table 3
Verification (at 20 8C) of lenses used for DSC
Lens type Lens code Parameters (20 8C)
BVP (D)
HEMA/VP 40% A6 3.01A7 2.51
HEMA/VP 55% B5 2.63B6 2.17
HEMA/VP 70% C5 2.85C3 3.55
VP/MMA 55% E5 2.57E6 2.87
VP/MMA 70% F4 3.18F5 2.61
HEMA 40% G6 2.84G7 2.96
HEMA/MAA 55% H5 2.97H6 3.25
HEMA/MAA 70% I5 3.00I6 3.21
Mean 2.89S.D. 0.34Melting endotherms of water in soft contact lenses were
determined using the DSC instrument described above.
Lenses were first equilibrated overnight in saline solution.
Mirejovsky et al. [10] reported that before scanning, all
samples were kept at 40 8C for 815 h. Mirejovsky et al.[11] found that 4 h of cooling were insufficient as compared
with 824 h. Pedley and Tighe [3] did not report such a
preparation of the samples.
The lenses to be studied by DSC were lightly blotted with
tissue to remove surface water and then hermetically sealed
in aluminium pans (Fig. 2). The contact lens samples were
then punched out with a cork borer to fit into the aluminium
sample pan. In order to achieve a high reproducibility of
results for material prone to fast dehydration, the blotted
samples were placed quickly onto a tared pre-weighed
sample pan and lid, sealed, weighed and immediately placed
in the DSC instrument. The aim of this procedure was to
avoid water evaporation before scanning. Two samples from
each material were analysed and the mean values of the two
results were taken. Good reproducibility of results for the
same lens (with samples cut in the centre and at the edge)
and between lenses has been reported by Mirejovsky et al.[11]. In separate experiments they also found that the results
are not affected by the thickness/surface ratio of the lens.
The samples were scanned from 40 8C to +30 8C with aheating rate of 5 8C/min. Mirejovsky et al. [10,11] scannedtheir samples from 40 8C to +10 8C with a heating rate of5 8C/min. Pedley and Tighe [3] scanned their samples from40 8C to +20 8C at a heating rate of 1.25 8C/min or 5 8C/min. The area under the melting peak and the heat of fusion
of pure water (79.72 cal/g = 340.6 J/g) were used to calculate
the percentage of free water. The amount of bound water was
obtained by subtracting the amount of free water from the
4 14.50 8.80 67
8 14.30 8.80 67
3 14.04 9.07
8 0.24 0.37total percent water content, whereby:
Total water content% free water content% bound water content%:
Fig. 2. Process for encapsulating the contact lens sample.
-
All water amounts were expressed as percentages of
the total weight of the hydrated polymer. The DSC
experiment was conducted in a masked and randomised
3. Results
The endothermic DSC curves for the eight different
contact lens materials used in this study (two samples per
material) are shown in Figs. 318. In these figures, two peaks
can be observed: one sharp peak (peak 1) at about 0 8C andanother broad peak (peak 2) at about 10 8C, respectively[9]. According to Tasaka et al. [9], peak 1 corresponds to the
free water in contact lenses. Peak 2 corresponds to the water
with partially restricted movement due to the presence of
fixed charges; that is, loosely bound water.
The area of each peak was estimated from the difference
between the endotherm curve and the straight line drawn in
the figure, using the computer program that controls the DSC
experiments. Certainly, the absolute value of the area
depends on how this line was drawn. However, any resultant
errors will be minimal because the portion of bound water is
much smaller than that of the total water. If it is assumed that
the heat of fusion of pure water is 340.6 J/g (79.72 cal/g), the
amount of free water of peaks 1 and 2 can be estimated.
I. Tranoudis, N. Efron / Contact Lens & Anterior Eye 27 (2004) 193208198manner in order to avoid experimental bias. The following
formula [25] was used to calculate the percentage of free
water:
C DHtrm
1
DHf106
where C = concentration of water (mg/g), DHtr = heat oftransition (mJ), m = sample weight (mg), DHf = heat fusionof water (340.6 J/g) and DHtr = 60ABEDqs; where A = peakarea (cm2), B = time base (min/cm), E = cell calibration
coefficient (mW/mV (dimensionless)) and Dqs = Y-axissensitivity (mV/cm).
The amount (DHtr/m) can automatically be estimatedusing a specific utility of the computer program that
controls the DSC measurements. To test the precision of
measurement of this technique, ten measurements were
made on samples taken from identical Acuvue lenses
(Vistakon, Jacksonville, FL, USA). The means and
standard deviations of these measurements were as
follows: for free water content 30.15 3.80%; for boundwater content 27.85 3.80%; and for free-to-bound waterratio 1.12 0.31.
2.4. Clinical study
Two lenses from each of the eight soft contact lens
groups were used in experiments concerning the stability
of water content and oxygen transmissibility1. Six sub-
jects were fitted with lenses for one day. In vitro measure-
ments of water content and oxygen transmissibility were
undertaken at 35 8C before lens fitting and after 6 h of lenswear.
In order to compare the difference of the parameters both
before and after wear, among the eight different materials
that were used in this study, the relative change in lens
parameter (%DP) was determined [26]. That is:
%DP P P0
P100
where P is the lens parameter before wear and P0 is the lensparameter after 6-h wear, giving the amount %DP a positivevalue. The relative change in lens water content (%DWC)and relative change in lens oxygen transmissibility
(%DDk/t) were calculated. The results of this clinical study,which have been reported previously [27], were used to
examine the relationships that exist among the water proper-
ties of soft contact lens materials.
1 Exponential terms and units of oxygen transmissibility (Dk/t) are 109
(cm/s) (mlO2/ml mmHg). In the exponential terms and the units for trans-missibility will be omitted.Fig. 3. Endothermic DSC curve for the HEMA/VP 40% (sample A6).Fig. 4. Endothermic DSC curve for the HEMA/VP 40% (sample A7).
-
I. Tranoudis, N. Efron / Contact Lens & Anterior Eye 27 (2004) 193208 199
Fig. 5. Endothermic DSC curve for the HEMA/VP 55% (sample B5).
Fig. 6. Endothermic DSC curve for the HEMA/VP 55% (sample B6).
Fig. 7. Endothermic DSC curve for the HEMA/VP 70% (sample C3).
Fig. 8. Endothermic DSC curve for the HEMA/VP 70% (sample C5).
Fig. 10. Endothermic DSC curve for the VP/MMA 55% (sample E6).
Fig. 9. Endothermic DSC curve for the VP/MMA 55% (sample E5).
-
I. Tranoudis, N. Efron / Contact Lens & Anterior Eye 27 (2004) 193208200
Fig. 11. Endothermic DSC curve for the VP/MMA 70% (sample F4).
Fig. 12. Endothermic DSC curve for the VP/MMA 70% (sample F5).
Fig. 14. Endothermic DSC curve for the HEMA 40% (sample G7).
Fig. 13. Endothermic DSC curve for the HEMA 40% (sample G6).
Fig. 15. Endothermic DSC curve for the HEMA/MAA 55% (sample H5).
Fig. 16. Endothermic DSC curve for the HEMA/MAA 55% (sample H6).
-
I. Tranoudis, N. Efron / Contact Lens & Anterior Eye 27 (2004) 193208 201Fig. 17. Endothermic DSC curve for the HEMA/MAA 70% (sample I5).These amounts, calculated from the area of the peaks in Figs.
318, are shown in Table 4. Two samples per material were
used and the calculated mean values are presented.
The amount of bound water is estimated from the
difference between the total amount of water measured
using the Atago CL-1 soft contact lens refractometer and the
amount of free water. The last column in Table 4 gives the
Fig. 18. Endothermic DSC curve for the HEMA/MAA 70% (sample I6).
Table 4
Total, free and bound water content (WC; %) and free-to-bound water ratio of t
Materials Mean
WC (%) Free WC (%)
HEMA/VP 40% 37.00 13.38
HEMA/VP 55% 49.50 21.31
HEMA/VP 70% 70.50 43.73
VP/MMA 55% 60.00 21.79
VP/MMA 70% 68.50 41.56
HEMA 40% 38.00 16.78
HEMA/MAA 55% 56.00 26.96
HEMA/MAA 70% 67.00 41.58estimated mean values of the free-to-bound water ratio for
each of the eight contact lens materials.
It is not possible to test statistically whether the
differences in the water properties among the eight different
soft contact lens materials are significant, due to the fact that
only two measurements were obtained for each material.
Considering the standard deviations of the measured values
obtained during the testing of the precision of this technique
(S.D. of free water content = 3.80%), it can be assumedthat there are real differences in the values of the water
properties tested among the eight materials used in this
Fig. 19. Equilibrium water content and the relative proportions of free and
bound water for the eight soft contact lens materials.study.
Fig. 19 shows the equilibrium water content and the
relative proportions of free and bound water for each type of
lens material. As total water content increases, the free water
content also increases. The amount of bound water content
remains almost constant for all the materials except the VP/
MMA 55%. This finding is consistent with the theory
presented earlier, whereby low water content soft contact
lenses would be expected to have the same amount of bound
water as lenses with much higher water contents.
he eight different soft contact lens materials
Bound WC (%) Free-to-bound WC (%)
23.62 0.565
28.19 0.765
26.77 1.635
38.21 0.575
26.94 1.540
21.22 0.795
29.04 0.930
25.42 1.640
-
I. Tranoudis, N. Efron / Contact Lens & Anterior Eye 27 (2004) 193208202
Fig. 20. Relationship between free-to-bound water ratio vs. free water content.3.1. Free-to-bound water ratio versus free WC
The free-to-bound water ratio of the materials measured
correlated significantly with their free water content (Fig.
20; R2 = 0.9155, p = 0.0002). The linear regression (R =0.9568) is positive, indicating that by increasing free water
content, the free-to-bound water ratio is increased.
Fig. 21. Relationship between free w3.2. Free WC versus WC
The free water content of the materials correlated
significantly with their water content (Fig. 21; R2 =
0.8575, p = 0.0010). The linear regression (R = 0.9260) ispositive, which indicates that increasing water content will
result in an increase in free water content.
ater content vs. water content.
-
I. Tranoudis, N. Efron / Contact Lens & Anterior Eye 27 (2004) 193208 203
Fig. 22. Relationship between free-to-bound water ratio vs. water content.3.3. Free-to-bound water ratio versus WC
The free-to-bound water ratio of the materials correlated
significantly with their water content (Fig. 22; R2 = 0.6366, p
= 0.0176). The linear regression (R = 0.7979) is positive,
demonstrating that by increasing water content, the free-to-
bound water ratio is increased.Fig. 23. Relationship between free water3.4. Free WC versus Dk/t
The free water content of the materials correlated
significantly with their Dk/t (Fig. 23; R2 = 0.7685, p =
0.0043). The linear regression (R = 0.8766) is positive,
indicating that increasing the free water content will increase
Dk/t.content vs. oxygen transmissibility.
-
ns &
ound3.5. Free-to-bound water ratio versus Dk/t
The free-to-bound water ratio of the materials correlated
significantly with their Dk/t (Fig. 24; R2 = 0.5999, p =
0.0240). The linear regression (R = 0.7745) is positive,
demonstrating that by increasing the free-to-bound water
ratio, Dk/t will be increased.
I. Tranoudis, N. Efron / Contact Le204
Fig. 24. Relationship between free-to-b3.6. %DWC versus free WC
Correlating the relative change in water content follow-
ing a 6-h wear period of the materials measured to their
free water content failed to reveal a significant relationship
(R2 = 0.0608, p = 0.5560).
3.7. %DWC versus free-to-bound water ratio
Correlating the relative change in water content follow-
ing a 6-h wear period of the materials measured to their free-
to-bound water ratio failed to reveal a significant relation-
ship (R2 = 0.0260, p = 0.7030).
3.8. %DDk/t versus free WC
Correlating the relative change in Dk/t following a 6-h wear
period of the materials used to their free water content failed to
reveal a significant relationship (R2 = 0.4209, p = 0.0818).
3.9. %DDk/t versus free-to-bound water ratio
Correlating the relative change in Dk/t following a 6-h
wear period of the materials measured to their free-to-4. Discussion
The calculation of the relative amounts of free and boundbound water ratio failed to reveal a significant relationship
(R2 = 0.2742, p = 0.1829).
Anterior Eye 27 (2004) 193208
water ratio vs. oxygen transmissibility.water is approximate, since exact heats of melting are
required for the calculation of the amount of free water from
the peak area and the measured heat/g of a wet sample. It can
be expected that, in polymers showing multiple endotherms,
a portion of the water melting below 0 8C would have alower heat of fusion than pure water. It is clear that the use of
the heat of fusion of pure water (an upper limit) for the
calculation of the amounts of free water would lead to a
slight overestimation of the amount of bound water [11].
In spite of these shortcomings, Fig. 19 reveals some
noticeable differences among soft contact lens materials.
The HEMA 40% material contains more free water than the
HEMA/VP 40%. In medium water content materials the
HEMA/MAA 55% has the highest amount of water melting
at 0 8C. HEMA/VP 55% and VP/MMA 55% have about thesame amount of free water content. In high water content
materials the HEMA/VP 70% has the highest amount of free
water. The VP/MMA 70% has about the same amount of free
water as the HEMA/MAA 70%. However, the differences
among materials are even more pronounced when the
patterns of melting endotherms (Figs. 318) are simply
compared. A close agreement between the two runs for each
material can be observed.
The terminology of free, partially bound and bound water
has been used freely. The different categories of water can be
-
ns &designated on the basis of three parameters: (1) the average
number of hydrogen bonds/molecule of water that are
formed, (2) the length of hydrogen bonds and (3) the angle of
hydrogen bonds in water. In free liquid water every molecule
is hydrogen bonded, on average to two or three nearest
neighbours. In the hexagonal ice structure, each water
molecule is hydrogen bonded to four nearest neighbours.
Presumably the length of the H-bond in free liquid water is
somewhat longer, thus weaker [12]. Another way of looking
at water structure is considering that the bond angles in ice
are tetrahedral, while in liquid (free) water the flexibility of
hydrogen bonds increases. This results in both bonding and
distortions. The distortion provides the absorption of energy
in the fusion process without decreasing the actual number
of hydrogen bonds (breaking the hydrogen bonds).
The DSC technique is rapid and accurate and measures
only the free water content, as the bound water does not
freeze when the temperature of the polymer is lowered
below the freezing point of water. The bound water forms
hydrogen bonds with the polymers atoms, rather than with
other water molecules, as would be necessary for the water
to freeze. Free water, however, can bond with other water
molecules to form ice crystals when the temperature is
lowered sufficiently. The most important finding of this
experiment is that differences in water/polymer interactions
among various soft contact lens materials can be identified
and analysed using a relatively simple technique. This work
demonstrates the usefulness of DSC measurements. The
melting endotherms are reproducible and indirectly give
insights into the extent of interactions of water with a
polymer matrix. As has been outlined earlierwater, which
does not participate in transport, is water which interacts
strongly with the polymer. In the DSC experiment, this water
corresponds to bound water. Inspection of Table 4 shows that
the amounts of bound water varied from 21% to 38 %,
depending upon the type of polymer. This implies that in
some materials, a portion of the water melting below 0 8Cwill not participate in water transport.
The results of the present study are in general agreement
with reported values [2,3,5,10,11] for similar types of
polymers (see Table 5). Table 5 presents data of the state of
water in soft contact lenses and hydrogels from other
researchers as well as literature from manufacturers. Total,
free and bound water contents have been expressed as
percentages in order to be able to compare the values of the
water properties presented in this table. Direct comparisons
between the present data and data quoted from different
references in Table 5 cannot be made due to the fact that (1)
lenses or hydrogels with different compositions are
presented; (2) the test conditions are different (temperature
range, heating rate); (3) the sample preparation may be
different and (4) in the report of Bausch and Lomb [2], the
method of obtaining the presented data is not specified.
There is obviously a distinction between the more
complex of the descriptions of water states shown in Table 1
I. Tranoudis, N. Efron / Contact Leand the experimental results under consideration, becausethe former are theoretical concepts, which do not necessarily
correspond to the results of any particular experimental
technique. Similarly, the distinction that different experi-
mental techniques make between bound and free water will
be affected to varying extents by the presence of
intermediate states of water [3].
The results of this study are consistent with the idea of a
continuum of water states between the primary statewhich
is hydrogen bonded to functional groups in the polymer
and the state whereby water is unaffected by its polymer
environment. In the latter case, water crystallises and
remelts in a manner indistinguishable from that of pure
water. There is thus a continuum of water states whose
crystallisation is somewhat affected by the environment and
takes place more slowly.
The freezing behaviour of water-swollen polymers has
been shown by a number of workers to be anomalous, in that
only a part of the available water freezes even when cooled
to very low temperatures. The use of DSC enables a
quantitative determination of the relative amounts of free
and bound water to be made. On the basis of this
information, coupled with the availability of hydrogen
bonding sites in each monomer unit and in light of the fact
that the bound water has remained unaffected (i.e. does not
freeze) on cooling to the temperatures of liquid nitrogen it
seems reasonable to identify bound water with water that is
directly associated by hydrogen bonding with the polymer.
The use of DSC as described here offers a valuable
method for the study of the states of water in polymers used
for soft contact lens materials. In agreement with the
literature, low water content materials exhibited a simple
endotherm and all water melted around 0 8C. Medium andhigh water content materials exhibited multiple-melting
endotherms, representing a broad range of interactions
between the water and the polymer, similar to that reported
by Pedley and Tighe [3] and Mirejovsky et al. [10,11]. A
portion of the water, which maintained the properties of the
free water melted at 0 8C, while some proportion of thewater, which weakly interacted with the polymer, had
already melted at a temperature below 0 8C.Brennan and Efron [28] suggested that the way in which
water is contained within the polymer matrix of the lensin
other words the free-to-bound water ratiomay govern the
extent of dehydration in that the free water component may
leave the lens more readily than the bound component. This
hypothesis was tested in the present study. Preliminary
correlations among free-to-bound water ratio, free water
content and water content confirmed the theory that the
higher water content lenses contain higher amounts of free
water and that the free-to-bound water ratio is increased by
increasing the free water content [2,3]. In addition, positive
significant correlations were revealed among oxygen
transmissibility, free water content and free-to-bound water
ratio.
The attempt to correlate relative change in water content
Anterior Eye 27 (2004) 193208 205following a 6-h wear period and relative change in oxygen
-
I. Tranoudis, N. Efron / Contact Lens & Anterior Eye 27 (2004) 193208206
Table 5
Water properties of soft contact lenses and hydrogels
Reference Trade name Hydrogel composition WC (%) Free-to-bound water ratio
Total Free Bound
Pedley and Tighea [3] MSS:ACM:Sa 50.00:50.00:0.00 39.6 12.4 27.2 0.456
49.50:49.50:1.00 33.5 6.8 26.7 0.255
48.75:48.75:2.50 31.3 4.4 26.9 0.163
48.00:48.00:4.00 29.1 2.1 27 0.078
47.50:47.50:5.00 29.2 1.1 28.1 0.039
47.00:47.00:6.00 28.1 0.7 27.4 0.026
46.25:46.25:7.50 33.1 0.7 32.4 0.022
45.00:45.00:10.00 32.1 0.1 32 0.003
43.75:43.75:12.50 34.7 0.4 34.3 0.012
42.50:42.50:15.00 33.8 0.3 33.5 0.009
Mirejovsky et al.b [10] Zero 4 34 11 23 0.478
Durasoft 3 48 27 21 1.286
Acuvue 52 32 30 1.067
Vistamarc 53 31 22 1.409
B&L 70 67 42 25 1.680
Mirejovsky et al.c [11] Zero 4 35 9 26 0.346
CSI 40 21 19 1.105
Durasoft 3 48 32 16 2.000
Hydrocurve II 50 28 22 1.273
Hydrosoft 52 33 19 1.737
Softcon EW 52 28 24 1.167
Acuvue 53 37 16 2.313
Vistamarc 53 35 18 1.944
SoftPerm 67 63 38 25 1.520
H 67 64 41 23 1.783
B&L 70 67 49 18 2.722
Durasoft 4 71 51 20 2.550
Permaflex 72 57 15 3.800
Bausch and Lombd [2] System 1, HEMA contains 0.2% MAA,
0.16% diethylene glycol methacrylate
and 0.01% EGDMA
18 0 18 0
25 0 25 0
30 3.6 26.4 0.136
35 10.5 24.5 0.429
38.5 14.2 24.3 0.584
40 17.2 22.8 0.754
45 24.3 20.7 1.174
50 31 19 1.632
System 2, HEMA (same as system 1)
contains 1 mole TEGDMA
22 0 22 0
25 0 25 0
30 0 30 0
35 0.4 34.6 0.012
38.5 19.9 36.6 0.052
40 6 34 0.176
45 13.5 31.5 0.429
50 21.5 28.5 0.754
System 3 based on polymacon
lens using HEMA
9 0 9 0
17 0 17 0
30 3.6 26.4 0.136
VP/MMA containing 70.66% VP
29.12% MMA 0.026%
EGDMA 0.19% AMA
14.5 0 14.5 0
18 0 18 0
40.8 0.9 39.9 0.023
56.7 23.2 33.5 0.693
74.1 54.8 19.3 2.839
77.5 58.9 18.6 3.116
Present study Vistagel 42 HC HEMA/VP 40% 37 13.38 23.62 0.565
Vistagel 55H HEMA/VP 55% 49.5 21.31 28.19 0.765
Vistagel 75 H HEMA/VP 70% 70.5 43.73 26.77 1.635
Vistagel 60 VP/MMA 55% 60 21.79 38.21 0.575
Vistagel 75 VP/MMA 70% 68.5 41.56 26.94 1.540
Vistagel 38 R HEMA 40% 38 16.78 21.22 0.795
-
ns &
26.96 29.04 0.930
e).
EGDM
A (altransmissibility following a 6-h wear period with the free
water content and free-to-bound water ratio failed to reveal
significant correlations. The findings of this study confirmed
the results obtained by Mirejovsky et al. [10] and Pedley and
Tighe [3], whereby lenses of high water content seem to have
a higher free water content as compared to lenses with low
water contents. Larsen et al. [18] correlated in vivo lens
dehydration to water mobility using proton NMR relaxation.
Fatt [29] proposed a different approach to predict in vivo lens
dehydration based upon swelling pressure and the calculation
of water transport coefficients in hydrogels. In both cases,
substantial differences in water properties were found
between high water content and low water content hydrogels.
Mirejovsky et al. [11] criticised both papers by noticing
that it is not clear whether these approaches are sensitive
enough to predict in vivo desiccation induced by materials
with similar water contents but slightly different composi-
tions. In order to answer this question, Mirejovsky et al. [11]
compared an unspecified experimental material B to the
Acuvue lens (Vistakon, Jacksonville, FL, USA). DSC
measurements indicated a larger proportion of free water in
the Acuvue lens than in the material B lens. The authors
then fitted these 2 materials to 14 subjects and corneal
staining with fluorescein was monitored immediately before
and after a 6-h wear period. At the end of the study, corneal
staining was present in 10 eyes wearing Acuvue and in 3
eyes wearing material B lenses. Although using a binomial
distribution, the two-tailed probability of this occurring by
change is low (p = 0.047), the above experiment cannot be
used to draw general rules about the aetiology of soft contact
lens dehydration.
Larsen et al. [18] demonstrated that proton NMR T1values can be used to predict on eye lens dehydration
I. Tranoudis, N. Efron / Contact Le
Table 5 ( Continued )
Reference Trade name Hydrogel composition
Vistagel 55MA HEMA/MAA 55% 56
Vistagel 70MA HEMA/MAA 70% 67
a Molar composition MAA:ACM:S (methacrylic acid:acrylamide:styrenb Figures extrapolated from Fig. 1 of Mirejovsky et al. [10].c Figures extrapolated from Fig. 1 of Mirejovsky et al. [11].d HEMA (2-hydroxy-ethyl methacrylate), MAA (methacrylic acid),
dimethacrylate), VP (vinyl pyrrolidone), MMA (methyl methacrylate), AMbehaviour. NMR results have often been discussed in terms
of fractions of water in various states, such as bound and
free. Such a discussion uses a site model, normally involving
two or three sites. For two sites, the relaxation time depends
on the fractions of free water (Xf) and bound water (Xb) in the
sample according to:
1
T1 Xf
T1f
Xb
T1b
where T1f and T1b are relaxation times for the free and bound
water, respectively. Since only one parameter (the relaxationtime) is measured and three are required in the above
equation, the problem is underestimated. Furthermore, there
are also assumptions concerning the relaxation mechanism
implicit in the site model, and these have recently been
seriously challenged by further NMR studies [16,17]. Pre-
sumably, this is why Larsen et al. [18] did not consider
estimating fractions of bound and free water from their data.
Larsen et al. [18] also admitted that the NMR signal
measured by them arises from both water and exchangeable
polymer protons hence the inclusion of a polymer
contribution may be required for the NMR results to
correlate with the on-eye dehydration data, so that a
parameter that characterises only lens water may not
correlate with dehydration data.
The DSC technique as applied here provides a rapid
and accurate method for measuring free water in polymers
used for soft contact lens materials, by virtue of its high
constant calorimetric sensitivity, superior baseline perfor-
mance and high precision and accuracy of calorimetric
measurements. The procedure is sensitive, specific and
requires only milligram quantities of contact lens material.
Using the DSC method revealed some noticeable differ-
ences among soft contact lens materials with approxi-
mately the same or different water contents. As a general
rule, it can be concluded that by increasing the total water
content, the bound water remains constant and the free water
increases.
Finally, by increasing the water content of a material,
oxygen transmissibility, free water content and free-to-
bound water ratio are also increased and the free water
content and free-to-bound water ratio cannot be used for the
prediction of dehydration. Research must be continued to
further understand the in vitro and in vivo behaviour of
41.58 25.42 1.640
A (ethylene glycol dimethacrylate), TEGDMA (tetra ethylene glycol
kyl methacrylate).Anterior Eye 27 (2004) 193208 207
WC (%) Free-to-bound water ratio
Total Free Boundhydrogel contact lens materials. Designing an optimal
hydrogel contact lens material requires the achievement of a
balance between hydrophilic and mechanical properties. By
using unique chemistries, it ought to be possible to achieve
an optimal balance of these properties without compromis-
ing the comfort of the patient.
Acknowledgement
Dr. Tranoudis was supported by a grant from the State
Scholarships Foundation, Republic of Greece.
-
References
[1] Kanome S. Fundamental chemistry and physical properties of polymer
materials. In: Iwata S, editor. MeniconToyos 30th anniversary
special compilation of research reports. Nagoya: MeniconToyo
Contact Lens Co. Ltd.; 1982. p. 10938 (Chapter 6).
[2] Bausch and Lomb. Free and bound water in water swollen polymer
systems and their correlation with lens water dehydration. Rochester:
Bausch and Lomb; 1994.
[3] Pedley DG, Tighe BJ. Water binding properties of hydrogel polymers
for reverse osmosis and related applications. Br Polym J 1979;11:130
6.
[4] Sung YK, Gregonis DE, John MS, Andrade JD. Thermal and pulse
NMR analysis of water in poly(2-hydroxyethyl methacrylate). J Appl
Polym Sci 1981;26:371928.
[5] Hatakeyema T, Yamouchi A, Hatakeyema H. Studies on bound water
in poly(vinyl alcohol) hydrogel by DSC and FT-NMR. Eur Polym J
1984;20:614.
[6] Collett JH, Spillane DEM, Pywell EJ. Infrared attenuated total reflec-
tance (ATR) in the characterisation of water structure within poly-
HEMA hydrogels. ACS Polym Reprints 1987;28:1412.
[7] Quinn FX, McBrierty VJ, Wilson AC, Friends GD. Water in hydrogels.
3-Poly hydroxyethyl methacrylate/saline solution systems. Macromo-
lecules 1990;23:457681.
[8] Lee HB, John MS, Andrade JD. Nature of water in synthetic hydro-
gels. J Colliod Interface Sci 1975;51:22531.
[14] Kwok S. Thermogravimetric quantification of water ligand binding in
soft contact lenses. Optom Vis Sci 1988;65:188S.
[15] Amano H, Kawai K, Takahashi K, Hayano S, Nagaoka S, Sogami M,
et al. Comparative H-NMR studies on the water structures in soft
contact lens and protein gel. J Jpn Contact Lens Soc 1984;26:23744.
[16] Roorda WE, de Bleyser J, Junginger HE, Leyte JC. Nuclear magnetic
relaxation of water in hydrogels. Biomaterials 1990;11:1723.
[17] Yamada-Nosaka A, Tanzawa H. H-NMR studies on water in metha-
crylate hydrogels. J Appl Polym Sci 1991;43:116570.
[18] Larsen DW, Huff JW, Holden BA. Proton NMR relaxation in hydrogel
contact lenses: correlation with in vivo lens dehydration data. Curr Eye
Res 1990;9:697706.
[19] Pescosolido N, Lupelli L, Brosio E, Delfini M, Aureli T. Preliminary
study on the dehydration of hydrogel contact lenses by NMR at low
resolution. Contact Lens J 1990;18:1015.
[20] Masters BR, Subramanian VH, Chance B. Rabbit cornea stromal hydra-
tion measured with NMR spectroscopy. Curr Eye Res 1983;2:31721.
[21] Quinn FX, Kampff E, Smyth G, McBriety VJ. Water in hydrogels 1. A
study of water in poly(N-vinyl-2-pyrrolidone/methylmethacrylate)
copolymer. Macromolecules 1988;21:31918.
[22] Smyth G, Quinn FX, McBrierty VJ. Water in hydrogels 2. A study of
water in poly(hydroxyethyl methacrylate). Macromolecules
1988;21:3198204.
[23] Castoro JA, Bettelheim FA. Distribution of the total and non-freezable
water in rat lenses. Exp Eye Res 1986;43:18591.
[24] Wang X, Bettelheim FA. Distribution of total and non-freezable water
I. Tranoudis, N. Efron / Contact Lens & Anterior Eye 27 (2004) 193208208[9] Tasaka M, Suzuki S, Ogawa Y, Kamaya M. Freezing and nonfreezing
water in charged membranes. J Membr Sci 1988;38:17583.
[10] Mirejovsky D, Patel AS, Rodriquez DD. Effect of proteins on water
and transport properties of various hydrogel contact lens materials.
Curr Eye Res 1991;10:18796.
[11] Mirejovsky D, Patel AS, Young G. Water properties of hydrogel
contact lens materials: a possible predictive model for corneal desic-
cation staining. Biomaterials 1993;14:10808.
[12] Bettelheim FA, Christian S, Lee LK. Differential scanning calori-
metric measurements on human lenses. Curr Eye Res 1983;2:8038.
[13] Bettelheim FA, Castoro JA, White O, Chylack Jr LT. Topographic
correspondence between total and non-freezable water content and the
appearance of cataract in human lenses. Curr Eye Res 1986;5:92532.contents of galactosemic rat lenses. Curr Eye Res 1988;7:7716.
[25] Baker KF, Cattiaux J. Thermal analysis high sensitivity determination
of clustered water in polyethylene by differential scanning calorime-
try. Paris: Du Pont Company; 1977.
[26] Brennan NA, Efron N, Truong VT, Watkins RD. Definitions for
hydration changes of hydrogel lenses. Ophthalmic Physiol Opt
1986;6:3338.
[27] Tranoudis I, Efron N. Parameter stability of soft contact lenses made
from different materials. Contact Lens Ant Eye 2004;27:11531.
[28] Brennan NA, Efron N. Hydrogel lens dehydration: a material-depen-
dent phenomenon? Contact Lens Forum 1987;12:289.
[29] Fatt I. A predictive model for dehydration of a hydrogel contact lens in
the eye. J Br Contact Lens Assoc 1989;12:1531.
Water properties of soft contact lens materialsIntroductionMethodsLensesDifferential scanning calorimetryExperimental procedureClinical study
ResultsFree-to-bound water ratio versus free WCFree WC versus WCFree-to-bound water ratio versus WCFree WC versus Dk/tFree-to-bound water ratio versus Dk/t%WC versus free WC%WC versus free-to-bound water ratio%Dk/t versus free WC%Dk/t versus free-to-bound water ratio
DiscussionAcknowledgementReferences