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: n.efron@manchester.ac.uk (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

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