wettahility of hydrogels hema

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J.

BIOMED. MATER.

RES.

VOL. 9,

PP. 315 326

(1975)

Wettahility of Hydrogels

I. Po ly (2-Hy droxy ethyl Methacrylate)

1JRAKK

J. HOLLY and JIIGUEL

F R E F O J O Eye Research

Ins t i tu t e

of

Re t i na Founda t i on Boston J lassachuse t t s

021

4

Summary

The wettability characteristics

of

the contact lens material, PHEMA, with

respect to water have been determined by using the sessile drop, and the captive

air

bubble techniques of contact angle goniometry.

It

is concluded that on

PH EMA gels water does no t spread spontaneously. Large hysteresis has been

observed in the advancing and receding contact angles. This suggests that, this

hydrogel surface is capable of changing it s free energy through reorientation of

th e polymer side chains and chain segments depending on the nature of the

adjacent phase. The water content of the gels does not appear to have an effect

on water wettability in the hydration range investigated. The minor wetta-

bility differences among the various gels studied were most likely due to dif-

ferences in surface structure and segmental mobility due to inherent variations

in the method of preparation. Small bu t consistent differences were found

between the contact angles measured by the captive bubble method and those

obtained by the sessile drop method, the former values being higher. These

differences may not be method-related artifacts and cannot be explained at the

present time.

INTRODUCTION

I n the past decade hydrophilic

soft

contact lenses made mostly of

crosslinked poly (Zhydroxyethyl methacrylate) polymer, PHEMA,

have become increasingly popular.lr2 The

PHEMA

gel

is

also

expected to be biocompatible, i.e. antithr~mbogenic,~o widespread

use as surface coating

of

durable synthetic materials

for

artificial

organs is predicted in the future.

PHEhlA

is but one of many

existing transparent hydrogels and

it

contains approximately

40Yo

water by weight a t equilibrium hydration. This equilibrium water

content of PHEMA gels can be varied between certain limits by

31.7

@

1975 by John Wiley

i

Sons, Inc.



316

TTOLLY

AN11

REFOJO

changing the conditions of polymerization and the density of cross-

linking.

The fact that the polymer matrix of a hydrogel such as PHEMA

consists of macromolecules having numerous hydrophilic sites and

can thus imbibe considerable amount of water, leads people to assume

that

PHEMA,

as well as the other hydrogels, have hydrophilic

surfaces.

It is,

in other words, expected that a water drop placed on

a clean hydrogel surface will spontaneously spread.

If

the adhesion

of water to the gel surface were a t least as strong as the cohesion of

water, then indeed, water would completely wet the gel.

There has been no detailed study published, to our knowledge,

on

the water wettability of hydrogels to confirm this assumption.

However, occasional statements in the literat~re,~nd experience

in handling such gels suggest that at least some hydrogels may not

be completely wettable by water. We have decided to investigate

the water wettability of hydrogels, since a basic knowledge of the

surface characteristics of such gels could provide understanding and

even help solving the often troublesome problems of surface con-

tamination and deposit build-up of therapeutic and cosmetic hydrogel

contact lenses.

I n this first part of the hydrogel wettability study, the wettability

characteristics of

PHEMA

hydrogels prepared by various methods

and having different water content are reported. In later reports,

the effect of water content and chemical composition and structure

on

the

mater wettability

of

various hydrogeis

will

be discussed.

A

basic study of hydrogel surface and the hydrogel-water interface and

that of biopolymer adsorption a t the hydrogel-water interface are

also in progress.

MATERIALS AND METHODS

Preparation of the Hydrogels

All

PHEMA

hydrogels studied, except PHEMA

IV

were prepared

by polymerizing and simultaneousiy crosslinking the hydroxyethyl

met,hacrylate monomer in solution with a redox initiator (Table I).

The polymerization was carried out between two clean soda-lime glass

plates separated by a silicone rubber gasket. In some cases, where

indicated in the Table, the glass plates were siliconized prior to use

in order

to

prevent adherence of the gel to the glass.



WATER WETTABILITY O F PHEPtIA GELS

317

TABLE I

Methods of Preparation of PHEMA Gels

6%

Ethylene Ammonium 10%

PH EM A HEMAb EDMA MAAb Glycol Water Persulfate DMAEAd

Gel. NO ^ (ml) (ml) (ml) (ml) (ml) (ml) (ml)

I 22 b b 11 11 0.7 0 .7

I1

6

b

b 2

1

0.2 0 .2

b b

2

0.2 0.2

11

6

IV 20 (bulk polymerized with 0.191 g isopropyl percarbonate)

b

2

__

0.2 0 .2

VIC 6

b

1 2 0.2 0.2

V 6 0.1

VIIC 6 0.2 2 0. 2 0. 2

&Conditionsof polymerization: Gel I, 3 hr a t 60°C; gels 11 111 V, VI, VII,

1 4 hr at 75°C and an additional hr a t

90°C.

Gel IV, 2 hr a t 45°C and

1

hr

at

W C ,

all in

a

circulating ai r oven.

b Commercial HEMA monomer contains about 0.1

yo

ethylen dimethacrylate,

EDMA, and about

3 0y0

methacrylic acid, MAA. In these syntheses, the com-

mercial HEMA was vacuum distilled and the main portion of the distillate used

for the polymerization without further purification

or

analysis. Th e distillation

removes inhibitors and polymers already formed during storage but leaves

EDMA and MAA approximately in the same proportion as in the raw material.

Only PHEMA V and VII contains additional crosslinking agent while PHEMA

VI contains additional acid moieties in the polymeric network.

c

Glass plates were Siliclad treated.

d 2-dimethylaminoethyl acetate.

The resulting gels were submitted to prolonged washing to leach

all the soluble by-products, then were stored in distilled water with

frequent changing of water for a t least one month prior to the

measurement. PHEMA

I V

was bulk polymerized with a free radical

initiator (see Table I) between siliconized glass plates, then swollen

to

equilibrium and washed as thoroughly as the other PHEMA gels

were.

PHEMA hydrogels with equilibrium water content ranging from

32y

to 43y0 have been prepared by varying the amount of cross-

linking agent in the polymerization mixture, and by varying the

reference degree of sw-elling (i.e. initial monomer dilution) in the

polymerization process.2



318

HOLLY AND REFOJO

Wettability Measurements

The wettability of the PHEMA gels and th at of the hard contact,

lens material, poly(methy1 methacrylate), PMMA, were determined

at

room temperature by contact angle goniometry using two tech-

niques. In the sessile droplet technique (water-in-air) the gel was

enclosed in an environmental chamber (Model B-100 Ram&Hart,

Inc., Mountain Lakes, New Jersey) with transparent windows, the

excess water was blotted from the gel surface with a clean, lipid-free

filter paper, and the gel was equilibrated with air saturated with

water vapor. A drop of water was placed on the gel surface and its

size was slowly increased t o obtain the advancing contact angle or

decreased until the drop receded in order

t o

obtain the receding

contact angle value (Fig. la).

In the captive bubble technique, the whole gel was immersed in

water in a plastic chamber built for this purpose, and a small air

bubble was placed on the bottom side

of

the gel by a curved capillary

pipette. The bubble was slowly increased or decreased in order

to

obtain the receding or advancing contact angle values, respectively

A/WATER IN AIR

AIR

A ‘ A

=//=// GEL

/ / = N

=

B/A IR- IN-WATER

=//

=//GEL

N

=// =

AIR AIR

ADVANCING

e,

RECEDING

en)

Fig.

1.

Schematic diagram

of

contac t angle measurements.



W A T E R W E T T A B I L I T Y O F PHEMA

GELS

319

(Fig.

Ib).

The contact angles were directly read on a N R L Contact

Angle Goniometer (Model A-100 Ram6-Hart, Inc., Mountain Lakes,

New Jersey,

to 1

degree accuracy.

RESULTS

A slow, monotonous variation of cont,act angle with time was

often observed in the first few minutes after placing the water

droplet on the gel. The advancing contact angles decreased some-

what due

t o

gradual spreading while the receding angles increased

to

a lesser extent (Fig.

2 .

Such time variation was also observed

L

0

I

30

2

5

10 2

T IM IN

M I N U T E S

Fig.

2 .

Time dependence

of

water contact angle on PHEMA gels

measured

in

the water-in-air system.



320

H O L L Y AND R E F O J O

for the contact, angles measured by the captive bubble technique

(Fig.

3 ) .

Table

I1

contains the mean of the contact angle measurements

at a given time (at 30 min both for advancing and receding contact

angles after the formation

of

the water droplet

or

air bubble), followed

by the standard error of the mean. Both the values obtained with

the sessile drop and the captive bubble techniques and the difference

bet,ween these two

for

the advancing and receding angles are listed.

A s

can be seen from the Table and also from Fig.

4,

there are signifi-

9

8

7

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W

u 6G

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W

z

50

U

I

3

40

4:

l-

2 30

z

20

10

*

1 1 I 1 I

1 2 5

10

2 30

TIME

IN MINUTES

Fig.

3.

Time dependence of watei contact angle on PHEMA gels measured

in the air-in-water system.



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322

HOLLY AND REFOJO

cant differences between the contact angles obtained by these two

techniques, the captive bubble values being consistently higher for

both the advancing and the receding contact angles.

The contact angle of water obtained with the various PHEMA

gels are plotted as a function of gel hydration in Fig. 4. The hard

contact lens material PMMA (equilibrium water content

=

1.5 )

is also included for comparison.

Two interesting observations can be made when considering these

contact angle data. First, the advancing contact angle on all the

gels is unexpectedly high.

For

example, some of the PHEMA gels

are even less wettable than PMMA. Secondly, there is a large

difference between advancing and receding angles for all the gels,

i.e. the contact angle hysteresis is unusually pronounced.

IPHEMA

V I I

I

I

I

0

I

I

I

I

I

I

I

*

I

WATER COlIENT IN

PER

CENI

Fig.

4.

Water contact angles on PHEMA

as a

function

of

the gel water

0)

easured by

The values obtained

for

PMMA are represented by

Solid shapes represent the

content.

the sessile drop technique.

a)

dd

(m

or

the two methods, respectively.

receding angles.

0)easured by the captive bubble technique.



W A T E R. W E T T A BIL IT Y

OF

P H E M A GELS

323

DISCUSSION

It is well known th at the polymeric network of the PHEMA

hydrogel contains sufficient number of binding sites for water, e.g.

hydroxyl groups,

so

that the resulting energy of water-polymer

interaction is sufficiently high to overcome, at least partially, the

hydrophobicity of the nonpolar parts of the polymer backbone.

When the polymer segments of such

a

gel is subjected to the

asymmetric molecular force field at the gel-air phase boundary,

it is energetically more favorable for the polymer chain segments

to orient in such

a

way as to expose the hydrophobic side groups

or the nonpolar parts of the polymer backbone toward the gaseous

phase arid to bury the polar sites in the aqueous phase within the

gel. This is

so

because the molecular force field of the water mole-

cules in the gaseous phase is much weaker than in the liquid phase.

Thus the gel surface appears hydrophobic in spite of the numerous

hydrophilic sites on the polymer molecules forming the matrix of

the gel. This must be why such large advancing contact angle

values are obtained with water on PHEMA hydrogels.

When such a PHEMA gel surface having the most hydrophobic

configuration possible is immersed in water, its structure becomes

unfavorable since it would have high interfacial tension against

water. So the polymer segments reorient again taking up confor-

mation similar to that in the gel interior in order to achieve minimal

interfacial tension. This is why the receding contact angle of water

is

so

much smaller on PHEMA gel than the advancing contact angle.

The surface structural changes cause the large hysteresis of the water

contact angle.

The extent to which

a

gel surface may become hydrophobic by

reorientation and conformational changes, depends

011

the chemical

structure of the polymer in the gel matrix and also on the mobility

of

the individual chain segments. In PHEMA swollen in water,

there is evidence of

a

secondary structure in addition to the covalently

linked primary structure. This secondary structure is a result of

water-induced ordering of polymer segments in the network.

6-8

Such

a

secondary structure formation may not be extensive at the

gel-air interface resulting in increased segmental mobility.

As

judged by the magnitude of the advancing water contact angle,

some

of

the PHEMA gels have

a

surface more hydrophobic even



324

HOLLY

AND

REFOJO

than that of PMMA, a considerably more hydrophobic polymer.

The receding water contact angle on PMMA, however, is much

larger than on PIIEMA gels, i.e. water contact angle hysteresis is

much smaller with PMMA (Fig.

4).

This is no doubt due to the

surface structure of PMMA which is “frozen” when compared to the

relatively mobile surface structure of PHEMA gel where the polymer

segments are plasticized by water.

Even if the surface structure of PHEMA gel could be immobilized

in the hydrophilic conformation tha t occurs when the gel is immersed

in water, the hydrogel would not, be completely wetted by water.

This

is shown by the finite receding contact angle of water. This

is not too surprising, if one considers that the PHEMA homopolymer

is not hydrophilic enough to be water-soluble, although its monomer,

2-hydroxyethyl methacrylate, is soluble in water.

Most of the conformational change takes place rapidly a t the gel

interface as

shown

by the consistency of the contact angle data

obtained by the sessile drop and the captive bubble techniques.

However, the time variation of the contact angles observed to last

several minutes could be due

to

further, relatively slow changes in

polymer conformation near the equilibrium state.

We have no explanation to offer for the consistent differences

observed between the contact angles measured by the captive bubble

technique and by the sessile drop technique. Similar differences

were also found for PMMA which also contains a small fraction of

water. Generally the contact angle values obtained in the air-in-

water system were larger except for two cases:

0 E

for PHEMA

V I I and

@ A

for PIIEMA I, and for both cases their errant behavior

can be attributed to some unusual circumstances connected with

the gel samples. (PHEMA VI I was transgressed with numerous

fine cracks that developed during swelling due to the internal stresses

in this highly crosslinked gel.)

The equilibrium water content of the PHENIA gels does not

seem to play a role in determining wettability. The conditions

of gel preparation as they influence surface structure and mobility

seem to be more important. This appears to be strange, since

surface segmental mobility certainly depends on crosslink density,

which in turn is a function of the extent

of

equilibrium hydration.

It

is important to remember, however, that the overall bulk crosslink



W A T E R W E T T A B I L I T Y

O F

P H E M A G EL S

325

density may bear littl reelation to surface crosslink density and

segmental entanglement structure.

CONCLUSIONS

In conclusion, we may state tha t since water does not spread

on

PHEMA spontaneously, this methacrylic hydrogel has a hydro-

phobic surface even when in equilibrium with saturated water vapor.

However, when immersed in water, the gel surface becomes less

hydrophobic. Thus a thin water film would be less apt to rupture

over

a

PHEMA

surface than the large advancing angle would seem

to indicate. The polymer chains a t the gel surface appear to have

sufficient mobility to reorient and the type of interface resulting

from these conformational changes depends on the nature of the

adjacent phase.

Such high surface mobility attributes special properties to the

PHEMA

hydrogel surface.

It

has moderate to low surface free

energy in air, while the gel-water interfacial tension is less than for

solids of comparable surface free energy. This is probably the

reason why lipids and other low energy surface contaminants, i.e.

denatured proteins and lipoproteins, are not more of

a

problem with

the hydrogel lenses especially if their surface is continuously covered

with tears.g

In order to reduce unwanted interaction with surface active

biopolymers and lipids, the hydrogel-water interfacial tension could

be further decreased by preparing gels with hydrophilic surface

groups that would cause minimal changes in the interfacial water

structure. Such a material should be quite biocompatible according

to the minimum interfacial tension hypotheses of biocompatibility

of Andrade and others.lOJ1

The knowledge

of

the surface chemical properties of the hydrogel

lenses is fundamental for the scientific formulation of more effective

cleaning, sterilizing, and wetting solutions and for the preparation

of new hydrogel contact lens materials with improved surface prop-

erties. Either through the adsorption of macromolecular wetting

solution components or by some change of the gel surface composition,

mobility, and structure,

a

minimal interfacial tension against water

may be achieved while retaining the mechanical strength of the gel.

A low gel-water interfacial tension would result in less adsorption and



326 HOLLY

A N D

REFOJO

denaturation

of

tear proteins, negligible lipid adsorption, and lipid-

protein interaction which all interfere with visual acuity and are

difficult to remove.

It

would also improve the gel wettability

by the tears resulting in higher stability for the tear

film

overlying

the hydrogel lenses.

This study

was

supported by Special Research Fellowship

No.

1

F03 EY-

550088-01,

in part by Research Grant No.

EY-00208,

and by

PHS

Grant

No.

EY-00327, all from the National Eye Instit.ute, and in part by the Massachusetts

Lions Eye Research Fund, Inc. The capable assistance

of

Fee-Lai Leong

is

also gratefully acknowledged.

References

1.

0. Wichterle and D. Lim, U.S. Pat.

3,220,960 (1965)

Re

27,401 (1972).

2.

M.

F. Refojo, Surv.

Ophthal.

16, 233 (1972).

3. J.

D.

Andrade,

Trans . Ame r . Soc . A r t i f . In t e rn . Org.

19,

1

(1973).

4.

A. S.

Hoffman and G. Schmer, Par oi Arterie l le

1(2 ), 95 (1973).

5.

M.

F. Refojo and

H.

Yasuda, J.

A p p l . Polymer

Sci. 9, 2424 (1965).

6.

M . F. Refojo, J . Polymer Sci. A-1 5

3101 (1967).

7. T. C. Warren and W. Prins,

Macromolecules 5,

506 (1972).

8. B.

D .

Ratmer and I. F. Miller,

J . Polymer Sci. A-1

10, 25 (1972).

9.

F. J.

Holly, in

Th e Preocu lar Te ar F i lm and D ry E y e Sy ndrome s

Int . Ophthal.

Clin., F.

J.

Holly and M. A . Lemp, Eds., Little, Brown and Co., Boston,

1973, p. 279.

10. J . D. Andrade,

M e d . I n s tr u m . 7 110

(1973).

11. J.

V . Maloney, D. Roher, E. Roth, and W .

A.

Latta,

Surgery

66,775 (1969).

Received Sept ember 9, 1974

wettahility of hydrogels hema

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