hydrophilic sponges based on 2-hydroxyethyl methacrylate. iii. effect of incorporating a hydrophilic...

Polymer International 42 (1997) 45È56

Hydrophilic Sponges Based on2-Hydroxyethyl Methacrylate. EffectIII.

of Incorporating a HydrophilicCrosslinking Agent on the Equilibrium

Water Content and Pore Structure

Anthony B. Clayton,a Traian V. Chirilaa* & Paul D. Daltona,b

a Department of Biomaterials and Polymer Research, Lions Eye Institute, 2 Verdun Street, Block A, 2nd Floor, Nedlands, WesternAustralia 6009, Australia

b Curtin University of Technology, School of Applied Chemistry, Kent Street, Bentley, Western Australia 6102, Australia

(Received 23 April 1996 ; revised version received 12 July 1996 ; accepted 28 July 1996)

Abstract : The e†ect of using a hydrophilic crosslinking agent (divinyl glycol,DVG) on 2-hydroxyethyl methacrylate (HEMA) sponge swelling and pore mor-phology was evaluated. Concentrations of crosslinking agent, redox initiatorsand HEMA (75È90 wt%) in the initial aqueous monomer mixture were varied.Anomalous sponge swelling behaviour, together with the formation of non-uniform stratiÐed sponges was rationalised in terms of the assumed disparate freeradical reactivities of DVG and HEMA. Environmental scanning electronmicroscopy indicated that the stratiÐed sponges did not exhibit suitable porosityfor biomedical use.

Key words : poly(2-hydroxyethyl methacrylate), sponges, phase separation, cross-linking, divinyl glycol.

INTRODUCTION

We have recently1h3 developed an artiÐcial cornea(keratoprosthesis) in which the peripheral skirt wasmade from a poly(2-hydroxyethyl methacrylate)(PHEMA) sponge attached to a transparent PHEMAgel inner core via an interpenetrating polymer network.PHEMA sponges resulted when the water content inthe monomer mixture was higher than 45 wt% andphase separation occurred at the time of polymeris-ation. Therefore, we called them “phase separationspongesÏ or “syneretic spongesÏ,4,5 in order to di†eren-tiate these sponges from those produced by proceduresnot involving phase separation. The function of thespongy rim is to provide an anchoring zone between thehost tissue and the non-porous clear core, aimed atreducing the incidence of prosthesis expulsion. The

* To whom all correspondence should be addressed.

sponges prepared in more than 75 wt% water, dis-playing pores larger than 10 km, were biocolonisedthrough cellular invasion when implanted in animals,both subcutaneously6 and in the corneas.7

The mechanical weakness of high water content het-erogenous hydrogels is a limiting factor in their applica-tion in the biomedical area. Previous workers8,9 havefound that the increase in diluent content in themonomer mixture is invariably accompanied by a rapiddecrease in ultimate tensile properties. This disadvan-tage is particularly inconvenient when the device isinserted into the cornea, because the sutures have con-sequently to be passed through the central core of theprosthesis in order to avoid the tearing apart of thespongy rim.

We have previously attempted10 to enhance themechanical strength of PHEMA sponges by copolymer-ising HEMA with 4-t-butyl-2-hydroxycyclohexyl meth-acrylate (TBCM), a hydrophilic strengthening monomer

45Polymer International 0959-8103/97/$09.00 1997 SCI. Printed in Great Britain(

46 A. B. Clayton, T . V . Chirila, P. D. Dalton

which has reportedly enhanced the mechanical strengthof poly(1-vinyl-2-pyrrolidinone) networks.11 Unfor-tunately, copolymerisation of TBCM with HEMAmonomer actually decreased the tensile strength atbreak of sponges produced in 70 wt% water, while nosigniÐcant improvement in tensile properties wasobserved for sponges produced in 80 wt% water with5 wt% TBCM. In the same work, a considerableenhancement of mechanical strength for PHEMAsponges was achieved by colonisation with tissue viasubcutaneous animal implantation, a procedure of ques-tionable feasibility.

The improvement of mechanical strength by incorp-oration of strengthening comonomers remains attrac-tive, owing chieÑy to the possible elimination ofimplantation problems. We have found12 that thehydrophilic monomer 1,5-hexadiene-3,4-diol (knowntrivially as divinyl glycol, DVG), is a suitable cross-linking agent for very high water content gels based on1-vinyl-2-pyrrolidinone. Over a certain range, theincrease in DVG concentration induced an increase inthe equilibrium water content of the gels. This wasprobably due to the intrinsic hydrophilic character ofDVG acting against the normal e†ect of crosslinking.While we have previously4,5 used only relatively hydro-phobic crosslinking agents in PHEMA sponge synthe-sis, such as ethylene glycol dimethacrylate (EDMA), and1,6-hexamethylene dimethacrylate (HDMA), the aim of

the present study is to assess whether DVG would alsobe a suitable crosslinking agent for the sponges.

While the possible strengthening e†ect of the DVGwill be the topic of a future report, this study evaluatesthe quantitative inÑuence of water, DVG and initiatorconcentration on the swelling behaviour of the resultingsponges. The retention of the porosity is also investi-gated.

EXPERIMENTAL

Materials

HEMA, supplied by Ubichem Ltd (UK), was vacuumdistilled prior to use. DVG, supplied by Polysciences,Inc. (USA), was used without further puriÐcation.Aqueous solutions (1, 5, 10 or 25% w/w) of ammoniumpersulphate, (supplied by BDH, UK) and(NH4)2S2O8sodium metabisulphite, (Merck, Germany),Na2S2O5were used together as redox initiators.

Polymerisation

Four series of sponges were prepared, di†ering in thewater/HEMA ratio in the monomer mixture, accordingto the compositions described in Table 1. The spongeswere prepared as cylindrical buttons by the polymeris-ation of HEMA in aqueous solution as previously

TABLE 1. Composition of monomer mixtures and code names of resulting polymers.

Crosslinking agent, divinyl glycol (DVG)

Composition of Crosslinking Initiators, Na2S

2O

5½(NH

4)S

2O

8water/HEMA agent (wt% of (each, wt% of monomer)

mixture monomer)

(wt%) 0·01a 0·05a 0·1b 0·25b 0·5c 1·0d 2·0d

75/25 0·5 G77 G78 G79 G80 G81 G82 G83

0·5 G39 G40 G41 G42 G43 G44 G45

1 G38 G46 G14 G47 G36 G37 G48

80/20 2 G49 G50 G51 G52 G53 G54 G55

3 – – G16 – – – –

4 – – G17 – – – –

5 – – G18 – – – –

0·5 G63 G64 G65 G66 G67 G68 G69

1 – – G19 – – – –

85/15 2 – – G20 – – – –

3 – – G21 – – – –

4 – – G22 – – – –

5 – – G23 – – – –

0·5 G70 G71 G72 G73 G74 G75 G76

1 – – G24 – – – –

90/10 2 – – G25 – – – –

3 – – G26 – – – –

4 – – G27 – – – –

5 – – G28 – – – –

Added as a 1, b 5, c 10 and d 25%w/w aqueous solutions.

POLYMER INTERNATIONAL VOL. 42, NO. 1, 1997

Hydrophilic sponges based on HEMA 47

described.4 The homogeneous liquid mixture of HEMA,water, initiator and crosslinking agent was distributedin polypropylene moulds Ðtted in a moulding unitdesigned in our laboratories. After replacing the air withnitrogen, the sealed moulding unit was placed in atemperature-controlled waterbath, where a three-stagecure cycle (10 h at each of 30¡C, 40¡C and 50¡C) wasused to e†ect polymerisation.

Equilibrium water content

After polymerisation, the sponge specimens were placedin separate containers with deionised water andhydrated for at least three weeks prior to measurements,with daily water changes.

Surface water on the sponges was removed by gentletissue blotting prior to weighing. After weighing, thesamples were dried for 24 h at 50¡C, followed by 6 h at40¡C in a vacuum oven, and then reweighed. Followingthis treatment, the specimens had reached constantweight.

The equilibrium water content (EWC), as weight per-centage, was calculated using eqn (1), where andww wdare the weights of a fully hydrated specimen and of adried specimen, respectively :

EWC\ 100(ww [ wd)/ww (1)

Cast water content

A large proportion of the sponges produced consisted ofstratiÐed, non-uniform material, with a water layerpresent on top of the sponge button after polymeris-ation. Immediately after removal from the mouldingunits, the specimens from each formulation were dividedinto two groups. One group of sponges (Group 1) wasnot hydrated immediately upon removal from themoulding unit, but was dried according to the dryingprotocol used for the EWC determination. The remain-ing sponges from that formulation (designated as Group2) were hydrated immediately after removal from thepolymerisation moulds. Weighing both groups ofsponges enabled the weight of unreacted monomerwithin the sponges at the end of the polymerisation tobe eliminated from the “castÏ water content (CWC)determination. CWC, i.e. the weight percentage of waterwithin the sponges immediately after polymerisation,was obtained from the equation :

CWC

\ 100[wc [ (w1d [ w2d) [ w1d]/[wc[ (w1d [ w2d)](2)

where is the cast weight, i.e. the weight of the spongewcimmediately after polymerisation, without any treat-ment, is the dry weight of the Group 1 sponges, andw1d

is the dry weight of the Group 2 sponges.w2dThe term in eqn (2) accounts for unre-(w1d[ w2d)

acted monomer present within the sponge at the com-

pletion of the 30 h polymerisation cycle, which ispresumably removed during sponge hydration.

Sol fraction

Measurement of and also enabled the amountw1d w2dof water-soluble solids (“sol fractionÏ) within the spongesto be calculated from the equation :

sol fraction \ 100(1 [ w2d/w1d) (3)

Sol fractions for sponges, which were hydrated imme-diately on completion of polymerisation (Group 2),were obtained by extrapolation from results for spongesof identical formulation, i.e. the initial dry weight of thesponge (including unreacted monomer) wasw1dobtained from the average ratio of to cast weightw1dfor sponges of identical formulation, which were driedimmediately after polymerisation. While the sol fractionobtained in this manner is a “theoreticalÏ value, thismeasurement should yield information on the e†ect ofthe sponge drying protocol on conversion, i.e. whetherfurther polymerisation of unreacted monomer occursduring the drying procedure.

Scanning electron microscopy

A number of sponges were examined by environmentalscanning electron microscopy (ESEM). This techniqueenables untreated samples containing water to be exam-ined, avoiding possible artefacts arising from samplepreparation procedures (e.g. dehydration and conduc-tive coating). An ESEM model E-3 (Electroscan Corp.,USA) was used in this study. To ensure suitable imageresolution, the microscope was operated at chamberpressures varying from 400 to 600 Pa, with an acceler-ating voltage of 20 kV. The polymer sponge sampleswere maintained at 8^ 2¡C using a cooling stage : thisprocedure enabled a water-saturated environment to bemaintained in the sample chamber, preventing possiblesample dehydration. Vertical cross-sections(approximately 1 mm wide) were cut from the spongebuttons, placed on the cooling stage and then coveredwith water droplets to avoid sample dehydration duringinitial chamber evacuation. The chamber pressure wasperiodically increased by 250 Pa in order to condensewater on the sample surface, again ensuring that nodehydration occurred.

RESULTS

Equilibrium water content

A large proportion of the sponges produced appearedto be macroscopically inhomogeneous, with a toughrubbery layer present under an opaque sponge layer.



TABLE 2. Stratification in PHEMA spongesa. Crosslinking agent, divinyl glycol

(DVG)

Composition of Crosslinking Initiator concentrationb

water/HEMA agent

mixture (wt% of

(wt%) monomer)

0·01 0·05 0·1 0·25 0·5 1·0 2·0

75/25 0·5 ½ ½ ½ ½ ½ ½ ½

0·5 ½ * * * * ½ ½

1 ½ * * * ½ ½ ½

80/20 2 ½ * * * ½ ½ ½

3 *4 *5 *0·5 ½ * É É * ½ ½

1 É

85/15 2 *3 *4 *5 *0·5 K É É É * ½ ½

1 É

90/10 2 É

3 É

4 É

5 *

a Key: ½¼severe stratification; *¼thin stratified layer discernible ; É¼stratification not

observed; did not occur.K¼polymerisation

b Concentrations of initiator stock solutions as indicated in Table 1.

Fig. 1. E†ect of the concentration of initiator at various dilutions of the monomer mixture on the cast water content of PHEMAsponges prepared with 0É5 wt% DVG. Error bars indicate the standard deviation obtained from quadruplicate sample weight

determinations.



Fig. 2. E†ect of the initial dilution of monomer mixture and the concentration of initiator on the equilibrium water content ofPHEMA sponges prepared with 0É5 wt% DVG: (a) hydrated immediately after polymerisation (Group 2) ; (b) hydrated after oven

drying (Group 1).

This stratiÐcation was most apparent at the lowest initi-ator concentration (0É01 wt%) and at initiator concen-trations higher than 0É25 wt%. The morphology of thesponges produced at 0É01 wt% initiator concentrationdi†ered from that of the stratiÐed sponges formed athigher initiator levels, in that hard, irregular lumps werepresent within the lower rubbery stratum.

Table 2 summarises visual observations on the extentof sponge stratiÐcation, manifested as a rubbery bottomlayer. Severe stratiÐcation was also invariably accompa-nied by the presence of a separated water layer on topof the cast sponge, and this amount of water appeared

to increase as stratiÐcation increased. While a rubberybottom layer could not be discerned in samples castwith 90 wt% water in the initial monomer mixture, theupper sponge layer appeared to be non-uniform, i.e.some collapse of the original cast volume was apparent.

QuantiÐcation of the extent of sponge non-uniformityis desirable ; however, owing to possible inaccuracy inthe measurement of the top water layer, this was notattempted. In preference, the water content of the lowersponge layer was measured. This water content, knownas “castÏ water content (CWC), as distinct from equi-librium water content (EWC), was determined by the



Fig. 3. E†ect of the concentration of initiator at various dilutions of the monomer mixture on the equilibrium water content ofPHEMA sponges prepared with 0É5 wt% DVG, hydrated immediately after polymerisation (Group 2).

method previously outlined, for all four sponge seriescontaining 0É5 wt% DVG as a crosslinking agent (Fig.1). Comparison of the extent of sponge stratiÐcationwith the CWC of highly stratiÐed samples showed astrong correlation between sponge stratiÐcation andlow (less than 65 wt%) CWC.

The EWC of the sponges was calculated from quad-ruplicate measurements using eqn (1). We have foundpreviously5 that plotting the EWC against the concen-tration of water in the initial monomer mixture facili-tates the interpretation of swelling behaviour ofsponges. If a diagonal line is assumed for such a plot,

Fig. 4. E†ect of the concentration of initiator on the equilibrium water content of PHEMA sponges prepared with 0É5 wt% DVGand 80 wt% water in the monomer mixture. Group 1 : sponges hydrated after oven drying. Group 2 : sponges hydrated immediately

after polymerisation.



Fig. 5. E†ect of the concentration of initiator and DVG on the equilibrium water content of PHEMA sponges prepared in 80 wt%water.

sponges which swell upon equilibration in water areplaced above the diagonal on these plots, while thosewhich deswell on equilibration appear below the diago-nal. Results for Group 2 sponges (i.e. those hydratedimmediately after polymerisation with 0É5 wt% DVG

concentration are shown in Fig. 2(a), where it is appar-ent that for a given initiator concentration deswelling ismore pronounced at higher diluent concentrations.

Group 1 sponges (i.e. those dried prior to initialhydration) exhibited a greater tendency to deswell than

Fig. 6. E†ect of the concentration of initiator on the sol fraction of PHEMA sponges prepared with 0É5 wt% DVG and 80 wt%water in the monomer mixture. Group 1 : sponges hydrated after oven drying. Group 2 : sponges hydrated immediately after

polymerisation.



Fig. 7. Composite ESEM micrograph of sample G14 (80 wt% water).



Fig. 8. ESEM micrographs of samples (a) G19 (85 wt% water) and (b) G24 (90 wt% water).

their Group 2 counterparts (Fig. 2(b)). The measuredEWCs for most of the Group 1 samples were also sig-niÐcantly less than those for sponges which werehydrated immediately, with more than half of theGroup 1 sponges having EWCs less than 70 wt%.

The inÑuence of initiator concentration on EWC ismore clearly shown in Fig. 3, where EWC is plottedagainst initiator concentration for the four series ofGroup 2 sponges containing between 75 and 90 wt%water in the monomer mixture. The sponges containing75 wt% water in the monomer mixture showed a steadyrise in EWC up to 1É0 wt% initiator, after which theEWC decreased. The hydration behaviour of spongescontaining 80, 85 and 90 wt% water in the initial

mixture appears to be more complex, however, with theEWC initially increasing at low initiator concentrations,decreasing after this initial rise, and then increasing upto 1É0 wt% initiator concentration, after which itremained approximately constant.

A complex dependence of sponge EWC on initiatorconcentration was found for all Group 1 sponges. Thehydration behaviour of these samples di†ered slightlyfrom that of the Group 2 samples, in that the EWC didnot remain constant after 1É0 wt% initiator concentra-tion but increased further up to 2É0 wt% initiator.

The di†erences in hydration behaviour betweenGroup 1 and Group 2 sponges are exempliÐed by theresults obtained for formulations containing 80 wt%



water in the monomer mixture (Fig. 4), where is appar-ent that, at low initiator concentrations, the EWCs forGroup 1 sponges are consistently lower than those forsponges hydrated immediately after polymerisation(Group 2).

The e†ect on EWC of higher concentrations of DVGwas investigated for formulations containing 80 wt%water in the monomer mixture. The hydration behav-iour for sponges at three DVG concentrations (Fig. 5)was similar, i.e. a rapid initial rise in EWC, followed bya decrease, then a further increase as the initiator con-centration increased to 1É0 wt%.

Sol fraction determinations for water/HEMA spongescontaining 0É5 wt% DVG and 80 wt% water in themonomer mixture (Fig. 6) were typical of resultsobtained for the other water contents, and showed thatsol fraction for Group 1 sponges increased as the initi-ator concentration increased. Group 2 sponges (i.e.sponges hydrated immediately after polymerisation)generally showed higher sol fractions than their Group1 counterparts for intermediate initiator concentrations(0É01 \ [I]\ 0É5 wt%).

Pore size

ESEM was used to examine the morphology of thesponges in cross-section in order to clarify the observedstratiÐcation. A typical example is the ESEM micro-graph for sponge formulation G14 (Fig. 7), which isoriented such that the base of the sponge is at the top ofthe micrograph.

ESEM shows that the stratiÐed layer has a cellularstructure, in which droplets of water are entrapped in acontinuous polymer matrix : this morphology is presentin a uniform layer up to about 250 km from the spongebase. Between 250 and 600km from the sponge base,regions of cellular morphology become interspersedwith domains which consist of polymer droplets withina continuous water phase. At distances greater than600 km from the sponge base, both cellular and polymerdroplet morphologies are still present ; however, there isa signiÐcant increase in the sponge volume occupied bywater.

Micrographs of the base of G19 and G24 sponges(Figs 8(a) and (b), respectively) did not show a “closedcellÏ stratiÐed layer, but a morphology which consistedof polymer droplets in a continuous water phase.

DISCUSSION

Cast and equilibrium water content

The results obtained in this study indicate that theinclusion of DVG as a crosslinking agent in the synthe-sis of PHEMA sponges leads to signiÐcant di†erences insponge swelling behaviour compared with that found

for sponges crosslinked with EDMA. One of the mostobvious di†erences is that most sponges prepared in thepresent work showed a high degree of stratiÐcation,which was invariably accompanied by cast water con-tents considerably below the water content of the initialmonomer mixture (Fig. 1).

The lowest CWCs for sponge samples containing 75,80 and 85 wt% water in the initial monomer mixturewere found at the highest initiator concentration(2É0 wt%). The value of CWC of about 40 wt% is closeto the EWC for PHEMA prepared by bulk polymeris-ation.13h16 This result suggests that phase separation inthese samples probably occurs before any signiÐcantnetwork gelation. The formation of a macroscopicallyheterogeneous sponge is clearly dependent upon thegelation of the network occurring prior to, or coincidentwith, phase separation, with the initially unstable dis-perse structure becoming Ðxed by concomitant cross-linking.17 Clearly, any factors which change the rate ofnetwork gelation with respect to the rate of phaseseparation would be expected to inÑuence the homo-geneity of the resultant sponge.

We have found previously4 that minor stratiÐcationoccurs in EDMA-crosslinked HEMA sponges at highinitiator concentrations, since an increase in the numberof inorganic ionic species in water is expected toenhance phase separation, as the water becomes apoorer solvent for the polymer.

The comparatively greater degree of sponge stratiÐ-cation seen in the DVG-crosslinked sponges comparedwith analogous EDMA-crosslinked systems may be dueto the lower free radical reactivity of allylic doublebonds compared with methacrylate groups.18 Reso-nance stabilisation of the allyl radical, together withdegradative chain transfer resulting from a-hydrogenabstraction,19,20 contributes to the low reactivity underradical conditions of monomers containing allyl moi-eties. Using DVG as a crosslinking agent would there-fore be expected to slow the rate of network gelationsigniÐcantly compared with that in sponges crosslinkedwith EDMA.

The variation in EWC with initiator concentrationfor the DVG-crosslinked sponges was generallycomplex. While the swelling behaviour manifested inplots of EWC versus weight percentage of diluent in theinitial monomer mixture (Figs 2(a) and (b)) agreesbroadly with that found for low (0É5 wt%) EDMAcontent, i.e. the sponge deswelling is more pronouncedat higher diluent concentrations,5 there are a number ofsamples which exhibit unusual swelling properties. Theabsolute EWC values for sponges containing 75 and80 wt% water are greater than any we have seen pre-viously in EDMA-crosslinked samples. No EDMAsponges containing more than 70 wt% water in themonomer mixture showed increases in water contentbeyond 70 wt% when equilibrated : in contrast, severalDVG-crosslinked sponges with 75 and 80 wt% diluent



did show increases in EWC when hydrated. A notableexample of this increase is the measured EWC of 95%for a sponge initially containing 75 wt% water, with aninitiator concentration of 2É0 wt% (Fig. 2(b)).

Plots of EWC versus initiator concentration (Fig. 3)showed anomalous swelling behaviour as the initiatorconcentration was increased, i.e. an initial rise in EWC,followed by a decrease. The EWC values in Group 1showed an increase between 1É0 and 2É0 wt% initiator,while the EWCs in Group 2 remained relatively con-stant. Swelling results5 for EDMA-crosslinked spongescontaining 3É25 wt% EDMA and 85 and 90 wt% waterin the monomer mixture also showed an anomalousdependence of EWC on initiator concentration, i.e. aninitial rise, followed by a decrease, then a Ðnal gradualincrease. This complex swelling behaviour onlyoccurred, however, as the initiator concentration wasincreased to 5 wt%, whereas in the present work thehighest initiator concentration used was 2 wt%.

When the results of the present study are consideredin combination with those from our earlier work onEDMA-crosslinked PHEMA sponges, it becomesapparent that the phenomena of sponge stratiÐcationand anomalous swelling behaviour may be related.

As such, consideration of the extent of sponge stratiÐ-cation (quantiÐed by measurements of cast watercontent) may be necessary in order to rationalise theundoubtedly complex variation of EWC with initiatorconcentration found for these sponges. A typicalexample of the variation of sponge EWC with initiatorconcentration is found for the series of sponges contain-ing 0É5 wt% DVG and 80 wt% water in the initialmonomer mixture, i.e. samples G39 to G45 (Fig. 4).

Considered in isolation, the low EWC of samples pre-pared at 0É01 wt% initiator concentration (G39) is diffi-cult to rationalise. The morphology of these samples diddi†er, however, from those prepared with higher levelsof initiator, in that the sponge inhomogeneities did notsimply consist of a rubbery base layer under an opaquesponge layer, but were characterised by hard, irregularopaque sections. It seems likely that so-called “popcornÏpolymerisation, reviewed recently,16 has occurred inthese samples. The low sponge EWC values, togetherwith the disappearance of the phenomenon at higherinitiator levels, are consistent with previous observ-ations.16

As the initiator concentration was increased tobetween 0É05 and 0É25 wt%, EWCs remained relativelyconstant, forming a small “peakÏ region in the plot ofEWC versus initiator concentration. It is possible thatat these initiator concentrations the rate of phaseseparation is close to that of gelation, and the spongestratiÐcation becomes minimal.

Interestingly, a similar peak region was observed inthe plot of CWC versus initiator concentration (Fig. 1) ;the trend shown by the portion of the CWC curve forcompositions containing less than 0É5 wt% initiator

resembles that for EWC plotted against initiator con-centration, i.e. an initial sharp increase to a peak region,followed by a more gradual decrease.

As the initiator concentration is increased further tobetween 0É25 and 0É5 wt%, the rate of phase separationrelative to gelation might increase. Phase separation iscertainly enhanced at higher initiator concentrations :Yasuda et al.21 observed that sponges could be formedat lower diluent concentrations if high initiator concen-trations were used.

At initiator concentrations higher than 0É5 wt%,however, the trends in CWC and EWC diverge, i.e. theCWC decreases further, while EWC either levels o†(Group 2) or increases further (Group 1). It is temptingto attribute the increase in EWC in the Group 1sponges (which have undergone a further thermal cycleon drying) to an increase in incorporation of the hydro-philic DVG crosslinking agent ; however, we have nodirect evidence of this.

The higher sol fractions obtained for Group 2 com-pared with Group 1 at intermediate initiator concentra-tions (0É01 \ [I]\ 0É5 wt%) (Fig. 6) indicate thatfurther monomer conversion occurs during the dryingprocedure used to determine CWC.

The general increase in sol fraction at higher initiatorconcentrations may be due to poorer crosslinking. Sincesponges synthesised with the highest initiator level(2É0 wt%) also had the lowest CWCs (i.e. they were themost stratiÐed), it is possible that a proportion of thehydrophilic DVG monomer remains in the upper waterlayer, unable to participate in network crosslinking.

Pore size

The “closed cellÏ morphology with water dropletsentrapped within a polymer matrix, as seen in the strati-Ðed G14 sponge in Fig. 7, resembles that previouslyfound in sponges containing 60 wt% water.4 While thepore openings are larger than 10 km, they are not con-tiguous over the whole specimen and hence are not suit-able for surgical applications.

At higher water contents in the initial monomermixture (Fig. 8), the extent of this “closed cellÏ morphol-ogy is very much less, and stratiÐcation is not observedin the 90 wt% water sponge.

CONCLUSIONS

The results obtained for DVG-crosslinked sponges inthis study indicate that the incorporation of DVG leadsto macroscopic inhomogeneities in the sponges. Theformation of a rubbery non-porous layer in mostsponge formulations is undesirable for their use as pros-thetic materials, because tissue ingrowth into theseregions would clearly be limited. While this rubberylayer is not present in samples produced with the



highest initial water content (90 wt%), these sponges aremuch weaker than those produced with 75 and 80 wt%water, hence are unlikely to be suitable for use as per-ipheral skirts in artiÐcial corneas.

One possible method of eliminating the stratiÐcationproblem could be to use a more active initiating system,e.g. tetramethyl ethylenediamine/ammoninum persul-phate in preference to sodium metabisulphite/ammonium persulphate, in an attempt to increase therate of network gelation.

Another possible approach could be to use a hydro-philic crosslinking agent containing methacrylic ratherthan allylic polymerisable groups. The use of a co-monomer with similar reactivity to HEMA may elimi-nate sponge stratiÐcation, as the network gelationwould occur at a rate similar to separation of thepolymer from the aqueous phase.

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