Improved Wettability of an Experimental Silicone Rubber DentureSoft Lining Material

Mark Waters,1 Robert Jagger2

1 Department of Basic Dental Science, University of Wales College of Medicine, Dental School, Heath Park, Cardiff CF4 4XY,Wales, United Kingdom

2 Department of Restorative Dentistry, University of Wales College of Medicine, Dental School, Heath Park, Cardiff CF4 4XY,Wales, United Kingdom

Received 25 March 1999; revised 12 July 1999; accepted 23 July 1999

Abstract: The purpose of this study was to improve the wettability of an experimentalsilicone rubber soft lining material (Sildent) to increase patient comfort. Sildent was modifiedby the addition of polyalkylene oxide poly(dimethylsiloxane) surfactants. The various surfac-tants were added directly to the polymeric matrix in the quantities 5, 10, 20% w/w. Thesurfactants were also added to a one part silicone sealant, which was then painted onto thesurface of already cured Sildent. Contact angle measurements were made on samples using adynamic contact angle analyzer. Results showed that incorporation of surfactants A (SilwetL7600) and B (Silwet L7607) effectively improved the wettability of Sildent. This improvementwas still evident after 6 months storage in distilled water at 37°C suggesting retention of thesurfactants matrix via physio-chemical bonding. Formulations with surfactants added directlyto the matrix showed unacceptable water absorption after 2 months in distilled water. Sampleswith surfactant charged sealant painted on the surface showed a lower water absorption. Inconclusion, Sildent formulations modified with polyalkylene silicone surfactants showed im-proved wettability compared to unmodified Sildent. Further work is needed to reduce wateruptake and determine the effect on key mechanical properties.© 1999 John Wiley & Sons, Inc. JBiomed Mater Res (Appl Biomater) 48: 765–771, 1999

Keywords: denture liners; silicone rubber; surfactants; wettability; dynamic contact angleanalysis

INTRODUCTION

Denture soft lining materials act as a cushion between thehard denture base and the supporting mucosa and are oftenused for patients with atrophic edentulous alveolar ridges. Itis important that these soft resilient materials be adequatelywetted by saliva, so that a lubricating layer can be set upbetween the material and the mucosa thus reducing frictionalproblems. Normally the effect of polishing, coupled with alubricating layer of saliva, prevents frictional problems withconventional acrylic denture base. The layer of saliva wettingthe surface of the denture also maximizes the retention. Thecombination of difficulty in polishing soft lining materialsand poor wettability by saliva can lead to frictional problemsand reduced retention.1 The problem of poor wettabilityhas been particularly associated with silicone soft liningmaterials.2-4

Investigations into the wettability characteristics of den-ture soft lining materials have been limited. Two techniqueshave generally been used to investigate contact angles. Thefirst involves the placing of a drop of liquid on a surface andadding or withdrawing liquid from the top to determineadvancing and receding contact angles.5 This technique wasused in studies to evaluate the wettability of soft liningmaterials.2,6 The second technique involves placing a drop ofliquid on an inclined surface thereby measuring the advanc-ing angle at the lower periphery and the receding angle at theupper periphery.7 This technique was also used to investigatethe contact angles of soft lining materials.1 More recently, adynamic contact angle analyzer has been introduced wherespecimens are immersed in liquid with the meniscus at theinterface characterizing the advancing and receding contactangles.4 This dynamic technique was used in the presentstudy.

Studies concerned with improving the wetting character-istics of silicone soft liners have been limited. Louka et al.2

treated the surface of soft liners by depositing a thin film ofsilica on the surface by treatment with silicon tetrachlorideand by cross-linking the surface molecules and bombarding

Correspondence to:Dr. MGJ Waters, Department of Basic Dental Science, Uni-versity of Wales College of Medicine, Dental School, Heath Park, Cardiff CF4 4XY,Wales, U.K.

© 1999 John Wiley & Sons, Inc. CCC 0021-9304/99/060765-07

765

with hydroxyl free radicals. Both treatments were successfulin the short term. However, within two weeks, the treatmentswere ineffective and the water absorption of the silica tetra-chloride treated samples was significantly increased. The useof plasma treatment to hydrophilize silicone rubber surfaceshas also been used.8,9 However, there is a question mark overthe longevity of the treatment due to the mobility of thesiloxane backbone. Polyzois et al.10 modified a maxillofacialsilicone rubber material by the incorporation of various sili-cone alkene oxide block co-polymers directly into the poly-meric matrix. Some of the surfactants proved reasonablysuccessful in reducing contact angle, but the long term sta-bility (beyond 30 days) and the effect on resultant propertieswas not tested.

It has previously been shown that an experimental siliconerubber material (Sildent) developed at Cardiff Dental Schoolhas the potential to be a successful soft lining material.11 Aproblem associated with the material, however, is its poorwettability by saliva, which may lead to frictional problemsand patient discomfort. The purpose of this study was, there-fore, to modify Sildent by the incorporation alkene oxidemodified poly(dimethylsiloxane) surfactants in order to im-prove the resultant wettability.

THEORETICAL CONSIDERATIONS

Contact Angle Measurement

When a liquid comes in contact with a solid, the liquid maywet the solid and thus spread completely across the surface.The contact angle (Fig. 1) characterizes this interaction be-tween a solid and a liquid surface at the interface, and is anexperimental representation of an important thermodynamicrelationship known as the Young equation, which relates thecosine of the angle,u, to the interfacial free energies of the

three interfaces (sv, solid/vapor;sl, solid/liquid; andlv, liq-uid/vapor). At equilibrium:

COSu 5gsv2 gsl

glv. (1)

By measuring the contact angles formed by different liquidsof known surface tension, quantitative values including sur-face energy parameters such as polar (acid-base) and nonpo-lar (dispersive) components can be calculated. The liquid onthe surface thus acts as a sensitive probe by interactingchemically with functional groups at the surface. The tech-nique suspends the material in an electrobalance, while thewetting medium scans along at a constant speed via a com-puter-controlled stage. The meniscus at the interface is char-acterized by the dynamic contact angle,u. Both advancingand then receding contact angles are measured as the stagemoves up (the sample) and then down. The difference be-tween these two contact angles is a universal property of mostsurfaces called contact angle hysteresis. Calculations of sur-face energies using this technique are derived directly fromthe Young equation given above.

A simple equation relates the cosine of the contact angle,theta, to the magnitude of the wetting force recorded by thebalance, the surface tension of the probe liquid and the wettedperimeter of the solid sample by the following equation12:

COSu 5F

p.v, (2)

whereF is the force (dynes),p is the perimeter (cm), andv isthe surface tension (dyne/cm).

Modification of Silicone Rubbers with Surfactants

The surfactants best suited to permanently improving thewettability of silicone rubber would be polyalkylene oxide-modified PDMS, more specifically linear PDMS to whichpolyethers have been grafted through a hydrosilation reac-tion. The process results in an alkyl-pendant (AP type) co-polymer, in which the polyalkylene oxide groups are attachedalong the siloxane backbone through a series of hydrolyti-cally stable Si—C bonds.

These products have the general formula:

CH3SiO(CH2SiO)x(CHSiO)ySiCH3

PPE

where PE5OCH2CH2CH2O(EO)m(PO)nZ

In the above formula EO represents ethyleneoxy, PO repre-sents 1,2-propyleneoxy, and Z can be either hydrogen or alower alkyl radical. By varyingx, y, m, andn, the percentageof EO, PO, and silicone can be varied.

Figure 1. Visual perspective of the contact angle, theta (u), as ob-served at the solid/liquid/vapor interface in a dynamic Wilhelmy-based experiment.

766 WATERS AND JAGGER

Polyalkylene oxide-modified poly(dimethylsiloxane) sur-factants are retained within a poly(siloxane) rubber matrix viaphysiochemical bonding. The exact nature of this bonding isunclear. However, these surfactants are unlikely to leach outof the rubber matrix and, therefore, are suitable for permanentmodification. Generally the siloxane portion is a lypophile(water repellant), while the alkene oxide portion is a hydro-phile (water attractant). By altering the amount of the poly-meric constituents, the desired surfactant properties can beachieved. Materials with ethylene oxide contents of 75% andhigher are freely soluble in water.10 The balance of hydro-philic/lyophilic properties is critical in achieving permanentlyimproved wettability. A surfactant with too low molecularweight and too high hydrophilic properties results in an initialimprovement in wettability, but diffusion of the surfactantinto the aqueous phase and, thus, loss of activity makes theeffect short-lived. Another factor that must be consideredwhen modifying silicone rubbers with surfactants is thatimparting hydrophilic properties leading to increased wateruptake may outweigh the advantage of improved wettability.

MATERIALS AND METHODS

Sildent consisted of a hydroxy end-blocked poly(dimethylsi-loxane) containing a hydrophobic silica. It was cross-linkedusing a mixture of alkoxy silanes and catalyzed using dicar-boxylate tin. All surfactants were alkylene oxide modifiedPDMS of the type described earlier (OSI Specialities, Dans-bury, UK). The surfactants A-D had the following makeup:surfactant A 25% ethylene oxide, molecular weight 4000;surfactant B 30% ethylene oxide, molecular weight 1000;surfactant C 30% propylene oxide, molecular weight 3000;surfactant D 27% ethylene oxide molecular weight 4000. Asstated earlier, the amount of alkylene oxide present coupledwith the molecular weight of the surfactants are the keyfactors in producing permanent improvements in wettability.Comparisons were made with Cosmesil maxillofacial siliconerubber (Principality Medical, Newport, UK) and Mollo-plast-B silicone rubber soft lining material (Regneri GmbH,Germany).

Sildent was cured by the addition of 12 drops of catalystand 18 drops of cross-linker to every 10 grams of elastomerbase and curing for 24 h at room temperature. Cosmesil andMolloplast-B were cured according to manufacturer’s in-structions.

Specimens (20 mm3 10 mm3 1 mm) were prepared inmoulds, which were formed by investing Perspex blanks ofappropriate size in 50:50 stone plaster mixture using a con-ventional dental flasking technique. The specimens were thenconstructed using the same procedures as would be usedclinically.

Incorporation of Surfactants Directly into Matrix

The surfactants were mixed into the elastomer base prior tothe addition of catalyst and cross-linker. The amounts added

were 5, 10, 20% by weight of the elastomer base. Thewettability characteristics of the various formulations weredetermined as described above. Five specimens were testedper formulation.

Coating the Surfaces with Surfactants

The surfactants were mixed directly into a one-part acetoxysealant in the amounts 5, 10, and 20%. This surfactant-charged sealant was then painted onto already cured experi-mental silicone rubber and the coated materials were testedfor wettability. As before, 5 specimens were tested per for-mulation.

Contact Angle Test Procedure

Care was taken not to handle the surfaces of the specimens inorder to reduce the chance of contamination. Testing wascarried out using a Cahn Dynamic Contact Angle Analysermodel 312 (Cahn Instruments, Inc. Cerritos, CA) linked to aDan 386 IBM compatible computer (Dan Technology plc,London). The perimeter of the samples was measured indi-vidually for each specimen and analyzed using the computersoftware for maximum accuracy. Care was taken that eachsample entered the wetting medium parallel to the surface.The specimens entered the wetting medium of distilled waterat a speed of 30 microns/sec (Fig. 2). The meniscus at thespecimen water interface is characterized by the dynamiccontact angle. A graphical display was shown on the com-puter as the experiment progressed. The buoyancy graph (Fig.3) shows the wetting force of the sample, as measured by themicro-balance as it moves first into (lower line) and then outof (upper line) the water. This enabled the advancing contactangle to be calculated by least squares analysis of the bottomline, and the receding contact angle to be calculated by leastsquares analysis of the top line using Eq. (2).

Water Uptake

Selected samples were placed in a desiccater containing phos-phorus pentoxide and calcium chloride until they achieved aconstant weight to an accuracy of 0.0001 g. The specimenswere then placed in glass screw-topped jars containing dis-tilled water and maintained in an oven at 37°C. The distilledwater was changed weekly. At recorded intervals, the speci-mens were removed, blotted to remove excess water, andweighed again, to an accuracy of 0.0001 g. Weights wererecorded for 2 months and 5 specimens were measured permaterial.

Statistical Analysis

The mean (X) and standard deviations (S.D.) within sampleswere calculated. A one-way analysis of variance, using theBonferroni method to produce correctedp values, was used toevaluate differences between materials. Ap value of,0.05was considered significant.

767IMPROVED WETTABILITY OF DENTURE SOFT LINING MATERIAL

RESULTS

Incorporation of Surfactants Directly intothe Polymeric Matrix of Sildent

The results were expressed as advancing contact angles as thewater advances over the material, and receding angle as thewater recedes back over the wetted surface. The equilibriumcontact angle is the average of the advancing and recedingangles.

The equilibrium contact angle has been highlighted as themost important parameter concerning wettability of soft lin-ing materials,1 and, therefore, was considered for comparisonpurposes using a one way analysis of variance followed bythe Bonferroni multiple comparison method.

The wettability of the surfactant modified materials wascompared to that of the original materials in Table I. When5% surfactant was incorporated into Sildent, surfactants Aand B had significantly lower contact angles than Sildent (A:p , 0.05; B: p , 0.01). However, after 6 months, aging at37°C the formulations containing surfactant D were alsosignificantly lower than Sildent (A:p , 0.001; B:p , 0.05;

D: p , 0.001). However, no modified formulation was sig-nificantly lower than Molloplast-B.

At the 10% level of surfactant incorporation, only formu-lations containing A and B were initially lower than Sildent(A: p , 0.05; B:p , 0.05). However after 6 months, agingat 37°C the formulations containing C and D were alsosignificantly lower than Sildent (A:p , 0.001; B:p , 0.01;C: p , 0.05; D:p , 0.05). The formulation containing 10%surfactant A had a equilibrium contact angle 5 degrees lowerthan Molloplast-B, after 6 months at 37°C; however, this didnot prove significant using the Bonferroni multiple compar-ison method.

Results for the formulations containing 20% surfactantsagain showed initial significant reductions over Sildent ex-cept for the formulation containing surfactant C (allp ,0.05). However after aging for 6 months at 37°C only theformulation containing surfactant D was significantly lowerthan Sildent (p , 0.05).

Coating of Surfactant Containing One-PartSilicone Sealant onto Sildent

Overall, the incorporation of surfactants A and B into thematrix of Sildent were deemed most successful in reducingequilibrium contact angle with no improvement being seenwith additions of more than 10% w/w of the surfactants.Incorporation of surfactants into the matrix does have theassociated problem of increasing water absorption. Thus,surfactants A and B were added to a one-part silicone sealantand coated onto cured Sildent in an effort to overcome thisproblem.

The coating did not produce the same reductions in equi-librium contact angles as seen with direct incorporation.However, there was significant reduction in the equilibriumcontact angle compared to Sildent at the 20% level initially(bothp , 0.05) and after 6 months 37°C at 5 and 10% levels(p , 0.05) (Table II). Again there was no significant im-

Figure 3. Buoyancy graph of force vs. sample position for dynamiccontact angle analysis.

Figure 2. Diagram showing specimen set up for dynamic contactangle analysis.

768 WATERS AND JAGGER

provement in adding 20% surfactant compared to 10% sur-factant suggesting that the 10% level is the optimum.

The high water uptake that the incorporation of the sur-factants caused after 2 months at 37°C is shown in Figure 4.The coated samples showed an improvement in water uptake;however, the water uptake was still very much higher thanSildent without surfactant.

DISCUSSION

Soft lining materials that are not sufficiently wetted by salivaare unable to achieve an adequate lubricating film betweenthemselves and the supporting mucosa, thus frictional prob-lems may arise.1,10Poly(dimethylsiloxanes) are generally lowsurface-energy solids exhibiting poor wettability and, thus, alarge increase in surface energy would be needed to achieveadequate lubrication and minimize irritation of the mucosa.10

The poor wetting characteristics of Sildent instigated theincorporation of polyalkylene oxide-modified poly(dimethyl-siloxane) surfactants within the material in an effort to im-prove wettability.

In this study, the advancing, receding, and equilibriumcontact angles are quoted. The significance of the differentcontact angles, however, must be clarified. When a liquidadvances over a solid, then the advancing contact angle is ofinterest, which is in effect the initial contact angle. Accordingto Zisman (1964),13 this advancing angle can be used as anindicator of total wettability. In the mouth, however, salivaalso recedes over the already wetted soft lining material,particularly in the case of the swallowing reflex, reseating thedenture.14 Thus, the receding angle is also of importancewhen assessing how a material to be placed in the mouth iswetted. It is probable, therefore, that the average of these twocontact angles (equilibrium contact angle) is the most rele-vant in the mouth, which has been suggested previously.1

TABLE I. Wettability of Surfactant Modified Formulations (Directly into Matrix) in Comparison with Original Materials, Initial Values,and After Aged for 6 Months at 37°C (n 5 5)

Materials

Advancing Angle(Mean6 SD)

Receding Angle(Mean6 SD)

Equilibrium Angle(Mean6 SD)

Initial 6 Months Initial 6 Months Initial 6 Months

Molloplast-B 92.16 3.7 90.26 7.1 59.06 3.3 51.26 2.1 75.66 3.3 65.36 3.2Cosmesil 105.46 4 78.36 3.1 57.26 2.0 62.16 3.0 81.36 2.7 70.26 2.8Sildent 106.36 7.7 96.76 4.8 60.16 4.4 62.26 5.9 83.26 5.9 79.56 5.4Sildent (5% surfactant A) 82.36 4.1 84.76 2.2 64.66 1.7 37.46 1.6 73.56 2.9 61.16 1.9Sildent (5% surfactant B) 76.06 0.9 84.26 4.8 67.96 1.3 52.26 1.4 71.96 1.2 68.16 3.1Sildent (5% surfactant C) 97.36 7.4 93.36 4.6 61.56 1.9 50.46 2.9 79.46 4.6 71.96 3.7Sildent (5% surfactant D) 94.16 7.4 80.56 5.1 60.76 1.6 42.26 1.2 77.46 2.0 61.46 3.9Sildent (10% surfactant A) 79.26 2.0 78.26 2.4 69.46 2.2 42.56 2.5 74.36 2.1 60.46 1.4Sildent (10% surfactant B) 84.86 2.5 77.96 4.7 67.76 1.3 59.96 0.8 74.66 1.9 68.96 2.8Sildent (10% surfactant C) 86.46 3.9 78.16 3.4 67.76 1.3 63.76 1.1 77.16 2.6 70.96 2.2Sildent (10% surfactant D) 80.36 3.5 82.46 3.3 67.96 2.1 62.26 2.7 74.16 2.8 71.86 2.9Sildent (20% surfactant A) 79.96 4.5 93.96 2.8 70.96 1.1 54.86 1.8 75.46 2.8 74.36 2.3Sildent (20% surfactant B) 78.66 3.1 94.46 3.9 71.76 1.1 56.86 0.6 75.26 2.1 75.66 2.3Sildent (20% surfactant C) 84.96 2.7 82.16 4.1 67.26 0.6 64.16 3.9 76.16 1.7 73.16 4.1Sildent (20% surfactant D) 78.46 3.3 84.56 6.2 69.16 1.2 55.96 3.1 73.76 2.3 70.26 2.1

TABLE II. Wettability of Surfactant Modified Formulations (Coated) in Comparison with Original Materials, Initial Values, andAfter Aging for 6 Months at 37°C (n 5 5)

Materials

Advancing Angle(Mean6 SD)

Receding Angle(Mean6 SD)

Equilibrium Angle(Mean6 SD)

Initial 6 Months Initial 6 Months Initial 6 Months

Molloplast-B 92.16 3.7 90.26 7.1 59.06 3.3 51.26 2.1 75.66 3.3 65.36 3.2Cosmesil 105.46 4 78.36 3.1 57.26 2.0 62.16 3.0 81.36 2.7 70.26 2.8Sildent 106.36 7.7 96.76 4.8 60.16 4.4 62.26 5.9 83.26 5.9 79.56 5.4Sildent (5% A coated) 85.46 4.0 93.36 2.5 70.56 0.6 54.76 0.9 77.96 2.3 74.06 1.7Sildent (5% B coated) 89.46 3.5 90.56 4.3 72.96 0.6 55.06 0.2 81.26 2.1 72.76 2.3Sildent (10% A coated) 77.66 2.4 85.36 3.9 73.16 1.2 52.86 2.6 75.46 1.8 69.16 3.3Sildent (10% B coated) 85.46 3.8 85.26 2.3 72.56 0.9 52.26 4.4 78.96 2.4 68.76 3.4Sildent (20% A coated) 77.46 2.3 90.96 2.6 71.96 1.2 61.16 2.6 74.66 1.8 76.06 2.6Sildent (20% B coated) 77.26 2.1 92.96 2.9 71.26 1.1 62.46 1.9 74.26 1.6 77.76 2.4

769IMPROVED WETTABILITY OF DENTURE SOFT LINING MATERIAL

Wettability measurements for the formulations containingsurfactants follow no clear trend and there is no direct rela-tionship between the amount of surfactant added and a re-duction in contact angles. Some general points can be made,however.

All the surfactant-charged formulations showed improvedwetting characteristics compared to Sildent and Cosmesilwithout surfactant. This would suggest that the modifiedmaterials would obtain a greater degree of lubrication be-tween soft liner and mucosa and, thus, greater patient com-fort.

In all cases, the equilibrium contact angle reduced after sixmonths in distilled water, suggesting that the surfactants wereretained within the matrix and did not leach out. It is as-sumed, therefore, that the siloxane portion of the surfactantreacts with the alkoxy silane cross-linker and the poly(di-methylsiloxane) polymer, retaining the surfactant chemicallywithin the matrix. The reduction in contact angle after 6months could be caused by water being absorbed into thematerials, thus providing additional adhesive forces with thewetting medium. Also, the reduction could be due to therelaxation of the flexible silicone chains and their consequentreorientation and alignment of the polar Si—O linkages ofboth the filler and polymeric matrix toward the aqueousinterface.10 The Si—O bond is 50% ionic, and polar interac-tions between these bonds and water provide the driving forcefor this orientation.15

Overall, surfactants A (Silwet L 7600) and B (Silwet L7607) were the most successful at improving wettability and,at the 5 and 10% levels, they produced formulations withcomparable wettability to Molloplast-B. As stated earlier, thesurfactants used in this study were nonhydrolyzable copoly-mers of a silicone and an alkylene oxide, where the PE groupmay be a long chain moiety giving a comb-like structure.Variations in the nature, ratio, and molecular weight of thesecomers determines the lypophile/hydrophile balance, whichis important for achieving a permanent improvement in wet-tability. Surfactants A and B have a high ethylene oxide

content (approaching 75%). Thus, they would be expected toinitially produce an improvement in wettability. Presumably,the fairly high viscosity of these surfactants coupled withtheir comb-like structures provides a good balance of prop-erties and the reaction with cross-linker and polymer matrixaccounts for the permanency.

A large contact angle hysteresis (advancing–receding an-gles) is seen for the unmodified material (Sildent), signifyinga large difference between the advancing and receding an-gles. The reason for the receding angle being lower than theadvancing angle results from differences in the adhesionbetween the probe liquid and the solid and the probe liquidand the liquid film. The hysteresis is greater, if the surfacebeing tested has pores and crevices capable of trapping theliquid as it advances. Thus, on receding, the surface includeswet areas that could result in a lower angle. Care was takenin the preparation of the samples to ensure that the surfaceswere as reproducible as possible. However, it must be as-sumed that there would be roughening of the surface, whichwould differ between samples. The relatively low standarddeviations seen for the contact angle measurements suggestthat the samples were adequately reproducible and, thus, it islikely that other factors have influenced the different hyster-esis between materials. As stated earlier, the functionalgroups at the surfaces of the polymers may reorient in orderto minimize surface-free energies in different environments,in this case water.16

Silicone rubbers have a high degree of chain mobility and,thus, it is not surprising that Sildent has a high contact-anglehysteresis. In general, introducing surfactants into the poly-meric matrix had the effect of reducing contact angle hyster-esis. The surfactants are, in effect, increasing surface energyand, thus, the normal hysteresis effect produced by reorien-tation of polar functional groups toward the aqueous phase isreduced. Another factor that could lead to hysteresis is pen-etration of the materials surface by the water molecules due totheir low molecular volume (18 mL gmol21). Water mole-cules can penetrate even well-packed surfaces, which would

Figure 4. Water uptake of Sildent with 10% surfactants A and B incorporated into the matrix andcoated on surface (n 5 5).

770 WATERS AND JAGGER

then swell and alter plate perimeter, thus introducing error tothe calculation of contact angle.

As feared, the water uptakes of the surfactant incorporatedformulations were unacceptably high for clinical usage, withvalues of approximately 20% at 2 months for 10% w/wsurfactants A and B (Fig. 4). The success of denture softlining materials is dependent on maintaining a permanentbond with the denture base. Water absorbing into the materialmay result in dimensional change, which may lead to stress atthe liner/denture base interface and reduce bond strength.Thus, any increase in water absorption through surfactantincorporation is detrimental to the clinical use of the material.In an attempt to reduce the high water absorption, a simplemethod was devised to incorporate the surfactants into asilicone sealant and simply paint it onto the cured rubber. Inthis way, only the surface was modified, with the bulk of thematerial still exhibiting hydrophobic properties. The wetta-bility was again improved, although not to the same extent asdirectly incorporating the surfactants, and there was a signif-icant reduction in the water uptake at 3 months (approximate-ly 10%; see Fig. 4). The water uptakes, however, were stillhigh compared to Sildent, because it was difficult to producea sufficiently thin coating. The water uptake was seen solelyin the coated layer causing swelling and de-bonding of thecoating from the silicone surface. This was confirmed bynoting that uncoated Sildent had negligible water uptake (Fig.4). A better approach may be to oxidize the surface of thesilicone rubber via plasma coating prior to coating withsurfactants. In this way, hydroxyl groups are created on thesurface of the silicone rubbers, which can react with thesiloxane portion of the surfactants, and which may impartmore permanent hydrophilic properties than plasma treatmentalone. This area will be the subject of future work.

Another factor that must be considered when modifyingsoft lining materials to become more hydrophilic is the pos-sible effect on micro-organism adhesion. Silicone soft liningmaterials accumulate the yeastCandida albicans,which isthe major etiological factor in chronic atrophic candidosis.Previous work has shown that the more hydrophobic thesilicone rubber, the less accumulation ofC. albicansandbacteria.9,17 Thus, by imparting hydrophilic properties toincrease patient comfort, the accumulation of yeast and bac-teria may increase.

CONCLUSIONS

Formulations have been produced with improved wettingcharacteristics maintained over a 6-month period. It would,therefore, be expected that these formulations would also be

wetted by saliva to a greater degree. This increased wettabil-ity may lead to a greater lubrication between the soft liner andthe mucosa, which in turn may lead to greater patient com-fort. High water uptake of the formulations has led to thedevelopment of a simple coating system, which reducedwater uptake. Further work is needed, however, to improvethe coating system and to establish the effect of surfactantincorporation on key mechanical properties and adhesion ofCandida albicans.

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771IMPROVED WETTABILITY OF DENTURE SOFT LINING MATERIAL