PEDIATRIC DENTISTRY/Copyright © 1982 byThe American Academy of PedodonticsNol. 4, No. 1 THEME

Composite and sealant resins -- past, present, andfuture

R. L. Bowen, DDS

AbstractComposite dental filling materiMs were developed in

response to the shortcomings of silicate cements andunfilied resins (based on .methyl methacrylate monomerand its polymer). A hybrid monomer, which came to beknown as "BIS-GMA" in the dental literature, wassynthesized; this molecule resembles an epoxy resinexcept that the epoxy groups are replaced bymethacrylate groups. BIS-GMA formulations canpolymerize rapidly under oral conch’tions, and they havepolymerization shrinkage less than that of methylmethacrylate. BIS-GMA resins are used as binders forglass, porcelain, or quartz particles to form relativelydurable direct esthetic filling materials. In combinationwith the acid-etch technique, developed elsewhere, BI~GMA formulations are used in the repair of fracturedincisor teetlz The combination is also useful to bindorthodontic brackets directly to teeth and for surgicalprocedures in which teeth are not properly placed orah’gned for eruption. This resin without filler is alsoused to prevent decay by the filling of developmentalpits and fissures in teeth which would otherwise have ahigh susceptibility to caries. Improvements in the glassfiller for composite resins may lead to greaterdurability in their chnical uses. Recent developments inadhesive bonding to teeth wili also widen the utility ofcomposites.

This is an informal essay that will give a broad

perspective and set the stage for more specific discus-sions in articles following.

Past HistoryComposite dental filling materials were developed

in response to the severe shortcomings of silicatecements.~ Silicate cement restorations were subject toacidic decays and were useful for only four to five yearson the average.3

Epoxy resins were being used in industrial applica-tions, and their intriguing properties suggested thatthey might have useful dental applications. 4 Theliquid epoxy resins could be mixed with a liquidhardener whereupon they would solidify at ordinarytemperature with very little hardening shrinkage;

they were very adhesive to most solid substances,and became physically strong and chemically inertpolymers. At the time, it seemed reasonable thatthese resins could be used as adhesive binders for par-ticles of porcelain, fused quartz, or other appropriateinorganic filler materials. It was thought that sucha mixture might be placed into a dental cavitypreparation where the maximally filled epoxy resinwould harden and adhesively bond the particlestogether and the "silica-resin" material to the cav-ity walls, thereby forming an esthetic, durablerestorative material. The major flaw in this schemewas that such materials did not harden quicklyenough for use as direct filling materials in dentistry.

This limitation called for the synthesis of a newmonomer which would resemble the epoxy resin soas to have relatively low hardening shrinkage, yet ex-hibit a rapid polymerization and hardening reaction.Methyl methacrylate polymerized rapidly and wasused, together with particles of its polymer as a filler,for direct dental fillings. The methyl methacrylatedirect filling resins were flawed primarily because oftheir large polymerization shrinkage, low stiffness,high coefficient of thermal expansion., and otherlesser faults.5.6

A hybrid monomer, "BIS-GMA," was thensynthesized.7 It was a large molecule that resembledan epoxy resin except that the epoxy ,groups werereplaced with methacrylate groups.8 Therefore, itcould polymerize rapidly under oral conditions, andyet its polymerization shrinkage was only about one-third as great as that of methyl methacrylate. Thisviscous liquid resin {BIS-GMA} could be used as abinder for glass, porcelain, or quartz particles to forma stiff, strong and relatively durable direct estheticfilling material. This resin came to be known as "BIS-GMA", an acronym that is more convenient than thelong chemical name of this molecule.

Dr. Michael Buonocore, Eastman Dental Center,at the University of Rochester School of Medicineand Dentistry, had discovered that acid etching ofdental enamel made its surface slightly rough and

10 COMPOSITE AND SEALANT RESINS: Bowen

porous -- thereby receptive to a micromechanicalbonding of polymerizable monomers.9 In his at-tempts to prevent decay from forming indevelopmental pits and fissures, he experimentedwith various resins as sealants. When BIS-GMAbecame available, he found it to work best.

Somewhat later, the acid-etching of enamel in andaround cavity preparations was found to bebeneficial.

Soon after Buonocore published his earlyfindings, 9,’° Regenos and other research-orientedclinicians ’’-’3 began using etching of enamel withphosphoric acid solutions as a means of attachmentand retention of direct filling resins. They discoveredthat fractured incisors could be repaired and restoredpromptly and esthetically without cutting the den-tin. Methods were also developed to use acid etchingand resins to bond orthodontic brackets directly toteeth, a procedure with advantages in many casesover the cementing of bands. Because the bondingto acid-etched enamel was effective with both the un-filled resins and composite materials containing in-organic reinforcing fillers, there was a gradual tran-sition toward the use of composites with most ofthese therapeutic procedures.

In surgical orthodontics, bonding to acid-etchedenamel allowed the more convenient and effective at-tachment of orthodontic buttons or pads with eyeletsto unerupted teeth that are not properly placed oraligned for normal eruption, or that have failed toerupt.

Present PracticesAlthough there is not uniform agreement about the

best agent for etching enamel for adhesive resin ap-plications, there are both experimentaP 5 andtheoreticalTM reasons for using phosphoric acid at a con-centration of about 30% as an etching agent. Thedentin surfaces should be protected from the applica-tion of an acid like this, because there is increasedpulp irritation -- not only from the acid treatmentitself, but also from subsequent response to com-posite restorations if a calcium hydroxide type ofliner is not used.

There is experimental evidence on both sides of thecontroversy regarding the use of an unfilled resinlayer applied to acid~tched enamel before the ap-plication of a composite material. For most photo-initiated systems, there is adequate free monomeravailable from the composite mixture to penetratethe small volume of the pores in the acid-etchedenamel surface. At the other extreme, with a very drymix of chemically activated composite (especially ifthe mixing and placement is not quick) it is quitepossible that the prior application of an unfilled resin,used sparingly, might give more reliable bonding.

There has been a trend, unfortunately, in many ofthe commercial composite formulations toward theuse of lower viscosity liquids and lower volumepercentages of reinforcing filler materials. This mayfacilitate mixing and placement by the use ofsyringes; however, the physical properties and valuesof the resulting restorations suffer from increasedhardening shrinkage, reduced stiffness, decreasedcolor stability, and other factors. Thick mixes (thatis, a minimum amount of slightly viscous monomerswith a maximum amount of inorganic reinforcingfiller) mixed thoroughly and placed immediately willgive the best results. The material hardens best ifprotected from air, which inhibits surface hardening.The maximum feasible time should be allowed forpolymerization before it is trimmed or finished.

After a composite restoration is finished to con-tour, it should be "etched" so as to remove debrisfrom surface air bubbles and to provide "extensionfor prevention" of the resin gIaze that is subsequentlyapplied. Currently it is difficult for the practitionerto apply a thin glaze or veneer of transparent and in-visible resin to etched enamel surfaces because of airinhibition of surface polymerization. If the inhibitedlayer is comparable in thickness to the desired glaze(polymerized layer), currently available materials aredifficult to use for this purpose; the glaze must beapplied as two layers, applied extra thickly, orcovered with something that will exclude at-mospheric oxygen during polymerization.

Composite materials were designed as a replace-ment for silicate cements, and not much thought wasgiven during their development as to their use inposterior restorations. The poor durability of com-posite materials on the occlusal surfaces of posteriorteeth contraindicates their use on the occlusal sur-faces of permanent dentition, unless esthetics is anoverriding consideration. In may cases occlusal com-posite restorations will remain in fairly good condi-tion for a couple of years, thereafter showing increas-ing loss of surface material. In the case of deciduousteeth, the question of the suitability of compositematerials for occlusal restorations involves a numberof variable factors including the estimated length oftime until the tooth is shed and replaced, the dura-bility of the particular composite material underthese conditions, esthetics, ease of placement, costof the materials, and other factors. Present practicesand associated rationale are more fully describedelsewhere in this journal.

Future PossibilitiesIt is justifiable to speak of future dental practice

because it takes many years for nascent technologyemerging from scientific research laboratories to bedeveloped and made commercially available for den-

PEDIATRIC DENTISTRY: Volume 4, Number 1 1 1

tists’ use. Therefore, some of the current researchsuccesses represent future improvements in patientcare.

The durability, repairability, and quality of the sur-face texture of composites are matters that arereceiving considerable research. The weakest link inthe physical integrity of composite materials is prob-ably the bonding between the organic resin and thesurfaces of the inorganic filler particles. Anythingwhich improves the strength and durability of thisinteraction could lead to restorations that arestronger and more durable.

One experimental approach toward getting a bet-

ter interaction between the organic resin and the in-organic reinforcement of a composite restorativematerial is the use of a three-dimensional glass fibernetwork.’7 This can be obtained by heating a cotton-like form of superfine glass fibers under pressure.This results in a sintering, or melting together of theglass fibers, where they contact one another, therebyforming a dense network with microscopic pores.This three-dimensional glass network can then bebroken into "particles" of suitable size. treated withan appropriate silane coupling agent, and combinedwith a hardenable liquid resin so as to form a com-posite restorative material. The hardening of theresin should form an interlocking composite of con-tinuous organic and inorganic phases; it is hoped thatthis will lead to improvements in properties such aspolymerization shrinkage, modulus of elasticity {stiff-ness}, and resistance to wear. It will be of con-siderable interest to follow the developments in thisresearch.

The development and use of "semiporous" glassfiller particles is a somewhat different experimentalapproach toward the objective of improving physicalproperties of composites by increasing the bondingbetween the inorganic and organic phases. In thiscase, only the surface of glass filler particles is madeporous. This is done by acid-etching the glass par-ticles lanalogous to the acid etching of enamel} to getsuperficial porosity into which the liquid resin canflow and polymerize. To obtain a glass that wouldhave the necessary X-ray opacity, the appropriaterefractive index, and would be capable of giving aporous surface when acid etched, a new kind of glasswas developed.’82° Experimental quantities of thisglass have been prepared by glass manufacturers,and samples have been made available to dentalmanufacturers for their appraisal research and prod-uct development.

The improvements in composite properties madeby one of these or other research activities willpossibly include improved surface texture of fin-ished composites with significantly greater durability

12 COMPOSITE AND SEALANT RESINS: Bowen

in the oral environment. An acid-etch treatment ofthe restoration surface as well as the adjacent enamelsurface could give interpenetrative bonding to a resinused as a glaze. This same mechanism may allow forbonding of repair increments.

The X-ray opacity of such restorations is expectedto be intermediate between that of dental amalgam,which is almost totally opaque, and quartz-filled com-posites which are practically radiolucent. With thisintermediate radiopacity, voids from trapped airunder restorations, air bubbles within the restora-tions that inevitably occur during the mxing andplacement of such materials, and secondary orunderlying decay associated with the restorationsbecome visible. Voids have been present in radiolu-cent and radiopaque restorations but; have goneundetected because of the extreme radiolucency ofthe one and the total radiopacity of the other.

To the extent that new composite material can bebonded to older restorations and to tooth surfaces,it might be unnecessary to remove all of a restora-tion to fill a void needing a remedy.

If the restorative materials continue to change forthe better, it is quite probable that the techniquesfor their proper use will also change. These changescan be expected to lead to modified cavity prepara-tions. For example, a classical cavity preparation fora gold foil restoration cannot be expected to be idealfor a future durable and adhesive composite resinmaterial.

One feature that will probably change the least isthe access form of the cavity. It will continue to benecessary to remove softened or discolored enamelor dentin, and access to do this will doubtless benecessary.

A novel method for discriminating between

carious dentin that should be removed and under-lying dentin that should be retained under a restora-tion is that of staining or dyeing the carious dentinwith a colored solution. For example, a 1% solutionof Acid Red in propylene glycol has been proposedby Fusayama2’ as an objective way of indicating thatpart of the carious layer of dentin that needs to beremoved.

The ideal retention form and resistance form ofprepared cavities will depend upon the degree andreliability of adhesive bonding between therestorative material and the hard tooth tissues, andon the physical properties of the completedrestoration.

For instance, it is currently recommended thatmost composite restorations utilize acid etching ofthe enamel and that the cavosurface should be abeveled, rounded, or chamfered configuration and nota ninety-degree angle. This extends the enamel sur-

face which, after acid etching, allows additional reten-tion and sealing of the restoration by penetration ofthe liquid resin into the microscopic pores etched in-to the ends of the enamel rods cut in making the bevelor chamfer. Since the tensile strength of the com-posite material is greater than the tensile strengthof human enamel,22 the tapering resin at the marginresists the stresses built up during hardeningshrinkage and increases subsequent margin integri-ty if there is good penetration of the resin into theetched enamel.

~-~I’rom an/n vitro study, there has been a research

report that the use of pyruvic acid might have cer-tain advantages over phosphoric acid as an etchantfor dental enamel.23 Although a number of studieshave been made comparing the relative merits of dif-ferent concentrations of phosphoric acid, there havebeen relatively few studies exploring the merits ofother kinds of acids {having various dissociation con-stants and pKa values} wherein the optimum concen-tration of each acid is determined. It would not besurprising if phosphoric acid were not only the onlyacid found suitable for the acid etching of enamel.

In acid etching enamel, the importance of washingwith clean water and the prevention of even theslightest contamination before, during, or after dry-ing with compressed air has not been adequately em-phasized either in the dental literature or in the in-structions supplied by dental manufacturers. Theterm "rinse," often used, is ambiguous in that it canbe interpreted to include the swishing of wateraround in the mouth by the patient after taking waterfrom a cup. Many sealants and acid-etched enamelapplications have failed because the washing of theacid-etched enamel surface was not done ade-quately and exclusively with clean water. Water-soluble crystals form on the surface while the acidis etching the enamel. These must be completelydissolved and removed during the washing step.

Furthermore, even the slightest trace of saliva,blood, or even tooth debris might negate the bonding.It was once thought that gross contamination withsaliva or other material, sufficient to clog the open-ings of the pores, was at fault. Now, it seems thateven invisible traces of saliva or other solublematerials can reduce bonding. This is probably dueto a lowering of the surface tension of the water onthe enamel when an attempt is made to dry it withcompressed air. Most semi-soluble materials, whichwould certainly include salivary solutes, traces ofblood, or other materials that might be found in themouth, can significantly lower the surface tension ofwater.

The mechanism by which the water in the poresof etched enamel is removed by an air stream has

just recently been explained by Asmussen andJ~rgensen.24 This emptying requires the high surfacetension of water, a small capillary {pore} diameter,and a wetting of the capillary walls by the water. Thediameters of the pores in acid-etched enamel are smallenough and water readily wets their surfaces; thedentist need not be concerned with these two factors.However, the surface tension of water can very easi-ly be lowered by contamination with any of a largenumber of things, and this is where the dentist’s at-tention needs to be focused. If the water is clean, thecompressed air stream over the surface will removethe superficial water and allow the automatic ejec-tion of the water, as vapor, from the depths of thepores. This happens because the surface tension insuch small pores lowers the pressure in the water somuch that it boils out.24

Resins that are sufficiently liquid to be pourableat room temperature will readily flow into these emp-tied pores by capillary action, but the resin cannotgo into the pores if the water has not come out ofthem first. Lowering the viscosity of the resin belowthat of a viscous but pourable liquid will undesirablyincrease its polymerization shrinkage.

Adhesive bonding to dentin is more difficult. Evenso, recent experimental research efforts give goodreason to predict that future clinical dentistry willbe able to utilize materials and methods for signifi-cant adhesive bonding to dentin as well as to enamel.

The scanning electron microscope has been

especially valuable to researchers in showing that thesurface of dentin that has been cut by rotary or handinstruments is covered by a smeared or disturbed sur-face layer. ~ Some workers removed this with thestrong acids used to etch enamel, but there isevidence that such treatment can cause pulp irrita-tion, especially if followed with a composite materialin the absence of a protective calcium hydroxideliner. ~ Others have found that the smearedlayer can beremoved without significant enlargement of the den-tinal tubular openings,27 or pulp irritation? ~ by thebrief application of isotonic concentrations of acidsof intermediate strength.

In the research laboratory, dentin surfaces can betreated with a mordant~ {a metallic salt solution}which chemically modifies the dentin surface andmakes it more receptive to adhesive materials. Othersolutions are then applied which lay down couplingagents~ that provide a basis for adhesion with a subse-quently applied composite resin. 31 Figures 1-3 showthe fractured surface of an adhesive bond to the den-tin of an extracted tooth. After storage in water fortwo days, breaking the bond required 1,910 poundsper square inch {13.2 MPa~ in tension.

It is quite possible that when these adhesive

PEDIATRIC DENTISTRY: Volume 4, Number 1 13

Figure 1. Scanning electromicrographof the dentin surface after breaking theadhesive bond. The bond strength was1,910 psi (13.2 MPa), and the fractureoccurred partly at the interface(adhesive failure) and partly throughthe composite resin (cohesive failure)leaving remnants of the compositematerial adhering to the dentin sur-face. Original magnification 47x.

Figure 2. Higher magnification of anarea of Figure 1. The circular featuresare regions in the composite contain-ing air bubbles when the material wasmixed and placed on the surface. Airbubbles represent sites of relativeweakness, and the fracture tends toleave the interface and involve thesevoids. Small depressions representingthe openings of dentinal tubules wherethe fracture involves the altered layerof surface dentin can be seen in otherareas. Original magnification 470x.

Figure 3. The surface in the upper partof this picture is the same fracturedsurface shown in Figures 1 and 2.However, in this case the dentinspecimen was fractured again toobserve the internal aspects of the den-tin underlying the adherent compositematerial. Filler particles and resinoverlie an altered dentin region con-taining enlarged and mostly-filled den-tinal tubule openings. With this bond-ing method, however, the dentinaltubules are neither enlarged nor filledwith resin "tags" to any significantdepth beneath the adhesive interface.Original magnification 600x.

materials become available to the dentist, thematerials, to be successful, will be accompanied bya very detailed description of the steps that must befollowed. The future might offer a tradeoff: a re-duced amount of cutting of sound enamel and den-tin (with the maintenance of greater strength and in-tegrity of the tooth crown due to a reduction, orpossibly an elimination, of classical retention formand resistance form); an elimination, of microleakageand marginal staining; and a lessened need foranesthetics to maintain patient comfort. Thesebenefits may well require precise adherence to exact-ing procedures in the application of the adhesiverestorative materials.

With the advent of effective adhesive bonding, itis quite possible that dentists can routinely applyesthetic or invisible protective coatings for entiretooth crowns as a logical extension of sealants andglazes. These can be used as protection against whitespots and smooth-surface carious lesions for newly-erupted (or perhaps not-so-newly-erupted) teeth. Suchthin veneers will doubtlessly wear away in areas sub-ject to mastication, brushing, interproximal contact,and vigorous flossing; these areas, however, are lessprone to the development of lesions. In regions thatare not self-cleansing (that is, tooth surfaces least ac-cessible to natural and artificial cleansing), cross-

linked polymeric coatings might remain intact forlong periods of time, protecting these areas from lowpH of plaque and nutrient stagnation. Preventivemeasures of this kind may not now be consideredcost-effective, at least as a public health measure;nonetheless, fluoridation, antibacterial dentrifices,and mouth rinses, improved levels of oral home care,and lessened busyness in dental practices may raisethe feasibility of some of these forms of dental treat-ment in many practices.

Composite and sealant resins will probably havea future. If research to improve these materials is notadequately supported, if dental manufacturers fail totransfer technology from the laboratory to themarketplace, if dental practitioners hold them in alow esteem (thinking them to be a quick and easy wayto provide second class dentistry), then the future ofcomposites and sealants may be dim — if not dismal.The more probable course is that of continued ad-vancements in the scientific laboratories, ready ac-ceptance and promotion by manufacturers, andcareful use by conscientious dentists who constitutethe majority of our colleagues. The placement of thebest possible composite restoration (including the useof materials yet to be developed) might be as demand-ing and give as much cause for pride, because of themerits of its serviceability, as have been the place-

14 COMPOSITE AND SEALANT RESINS: Bowen

ment of top-rate gold foil, porcelain inlay, silver

amalgam, or other traditional restorations.Future improvements in the level of oral hygiene,

the use of fluorides, antiseptics, sealants, and pro-tective coatings to prevent decay, together with im-provements in composites to repair traumaticallydamaged or malformed teeth, can lead to bettergeneral health because of better oral health.

Dr. Bowen is associate director of the American Dental Associa-tion Health Foundation, National Bureau of Standards. Reprintrequests should be sent to him at: National Bureau of Standards,Washington, D.C. 20234.

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13.Doyle, W. A. Acid etching in pedodontics, in Dent Clinics ofNorth America. Philadelphia, W. B. Saunders Co., 1973, pp93-104.

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17.Ehrnford, L. A method for reinforcing dental compositerestorative materials. Odont Revy 27:51, 1976.

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21.Fusayama, T. New Concepts in Operative Dentistry. Chicago,Quintessence Pub. Co., Inc., 1980, pp 45-59.

22.Bowen, R. L. and Rodriguez, M. S. Tensile strength andmodulus of elasticity of tooth structure and several restorativematerials. JADA 64:378, 1962.

23.Oshawa, T. Studies on solubility and adhesion of the enamelin pretreatment for caries preventive sealing. Bull Tokyo DentCol 13:65, 1972.

24.Asmussen, E. and J~rgensen, D. K. The stability of water inthe pores of acid etched human enamel. Acta Odont Scand36:43, 1978.

25.Eick, J. D., Wilke, R. A., Anderson, C. H., and Sorensen, S.E. Scanning electron microscopy of cut tooth surfaces and iden-tification of debris by use of the electron microprobe. J DentRes 49:1359, 1970.

26.Aida, S., Matsui, K., Hirai, Y., and Ishikawa, T. A clinico-pathological study of pulpal reaction to acid etching withphosphoric acid solution at various concentrations. Bull TokyoDent Coll 21:163, 1980.

27.Bowen, R. L. Adhesive bonding of various materials to hardtooth tissues XIX. Solubility of dentinal smear layer in diluteacid buffers. Intl Dent J 28:97, June 1978.

28.MjSr, I. A. Henste~Pettersen, A., and Bowen, R. L. Biologicalassessments of experimental cavity cleaners. (Manuscript inpreparation.)

29.Bowen, R. L. Adhesive bonding of various materials to hardtooth tissues VII. Metal salts as mordants for coupling agents.in Dental Adhesive Materials, Moskowitz, H. D., Ward, G. T.,and Woolridge, E. D. (eds}. New York City, Prestige GraphicServices, 1974, pp 205-221.

30.Misra, D. N. and Bowen, R. L. Adhesive bonding of variousmaterials to hard tooth tissue XII. Adsorption ofN-(2-hydroxy-3-methacryloxpropyl)-N-phenylglycine INPG-GMA} on hydroxyapatite. J Coll Interface Sci 61:14, 1977.

31.Bowen, R. L., Cobb, E. N., and Rapson, J. E. Adhesive bond-ing of various materials to hard tooth tissues XXV. Improve-ment in bond strength to dentin. (Manuscript in preparation.}

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