Commercially Available Fluoride-Releasing Restorative Materials: a Review and a Proposal for Classiﬁcation

materials

Review Commercially Available Fluoride-Releasing Restorative Materials: A Review and a Proposal for Classiﬁcation

Philippe Francois 1,2, Vincent Fouquet 1,3, Jean-Pierre Attal 1,4 and Elisabeth Dursun 1,5,*

1 Innovative Dental Materials and Interfaces Research Unit (URB2i, UR4462), Faculty of Health, Paris University, 1 rue Maurice Arnoux, 92120 Montrouge, France; [email protected] (P.F.); [email protected] (V.F.); [email protected] (J.-P.A.) 2 Bretonneau Hospital, 23 rue Joseph de Maistre, 75018 Paris, France 3 Louis Mourier Hospital, 178 rue des Renouillers, 92700 Colombes, France 4 Charles Foix Hospital, 7 avenue de la République, 94200 Ivry-sur-Seine, France 5 Henri Mondor Hospital, 1 rue Gustave Eiﬀel, 94000 Créteil, France * Correspondence: [email protected]; Tel.: +33-1-58-07-67-25

Received: 4 April 2020; Accepted: 11 May 2020; Published: 18 May 2020

Abstract: Resin composite and glass ionomer cement (GIC) are the most commonly used dental materials to perform direct restorations. Both have specific characteristics that explain their popularity and their limits. More than 20 years ago, the first attempt (followed by others) to combine the advantages of these two families was performed with compomers, but it was not very successful. Recently, new formulations (also called ‘smart materials’) with claimed ion release properties have been proposed under different family names, but there are few studies on them and explanations of their chemistries. This comprehensive review aims to gather the compositions; the setting reactions; the mechanical, self-adhesive, and potential bulk-fill properties; and the ion release abilities of the large existing families of fluoride-releasing restorative materials and the new restorative materials to precisely describe their characteristics, their eventual bioactivities, and classify them for an improved understanding of these materials. Based on this work, the whole GIC family, including resin-modified and highly viscous formulations, was found to be bioactive. Cention N (Ivoclar Vivadent, AG, Schaan, Lietschentein) is the first commercially available bioactive resin composite.

Keywords: GIC; RM-GIC; HV-GIC; compomer; giomer; nanoionomer; Activa BioActive Restorative; Sureﬁl One; Cention N; bioactive materials

1. Introduction A trend regarding the development of new hybrid restorative materials that combine resin composites and glass ionomer cements (GICs) has been observed not only in the scientiﬁc literature but also in the dental industry. The approach involves combining the advantageous properties of adhesive–resin couple composites (mechanical strength, esthetics, and high bond strength) and GICs (self-adhesive properties, moisture tolerance, and ion release) [1]. This almost chimeric intention already resulted in the resinous and still self-adhesive variant of conventional GICs, namely resin-modiﬁed glass ionomer cements (RM-GICs), as well as ion releasing but still non-adhesive variants of resin composites, namely compomers and giomers [2], approximately 20 years ago. RM-GICs have been extensively tested in recent research and showed both good mechanical characteristics [3] and acceptable bond strength values [4]. These hybrid products, with their well-studied and well-described chemistry, have achieved variable clinical success, but their use has decreased in daily practice due to their limitations compared

Materials 2020, 13, 2313; doi:10.3390/ma13102313 www.mdpi.com/journal/materials MaterialsMaterials 20202020, ,1313, ,x 2313 FOR PEER REVIEW 2 2of of 29 28 compared to the performance of adhesive-composite resin couples and recent generation of high- to the performance of adhesive-composite resin couples and recent generation of high-viscosity glass viscosity glass ionomer cements (HV-GICs) [5]. ionomer cements (HV-GICs) [5]. Recently, three new hybrid composites that claim to release fluoride ions were introduced to the Recently, three new hybrid composites that claim to release fluoride ions were introduced to market: Activa BioActive Restorative (Pulpdent Corporation, Watertown, MA, USA), Cention N the market: Activa BioActive Restorative (Pulpdent Corporation, Watertown, MA, USA), Cention N (Ivoclar-Vivadent, AG, Schaan, Liechtenstein), and Surefil One (Dentsply-Sirona, Konstanz, (Ivoclar-Vivadent, AG, Schaan, Liechtenstein), and Surefil One (Dentsply-Sirona, Konstanz, Germany). Germany). Some of these materials, like those mentioned above, are sometimes called bioactive due Some of these materials, like those mentioned above, are sometimes called bioactive due to their ionic to their ionic release, although the use of this term is controversial. It seems that only a “restorative release, although the use of this term is controversial. It seems that only a “restorative material capable material capable of inducing bio-remineralization by means of a sufficiently significant ionic release” of inducing bio-remineralization by means of a sufficiently significant ionic release” can be considered can be considered bioactive [6]. bioactive [6]. The aim of this article is to provide a review of the chemical reactions responsible for the initial The aim of this article is to provide a review of the chemical reactions responsible for the initial setting and ion release of these new formulations. To better understand their chemical and physical setting and ion release of these new formulations. To better understand their chemical and physical properties, the release capacities and indications of the different generations of GICs, compomers, properties, the release capacities and indications of the different generations of GICs, compomers, and giomers are described, leading to a classification proposal based on the composition and setting and giomers are described, leading to a classification proposal based on the composition and setting reaction of these new materials. reaction of these new materials. For the sake of simplification, reactive fillers (i.e., those activated by acids), whose compositions For the sake of simplification, reactive fillers (i.e., those activated by acids), whose compositions vary according to the manufacturer, are called fluoro-alumino-silicate (FAS) fillers. These different vary according to the manufacturer, are called fluoro-alumino-silicate (FAS) fillers. These different compositions likely produce different ion release behaviors. In the proposed diagrams, only the compositions likely produce different ion release behaviors. In the proposed diagrams, only the releases of calcium, aluminum, and fluoride ions are represented. Figure 1 illustrates the legend that releases of calcium, aluminum, and fluoride ions are represented. Figure1 illustrates the legend that contains a proposal to describe the chemistry of all the materials discussed in this review. contains a proposal to describe the chemistry of all the materials discussed in this review.

Figure 1. Legends for the illustrations presented in the following ﬁgures. Figure 1. Legends for the illustrations presented in the following figures. 2. Conventional GICs and Their Evolution: HV-GICs and RM-GICs 2. Conventional GICs and Their Evolution: HV-GICs and RM-GICs From a semantic point of view and in relation to ISO standards, GICs should be called polyalkenoate cementsFrom [7 ].a However,semantic thepoint term of GICview that and was in used relation by inventors to ISO [8standards,] is commonly GICs used should and acceptedbe called in polyalkenoatethe scientiﬁc literature cements [7]. [9]. However, the term GIC that was used by inventors [8] is commonly used and accepted in the scientific literature [9]. 2.1. Conventional GICs

2.1.1. Composition and Chemical Reactions Conventional GICs, meaning those in their original classical form, are produced by an acid–base reaction from a powder–liquid mixture [8]. These are complex materials [10] with widely varying compositions from one formulation to another [11].

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2.1. Conventional GICs

2.1.1. Composition and Chemical Reactions Conventional GICs, meaning those in their original classical form, are produced by an acid–base reactionMaterials 2020 from, 13 ,a 2313 powder–liquid mixture [8]. These are complex materials [10] with widely varying3 of 28 compositions from one formulation to another [11]. The liquid,liquid, in in addition addition to containingto containing water, water, consists consists mainly mainly of polyacrylic of polyacrylic acid [8]. Other acid polyacids—[8]. Other polyacids—suchsuch as tartaric, itaconic, as tartaric, maleic, itaconic, or tricarballylic maleic, acid—mayor tricarballylic also be addedacid—may or even also replaced be added by polyacrylic or even replacedacid to modulate by polyacrylic the reaction acid or to the modulate rheological the properties reaction or of thethe materialrheological [12,13 properties]. Schematically, of the thematerial liquid [12,13].is an aqueous Schematically, solution containingthe liquid is a polyacrylican aqueous acid solution with numerouscontaining ionized a polyacrylic carboxyl acid functional with numerous –COOH ionizedgroups, whichcarboxyl are functional therefore in–COOH the form groups, of COO–. which However, are therefore some formulations in the form placeof COO–. the acid However, groups somein lyophilized formulations form place directly the acid into thegroups powder. in lyophilized These groups form aredirectly secondarily into the activatedpowder. These by the groups liquid, arewhich secondarily almost exclusively activated by contains the liquid, water which [14]. almost exclusively contains water [14]. The powder contains basic reactive FAS fillers.fillers. However, However, this this name name is is simplistic simplistic because, because, depending on on the the composition chosen chosen by by the the manu manufacturer,facturer, other other elements elements may may be be incorporated, incorporated, such as strontium, strontium, phosphate, zinc zinc,, calcium, or sodium [11,15,16]. [11,15,16]. In In addition addition to to playing playing a a major major role role inin the acid–base reaction reaction that that sets sets the the material, material, these these FAS FAS fillers fillers play play a a role in the final final mechanical properties of the material. To To preserve preserve their their reac reactivitytivity and and given given the the absence absence of of a a resin resin in in their formulation,formulation, the FAS fillersfillers used in conventional GICsGICs onon thethe marketmarket areare non-silanated.non-silanated. Two commercial forms exist for these materials: a a manual manual powder–liquid powder–liquid mixture mixture or a predosed predosed capsule that needs to be mechanically vibrated. Fi Figuregure 22 explainsexplains thethe locationslocations ofof thethe compartmentscompartments (powder or liquid) of the material.

Figure 2. Conventional GIC in its storage medium. The powder contains FAS fillers that are not Figuresilanated, 2. Conventional whereas the liquid GIC containsin its storage water medium. and ionized The polyacrylic powder contains acids. FAS fillers that are not silanated, whereas the liquid contains water and ionized polyacrylic acids. During the mixing of the powder and liquid, the acid–base reaction begins and consists of an attack of theDuring FAS fillers the mixing by the polyacrylicof the powder acid and followed liquid, by the an ionacid–base release reaction according begins to their and composition consists of [ 17an]. attackThis release of the of FAS calcium fillers and by aluminum the polyacrylic ions in acid particular followed initiates by an a gellingion release reaction according via ionic to bonds their compositionwith the ionized [17]. carboxylThis release ions of of calcium the polyacrylic and aluminum acids. Polyacrylateions in particular salts, initiates first of calciuma gellingand reaction then viaof aluminum,ionic bonds are with thus the formed ionized in carboxyl the course ions of of the the reaction polyacrylic [18]. acids. During Polyacrylate the first few salts, minutes first of calciumthe reaction, and then the materialof aluminum, remains are verythus formed sensitive in to the a possiblecourse of alteration the reaction in [18]. its water During balance the first [19 few,20]. minutesIn addition, of the a silicicreaction, gel the resulting material from remains the acid–base very sens reactionitive to a is possible formed alteration on the surface in its of water the partially balance [19,20].reacted In FAS addition, fillers, allowing a silicic gel them resulting to be fixed from well the toacid–base the matrix reaction and protecting is formed them on the from surface hydrolysis of the partiallyby considerably reacted increasingFAS fillers, the allowing insolubility them ofto thebe fixed cement; well this to reactionthe matrix is essentialand protecting to the them stability from of hydrolysisthe GIC [14 by,21 considerably] (Figure3). This increasing resistance the toinsolubility hydrolysis of is the very cement; important, this reaction as this materialis essential contains to the stabilitybetween of 20% the and GIC 25% [14,21] water (Figure in its 3). final This structure resistance [22 ].to hydrolysis is very important, as this material containsThis between setting reaction20% and is25% very water slow. in its Despite final structure a relatively [22]. short working time of a few minutes, acceptable mechanical properties need more time to be reached and are largely established after 24 h. The setting reaction continues over several weeks or even months; this is called the maturation process [14,23–25]. Due to their purely acid–base reaction, these materials have a true bulk-fill setting reaction. Materials 2020, 13, 2313 4 of 28

Materials 2020, 13, x FOR PEER REVIEW 4 of 29

Figure 3. Ion release processes from a conventional GIC once it is in a moist environment (i.e., an oral Figure 3. Ion release processes from a conventional GIC once it is in a moist environment (i.e., an oral environment). When the powder and liquid are mixed, the acid–base reaction is initiated, the setting environment).of the material When begins, the powderand the FAS and fillers liquid are are partially mixed, attacked. the acid–base A silicic reactiongel is partially is initiated, formed theon the setting of theFAS material filler surface. begins, The and released the FAS calcium fillers and are partiallyaluminum attacked. ions are able A silicic to form gel ionic is partially bonds with formed the on the FASionized filler carboxylic surface. Thegroups. released Fluoride calcium ions are and also aluminum released. In ions water, are calcium, able to aluminum, form ionic and bonds fluoride with the ionizedions carboxylic (and eventually groups. other Fluoride ions) are ions able are to be also exchanged released. with In water, the oral calcium, environment. aluminum, and fluoride ions (and eventually other ions) are able to be exchanged with the oral environment. This setting reaction is very slow. Despite a relatively short working time of a few minutes, 2.1.2.acceptable Mechanical mechanical Properties, properties Ion Release need more in the time Oral to Environment, be reached and and are Indicationslargely established as a Definitive after 24 Restorativeh. The Materialsetting reaction continues over several weeks or even months; this is called the maturation Theseprocess conventional [14,23–25]. GICs have low flexural values [26] and a high propensity to wear [27]. Due to their purely acid–base reaction, these materials have a true bulk-fill setting reaction. The ion release of GICs in the oral environment varies depending on the composition of the FAS fillers.2.1.2. Mechanical However, Properties, fluoride, Ion aluminum, Release in the and Oral calcium Environment, ions are and considered Indications toas a be Definitive the main ions releasedRestorative [17,25] (FigureMaterial3 ). In the majority of studies, conventional GICs are the materials that release the highest level of fluoride among the studied materials [28]. An ion release peak is observed in the These conventional GICs have low flexural values [26] and a high propensity to wear [27]. first hoursThe of ion material release placementof GICs in the [29 oral–34 environment]. This release varies tends depending to decrease on the overcomposition time until of the it FAS reaches a plateau.fillers. ThisHowever, material fluoride, is capable aluminum, of recharging and calcium itself, ions are especially considered with to be fluoride the main ions ions from released the oral environment[17,25] (Figure [28,34 ,3).35 ].In This the majority ion release of studies, has been conventional shown to induce GICs are remineralization the materials that of therelease underlying the hard dentalhighest tissueslevel of fluoride in many amongin vitro thestudies studied [materials36,37]. Therefore, [28]. An ion conventional release peak is GICs observed can bein the considered first bioactivehours restorative of material materials.placement [29–34]. This release tends to decrease over time until it reaches a plateau. Today,This material these conventionalis capable of recharging GICs are itself, virtually especially no longer with fluoride used for ions definitive from the restorations.oral environment The last indication,[28,34,35]. with This a satisfactory ion release has success been rate,shown is to prophylactic induce remineralization fissure sealing of the in underlying pediatric dentistry hard dental [38 ,39]. tissues in many in vitro studies [36,37]. Therefore, conventional GICs can be considered bioactive 2.2. High-Viscosityrestorative materials. Glass Ionomer Cements (HV-GICs) Today, these conventional GICs are virtually no longer used for definitive restorations. The last 2.2.1.indication, Composition with and a satisfactory Chemical Reactionssuccess rate, is prophylactic fissure sealing in pediatric dentistry [38,39]. Although the term HV-GIC is widely used in international publications, some authors continue to place2.2. theseHigh-Viscosity products Glass in Ionomer the category Cements of (HV-GICs) conventional GICs [40–42]. These HV-GICs incorporate small FAS fillers in addition to others to increase the speed of the reaction. The latter benefit from a surface2.2.1. treatment Composition (patented), and Chemical which increasesReactions their reactivity. The powder/liquid ratio increases [43–49], as does theAlthough molecular the weightterm HV-GIC of polyacrylic is widely used acid in [50 international]. publications, some authors continue Twoto place commercial these products forms in exist: the category manual of powder–liquid conventional GICs mixtures [40–42]. or predosedThese HV-GICs capsules incorporate that need to be mechanically vibrated. Figure4 explains the locations (powder or liquid) of the main components of the material. Materials 2020, 13, x FOR PEER REVIEW 5 of 29 Materials 2020, 13, x FOR PEER REVIEW 5 of 29 small FAS fillers in addition to others to increase the speed of the reaction. The latter benefit from a smallsurface FAS treatment fillers in (patented), addition to which others increases to increase their the reactivity. speed of theThe reaction. powder/liquid The latter ratio benefit increases from [43– a surface49], as treatmentdoes the molecular (patented), weight which of increases polyacrylic their acid reactivity. [50]. The powder/liquid ratio increases [43– 49], asTwo does commercial the molecular forms weight exist: of manualpolyacrylic powder–liqui acid [50]. d mixtures or predosed capsules that need to beTwo mechanically commercial vibrated.forms exist: Figure manual 4 explainspowder–liqui the dlocations mixtures (powder or predosed or liquid) capsules of thatthe needmain toMaterialscomponents be mechanically2020, 13 of, 2313 the material.vibrated. Figure 4 explains the locations (powder or liquid) of the main5 of 28 components of the material.

Figure 4. HV-GIC in its storage medium. The powder contains nonsilanated FAS fillers in which very Figure 4. HV-GIC in its storage medium. The powder contains nonsilanated FAS ﬁllers in which very small FAS fillers are added to speed up the reaction and increase the powder/liquid ratio. The liquid Figuresmall FAS4. HV-GIC ﬁllers arein its added storage to speedmedium. up theThe reaction powder andcont increaseains nonsilanated the powder FAS/liquid fillers ratio. in which The liquidvery contains water and ionized polyacrylic acids. smallcontains FAS water fillers and are ionizedadded to polyacrylic speed up acids.the reaction and increase the powder/liquid ratio. The liquid contains water and ionized polyacrylic acids. WhenWhen mixingmixing thethe liquidliquid powder,powder, thethe stepssteps andand settingsetting characteristicscharacteristics areare thethe samesame asas thosethose previouslypreviouslyWhen describedmixing described the for for liquid conventional conventional powder, GICs GICsthe (Figure steps (Figure5 ).and They 5) setting. They also maintain alsocharacteristics maintain a true ‘bulk-fill’a aretrue the ‘bulk-fill’ reaction.same as reaction. Duethose to previouslychemicalDue to chemical innovations, described innovations, for the conventional initial the setting initial GICs time setting is(Figure reduced time 5) is. to Theyreduced limit also the to maintain water limitbalance the a watertrue sensitivity ‘bulk-fill’ balance [44sensitivity reaction.,49]. Due[44,49]. to chemical innovations, the initial setting time is reduced to limit the water balance sensitivity [44,49].

FigureFigure 5.5. Ion release from an HV-GIC onceonce itit isis inin aa moistmoist environmentenvironment (i.e., an oral environment). FigureTheThe mechanismmechanism 5. Ion release ofof thethefrom reactionreaction an HV-GIC andand ionion once exchangeexchange it is in a withwi moistth thethe environment oraloral environmentenvironment (i.e., an areare oral thethe environment). samesame asas thatthat Thepreviouslypreviously mechanism describeddescribed of the forfor reaction conventionalconventional and ion GIC.GIC. exchange with the oral environment are the same as that previously described for conventional GIC. InIn addition,addition, the the use use of theseof these materials materials is associated is associated with awith photopolymerizable a photopolymerizable varnish varnish that isolates that themisolatesIn from addition, them the from oral the the environment use oral of environment these during materials during the initialis asso the stagesciatedinitial ofstageswith the a settingof photopolymerizable the setting reaction, reaction, thus reducing thusvarnish reducing theirthat isolatessensitivitytheir sensitivity them to from alterations to the alterations oral in environment the in water the water balance during balance [51 the,52 in[51,52].].itial stages of the setting reaction, thus reducing their sensitivity to alterations in the water balance [51,52]. 2.2.2. Mechanical Properties, Ion Release in the Oral Environment, and Indications as a Definitive Restorative Material All these modifications substantially increase the flexural strength of the material and thus reduce the risk of cohesive fracture of the material, which is reported to be the major cause of failure of conventional GICs [53–55]. Their wear resistance is now clinically acceptable [56]. Materials 2020, 13, x FOR PEER REVIEW 6 of 29

2.2.2. Mechanical Properties, Ion Release in the Oral Environment, and Indications as a Definitive Restorative Material All these modifications substantially increase the flexural strength of the material and thus reduce the risk of cohesive fracture of the material, which is reported to be the major cause of failure ofMaterials conventional2020, 13, 2313GICs [53–55]. Their wear resistance is now clinically acceptable [56]. 6 of 28 The release and recharge mechanisms of HV-GICs are similar to those described for conventional GICs (Figure 5), including a release peak at placement. However, HV-GICs generally The release and recharge mechanisms of HV-GICs are similar to those described for conventional appear to release less fluoride than their precursors, conventional GICs [28,57]. This ion release has GICs (Figure5), including a release peak at placement. However, HV-GICs generally appear to release been shown to induce remineralization of the underlying hard dental tissues in many in vitro studies less fluoride than their precursors, conventional GICs [28,57]. This ion release has been shown to [18,36,58–60]. Therefore, HV-GICs can be considered bioactive restorative materials. induce remineralization of the underlying hard dental tissues in many in vitro studies [18,36,58–60]. Compared with conventional GICs, these materials have wider indications for use in restoration. Therefore, HV-GICs can be considered bioactive restorative materials. Thus, they can be successfully used as restorative materials for Class I and Class II limited occlusal Compared with conventional GICs, these materials have wider indications for use in restoration. restorations in adults [61], cervical restorations [62], in children, or as an intermediate base in the Thus, they can be successfully used as restorative materials for Class I and Class II limited occlusal sandwich technique [63]. restorations in adults [61], cervical restorations [62], in children, or as an intermediate base in the sandwich technique [63]. 2.3. Resin-Modified Glass Ionomer Cements (RM-GICs) 2.3. Resin-Modified Glass Ionomer Cements (RM-GICs) 2.3.1. Composition and Chemical Reactions 2.3.1.RM-GICs Composition retain and the Chemicalsame acid–base Reactions reaction as GICs combined with a radical polymerization reactionRM-GICs of methacrylate retain the monomers same acid–base [64]. Monomers reaction asare GICs added combined to the liquid with (typically a radical 2-hydroxyethyl polymerization methacrylatereaction of methacrylate (HEMA)), as monomers well as photoinitiators [64]. Monomers such are addedas camphorquinone. to the liquid (typically The purpose 2-hydroxyethyl of resin additionmethacrylate is to (HEMA)),decrease the as wellsetting as photoinitiatorstime, enhance suchthe asmechanical camphorquinone. properties, The and purpose decrease of resinthe sensitivityaddition is of to the decrease material the to setting early aqueous time, enhance or salivary the mechanical contamination properties, compared anddecrease to conventional the sensitivity GICs [28].of the material to early aqueous or salivary contamination compared to conventional GICs [28]. TheThe fillers fillers are are silanated silanated FASs FASs [65–67]. [65–67]. This This silanization silanization treatment treatment of of reactive reactive fillers fillers has has not not been been adequatelyadequately communicated communicated by by manufa manufacturers,cturers, which which theoretically theoretically allows allows the the binding binding of of these these FAS FAS particlesparticles to to the the resin resin matrix, matrix, further further increasing increasing the the cross-linking cross-linking of the of theresin resin network, network, improving improving the finalthe finalmechanical mechanical properties properties of the of material the material and modu andlating modulating the solubilization the solubilization of the reactive of the reactive fillers [68–71].fillers [ 68However,–71]. However, the stability the stability of the ofsilanization the silanization of FAS of fillers FAS fillers and its and impact its impact on ion on release, ion release, in parallelin parallel with with the theacid–base acid–base reaction, reaction, is unclear is unclear and and has has not not been been studied. studied. TwoTwo commercial commercial forms forms of of these these materials materials are are ava available:ilable: manual manual powder–l powder–liquidiquid mixtures mixtures or or the the predosedpredosed capsules capsules to to be be mechanica mechanicallylly vibrated. vibrated. Figure Figure 66 explainsexplains th thee locations locations of the compartments (powder(powder or or liquid) liquid) of of the the main main components components of of the the material. material.

Figure 6. An RM-GIC in its storage medium. The powder, in contrast with GICs and HV-GICs, contains Figure 6. An RM-GIC in its storage medium. The powder, in contrast with GICs and HV-GICs, silanated FAS ﬁllers; the liquid contains the same components as those in the GIC liquid but with contains silanated FAS fillers; the liquid contains the same components as those in the GIC liquid but HEMA monomers added to the formulation. with HEMA monomers added to the formulation. When mixing a powder and liquid, two reactions occur: an acid–base reaction, similar to that of a conventionalWhen mixing GIC, a and powder a polymerization and liquid, two reaction reactions triggered occur: by an light acid–base activation reaction, (Figure similar7). The to settingthat of aof conventional this material GIC, can and be describeda polymerization as a dual reaction setting tr process.iggered by However, light activation these materials (Figure 7). have The a setting radical resin polymerization reaction that is activated only by light energy; therefore, to ensure a true dual setting throughout the thickness of these materials, they must be laminated in 2 mm layers. These materials do not allow a true bulk-ﬁll reaction due to the absence of chemical initiators in the resin polymerization process. Materials 2020, 13, x FOR PEER REVIEW 7 of 29 of this material can be described as a dual setting process. However, these materials have a radical resin polymerization reaction that is activated only by light energy; therefore, to ensure a true dual setting throughout the thickness of these materials, they must be laminated in 2 mm layers. These Materialsmaterials2020 do, 13 not, 2313 allow a true bulk-fill reaction due to the absence of chemical initiators in the 7resin of 28 polymerization process.

Figure 7. IonIon release release from from an an RM-GIC once once it it is is in a moist environment (i.e., an oral environment). When the powder and liquid are mixed, the acid–base reactionreaction is initiated, the setting of the material begins, andand thethe FAS FAS fillers fillers are are partially partially attacked: attacked: a silicic a silicic gel isgel partially is partially formed formed on the on FAS the filler FAS surface. filler Thesurface. released The released calcium calcium and aluminum and aluminum ions are ions able are to form able ionicto form bonds ionic with bonds ionized with ionized carboxylic carboxylic groups. Fluoridegroups. Fluoride ions are alsoions released. are also Areleased. second reactionA second of reaction resin polymerization of resin polymerization is activated is when activated the material when isthe light-cured: material is light-cured: monomers can monomers copolymerize can copolymeri with otherze monomers with other ormonomers silanated or FAS silanated fillers. FAS At the fillers. end ofAt thethe reaction,end of the two reaction, different two interpenetrated different interpenet networksrated are networks produced arewithout produced covalent without or covalent ionic links or betweenionic links both. between In water, both. calcium, In water, aluminum, calcium, alumin and fluorideum, and ions fluoride (and eventually ions (and eventually other ions) other are able ions) to beareexchanged able to be exchanged with the oral with environment. the oral environment.

Even ifif these these two two reactions reactions coexist, coexist, they compete they co withmpete each with other. each As soonother. as resinAs polymerizationsoon as resin ispolymerization initiated, the acid–baseis initiated, reaction the acid–base is limited reaction [72–74 ].is Thelimited polymerization [72–74]. The reaction polymerization is also impacted reaction byis thealso acid–base impacted reaction by the [acid–base75], and the reaction unreacted [75], HEMA and the monomers unreacted cause HEMA greater monomers water absorption cause greater than thatwater of absorption HV-GIC due than to theirthat hydrophilicityof HV-GIC due [76 to]. Thistheir e ﬀhydrophilicityect increases the [76]. sensitivity This effect of the increases material the to hydrolysissensitivity of [77 the]. material to hydrolysis [77].

2.3.2. Mechanical Properties, Ion Release in the Oral Environment, and Indications as a DefinitiveDefinitive Restorative Material The mechanical properties of RM RM-GICs,-GICs, especially in flexion, flexion, are increased compared with those of conventional GICs and HV-GICs [[26,78].26,78]. However, their resistance to wear in areas of mechanical stress remains low [[27].27]. The releaserelease and and recharge recharge mechanisms mechanisms of RM-GICs of RM-GICs are similar are tosimilar those describedto those for described conventional for GICs,conventional in particular, GICs, exhibitingin particular, a release exhibiting peak when a release they arepeak installed when (Figurethey are7). installed However, (Figure RM-GICs 7). appearHowever, to RM-GICs release less appear fluoride to release than resin-freeless fluoride GICs, than conventional resin-free GICs, GICs, conventional and HV-GICs GICs, [ 28and,57 HV-,79]. TheGICs polymerized [28,57,79]. The resin polymerized matrix limits resin ionmatrix exchange limits ion with exchange the external with the environment external environment [57]. This [57]. ion releaseThis ion has release been has shown been to shown induce to remineralization induce remineralization of underlying of underlying hard dental hard tissues dental in tissues many inin many vitro studiesin vitro [ 37studies,80–82 ].[37,80–82]. Therefore, Therefore, former RM-GICs former can RM-GICs be considered can be bioactive considered restorative bioactive materials. restorative materials.Given the limitations mentioned above and the advantages of HV-GICs, their indications are becomingGiven increasingly the limitations limited mentioned [83]. However, above and they the can advantages still be used of HV-GICs, with good their success indications rates as are an intermediatebecoming increasingly base in the limited sandwich [83]. technique However, [84 ],they for thecan restoration still be used of cervicalwith good lesions success [62], forrates primary as an teeth under certain conditions, or when the operator wishes to have a controlled set of the material, which is not possible with conventional GICs and HV-GICs. Materials 2020, 13, x FOR PEER REVIEW 8 of 29 intermediate base in the sandwich technique [84], for the restoration of cervical lesions [62], for primary teeth under certain conditions, or when the operator wishes to have a controlled set of the material, which is not possible with conventional GICs and HV-GICs. Materials 2020, 13, 2313 8 of 28

2.4. Special Cases of Vitremer and Its Evolution: Ketac Nano (Also Called Ketac N100) 2.4. Special Cases of Vitremer and Its Evolution: Ketac Nano (Also Called Ketac N100) 2.4.1. Composition and Chemical Reactions 2.4.1. Composition and Chemical Reactions To describe the chemistry of these materials, many international studies evaluated the behavior of VitremerTo describe (3M ESPE, the chemistry St. Paul, of MN, these USA). materials, Vitremer many appeared international in 1991, studies one evaluated year after the the behavior first RM- of GIC;Vitremer it was (3M immediately ESPE, St. Paul, categorize MN, USA).d as a Vitremermember appearedof this family in 1991, despite one its year chemical after the characteristics first RM-GIC; [15]it was due immediately to its similar categorized behavior in as terms a member of mechanical of this family properties despite and its chemical fluoride characteristics release [24,55,85–89]. [15] due Ketacto its similarNano (3M behavior ESPE, in St. terms Paul, of MN, mechanical USA) is properties an evolution and of fluoride Vitremer release that [24was,55 introduced,85–89]. Ketac in 2007. Nano We(3M chose ESPE, to St. specifically Paul, MN, USA)describe is anthe evolution chemistry of Vitremerof these materials that was introducedbecause they in 2007.represent We chose major to evolutionsspecifically in describe the chemistry the chemistry of RM-GICs of these, which materials will serve because as elements they represent to justify major our classification evolutions in and the tochemistry understand of RM-GICs, the chemistry which of will the servenew commercial as elements products. to justify our classification and to understand the chemistryThe Vitremer of the new matrix commercial is based products. primarily on the use of functionalized high molecular weight polyacrylicThe Vitremer acids. Schematically, matrix is based they primarily are the same on the polyacrylic use of functionalized acids as those highfound molecular in the matrix weight of HV-GICspolyacrylic and acids. RM-GICs, Schematically, but some they–COOH are thegroups same ar polyacrylice substituted acids by asmethacrylate those found groups in the with matrix a carbon–carbonof HV-GICs and double RM-GICs, bond, butallowing some a –COOH polymerization groups arereaction substituted (called byVitrebond methacrylate copolymer groups by with the manufacturer).a carbon–carbon This double is the first bond, major allowing innovation a polymerization of this product, reaction as it theoretically (called Vitrebond allows copolymerimproved cross-linkingby the manufacturer). between resinous This is and the firstpolyacid major networ innovationks, although of this no product, studies ashave it theoretically been conducted allows to investigateimproved cross-linkingthis particular between issue. The resinous product and also polyacid contains networks, other acids although frequently no studies found have in GICs, been HEMAconducted monomers to investigate identical this to particular those in other issue. RM-GIC The products and also water contains to activate other the acids acid–base frequently reaction. found Finally,in GICs, it HEMA contains monomers chemopolymerization identical to those agents in otherin addition RM-GICs to andphotopolymerization water to activate theagents acid–base. This chemopolymerizationreaction. Finally, it contains ensures chemopolymerization a theoretical deep resi agentsn setting in addition between to photopolymerizationthe methacrylate groups agents. of theThis functionalized chemopolymerization polyacrylic ensures acids. a theoreticalThis development deep resin is settingan advance between compared the methacrylate to the classic groups RM- of GICs;the functionalized therefore, it polyacrylic is a theoretical acids. This bulk-fill development reaction is an[90]. advance However, compared studies to theon classic the depth RM-GICs; of polymerizationtherefore, it is a theoreticalof Vitremer, bulk-fill compared reaction with [90]. However,other RM-GICs studies onwithout the depth chemopolymerization of polymerization of activators,Vitremer, comparedhave not withreported other any RM-GICs obvious without differences chemopolymerization and still recommend activators, a layering have not technique reported [85,91].any obvious differences and still recommend a layering technique [85,91]. TheThe fillers fillers are are silanated silanated FAS. FAS. DespiteDespite the the innovations innovations described described above, above, the the behavi behavioror of of the the material material in in the the short short and and long long term term remainsremains similar similar to to that that of of the the RM-GICs RM-GICs that that do do not not benefit benefit from from these these innovations innovations or or whose whose trade trade secretssecrets have have not not been been disclosed. disclosed. ThisThis material material is is in in the the form form of of a a powder–liquid powder–liquid mixture mixture that that is is spatulated spatulated manually. manually. Figure Figure 8 explainsexplains in in which which compartments compartments (powder (powder or or liquid) liquid) the the main main components components of of the the material material are are located. located.

FigureFigure 8. 8. VitremerVitremer in in its its storage storage medium. medium. The The powder powder contains contains silanated silanated FA FASS fillers, ﬁllers, and and some some chemopolymerizationchemopolymerization components components are are added. added. The The liquid liquid contains contains water, water, an an ionized ionized modified modiﬁed polyacrylicpolyacrylic acid acid with with photopolymerizable photopolymerizable groups groups,, camphorquinone camphorquinone as as a a photoinitiator, photoinitiator, and and some some chemopolymerizationchemopolymerization components. components.

Ketac Nano, which was introduced in 2007 [15], is the chemical evolution of Vitremer and is provided in the form of a single-use self-mixing capsule. It represents an evolution in terms of Materials 2020, 13, x FOR PEER REVIEW 9 of 29 Materials 2020, 13, x FOR PEER REVIEW 9 of 29 Materials 2020, 13, 2313 9 of 28 Ketac Nano, which was introduced in 2007 [15], is the chemical evolution of Vitremer and is Ketac Nano, which was introduced in 2007 [15], is the chemical evolution of Vitremer and is provided in the form of a single-use self-mixing capsule. It represents an evolution in terms of provided in the form of a single-use self-mixing capsule. It represents an evolution in terms of manipulation. This material always requires the application of a primer on the tooth surface before manipulation. This This material material always always requires requires the the application application of of a a primer on the tooth surface before being inserted into the cavity and does not have the chemopolymerization activators contained in being inserted inserted into into the the cavity cavity and and does does not not have have the the chemopolymerization chemopolymerization activators activators contained contained in Vitremer; it must therefore be laminated in 2 mm layers as it has no bulk-fill properties [92]. The Vitremer;in Vitremer; it must it must therefore therefore be laminated be laminated in 2 inmm 2 mmlayers layers as it ashas it no has bulk-fill no bulk-fill properties properties [92]. [The92]. defining feature of this material, in addition to possessing a more varied monomeric composition, is Thedefining defining feature feature of this of thismaterial, material, in addition in addition to possessing to possessing a more a more varied varied monomeric monomeric composition, composition, is that it contains silanized nanofillers and nanoclusters. The idea of this modification is to improve its thatis that it contains it contains silanized silanized nanofillers nanofillers and and nanoclusters. nanoclusters. The The idea idea of this of this modification modification is to is improve to improve its mechanical properties, as shown in the literature on experimental RM-GICs [93]. This concept, as mechanicalits mechanical properties, properties, as shown as shown in the in theliterature literature on onexperimental experimental RM-GICs RM-GICs [93]. [93 This]. This concept, concept, as shown below, is also used for Activa BioActive Restorative and Surefil One. Figure 9 explains the shownas shown below, below, is isalso also used used for for Activa Activa BioActive BioActive Re Restorativestorative and and Surefil Surefil One. One. Figure Figure 9 explainsexplains thethe compartments (paste/paste) in which the main components of the material are located. compartments (paste(paste/paste)/paste) in which the main componentscomponents ofof thethe materialmaterial areare located.located.

FigureFigure 9. 9. KetacKetac Nano Nano in in its its storage storage medium. medium. The The first first paste paste contains contains silane-treated silane-treated reactive reactive FAS FAS fillers fillers Figure 9. Ketac Nano in its storage medium. The first paste contains silane-treated reactive FAS fillers andand unreactive unreactive fillers, fillers, as as well well as as HEMA HEMA monomers. monomers. The The second second pa pasteste contains contains water, water, an an ionized ionized and unreactive fillers, as well as HEMA monomers. The second paste contains water, an ionized modifiedmodified polyacrylic polyacrylic acid acid with with photopolymerizable photopolymerizable groups, groups, camphorquinone camphorquinone as a as photoinitiator, a photoinitiator, a modified polyacrylic acid with photopolymerizable groups, camphorquinone as a photoinitiator, a blenda blend of monomers of monomers including including HEMA, HEMA, and and silane-treated silane-treated unreactive unreactive fillers. fillers. blend of monomers including HEMA, and silane-treated unreactive fillers. UponUpon activation activation of of the the mixture, mixture, the the setting setting reacti reactionon of of these these two two materials materials is is similar similar to to that that of of Upon activation of the mixture, the setting reaction of these two materials is similar to that of RM-GICsRM-GICs (Figure (Figure 10). 10 ).Because Because of ofthe the attack attack of the of thecarboxylic carboxylic acids acids on the on functionalized the functionalized polyacid, polyacid, ion RM-GICs (Figure 10). Because of the attack of the carboxylic acids on the functionalized polyacid, ion salting-oution salting-out occurs occurs due dueto the to thesilanized silanized FAS FAS fillers, fillers, and and a layer a layer of ofsilicic silicic gel gel partially partially forms forms on on the the salting-out occurs due to the silanized FAS fillers, and a layer of silicic gel partially forms on the surface.surface. The The parts parts of of the the reactive reactive filler filler that that remain remain silanized silanized enable enable its its linkage linkage to to the the resin resin network. network. surface. The parts of the reactive filler that remain silanized enable its linkage to the resin network.

Figure 10. Ion release from Vitremer (left) or Ketac Nano (right) once they are in contact with a moist Figure 10. Ion release from Vitremer (left) or Ketac Nano (right) once they are in contact with a moist environmentFigure 10. Ion (i.e., release oral fromenvironment). Vitremer (left)When or the Ketac powder Nano and (right) liquid once (or they pastes are in for contact Ketac with Nano) a moist are environment (i.e., oral environment). When the powder and liquid (or pastes for Ketac Nano) are mixed,environment the acid–base (i.e., oral reaction environment). is initiated, When the the setting powder of andthe liquidmaterial (or begins. pastes forOnce Ketac the Nano) FAS fillers are mixed, are mixed, the acid–base reaction is initiated, the setting of the material begins. Once the FAS fillers are partiallythe acid–base attacked, reaction a silicic is initiated, gel partially the setting forms of on the the material FAS filler begins. surface. Once The the FASreleased fillers calcium are partially and partially attacked, a silicic gel partially forms on the FAS filler surface. The released calcium and aluminumattacked, aions silicic are gel able partially to form forms ionic on bonds the FAS with filler ionized surface. carboxylic The released groups. calcium Fluoride and aluminum ions are also ions aluminum ions are able to form ionic bonds with ionized carboxylic groups. Fluoride ions are also released.are able toA formsecond ionic reaction bonds withinvolving ionized resin carboxylic polymerization groups. Fluoride occurs ionsduring are alsomixing released. for Vitremer A second released. A second reaction involving resin polymerization occurs during mixing for Vitremer (chemopolymerization)reaction involving resin or polymerization during light curing occurs fo duringr Ketac mixing Nano. forMonomers Vitremer can (chemopolymerization) copolymerize with (chemopolymerization) or during light curing for Ketac Nano. Monomers can copolymerize with silanatedor during FAS light fillers curing and for other Ketac monomers Nano.Monomers for both ma canterials copolymerize and silanated with unreactive silanated fillers FAS fillersfor Ketac and silanated FAS fillers and other monomers for both materials and silanated unreactive fillers for Ketac Nano.other At monomers the end forof the both reaction, materials two and interconnect silanated unreactiveed networks fillers are forobtained Ketac Nano.with covalent At the endlinks of Nano. At the end of the reaction, two interconnected networks are obtained with covalent links betweenthe reaction, both twodue interconnected to the modified networks polyacrylic are obtained acid (Vitrebond with covalent copolymer). links between In water, both duecalcium, to the between both due to the modified polyacrylic acid (Vitrebond copolymer). In water, calcium, aluminum,modified polyacrylic and fluoride acid ions (Vitrebond (and eventually copolymer). othe Inr water,ions) calcium,are able aluminum,to be exchanged and fluoride with ionsthe oral (and aluminum, and fluoride ions (and eventually other ions) are able to be exchanged with the oral environment.eventually other ions) are able to be exchanged with the oral environment. environment.

Materials 2020, 13, 2313 10 of 28

For Ketac Nano, an additional bond between the resin matrix and the silanized nanoﬁllers/nanoclusters can be expected to improve its mechanical and optical properties. The setting kinetics of Vitremer and Ketac Nano are comparable to those of other RM-GICs [74]. Competition between resinous and acid–base reactions is suspected for both materials but has not yet been studied.

2.4.2. Mechanical Properties, Ion Release in the Oral Environment, and Indications as a Definitive Restorative Material Vitremer and Ketac Nano have a fluoride release profile close to that of RM-GICs [30,88] (Figure 10), and the release peak at the time of their application is preserved. This ion release was demonstrated in many in vitro studies with Vitremer to induce remineralization of the underlying hard dental tissues [94]. No study has focused on the potential of Ketac Nano to induce remineralization. Therefore, Vitremer can be considered a bioactive restorative material. Even if Ketac Nano properties are close to those of Vitremer, information is lacking about its potential bioactivity. The indications of Vitremer and Ketac Nano are identical according to the manufacturer and are just as limited as those of the RM-GIC. These materials can reasonably be used in the sandwich technique for the restoration of cervical lesions [95,96] or for primary teeth. Whereas Vitremer is a material with a long clinical record of accomplishment, there are few clinical studies for Ketac Nano [97–99].

2.4.3. Update on the Classification Because of the chemistry of Vitremer, it seems consistent to classify it with RM-GICs. It has already been extensively studied in the literature, and its performance is consistent with that of the other products in this family. Its chemistry is not fundamentally different from that of RM-GICs since it is based on reactive FAS fillers and a polyacrylic acid, albeit a functionalized polyacrylic acid. The addition of chemoinitiators and silanization of FAS fillers appear to have only produced a marginal effect. Ketac Nano is sometimes classified as a nanoionomer because of the terminology provided by the manufacturer and because the nanoparticles and nanoclusters it contains are analogous to nanohybrid composites [100]. Because its chemistry is close to that of Vitremer and as the strategy of adding silanized nonreactive particles has already been proposed to create reinforced RM-GICs [93], it seems reasonable and consistent to classify it in the category of RM-GICs, especially because the increase in its clinical indications is limited and few studies classifying it as a nanoionomer have been published.

3. Fluoride-Releasing Composites

3.1. Compomers The term ‘compomer’ is a combination of ‘composite’ and ‘ionomer’. In addition to this generic term, these materials are also called polyacid-modified composite resins [6]. After a long debate on terminology [101], the term compomer finally became a part of scientific language [102].

3.1.1. Composition and Chemical Reactions These compomers have two major differences with resin composites: one involves organic monomers and the other involves inorganic fillers [102]. The organic resin component in compomers is similar to that possibly contained in a composite, with a base around bisphenol A-glycidyl methacrylate (bis-GMA) and other monomers added to modify its rheological and polymerization properties (such as triethylene glycol dimethacrylate (TEGDMA) and urethane dimethacrylate (UDMA)) [103]. As the vast majority of these formulations are photopolymerizable, they also contain photoinitiators, such as camphorquinone [104]. In addition, a small proportion of functional monomers with carboxylic acid (–COOH) groups is added to the chemical composition, which indicates the first major characteristic of compomers, hence the name Materials 2020, 13, x FOR PEER REVIEW 11 of 29

Materials 2020, 13, 2313 11 of 28 addition, a small proportion of functional monomers with carboxylic acid (–COOH) groups is added to the chemical composition, which indicates the first major characteristic of compomers, hence the name‘polyacid-modified ‘polyacid-modified composite composite resins’. resins’. As this As material this material contains contains no trace ofno watertrace inof its water composition, in its composition,the acid groups the acid are dehydrated groups are indehydrated the –COOH in form, the –COOH and once form, the material and once has the polymerized, material has it is polymerized,perfectly incorporated it is perfectly into theincorporated resin matrix into [103 the]. Despite resin matrix the addition [103]. of Despite dehydrated the addition acid, the finalof dehydratedmaterial obtained acid, the is final highly material hydrophobic. obtained is highly hydrophobic. TheThe second second unique unique feature feature of ofcompomers compomers regarding regarding the the inorganic inorganic mineral mineral component component is isthe the presencepresence of of silanated silanated reactive reactive FAS FAS fillers fillers [105,106] [105,106 in] in addition addition to to silanated silanated nonreactive nonreactive quartz quartz or or silica silica fillers,fillers, which which constitute constitute the the major major part part of ofthe the matrix; matrix; these these reactive reactive fillers fillers can can bind bind to tothe the resin resin matrix, matrix, improvingimproving the the mechanical mechanical properties, properties, and and releasing releasing fluoride fluoride ions ions under under certain certain conditions. conditions. TheseThese materials materials come come in inthe the form form of ofsingle-use single-use compules compules or or reusable reusable tubes. tubes. Figure Figure 11 11 shows shows the the mainmain components components of ofa compomer a compomer in in its its storage storage medium medium before before polymerization. polymerization.

Figure 11. A compomer in its storage medium. This material contains, schematically, silane-treated Figurereactive 11. FASA compomer fillers and in unreactive its storage fillers, medium. a blend This of monomersmaterial contains, including schemati dehydratedcally, acidic silane-treated monomers reactiveand camphorquinone, FAS fillers and but unreactive does not containfillers, anya bl water.end of monomers including dehydrated acidic monomers and camphorquinone, but does not contain any water. The setting reaction of these materials is initiated by photopolymerization, where the energy suppliedThe setting causes reaction the activation of these of materials photoinitiators is initiated and results by photopolymerization, in radical polymerization wherethat the isenergy similar suppliedto that ofcauses a resin the composite activation [of45 ph].otoinitiators A resin network and results is formed, in radica and covalentl polymerization bonds are that formed is similar with tosilanated that of a FASresin and composite silanated [45]. nonreactive A resin networ fillers (Figurek is formed, 11). Compomersand covalent do bonds not exist are atformed the moment with silanatedwith bulk-fill FAS and properties, silanated and nonreactive they have fillers to be laminated(Figure 11). in Compomers 2 mm layers. do not exist at the moment with bulk-fill properties, and they have to be laminated in 2 mm layers. 3.1.2. Mechanical Properties, Ion Release in the Oral Environment, and Indications as a Restorative 3.1.2.Material Mechanical in Use Properties, Ion Release in the Oral Environment, and Indications as a Restorative MaterialCompomers in Use have initial mechanical properties that are roughly comparable to those of resin compositesCompomers [106 –have108], initial but their mechanical performance properties decreases that significantly are roughly over comparable time due to to those hydrolysis of resin and compositessolubilization [106–108], of the fillersbut their [106 perf,109ormance,110]. The decreases stability significantly of the silanization over time of FAS due fillers to hydrolysis in parallel and with solubilizationthe acid–base of reactionthe fillers of [106,109,110]. FAS feeds is, The as with stability Vitremer of the and silanization Ketac Nano, of FAS subject fillers to in questions parallel with in the theevent acid–base of hydrolysis reaction [ 106of FAS] and feeds has not is, as been with studied. Vitremer and Ketac Nano, subject to questions in the event ofCompomers hydrolysis with[106] no and water has not in their been composition studied. exhibit ion release based solely on the absorption of waterCompomers that occurs with afterno water contact in with their the composition oral environment exhibit [45ion]. Comparedrelease based with solely that ofon a the resin absorptioncomposite, of this water water that absorption occurs after is facilitatedcontact with by silanatedthe oral environment FAS fillers, which [45]. increaseCompared water with absorption that of a resincompared composite, with that this of water silanated absorption nonreactive is facilita fillersted[ 105by silanated]. This process FAS fillers, occurs which at the increase periphery water of the absorptionmaterial and,compared when itwith comes that into of contactsilanated with nonr a dehydratedeactive fillers acid, [105]. causes This its process activation occurs in its at ionized the peripheryform (COO– of the+ materialH+); the and, proton when that it is comes released into is contact able to with attack a dehydrated the surface of acid, the causes FAS fillers its activation and induce inion its release;ionized inform particular, (COO– fluoride+ H+); the is releasedproton that [101 is] (Figurereleased 12 is). able to attack the surface of the FAS fillers and induce ion release; in particular, fluoride is released [101] (Figure 12).

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Materials 2020, 13, x FOR PEER REVIEW 12 of 29

Figure 12. IonIon release from a compomer once it is inin contactcontact withwith thethe oraloral environment.environment. When the material is light cured, a resin polymerization reaction is initiated,initiated, and monomers can copolymerize with otherother monomers, monomers, silanated silanated FAS FAS fillers, fillers, and unreactive and unreactive fillers. Thefillers. acidic The groups acidic remain groups dehydrated, remain dehydrated,and no acid–base and no reaction acid–base occurs reaction in the occurs setting in the reaction setting of reaction the material. of the material. When placed When in placed a moist in aenvironment moist environment (i.e., oral (i.e., environment), oral environment), water sorption water sorption occurs, andoccurs, dehydrated and dehy acidicdrated monomers acidic monomers located locatedin the periphery in the periphery of the material of the canmaterial release can protons release to protons attack silanated to attack FAS silanated fillers. FAS This mechanismfillers. This mechanismleads to the releaseleads to of the calcium, release aluminum, of calcium, and aluminum fluoride, and ions. fluoride These ions ions. do These not participate ions do not in participate the setting inmechanism the setting of mechanism the material. of the material.

The ion release of these materials is very low an andd much lower than that of conventional GICs, GICs, HV-GICs, and RM-GICs. In addition, the initial peak in the the fluoride fluoride release release observed for conventional conventional GICs, HV-GICs, and RM-GICs is no longer observed for these materials, which are still capable of recharging with fluoride fluoride [31,79,111–114]. [31,79,111–114]. Compomers Compomers appear appear to to have have an an ion ion release close to that of giomers [31,79]. [31,79]. This This small ion release has not been shown to induce remineralization of underlying hard dentaldental tissues tissuesin in vitro vitro[115 [115].]. Compomers Compomers require require an adhesive an adhesive to be bondedto be bonded on the tooth on the structure, tooth whichstructure, could which impede could ion impede diffusion ion to diffusion induce remineralization.to induce remineralization. Therefore, Therefore, compomers compomers cannot be cannotconsidered be considered bioactive restorativebioactive restorative materials. materials. Although theythey appearappear to to have have a lowera lower long-term long-term success success rate rate than than that that of composite of composite resins resins in some in somestudies studies [116], compomers [116], compomers can be reasonably can be used reasonab long termly used for cervical long restorationsterm for cervical [95,117, 118restorations], anterior [95,117,118],restorations [anterior119,120], restorations as an intermediate [119,120], base as underan intermediate adult posterior base under composite adult restorations posterior composite [121,122], restorationsor for restorations [121,122], in pediatric or for restoratio dentistryns [ 121in pediatric,123,124]. dentistry [121,123,124].

3.1.3. Update Update on on the Classification Classification As their setting chemistry is comparable to that of of resin resin composites composites (absence (absence of of water, water, setting by resin polymerization,polymerization, and and silanized silanized fillers), fillers), where theywhere differ they only bydiffer a mechanism only by of postpolymerizationa mechanism of postpolymerizationion release by water ion absorption, release by we water propose absorption, to classify we compomerspropose to classify in a family compomers called ion-releasing in a family calledcomposites ion-releasing (IRCs). composites (IRCs).

3.2. Giomers Giomers The term ‘giomer’ is is an English combination of of ‘glass ‘glass ionomer ionomer cement’ cement’ and and ‘resin ‘resin composite’. composite’. Unlike compomerscompomers that that incorporate incorporate lyophilized lyophilized modified modified resin resin groups groups and initially and initially inactivated inactivated reactive reactivefillers, giomers fillers, giomers use dehydrated use dehydrated and silanized and sila preactivatednized preactivated reactive fillersreactive [125 fillers,126]. [125,126].

Materials 2020, 13, x FOR PEER REVIEW 13 of 29 Materials 2020, 13, 2313 13 of 28 3.2.1. Composition and Chemical Reactions The resin matrix in a giomer is similar to that possibly contained in the composite, with a base 3.2.1. Composition and Chemical Reactions around bis-GMA and other monomers added to modify its rheological and polymerization properties [127]. NoThe functional resin matrix acid ingroups a giomer or dehydrated is similar acid to that groups possibly are incorporated contained in inthe the composite,composition; with the a materialbase around therefore bis-GMA lacks adhesive and other potential monomers and requ addedires to the modify use of itsan rheologicaladhesive. Setting and polymerization is performed byproperties photopolymerization, [127]. No functional and the acidphotoinitiators groups or dehydratedfound are simila acidr groups to those are contained incorporated in resin in the compositescomposition; [128]. the The material main therefore feature of lacks a giomer adhesive is that potential in addition and requires to the thenonreactive use of an adhesive.silanized Settingglass fillersis performed of the resin by composite, photopolymerization, it incorporates and reac thetive photoinitiators fillers of preactivated found are similarFAS (i.e., to coated those contained with SiO2 in gel),resin similar composites to those [128 contained]. The main in HV-GICs feature of after a giomer the setting is that reaction in addition [127]. to For the this nonreactive purpose, silanized before theirglass incorporation fillers of the into resin a composite,material that it incorporatesalready contains reactive organic fillers monomers of preactivated and silanized FAS (i.e., nonreactive coated with glassSiO 2fillers,gel), similarthe FAS to fillers those containedare pre-etched in HV-GICs with polyacrylic after the settingacid to reactioncover them [127 ].with For a this silicic purpose, gel, dehydratedbefore their by incorporationfreeze-drying, intoand functionalized a material that by already silanization contains to allow organic their monomers copolymerization and silanized with thenonreactive resin monomers glass fillers, and theto make FAS fillers them are suitable pre-etched for ion with release polyacrylic in contact acid to with cover water them withwhen a it silicic is absorbedgel, dehydrated into the material by freeze-drying, [129]. and functionalized by silanization to allow their copolymerization withThe the fillers resin obtained monomers at andthe end to make of this them treatmen suitablet are for called ion release pre-reactive in contact glass with ionomer water particles when it is (PRGs).absorbed According into the to material the volume [129]. of activation obtained from the FAS filler at the end of the treatment, a distinctionThe fillers is possible obtained between at the end S-PRG of this fillers treatment (only arethe calledsurface pre-reactive of the filler glass is activated ionomer particlesby the chemical(PRGs). treatment) According toand the F-PRG volume fillers of activation (the entire obtained filler or from almost the the FAS entire filler atfiller the has end been of the activated treatment, bya the distinction chemical is treatment) possible between [129]. For S-PRG restoration fillers (only giomers, the surface S-PRG of technology the filler is is activated used to bytreat the reactive chemical fillers.treatment) and F-PRG fillers (the entire filler or almost the entire filler has been activated by the chemicalThese treatment)materials, [marketed129]. For restorationby the Shofu giomers, Compan S-PRGy, are technology provided is usedin the to form treat reactiveof single-use fillers. compulesThese or reusable materials, tubes. marketed Figure by13 shows the Shofu the ma Company,in components are provided of a giomer in thein its form storage of single-usemedium beforecompules polymerization. or reusable tubes. Figure 13 shows the main components of a giomer in its storage medium before polymerization.

Figure 13. A giomer in its storage medium. This material contains, schematically, a silane-treated partially Figure 13. A giomer in its storage medium. This material contains, schematically, a silane-treated pre-reacted FAS filler (S-PRG), an unreactive filler, and a blend of monomers and camphorquinone. It does partially pre-reacted FAS filler (S-PRG), an unreactive filler, and a blend of monomers and not contain any water. camphorquinone. It does not contain any water. The setting reaction of a giomer occurs by photopolymerization; this process is similar to that in aThe composite setting reaction or compomer of a giomer that involves occurs by the photopolymerization; creation of a resin network this process and theis similar establishment to that in of a covalentcomposite bonds or compomer with silanized that reactiveinvolves S-PRGthe creati andon silanized of a resin nonreactive network ﬁllersand the (Figure establishment 14). Giomers of covalentexist in bonds laminable with or silanized bulk-ﬁll reactive versions. S-PRG and silanized nonreactive fillers (Figure 14). Giomers exist in laminable or bulk-fill versions.

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Materials 2020, 13, x FOR PEER REVIEW 14 of 29

Figure 14. Diagram of ion release from a giomer once it is in contact with the oral environment. When Figure 14. Diagram of ion release from a giomer once it is in contact with the oral environment. When the the material is light cured, a resin polymerization reaction is initiated, and monomers can material is light cured, a resin polymerization reaction is initiated, and monomers can copolymerize withcopolymerize other monomers, with other silanated monomers, S-PRG silanated fillers, and S-PRG unreactive fillers, fillers.and unreactive No acid–base fillers. reaction No acid–base occurs inreaction the setting occurs reaction in the setting of the material.reaction of When the materi placedal. in When a moist placed environment in a moist ( i.e.,environment oral environment (i.e., oral), waterenvironment), sorption water occurs, sorption and S-PRG occurs, fillers and are S-PRG able fillers to release are able calcium, to release aluminum, calcium, and aluminum, fluoride ions.and Thesefluoride ions ions. do These not participate ions do not in theparticipate settingmechanism in the setting of mechanism the material. of the material.

3.2.2. Mechanical Properties, Ion Release in the Oral Environment, andand IndicationsIndications asas aa RestorativeRestorative Material inin UseUse Studies on thethe initialinitial propertiesproperties ofof giomersgiomers reportedreported initialinitial mechanicalmechanical propertiesproperties that are comparablecomparable withwith thosethose ofof aa resinresin compositecomposite [[130,131].130,131]. As withwith compomers,compomers, the the ion ion release release of thisof this material material is based is based on the on absorption the absorption of water of thatwater occurs that onceoccurs it once comes it intocomes contact into contact with the with oral the environment oral environment [127,132 [127,132].]. This water This absorptionwater absorption occurs occurs at the peripheryat the periphery of the material,of the material, and when and itwhen comes it intocomes contact into contact with an with S-PRG an filler,S-PRG it filler, causes it ioncauses release; ion thisrelease; S-PRG this filler S-PRG must filler be viewedmust be as viewed more reactive as more than reactive a conventional than a conventional FAS filler. FAS Therefore, filler. thereTherefore, is no needthere foris no an need acid for to cause an acid the to activation cause the ofactivation the reactive of the filler, reactive unlike filler, for compomersunlike for compomers (Figure 14). (Figure 14). However, the ion release of these materials is shown to be very low and much lower than that of HV-GICsHowever, and RM-GICs. the ion release It is comparable of these materials to that ofis compomers,shown to be andvery as low with and compomers, much lower no than fluoride that releaseof HV-GICs peak isand observed RM-GICs. after It the is placement comparable of giomersto that [of31 ,compomers,79,111,112]. However,and as with giomers compomers, are capable no offluoride being rechargedrelease peak with is fluorideobserved ions after [133 the]. Thisplacemen smallt ionof giomers release ability [31,79,111,112]. that induces However, remineralization giomers ofare the capable underlying of being hard recharged dental tissues with fluoride has not beenions studied.[133]. This However, small ion as release with compomers, ability that giomersinduces requireremineralization an adhesive of tothe be underlying bonded on thehard tooth dental structure, tissues which has not could been impede studied. ion diHowever,ffusion to as induce with remineralization.compomers, giomers Starting require from an this adhesive base, giomers to be bonded cannot on be the considered tooth structure, bioactive which materials. could impede ion diffusionAlthough to they induce appear remineralization. to have a lower Starting long-term from success this base, rate thangiomers resin cannot composites be considered in some studiesbioactive [128 materials.], giomers can be reasonably used for cervical restorations [134], occlusal restorations [134], or restorationsAlthough inthey pediatric appear dentistry.to have a Reservationslower long-ter werem success expressed rate by than some resi authorsn composites on the in esthetic some renderingstudies [128], of giomers giomers in can the be long reasonably term [135 ].used for cervical restorations [134], occlusal restorations [134], or restorations in pediatric dentistry. Reservations were expressed by some authors on the 3.2.3. Update on the Classification esthetic rendering of giomers in the long term [135]. Given that the setting chemistry of giomers is comparable to that of resin composites (an absence of3.2.3. water, Update setting on bythe resin Classification polymerization, and silanized fillers), and they differ only by a mechanism Given that the setting chemistry of giomers is comparable to that of resin composites (an absence of water, setting by resin polymerization, and silanized fillers), and they differ only by a mechanism

Materials 2020, 13, 2313 15 of 28 of postpolymerization ion release by water absorption, we propose to classify them in the family of ion-releasing composites (IRCs). These giomers are ultimately similar to compomers. S-PRG technology is used to produce additional reactive silanized FAS particles to avoid the use of a dehydrated acid, which is present in compomers.

4. New Bioactive Composites These materials were isolated from PubMed studies on ﬂuoride-releasing materials. The names and compositions of these materials are given in Table1. They have all been recently introduced and have therefore received little or no attention in the scientiﬁc literature.

Table 1. Composition of new ﬂuoride-releasing materials

Name Composition Powder: silanated bioactive glass and calcium, silanated silica, and sodium fluoride Activa BioActive Restorative Liquid: diurethane modified by the insertion of a hydrogenated polybutadiene and other methacrylate monomers, modified polyacrylic acid, and water Powder: barium aluminum silicate glass, ytterbium trifluoride, isofiller, calcium barium aluminum fluorosilicate glass, and calcium fluorosilicate glass Cention N Liquid: urethane dimethacrylate, tricyclodecane dimethanol dimethacrylate, tetramethyl-xylylen diurethane dimethacrylate, polyethylene glycol 400 dimethacrylate, Ivocerin, and hydroxyperoxide Powder: silanated aluminum-phosphorus-strontium-sodium-fluoro-silicate glass, Surefil One dispersed silicon dioxide, ytterbium fluoride, and pigments Liquid: acrylic acid, polycarboxylic acid, bifunctional acrylate, self-cure initiator, camphorquinone, and stabilizer

4.1. Activa BioActive Restorative To describe the chemistry of this material that was introduced in 2013, we have a few independent sources of scientiﬁc publications and information from the patent for this product [136]. Activa BioActive Restorative is referred to by its manufacturer and some authors as a “bioactive composite” [137], but it is considered by others to be an RM-GIC [138,139].

4.1.1. Composition and Chemical Reactions The Activa BioActive Restorative liquid contains a high molecular weight polyacrylic acid similar to that used in HV-GICs and RM-GICs and not modified by methacrylate polymerizable groups, as in Vitremer or Ketac Nano. Urethane dimethacrylate monomers (called Embrace resin by the manufacturer) and dimethacrylate phosphate (acids) are added. To initiate the polymerization reaction, the material contains photoinitiators and chemical initiators. Finally, this liquid contains a small proportion of water. The fillers are silanized FAS fillers and silanized nonreactive fillers that are able to bond with the resin matrix and play a role in the wear resistance and esthetics of the material. This material is in the form of a self-mixing syringe and is theoretically a true bulk-fill material. Figure 15 explains the compartments (powder/liquid) in which the main components of the material are located. A double-setting reaction occurs during mixing. An acid–base reaction occurs, with polyacrylic acid and dimethacrylate phosphate monomers attacking silanized FAS fillers. This acid–base reaction causes an ion release similar to that described for previous materials. Along with this acid–base reaction, Materials 2020, 13, x FOR PEER REVIEW 16 of 29

Materials 2020, 13, 2313 16 of 28

a resin polymerization reaction is activated upon mixing by the chemopolymerization activators and then completed by photopolymerization. The silanized FAS ﬁllers are theoretically capable of binding to the resin matrix, as are silanized nonreactive ﬁllers. Materials 2020, 13, x FOR PEER REVIEW 16 of 29

Figure 15. Diagram of Activa BioActive Restorative in its storage medium. The powder contains silanated FAS fillers, some chemopolymerization components, and silanated unreactive fillers. The liquid contains water, polyacrylic acid, a blend of monomers including phosphate dimethacrylate monomers, camphorquinone as a photoinitiator, and some chemopolymerization components.

A double-setting reaction occurs during mixing. An acid–base reaction occurs, with polyacrylic acid and dimethacrylate phosphate monomers attacking silanized FAS fillers. This acid–base reaction causes an ion release similar to that described for previous materials. Along with this acid–base

reaction, a resin polymerization reaction is activated upon mixing by the chemopolymerization activatorsFigureFigure and15. DiagramDiagramthen completed of of Activa Activa by BioActive BioActive photopolymerizat Restorative Restorative ion.in in its The storage silanized medium. FAS The The fillers powder powder are contains containstheoretically capablesilanatedsilanated of binding FAS FAS fillers, ﬁllers,to the some resin some chemopolymerization matrix, chemopolymerization as are silanized components, components, nonreactive and andsilanated fillers. silanated unreactive unreactive fillers. ﬁllers. The liquidThe liquidions contains released contains water, water,by polyacrylic the polyacrylic acid–base acid, acid, reactiona blend a blend ofis of monomerseither monomers exchanged including including with phosphate phosphate the external dimethacrylate dimethacrylate environment or createsmonomers,monomers, ionic camphorquinone camphorquinonebonds between as aspolyacrylic a a photoinitiator, photoinitiator, acids, and but some trivalent ch chemopolymerizationemopolymerization ions, such as aluminum, components. also bind to polyacrylic acid and dimethacrylate phosphate, thus cross-linking the resin and ionic networks AThe double-setting ions released reaction by the acid–baseoccurs during reaction mixing. is either An acid–base exchanged reaction with the occurs, external with environment polyacrylic (Figure 16). acidor creates and dimethacrylate ionic bonds between phosphate polyacrylic monomers acids, attacking but trivalent silanized ions, FAS such fillers. as aluminum,This acid–base also reaction bind to The same mechanisms as those described for RM-GICs regarding competition between the acid– causespolyacrylic an ion acid release and dimethacrylate similar to that phosphate, described thus for cross-linking previous materials. the resin and Along ionic with networks this (Figureacid–base 16). base and resin polymerization reactions are suspected to be present in this material. reaction, a resin polymerization reaction is activated upon mixing by the chemopolymerization activators and then completed by photopolymerization. The silanized FAS fillers are theoretically capable of binding to the resin matrix, as are silanized nonreactive fillers. The ions released by the acid–base reaction is either exchanged with the external environment or creates ionic bonds between polyacrylic acids, but trivalent ions, such as aluminum, also bind to polyacrylic acid and dimethacrylate phosphate, thus cross-linking the resin and ionic networks (Figure 16). The same mechanisms as those described for RM-GICs regarding competition between the acid– base and resin polymerization reactions are suspected to be present in this material.

Figure 16. Diagram of ion release from Activa BioActive Restorative once it is in contact with a moist Figure 16. Diagram of ion release from Activa BioActive Restorative once it is in contact with a moist environment (i.e., oral environment). When the powder and liquid are mixed, the acid–base reaction environment (i.e., oral environment). When the powder and liquid are mixed, the acid–base reaction is initiated, the setting of the material begins, and the FAS fillers are partially attacked. A silicic gel is initiated, the setting of the material begins, and the FAS fillers are partially attacked. A silicic gel partially forms on the FAS fillerfiller surface.surface. The releasedreleased calcium and aluminum ions are able to form ionic bonds bonds with with ionized ionized carboxylic carboxylic groups. groups. Fluoride Fluoride ions ions are arealso also released. released. A second A second reaction reaction of resin of resinpolymerization polymerization occurs occurs during during mixing: mixing: monomers monomers can can copolymerize copolymerize with with silanated silanated FAS FAS fillers, fillers, silanated unreactive fillers, fillers, and other monomers. At At the the end of the reaction, we obtain two different different interpenetrating networks with theoretical ionic links between both with trivalent ions. In water, interpenetrating networks with theoretical ionic links between both with trivalent ions. In water, calcium, aluminum, and fluoride fluoride ions (and eventua eventuallylly other ions) are able to be exchanged with the Figure 16. Diagram of ion release from Activa BioActive Restorative once it is in contact with a moist oral environment. environment (i.e., oral environment). When the powder and liquid are mixed, the acid–base reaction is initiated, the setting of the material begins, and the FAS fillers are partially attacked. A silicic gel partially forms on the FAS filler surface. The released calcium and aluminum ions are able to form ionic bonds with ionized carboxylic groups. Fluoride ions are also released. A second reaction of resin polymerization occurs during mixing: monomers can copolymerize with silanated FAS fillers, silanated unreactive fillers, and other monomers. At the end of the reaction, we obtain two different interpenetrating networks with theoretical ionic links between both with trivalent ions. In water, calcium, aluminum, and fluoride ions (and eventually other ions) are able to be exchanged with the oral environment.

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The same mechanisms as those described for RM-GICs regarding competition between the acid–base and resin polymerization reactions are suspected to be present in this material.

4.1.2. Mechanical Properties, Ion Release in the Oral Environment, and Indications as a Restorative Material for Use Regarding the initial evaluation of the mechanical properties of Activa BioActive Restorative, its in vitro wear was shown to be comparable to that of a hybrid composite and inferior to that of RM-GICs and HV-GICs [137,140]. Its initial flexural strength is superior to those of RM-GICs and HV-GICs. The results for composites and their derivatives (compomers and giomers) are contradictory but appear to be similar [137,138,141]. The fluoride release of Activa BioActive Restorative appears to be lower than that of RM-GICs and HV-GICs [142,143] and close to that of compomers and giomers, both quantitatively and qualitatively; it lacks an initial release peak but possesses the capacity to be recharged by fluoride [137,142]. In addition to the release of calcium and fluoride, the release of phosphate is noted by the manufacturer. This ion release has not been found to induce remineralization of underlying hard dental tissues in vitro. Therefore, Activa BioActive Restorative cannot be classified as a bioactive material at the moment [144]. Due to its recent introduction, no long-term clinical studies are available on this material. The manufacturer specifies this material for all direct anterior and posterior restorations. It is also indicated as an intermediate base. Two short-term clinical studies report different behaviors for posterior restorations for use in adults. In the absence of an adhesive (possibly an old protocol that is not currently recommended by the manufacturer), the short-term results obtained with Activa BioActive Restorative are contradictory. One study reported good short-term behavior, and the other reported an unacceptable failure rate [139,145]. The new protocol proposed by the manufacturer involves the use of an adhesive system prior to the application of Activa BioActive Restorative.

4.1.3. Update on the Classification This material is therefore similar to RM-GICs, exhibiting a double-setting reaction and presenting similarities to Vitremer (setting of a dual resin matrix and silanization of FAS fillers) and Ketac Nano (presence of silanized nonreactive fillers). It also has the distinction of using a dimethacrylate phosphate monomer that, once ionized, is theoretically able to cross-link resin and acidic networks through aluminum cations. Despite these innovations, Activa BioActive Restorative could be classified as an RM-GIC. It water and are not derived, for the most part, from an acid–base reaction. However, this material is quite different from the first generation of RM-GICs due to its reinforced formulation.

4.2. Cention N To describe the chemistry of this material, we have a few independent sources as well as the product patent [146]. Please note that unlike the other products described in this article, Cention N is currently only marketed in Asia. The manufacturer places it in a new family that was apparently derived from the family of composites called alkasites. This name is used in the ﬁrst international publications on this product.

4.2.1. Composition and Chemical Reactions The liquid of Cention N is composed of the association of four monomers commonly found in the composition of resin composites. It does not contain any acidic monomer or water; the material is therefore devoid, a priori, of adhesive potential (the manufacturer indicates this with an adhesive in nonretentive cavities). The liquid also contains photopolymerization and chemopolymerization activators and is therefore, theoretically, a true bulk-ﬁll material. Materials 2020, 13, 2313 18 of 28

The defining feature of this material lies in the composition of its powder, particularly in the reactive fillers that it incorporates. In addition to nonreactive silanized fillers, Cention N has reactive Materials 2020, 13, x FOR PEER REVIEW 18 of 29 silanized FAS fillers similar to those used in GICs (calcium-barium-aluminum-fluorosilicate-glass) and silanizedand silanized fillers advertised fillers advertised as highly as reactivehighly reactive particularly particularly in an in acidic an acidic environment, environment, which which strongly Materials 2020, 13, x FOR PEER REVIEW 18 of 29 resemblesstrongly FAS resembles [144] (calcium FAS [144] fluorosilicate (calcium fluorosilicate glass) fillers. glass) These fillers. fillers These are thefillers origin are the of the origin name of the ‘alkasite’ givenname byand the ‘alkasite’ silanized manufacturer. given fillers by advertised the manufacturer. as highly reactive particularly in an acidic environment, which ThisstronglyThis material material resembles is in is thein FAS the form form[144] of of(calcium a powder–liquida powder–liquid fluorosilicate mixture mixture glass) andfillers. must These be be spatulatedfillers spatulated are the manually. manually. origin of Figure the Figure 17 explains17 explainsname compartments ‘alkasite’ compartments given (powder by (powder/liquid)the manufacturer./liquid) in whichin which the the main main components components of of the the material material are are located. located. This material is in the form of a powder–liquid mixture and must be spatulated manually. Figure 17 explains compartments (powder/liquid) in which the main components of the material are located.

Figure 17. Diagram of Cention N in its storage medium. The powder contains, schematically, Figure 17. Diagram of Cention N in its storage medium. The powder contains, schematically, silane- silane-treated FAS filler, silanated ‘alkasite’ filler, unreactive filler, and some chemopolymerization treated FAS filler, silanated ‘alkasite’ filler, unreactive filler, and some chemopolymerization components.Figure 17. The Diagram liquid of containsCention N ain blendits storage of medium. monomers, The powder Ivocerin contains, as a schematically, photoinitiator, silane- and some components.treated FAS The filler, liquid silanated contains ‘alkasite’ a blend filler, of monounreactivemers, filler,Ivocerin and as some a photoinitiator, chemopolymerization and some chemopolymerization components. It does not contain any water. chemopolymerizationcomponents. The liquid components. contains Ita doesblend not of containmonomers, any water.Ivocerin as a photoinitiator, and some chemopolymerization components. It does not contain any water. TheThe radical radical polymerization polymerization reaction reaction startsstarts with with a a chemopolymerization chemopolymerization reaction reaction as soon as soonas the as the liquid–powderliquid–powderThe radical mixture mixture polymerization is is mixed mixed and and reaction is is completed completed starts with by by a photopolymerization.chemopolymerization photopolymerization. reaction This This setting as settingsoon reaction as reactionthe is is similarsimilarliquid–powder to that to that of aof composite, amixture composite, is mixed giomer, giomer, and isoror completed compomer,compomer, by withphotopolymerization. with the the crea creationtion of a ofThis resin a setting resin network networkreaction and is theand the establishmentestablishmentsimilar to of that covalentof covalentof a composite, bonds bonds with withgiomer, reactive reactive or compomer, and nonreactive nonreactive with the silanizcrea silanizedtioned of fillers a fillers resin (Figure network (Figure 18). and18). the establishment of covalent bonds with reactive and nonreactive silanized fillers (Figure 18).

Figure 18. Diagram of ion release from Cention N once it is in contact with the oral environment. FigureFigure 18. 18.Diagram Diagram of of ion ion release release fromfrom Cention N N once once it itis isin incontact contact with with the oral the oralenvironment. environment. When the material is mixed, a resin polymerization reaction is initiated due to the chemical initiator, When the material is mixed, a resin polymerization reaction is initiated due to the chemical initiator, When theand material monomers is can mixed, copolymerize a resin polymerizationwith other monomers, reaction silanated is initiatedFAS filler, duesilanated to the alkasite chemical fillers, initiator, and monomers can copolymerize with other monomers, silanated FAS filler, silanated alkasite fillers, and monomersand unreactive can copolymerize fillers. No acid–base with otherreaction monomers, occurs in the silanated setting reaction FAS filler, of the silanated material. alkasite When fillers, and unreactive fillers. No acid–base reaction occurs in the setting reaction of the material. When and unreactiveplaced in fillers. a moist No environment acid–base (i.e., reaction oral occursenvironment), in the settingwater sorption reaction occurs of the in material.FAS fillers, When and placed placed in a moist environment (i.e., oral environment), water sorption occurs in FAS fillers, and in a moistalkasite environment fillers are able (i.e., to oral release environment), calcium, aluminum, water sorptionand fluoride occurs ions in(and FAS eventually fillers, andother alkasite ions). fillers alkasite fillers are able to release calcium, aluminum, and fluoride ions (and eventually other ions). are ableThese to release ions do calcium, not participate aluminum, in the setting and fluoride mechanism ions of (and the material. eventually other ions). These ions do not These ions do not participate in the setting mechanism of the material. participate in the setting mechanism of the material.

Materials 2020, 13, 2313 19 of 28

4.2.2. Mechanical Properties, Ion Release, and Clinical Performance Once placed in the oral environment, especially in an acidic environment [144], Cention N releases ions, especially ﬂuoride, due to the absorption of water, as with giomers or compomers (Figure 18). The ion release of Cention N was shown to be superior to that of Activa BioActive. An in vitro study reported that Cention N is capable of forming apatite on its surface and thus remineralizing the underlying dentin when used without the application of a dental adhesive [144]. This material could therefore be considered bioactive and would thus be the ﬁrst derivative of a composite resin with a proven bioactivity. This material is indicated by the manufacturer for all temporary tooth restorations, for occlusal and proximal restorations of posterior teeth, and for cervical restorations in combination with an adhesive. However, no long- or short-term clinical studies have examined its performance due to its relatively short time in the market.

4.2.3. Update on the Classification This material, by its composition, is therefore very similar to resin composites, such as compomers and giomers. It is even more similar to the latter, as each of these materials is based on reactive fillers that do not require acids for their activation (S-PRG FAS fillers for giomers and calcium fluorosilicate fillers for Cention N). However, this material is capable of producing remineralization of the underlying hard tissue with which it is in contact. To date, this is the only commercially available ion-releasing composite that has been independently and scientifically proven to induce remineralization of the underlying dentin. We therefore propose classifying it in the family of bioactive composites. However, reservations are expressed about the bioactivity of this material when an adhesive is used before the placement of this material.

4.3. Sureﬁl One To describe the chemistry of this material, the only available independent source is its patent [147]. No independent studies have yet been published. The manufacturer classiﬁes this material in the family of self-adhesive hybrid composites.

4.3.1. Composition and Chemical Reactions The Surefil One liquid consists primarily of a high molecular weight polyacrylic acid functionalized with polymerizable groups (called MOPOS by the manufacturer). Schematically, this polyacrylic acid resembles the Vitrebond copolymer found in Vitremer and Ketac Nano. Additionally, monomers are found in the liquid with two photopolymerizable ends (called BADEP by the manufacturer), which are added to act as a cross-linker between functionalized polyacrylic acid chains. Finally, photopolymerization and chemopolymerization agents and a certain proportion of water are present in the composition. The fillers are silanized FAS fillers and silanized nonreactive fillers (sub- and supramicron in size) that bind with the resin matrix and play a role in the wear resistance and esthetics of the material. This material is therefore reminiscent of Ketac Nano, with a dual grip and a different load size distribution. This material is provided in the form of a single-use capsule that must be mechanically vibrated and is theoretically a true bulk-fill material. Figure 19 explains the compartments (powder/liquid) in which the main components of the material are located. Materials 2020,, 1313,, 2313x FOR PEER REVIEW 2020 of of 2829 Materials 2020, 13, x FOR PEER REVIEW 20 of 29

Figure 19. SurefilSurefil One One in inits itsstorage storage medium. medium. For RM-GICs, For RM-GICs, the powder the powder contains contains silanated silanated FAS fillers, FAS Figurefillers,some chemopolymerization19. some Surefil chemopolymerization One in its storage componen medium. components,ts, and For silanated RM-GICs, and silanated unreactive the powder unreactive fill ers.contains fillers.The liquidsilanated The contains liquid FAS contains fillers, water, somewater,an ionized chemopolymerization an ionized modified modified polyacrylic componen polyacrylic acidts, acid withand with silanatedphotopolymerizable photopolymerizable unreactive fill groups,ers. groups, The a liquid ablend blend contains of of monomers, water, ancamphorquinone ionized modified asas aa polyacrylic photoinitiator,photoinitiator, acid andan withd somesome ph chemopolymerizationchemopolymerizationotopolymerizable groups, components.components. a blend of monomers, camphorquinone as a photoinitiator, and some chemopolymerization components. During mixing,mixing, the the same same mechanisms mechanisms as thoseas those described described for Ketacfor Ketac Nano Nano occur, occur, including including an attack an ofattack theDuring silanizedof the mixing, silanized FAS the filler FAS same by filler the mechan by functionalized the ismsfunctionalized as those polyacrylic described polyacrylic acid for andacid Ketac theand subsequentNano the subsequent occur, ion including salting-out.ion salting- an attackTheseout. These of released the releasedsilanized ions areions FAS either arefiller either exchanged by the exchanged functionalized with the with external polyacrylicthe external environment acid environment and or the create subsequent or ionic create bonds ionionic salting- between bonds out.thebetween functionalizedThese the released functionalized polyacids.ions are eitherpolyacids. Along exchanged with Along the with acid–base the externalacid–base reaction, environment reaction, the resin the polymerization or resin create polymerization ionic reaction bonds betweenisreaction initiated theis initiated byfunctionalized the chemopolymerization by the chemopolymerizationpolyacids. Along activators with activators the as acid–base soon as as soon itreaction, is as mixed; it is themixed; it resin is completedit polymerizationis completed by the by reactionphotopolymerizationthe photopolymerization is initiated by reactionthe reactionchemopolymerization (Figure (Figure 20). 20). activators as soon as it is mixed; it is completed by the photopolymerization reaction (Figure 20).

Figure 20.20.Ion Ion release release from from Surefil Surefil One onceOne in once contact in withcont aact moist with environment a moist environment (i.e., oral environment). (i.e., oral Figure 20. Ion release from Surefil One once in contact with a moist environment (i.e., oral Whenenvironment). the powder When and the liquid powder are mixed,and liquid the acid–baseare mixed, reaction the acid–base is initiated, reaction the is setting initiated, of the the material setting environment). When the powder and liquid are mixed, the acid–base reaction is initiated, the setting begins,of the material and the begins, FAS fillers and the are FAS partially fillers attacked.are partially A silicicattacked. gel A is silicic partially gel is formed partially on formed the FAS on filler the of the material begins, and the FAS fillers are partially attacked. A silicic gel is partially formed on the surface.FAS filler The surface. released The calcium released and calcium aluminum and aluminum ions are able ions to are form able ionic to form bonds ionic with bonds ionized with carboxylic ionized FAS filler surface. The released calcium and aluminum ions are able to form ionic bonds with ionized groups.carboxylic Fluoride groups. ions Fluoride are also ions released. are also released A second. A reaction second reaction of resin polymerizationof resin polymerization occurs duringoccurs carboxylic groups. Fluoride ions are also released. A second reaction of resin polymerization occurs mixing,during mixing, where monomers where monomers can copolymerize can copolymerize with silanated with FASsilanated fillers, FAS silanated fillers, unreactive silanated fillers,unreactive and during mixing, where monomers can copolymerize with silanated FAS fillers, silanated unreactive otherfillers, monomers. and other monomers. At the end ofAt the the reaction, end of twothe reac interconnectedtion, two interconnected networks are obtainednetworks with are obtained covalent fillers, and other monomers. At the end of the reaction, two interconnected networks are obtained linkswith betweencovalent bothlinks duebetween to the both modified due to polyacrylic the modifi acided polyacrylic (MOPOS). acid In water, (MOPOS). calcium, In aluminum,water, calcium, and with covalent links between both due to the modified polyacrylic acid (MOPOS). In water, calcium, fluoridealuminum, ions and (and fluoride eventually ions other (and ions) eventually are able othe to ber ions) exchanged are able with to the be oralexchanged environment. with the oral aluminum,environment. and fluoride ions (and eventually other ions) are able to be exchanged with the oral environment.

Materials 2020, 13, 2313 21 of 28

The same competitive mechanisms described for RM-GICs between acid–base and resin polymerization reactions are suspected in this material.

4.3.2. Mechanical Properties, Ion Release in the Oral Environment, and Indications as a Restorative Material for Use The special feature of Surefil One is that it is indicated by the manufacturer for use in all types of restorations. In vitro and clinical studies are required to assess the accuracy of these indications. This material, which consists partly of water, theoretically promotes water and ion exchange with the oral environment. In particular, this effect leads to the release of fluoride, aluminum, and calcium ions (and probably other ions due to the composition of the reactive fillers) (Figure 20). This ion release has not yet been studied, especially its ability to induce remineralization of the underlying hard dental tissues. Therefore, Surefil One cannot be classified as a bioactive material for the time being. To date, there are no long-term clinical studies that can be used to determine the performance of these materials.

4.3.3. Update on the Classification This material, aside from the distribution of the silanized nonreactive fillers, is very similar chemically to Ketac Nano. In addition, like Activa BioActive Restorative and Ketac Nano, this material contains water, and a significant portion of its setting is based on an acid–base reaction. As such, Surefil One represents a new evolution of RM-GICs, although it appears to have broader indications than the first generations. In the patent, the manufacturer acknowledges that the material is closely related to the GIC family [147].

5. Conclusions The new fluoride-releasing materials represent a chemical evolution of adhesive–resin composite couples, compomers, giomers, and the different families of GICs used for years. These innovations correspond to something more than a simple marketing advertisement. However, although the reactions of these materials are becoming increasingly hybridized among the different families described and increasingly chemically complex, we see many similarities between some of them and those of RM-GICs (for Surefil One, Activa BioActive Restorative, and Ketac Nano). These new materials represent modern evolutions of older material families and should continue to be classified among these families. Whereas the former RM-GIC was shown to be bioactive, the conservation of the bioactive properties of these new formulations has to be proven. Compomers and giomers differ from resin composites only by a mechanism of postpolymerization ion release by water absorption; we suggest classifying them in a unique family called ion-releasing composites (IRCs). The Cention N chemistry is close to that of the IRCs; this material is actually the only material that has been proven to be a real bioactive composite when used without a dental adhesive and should be classified in a new family (Figure 21). Over time, these new formulations may tend to converge toward a composition that will provide the best time/efficiency/activity ratio. Given the lack of clinical experience with these materials, caution should be exercised in their systematic use in all patients. This paper provides a comprehensive overview including new materials: further work is needed to definitively validate some of the chemistries explained here, as well as to determine even more precisely the bioactivity of these materials. Materials 2020, 13, 2313 22 of 28 Materials 2020, 13, x FOR PEER REVIEW 22 of 29

Figure 21. ClassificationClassification of of the the fluoride-releasing fluoride-releasing ma materialsterials according to: the the importance importance of of the acid/baseacid/base reaction or resi resinn polymerization in in their their setting, setting, their their bioactivity, bioactivity, their their bulk-fill bulk-fill properties, properties, and their bioactivity.

Over time, these new formulations may tend to converge toward a composition that will provide theAuthor best Contributions: time/efficiency/activityP.F. conducted ratio. the primary Given literaturethe lack search, of clinical performed experience the chemistry with analysis,these materials, and wrote cautionthe majority should of this be manuscript.exercised in V.F. their wrote systematic a part of thisuse manuscriptin all patients. and contributed to the proofreading of the manuscript. J.-P.A. provided an expert opinion on fluoride-releasing materials. E.D. provided an expert opinion on fluoride-releasingThis paper provides materials a comprehensive and contributed tooverview the writing including of the manuscript. new materials: All authors further have work read andis needed agreed toto thedefinitively published validate version of some the manuscript. of the chemistries explained here, as well as to determine even more preciselyFunding: Thisthe bioactivity research received of these no externalmaterials. funding. Acknowledgments: The authors would like to thank the Research and Development teams of 3M ESPE, AuthorDentsply-Sirona, Contributions: GC Corporation, P.F. conducted Ivoclar-Vivadent, the primary Pulpdentliterature Corporation, search, performed and SDI the for chemistry their contribution analysis, to and the wroteunderstanding the majority of some of this points manuscript. of the chemical V.F. wrote mechanisms a part of this involved manuscript in these and products. contributed to the proofreading of the manuscript. J.-P.A. provided an expert opinion on fluoride-releasing materials. E.D. provided an expert Conflicts of Interest: The authors declare no conflicts of interest. opinion on fluoride-releasing materials and contributed to the writing of the manuscript. All authors have read and agreed to the published version of the manuscript. References Funding: This research received no external funding. 1. Klee, J.E.; Renn, C.; Elsner, O. Development of Novel Polymer Technology for a New Class of Restorative Acknowledgments:Dental Materials. TheJ. Adhes.authors Dent. would2020 like, 22 ,to 35–45. thank [PubMed the Rese] arch and Development teams of 3M ESPE, Dentsply-Sirona,2. Ilie, N.; Hickel, GC R.Corporation, Resin composite Ivoclar-Vivadent, restorative materials. PulpdentAust. Corporation, Dent. J. 2011 and, SDI56, 59–66.for their [CrossRef contribution][PubMed to the] understanding of some points of the chemical mechanisms involved in these products. 3. Imataki, R.; Shinonaga, Y.; Nishimura, T.; Abe, Y.; Arita, K. Mechanical and Functional Properties of a Novel ConflictsApatite-Ionomer of Interest: The Cement authors for declare Prevention no conflicts and Remineralization of interest. of Dental Caries. Materials 2019, 12, 3998. [CrossRef][PubMed] References4. Hung, C.-Y.; Yu, J.-H.; Su, L.-W.; Uan, J.-Y.; Chen, Y.-C.; Lin, D.-J. Shear Bonding Strength and Thermal Cycling Effect of Fluoride Releasable/Rechargeable Orthodontic Adhesive Resins Containing LiAl-F Layered 1. Klee, J.E.; Renn, C.; Elsner, O. Development of Novel Polymer Technology for a New Class of Restorative Double Hydroxide (LDH) Filler. Materials 2019, 12, 3204. [CrossRef] Dental Materials. J. Adhes. Dent. 2020, 22, 35–45. 5. Montag, R.; Dietz, W.; Nietzsche, S.; Lang, T.; Weich, K.; Sigusch, B.; Gaengler, P. Clinical and Micromorphologic 2. Ilie, N.; Hickel, R. Resin composite restorative materials. Aust. Dent. J. 2011, 56, 59–66, doi:10.1111/j.1834- 29-year Results of Posterior Composite Restorations. J. Dent. Res. 2018, 97, 1431–1437. [CrossRef] 7819.2010.01296.x. 6. Vallittu, P.; Boccaccini, A.R.; Hupa, L.; Watts, D.C. Bioactive dental materials—Do they exist and what does 3. Imataki, R.; Shinonaga, Y.; Nishimura, T.; Abe, Y.; Arita, K. Mechanical and Functional Properties of a bioactivity mean? Dent. Mater. 2018, 34, 693–694. [CrossRef] Novel Apatite-Ionomer Cement for Prevention and Remineralization of Dental Caries. Materials (Basel) 7. ISO 9917-1:2007. Dentistry—Water-Based Cements—Part 1: Powder/Liquid Acid-Base Cements; ISO: Geneva, 2019, 12, doi:10.3390/ma12233998. Switzerland, 2003. 4. Hung, C.-Y.; Yu, J.-H.; Su, L.-W.; Uan, J.-Y.; Chen, Y.-C.; Lin, D.-J. Shear Bonding Strength and Thermal Cycling Effect of Fluoride Releasable/Rechargeable Orthodontic Adhesive Resins Containing LiAl-F Layered Double Hydroxide (LDH) Filler. Materials (Basel) 2019, 12, doi:10.3390/ma12193204.

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