An Insight into Enamel Resin Infiltrants with Experimental Compositions

Resin infiltration is a conservative treatment of initial enamel carious lesions. Only one infiltrant material is available on the market (Icon, DMG), and research is now investigating new chemical compositions so as to further exploit the benefits of the resin infiltration technique. A literature search of the articles testing the effects of different formulations on mechanical properties, resin penetration ability, remineralizing, and antibacterial activities was conducted. Of 238 articles, 29 resulted in being eligible for the literature review. The formulations investigated were all different and consisted in the inclusion of hydrophobic monomers (i.e., BisEMA, UDMA), solvents (ethanol, HEMA), alternative etchants (PAM) or molecules with antibacterial or bioactivity features (i.e., AgNP, YbF3, MTZ, chitosan, DMAMM, HAp, MC-IL, NACP, PUA, CHX) and microfilled resins. Information on the long-term performances of the tested experimental materials were scarce. The combination of TEGDMA with hydrophobic monomers and the inclusion of a solvent alternative to ethanol reinforced mechanical properties of the materials. Hybrid-glass materials demonstrated an enhanced remineralization capacity. Techniques such as tunnelization increased the penetration depth and preserved the recourse to less-conservative treatments. Combining the min-invasive infiltrant approach with remineralizing and bacteriostatic properties would be beneficial for therapeutic and economical aspects, according to the principles of minimally invasive dentistry.


Introduction
Caries is a chronic disease with the highest incidence worldwide, entailing social and economic implications [1]. An imbalance between demineralization/remineralization processes initiates the carious development through the continuous loss of OH⁻ ions from the apatite crystals of dental enamel. When physiological remineralization fails to compensate for enamel demineralization, translucent, non-cavitated areas of enamel characterized by increased porosity are formed and clinically known as white spot lesions (WSLs). At this stage, preventive protocols can be implemented with individual oral hygiene instructions and diet counseling to avoid bacterial proliferation, which would invariably lead to the formation of cavitated carious lesions. Various treatments can assist with home oral and dietary routines, such as fluoride, xylitol or caseinates [2][3][4]. Despite improvements in the clinical behavior of these materials, in the presence of WSLs, they can only act superficially, as they do not have the ability to reach deep into the demineralized enamel [5]. Moreover, the patient's cooperation is mandatory to maximize the effects of such treatments.
The use of a resin capable of infiltrating the porosity of demineralized enamel, penetrating, and reaching its depth was first introduced in 2009, with Icon (DMG, Hamburg, Germany) representing the pioneer and, at the moment, the solely patented product present on the market [3,[5][6][7]. Icon is a low viscosity, unfilled light-curable solution based on triethylene glycol dimethacrylate (TEGDMA) diluted in ethanol and containing camphoroquinone (CQ) as a photoinitiator. The infiltration technique consists of a first step of hydrochloric acid etching necessary to produce changes in the enamel structure by removing organic material, opening interprismatic spaces and providing mechanical interlocking with the resin [7,8]. In order to more uniformly clean the surface and improve the penetration and retention of the resin, deproteination (i.e., 1% NaOCl) has been recommended before the etching procedures [9,10]. A low viscosity resin is then guided by capillary forces to penetrate deeply into the carious lesion and occlude the dentinal orifices, thus sealing the enamel pores and possibly arresting the caries progression [11,12]. The quality and quantity of resin depth penetration and bond are driven by the chemical composition and mechanical properties of the infiltrant material [12].
Several laboratories [13,14] and clinical studies [12,15] have supported the use of this technique over standard filling practices or resin groove sealants [10,16,17]. Besides providing drill-free treatments, the resin infiltration technique also concurs to decrease tooth sensitivity and improve the esthetics by reducing the pearly white effect typical for the WSLs [18][19][20][21], demonstrating a higher masking effect on labial WSLs when compared to conventional remineralitazion approaches (i.e., hydroxyapatite-and fluoride-based products) [22].
From an esthetic point of view, in the case of mismanipulation or in the case of noncompliance with the instructions for use, the infiltrant resin could be prone to dye absorption possibly jeopardizing the color stability [23,24]. The Icon has undoubtedly allowed for a minimally invasive restorative approach, postponing as much as possible more aggressive treatments thanks to its higher ability to penetrate into the carious lesion compared to sealants or flowable composites [16]. It was noteworthy that some studies observed incomplete resin infiltration of the entire depth of the carious lesion, casting doubts on the tightness of the seal over time [25,26]. However, in those studies, the presence of an organic matrix not completely removed by the sole etching procedures and possibly hampering material's infiltration, was not taken into consideration [9,10].
Considering the appealing opportunity provided by the resin infiltration technique, the research is continuously moving to expand its therapeutic spectrum by testing new approaches and experimental formulations. Designing a material combining the infiltration mechanism with remineralizing capacity and bacteriostatic activity would be beneficial in further paralyzing the progression of the carious lesion into the underlying dentin [8,[27][28][29][30][31]. The technology of the penetrating resins is generating more and more interest among clinicians and researchers, moving toward new formulations that could enhance the strategy established with Icon. In light of these considerations, this literature review aimed to provide a comprehensive perspective of the experimental possibilities currently available to exploit the benefits of the resin infiltrant approach.

Materials and Methods
Two independent investigators conducted a comprehensive literature review of the 2 databases, Pubmed and Scopus (C.M and T.M.). The following keywords were used: "resin infiltration", "resin infiltrations", "resin infiltrant", "resin infiltrants", and "experimental", along with a combination of the different terms. The limit search was set from 2009, when DMG commercialized the first (and unique) infiltrant resin (Icon). Only articles in the English language were taken into consideration. Posters and conference proceedings were not considered for the analysis. The last search was conducted in September 2022. A first selection of the articles was made with the reading of the titles and abstracts. Those studies not considered suitable for inclusion, that is, those articles that did not have the objective of studying experimental formulations of infiltrating resins, were not taken into consideration for the reading of the full text. The following data were extracted from the full-text readings: the author and year of publication, the main country in which the research was conducted (coinciding, where different nationalities are present, to that of the first author), the type of infiltrating resin used (usually taken as a reference for the control group), the objective of the research, experimental formulations of the infiltrating resin, tests performed, and conclusions of the studies. These variables were collected in a custom-made table in MS Excel.

Results
Of the 238 titles obtained from the first research, those studies not considered relevant to the purpose of the review, i.e., those studies that did not have as their object the infiltrating resins and their experimental formulations, were eliminated. In addition, duplicate articles were also subtracted. A total of 20 articles were considered eligible, with an additional 9 studies included after a cross-sectional analysis of the pertinent bibliography. Finally, 29 articles were considered for inclusion in the review process. The data obtained from the included articles after the review process are presented in Table 1.
In general, the formulation containing TEGDMA/UDMA positively enhanced the mechanical properties, while the inclusion of ethanol as a diluent resulted in the most deleterious effects. Similarly, pre-heating affected the mechanical properties of the resins investigated [12,[30][31][32][33]. Techniques such as tunnelization were able to increase the penetration ability of Icon [28]. The inclusion of organic fillers into Icon formulation demonstrated good penetration ability with enhanced sealing ability, both in interproximal [31] and occlusal [8] cavitated carious lesions with different ICDAS (2,3 and 5, respectively). The remineralization activity could be promoted by the inclusion of hybrid glass with a high resistance to acidic/cariogenic challenges [52].
Only three studies included the aging of the specimens, although with a limited period of time [11,41,51]. No long-term studies were reported, nor were clinical trials present on the use of experimental infiltrant materials with chemical formulations different from Icon.

Discussion
The benefits of resin infiltration for the treatment of cavitated and non-cavitated carious lesions, both on interproximal and occlusal dental tissue, have been widely accepted [17,32,50,52,53].
From a chemical point of view, Icon is a low viscosity, unfilled light-curable (CQ) solution based on TEGDMA diluted in ethanol. The absence of fillers favors its very low viscosity, giving it the ability to penetrate deeper spaces than a more viscous resin [10,28,31,50]. The prevalence of TEGDMA in Icon composition, considered a highly hydrophobic molecule par excellence, influences the interaction of the resin with the enamel tissue [35,50]. The presence of ethanol has been considered the most significant limitation in resin infiltrant's composition, as it may hamper the penetration of the resin up to the entire depth of the carious lesion [39] and might not be completely evaporated, possibly compromising the polymerization reaction [47,54]. In light of the aforementioned considerations, researchers continuously move to expand the possible use of Icon by investigating alternative resin formulations to prevent caries progression, postpone aggressive treatments and, finally, fulfill the minimum intervention dentistry requirements [27,28,49].
When formulating novel molecules, it is fundamental to consider their physicalmechanical characteristics, which must correspond to well-defined physical and biological requirements [55]. The new-designed infiltrant materials are requested to protect the infiltrated tissue and the adjacent demineralized area from recurrent bacterial attacks, thus preventing caries progression. To this end, an ideal resin infiltrant material should possess specific mechanical properties to interact with the demineralized/decalcified tooth tissues, withstand occlusal forces or functional habits (i.e., brushing) and prolong the effectiveness of the seal over time (Table 2). Even though this concept could be hard to obtain in a single product, nanotechnology, and the possibility of introducing bioactive fillers into the mate-rial's composition, it represents feasible strategies to fulfill the requirements of minimally invasive dentistry and impart bioactivity. Table 2. Template summarizing an ideal resin infiltrant material according to the information found in the literature regarding formulations and techniques. This literature review helped unfold limits and possibilities of the experimental formulations. It was not possible to extrapolate clear indication regarding the best materials to increase the antibacterial capacity and color stability of a potential novel resin infiltrant due to a variety of molecules tested with no clear standardization of formulations and setup of the studies.

Mechanical Properties
The mechanical properties and physical-chemical characteristics regulate the therapeutic effects of the material. Accordingly, any improvement in the material's properties would result in an increased therapeutic effect.
The degree of conversion (DC) of a material assumes importance not only for the degree of loose carbon-carbon bonds, but also because it, in turn, conditions many parameters such as water sorption and solubility, surface hardness, color stability, and chemomechanical properties [56]. The basic characteristic that an infiltrating resin must possess is a DC of not less than 50%. The DC of Icon was estimated around 55% [43,57], eventually limited by the high concentration of TEGDMA, which is characterized by a rapid vitrification effect that could limit the polymeric conversion of the resin [58]. Some authors have speculated that the high amount of TEGDMA and the absence of filler contents in Icon formulations could affect the resin micro-hardness [35,59], as well as its high molecular weight might influence the solubility rate of the resin [60].
The DC is directly related to the water sorption of a resin-based material, according to which the lower the DC, the higher the water sorption [61]. The water sorption/solubility regulates several physio-chemical mechanisms, possibly negatively influencing the structure of the material. A degraded polymer matrix would reduce the longevity and function of resin-based materials [54,[62][63][64].
TEGDMA, the main component of Icon, is a highly hydrophilic monomer, possibly exposing the resin to premature hydrolytic degradation [28,65]. The inclusion in Icon of additional monomers (i.e., hydrophobic) could enhance the material's mechanical properties and maximize its therapeutical potential [58]. Accordingly, more hydrophobic, lowviscosity monomers, i.e., UDMA and BisEMA, have been tested [12,30,32,47]. Studies have demonstrated that the inclusion of such hydrophobic monomers into the resin infiltrant Icon did not impair its mechanical properties but could be proposed as an enhanced formulation [32]. Among these chemical combinations, TEGDMA/UDMA resulted more than others in improved DC, E-modulus, surface hardness, and bond strength [12,30,32].
Previously, an increase in enamel surface hardness after using Icon as infiltrating resin has been observed [66]. The experimental formulations have demonstrated a beneficial effect in increasing the surface hardness of the enamel with which the resin comes into contact. This feature becomes even more necessary in the case of a tooth affected by developmental defects (i.e., MIH, amelogenesis imperfecta, etc.) [67]. In those teeth, the resin has demonstrated better effects than resinous sealants when used in cavities with ICDAS 1 and 2 [68,69]. Since the teeth with developmental defects are highly exposed to premature fracture [4], increasing the surface hardness would allow them to increase the possibilities of maintaining the dental structure over time.
The type of solvent has been considered the most influencing factor on the mechanical properties of experimental resin infiltrants [12,32,38,39]. In particular, the presence of ethanol in their composition could impair the mechanical properties, possibly due to an interference with the polymerization of the material resulting in reduced cohesive strength, increased softening, higher water sorption, and contact angle [12,32,39], and finally influencing the depth of penetration of the resin [12].

Penetration Depth
A penetration coefficient (PC) equal to or higher than 100 Cm/s is essential to achieve a uniform penetration into WSLs [12,49]. Materials mainly composed of TEGDMA with PC higher than 200 Cm/s demonstrated a higher penetration ability and the chance to inhibit the progression of the carious lesion with respect to other materials in which more hydrophobic monomers and solvents have been added [11]. However, when the same resin formulations were used on primary teeth, no differences in penetration depth were observed between resins with different penetration coefficients [50]. Notwithstanding these results on primary teeth, as previously affirmed for permanent teeth [11], resins with higher penetration coefficient and ethanol-free should be preferred in order to achieve deeper resin infiltration [11,50].
The penetration depth is directly correlated with the ability to arrest the caries progression [7,14,70]. Ideally, an infiltrant resin should possess a high PC in a clinically reasonable application time [11]. Taking into account the first aim of infiltration therapy able to postpone less conservative restorative procedures, re-application of the resin into already infiltrated areas has been proposed to reinforce the beneficial effects of Icon [10,28]. Moreover, the inclusion of micro-organic fillers into Icon composition could provide a better sealing and contribute to prolonging the durability of the resin infiltrant over time [31]. Regarding the alternative formulations found in the present literature review, many attempts made to increase resin penetration capacity consisted in the inclusion of monomer mixtures such as BisGMA, HEMA, or BisEMA. According to the results, the monomer combination often resulted in the worsening of the material's mechanical properties and did not always improve penetration depth. In order not to interfere with viscosity and penetration, the filler concentration should not exceed the 15%wt [34,71]. This should be taken into consideration when formulating novel resin infiltrant formulations. Alternative techniques have been proposed to overcome the problem of the incomplete penetration depth, such as double internal/external infiltration [10,28] or resin pre-heating [38]. The internal/external infiltration has been considered a viable technique to treat proximal lesions extending to dentin [10,28], contributing to the occlusion and stabilization of caries lesion porous areas and resulting in the preservation of the proximal contact area [10]. This technique should be further explored to increase the lifetime of the infiltrating material. Instead, resin preheating, in combination with alternative solvents, did not improve mechanical properties nor increased the penetrability into the enamel [38]. The solvent should increase the PC of the material into the enamel rods; however, structural inhomogeneities in the form of unpolymerized areas within the resin attributing to the type of solvent, of which ethanol was considered the major culprit, have been observed [11,32,38].

Antibacterial Activity
Different molecules with previously proved bacteriostatic capacity have been included in the original formulation of Icon to evaluate their effects on the mechanical properties and assess their validity as antibacterial products [18,21,28,30,[33][34][35][36][37]39,40,44,51]. The review has highlighted a heterogeneity of molecules tested, and it was challenging to outline which is the most performing formulation for the purpose of antibacterial activity. All in-vestigated experimental formulations resulted in enhanced antibacterial activity in a short period of time. However, these results need to be confirmed by long-term studies.

Remineralization Activity
The combination of a microinvasive infiltrant approach with remineralization capacity would represent a therapeutic possibility to postpone restorative procedures over time, with consequent cost savings for both patients and health institutions [72]. Several strategies have been proposed to furnish Icon with certain bioactivity by blending bioactive fillers and fluoride complexes into the resin composition. The formation of fluorapatite complexes has been confirmed to increase the resistance of dental tissues to acid attacks [56]. The inclusion of fluoride-containing complexes in composite resins and orthodontic cements has demonstrated promising results in preventing the formation of WSLs and reducing bacterial biofilm rooting. Hashemian et al. evaluated the effects of remineralizing fillers such as fluorohydroxyapatite (FHA) and fluoride-doped bioactive glass (FD-BG) added to an experimental TEGDMA-based infiltrating resin [49]. According to the authors, promising remineralizing results have been achieved with the experimental formulations without affecting the resin's mechanical properties. Very recently, Dai et al. have evaluated the re-demineralizing ability of nanoparticles of amorphous calcium phosphate (NACP)based compound [43]. The incorporation of 30% NACP into the Icon formulation resulted in increased mechanical properties, optimal biocompatibility, and the possibility of Ca and P ion release over a period of 14 days [43]. Many factors may influence the material's bioactivity in the oral cavity, such as the saliva and patient's compliance [73], and these should be considered in future studies.

Color Stability
WSLs represent the first manifestation of an initial, non-cavitated carious lesion. The initial subsurface demineralization promoted by a bacterial acidic attack leads to the formation of micropores between enamel rods, mineral loss, and increased surface roughness. From an optical point of view, this phenomenon results in light refractive alterations creating the typical pearly effect of [74]. WSLs represent a critical esthetical issue, often creating discomfort and consequences in the social life of adolescents and adults. The resin infiltration technique ameliorates the white appearance by filling the spaces between enamel rods [3,51]. One study experimented different combinations of TEGDMA with bioactive glass or nano-fluorohydroxyapatite. Irrespective of the percentage, the latter obtained good masking effects of artificial WSLs maintained after 14 days [48]. Among the studies analyzed, only two focused on the effect of Icon and the experimental formulation on in vitro enamel optical properties [43,51]. Neither the quaternary ammonium compound [51] nor the calcium phosphate nanoparticles [43] investigated altered the chameleon performance of resin infiltration, presuming the effectiveness of these formulations from an esthetical point of view. Considering the corroborated results on the antibacterial activity of quaternary ammonium compounds [75] and the remineralizing effects of CaP nanoparticles [24], it would be worthwhile to push additional investigations in their direction.

Root Caries
Considering the lengthening of the average life span and the increasing possibility of preserving many teeth in the oral cavity, geriatric dentistry is becoming increasingly popular. Dental tissues undergo a number of physiological changes over time, of which gingival retraction, root exposure, and the possibility of developing non-carious cervical lesions (NCCLs) are the primary consequences of the aging process [76,77]. In the presence of non-optimal hygiene and pathological situations that occur with age (diabetes, drug intake, etc.), the patient is easily exposed to the development of caries. In this case, a minimally invasive therapeutic intervention, such as the resin infiltration technique, could be an optimal clinical option in the case of a debilitated or poorly cooperative elderly patient.
Taking these aspects into consideration, the effect of several experimental infiltrating resins has also been tested on cement and root dentin [45,46]. A pilot study suggested the possible use of an experimental resin composed of a mixture of TEGDMA, HEMA, DMAEMA and antibacterial components able to penetrate root tissues, not only enamel/cementum but also at the root/dentin level [45]. Contrarily, no differences between experimental blends and commercial solutions were identified when YF3 was included to increase the coefficient penetration into root caries [46].

Conclusions
The resin infiltration technique, with its pioneer product Icon, has expanded the dogma of minimally invasive dentistry. Great efforts are continuously made in the material's technology to provide a material with increased potentials. Within the limits of this study, the combination of TEGDMA and hydrophobic monomers (i.e., TEGDMA/UDMA), other solvents alternative to ethanol, and molecules with antibacterial capacities seems to be the direction to which research is directed. Techniques such as tunnelization or the inclusion of microfiller or hybrid glasses resulted in improved penetration depth, sealing ability, and remineralizing effects. However, the heterogeneity of the experimental compositions present in the literature does not allow for a standardization of the formulation.