EDTA-functionalized silica nanoparticles as a conditioning agent for dentin bonding using etch-and-rinse technique

The tight bonding between resin-based composite materials and dental tissues via adhesives is the basis of oral prosthodontic techniques, including orthodontic treatments, caries filling, indirect prosthesis and fixed prosthesis [1], [2], [3], [4]. The insufficient durability of dental adhesion considerably influences the performance of adhesive restorations over time [5,6]. However, compared with stable enamel bonding, the durability and stability of dentin bonding face more challenges [7].

Dentin is a collagen-based mineralized tissue with high complex hierarchical structure, consisted of 45 vol% minerals and 33 vol% organic materials [8]. Dentin collagen micro-fibrils are composed of regular arrangement of collagen molecules and form a 67 nm periodicity (one D-period) with alternating overlap zone (0.46 D) and gap zone (0.54 D) [9,10]. The minerals in dentin can be divided into extrafibrillar minerals and intrafibrillar minerals based on their positions relative to the collagen fibrils, the former located in the spaces between the fibrils, the latter mainly located in gap zone of the collagen molecules [11,12]. Although more than half of the minerals in dentin are extrafibrillar minerals, intrafibrillar minerals are the key to maintain the mechanical properties of dentin and topography of collagen fibrils [11].

Contemporary dental bonding techniques are mainly including the etch-and-rinse (E&R) and self-etch (S&E) bonding techniques [13]. Although the E&R technique is still the gold standard of dental adhesion, it is more technically sensitive [14,15]. When the dentin surface is etched with phosphoric acid, the dentin matrix on the top surface is completely demineralized and the elastic modulus is reduced [16,17]. After blowing dry the dentin surface, the completely demineralized collagen fibrils will collapse to form an impermeable collagen layer [18]. The theory of wet-bonding proposes that proper moisture can keep the acid-etched-dentin collagen fiber network fluffy open to facilitate the infiltration of adhesives resin monomers and form a hybrid layer (HL) [19,20]. However, there are still some problems that cannot be eliminated when using wet-bonding strategy, such as the optimal moisture conditions during bonding, phase separation, suboptimal polymerization and the residual water will affect the durability and stability of dentin bonding [21], [22], [23], [24]. Self-etch adhesives containing water and hydrophilic monomers with acidic functional groups also based on the concept of wet-bonding [25]. Studies have confirmed that collagen fibrils have molecular sieve effect due to the hierarchical arrangement of collagen molecules [16]. Molecules smaller than 6 kDa can access to the intrafibrillar water compartment freely, while molecules between 6-40 kDa can partially penetrate and those lager than 40 kDa are completely excluded [26,27]. The molecular weights of phosphoric acid and acidic monomers are much smaller than 6 kDa, leading to intrafibrillar demineralization regardless of whether E&R or S&E techniques are adopted. Due to the nanostructure of dentin collagen fibrils, complete infiltration of resin monomers is impossible by E&R technique or S&E technique [28]. The intrafibrillar spaces is too small for resin monomers to completely displace water within intrafibrillar compartments of completely demineralized dentin fibrils [29], leading to imperfect HL. In addition, the intrafibrillar demineralization by acid conditioners can expose and activate the endogenous protease within the collagen matrix regardless of E&R or S&E procedure adopted, which will further degrade the collagen fibrils within HL [30].

It is considered that preserving the intrafibrillar minerals will improve the durability of dentin bonding. Studies have achieved partial dentin demineralization by using phosphoric acid with low concentration [25] and some studies have shown that the ethylenediaminetetraacetic acid (EDTA) as a substitute for phosphoric acid for etching enamel and dentin [31], [32], [33], [34]. However, the molecular weights of phosphoric acid and EDTA are smaller than 6 kDa, leading to intrafibrillar demineralization. Recently, chelators with molecular weights greater than 40 kDa have been proposed for extrafibrillar demineralization, such as chitosan, glycol chitosan-EDTA, carboxymethyl chitosan, sodium salt of polyacrylic acid [16,[35], [36], [37]]. In this strategy, only extrafibrillar minerals were selectively removed and retained the intrafibrillar minerals, thus maintaining the mechanical properties of collagen fibrils and creating the spaces for the infiltration of resin. The above experiments showed the potential of extrafibrillar demineralization in dentin bonding. All the above studies are the synthesis of polymeric conditioners to achieve dentin extrafibrillar demineralization [16,[35], [36], [37]].

In order to achieve the extrafibrillar demineralization of dentin collagen, the intrafibrillar minerals should be retained, which mainly locate in the gap zone [11]. Based on these bases, it is surmised that a conditioning agent with a diameter larger than the gap zone (approximately 36.18 nm) can achieve extrafibrillar demineralization of dentin. At present, there has been no study on the combination of inorganic and inorganic materials or inorganic materials as dentin-conditioning agent for extrafibrillar demineralization. As inorganic materials, silica nanoparticles (SiO2) have large surface area, controllable particle size, good biocompatibility, uniform pore size, and easy to modified-surface [38,39]. EDTA can be used as a milder dentin demineralization agent to avoid the denaturation of collagen and inhibit the activity of MMPs [40,41]. N- [(3- trimethoxysilyl) propyl] ethylenediamine triacetic acid (TMS-EDTA) is an EDTA derivative in which a carboxylic group is replaced with an active trialoxy silane, which binds to the surface of SiO2 [42]. TMS-EDTA covalently bonded to nanoparticles has better long-term stability than EDTA directly bonded to the surface of nanoparticles through electrostatic interactions [43,44]. The purpose of this study was to evaluate if EDTA-functionalized SiO2 (SiO2-EDTA, 50 nm) may be used as a dentin-conditioning agent for etch-and-rinse technique to achieve extrafibrillar demineralization characterized by more intrafibrillar minerals and promote the durability of dentin bonding. Accordingly, 3 null hypotheses were tested in the present study: (1) SiO2-EDTA would not selectively demineralize dentin to retain intrafibrillar apatite crystallites when compared with EDTA; (2) The dentin surface conditioned with SiO2-EDTA would not achieve dentin bond strength comparable to that conditioned with EDTA; (3) SiO2-EDTA would have no effect on the collagenase activity and nanoleakage within HL when compared with EDTA.

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