Since the introduction of three-step etch-and-rinse adhesive systems, dental manufacturers have been striving to simplify bonding procedures while maintaining bond strength that meets clinical requirements [1]. Although newer products have made application easier and faster, this does not necessarily translate to stronger and more durable resin-dentin bonds, when compared with previous generation products [2], [3]. The resin-dentin interface remains the weak link in the longevity of direct and indirect tooth restorations. Interfacial integrity deteriorates over time, resulting in retention failure and ultimately restorative failure [4], [5]. Therefore, the development of user-friendly and effective dentin bonding systems is still an ongoing pursuit.

Dentin possesses a hierarchical hard tissue structure that comprises minerals, type I collagen fibrils, non-collagenous proteins and water [6]. Minerals present in dentin may be divided into inter- and intrafibrillar minerals. Interfibrillar minerals are located in the spaces between the fibrils, whereas intrafibrillar minerals are predominantly present within the gap zones that separate the collagen molecules [7].

Dentin bonding involves the use of acids, chelating agents or acidic resin monomers to demineralize the inter- and intrafibrillar minerals within the dentin surface [8]. This demineralization process creates a moist, microporous collagen network that resin monomers can infiltrate and polymerize in-situ to create a hybrid layer that is critical for dental adhesion [9], [10]. Ideally, adhesive resin monomers should completely replace the free and loosely-bound water from the inter- and intrafibrillar water compartments of the demineralized collagen fibrils. When the intrafibrillar water is completely displaced, the adhesive resin monomers should be able to separate the demineralized collagen molecules to prevent their hydrolysis or enzymatic degradation. Nonetheless, the dentin's organic matrix boasts intricate hierarchically organized nanostructures. At the fibrillar level, the interfibrillar spaces in the acid-etched dentin are occupied by highly hydrated, negatively charged proteoglycans, which collectively create a hydrogel-like matrix within this region. This "molecular sieving" effect restricts the deeper penetration of larger dimethacrylates, such as BisGMA, towards the base of the hybrid layer, which leads to lots of exposed unencapsulated collagen fibrils at the bottom of the hybrid layer [11], [12]. At a more microscopic level, the intrafibrillar spaces has been determined to fall within the range of 1.26–1.33 nm, while a single fully extended monomer, like triethyleneglycoldimethacrylate (TEGDMA), spans a length of about 2 nm. Compounded by the hydrophobic nature of these monomers, the existing adhesive components are unable to completely displace the water residing within the intermolecular spaces of the collagen fibrils [13], [14], [15]. Consequently, the resulting hybrid layer contains mineral-depleted, resin-sparse and water-rich zones within collagen fibrils [16]. Previous studies reported that conditioners and adhesives may expose and activate endogenous dentin proteases such as matrix metalloproteinases (MMP) and cysteine cathepsins during the bonding process [17], [18]. These endogenous proteases can degrade the denuded collagen fibrils in the hybrid layer in the presence of water, resulting in disruption of the integrity of the resin-dentine interface [19].

Different techniques have been used to improve the durability of resin-dentin bonds. These techniques include the use of MMP inhibitors and collagen cross-linking agents [19]. However, these approaches do not address the water trapped within the intrafibrillar spaces of collagen fibrils. These entrapped water molecules represent a significant impediment to optimal dentin bonding [20]. Even in the absence of enzymatic degradation, the denuded collagen fibrils in the hybrid layer remain weak and flimsy. They are vulnerable to fatigue stress, which can lead to bond failure over time in vivo [20], [21].

Biomimetic remineralization is a process that imitates natural biomineralization by replacing the water in the demineralized collagen matrix with apatite crystallites both within and between the collagen fibrils [22]. This post-bonding technique enhances the longevity of resin-dentin bonds by allowing apatite crystallites to re-occupy the intrafibrillar spaces and protect the collagen matrix from enzymatic degradation [20], [23]. However, the inclusion of non-adhesive remineralization components in dentin adhesives may weaken the adhesive resin network after polymerization. Such a compromise adversely affects immediate and post-aging bond strength [24].

Socrates' adage "prevention is better than cure" holds true for many facets of medicine. These words of wisdom open a new vista in the realms of dentin bonding. By selectively demineralizing dentin to remove only the interfibrillar minerals while leaving intrafibrillar minerals intact, the need to replace intrafibrillar water with resin monomers may be avoided. Mild demineralization agents such as 5-wt% phosphoric acid, ethylenediamine tetra acetic acid, and mild acidic functional monomers have been used to create partially-demineralized dentin surfaces for improved bonding [25], [26], [27], [28]. However, size exclusion theory reveals that only molecules smaller than 6 kDa can infiltrate the intrafibrillar compartments. [29], [30]. In addition, molecules larger than 40 kDa are completely excluded from all intrafibrillar water compartments [29], [30]. Because mild demineralization agents with molecular weights lower than 6 kDa are small enough to penetrate the intrafibrillar spaces, it is impossible to limit demineralization to extrafibrillar minerals during bonding procedures to dentin.

Based on the aforementioned size-exclusion characteristics of collagen fibrils, a novel selective demineralization strategy has been developed using synthetic and natural polymeric chelators with molecular sizes larger than 40 kDa [31], [32], [33], [34]. The first chelator tested was the sodium salt of poly(acrylic) acid; this was followed by the introduction of more biocompatible chitosan and chitosan derivatives [31], [32], [33]. To reduce dentin conditioning time, Guo et al. designed a novel chitosan derivative by conjugating glycol chitosan with EDTA. This derivative was functional within a clinically-acceptable time frame (30 s), while retaining the favorable properties of chitosan such as its antimicrobial activity and biocompatibility [34]. The water-soluble EDTA-conjugated glycol chitosan removes minerals from the extrafibrillar spaces only for the infiltration of adhesive resin monomers. The resultant dentin bond strength was equivalent to that achieved using phosphoric acid-etching prior to bonding with the same etch-and-rinse adhesive. Follow-up studies identified that the selective demineralization strategy achieves more durable resin-dentin bond than those created with the conventional etch-and-rinse approach [35], [36], [37], [38].

To date, the experimental extrafibrillar dentine demineralization concept has primarily been adopted for the etch-and-rinse approach. Its application in the self-etch approach remains largely unexplored. Self-etch adhesive systems eliminate the need for a separate acid-etching step of the enamel and dentin substrates, making them more user-friendly, less technique-sensitive and more extensively used by clinicians [10]. In additional, because the rinsing step is eliminated, the so-formed resin-dentin interface contains more functional molecules of the conditioner compared to the etch-and-rinse approach.

Thus, the objective of the present study was to synthesize a light curable chitosan derivative with selective extrafibrillar demineralization capability, and to investigate its potential use in the self-etch bonding approach. Three null hypotheses were tested: 1) the chitosan derivative does not retain intrafibrillar minerals when it is used for conditioning dentin; 2) there is no difference between the dentin bond strength achieved with a polymeric chelator-based primer prepared from the chitosan derivative, and those achieved using commercially available self-etch adhesive or etch-and-rinse adhesive; 3) the chitosan derivative has no antibacterial activity or anti-gelatinolytic activity.