Study Of Functionalized Monomer With Aldehyde-based Modification To Improve Dentin Bond Durability

Robust and durable dentin bonding depends on the long-term stability of the hybrid layer.The hydrolysis of the resin matrix in aqueous environments and the degradation of the dentin matrix initiated by matrix metalloproteinases(MMPs)and cysteine cathepsin have been identified as the predominant ingredients responsible for the deterioration of the hybrid layer.Therefore,the enhancement of the hydrolysis resistance of the hybrid layer along with the infiltration of the adhesives so as to the protection of the dentin collagen matrix is considered to be the main direction to improve the durability of the dentin bonding.The current commercial adhesives have no functional components that specifically aim at chemical bonding with collagen fibers.Exogenous cross-linking enhances the biomechanical properties,stability,and resistance to collagenase degradation by inducing the formation of additional inter-and/or intramolecular cross-links through non-enzymatic reactions,thereby improving the bonding stability.The essential issue of the absence of resin–collagen covalent bonding can be addressed by chemically derivatizing the acrylate ester ends in a proven cross-linker molecule.Inspired by the rapid chemical binding of aldehyde to proteins,we designed and synthesized a hydrophobic aldehyde-based functional monomer,4-formylphenyl acrylate(FA).It has been investigated in three aspects of its application as a primer for etch-and-rinse adhesives,as an induced bionic mineralization,and as a dilution system in the experimental adhesives,capitalizing on the molecule’s properties of chemically binding to collagen fibers.This study was conducted to validate the potential of novel monomers in the field of dentin bonding,to compensate for the deficiency in components that specifically bind collagen fibers in commercial adhesives,and to provide new perspectives for improving the stability of dentin bonding.The toxicity of small molecule monomers could be reduced by imparting curable properties to this compound.Based on such rationale,in Chapter 2,we synthesized and characterized FA monomers by grafting acrylic groups onto formaldehyde molecules,and the products were characterized using fourier infrared spectroscopy(FTIR),nuclear magnetic resonance(~1H NMR),and high resolution mass spectrometry(HRMS).To verify the cytotoxic effect of FA monomer,the in vitro cytotoxicity of FA was detected by MTT assay and live/dead cell staining technique.The in vivo cytotoxicity was detected by gastric perfusion in SD rats.FA was subsequently prepared in different gradient concentrations of dentin primers and the chemical binding of FA to dentin collagen was investigated by attenuated total reflection Fourier transform infrared spectroscopy(ATR-FTIR),thermal weight loss analysis(TGA),and collagenase degradation experiments.The optimal group was selected according to the effect of FA primer on the rate of conversion of the two universal adhesives,and then grouped according to the treatment time gradient to examine the dentin bond strength and the degree of nanoleakage before and after aging.The optimal processing time was selected,and laser confocal microscopy(CLSM)and scanning electron microscopy(SEM)were used to observe the dentin bonding interface.FTIR,1H NMR and HRMS results showed that FA was successfully synthesized,and the results of ATR-FTIR,TGA and collagenase degradation experiments showed that as a binding bridge,FA achieved covalent bonding between acrylate-based dental adhesive and collagen matrix,improved the physicochemical properties and resistance to enzymatic degradation of collagen fibers,and compensated for the deficiency of chemical bonding with dentin collagen of currently available commercial adhesives.Through its exceptional infiltration capability,FA primer has the potential to modify the dentin surface and induce deep penetration of the bonding resin monomer into the interface of dentin,which in turn improves the sealability and resistance to degradation of the bonding interface and enhances the stability of dentin bonding without compromising the conversion rate of the two commercial bonding agents.More importantly,FA possesses good biocompatibility.The essential for the formation of a stable binding is the elimination of water within the bonding interface.One function of the presence of apatite within mineralized collagen fibers is to exclude water molecules from the mineral matrix.Based on the characteristic polyanion-rich structure of human dentin non-collagenous proteins(NCPs),and relying on the promising infiltration of FA monomer into the demineralized dentin interface and its ability to specifically bind the collagen matrix,in Chapter 3,we co-polymerized FA monomer with acrylic acid(AA)to synthesize polyanion-aldol block copolymers containing different functionalities.Through the coordinated binding of the aldehyde group of this molecule to the specific site of collagen,the anion-rich ions can be bound to the collagen fibers.On the one hand,it attracts calcium and phosphorus ion accumulation,on the other hand,it promotes the formation of amorphous calcium phosphate(ACP),thus inducing in situ mineralization of demineralized dentin.The products were first characterized by ~1H NMR and gel permeation chromatography(GPC),followed by ATR-FTIR and high performance liquid chromatography(HPLC)to verify the binding of FA-AA to collagen;finally,the degree of in situ mineralization was examined by transmission electron microscopy(TEM)observation of charged particles in solution,zeta potential analysis,surface hardness test of demineralized dentin and SEM observation.The ~1H NMR and GPC results indicated that the FA-AA copolymer was successfully synthesized.The ATR-FTIR and HPLC results confirmed the covalent binding ability of FA-AA copolymer to collagen fibers.The TEM results showed that FA-AA was capable of inducing ACP formation,and the crystalline type of the formed particles increased with the increased AA functionalities,creating an increase in the anisotropy of the formed crystals.The particles induced by FA-AA exhibited significantly enhanced negative electrical properties.Finally,demineralized dentin showed varying degrees of mineralization in the presence of FA-AA,and thus its surface hardness increased significantly.We found that the liquid nature of FA gives it permission to act as a dilution monomer for adhesive components,thus reducing the hydrophilic monomer content and improving the hydrolysis resistance of the adhesive.Therefore,in Chapter 4,we incorporated different quantities of FA into the experimental adhesive system in order to gradient reduce the content of hydrophilic monomer hydroxyethyl methacrylate(HEMA).The curing performance of the experimental adhesives was first tested;followed by the examination of the physical and chemical properties(viscosity,flexural strength,tensile strength,surface hardness,thermal stability)after curing.The water absorption/dissolution rate and static water contact angle tests were used for the detection of their hydrophobic properties.The infiltration and inhibition of proteolytic enzyme activity were measured by CLSM observation of resin tags morphology and in situ enzyme spectrometry tests,and the optimal FA content was screened.Finally,microtensile strength tests(μTBs)were carried out and SEM interfacial observation was performed to verify the test results.The results showed that the incorporation of FA significantly improved the polymerization conversion,flowability and mechanical properties of the experimental adhesive,which reached the maximum at 30%.There was no significant decrease in the glass transition temperature of the experimental adhesives with the incorporation of FA.FA-modified adhesives showed significantly lower water absorption/dissolution rates and exhibited significantly higher static water contact angles.The CLSM results showed that the FA-modified experimental adhesives had good infiltration into the dentin,with significant and high number of lateral branches of resin tags forming within the hybrid layer,and the ability to inhibit proteolytic activities at a low concentration(10%).Finally,the FA-modified experimental adhesive achieved higherμTBs and better bonding stability.In summary,utilizing FA’s ability to specifically covalently bind dentin collagen matrix,we investigated and confirmed its value for applications in primer,biomimetic mineralization and dilution systems.This new functional monomer not only can fill the blank of monomers that specific binding dentin organic components in commercial adhesives,but also can reduce the risk of hydrolytic degradation of current diluted monomers.While it still has a high potential for biomimetic mineralization.These advantages help contribute to the promotion of further applications of FA in the adhesive dentistry.