Synthesis And Application Of Self-Healing Microcapsules Enhanced With Novel Nano-Antibacterial Inorganic Fillers In Dental Resin

Background and Objective:Dental resin composites have been the most popular materials for caries restoration owing to their better aesthetic property,simplicity of usage,and adequate mechanical durability,and have become one of the most commonly used materials for caries restoration.However,catastrophic failure of resin restorations occasionally occurs because of microcracks and secondary caries.In recent years,the development of multifunctional self-healing materials provided a new approach to solving these problems.Self-healing material mimics the self-healing phenomenon of biological organisms and repairs microcracks and mechanical damage of the matrix without external intervention.Among them,self-healing materials based on microcapsules are widely used and can be applied to dental resin materials.When microcracks occur,the microcapsules inside the matrix are punctured by the expanded microcracks,releasing the self-healing liquid inside the capsule.The healing liquid flows to the target area via the capillary effect and contacts with pre-set catalysts,triggering polymerization and repairing the microcrack.In addition,adding antibacterial agents to self-healing materials to combat secondary caries and resin fracture,extends the service life of clinical restoration materials.In the previous study,our group synthesized self-healing microcapsules with poly(ureaformaldehyde)(PUF)as capsule shell and triethylene glycol dimethacrylate(TEGDMA)as core material by in situ polymerization method.The experimental results showed that dental resin containing self-healing microcapsules extended great self-healing capability,which was also suitable for adhesives and cements.However,the shell of self-healing microcapsule(SHMC)is usually organic material,which is easy to deform and break during the resin preparation process,affecting microcapsule formation quality.In addition,traditional selfhealing microcapsules only have a single-time self-healing effect.After microcapsules rupture,the shell left in the dental resin may become a new weak point.Hence,the key problem is to strengthen the shell of the microcapsule and endow them with other functions besides self-healing.Therefore,this study synthesized novel self-healing microcapsule enhanced with nano-antibacterial inorganic fillers(NIFs).NIFs were obtained by grafting dimethyl octadecyl[3-(trimethoxysilyl)propyl]ammonium iodide to a nano-silica particle.Then,synthesized novel antibacterial and self-healing dental resin containing novel microcapsules.Explored the novel antibacterial and self-healing dental resin to provide the scientific basis for the expansion of multifunctional dental resin.Materials and Methods:1.Synthesis and characterization of novel nano-antibacterial inorganic fillers-enhanced self-healing microcapsules(1)Synthesis and characterization of novel nano-antibacterial inorganic fillers.First,Dimethyloctadecy[3-(trimethoxysilyl)propyl]ammonium chloride was reacted with potassium iodide to obtain the iodinated quaternary ammonium salt.Then,the iodinated quaternary ammonium salt was grafted onto nano-silica to prepare iodinated quaternary ammonium salt-modified nano-silica.A coupling agent was used to modify the surfaces of NIFs.The surface morphology was observed by field emission scanning electron microscope(FESEM),and the chemical structure was analyzed by Fourier transform infrared spectroscopy(FTIR).(2)Synthesis and characterization of novel microcapsules.NIFs-enhanced multifunctional PUF-TEGDMA microcapsules were synthesized by in situ polymerization and liquid-phase precipitation.The NIFs with mass fractions of formaldehyde of 0%,5%,10%,15%,20%,or 30%were added to PUF-TEGDMA microcapsules to prepare novel NIFs-enhanced antibacterial self-healing microcapsules,denoted herein as M-0,M-5,M-10,M-15,M-20,and M-30,respectively.The surface morphology,formation process,and thickness were explored by optical microscope(OM)and FE-SEM.The chemical composition was identified by energy dispersive spectroscopy(EDS)and FTIR.The thermal stability was analyzed by the thermogravimetric analyzer.The relationship between NIFs addition amount and yield and particle size distribution of microcapsules was also explored.The surface morphology,roughness,and mechanical properties were measured by an atomic force microscope(AFM).Streptococcus mutans was selected to evaluate the antibacterial property of microcapsules.The optimal addition amount of NIFs was determined by considering morphology,mechanical properties,and antibacterial property comprehensively.The self-healing capability and biosafety of antibacterial and self-healing dental resin containing 7.5 wt%novel microcapsules were preliminarily evaluated.2.Synthesis and characterization of novel antibacterial and self-healing dental resin(1)Synthesis of novel antibacterial and self-healing dental resin.The dental resin matrix was prepared by mixing TEGDMA and Bis-GMA in a mass ratio of 1:1,and novel microcapsules were added to the resin matrix at different mass fractions of 0%,5%,10%,15%,and 20%to prepare antibacterial and self-healing dental resin,denoted herein as M0,M5,M10,M15,M20.Another positive control group was added with a mass fraction of 10%NIFs,denoted as N10.(2)Characterization of novel antibacterial and self-healing dental resin.The hydrophilicity of the resin samples was evaluated by measuring the contact angle.The antibacterial property of the resin material was tested using a spread plate method,electron microscopy observation,fluorescent staining and cell counting kit-8(CCK-8).The mechanical properties of the resin material were tested using a three-point bending method.The optimal amounts of microcapsules were determined based on results of hydrophilicity,antibacterial property and mechanical properties.The self-healing performance of the resin samples was detected by the single edge V-notched beam(SEVNB)method.The fracture surface was observed by FE-SEM to analyze the self-healing mechanism.The biosafety of the resin samples was evaluated using CCK-8.3.Dynamic mechanical properties and finite element analysis of novel antibacterial and self-healing dental resin(1)Dynamic mechanical properties of novel antibacterial and self-healing dental resin.The dynamic mechanical properties(the storage modulus,loss modulus,and glass transition temperature)of M0,M5,M10,M15 and M20 dental resin were analyzed using a dynamic mechanical analyzer(DMA).(2)Finite element analysis of novel antibacterial and self-healing dental resin.The finite element analysis(FEA)software ABAQUS was used to establish the mechanical analysis model of microcapsules after matrix cracking,define reasonable boundary conditions,optimize the mesh division,explore the stress state of microcapsules with different thickness,particle size,and Young’s modulus by using the control variable method,and analyze the influence of various parameters of microcapsules on the cracking of microcapsules.Results:1.Novel nano-antibacterial inorganic fillers-enhanced self-healing microcapsules were successfully synthesized(1)Dimethyl octadecyl[3-(trimethoxysilyl)propyl]ammonium iodide was successfully grated to a nano-silica particle to synthesize NIFs.FE-SEM results showed that the morphology of the filler was regular spherical particles.FTIR showed that long-chain alkyl quaternary ammonium salt was successfully grafted onto the surface of nano-silica.(2)Novel antibacterial and self-healing microcapsules were successfully synthesized.Optical microscope,FE-SEM,and AFM results showed that the novel microcapsules were regular spherical shapes with PUF and NIFs particles deposited on the surface.EDS results further verified the presence of NIFs in microcapsules.FTIR results showed that the microcapsules retained the characteristic groups of both shell material PUF and core material TEGD MA.The microcapsules showed great thermal stability with an initial evaporation temperature of about 170℃.With increasing NIFs addition amount,the yield and particle size of microcapsules decreased,and the number of NIFs particles deposited on the surface of microcapsules increased.Taking into account the microcapsule morphology,particle size and yield,M-10 microcapsules were selected for subsequent experiments.AFM experimental results showed that with increasing NIFs addition amount,the surface roughness and mechanical properties of microcapsules increased significantly(P<0.05).Antibacterial experimental results showed that M-10 microcapsules had a great inhibitory effect on Streptococcus mutans(P<0.05).Antibacterial activity increased with increasing concentrations of M-10 microcapsules,and the presence of M-10 microcapsules reduced the CFU by 2-3 orders of magnitude compared to the control group.The self-healing experiments showed that the resin containing 7.5%mass fraction M-10 microcapsules exhibited good selfhealing properties.The results of CCK-8 showed that the resin containing 7.5%mass fraction M-10 microcapsules had good biosafety.2.Novel antibacterial and self-healing dental resin was successfully preparedThe hydrophilicity of novel antibacterial and self-healing resin increased with the increase of microcapsule content.When the mass fraction of microcapsules reached 15%,the hydrophilicity of the resin increased significantly(P<0.05).The antibacterial experiment results showed that,with the increase of microcapsule addition amount,the antibacterial property of resin increased significantly(P<0.05).The bacteriostasis rates of M0,M10,M20 and N10 groups were:0%,80.3%,84.2%and 93.7%respectively.The mechanical property of novel antibacterial and self-healing resin decreased with the increase of microcapsule content.When the mass fraction of microcapsules reached 15%,the flexural strength and elastic modulus of resin decreased significantly(P<0.05).The addition of microcapsules to the resin must not hinder the mechanical properties,so the M10 group was selected for subsequent experiments.CCK-8 results showed that M10 group had good biosafety,and there was no difference from negative control group(P<0.05).The self-healing experiment results showed that the selfhealing efficiency of M10 groups was 66.1%.Compared with Mo group,the self-healing efficiency of M10 group increased significantly(P<0.05).Based on the above results,the optimal addition amount of microcapsules in dental resin was 10%mass fraction,which could achieve good antibacterial and self-healing function without affecting the hydrophilicity,mechanical properties and biosafety of dental resin.3.The addition of novel microcapsules affected the dynamic mechanical properties of dental resin to a certain extent,and the thickness and particle size of microcapsules affect the stress distribution in dental resin(1)DMA test results proved that the addition of novel microcapsule affected the storage modulus,loss modulus,and glass transition temperature of novel antibacterial and self-healing dental resin to a certain extent.(2)The FEA of novel antibacterial and self-healing dental resin.According to FEA results,the smaller thickness and the larger the particle size of the microcapsule,the greater the stress distribution in the resin matrix and on the microcapsules,the more obvious the microcapsule deformation,and the more favorable the microcapsule rupture.Young’s modulus of microcapsules has almost no effect on the stress distribution in the resin matrix.Conclusions:1.Novel NIFs-enhanced microcapsules had good thermal stability,mechanical properties,antibacterial properties,self-healing properties,and good biosafety.2.Antibacterial and self-healing dental resin containing novel microcapsules presented a strong bacteriostasis rate and excellent self-healing efficiency.The novel antibacterial and self-healing dental resin is a promising candidate for use in clinical practice to combat secondary caries and restoration fractures.3.The addition of novel microcapsules affected the dynamic mechanical properties of dental resin to a certain extent,and the content of microcapsules in the resin needs to be strictly controlled in clinical applications.According to FEA results,microcapsules with small capsule wall thickness and large particle size are selected as much as possible within a reasonable range to improve the microcapsule response to microcracks.