Abstract:

? Objective To prepare novel dental composite resins using zinc（Zn）- and nitrogen（N）- modified titanium dioxide （TiO?） nanoparticles （NPs） and mesoporous alumina （Al?O?，r type， 20 mm） NPs as reinforcing fillers， systematically evaluating their antibacterial activity， mechanical strength， basic performance， and biosafety to obtain the dental composite resins with excellent antibacterial activity and mechanical strength. Methods Zn-N-TiO? NPs and mesoporous Al?O? NPs were added into a resin matrix at varying mass ratios to prepare five composite resins： control group （no filler）， group 0 （Zn-N-TiO?∶Al?O?=1∶0）， group 1 （Zn-N-TiO?∶Al?O? = 1∶1）， group 2 （Zn-N-TiO?∶Al?O?=1∶2）， and group 3 （Zn-N-TiO?∶Al?O?=1∶3）. Plate colony counting method was used to detect the number of adhered bacteria on composite resin surfaces in various groups and calculate the antibacterial rate； scanning electron microscope （SEM） was used to observe the morphology of adhered bacteria in various groups； universal testing machine was used to measure flexural strength （FS） and elastic modulus （EM） of composite resins in various groups； SEM was used to observe fracture surface morphology of composite resins in various groups； microhardness tester was used to determine Vickers microhardness of the composite resins in various groups； Fourier transform infrared spectroscope was used to detect double bond conversion rate （DC） after 20 s photocuring and calculate curing depth； water contact angle meter was used to measure water contact angle （WCA）， water sorption property （WSP）， and water solubility level （WSL） of composite resins in various groups； cell counting kit-8 （CCK-8） method was used to evaluate relative growth rate （RGR） of the mouse fibroblast L-929 cells cultured in composite resin extracts on days 1， 3， and 5 and determine in vitro cytotoxicity grade. Results The plate colony counting results showed that compared with control group， the colony counts on agar plates in the other groups were significantly reduced， with group 1 showing the lowest count. The SEM images results showed densely distributed and morphologically intact Streptococcus mutans in control group； small clusters of bacteria with depressed cell membranes in group 0 and group 3； sparsely distributed bacteria with obvious membrane shrinkage and cytoplasmic leakage in group 1 and group 2. No statistically significant difference in colony counts was found between group 1 and group 2 （P>0.05）， but both were lower than the other groups （P<0.05）. All the composite resins in experimental groups exhibited >85% antibacterial rates， with group 1 and group 2 exceeding 99%.The composite resins in group 0 showed the lowest FS. With addition of mesoporous Al?O?， the FS of the composite resin in group 1 and group 2 were significantly increased， with the composite resin in group 2 showing the highest FS among all groups. Although the FS of the composite resin in group 3 was lower than that in group 2， but it remained higher than other groups （P<0.05）. The SEM images results showed that in control group， the smooth-surfaced sillicon dioxide （SiO?） particles exhibited clear fracture interfaces with resin matrix， with >50% particle exposure； the composite resin in group 0 showed similar morphology and large Zn-N-TiO? agglomerates with tight filler-matrix bonding； the composite resin in group 1， 2， and 3 showed resin adhesion to SiO? surfaces （<50% particle exposure） and uneven fracture surfaces. Fractured SiO? spheres were observed in group 2. Filler distribution was uniform in group 1 and group 2， while the minor NP agglomeration occurred in group 3. The composite resin in control group showed the lowest EM. The EM was significantly improved in experimental groups， with group 3 having the highest value. Group 0 exhibited the lowest Vickers microhardness， showing statistically significant differences among other groups （P<0.05）. The Vickers microhardness of the composite resion was gradually increased with the rising of Al?O? content.The resins in group 2 and group 3 achieved >45 HV hardness， representing increases of 29.73% and 33.82% compared with control group， and 51.34% and 56.28% compared with group 0. No significant differences in DC of the composite resin were found among groups （P>0.05）. The depth of cure for all composite resin groups exceeded 4 mm， with no significance differences observed between various groups （P>0.05）. The composite resin in group 0 showed the smallest WCA. The hydrophobicity of the composite resion was increased with the rising of Al?O? content， but all the WCA values remained <80°. The composite resin in group 3 had the largest WCA without statistical significance compared with group 2 （P>0.05）. Filler incorporation reduced the water sorption/solubility. The composite resin in the CCK-8 assay results showed the composite resins in all groups had RGR>75%， meeting in vitro safety standards. Conclusion Reinforcing fillers impart superior antibacterial activity and mechanical properties to composite resins. Under experimental conditions， group 2 composite resin achieves optimal comprehensive performance in antibacterial efficacy and mechanical strength， demonstrating promising clinical application potential.

Key words: ? Composite resin, Titanium dioxide, Mesoporous alumina, Antibacterial property, Mechanical strength