الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Secondary decay occurs most commonly at the interface between the restoration and the cavity prepared. The tooth structure is demineralized following the invasion of acid producing bacteria, such as S. mutans, when fermentable carbohydrates are present.(238) Therefore, an effective antibacterial/bactericidal restorative material would be in the ideal location to prevent secondary decay,(238-240) especially since it has been shown that cariogenic bacteria, mainly S. mutans, adhere to restorative materials.(241)
Several attempts have been made to introduce antimicrobial properties in restorative materials, but some of these attempts resulted in compromised physical properties of the novel material. The changes in composition carried out to introduce antibacterial properties or leaching of particles may affect the material‘s strength, making it, then unsuitable as a restorative material or restrict its use to non- load bearing areas. Another issue may be problems with changes in color, as seen with the introduction of silver nanoparticles to composite resins, which may in turn restrict the use of the material to posterior restorations.(64)
Several types of nanoparticles have been used to impart antimicrobial properties to dental materials. Zinc oxide is an inorganic compound popularly in daily applications and it has been integrated in polymeric matrices in order to provide antimicrobial activity.(20) zinc oxide nanoparticles are reported by several studies as non-toxic to human cells.(72)
Chitosan is a natural biocompatible, nontoxic biopolymer polysaccharide formed by the deacetylation of chitin and is utilized in biomedical applications due to its high biocompatibility and antimicrobial properties. It has an in vitro antibacterial effect on S. mutans.(242)
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Since Chitosan has tremendous ability to form metal complexes with zinc metal because of its amine group and hydroxyl group. Currently, chitosan- ZnO complex attracted great interest.(100)
The current study was conducted to evaluate the effect of blending microhybrid composite (group I) with Zinc oxide nanoparticles (group II), Chitosan nanoparticles (group III) and Chitosan/ZnO nanoparticles (group IV) in terms of antibacterial activity, microhardness, flexural strength, flexural modulus, color stability, water sorption, solubility and degree of conversion under different aging periods.
The tests were performed after 24 hrs, 2 weeks and 12 weeks storage in pyrogen free distilled water, an extra (6 weeks) period was done for the bacteriological tests.
7.1 Preparation and characterization of the nanoparticles
Zinc oxide nanoparticles were purchased as dispersion, Chitosan nanoparticles were prepared using ionic gelation method, Chitosan/ ZnO nanoparticles were prepared by chemical precipitation method.
The nanoparticles were characterized in the terms of: particle size, zeta potential, shape, morphology and functional group determination.
7.2 Determination of nanoparticles’ minimum inhibitory concentration (MIC)
The MIC was determined by broth microdilution method according to the clinical and laboratory standards institute (CLSI) recommendations. A standard S. mutans (ATCC#25175) was used to monitor nanoparticles potency for quality control. The bacteria were supplied as actively growing culture in Trypticase soy yeast extract medium.
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7.3 Blending of nanoparticles with microhybrid composite resin
Based on the bacteriological studies using MIC and direct contact test, the percentage by weight of ZnO, Chitosan and Chitosan/ZnO nanoparticles that provided an antibacterial effect was calculated. Those percentages were then chosen for addition to the composite resins in all the tested specimens.
The nanoparticles were dispersed mechanically to the microhybrid composite resin in a dark room. SEM-EDX (Scanning electron microscopy with an energy dispersive X-ray analytical system) analysis was performed to confirm the homogeneity of the distribution of the nano-particles in the composite resins and provide chemical microanalysis.
7.4 Bacteriological study
7.4.1 Disc diffusion test: (n=80)
Eighty disc shaped specimens were fabricated, twenty specimens for each group. Then the twenty of each group were divided according to the previously mentioned subgroups so each subgroup contained five specimens.
Each specimen was placed on Brain Heart Infusion ―BHI‖ agar plates which was inoculated with the inoculum for 48 hrs and inhibition zones around the composite discs was measured and statistically analyzed.
7.4.2 Direct contact inhibition of composites evaluated by scanning electron microscopy: (n=48)
Forty eight disc shaped specimens were fabricated, twelve specimens for each group. Then the twelve of each group were divided according to the previously mentioned subgroups so each subgroup contained three specimens.
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Each specimen was inoculated with a bacterial solution and incubated for 1 h at 37 °C, then wells of the 96-well plate were filled with BHI broth without sucrose and allowed to culture for an additional 23 h. Twenty four hours fixation was performed in glutaraldehyde then dehydration was completed in a gradient ethanol series. Samples was visualized with scanning electron microscopy.
7.5 Mechanical properties
7.5.1 Microhardness test (n=60)
Sixty disc shaped specimens were fabricated, twenty specimens for each group. Then the twenty of each group were divided according to the previously mentioned subgroups so each subgroup contained five specimens.
The Vicker‘s hardness number was determined using a digital microhardness tester, then the hardness number for the top surface, lower surface and the hardness ratio were statistically analyzed.
7.5.2 Flexural strength test (n=60)
Sixty bar-shaped shaped specimens were fabricated, twenty specimens for each group. Then the twenty of each group were divided according to the previously mentioned subgroups so each subgroup contained five specimens.
Each specimen was submitted to the three-point bending test in the universal testing machine, then load was applied till the specimen fractures and the fracture loads were recorded. Flexural strength and flexural modulus values were calculated and statistically analyzed.