Preparation And Anti-caries Properties Of Nano-hydroxyapetite Modified Composites

Dental caries is one of the most common human diseases caused by bacteria, the origin reason of the dental caries is that the oral germ such as streptococcus mutans and its plaque produce acids that make the enamel deminerization. Fluoride can promote the eroded enamel to reminerization, harden the enamel and inhibit s. mutans to make acids, so it is the most popular substance used to anti-caries currently. However, Fluoride has toxicity and side effect, which lead to fluorosis because of over use. therefore, it raises controversy that fluoride is used in daily oral care products like toothpaste. So it is a focus for oral science to research new remedy or materials for anti-caries safely and effectively. Hydroxyapatite (HAP) is considered as the main mineral constituent of bone and dental enamel, synthetic pure HAP nanoparticle is similar in crystal lattice to the natural bone, and is broadly used as bone substitutes for its excellent bioactivity, biocompatibility and osteoconduction, moreover, nano-HAP is one of the favorite research topics of anti-caries materials applied to reminerization of enamel. However, synthetic HAP has low antibacterial property, but high crystallinity and stability, which lead to low effect of anti-caries.In this paper, nano-HAP and its modified materials doped by strontium, zinc and cerium (MCaHAP,M= Sr,Ce,Zn) were prepared by sol-gel-supercritical CO2 drying(SCFD-CO2); nano-HAP and nano-MCaHAP modified by chitosan(CS) can be produced by blend-in situ precipitation–SCFD. The influence of processing parameters on diameter and specific surface area of nano-HAP were studied, finally the optimizing technical parameters were obtained. The structure of nano-HAP and its modified materials were characterized by element content analysis, XRD,FT-IR, TEM and SEM. Results shows that the nano-HAP particles produced by sol-gel-SCFD-CO2 have more uniform particle size distribution, less mean diameter and larger specific surface area than the HAP particles dried by normal drying way. With the optimizing parameters as follows, the concentration ratio of reactants between Ca-salts and P-salts is 0.5/0.3(mol?L-1), pH 10, reacting temperature 60℃, reacting time 3 h, aging time 8 h, drying temperature 45℃, drying pressure 15 MPa, drying time 4 h, SCF-CO2 flow 15L?h-1, the nano-HAP and its metals doped materials that are highly pure, uniformly dispersed, measured between 30 nm～100nm and needle shaped can be produced. According to the set atomic ratio of XM = M/[Ca+M], Sr2+, Ce3+ and Zn2+ with XZn less than 0.15 can be quantitatively doped into the structure of HAP and maintain its hexagonal crystal structure, and form substitional solid solution with confirmed stoichiometric proportion. But with increase of XZn , the zinc doped compound will no more maintain its hexagonal crystal structure. After in situ combined with CS, nano-MCaHAP are still well dispersed and uniformly shaped like slight shuttle with the radial size of 20～50 nm and length of 100 nm. , and has porous and reticular surface structure.The antibacterial properties of nano-HAP modified materials were researched by the tests of inhibition zone, antibacterial ratio, minimal inhibitory concentration(MIC) on Escherichia coli, Staphylococcus aureus, Lactobacillus, Streptococcus Mutans . The tests results indicate that the antibacterial properties increase after nano-HAP are doped by Sr2+,Zn2+,Ce3+ and combined with CS, and bigger is the XM , more increase the antibacterial properties. As S. mutans was the test bacteria, the effects of modification of nano-HAP on the metabolizabilities of S. mutans, such as adhering ability, acidogenicity, WIG(water insoluble glucan) generation, were discussed. The results show that the nano-HAP modified materials have the better abilities inhibiting the metabolizabilities of S. mutans.The remineralization of nano-HAP modified materials on decayed enamel was studied by testing the changes of Micro Hardness Number (MHN) of the decayed enamel and the thickness of demineralized layer of enamel surface. The results find that nano-HAP can strengthen the MHN value of enamel surface, lessen the demineralization of enamel and have better abilities to promote the remineralization of enamel after nano-HAP are doped by Sr2+,Zn2+,Ce3+ and combined with CS, the more is the doped strontium and cerium, the more does the remineralization strengthen, but doped zinc gives no stronger remineralization. Under different pH values of 6.8 and 4.5 respectively, the adsorption capacities of Bovine Serum Albumin (BSA) on nano-HAP modified materials were investigated. Results indicates that the nano-HAP modified materials have stronger adsorption to BSA at the pH value of 4.5, and the adsorption capacities increase after nano-HAP are doped by Sr2+,Zn2+,Ce3+ and combined with CS. As XSr,,XCe increase, the adsorption capacities of nano-HAP modified materials enhance. But doped zinc can't obtain a stronger adsorptive ability, among the test materials, nano-CeCaHAP/CS has the best adsorption to BSA.By the test of acute toxicity and cumulative toxicity, the safety for use of nano-CeCaHAP/CS composite was studied. Results show that LD50, the acute toxicity of nano-CeCaHAP/CS through the mouth of NIH little mouse, is bigger than 5000 mg.kg-1 that indicates its non-toxicity for actual use; and the cumulative toxicity index K = LD50(n) / LD50(1) of NIH little mouse is bigger than 5 that demonstrate its feeble cumulation, so it can be confirmed that nano-CeCaHAP/CS composite has high safety for actual use. The nano-CeCaHAP/CS was mixed into toothpaste formulation, and compared its properties of anti-caries with that of fluoride toothpaste. It is found that the toothpaste with 1% nano-CeCaHAP/CS has better effects than the toothpaste with 0.15% NaF on fighting bacteria causing dental caries, inhibiting the metabolizabilities of S. mutans, and promoting the remineralization of enamel.