Research On Dental Restorative Material Of Nano-hydroxyapatite Crystals And Composite Resin

There are more than 40 percent of the whole population in our country who suffer from their caries. If one patient has two caries, there would be more than 1.3 billion caries which need repairing. Therefore, it would urgently demand a huge amount of dental restorative materials said by professor ZHU Xi-tao. Amalgam is becoming less and less relevant as a standard restorative. Patients'increased health-awareness, their desire for enhanced esthetics, continued discussions on amalgam and recent developments in materials' research are all reasons for developing new dental restoratives. Composite resin, invented by Bowen R L, an American scholar in 1962, in which Bis-GMA resin was taken as the matrix and silica etc. as the fillings, has already becoming a kind of useful materials instead of amalgam to repair hard tissue in clinical application. But the main problems incurred with composite fillings in the posterior region to date have been their tendency to form marginal gaps due to polymerization shrinkage and lack of strength and bioactivity. Apart from polymerization shrinkage, contact with the opposingdentition also causes elastic deformation of the filling material, which adversely affects its bond strength on the tooth structure. The above mentioned shortagesassociated with the current composite resin limit its further widespread application.As we know, various biological materials existing in nature, such as shells, pearls, bone and teeth, are all inorganic/organic composites with good mechanical properties. In order to fabricate biomedical materials which imitate human hard tissue to great extent, hydroxyapatite (HA)/polymer composites have attracted much attention because such composites may have osteoconductivity as a result of the presence of HA. HA has been extensively investigated due to its excellent biocompatibility, bioactivity and osteoconductivity as well as its similarity to the main mineral component of bone and teeth. According to the principle of biomimesis, if we could synthesize a kind of HA which has a similar composition, crystal structure, morphology, and crystalinity to that of human teeth, and then mix them with the current composite resin, some expected functions would be exhibited when it is applied to repair human teeth, such as the effect of biological function. Furthermore, the mixed HA would be conducive to the teeth reconstruction and the ability of anti-caries.In this research, the properties of CHARISMA(?) composite resin and the composition and structure of human posterior teeth were investigated firstly. Then the teeth-like nano-hydroxyapatite were fabricated. In addition, the interaction between n-HA and silane coupling agent was studied. On the basis, the n-HA/CHARISMA(?) composite was prepared by melting mixation technique.1. CHARISMA(?) resin is one kind of composites which consists of 71.44wt% inorganic filling materials and 28.56wt% organic phase. The inorganic materials are made of amorphous silica, barium and aluminum glass and titanium'oxide. The organic phase is a substance which contains: -OH, -C=O, -CH3 and -CH2. The testing results show that CHARISMA? resin matchs the basic requirement for composite resin according to GB11749-89, while its bioactivity and biocompatibility need to be further improved since there is no calcium and phosphate ions present in the resin.2. The crystal morphology and composition of human dental apatite were investigated using IR, XRD and TG techniques. The results show that two carbonate substitution types (A and B) exist in human posterior dental apatite, of which, carbonate apatites (A-type, substitution for -OH) have increased lattice constants a, and B-type, substitution for PO43-, are usually characterized by reduced a lattice constant. With the increase of calcining tempreture, the carbonates located in both A and B type substitution sites escaped from the dental apatite lattice and A-type carbonate disappeard in relatively low sinter temperature, but appeared again with the further increase of tempreture; while the B-type carbonate existed all the time with tempreture increase although its quantity decreased. XRD spectra show that there is an extensive degree of peak broading for the human dental apatite dried at 80 ℃ and the peaks become relatively sharp and well-resolved at high temperature, indicating that the crystal size and crystallizability of the teeth apatite increases with the thermal treatment temperature. Thermo gravimetric analysis showed that there were different types of water loss at temperature between 25℃ and 1000℃.The dentine contains more water and organic material, so its weigt loss was greater than that of enamel. The apatite crystals in enamel linked much closer than those of dentine. Therefore there would need more energy to break enamel than dentin. Those analyse provide a promising perspective for the tooth-like restorative material preparation.Nano-apatite crystals were made from hydrothermal treatment at 145 ℃ under normal pressure. The crystal morphology and composition of the synthesized nano-HA were investigated using TEM, IR, XRD, ICP and XPS techniques and were also compared with those of human posterior teeth. The results indicate that nano-apatite crystals have a similar composition, crystal structure, morphology and crystalinity to those of human teeth, and also containing CO32- and Na+ ions. From the view of biomimesis, the CO32- and F ions or groups should be introduced to the nano-apatite to mimic the biological apatite as much as possible. The research provides a wy for further n-HA/dental resin composites preparation.3. n-HA surface wae treated with silane. The interface of n-HA and silanecoupling agent was investigated. An effective surface modification method was developed to treat n-HA crystals slurry by using γ-(trimethoxysilyl) propyl methacrylate as a coupling agent. FT-infrared spectroscopy (FT-IR), Raman spectroscopy and X-ray photoelectron spectrum (XPS) were used to characterize the silane-modified n-HA. The experimental results showed that peaks of hydroxyl group of n-HA decrease greatly after n-HA treated by silane, implying the occurrence of possible chemical reaction between n-HA and silane. Moreover, several surface modification mechanism is involved, including hydrogen bond chemical bond as well as the charge effect. The analysis of n-HA/silane interface is important for further n-HA/polymer or n-HA/dental resin composites preparation.4. The n-HA/CHARISMA(?) resin was fabricated by melting mixation technique under the help of the diluent TEGDMA. The testing results showed that the proerties of the CHARISMA(?) resin could be changed by adding n-HA crystals, some characteristics were improved after the mixation of n-HA into CHARISMA(?) resin. The usage of diluent TEGDMA helps to enlarge the inorganic content of the composite but decreases the properties of composite. n-HA crystals should be added in the process of CHARISMA(?) resin formation.