Design, Preparation, Properties And Dental Appalication Of PMMA-ZrO₂ Biomimetic Nanocomposites

The technology of dental computer aided design/computer aided manufacturing (CAD/CAM) is considered as a revolutionary breakthrough in the field of dental prosthodtics due to its automation, high efficiency and economical reasons. Machinable material suitable for dental CAD/CAM has aroused great interests in the world recently, however, the search for a proper material to meet the requirements of dentists and patients is still going on.With a thorough analysis of the oral environment and the requirements of CAD/CAM material, we were inspired by the structures of natural tough composites such as enamel and nacre. Organic-inorganic nanocomposites very similar to enamel were firstly proposed and designed, which could be an ideal candidate for CAD/CAM material. The composites were composed of inorganic phase and organic phase. The inorganic phase provided strength and stiffness, organic phase provided toughness and flexibility, and the two phases were both continuous phase and mixed on nano-scale. An original processing route was used to prepare PMMA- ZrO₂ for the first time. Partially sintered ZrO₂ ceramics (PSZC) with nanometer sized porous network structure were obtained via isostatic pressing and partially sintering of Zirconia nanopowder (tetragonal Zirconia polycrystals stabilized with 3mol-% yttria); PMMA-ZrO₂ organic-inorganic nanocomposites were prepared by vacuum infiltrating the methyl methacrylate (MMA) resin solution into the PSZC, followed by in-situ polymerization and solidification.ZrO₂ nanopowder was pressed to form a compact, which was then sintered at different temperatures yielding PSZC. The factors determining the densities of the ceramics were studied. The open porosity , diameters of the pores and grain size formed in the ceramics of different densities as well as their mechanical properties were also studied. The results indicated that the density of PSZC was influenced by the density of the compact and sintering temperature. The pores formed in PSZC are all open pores, with a diameter between 60nm～130nm and a grain size between 120nm～180nm. A neck was observed between neighboring grains. The obtained PSZC-75% showed good mechanical properties, with a bending strength of 222.05 MPa, a fracture toughness of 2.03 MPa·m^1/2, an elastic moldulus of 65.70 GPa, and a Vicker's hardness of 3.91 GPa. If the density of PSZC is higher than 81%TD, it can not be machined on dental CAD/CAM system.A liner relationship was found between the relative elastic moldulus and the relativedensity for PSZC. It can be described by the following equation: E^* = aρ_rd+b; The bending strength was also found to be directly related to the relative density with an equation:σ= cρ_rd + d; where a, c are constants related to compact material properties,and b and d are constants associated with pore properties. These equations are applicable for PSZC produced in the range after sintering initiated and before closed pore formed. These equations can be used to predicte the mechanical properties of PSZC formed in that region. No other reports were found on the linear relationship between bending strength and relative density.factors affecting the preparation of PMMA-ZrO₂ composites (PZC) were studied. It was found that the shape and viscosity of the organic resin played an important role. Other factors, such as atmosphere and temperature, were also found to influence the polymerization of MMA. The MMA resin solution was introduced into the open pores of PSZC with coupling treatment by infiltrating in vacuum for in-situ polymerization and solidification. It was found PSZC with 60%TD～83%TD can be used for successful preparation of PZC using the technology described above.The mechanical properties of PZC were tested and compared to that of PSZC. PZC of different densities were machined using a dental CAD/CAM system, the machinability was investigated as a function of densities. The machinability of PZC and PSZC were compared based on observation of the cutting surface. Scanning electron microscopy (SEM) was employed to observe the fracture surface of PZC, so that the status of organic phase inside the material and its effect could be gleaned.The results indicated that the mechanical properties of PZC increases as the density of PSZC increases. The bending strength, fracture toughness, elastic moldulus, and Vickers hardness of PZC-75% were 258.05 MPa , 4.64MPa.m^-/2, 66.58GPa, and 4.40GPa, respectively. Compared to PSZC-75%, these factors increased 16.21 %, 128.57%, 1.33%, and 12.53%, respectively. Thereinto, the fracture toughness of PZC-75% improved significantly, 2.3 times higher than that of PSZC-75%. PZC-65%, PZC-70% and PZC-75% can be machined on dental CAD/CAM system, therefore, they are suitable material for CAD/CAM applications. The time needed to shape a premolar crown was between 13min～19min. The cutting surface of PZC is flat and smooth, rarely showing broken pieces on the edge; while for PSZC, the cracking on the edge is relatively severer. Observing both samples under an optical microscope, we found that the marks remaining on the cutting surface of PSZC were thicker than that of PZC.SEM images showed that the grains on the fracture surface of PZC were surrounded by a large portion of organic phase with shapes of mountain ridges and basins. Based on the great augment in mechanical properties, especially fracture toughness of PZC compared to PSZC, a structure model was built and a toughness mechanism was constructed.The PZC blocks were machined on the dental CAD/CAM. The premolar crown prepared from PZC material showed a precise figuration and no fine cracks. Polishing renders the restoration a flat and smooth surface, and it could be wore by the patient right after being polished. The restoration had good suitability, accuracy. The biocompatibility and biosafety of PZC were preliminarily evaluated by haemolysis test and short-term systemic toxicity test. The results of tests showed that the material had excellent biocompatibility for dental clinical application.The bending strength and fracture toughness of PZC are significantly higher than those of commercial CAD/CAM machinable materials. The application of this novel composites for dental restoration in a CAD/CAM system is characterized by its high efficiency, durability, and exquisiteness. We are sure that it will play an important role in the field of dental CAD/CAM materials, and assist development of CAD/CAM in prosthodtics in the future.