Permanent magnetic materials used in the field of oral medicine has been more than 40 years of history, the first use of permanent magnets to increase the retention of the retention force. In the late 1970s, some scholars (Kawata, 1977) introduced magnetic force into the field of orthodonics, made use of the principle of magnetism, the use of magnetic force and the traditional Dentistry to achieve rapid movement of teeth, correction of the wrong, the purpose of deformity [4]. Since the magnetic for the first time for orthodontics, after 20 years of research, permanent magnet in the field of orthodontic applications has been a great development, the application of a variety of methods have been applied, the scope of application has been very wide. But the magnetic force in the fixed orthodontic has obvious limitations: most of the magnets due to limited power is not suitable for fixed orthodontic, magnetic orthodontics and more used in the activities of correction device; magnetic force will increase with the distance between the magnets, Not enough to move the larger distance of the teeth, so scholars in recent years, mainly focused on the magnetic field of histological reactions, biological effects and magnetic function appliance design and other aspects of research [5].

1 the development of medical NdFeB magnets in the rare earth permanent magnet material is applied before the mouth commonly used magnetic materials are mainly ferrite (Ferrite), aluminum nickel cobalt (AlNiCo) permanent magnet, they are not magnetic, need to increase the volume of capacity Increase the magnetic force, and thus used in oral limited. Platinum-cobalt (Pt-Co) permanent magnet wear resistance and corrosion resistance, magnetic Ye Hao, but the value is expensive, not perfect, limited to experimental applications [6]. The first generation of rare earth permanent magnetic materials in the late 1960s was successfully developed. 1979 Tsutsui on the first generation of rare earth cobalt permanent magnet as a dental application materials, physical properties, corrosion resistance and toxicology research, that the material than the previous few have obvious advantages. The second generation rare earth permanent magnets are samarium cobalt 2:17 (SmCo2: 7) permanent magnets appearing in the 1970s. The third generation is Japan's first to develop a more successful magnetic NdFeB permanent magnet. The physical properties of the material is good, the magnetic force is large, the volume is small, and the physical and chemical effects of the mouth are small, so far the magnetic energy product and the highest coercivity permanent magnet material [7-10]. Recently, the N48 new NdFeB permanent magnet (size 2mm × 3mm × 4mm) [11], which is similar to the size of the brackets, is used to test the magnetic properties and mechanical properties of the magnet. The residual magnetic properties of N48 NdFeB The density is 13.73kGs, the coercivity is 13.00kOe, and the maximum energy product is 46.13MGsOeO. The performance is obviously better than that of the conventional magnet (platinum cobalt, samarium cobalt, nickel cobalt and so on), especially the maximum energy product. Magnet surface magnetic field strength of 3.38T (3380Gs), the surrounding magnetic field strength distance of 7mm down to 0.01T, 20mm from the time down to zero, indicating that the size of 2mm × 3mm × 4mm N48 NdFeB magnets produce a strong magnetic field Large range. Within 7 mm of the magnet, the magnetic field can have an effect on the alteration of the periodontal tissue. 2mm × 3mm × 4mm size of the N48 NdFeB magnetic force effective range of 5mm, far greater than the previous scholars to study the use of larger magnets 0.7 ~ 3.0mm effective range of action. And in the 0 ~ 5mm, the repulsive force of 288 ~ 36 g, the attraction of 456 ~ 40 g, this force can fully meet the needs of clinical mobile teeth, and all scholars use the magnet in the smallest and magnetic force is the largest of. The above results show that the excellent performance of N48 NdFeB permanent magnets can meet the needs of clinical fixation and orthodontics, and the magnetic brackets can be completely or substantially replace the mechanical correction force to achieve the purpose of promoting tooth movement and reducing the cost of orthodontic treatment. The

2 Preparation of medical magnetic ring Usually there are two kinds of medical magnetic ring preparation method: the first is the use of magnet block block into the overall ring, this method will be divided into several parts of the ring, respectively, for each part of the orientation into the magnetic field orientation The This method produces a non-uniformity of the magnetic ring, which seriously affects the overall magnetic properties of the magnetic ring and causes the magnetic ring to crack due to the different shrinkage rates of the different orientation parts during the sintering process [12-13]. The second is the use of isotropic magnetic powder bonded into a magnetic ring, the magnetic ring because the magnet density is low, so the magnetic properties are very low, magnetic energy product in the 6 ~ 8MGOe, so that the magnetic ring in use is limited. And because of the special nature of the use of medical magnetic ring, making its easy to corrode its use. Especially in the preparation of smaller inner diameter magnetic ring, the magnetic powder orientation is not complete, seriously restricting the magnetic materials in the development of biomedical [14-15]. At present, the latest technology used in the preparation of medical magnetic ring is radiation-oriented technology, the use of the technology to produce magnetic ring can not only make the magnetic powder uniform orientation and the production of smaller magnetic ring can also get a larger orientation field, the magnetic ring The magnetic properties and consistency are greatly improved [16-17]. Because the magnetic ring has this superiority, it has been widely studied and used, is now one of the main technology to prepare medical magnetic ring.

3 medical NdFeB magnets surface modification of the mouth is a complex biochemical environment, the magnet in the mouth will be subject to strong corrosion, light caused by magnetic decline, heavy free metal ions will lead to local mucosal pigmentation, oral mucosa Fibroblast proliferation rate was significantly reduced. Magnet corrosion not only with the magnet itself, the characteristics of the elements, but also with the oral environment such as saliva, bacteria. Wilson et al [18] found that the quality of NdFeB discs decreased by 3.2% after 21 days of hemolytic streptococcus. Corrosion is closely related to the biocompatibility of the alloy. The systemic or topical toxicity, sensitization and carcinogenesis of the alloy are all caused by the release of the elements in the corrosive. Dental alloys may cause toxic biological reactions in the oral cavity. Corrosion as a prerequisite. Therefore, the magnet must be completely isolated from the saliva, improve the corrosion resistance of the magnet is an important guarantee for its long-term safe application in the oral cavity. Hai et al. [19] suggested that titanium nitride plating on stainless steel surface can significantly improve the corrosion resistance of magnetic stainless steel. Jie Baosheng et al [20] in the surface of NdFeB titanium nitride, the study that anti-corrosion effect is good. Titanium nitride is a non-metallic oxidized ceramic, golden yellow, acid and alkali resistant, with excellent conductivity and superconductivity. Titanium nitride coating can enhance the wear resistance of metal alloys, titanium nitride film can enhance the corrosion resistance of metal alloys, slow down the release of metal ions, metal materials to reduce the toxicity of the surrounding medium, titanium nitride film does not change The overall physical structure of the magnetic alloy does not affect the magnetic force [21-22]. Due to the excellent physical and chemical properties of titanium nitride film, corrosion resistance, so the NdFeB outer ring titanium nitride film. The use of pulsed DC PCVD technology plating, can successfully solve the medical NdFeB magnetic ring corrosion problem, making the magnetic ring has a good cell compatibility. From the ability to meet the magnetic field in the medical field of performance requirements, so that the use of biomedical applications have a broader prospect.

4 Conclusion NdFeB permanent magnet excellent performance and titanium nitride good biocompatibility of the titanium nitride film titanium NdFeB magnets in the field of biomedical has a broader application prospects.

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