Why is NiTi alloy the only memory alloy that can be used in human body?

Nickel-titanium alloy is a shape memory alloy, and it is a special alloy that can automatically restore its plastic deformation to its original shape at a certain temperature.

Nickel-titanium alloy is a shape memory alloy, and it is a special alloy that can automatically restore its plastic deformation to its original shape at a certain temperature. Its expansion rate is above 20%, its fatigue life reaches the 7th power of 1* 10, its damping characteristic is 10 times higher than that of ordinary spring, its corrosion resistance is better than the best medical stainless steel at present, and it can meet various engineering and medical application requirements, so it is an excellent functional material.

Memory alloy not only has unique shape memory function, but also has excellent characteristics such as wear resistance, corrosion resistance, high damping and superelasticity.

Performance and characteristics

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(I) Phase transformation and properties of NiTi alloy

As the name implies, NiTi alloy is a binary alloy composed of nickel and titanium. Due to the change of temperature and mechanical pressure, there are two different crystal structure phases, namely austenite phase and martensite phase. The transformation sequence of NiTi alloy during cooling is parent phase (austenite phase) -R phase-martensite phase. The R phase is rhombic, and when the temperature is higher (higher than the initial temperature of austenite) or when the load is removed (removed by external force), the austenite is cubic and hard. The shape is relatively stable. Martensite is in a relatively low temperature (less than Mf: the temperature at which martensite terminates) or a loaded (activated by external force) state, hexagonal, malleable, repetitive, unstable and easy to deform.

(2) Special properties of NiTi alloy

1, shape memory Shape memory means that when the parent phase of a certain shape is cooled from above Af temperature to below Mf temperature to form martensite, the martensite is deformed below Mf temperature, and then heated below Af temperature, and the material will automatically recover its shape in the parent phase with the reverse phase transformation. Shape memory effect is actually a thermally induced phase transformation process of nickel-titanium alloy.

2. Superelasticity The so-called superelasticity refers to the phenomenon that the sample produces strain far greater than the elastic limit strain under the action of external force, and the strain can automatically recover when unloading. That is to say, in the parent phase, stress-induced martensitic transformation occurs due to the action of external stress, so the alloy shows different mechanical behavior from ordinary materials, and its elastic limit is much larger than that of ordinary materials, so it no longer obeys Hooke's law. Compared with shape memory characteristics, superelasticity has no participation of heat. In a word, hyperelasticity means that the stress does not increase with the increase of strain within a certain deformation range, and hyperelasticity can be divided into linear hyperelasticity and nonlinear hyperelasticity. The stress-strain relationship of the former is close to linear. Nonlinear hyperelasticity refers to the result of stress-induced martensite transformation and its inverse transformation in a certain temperature range above Af, so nonlinear hyperelasticity is also called phase transformation pseudoelasticity. The pseudoelasticity of nickel-titanium alloy can reach about 8%. The superelasticity of NiTi alloy can be changed with the change of heat treatment conditions. When the archwire is heated to 400? Above c, superelasticity begins to decline.

3. Sensitivity of temperature change in oral cavity: The orthodontic force of stainless steel wire and CoCr alloy orthodontic wire is basically not affected by the temperature in oral cavity. The orthodontic force of superelastic NiTi alloy orthodontic wire changes with the change of oral temperature. When the deformation is constant. With the increase of temperature, the correction force increases. On the one hand, it can accelerate the tooth movement, because the temperature change in the oral cavity will stimulate the blood flow of the blood stasis part caused by the orthodontic device, so that the repair cells can get sufficient nutrition during the tooth movement and maintain their vitality and normal function. On the other hand, orthodontists cannot accurately control or measure the orthodontic force in the oral environment.

4. Corrosion resistance: Research shows that the corrosion resistance of NiTi wire is similar to that of stainless steel wire.

5. Anti-toxicity: Nickel-titanium shape memory alloy has a special chemical composition, that is, it is an atomic alloy of nickel and titanium, containing about 50% nickel, which is known to cause cancer and promote cancer. Generally speaking, the oxidation of titanium on the surface layer acts as a barrier, which makes NiTi alloy have good biocompatibility. TiXOy and TixNiOy in the surface layer can inhibit the release of Ni.

6. Soft orthodontic force: At present, commercial dental orthopedic wires include austenitic stainless steel wire, cobalt-chromium-nickel alloy wire, nickel-chromium alloy wire, Australian alloy wire, gold alloy wire and? Titanium alloy wire. The load-displacement curves of these orthodontic wires under the conditions of tensile test and three-point bending test. The unloading curve platform of NiTi alloy is the lowest and the flattest, which shows that NiTi alloy can provide the most lasting and gentle orthodontic force.

7. Good shock absorption characteristics: The greater the vibration caused by mastication and bruxism to the arch wire, the greater the damage to the root and periodontal tissue. The experimental results of different arch wire attenuation show that the amplitude of stainless steel wire is larger than that of superelastic NiTi wire, and the initial amplitude of superelastic NiTi arch wire is only half that of stainless steel wire. Good vibration and shock absorption characteristics of arch wires are very important for tooth health, while traditional arch wires, such as stainless steel wires, often increase root absorption.

(III) Classification of NiTi alloy wires

1) 1940, gold bow wire, cobalt-chromium alloy wire, stainless steel round wire.

2) 1960, Martensite stabilized alloy: Most of them are made of NiTi alloy after deformation in martensitic state. This kind of arch wire has low rigidity and can produce light orthodontic force. There is no martensitic transformation caused by stress or temperature, so it does not show memory effect and superelasticity.

3) During the period of1980, both China NiTi alloy and Japanese NiTi alloy arch wires are austenitic activated alloys, that is, they are in an austenitic state in any state, but there is no martensitic state caused by temperature when placed in or outside the mouth. Martensite state can only be caused by stress and has superelasticity, but it has no shape memory function. This kind of arch wire has good resilience and low stiffness, and can produce weak orthodontic force. The characteristic of Zoda is that the force generated from the initial stage to the final stage is constant, and the effect is good when the teeth are irregular in the early stage. The disadvantage is that it can not be bent at room temperature and is not easy to weld. If the company is used as the main arch wire, it will often cause bad expansion or contraction of the dental arch, and it is difficult to establish a good arrangement of premolars and molars.

4) 1990, Martensite-activated NiTi alloy: that is, TTR is lower than or very close to the oral temperature, which exists in a multi-element state at room temperature and is easy to deform. When put into the mouth, martensite caused by stress and room temperature is transformed into austenite at the same time, that is, it has shape memory function and superelasticity. It is easy to deform at or below room temperature (about 25℃), but it will return to its original preformed shape after reaching a certain temperature (about 32℃), showing the characteristics of shape memory and superelasticity. Smart brand of Beijing Shengmate Technology Co., Ltd. and NitinolHA brand of 3M Company are typical representative products. Because of this characteristic, the heat-activated nickel-titanium archwire is easy to be shaped and placed in brackets when it is maintained at or below room temperature, and it can generate shape restoring force when it is activated by body temperature in the mouth, providing necessary strength for orthopedics. Because the heat-activated nickel-titanium orthopedic wire has the characteristics of "softening when it is cold and becoming elastic when it is activated by heat", under the guidance of doctors, patients can change their orthodontic force by taking cold and hot water in their mouths, which is more convenient for orthodontists to correct and reduces the discomfort of initial correction.

5) Grading thermodynamics: adding thermodynamic nickel-titanium alloy: TTR temperature is higher than the oral temperature, about 40℃, which makes the nickel-titanium arch wire still in a pluralistic state when it is put into the oral cavity, and the arch wire is soft, and austenite phase transformation can only occur when the oral cavity contains hot water. Therefore, the orthodontic force is weak, and it can be used as the initial arch wire for adults and patients with periodontal disease. Both the copper-containing nickel-titanium wire produced by Omcroft Company and the Japanese low-lag L-H nickel-titanium bow wire have this property.

(4) Clinical application of 4)NiTi alloy wire:

1, used for early alignment and leveling of patients' dentition. Because NiTi alloy arch wire has superelasticity, shape memory performance and low stress-strain curve, it is usually used as the arch wire initially incorporated into orthodontic system in clinic, thus greatly reducing the discomfort of patients. At present, there are several different straight wire bow techniques. For MBT technology, it is recommended to use 0.0 16 inch heat-activated NiTi alloy archwire (HANT wire), for DEMON self-locking bracket technology, it is recommended to use copper-containing heat-activated NiTi alloy archwire (phase transition temperature is about 40 degrees), and for O-PAK orthodontic technology, it is recommended to use 0.0 16 inch super-elastic NiTi alloy archwire for early alignment and leveling.

2. Nickel-titanium spring: Nickel-titanium push spring and tension spring are a kind of spring used in orthodontics, which have the characteristics of nickel-titanium superelasticity and are suitable for opening gaps between teeth and pulling teeth in different directions in orthodontics. Nickel-titanium spiral spring can be extended by 1mm, which can produce about 50g force. Nickel-titanium spiral spring has high elasticity and can produce soft and stable continuous force in tension. The force attenuation is very small, which can produce ideal orthodontic force to meet the needs of clinical tooth movement. Meet the physiological requirements. NiTi steel wire tension spring has high elasticity and extremely low permanent deformation rate. Compared with stainless steel wire with the same diameter, its release correction force is 3.5-4 times different. Therefore, in the application of orthodontic treatment, patients not only have mild pain, soft and lasting sensory strength, but also reduce the follow-up time, shorten the course of treatment and improve the curative effect. It is an excellent new mechanical device in orthodontic treatment.

3.L-H archwire was developed by Dr. Soma in Japan and produced by Tomy Company. "LH" is named after "LowHysteresis", that is to say, the stress generated when the archwire is ligated to the bracket, that is, when the archwire is activated, is not much different from the stress generated when the tooth moves, that is, when the archwire slowly recovers to its original state. That is, the lag is very small. SOMA et al. compared the stress-strain curves of LH arch wire and other NiTi alloy wires. The lag range of L-H arch wire is the smallest, which makes the arch wire have the advantages of low load and continuous light force. At the same time, the initial slope of the curve is low, which indicates that the rigidity of the arch wire is low. The hysteresis curve of other nitinol arch wires shows that they are relatively hard. Obviously, L-H archwire has obvious mechanical advantages. Because the content of titanium in the Ni-Ti component of LH wire is higher than that of the general Ni-Ti arch wire, it is called Ti-Ni wire, and the experiment proves that it has strong shock absorption effect. Another feature of LH Nitinol wire is that it can be bent and heated by a heat treatment instrument. Therefore, LH nickel-titanium alloy wire can also have an arch wire at the top and an arch wire at the bottom, and then heat it with a heat treatment instrument to strengthen the hardness. At present, L-H archwire is used to treat open jaw, crooked jaw and malocclusion in clinic, because it is stable, soft and effective. At the same time, J hook is often used to improve the weakness of arch wire. Although MEAW technology has an ideal effect on correcting the above malocclusion, the complicated bending of arch wire often makes many doctors flinch. Therefore, some doctors use the rocking chair NiTi arch wire to vertically pull the front teeth, which is similar to a mechanical system. Although this has a similar effect, they always feel that the movement of individual teeth is not as good as MEAW technology, because the rocking chair NiTi arch wire is a continuous arch wire, and there is no way to bend it. Therefore, the angle of bracket bonding and the curvature of arch wire rocking chair determine the angle of each tooth, which is different from that of each tooth in MEAW technology. Bending the rocking chair with LH nickel-titanium and then bending it in the mouth with arch wire shaper is quite ideal.

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