The application of titanium-nickel memory alloys in medical fields

The two characteristics of titanium-nickel alloy after different treatments each have their own unique features. When these features are applied in medical practice, they can be made into medical sutures, which can be used for non-damaging muscle and intradermal sutures according to clinical suturing requirements. For those that need to be shaped, temperature-memory metal sutures can be used; for those that do not, super-elastic metal sutures can be used. After the wound heals, the metal sutures can be removed. It is a medical product that does not cause damage after surgery, aiming to leave no foreign objects in the human body and prevent the wound from having hard ridges, pinholes or other adverse conditions.

Special properties of nickel-titanium alloys:
Shape memory property (shapememory) Shape memory occurs when a parent phase of a certain shape is cooled from above Af temperature to below Mf temperature to form martensite. If the martensite is deformed at temperatures below Mf and then heated to below Af temperature, accompanied by reverse phase transformation, the material will automatically restore its shape as it was in the parent phase. In fact, the shape memory effect is a thermally induced phase transformation process of nickel-titanium alloys.

superelastic refers to the phenomenon where a specimen, under the action of an external force, generates a strain far greater than the elastic limit strain, and the strain can automatically recover upon unloading. That is, in the parent phase state, due to the effect of the applied stress, stress-induced martensitic transformation occurs, thus the alloy exhibits mechanical behavior different from that of ordinary materials. Its elastic limit is much greater than that of ordinary materials, and it no longer follows Hooke's Law. Compared with the shape memory property, superelasticity has no thermal involvement. In summary, hyperelasticity refers to the condition within a certain deformation range where stress does not increase with the increase of strain. Hyperelasticity can be classified into two types: linear hyperelasticity and nonlinear hyperelasticity. In the stress-strain curve of the former, the relationship between stress and strain is close to linear. Nonlinear superelasticity refers to the results of stress-induced martensitic transformation and its inverse transformation occurring respectively during loading and unloading within a certain temperature range above Af. Therefore, nonlinear superelasticity is also known as phase transition pseudo-elasticity. The phase transformation pseudo-elasticity of nickel-titanium alloys can reach about 8%. The superelasticity of nickel-titanium alloy can change with the variation of heat treatment conditions. When the bow wire is heated above 400 ° C, the superelasticity begins to decline.

Sensitivity to temperature changes in the oral cavity: The orthodontic force of stainless steel wires and CoCr alloy orthodontic wires is basically not affected by the temperature in the oral cavity. The corrective force of the super-elastic nickel-titanium alloy dental orthodontic wire varies with the change of oral temperature. When the deformation amount is constant. When the temperature rises, the corrective force increases. On the one hand, it can accelerate the movement of teeth. This is because the temperature changes in the oral cavity will stimulate the blood flow in the areas where the blood flow stagnates due to the capillary stagnation caused by the orthodontic device, thereby ensuring that the repair cells receive sufficient nutrition during the movement of the teeth and maintaining their vitality and normal function. On the other hand, orthodontists cannot precisely control or measure the orthodontic force in the oral environment.

Recently, there has been a kind of non-invasive cosmetic skin suture needle. The feature of this needle is that a titanium-nickel memory alloy wire is connected to the rear end of the suture needle as the suture thread and is plated with metallic zinc. Due to the temperature memory property of memory alloy wire, when it reaches the originally set temperature, it forms a straight line, demonstrating the elastic performance of the metal material, which is not possessed by ordinary metal materials. Using this method for intra-skin suturing makes the healing of surgical wounds relatively ideal.

The suture shaping and muscle binding have been achieved. The suture shaping and muscle binding are carried out by using the various suture needles mentioned above, through suture removal and live joint binding, to pull out the metal wire inside the healed wound in a straight line, leaving no foreign objects in the human body. The feature of being able to pull out metal wires lies in the temperature memory of the metal and its super-elasticity that is not affected by temperature.