Is Nitinol a biomaterial?

(1) Physical and chemical properties of nitinol alloy

Biomedical Metal materials are implanted into human body as foreign bodies. On the one hand, its basic physical properties should be investigated. For example, when selecting implant materials, we should consider whether their density and elastic modulus are similar to those of surrounding tissues and whether they are magnetic. The density of nitinol alloy is relatively small, which is similar to that of bone tissue, so it is more suitable for hard tissue repair; Its elastic modulus is only 1 / 4 of that of ordinary stainless steel, which is close to human bone, which can greatly reduce the stress shielding effect and avoid osteoporosis; Nickel titanium alloy is non-magnetic and can be used for nuclear magnetic resonance imaging. It will not be affected by external magnetic field after synchronous implantation into human body. In addition, nitinol shape memory alloy can also be made into porous materials. Porous nitinol alloy has low density and is more suitable for compatibility with bone mechanics.Biomedical Metal materials are implanted into human body as foreign bodies. On the one hand, its basic physical properties should be investigated. For example, when selecting implant materials, we should consider whether their density and elastic modulus are similar to those of surrounding tissues and whether they are magnetic. The density of nitinol alloy is relatively small, which is similar to that of bone tissue, so it is more suitable for hard tissue repair; Its elastic modulus is only 1 / 4 of that of ordinary stainless steel, which is close to human bone, which can greatly reduce the stress shielding effect and avoid osteoporosis; Nickel titanium alloy is non-magnetic and can be used for nuclear magnetic resonance imaging. It will not be affected by external magnetic field after synchronous implantation into human body. In addition, nitinol shape memory alloy can also be made into porous materials. Porous nitinol alloy has low density and is more suitable for compatibility with bone mechanics. It has been widely used in surgical repair.

(2)Nitinol alloy biocompatibility

Biocompatibility refers to the concept of various biological, physical and chemical reactions after the interaction between materials and organisms. Generally speaking, it refers to the compatibility limit between the material and the human body after it is implanted into the human body, that is, whether it will cause toxic effects on human tissues.

• Nickel biocompatibility

Nickel is an essential trace element in normal body. It is mainly absorbed by respiratory tract and is well absorbed. Metal nickel is not easy to be absorbed from digestive tract or skin. The content of nickel in normal human serum was 0.026 mg / L, and the content in hair was the highest, up to (0.64 ± 0.44) mg / g. The content of nickel in normal human body is 6 ~ 10 mg, which is mainly distributed in kidney, lung, brain, cartilage, spinal cord, connective tissue and skin. The World Health Organization points out that the daily nickel intake of adults is 0.4 ＾ rag, which can meet the physiological needs, and its important thing is excretion through feces, urine and sweat. Low or high nickel intake will lead to diseases. Lack of nickel will cause slow growth, weight loss, interfere with iron, fat, glucose and liver glucose metabolism in the body, disrupt the process of calcium entering the bone, reduce the ratio of bone length to width, inhibit the activities of several enzymes in the heart, liver and kidney, and worsen the myocardium and skeletal muscle; Nickel intake or nickel release in implants exceeding a certain concentration will also cause serious consequences. High concentration nickel has obvious cytotoxic effect on cells and can damage cells. Implantation of pure nickel in muscle or bone will cause serious local tissue stimulation and necrosis, and has high potential carcinogenicity and toxicity.

• Biocompatibility of titanium

Titanium is necessary and harmless to human body. The content of titanium in normal human tissues is not more than 15 mg. It is generally believed that pure titanium can be well tolerated by local tissues and will not induce toxic or inflammatory reactions. Under appropriate conditions, titanium can bind to bone, and titanium can inhibit bacteria without significantly activating or inhibiting various toxic enzymes. Titanium particles are released from the implant passivation layer, mainly insoluble titanium oxide or secondary oxide, which are considered to be bioinert. When the titanium passivation layer is scratched, due to the strong oxidation ability of titanium, it can be repaired soon. This behavior protects the alloy and stops the formation of other compounds except oxides.

• Biocompatibility of nitinol alloy

Biocompatibility is the most important factor whether shape memory alloys can be used in human body. With good biocompatibility, the material will not cause allergic reaction in the organism and will not release any ions into the blood of the organism; In the long-term existence of organisms without harmful reactions. Biocompatibility and surface properties of materials are closely related to biological inflammation and allergic reaction. Many factors, such as patient’s health status, age, immune status and material properties (surface roughness, porosity, element toxicity) can affect human inflammation and allergic reaction.

Cell culture and proliferation experiment,nitinol alloy has good biocompatibility with human embryonic epithelial cells and human embryonic palatal mesenchymal cells;

Biocompatibility test of soft tissue in vivo,nitinol alloy has good compatibility with blood vessels, muscles, nerves and bones.

For Nitinol, which has nearly 50% nickel,as a long-term implant, its nickel dissolution will have a certain toxicity to the body. At present, the important way to solve nickel ion precipitation is to modify and modify the surface of memory alloy. There are two important ways: one is to attach a protective barrier on the surface of memory alloy to prevent nickel ion dispersion; The other is to increase the thickness of oxide layer to resist nickel ion dispersion. The first coating has corrosion resistance and high strength, but it still has chemical reaction after long-term implantation. After oxidation reaction, the mechanical, electrical and electrochemical properties of the material surface also change synchronously; The second oxide layer has little effect on the shape memory function and hyperelasticity of the memory alloy, but it has the defects of increased nickel ion concentration in the outer layer and poor wear resistance after oxidation. Therefore, choosing a material with higher inertia, stronger strength and good biocompatibility to coat the surface of nitinol memory alloy may be the core to solve this problem.