Stainless steel has excellent comprehensive performance, is the most widely used medical metal materials. However, the use of ordinary medical stainless steel is also found in the process of its limitations, such as stainless steel tends to serve in the cold working state, such as 316L stainless steel is often used to maintain a cold deformation of about 30%, and when the deformation is greater than 20%, the deformation of defects generated will significantly weaken the stainless steel resistance to pitting corrosion. In addition, there are also problems such as implant fracture due to insufficient strength and toxic reactions caused by metal ion leaching.
In order to improve its safety and effectiveness, on the basis of traditional austenitic stainless steel, new stainless steels with more excellent mechanical properties, corrosion resistance, biocompatibility and biological functions have been developed. Among them, nickel-free stainless steel has excellent comprehensive performance, and has become the future development trend of medical stainless steel. Nickel-free stainless steel effectively avoid the nickel ion leaching on the human body sensitization, teratogenic and other hazards, and has exceptionally excellent mechanical properties and local corrosion resistance.
Nickel-free stainless steel with high N content (more than 0.4%, mass fraction) instead of Ni elements to stabilize the austenitic organization of the stainless steel, so it is also known as high nitrogen nickel-free stainless steel. In order to increase the solid solubility of N in steel, an appropriate amount of Mn is generally added. Existing high-nitrogen nickel-free stainless steel including P558 alloy, P2000 alloy, BioDur108 alloy, BIOSSN (Fe-18Cr-15Mn-2Mo-(0.5~1.0)N) and so on.
- 1.Mechanical properties
N as an interstitial atom has a strong strengthening effect on stainless steel. The strength of nickel-free stainless steel increases linearly with increasing N content while hardly reducing plasticity. In order to obtain more excellent mechanical properties, medical nickel-free stainless steel is often selected higher N. N content of 0.9% (mass fraction) of the BIOSSN stainless steel, its plasticity is comparable to the 316L stainless steel, the strength of more than two times that of the 316L stainless steel, and its work-hardening ability is excellent. In addition, BIOSSN stainless steel also has excellent wear resistance. Therefore, higher N content of nickel-free stainless steel as implant material has higher biomechanical safety. However, N-containing stainless steel has low-temperature brittleness, and the addition of Cu can reduce its toughness and brittle transition temperature, but can not eliminate its low-temperature brittle behavior.
- 2. Corrosion resistance
The corrosion resistance of medical stainless steel, especially pitting resistance, has a great impact on the safety of the implant, such as the femoral stem due to fatigue-induced fracture failure is related to the stress concentration generated by the pitting pit. Clinical applications show that stainless steel hip prosthesis implanted several years after the surface pitting, and triggered aseptic loosening and failure.
The N element has a strong influence on improving the pitting resistance of stainless steel, the pitting resistance of nickel-free stainless steel increases linearly with the N content, and its pitting resistance potential even reaches two times that of conventional stainless steel. The special corrosion resistance of nickel-free stainless steel is related to its special passivation film structure containing N-enriched layer.
The unfavorable effect of severe cold deformation on pitting resistance is evident at low N contents in nickel-free stainless steels. When the N content continues to increase, this adverse effect is significantly weakened. When the N content is greater than 0.9% (mass fraction), the adverse effect of cold deformation is completely eliminated.
- 3.Biocompatibility
Nickel-free austenitic stainless steel replaces Ni with N, avoiding the release of nickel ions harmful to the human body. Compared with traditional Ni containing stainless steel (such as 316L) and NiTi alloys, high-nitrogen nickel-free stainless steel has a higher cellular value-added rate, with excellent biocompatibility. In vivo bone implantation experiments in animals show that the bonding of nickel-free stainless steel with bone tissue is superior to that of high-nitrogen stainless steel containing Ni and Ti6Al4V alloy.
It was concluded that high-nitrogen nickel-free stainless steel promoted the differentiation of osteoblasts. Nickel-free stainless steel was implanted into rabbit tibiae, and the results showed that the binding ratio of nickel-free stainless steel to bone tissue, the area of new bone, and the osseointegration force were significantly better than those of conventional 316L stainless steel. The study also concluded that the N and Mn elements in the steel together promote the ability of nickel-free stainless steel for osteoinduction and long-term osseointegration. Scholars studied platelet adhesion and dynamic clotting time on the surface of high-nitrogen nickel-free stainless steel, and found that compared with conventional 316L stainless steel, high-nitrogen nickel-free stainless steel has a more superior blood compatibility, which may be due to the fact that nickel-free stainless steel avoids the reaction in which nickel ions bind to human serum proteins and lead to blood denaturation. In addition, the nickel-free stainless steel stent had a faster rate of endothelialization and a lower degree of endothelial hyperplasia compared with conventional 316L stainless steel.
- 4. Application of medical nickel-free stainless steel
In terms of medical devices, high-nitrogen nickel-free stainless steel as a medical material has been developed in the field of orthopedics and vascular disease treatment of related products, and began clinical applications. The United States developed a hollow screw system, which has a deep thread, large aperture characteristics, and does not reduce the strength of the screw, has been used in a large number of clinical applications. Canada developed coronary stents with lower mesh wire thickness and surface coverage, and the effect of clinical trials is obvious. After 4 months of implantation of the new coronary stent developed in China, no obvious restenosis occurred in 30 patients, avoiding the harm to human body caused by the dissolution of Ni ions in the stent currently used in clinical practice, and at the same time, it also has the advantages of thin mesh filaments, uniform deformation, excellent flexibility and support, and so on.
