Currently commercialized coronary stents are mainly made of the medical 316L stainless steel and cobalt-based alloy(L605) due to their good combination of properties,especially excellent mechanical properties.However,the presence of high quantity of nickel and/or cobalt elements,the agents known to trigger the toxic and allergic responses,in these materials has caused many clinic concerns.The potential adverse effect of nickel ions release has prompted the development of high nitrogen nickel-free austenitic stainless steels for medical application.Nitrogen in steel is not only to replace the nickel but also improve the properties of steel.In this paper,the harmfulness and release of nickel from metallic stents,and the advantages in mechanical properties and hemocompatibility of high nitrogen nickel-free stainless steels for coronary stents are reviewed.Apart from the highlight of nickel-free,the superiority of high strength and better hemocompatibility of high nitrogen nickel-free stainless steels can guarantee to manufacture thinner strut coronary stents with remarkable anticoagulation ability.High nitrogen nickel-free stainless steels as a promising coronary stents material will attract more and more clinical doctors and stents makers to bring them into clinical application.
Most commercialized coronary stents are made of 316L stainless steels due to its good combination of properties, and currently some new stents are made of cobalt-based alloy owing to its higher mechanical properties. However, the presence of high quantity of nickel and/or cobalt elements in these materials, which are known to trigger the toxic and allergic responses, has caused many concerns. Nickel-free austenitic stainless steels have been developed in order to solve these problems. In this paper, based on the development of a new Fe- Cr-Mn-Mo-N type high nitrogen nickel-free austenitic stainless steel, properties such as mechanical property, corrosion resistance in Hankls solution, and in vitro blood compatibility including the kinetic clotting time and the platelets adhesion, were investigated in comparison to the above two conventional materials, a 316L stainless steel and a Co-28Cr-6Mo alloy. The results showed that the new high nitrogen steel possessed better combination of mechanical properties, corrosion resistance and blood compatibility than those of 316L steel and the Co-28Cr-OMo alloy, and can be a promising alternative material for manufacture of coronary stents.
Stable austenitic structure in medical stainless steels is basically required for surgical implantation. A weak magnetism was found in a high nitrogen nickel-free austenitic stainless steel for cardiovascular stent application. This magnetic behavior in high nitrogen stainless steel was investigated by optical microscopy, X-ray diffraction （XRD）, electron probe microanalysis （EPMA） and superconducting quantum interference device （SQUID）. The results showed that the magnetism came from the composition segregation of ferrite formation elements such as Cr and Mo in the steel and some 6-ferrites were locally formed during the pressurized electroslag remelting process. The magnetism of high nitrogen stainless steel could be eliminated by a proper high temperature gas nitriding （HTGN）.
Cytocompatibility of high nitrogen nickel-free stainless steel （HNS） with different nitrogen content was evalu- ated and compared with a conventional austenitic stainless steel 317L. The MTT assay （3-（4,5-dimethylthiazol- 2-yl）-2,5-diphenyltetrazolium bromide） was performed on MG63 osteoblasts to assess the cytotoxicity. The expression of selected marker typical of differentiated osteoblasts, such as alkaline phosphatase activity （AKP）, was also monitored in MG63 cells cultured on the tested materials. As a result, HNS had higher cell growth than 317L; meanwhile the cytocompatibility was increased with increasing nitrogen content. Furthermore, HNS enhanced osteoblasts differentiation, as confirmed by AKP activity. Overall these facts indicated that HNS had higher cytocompatibility than 317L and the nitrogen content contributed to the higher cytocompatibility of HNS. The influence of nitrogen on surface energy further explained the cytocompatibility of HNS.
Tianchi MaPeng WanYuyan CuiGuirong ZhangJiqiang LiJihui LiuYibin RenKe YanggLi Lu
