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Vacuum Annealing Heat Treatment of Metal Coronary Stents

The vacuum annealing heat treatment of metal coronary stents is a key process and a special process in its manufacturing process. The performance of metal coronary stents can be optimized by vacuum annealing heat treatment, including reducing the tensile strength and hardness of the material, increasing the elongation at break, and reducing the thickness of the stent. The control level of the vacuum annealing heat treatment process determines the performance of the metal coronary stent during and after implantation in the human body. It is recommended to control the elongation at break of the metal coronary stent after annealing and heat treatment 50% or more, and the grain size grade is above 8.

The combined action of bad living habits and various factors such as body mass index, high blood pressure and high blood lipids can lead to coronary atherosclerotic lesions in the human heart blood vessels, and in severe cases, the coronary blood vessels of the heart will be blocked by thrombosis, which will eventually lead to human beings in the world today. Cardiac myocardial infarction is one of the leading causes of death. The emergence of coronary stents has revolutionized the treatment of coronary artery thrombosis. Compared with drug therapy and heart bypass surgery, coronary stent intervention has quicker effects, It has the advantages of high success rate, less trauma, and no need for general anesthesia. Usually, you can get out of bed 24 hours after surgery. With the widespread application of percutaneous coronary intervention (PCI) in clinical practice, the mortality rate of coronary heart disease has decreased significantly.

1.1 Coronary stent raw materials

The first generation of coronary stents are bare metal stents without drug coating (BMS, Bare-Metal Stent), usually made of 316LVM stainless steel. For restenosis, patients need lifelong medication to combat these shortcomings. The second-generation coronary stent is a drug-eluting stent (DES, Drug-Eluting Stent), which adds a drug coating on the basis of the first-generation bare metal stent. In addition to 316LVM stainless steel, L605 and MP35N cobalt-chromium alloys, platinum-chromium alloys, etc. also appeared. Among them, the drug carrier between the drug and the metal stent is divided into two types: absorbable and non-absorbable. The second-generation stent has greatly improved the first generation of stents. The first-generation bare metal stents have various shortcomings, but the mortality rate caused by late thrombosis is relatively high to a certain extent. The third-generation coronary stents are degradable and absorbable stents for the human body, and the materials used are mainly divided into high molecular polymers and metals. There are two major types of alloys. The representative of high molecular polymers is polylactic acid PLA material; the absorbable and degradable alloys include magnesium alloy, zinc alloy, pure iron, etc. Although there are so many stent materials, the current mainstream stent material in the market is still the most The traditional 316LVM stainless steel and L605 are equally divided in the market.

1.2 Manufacturing process of coronary stent

The manufacture of coronary stents is an art involving multiple professional fields, starting from selecting the best raw materials, cutting out complex patterns through high-precision lasers, and then performing surface treatment to ensure the highest quality ideal products. In terms of the selection of stent raw materials, the two most common metal pipes are 316LVM stainless steel pipe and L605 cobalt-chromium alloy pipe. Both kinds of pipes can choose to use seamless cold drawn pipe or annealed pipe for the processing and production of coronary stent. Specific The production process is also different.

2.Vacuum annealing heat treatment process of metal coronary stents

2.1 Vacuum heat treatment

The process of using a vacuum furnace to place the parts in a vacuum state for heating, heat preservation, cooling and other heat treatment is called vacuum heat treatment. In a vacuum state, the vacuum furnace heats the parts by radiation, so the vacuum heat treatment has the advantages of slow heating speed and uniform temperature. Compared with other heat treatments, vacuum heat treatment of metal materials has the advantages of vacuum protection to prevent oxidation, vacuum degassing and degassing, and purification and degreasing of metal surfaces. Common vacuum heat treatment processes include vacuum annealing, Vacuum quenching, vacuum tempering, vacuum carburizing, etc.

2.2 Vacuum annealing of metal coronary stents

Laser cutting and electropolishing are generally considered to be the main quality influencing factors in the manufacturing process of coronary stents, since both laser cut and electropolished coronary stents can be visually inspected by experienced quality inspectors using a stereo microscope. It is easy to judge whether its quality is qualified or not. In fact, the annealing heat treatment of coronary stents is another crucial critical quality factor in its manufacturing process. Because the annealing process also has a considerable impact on the mechanical properties of the coronary stent and the performance of implantation in the human body, and is also a key process and special process in the manufacture of the stent. When the metal coronary stent is opened by the balloon in the coronary vessel Plastic deformation will occur, and the corresponding plastic strain is as high as 20 to 30%. In order to make the stent expand and deform in the blood vessel according to the design intention, the performance of the metal coronary stent should be optimized by thermomechanical treatment and annealing. Use cold drawing It is better to anneal the bracket after laser cutting the tube as the raw material tube, because the hardened cold-drawn tube can avoid the damage caused by improper handling of the softened annealed tube, and it also helps to release the residual stress of the tube drawing and eliminate the Undesirable effects of the heat-affected zone produced by laser cutting. In addition, compared with the annealing heat treatment of the raw tube with a length of 1~2 m, the length of the coronary stent after laser cutting is only 8~40 mm, and the annealing heat treatment of the coronary stent can better optimize the control of the micron dimensional accuracy of the coronary stent. vascular stent to obtain the desired mechanical properties and microstructure.

3 Performance evaluation of metal coronary stents

Annealing heat treatment determines the final properties of coronary stent materials, such as grain size, tensile strength, elongation at break, corrosion resistance, surface appearance quality, etc. The performance changes brought about by these annealing heat treatment processes cannot be determined by a stereo microscope. To confirm by visual appearance inspection, more expensive testing and analysis methods are required for detection. The testing and analysis methods commonly used to confirm the performance and quality of coronary stents after annealing and heat treatment include tensile testing, microhardness testing, metallographic structure analysis, etc.

4 Conclusion

The vacuum annealing heat treatment of metal coronary stents is a special process in the manufacturing process of coronary stents. The performance of metal coronary stents can be optimized by vacuum annealing heat treatment, including reducing the tensile strength and hardness of the material, increasing the elongation at break, thinning The grain structure, etc, determines the performance of the metal coronary stent during and after implantation in the human body. It is recommended to control the elongation at break of the metal coronary stent after annealing and heat treatment to more than 50%, and the grain size grade is 8 level or above.