Intravascular stents are mainly made of biomedical metals, alloys or medical macromolecule materials, which are manufactured by special processing, and are used as medical devices for treating stenosis or occlusion of human blood vessels. Among metal stents, airbag expansion stents made of stainless steel or cobalt-chromium alloys, and self-expanding stents made of nickel-titanium alloy Nitinol are products already on the medical market.
With the widespread use of nickel-titanium alloy self-expanding stents in the interventional treatment of diseases such as vascular stenosis, the manufacturing technology of nickel-titanium alloy stents has also been developed, and roughly experienced three different manufacturing technologies. The self-expanding stent is a helical coil-shaped structure, and a braided mesh-structured stent appeared later. The newer technology currently is laser-cut tubular stent.
High geometric accuracy and customized
Lab22's additive laboratory has achieved many results in the field of medical device 3D printing research and development, including personalized 3D printed sternum implants and heel implants.
The Lab22 team has been exploring the feasibility of 3D printing Nitinol self-expanding stents in the laser melting additive manufacturing process in selected areas. Selective laser melting 3D printing technology can create complex vascular stents with high geometric accuracy, and at the same time it is easy to realize on-demand manufacturing of customized stents for patients.
The Lab 22 team also said that using SLM to develop Nitinol stents is challenging. The nitinol material they used was Nitinol, which is a shape memory alloy that exhibits superelasticity when pressed. The material has a unique crystal structure, which changes when subjected to pressure or heating. The two different phases of the alloy (martensite and austenite) are determined by temperature, and the phase transition temperature is extremely sensitive to the manufacturing conditions of the stent. In order for the stent to exhibit self-expansion, the transition temperature needs to be 37 ° C below body temperature. In addition, the process parameters of SLM 3D printing require ultra-fine mesh structures suitable for the manufacture of stents, including thin struts with dimensions of 80-200 μm.
According to Lab 22 Additive Manufacturing Laboratory, additive manufacturing technology provides freedom for stent design, design developers can develop the proximal and distal diameters of blood vessels of specific sizes as needed, and can also manufacture larger sizes through this process The new shape of the stent, cross branches and proximal and distal blood vessels. In addition, 3D printing technology will enhance the customized production capacity of vascular stents, reduce inventory, and increase the effective utilization of resources. For patients, this type of 3D printed special vascular stent has better vascular compliance and is expected to improve the patient experience.
Nitinol self-expanding stents have roughly undergone the evolution of three technologies: spiral coil-like structures, braided mesh-like stents, and laser-cut tubular stents.
Performance index of medical nickel-titanium alloy stent
The coil-shaped structure stent is formed by winding a nickel-titanium alloy wire. The stent is simple and flexible to manufacture, but the disadvantage is that the strength is insufficient and the coverage is low, and post-restenosis is easy to occur. This method has been gradually eliminated. The mesh structure bracket is made of nickel-titanium alloy wire, which has good elasticity but poor strength and is prone to displacement. This type of bracket is now used less. Laser-cut tubular stent is currently the most widely used type in clinical practice. The manufacturing method is laser engraving. This kind of stent better avoids the shortcomings of the previous two generations of technology. There is no welding point structure and the contact with the diseased lumen Surface contact has a strong force on the inner wall of the diseased lumen and is not prone to displacement. At the same time, the structure is strong and the wall thickness is small.
The manufacturing technology of Nitinol self-expanding vascular stent is still continuing to develop. At present, the common manufacturing methods of nickel-titanium alloy stents still have the problems of high production cost, limited structural design of the stent, difficulty in achieving complex shapes, and the failure to achieve key performances such as accuracy and finish.
Selective laser melting, as a new additive manufacturing process for nickel-titanium alloy self-expanding stents, has not yet become a mature commercial vascular stent manufacturing technology, but research institutions have conducted research on this application for many years. Complex design and development of improved nickel-titanium alloy powder materials and their additive manufacturing processes.
Typical research achievements in the field of additive manufacturing of nickel-titanium alloy stents in China.
Process development and printing material preparation
Nanjing University of Aeronautics and Astronautics has developed a method for preparing a shape memory alloy (nickel-titanium alloy) vascular stent based on automatic powder coating (selected laser melting 3D printing) laser combined processing technology. This method is based on the three-dimensional data model of the parts to be processed. The high-energy laser beam melts the mixed powder system and spreads the powder layer by layer, fusing layer by layer to superimpose and accumulate, until the vascular stent blank of the net structure is finally formed, and then undergoes electrochemical polishing to meet the specific surface roughness requirements.
The vascular stent prepared by this method relies on the unique superelastic function and shape memory effect of the shape memory alloy, which can effectively reduce the incidence of vascular restenosis in clinical application of the vascular stent; through mechanical properties and simulated biological environment tests, the vascular stent has good The biological tissue and blood compatibility are in line with the medical application conditions; and based on the advantages of ultra-high manufacturing precision of laser combined processing technology and the protection of inert gas during the forming process, it can effectively overcome the processing surface roughness, burrs and oxidation during the preparation of traditional vascular stents problem.
More flexible design: different expansion coefficients in different parts
South China University of Technology is based on the development of a three-dimensional vector-expanded cardiovascular stent with memory effect. The stent is made of metal material with vector expansion effect through metal printing. It includes a plurality of concave hexahedral grids arranged uniformly along the axis. The mesh loop wire composed of units, and the support body is composed of multiple layers of the mesh loop wire along the array. The mesh loop wires located at both ends of the rack body are respectively connected with an upper support ring and a lower support ring, which are supported by the upper The ring and the lower support ring fix the bracket body. The manufacturing method is to control the austenite-martensite transformation temperature by controlling the energy density of laser selective melting, forming different parts of the matrix structure with different energy densities, and achieving the controllable deformation of the shaped cardiovascular stent based on temperature-dependent changes The different parts of the stent have different expansion coefficients based on temperature, making the stent more suitable for the specificity of the shape of the blood vessel and the thermal expansion and contraction.
In addition, South China University of Technology has also developed a 4D printing method that adjusts the functional characteristics of nickel-titanium alloys in situ. This method uses metal additive manufacturing to achieve additive manufacturing of a mixed powder of nickel-titanium alloy powder and nano-scale nickel powder. By adjusting the mixing ratio of the nickel-titanium alloy powder and the nano-nickel powder, the nickel-titanium atomic ratio of the nickel-titanium alloy can be precisely adjusted, and finally the phase transition temperature and functional characteristics can be adjusted to expand the industrial application field of the nickel-titanium alloy.
The performance indicators of medical nickel-titanium alloy stents show that nickel-titanium alloy stents, as an implantable medical device, have very strict requirements for various performance indicators. No matter how their manufacturing technology evolves, they need to meet 6 basic performance indicators. . Whether additive manufacturing technology has developed into a new generation of nickel-titanium alloy self-expanding stent manufacturing technology.