Biomedical titanium alloys have now become the main raw material for surgical implants and orthopedic devices worldwide. The 3D printing technology can personally customize biomedical materials according to the needs of different patients and precisely control the microstructure. Therefore, combining this emerging technology with biomedical materials is a major research trend in future biohistological engineering. In recent years, different medical materials have been successively prepared using 3D printing technology for animal tissue repair and other experiments. This article briefly reviews the research status and progress of 3D printed titanium alloy biomaterials.
1
introduction
Biomedical metal materials are a class of biologically inert materials that are widely used in surgical implants and orthopaedic devices in the field of orthopedics. At present, commonly used medical metal materials mainly include cobalt-based alloys, stainless steels, and titanium-based alloys. There are also memory alloys, precious metals, and pure metals such as antimony, bismuth, and zirconium [1]. Among them, titanium-based alloys are widely used in biomedical applications as load-bearing implants due to their advantages of light weight, high strength, good corrosion resistance in physiological environments, excellent fatigue resistance and low elastic modulus [2]. Due to the poor matching between the implant and the affected area in clinical practice, the effect of surgery and the life of the implant are affected. According to the patient's condition, tailoring individualized surgical implants with specific structures and satisfying biosafety requirements has become a research hotspot for medical materials. Most of the existing metal implants use molds, turning and milling and other traditional mechanical processing methods to shape and cut raw materials. The cost is high, the smelting process is long and difficult, the process is complicated, and the purpose of individualized treatment cannot be met. With the rapid development of materials science and computer-aided engineering, 3D printing technology has provided new ideas for the realization of personalized treatment methods.
2
3D printing technology overview
3D printing technology, which is a kind of rapid prototyping technology, is based on digital model files. It uses software to layer discrete and numerically controlled molding systems, and uses hot melt nozzles, laser beams, and other methods to bond powdery metals or plastics. The materials are stacked layer by layer and finally overlaid to form objects [3]. "Layered manufacturing, layer by layer superposition" is its core principle [4]. Currently available 3D printing technologies include: Electron Beam Melting (EBM), Selective Laser Sintering (SLS), Direct Metal Laser Sintering (DMLS), Melt Stacking (FDM), Laser Cladding Technology (LENS), Three-dimensional lithography (SLA), three-dimensional printing (3DP), DLP laser molding technology, UV UV molding technology, LOM layered solid manufacturing technology, etc. [5]. Materials commonly used for 3D printing include: metals, ceramics, and polymer materials. After decades of development, 3D printing technology has been widely used in industrial design, automotive, aerospace, construction, medical, and education. This digital manufacturing model breaks through the limitations of traditional processes, shortens the time-course of product design and production, simplifies the complexity of manufacturing, and can fully meet the requirements and goals of personalized customized services.
3
3D printing titanium alloy process
Compared with traditional techniques, the advantages of using 3D printing technology to create personalized surgical implants are mainly reflected in: 3D printing free-form features can quickly and accurately customize the internal implant, can overcome the shape of the traditional universal implant Incompatibility with the human body and its mechanical properties are not up to the standard of the problem [6]; in the manufacture of complex structures and difficult-to-manufacture products, the custom-built microstructure, especially the porous structure, not only can meet specific physical and chemical properties, but also Enhances biological tissue compatibility. This series of advantages can effectively overcome the ubiquitous stress shielding and low biological activity of implants. The most commonly used and most widely used 3D printing titanium alloys are SLM technology and EBM technology.
Selective Laser Melting (SLM) [7] uses a laser as a heat source to selectively irradiate pre-powdered powder material for rapid melt forming. Its working principle is mainly in the inert gas protection environment, the instrument and equipment according to the system design mode generated by the fill scan path to control the laser beam to select the region melting layer of powder. Then the platform is moved down, powdered and sintered again, and circulated back and forth until it is integrally formed. The inert gas protection prevents the metal from reacting with other gases at high temperatures. SLM technology has a wide range of molding materials, material saving and recovery, and no need to design and prepare complex support systems. This series of advantages makes the application of SLM technology more and more extensive. However, SLM also has some drawbacks: Because of the limited laser power and scanning oscilloscope deflection angle, there is a limit to the range of parts sizes prepared by SLM; high-power lasers and high-quality optical equipment are expensive to manufacture, which to a certain extent Increased economic burden; Due to the use of powdered materials in SLM technology, the surface quality of molded parts may be problematic. This requires secondary processing of the product for subsequent work; spheroidizing and warping may also occur during processing. The flaws require further rigorous optimization of the machining program [8].
Electron beam melting (EBM) is a process method in which an electron beam is used as a heat source in a vacuum environment to melt the metal powder layer by layer to add materials. Its working principle is: pre-powdering, high-energy electron beam after focusing to produce high-energy deflection in the local micro-region to produce a high temperature and even the melting of the powder layer, the continuous scanning of the electron beam to generate energy so that the molten pool between the fusion and solidification, Connected into linear and planar metal layers. After the current layer is processed, repeat the powder coating operation until it is formed. In the production process, EBM uses a vacuum melting environment to ensure both the high strength of the material and the oxidation of the alloy. Compared with SLM, EBM's main advantages lie in [9~10]: Efficiently generated electron beam power consumes less power and has a higher output rate, resulting in a higher total total power of the whole machine; the deflection of the electron beam is not further increased by moving parts of the equipment. The scanning speed; good thermal environment makes 3D printed parts shape stability can be guaranteed, and to ensure its static mechanical properties, to meet biological requirements, and metal powder can also be recycled.
4
The current status and progress of 3D printed surgical implants
The use of 3D printed surgical implants and orthopaedic devices has a good application prospect in the field of orthopedics. Nowadays, more and more 3D printed implant materials such as hearing aids, artificial limbs, orthopedic surgery personalization guides, artificial joints, artificial external ear, and personalized implants are used in clinical individualized treatment.
According to reports, in 2014, Peking University researchers successfully implanted a 12-year-old boy with a 3D printed artificial spine, which was designed with a micro hole in mind. This is the first case in the world. In the same year, doctors and scientists installed special 3D-printed prosthetic limbs for a 5-year-old girl in Scotland, England. The Oral Specialties Center of the 411th Hospital of the People's Liberation Army used EBM technology to successfully customize and implant an anatomical high-individualized mandibular titanium alloy implant for the treatment of a mandibular resection patient. Mandibular resection and individualized functional repair were completed at one time. The defect mandible was individually repaired and reconstructed. The postoperative results were satisfactory. Lethaus B et al. [12] used 3D printing technology to reconstruct bone and microvascular flaps in 20 patients who underwent mandibular resection. This shortened the operation time and improved the quality of surgery. The results were good. In recent years, similar news and research have emerged in an endless stream, which fully reflects the good application prospects of 3D printing in the medical field.
In terms of orthopaedic products, surgical implant materials for 3D printing have also gradually become commercialized and market-oriented. The 3D-printed acetabular cup for hard tissue supports developed by the Italian company Adler Ortho and Lima-Lto in 2007 passed the CE certification. In 2010, the FDA certification of the United States passed the same product of Exactech. In 2009, the all-titanium vertebral fusion cage produced by AMT in the United States using 3D printing also passed the EU CE certification. In 2013, the first bio-printed skull implant product in the United States received FDA approval. This is the world's first personalized 3D printed PEEK skull implant. On this basis, in 2014, the United States, Oxford Corporation received FDA approval for 3D printing of the maxillofacial bone products (510K model) [13]. In addition, it was reported that in September 2015, the artificial hip joint for 3D printed human implants jointly developed by Beiyisanyuan and Beijing Aikang Yicheng Medical Equipment Co., Ltd. had been approved by the State Food and Drug Administration. The 3D printing of the hip joint into the "mass production phase" means that China's 3D printed implants have also entered the stage of productization.
3D printing technology has shown more and more important role in medical technology innovation. The research and application of a variety of personalized implant implants, prostheses, dental implants, etc. have also become increasingly widespread. Therefore, the evaluation of biosafety evaluation of implants prepared with this new type of technology will increasingly require attention.
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3D Printing Titanium Biosafety Study
The safety of biomedical materials is mainly reflected in the interaction between tissues and materials. In order to meet the criteria for implanted devices, biomedical metallic materials must require that the response caused by the implantation of the human body be at an acceptable level, and at the same time, it must not cause qualitative changes in the structure and performance of the material. The interaction between the human body and the implant is mainly reflected in its biocompatibility and biological function. Therefore, after implantation in the human body, the implant should not cause adverse reactions such as allergy, inflammation, and chemical reactions in human cells, blood, and organs, or human body foreign body rejection. At the same time, it also requires the need for long-term implantation of the implant must have good static mechanical properties, that is, sufficient strength, suitable elastic modulus, high stability, good corrosion resistance and durability [14]. Titanium alloy surgical implants are now widely used clinically, and biocompatibility research is also quite mature. Therefore, the safety of 3D printed titanium alloy parts mainly focuses on its biomechanical safety.
Whether the metal implant of 3D printing has the same mechanical properties, corrosion resistance and biocompatibility as general implant products of traditional techniques, whether the partial static mechanical properties of the alloy implant can meet clinical application and national standards These studies are still in progress [15]. It has been found that some of the static mechanical properties of 3D printed titanium alloy implants can meet clinical needs. Lock Hongbo [16] Employing EBM technology to prepare Ti6Al4V specimens for direct drawing and hot isostatic pressing followed by tensile and hardness tests, it was found that the strength exceeded the forging standards. The corrosion resistance of Co-Cr-Mo alloys prepared by researchers using SLM technology is similar to that of conventionally prepared alloys [17], and the amount of ion-extracted 3D printing in simulated saliva environments is less than that of conventional process alloys [18]. EOS company through the DMLS technology prepared by Ti6Al4V products after reasonable processing, found that it has no weaker than the traditional static mechanical properties and fatigue resistance. Researchers [19,20] implanted porous titanium interbody fusion cages made from EBM into goats and achieved good results in sheep cervical fusion models. The bone-material interface is better than PEEK fusion cages.
Clinically, the elastic modulus and other mechanical properties of titanium alloys do not match the properties of human bones. This can lead to “stress shielding†of the bone tissue around the implant and cause osteoporosis, resulting in bone resorption and planting. Into the body loose and fall off and the problem of failure. In order to reduce this problem of titanium alloy implants, porous structural implants have entered the researchers' line of sight. Now studies have shown that 3D printing can adjust the elastic modulus and mechanical properties of implants by adjusting the microstructure of the implants with high precision to match the human bone tissue, and further improve the biology on the basis of ensuring the physiological load is appropriate. Mechanical function [21]. Li X [22] used EBM forming technology to manufacture a controllable structure of Ti6Al4V implants. Scanning electron microscopy (SEM) revealed that the internal void structure was consistent with the theoretical design and achieved the precise control of the structure of 3D printed products by EBM. The mechanical properties test showed that at a porosity of 60.1%, the corresponding compressive strength is 163 MPa and the elastic modulus is 14 MPa, which is similar to human bone. In vitro cell culture has also been found to have good cell compatibility. Parthasarathy J [23,24] used the EBM technology to optimize the preparation of porous scaffolds through design parameters and assessed their mechanical properties. He found that the biomechanics of the porous materials designed have a high degree of superiority in the compatibility and implant matching of simulated implants. Sex. Taniguchi N [25] implanted porous titanium with porosity of 300 μm, 600 μm, and 900 μm into the sacrum of the rabbit using SLM to study the effect of porosity on bone ingrowth. It was found that the ingrowth of bone tissue was better when the porous structure was 600 μm. More capacitive.
From the perspective of bone growth, a stent with adjustable porosity and pore size will be more conducive to the transmission and transmission of nutrients in the human body. It will also promote bone ingrowth and increase the binding of implants to the bone bed. And prolong the service life of the prosthesis, so as to obtain better medical effect than solid structure titanium alloy [26]. In recent years, porous titanium alloys are gradually considered to be the most ideal clinical novel hard tissue repair and replacement materials. The application of 3D printed titanium alloy implants with various microstructures or through-structures has also opened up new prospects.
6
Current problems and prospects
Currently, 3D printing has achieved great research progress and achievements in the preparation of surgical implants and orthopaedic devices. However, this technology is still in the initial stage of development in the field of biomedicine. There are still many challenges to achieve the large-scale clinical application of this technology. First, the limited conditions of materials, information, and control technologies are a major difficulty in the development of 3D printing. 3D printing requires metal powders with high purity, good sphericity, small particle size, narrow distribution, low oxygen content, good plasticity and fluidity, etc. [3], but some metal and ceramic materials are now suitable for bone tissue scaffolds. Can not be processed into the ideal particle size for 3D printing, and its temperature control, particle fusion and bonding pathways have yet to be breakthrough [27]. At present, the most commonly used is titanium alloy powder, and other materials have great limitations. The integration of CAD/CAPP/RP supporting software required for 3D printing needs further improvement and optimization. Second, the accuracy, speed, and efficiency of 3D printing have yet to be improved. The printing efficiency is far from being suitable for large-scale production. Due to the limitations of the powder raw materials, the preparation process level, and the conditions of the equipment itself, 3D printing is still difficult to realize high-precision molding, and post-processing is also required for optimization. Therefore, it is also important to achieve rapid manufacturing based on the high-precision quality of 3D printed products. Furthermore, the research is costly and costly. 3D printing equipment is expensive. At this stage, the source of printing materials is single and expensive, the introduction of advanced technology is difficult, the daily maintenance cost is high, and the existing intellectual property protection mechanism is difficult to adapt to the future development of the industry, which limits the development and promotion of the 3D printing industry chain. .
Facing the general trend, it can be irreversible; in the face of opportunities, it can't be used. Although the 3D printing process technology is still in the development stage, as a pioneering and emerging technology, 3D printing has infiltrated into various fields of clinical medical applications, and its development prospect is beyond any doubt. The first registration approval of the “hip†in 3D printing in China is a reflection of the country’s strong emphasis on this technology. During the “Thirteenth Five-Year Plan†period in China, the topics of 3D printing medical product research and development were included in important projects such as biomedical materials and tissue repair alternatives, additive manufacturing, and laser manufacturing. This also means that China’s 3D printing medical products will be developed in the future. There will be more development and application. At this stage, investment in the field of 3D printing should focus on strengthening innovation and research and development, technology introduction and storage [28]. It is believed that in the near future, with the continuous optimization and improvement of materials technology, information technology, and control technology, 3D printing technology will become increasingly sophisticated and mature, bringing happiness to more patients in the medical field.
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