3D printing technology, also called additive manufacturing or incremental manufacturing, is based on the data of 3D mathematical models, mainly using continuous layered printing and the final formation of 3D solids through the superposition of layers. 3D printing's main advantages include: improving manufacturing accuracy, simplifying the tedious production process, saving processing materials and human resources, economic and environmental protection, shorten production time, improve production efficiency, and achieve personalized production. In the field of dentistry, 3D printing technology is being more and more widely used as well as researched, in which prosthodontics, dental implantology, orthodontics, and endodontics are involved, and also includes the production of dental implant guides, craniomaxillofacial and plastic surgery implants, and the manufacture of endocrowns and skeletons for implants and dental restorations, etc.
With the continuous development of dentistry, the study of 3D printing materials has become the focus of many researchers; in recent years, major breakthroughs have been made, and new 3D printing materials are continuously applied in the field of dentistry. In this paper, we will review the 4 major 3D printing materials in the field of dentistry, namely, metals, polymers, ceramics and bioactive materials.
1.1 Metal materials
Oral medical metal products require metal materials with good mechanical properties, chemical properties, biocompatibility and corrosion resistance, etc.. The requirements for raw materials are also very high, including high purity, low oxygen content, fine powder size, good plasticity, good fluidity and other characteristics. At present, the main applications in the field of dentistry 3D printing metal powder materials include: titanium, titanium alloy, cobalt-chromium alloy, stainless steel, etc.. Among them, titanium and titanium alloy materials have the advantages of low density, high precision and high strength, and the material has good biocompatibility, which is regarded as the ideal 3D printing metal material in the field of dentistry. Especially, it is widely used in the fields of restoration of oral and maxillofacial areas, restoration of dental tissues and related implant manufacturing.
Due to the defects of some properties of pure titanium, for example, the strength of pure titanium is not as strong as titanium alloy, and the elastic modulus of pure titanium is higher than that of bone tissue, which can easily lead to the mechanical stress of incompatibility between titanium implants and bone tissue. 3D printed cobalt-chromium alloy is also a common restorative material in the field of dentistry. Manufactured using 3D printing technology, the artificial teeth are then added using prosthetic techniques so that the restoration has a good fit once it enters the mouth. Since the cobalt-chromium alloy brackets used and the artificial teeth added are made of different materials, it is basically impossible to print a complete restoration in one go based on the current technological facilities.
Traini et al. formed gradientized Ti-6Al-4V titanium alloy porous dental implants with more optimized physicochemical properties, with tensile strength, section shrinkage and elongation up to AMs 4999 (the relevant standard published by the American Society for Materials on 3D printed titanium alloys). The laser sintering of titanium alloy specimens and the measurement of the elastic modulus of the porous layer on the surface of the specimens and the dense layer on the inside of the specimens showed that the former was close to the bone cortex and the latter was close to the machined titanium metal, indicating that this method of processing titanium alloy implants could reduce the surface stress and contribute to the long-term stability of the implants.
Mangano et al. used laser-sintered narrow-diameter implants for posterior dental implant restoration treatment in patients with a retention rate of 100.0% and a success rate of 94.6% in 37 implants after 2 years of follow-up. In terms of physical and mechanical properties, biological corrosion resistance and compatibility, it is necessary to study in depth whether the 3D printed metal products in question are the same as those manufactured by traditional processes and whether they follow national standards. At present, the emerging metal materials in the field of dentistry are still in the state of in vitro research, especially as the performance of oral implant materials still have a lot of research space.
At present, 3D printing technology continues to develop, with constantly optimized equipment performance and diverse metal printing materials, metal 3D printing technology will also be more widely used in various fields of dentistry.
1.2 Polymer materials
Polymer materials have become the basic mature printing materials in the field of 3D printing, plastic as a representative of polymer materials, with better thermoplasticity, fluidity and fast cooling bonding and its rapid curing properties. In addition, because of their good adhesive properties, polymers can make it possible to form new composite materials with ceramics, glass, fibers, inorganic powders, metal powders, etc. In dentistry, polylactic acid, polycaprolactone, poly(dihydroxypropyl fumarate), etc. belong to the more common 3D printing materials. Polylactic acid (PLA) is an environmentally friendly material with good biodegradability, which can be completely degraded by microorganisms in nature under specific conditions, eventually generating carbon dioxide and water, which will not cause environmental pollution and is very beneficial to environmental protection, and is recognized as an environmentally friendly material. It also has a translucent and glossy texture, making it an ideal material for 3D printing in the field of dentistry. Polyetheretherketone (PEEK) is a thermoplastic polymer that is currently used to make 3D printed satellites, 3D printed car parts, and is beginning to make a real impact in the 3D printing industry.
The advantages of PEEK materials include, ① PEEK material elastic modulus and human bone is similar to the stress integrity of the skull after repair; ② X-ray transmission performance, will not produce metal artifacts, does not affect medical imaging, easy to detect post-operative recovery; ③ using 3D printing PEKK material made of structures than with traditional PEEK has better antibacterial properties, can be high temperature sterilization and reuse; ④ PEEK itself is highly inert, with very little scalp irritation, low rejection, and high stability. It is currently used to manufacture denture parts.
In terms of the development status of 3D printing technology, light-curing three-dimensional molding belongs to the earliest development and the most mature technology, and has been widely used. 3D printing photosensitive resin, that is, light-curing resin, UV resin, is a polymer material widely used in the field of dentistry. For the field of dentistry, liquid resin materials need to have excellent stability, low viscosity, curing quickly and to a high degree. It has been found that liquid photosensitive resins can be printed as biodegradable tissue engineering scaffolds, and the scaffolds formed by light-curing additive manufacturing technology have the same mechanical properties as human cancellous bone, and have the effect of promoting fibroblast adhesion and differentiation. The rapid development of light-curing resin materials continues to promote the progress of dentistry, which is conducive to more personalized and precise dentistry.
1.3 Ceramic materials
Ceramic materials in the field of dentistry require good aesthetics and biocompatibility, with low density, high strength, high hardness, high temperature resistance, corrosion resistance, good chemical stability and other excellent physical and chemical properties, which are widely used in mechanical engineering, aerospace, biomedical and other industries. Due to its excellent mechanical and aesthetic properties, it is now also used as a restorative dental material. 3D printed zirconia ceramics can be used for more than 90% material utilization, which is relatively low cost.
3D printed zirconia can reduce material waste and environmental pollution, and can be bionic in mechanical properties such as hardness by printing special internal structures. Early 3D printed manufacturing of zirconia is mainly based on laser sintering method, but there are problems such as low densities and forming efficiency of the fabricated parts, surface roughness and cracks. Light-cured formed ceramics have good surface quality and controllable structural accuracy, and are rapidly becoming a research hotspot. At present, there are still some problems in the 3D printing process of zirconia materials, such as high internal stress, easy to produce cracks after sintering and large volume shrinkage, which may affect their mechanical properties and clinical suitability, and the ceramic materials and their processing processes still need further research.
1.4 Biological tissue materials
The use of 3D printing materials and technologies to produce human cells, tissues, and organs with good biological functions has been the pursuit of many scholars. Scholars have continued to explore 3D printing technology and have closely combined it with biological tissue engineering techniques to produce artificial cells, tissues, and organs with biological functions to replace human defective tissues that need to be repaired. Hydrogels are water-soluble polymers that are created using chemical or physical cross-linking and are 3D network structures. Hydrogels have excellent biocompatibility, allow the construction of tissue engineering scaffolds, and can be processed to form carriers for controlled release of drugs. However, currently, 3D mapped bio-written manufactured hydrogels have low stiffness, which may lead to structural breakdown or limit shape complexity, so recent advances in 3D printed biomaterials will advance the progress and development of the 3D printed biomaterials field.
In the field of dentistry, 3D printed products play an important role in dentistry and oral surgery, both in terms of patient personalized biologic tissue materials and in terms of existing finished products. At present, 3D printing technology basically realizes the bioprinting of human dental pulp cells (hDPCs), which lays the foundation for the wider application of 3D bioprinting technology to dental tissues. Furthermore, the fusion of artificial bone material hydroxyapatite with photosensitive polymers can be used to create biologically active bone tissue engineering scaffolds.
In implantology, 3D printing of personalized implants has become the trend for immediate implantation. Modification of the surface of titanium implants can promote the growth and differentiation of osteoblasts and implants with better characteristics. The roughness of the micron surface generated by 3D printing technology is more easily recognized by specific cells. Implants with micro-nano composite structures promote cell proliferation and extension, while facilitating cell differentiation towards osteogenesis. In the physiological three-dimensional bionic environment provided by the micro-nano composite structure, it is more conducive to cell extension and thus better proliferation and differentiation.
2. Development Prospects
Digital 3D printing technology provides advanced technical support for the development of dentistry. In the face of the continuous development and innovation of 3D printing technology, China's government departments have been paying attention to and formulating the development plan of 3D printing technology, fully combining the new development stage of 3D printing technology at home and abroad as well as the new situation, new opportunities and new needs, providing policy promotion for the development of 3D printing in dentistry. and guarantee. At present, 3D printing digital dentistry, with its advantages of efficiency, accuracy and safety, can meet the demand for complex, highly customized and rapidly produced oral products. In the field of restorative dentistry, crowns and other restorations of various materials are widely used, with a high degree of simulation in both tooth color appearance and semi-transparency and fluorescence effects, etc., bringing patients a high-quality medical experience and more efficient and satisfactory restorative results.
In the field of dental teaching, 3D printing, as an important technology for the future development of the field of dentistry, should allow students to contact and understand the digital dental technology and process as early as possible to open up students' thinking and creativity, and also make the teaching mode more advanced and novel. In recent years, 3D printing digital dental technology has brought high precision and low cost dental data and products to dentistry. 3D printing in the field of dentistry will also increasingly enhance the application and development, so the research and innovation of oral 3D printing materials is more urgent. Although at present, there are still some problems with 3D printing materials in the field of dentistry, such as: most of the printing materials rely on imports, resulting in high prices; 3D printing technology and equipment still need to be improved and perfected; the quality and use of 3D printing materials need further investigation and research; it is also necessary to establish relevant norms and standards for the physical and chemical properties of 3D printing materials in the field of dentistry. The relevant norms and standards need to be established to regulate the physicochemical properties, mechanical properties, and biological safety of 3D printing materials in dentistry.
In order to promote the wider use of 3D printing technology in the field of dentistry, the research and development of biological safety, mechanical properties and printing technology need to be further developed. Biodegradable materials and 3D printing of living cells, 4D printing and other high technology are important directions for future 3D printing exploration in the field of dentistry. At the same time, during the research process, the relevant national policies should be fully grasped to guarantee, of course, the binding force of CFDA (State Food and Drug Administration) policy needs to be considered, mainly limited by the certification mechanism of medical materials and the cycle of new materials, etc. It is believed that in the near future, with the development and innovation of 3D printing materials and supporting material processing methods, 3D printing materials and technologies in the field of dentistry will bring revolutionary development to dental clinical applications.