Rapid prototyping manufacturing technology (RPMT), also known as rapid prototyping and just-in-time manufacturing, originated in Japan in the 1980s and soon spread to the United States and Western Europe, and has been a major breakthrough in the field of manufacturing technology in the past 20 years. In 1990, RPMT started to be used in the field of medicine, and around 1992, it was used in the field of dentistry, when Klein et al. pointed out that the fast and accurate characteristics of RPMT would be useful in the field of medicine, including dentistry, after comparing with the traditional lathe processing technology, which has been proved in the rapid development of RPMT.
1 Development of RPMT
RPMT is developed with CAD/CAM technology, CNC technology, laser processing technology, and material technology, and is closely related to mechanical engineering, inspection technology, electronics and information technology, and can be combined with special processing methods such as electroforming, arc spraying, plasma spraying, plasma melt forming, casting, precision casting, and EDM. In the 1980s, the U.S. 3D System was the only company that produced rapid prototyping equipment, and by the end of 1996, more than 1,400 sets had been installed worldwide, and the direct economic income generated by RPMT in 1998 was as high as $1 billion. The first domestic article on RPMT was published in 1993, and the Rapid Prototyping Group was established in 1994 under the leadership of Tsinghua University, and as the research on RPMT deepened, its application in various fields rapidly began. At present, the highest accuracy can reach 0.001mm, layer thickness ±0.005mm, and the maximum size of formed parts can reach 800mm×1600mm×500mm (such as Tsinghua University’s SSM-1600) with a speed of several hours to tens of hours/piece using specialized molding equipment.
2 RPMT classification and respective characteristics
With the continuous improvement of this technology, scholars have manufactured a variety of RPMT devices with different principles and structures, which have improved their accuracy and speed, and accordingly, the depth and breadth of RPMT applications in the medical field have increased. RPMT is usually classified according to the manufacturing process principle, and several more mature technologies that have been applied in the dental field are introduced as follows.
(1) Stereo printing molding (Stereo Lithography Apparatus, SLA) also known as photosensitive liquid phase curing, three-dimensional lithography, three-dimensional modeling, etc.. This technology is the most mature, the most applied one. SLA method can produce fine prototypes with good surface quality, and can directly manufacture plastic parts. Most of the parts are transparent. SLA can also be used for miniature manufacturing, and the Kyushu Institute of Technology in Japan has produced a model of about 50μm. The disadvantage is that SLA method is only applicable to the production of in vitro models, and it is difficult to generate microstructures with biological activity; there are volume changes in the molding, increasing the difficulty of control; SLA equipment is more expensive, and the cost of photosensitive resin is higher. In recent years, the development of some domestic equipment such as the LPS and CPS series SLA machines of Xi’an Jiaotong University and the corresponding photosensitive resin has led to a significant reduction in the cost of the parts.
(2) laminated entity manufacturing (Laminated Object Manufacturing, LOM). This method can manufacture large-size prototypes, low price of equipment and molding materials, no internal stress and deformation of the model, high precision; high strength and stiffness; short production time. Compared with SLA, it is more suitable for making complex free-form surfaces because no support is needed. The disadvantage is that the weather resistance and bonding strength of the material are closely related to the selected base material and adhesive type; the waste separation is time-consuming. Domestic Tsinghua University’s SSM and Huazhong University of Technology’s ZIPPY series are the better LOM models. At present, the LOM process is developing towards the diversification of optional materials (such as metal sheet and ceramic materials, etc.).
(3) Selective domain laser sintering (Selected Laser Sintering, SLS). The method generally does not add binder and no subsequent processing, so it can form a high-strength model; no support; high model accuracy (particle size of less than 0.1mm up to ± 0.01mm), such as the use of wax powder can be directly manufactured precision casting wax mold. The early SLS method is difficult to remove the powder between the pores and it is difficult to perform the manufacturing of the cell carrier framework structure, but the development of SLA now allows the user to adjust the internal microstructure (pores and pore size) of the sintered product by controlling the parameters. the shortcomings of SLS are that it is difficult to accurately control the absorbed power per unit area in sintering; sometimes the surface of the model is relatively rough and needs to be properly baked and cured and polished. In China, there are models such as HRPS-Ι of Huazhong University of Technology.
(4) fused deposition modeling (Fused Deposition Modeling, FDM) also known as melt stacking method, fused extrusion into the mold, etc.. The method does not use laser, low cost, small size, fast production speed, no pollution. The disadvantage is that the accuracy is relatively low; there is also volume change; the FDM method is only suitable for making scaffolds that do not add active substances such as growth factors during processing because of the need for heating. In China, there are models such as MEM-250 of Tsinghua University.
(5) three-dimensional spray bonding (Three-dimensional Printing and Gluing, TDP) also known as three-dimensional printing, ceramic shell method. The method can change the material in the manufacturing process to produce a variety of different materials, colors, mechanical properties, thermal properties of the combination of composite or non-homogeneous material models. tdp has a wide range of applications, especially for the production of ceramic mold; lower cost; the speed of the parts is very fast. It is the most suitable for making non-homogeneous and porous structures, achieving functional gradient material stacking and forming, and it is promising to be the main process method for preparing bioengineered scaffolds (fine structure spray stacking and forming). The main problem is that the accuracy and surface roughness are slightly poor, and some materials are easily deformed or even cracked.
3 Application of RPMT in the field of dentistry
In general, there are three stages: primary stage: biological solid model for diagnostics and operation; intermediate stage (compatible biological model): implant for therapeutics and rehabilitation engineering; advanced stage (advanced biological model): artificial organ (“real” bone that can participate in metabolic processes). Currently, the main focus is on the first two stages.
3.1 Applications in the field of prosthodontics
In other areas of dentistry, RPMT also poses a challenge to traditional processes. In the field of restorative dentistry, there are many examples of the use of RPMT technology to create 3D models of the patient’s crown and alveolar bone, and to design, fabricate, and fit the denture based on the models. Wu et al. used the RPMT method to assist in the fabrication of cast titanium crowns and optimized the design of the cast channel using commercial software prior to casting. He believes that this technique has great potential to replace the traditional steps of “taking impressions and waxing”. J. Grau et al. of MIT, USA, used TDP technology to prepare an alumina ceramic mold for powder casting to replace the traditional plaster mold because it has higher strength and can be heated to several hundred degrees to reduce drying time and. Domestic Gao Bo et al. used the LOM method to produce a whole tooth model with good geometric similarity, which laid the foundation for further application of laser sintered metal or ceramic powder for direct fabrication of oral restorations.
3.2 Application in the field of oral implant
When making implant prostheses, imaging techniques such as CT are of great help in thorough surgical planning, and the application of RPMT in the oral field makes these digital images even more useful: Sarment et al, found that when implant surgery was performed under the guidance of CT images only, the preoperative plan differed from the postoperative one by an average of 1.5 mm. at the alveolar ridge implantation point and 2.1 mm. at the apex of the intraosseous implant; in Sader et al. used the visualized entity of RPMT to predict the maxillofacial profile of patients with severe maxillary alveolar bone atrophy after maxillary sinus elevation and implant placement in 23 patients and guided the surgery, and all patients were satisfied with the results.
3.3 Application in the field of internal dentistry and orthodontics
Kim et al, reported on a patient treated in endodontics with hypothyroidism for 1 year who was found to have multiple lateral paraprosthetic invasive root resorption, and he quickly clarified the location and area of occurrence after a set of teeth was fabricated using RPMT. The maintenance of healthy periodontal membrane with active cells plays a very important role in the success of dental autotransplantation, thus reducing the in vitro operation time is significant. lee et al. made a donor tooth model using RPMT, and then took the donor tooth for transplantation after the recipient area was measured appropriately, resulting in a reduced operative time and good periodontal membrane in a total of 22 transplanted teeth.
RPMT is also useful in orthodontics. For example, Wiechmann D et al. used RPMT to create individualized orthodontic brackets for patients, reducing the size of the brackets for patient comfort and reducing the incidence of accidental bracket loss.
3.4 Applications in the field of oral and maxillofacial surgery
SLA, LOM, SLS, FDM, TDP and other techniques have been used in this field and have played an important role in assisting diagnosis (fracture, joint ankylosis and even obstructed teeth), planning, simulating surgery and treatment. For example, Mingguo Qiu et al, had used the LOM method to produce a physical paper model of the temporal bone, which can be used for preoperative design of complex ear neurosurgery and also to simulate surgical operations.
For the problem of hard tissue replacement of large defects (e.g., 14.7 cm × 12.0 cm,) of craniofacial bone tissue caused by congenital defects, trauma, post-craniectomy decompression, infection, etc. The production of individualized pseudo-replicas by LOM (e.g., Ono et al., who repaired large and complex jaw defects with HA ceramics in 9 patients) or FDM (e.g., Eppley et al., who performed cranial reconstruction in 13 patients) can greatly save the operative time and patient exposure time to radiation, reduce intraoperative and postoperative complications, and shorten the patient’s hospital stay. Statistics have been done: the application of RPMT increases the correct diagnosis rate by 29.60%, the operation accuracy by 36.23%, and the operation time by 17.63%, making it possible to complete complex orthopaedic surgery in a single operation, which was originally done in several operations.
RPMT has also recently become a major part of cell carrier scaffolds in tissue engineering, which is undoubtedly an extremely important aspect of tissue engineering. “The defective “cranial bone” with certain porosity was used as a scaffold together with fibrin glue, which was implanted after 3 days of co-culture with osteoblasts, and the morphology recovered well after 3 months, with the strength reaching 60% of normal bone and the yielding force reaching 85-90% of normal bone. Similar experiments have been conducted at Tsinghua University in China, in which polylactic acid (PLA) with a molecular weight close to 100,000 was compounded with HA, collagen and BMP to produce a porous cylinder with a diameter and height of 5 mm by TMF technology, which was placed into the radial defect in dogs and found to be significantly beneficial to bone tissue healing.
4 Outlook
RPMT is particularly suitable for the direct production of small-lot, complex (e.g., fluted, convex shoulder, hollow, nested, etc.), shaped products; equipment with different process principles is easily modularized and interchangeable; remote manufacturing services are available with the help of the Internet; a wide range of materials are available (e.g., resin, plastic, paper, paraffin, film, metal or ceramic powder, foil, silk, etc.); the manufacturing process is vibration- and noise-free and basically There are no offcuts, it is an environmentally friendly manufacturing technology, theoretically the utilization rate of raw materials can reach 100%, etc. Therefore, the field of dentistry is currently being developed in foreign countries, and has a broad prospect and huge development space.