A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 2 1 9 more successful and effective operations. 3D printing could be applied not only to building organ models for surgical simulations, but also to the production of real organs using induced pluripotent cells. In fact, these efforts are already underway. Artificial limbs are another application area with significant potential for growth. The use of artificial limbs has been restricted compared with potential needs, perhaps due to the relatively high cost and restricted functionality. The introduction of AM has decreased costs in addition to providing the ability to customize limb design and increase usability. Various practical limbs have already been produced by vat polymerization. Regarding implants,metallicmaterials, titanium alloys, cobalt-chromium alloys, and stainless steels have been used for artificial bones and dental crowns. These metallic materials provide mechanical strength but often cause corrosion and allergy problems. Ceramics could serve as a substitute for metallic implants. Bioactive ceramics such as hydroxyapatite, or bioinert ceramics such as alumina and others could be used. These ceramic powders are mixed into photopolymers such as acryl resin and could be hardened by UV light. From the standpoint of regenerative medicine, 3D printing of living cells is currently attracting attention. AM is being investigated so that realistic artificial human tissues can be constructed via 3D bioprinting. Three main AM processes are now used for this purpose: material jetting bioprinting, material extrusion bioprinting, and vat polymerization bioprinting, according to ASTM International standard F 2792 and ISO standard 17296-2:2015 (Fig. 4). BIOPRINTING Vat polymerization bioprinting is classified into three categories: stereolithography (SLA), digital light processing (DLP), and two-photon polymerization (2PP). Thus far, vat polymerization bioprinting processes have been represented by SLA and DLP (Fig. 5). For both processes, laser light is irradiated onto photo-crosslinkable resins. The difference between the two processes depends on the light source and how it is controlled to selectively illuminate and cure the resin. In SLA, the laser is scanned to draw each layer. On the other hand, each layer is drawn only once in DLP. Because DLP does not require the scanning of laser light, the printing rate is faster than in SLA. However, SLA can produce objects with more accuracy. Therefore, SLA is better suited to building very fine small parts one at a time or to making multiple products with higher accuracy. On the other hand, DLP is more suitable for producing a very complicated small part or relatively big parts without high resolution. Generally, epoxy-based, acrylic-based, and infused polymers are used. As for the 2PP process, conventional one-photon absorbance is not used. Instead, two-photon absorbance is used, in the near infrared by femtosecond-pulse laser irradiation. This method makes it possible for one to adjust and focus the laser beam on a very narrow local part for photo-crosslinkable reactions. Therefore, high-resolution 3D models are possible. Whichever process is used, the bioink is arrayed in a certain arbitrary position and the scaffold is fixed there for all processes. Vat polymerization is used to produce the scaffold. As it is significantly different from other applications, the use of AM for tissue engineering restricts materials selection for vat polymerization in light of dimensional stability, biological compatibility, and biodegradability. To satisfy Fig. 4 — Medical applications of vat polymerization. Fig. 5 — Classification of 3D bioprinting.
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