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 8 liquid states. Oligomers include epoxides, urethanes, polyethers, and polyesters, and they are often functionalized by an acrylate. Examples of monomers are styrene, vinylpyrrolidone, and vinyl ethers. When UV light irradiates photopolymers in their liquid state, a PI is not always needed. In practice, however, a PI drives the crosslinking process. As shown in Fig. 2, a PI decomposes into reactive species and activates polymerization of specific functional groups of oligomers. During the curing process, crosslinking leads to formation of a thermoset network of polymers that are hardened. The PI is a very important component, because photopolymerization will usually not start without it. It is classified into two categories from the viewpoint of the mechanism that drives photopolymerization. In one, the PI brings about cationic polymerization. This category includes epoxy compounds, oxetane compounds, and vinyl ether compounds. In this process, UV light excites the chemicals to produce cationic radicals, which lead to polymerization. Onium salts such as iodonium and sulfonium salts, and organometallic salts such as ferrocenium and pyridinium salts, are examples of PIs in this category. Figure 3 shows the types of chemicals in each category. The other type is a PI based on a free radical mechanism. This category includes polyurethane acrylate, epoxy acrylate, ester acrylate, and other acrylate compounds. When UV light is irradiated on the PI, it produces free radicals, which induce crosslinking reactions of oligomers and monomers. Two mechanisms have been proposed for the production of free radicals. The first mechanism is the photo- fragmentation of the radicals (PI*) pro- duced by UV light. The free radical is produced as shown: hv PI PI* The produced radicals make their own photofragmentation occur: PI* P + I The second mechanism is hydrogen abstraction: PI* + R-H PI-H + R MEDICAL APPLICATIONS Four main categories exist for AM processes used for applications within the medical field. These include modeling body parts, producing artificial limbs, making implanted devices, and modeling body tissue. The first two types are for use outside of the human body, while the latter two are for internal applications (Fig. 4). Each category has its own detailed classes. According to the specific requirements for each category, the properties of the materials used also differ. Usually, materials to be used inside the body require a relatively long time frame for research and development, to ensure they are safe for human use and to meet legal regulations. Therefore, the cost to make these products is generally high. Products used outside of the body have fewer restrictions and are easier to market. For example, mouthpieces for orthodontics and surgical guides for oral surgery are often produced by AM. Another promising application is the modeling of internal organs for surgeons who could use the 3D models to learn detailed operation steps in vitro. This could enhance a surgeon’s ability to properly schedule an operation, obtain informed consent more easily, and be better prepared to perform the operation. Surgeons could order organ models based on a patient’s personal data obtained from computed tomography or magnetic resonance imaging and virtually simulate the real operation ahead of time. As a result, surgeons could improve their skills, potentially leading to Fig. 2 — Schematic of photopolymerization. Fig. 3 — Classification of photoinitiators.
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