## 2 Three Dimensional Printing Technologies
### Main Features  
SLM technologies are commonly the most expensive due to the high cost of the base material (fine metal powder) and to the presence of one or more high power laser sources in the machine. MJ technology is also commonly expensive, especially in terms of the material's cost. FFF machines are the cheapest both in terms of machine and material cost. They are also capable of processing the widest range of materials (thermoplastic polymers). On the other hand, FFF machines feature the lowest printing quality. BJ, MJ and some SLM machines working with polymers can feature full color resolution, offering great advantages in terms of anatomical modeling. BJ, SLS and some SLM working with polymers can avoid the use of support structures to be removed after the printing, thanks to their functioning principles structure [15].  
The application of the aforementioned technologies varies according to their technical features (e.g., minimum detail resolution, accuracy, speed of production) and to the materials' specifications (e.g., level of biocompatibility, mechanical properties) [16]. Table 1 shows the most relevant 3D-printable materials, their applications in dentistry and the major features related to their use [15; 16; 26; 27; 28; 29; 30; 31; 32; 33; 34].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Materials** & **Applications in Dentistry** & **General Features** \\ \hline \multirow{7}{*}{Bio-compatible photopolymer resins} & & Rigid and deformable polymers certified for contact with skin and mucous tissue for a prescribed time. Materials and machines devoted to the dental field are available in this category (e.g., 3D printed crown and bridge materials \(\rightarrow\) flexural strength: 100–140 MPa; flexural modulus: 0.5–2 GPa; Vickers hardness: 4–20 MPa). Available in SLA and MJ technologies. & \\  & & Photopolymer resins used for 3D printing must be acknowledged by Food and Drug Administration (FDA); their monomeric form is typically quite toxic even though the properly polymerized resin is not. \\ \hline \multirow{7}{*}{Photopolymer resins} & & Rigid and deformable polymers to be used in SLA or MJ technologies. \\  & & Wide range of mechanical properties are available, but usually it is not possible to reach high mechanical performance (beside the use of material filled with ceramic or metal powder to be sintered after the printing). \\  & & E.g., 3D-printed acrylic resin \(\rightarrow\) flexural strength: 63.93–113.16 MPa; Vickers hardness: 25.16–34.62 VHN; surface roughness: 0.12–5.77 \(\upmu\)m. \\ \hline \multirow{7}{*}{Co-Cr} & \multirow{7}{*}{Crowns, implants, partial dentures, hyrax, lingual arch, transpalatal arch, retainers} & High density (Co-Cr: 10 g/cm\({}^{3}\); Titanium: 4.5 g/cm\({}^{3}\)), hardness (Vickers hardness Co-Cr: 550–800 MPa; Vickers hardness Titanium: 830–3420 MPa) and elastic modulus higher than bone structures (Co-Cr: 220 GPa; Titanium: 110 GPa; Cortical bone: 18–20 GPa; Traclecular bone: 10–14 GPa). Biocompatible and implantable materials available in SLM/EBM machines. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Groups of materials printable with the different 3D-printing technologies, their applications in dentistry, and major features.