*2.4. Selective Laser Sintering*

SLS operates without a mold through a computer-controlled laser beam, powder bed, a piston assuring a vertical movement, and a roller to spread continuously powder layers [55]. This technique allows the realization of polymeric, metallic, and ceramic parts. SLS implies solid and semisolid consolidation procedures at a sintering temperature usually lower than the melting point. In the semisolid process suitable for treating low melting point polymer, as PCL, polyglycolide, PLA and poly(l-lactic) acid (PLLA), partially melted powder particles produce a certain volume of the liquid phase, which glues other solid elements. Microsphere-based hydroxyapatite (HA)/PCL scaffolds realized by SLS, shows a highly ordered porous structure [56]. Polyamide/HA composite platforms with porosities ranging from 40% to 70% and with a maximum tensile strength of 21.4 MPa were obtained by SLS [55,57]. Although the low near-infrared laser absorptivity of oxide ceramics, the direct SLS of ceramics throughout powder coating adds to the low melting point or composites ceramics has been done [58]. Many sacrificial binders as waxes, thermoplastics, long-chain fatty acids or sometimes a combination of binders as thermoset/semi-crystalline PA-11 or wax/PMMA are used for the

realization of porous 3D structured materials as graphite and composite ceramic Al2O3-ZrO2-TiC [59–61]. A high-energy laser beam increasing the temperature on the surface promotes the particle interaction to each other before sintering together, while the material on the grain borderline continues to diffuse into the pores, stimulating densification activities. Since SLS is characterized by a high heating rate and short holding time, it results as an excellent alternative in producing scaffolds supported by low-dimensional nanomaterials such as graphene and carbon nanotubes [62].
