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Additive manufacturing of bone scaffolds
scaffold being built. an introduction of low melting point infiltration into SLS
SLS usually refers to solid or semisolid consolidation is an alternative method to promote the liquid phase
mechanisms at a sintering temperature lower than melting formation and the crystallite rearrangement, thus enhancing
point . In semisolid sintering, powder particles are the densification behavior. Duan et al. [109] introduced CaO-
[93]
partially melted and form a small amount of liquid phase, Al O -SiO as a liquid phase into HA scaffolds in SLS.
3
2
2
which bonds other solid particles to become solid parts. SLS The improved densification in sintering led to enhanced
with semisolid consolidation mechanisms is suitable for mechanical properties, with the compression strength,
processing low melting point polymer, such as polylactic fracture toughness, and hardness increased by 105%,
acid (PLA) , polyglycolide , poly(L-lactide) (PLLA) , 63%, and 11%, respectively. Liu et al. [110] also reported an
[96]
[95]
[94]
and poly(ε-caprolactone) (PCL) . Researchers also used increase of 18.18% in fracture toughness after introducing
[97]
SLS to process the polymer and ceramic composites. Du PLLA as a liquid phase into β-TCP scaffolds.
et al. reported that a microsphere-based hydroxyapatite
(HA)/PCL composite scaffolds constructed by SLS [98,99] , 3.2. SLM
which shows a highly ordered porous structure, as shown in SLM is initially designed for 3D freeform fabrication of
Figure 6A. Scanning electron microscope (SEM) images metals [104] . Compared with SLS, SLM applied higher energy
confirmed that the microspheres were well connected density laser to fulfill a complete melting/solidification
through laser sintering in the composite scaffolds. In vitro mechanism [111] . Thus, SLM-derived parts normally have
assays showed that the scaffolds promoted cell adhesion, an improved surface quality, density, and resultant superior
cell proliferation, as well as cell differentiation (Figure 6B). mechanical strength. In consideration of the high-energy
In addition, in vivo assay demonstrated an excellent density, SLM is mainly applied to process metal scaffolds.
histocompatibility and promotion of new vascularization Čapek et al. [112] prepared a highly porous (87 vol.%) 316 L
tissue. Kumaresan et al. [100] successfully applied SLS to stainless steel scaffold as joint replacement by SLM.
fabricate polyamide/HA composite scaffolds with porosities The scaffolds exhibited similar mechanical properties to
ranging from 40% to 70%. With an optimal HA content of those of trabecular bone with a compressive modulus of
15 wt.%, optimal mechanical properties with a maximum elasticity 0.15 GPa and compressive yield strength of 3
tensile strength of 21.4 MPa and a compression stress of MPa. Weißmann et al. [113] reported that Ti-6Al-4V porous
25.2 MPa were obtained. scaffolds are formed with 3.4 and 26.3 GPa and porosity
SLS can also process bioceramic scaffolds, such ranging from 54% to 60%. Besides, it also revealed a clear
as HA [101] , β-tricalcium phosphate (β-TCP) [102] , and influence of the unit cell orientation on elastic modulus,
bioglass [103] . It is well known that bioceramics commonly compressive strength, and strain. Wang et al. [114] evaluated
have higher melting point as compared to polymers. the effects of parametrical variations on the mechanical
Thus, SLS of bioceramic is generally based on the solid properties of SLM-derived scaffolds. Results revealed
consolidation mechanism [104] . Specifically, high-energy that porous design could reduce the effective modulus
laser beam acts on the ceramic particles, increases the of scaffolds by 75–80%. Shah et al. [115] used SLM to
surface temperature, and promotes the particles approach produce Ti-6Al-4V scaffolds for load-bearing orthopedic
to each other before sintering together. Meanwhile, the applications. The bone/scaffold interface was studied
material on the grain boundary continues to diffuse to after implantation in an adult sheep for 6 months. The
the pores, which promotes the densification behaviors. compressive strength ranged from 35 to 120 MPa as the
Nevertheless, SLS has a very short sintering period, due porosity ranged from 55% to 75%.
to its high heating rate and short holding time [105] . Such a Recently, biodegradable metals, mainly Mg and Zn
short sintering period can effectively restrain the diffuse alloys, have drawn an increasing interest of researchers,
of grain boundaries, which enables SLS to obtain ceramic due to their inherent degradability as well as close
scaffolds with nanoscale grains. Moreover, SLS exhibits mechanical properties to that of natural bone [116-119] .
great potential in fabricating scaffolds reinforced by At present, the main problem of Mg alloys as bone
low-dimensional nanomaterials (LDNs), such as carbon implants is their poor corrosion resistance. Surprisingly,
nanotubes, graphene, and boron nitride nanotubes [106] . recent studies revealed that SLM-processed Mg alloys
This is because the short sintering period combined exhibited improved corrosion resistance. This could
with low sintering temperature can avoid destructing the be ascribed to that SLM involved a rapid solidification,
structure of LDNs [107] . Mechanical tests and in vitro cell resulting in grains refinement and reduced composition
culture confirmed their enhanced mechanical properties segregation, both of which made contribution to the
and improved biological properties, respectively [105,108] . enhanced corrosion resistance of biodegradable Mg
Special care should be taken with regard to the limited liquid alloys [4,120-122] . However, SLM of Mg alloys is still
formation in SLS, which causes insufficient densification technically challenging. This is because Mg possesses a
and heterogeneous microstructures. To solve this problem, very active chemical property and is flammable even at

8 International Journal of Bioprinting (2019)–Volume 5, Issue 1