ADDITIVE MANUFACTURING FOR BONE LOAD BEARING APPLICATIONS - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 240

11.5.2.2 Solid- or Slurry-based AM Approaches
Solid- or slurry-based AM methodologies rely on extrusion of molten materials or solids suspended
in slurries. The most common method is FDM. A typical FDM machine deposits molten materials
extruded through a heated nozzle displaced in the x - y plane to produce one layer following a specific
geometric path based on CAD data. The layer is then displaced in the z -direction and a new layer can
be built sequentially ( Hutmacher et al., 2004 ). LDM and robocasting are methods that have a non heat-
ing liquefying deposition head to deposit a slurry that solidifies due to a low-temperature manufactur-
ing environment ( Xiong et al., 2002 ) or due to chemical setting ( Dorj et al., 2013 ), respectively. Dorj
et al. ( Dorj et al., 2013 ) used a robocasting technique to prepare PCL/HA composites with increased
mechanical properties by incorporating positively charged carbon nanotubes into the slurry-producing
samples with a pore size of ∼ 230 m m, with a compressive strength of up to 50 MPa, showing a con-
siderable increase in mechanical properties with the addition of the carbon nanotubes. Liu et al. ( Liu
et al., 2009 ) used a PLGA/TCP slurry, while Xiong et al. ( Xiong et al., 2002 ) employed PLLA/TCP
slurry in the LDM process to produce bone substitutes with promising biological and mechanical prop-
erties. The most important benefit of these methods is the fact that there is a low risk of residual build
materials being trapped inside the part. There is, however, a limited selection of materials that can be
used for fabrication using these approaches ( Hutmacher et al., 2004 ). These techniques also require the
integration of extra support structures for building complex shapes with overhanging features. Using
these methods, the nozzle dimension is one of the key limiting parameters that influence the feature
size, as well as in dictating the speed of fabrication of the components.
11.5.2.3 Powder-based AM Approaches
Another class of AM techniques applies to powder-based materials. One such method is SLS, where
a laser beam, typically a CO 2 laser, is used to locally raise the temperature of composite powders up
to the glass transition temperature, resulting in fusion of neighboring particles and previous deposi-
tion layer ( Yang et al., 2002 ). The powder can be a polymer, ceramic, metal, or composites ( Leong et
al., 2003 ). The laser is displaced in the x - y plane to create a layer based on CAD slice data. A roller
mechanism spreads a new layer of powder and the process is repeated. The benefit of SLS is that there
is generally no need for postprocessing steps. The process also allows for a wide variety of materials
to be used.
For load bearing applications, researchers have looked at ways of using SLS to produce bioceramic
or bioactive glass bone substitutes with appropriate mechanical and structural properties. For bioc-
eramics, the task at hand is to enable the laser beam to bring the exposed powder ceramic material up
to the sintering temperature to fuse particles together, without creating undesirable crystalline phases
or inducing cracks in the part. Liu et al. ( D. Liu et al., 2013 ) have investigated the appropriate material
composition of a b -TCP powder blended with small amounts of PLLA (0.5-3 wt%) that would induce
a liquid phase during sintering to decrease the sintering temperature of b -TCP, in order to avoid a -TCP
phase transitions, achieving an increase of 18.8% in fracture toughness (1.43 ± 0.02 MPa m 1/2 ) and
4.5% in compressive strength (17.67 ± 0.04 MPa) when compared to the pure sintered b -TCP. Shuai et
al. have also looked at improving the mechanical properties of pure b -TCP constructs by changing the
SLS fabrication parameters, resulting in 3.59 GPa hardness and 1.16 MPa m 1/2 fracture toughness ( Sh-
uai et al., 2013 ). SLS fabrication using bioactive glasses typically results in weaker constructs, intended
for maxillofacial applications, such as Bioglass 58S and poly (D, L) lactide (PDLLA) scaffolds with a
maximum reported compressive strength of 1.68 MPa ( Pereira et al., 2014 ). Similarly, Bioglass® 45S5