ADDITIVE MANUFACTURING FOR BONE LOAD BEARING APPLICATIONS - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 242

as embedded interconnected channels ( Hutmacher, 2000; Stevens et al., 2008; Hollister, 2005; Hut-
macher et al., 2004; Castilho et al., 2013 ). For some applications, such as tissue scaffold design, the
inherent microporosity resulting from the arrangement of powder particles in the AM process is also a
highly beneficial feature ( Hollister, 2005; Hutmacher et al., 2004; Leong et al., 2003 ).
One of the most pressing drawbacks is that the smallest feature size of conventional powder-based
3DP techniques is limited by the binder droplet volume ( Butscher et al., 2011 ), powder particle size
with appropriate flowability ( Butscher et al., 2011; Leong et al., 2003; Yang et al., 2001 ), powder com-
paction force ( Shanjani, 2011 ), and the high potential for having trapped particles inside cavities formed
within the part ( Butscher et al., 2011; Hutmacher et al., 2004; Leong et al., 2003; Yang et al., 2002 ).
Overall, it is difficult to achieve features below 500 m m in size using this fabrication method ( Butscher
et al., 2011; Hutmacher et al., 2004; Leong et al., 2003; Yang et al., 2002; Castilho et al., 2011 ). This
issue becomes even more pressing in manufacturing constructs with complex conformal channels, as
it becomes increasingly difficult to remove trapped support materials from features within the parts.
Other emerging limiting aspects of conventional powder-based 3DP techniques are the use of only
one powder size or powder type during manufacturing, the application of a constant compaction force
during powder layer spreading, the utilization of a single layer thickness setting throughout the part,
and the lack of control over the grayscale gradient of binder volume dispersed within each layer. These
aspects impose limitations in manufacturing of porous scaffolds with heterogeneous or functionally
graded properties as required in various industrial and biomedical applications.
To address the current limitations in 3DP technology, a mechatronic system was designed by the
authors to control the fabrication of functionally graded internal features, porosities, and material prop-
erties of parts. To this end, a variable porosity AM system via 3DP was developed as seen in Figure 11.6 ,
where a collection of control modules were incorporated to achieve the required performance. Such
controlled devices include a counter-rotating roller module, multiple supply bed selection and alignment
module, sacrificial porogen particle insertion module, sacrificial polymer deposition module, UV cur-
ing module, and an environment control module. The sacrificial porogen particle insertion module and
sacrificial polymer deposition module assemblies are used to produce a controlled porous feature size in
the range of 100-500 m m, which can be achieved by selectively depositing either porogens or sacrificial
polymeric structures on specific layers that can thermally disintegrate during postprocessing, leaving
behind controlled porosities and/or interconnected channels. This approach allows for control of inter-
nal features by preventing loose support powder material from being trapped inside complex cavities of
parts, with results available in literature ( Vlasea et al., 2013; Vlasea and Toyserkani, 2013 ). The system
has been tested in the context of fabricating porous bone substitutes using CPP bioceramic powder.
The newly developed multiscale 3DP system allows for the capability of selecting between three
powder feed compartments during runtime to produce layers with different material composition at
selected locations throughout the part. To the author's knowledge, such a system is not commercially
available, nor is it discussed in the literature thus far. As a preliminary work presented as a case study
here, to evaluate the performance of the multipowder system configuration, two powder particle sizes
were selected, CPP bioceramic powder with 75-150 m m particle size (hereafter referred to as large),
and < 75 m m particle size (hereafter referred to as small), with a morphology and particle distribu-
tion shown in Figure 11.7 . Three categories of parts were manufactured, with small, large, and dual
(50/50) powder composition, respectively. The large CPP and small CPP powders were blended, re-
spectively, with 10% polyvinyl alcohol (PVA) powder (Alfa Aesar, Ward Hill, MA) and with particle
size < 63 m m. The fabricated test parts were 4 mm in diameter and 6 mm tall, printed with a layer