ADDITIVE MANUFACTURING FOR BONE LOAD BEARING APPLICATIONS - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 238

composition ( Rahaman et al., 2011 ). Bioactive glasses generally have an inherent brittleness ( Rahaman
et al., 2011 ) and it is also difficult to form them into porous 3D bone substitute structures or scaffolds
( Rahaman et al., 2011; Gerhardt and Boccaccini, 2010 ).
There is a general trend toward producing composites of bioceramic or bioactive glass mixed
with natural or synthetic polymers in order to meet the complex mechanical requirements in terms of
compressive, tensile, shear, and fatigue properties for load bearing applications, as well as maintain-
ing the biocompatible and bioresorbable characteristic of the bone substitute. Natural polymers such
as chitosan, agarose, or collagen as well as synthetic polymers such as poly(ε-caprolactone) (PCL),
poly(lactide-co-glycolide) (PLGA), poly(L-lactic acid) (PLLA), poly(L-lactide) (PLL), and poly (pro-
pylene fumarate) (PPF) are commonly used. The diversity of research done on composites, such as HA/
PCL ( Dorj et al., 2013 ), HA/PLGA ( Kim et al., 2006 ), HA/PLLA ( Wei and Ma, 2004 ), HA/ b -TCP/
collagen ( Maté-Sánchez de Val et al., 2014 ), HA/ b -TCP/agarose ( Sánchez-Salcedo et al., 2008 ), HA/
chitosan ( Zhang et al., 2003 ), b -TCP/PLLA ( D. Liu et al., 2013 ), b -TCP/chitosan ( Dessì et al., 2013 ),
bioglass/PLL ( Zhang et al., 2004 ), and many more, are popular and can be used to control the resorp-
tion rate and mechanical properties of the material system. The prolific work done in this field may
suggest that the optimal bone substitute scaffold material, structure, and properties have not yet been
achieved. There are still limitations in achieving an acceptable range in mechanical and biochemical
properties for resorbable ceramic and ceramic composites ( Bohner et al., 2012 ). The key to solving this
issue lies not only in considering the material itself, but also in implementing the appropriate complex
3D internal architecture of the construct from a structural and morphological point of view ( Bohner
et al., 2012; Butscher et al., 2011 ).
11.5.2 AM OF BIOCERAMIC MATERIALS: SEVERAL TECHNIQUES,
LIMITATIONS, AND OPPORTUNITIES
Materials with controlled porous internal properties and complex 3D external characteristics are re-
ferred to as designer structures ( Hollister, 2005 ). AM approaches are being refined toward achieving
the goal of fabricating such designer structures in the context of producing bone substitutes. AM meth-
odologies allow for parts to be built incrementally, layer-by-layer, based on information provided from
a computer-aided design (CAD) program ( Leong et al., 2003; Hutmacher et al., 2004 ). AM technolo-
gies strive to have each layer built to have the specific morphological configuration that would result in
the desired micro- and macrostructure of the final part. In essence, AM offers the possibility of auto-
mated manufacturing of highly reproducible custom-shaped parts with controlled internal structure and
custom external 3D architecture ( Hutmacher et al., 2004 ). There are various AM techniques that can
be used to construct bioceramic bone substitutes, depending on the type of raw materials used. Based
on the ASTM F2792-12a standard terminology for AM technologies ( ASTM International 2012 ), the
general categories of AM methodologies are: (i) liquid-based methods such as direct light processing
(DLP) and stereolithography (SL); (ii) solid or slurry extrusion-based methods such as fused deposition
modeling (FDM) and low temperature deposition modeling (LDM); and (iii) powder-based methods
such as SLS, DMLS, EBM, 3DP. These methods are summarized in Figure 11.5 .
11.5.2.1 Liquid-based AM Approaches
One class of AM systems relies on photopolymerization of liquid-based materials. In this AM fabrica-
tion approach, radiant energy is used to excite and initiate the polymerization process in low molecular