ADDITIVE MANUFACTURING FOR BONE LOAD BEARING APPLICATIONS - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 237

Dicalcium phosphate (DCP) and dicalcium phosphate dihydrate (DCPD), commonly referred to
as monetite and brushite respectively, are generally resorbed by direct chemical dissolution, and have
some of the higher resorption rates among the calcium phosphates ( Bohner et al., 2012 ). It has been
found that DCP has a higher resorption rate in vivo when compared to DCPD, as hydroxyapatite is
formed and precipitated during the degradation process for DCPD ( Gbureck et al., 2007 ). These ma-
terials by themselves are brittle, with mechanical properties below the required limits for bone aug-
mentation ( Alge et al., 2012 ). To mitigate the fast resorption rate of this class of calcium phosphates,
research groups have used DCPD as a cement, reinforced with a polymer, for example poly(propylene
fumarate) ( Alge et al., 2012 ), to increase mechanical properties, while maintaining biocompatibility
and osteointegration capabilities.
Tricalcium phosphate (TCP) is generally produced in three allotropic forms, b -TCP, a -TCP, and a9 -
TCP, differing in crystalline structure and mechanical properties ( D. Liu et al., 2013 ). The b -TCP for-
mulation is appropriate for bone scaffolding due to the biological response and mechanical properties
of this allotropic phase, with care given to avoid the a -phase during sintering or material preparation
( D. Liu et al., 2013 ). b -TCP has been used as granules, as an injectable paste ( Matsuno et al., 2008 ),
or mixed with HA as it has proven osteoconductive and resorbable properties ( Vorndran et al., 2008 ).
Products such as the Integra MozaikStrip TM porous scaffolds (Integra, 80% TCP, 20% Type I collagen)
and Integra MozaikPutty TM (Integra, 80% TCP, 20% Type I collagen) are commercially available for
repairing small bone defects and voids. Ongoing research focuses on manufacturing methodologies
that would enable TCP to be used in load bearing applications ( Vorndran et al., 2008 ) as well as on
biological augmentation using embedded biomolecules or cells to improve osteointegration ( Kim et al.,
2014 ). TCP is a very popular bioceramic that promotes long-term osteointegration at a rate similar to
allografts ( Kim et al., 2014 ). Concerns have been raised over products such as GeneX® paste com-
posed of calcium sulfate and b -TCP due to inflammation response ( Friesenbichler et al., 2014 ).
For calcium polyphosphate (CPP), the decreased Ca:P ratio allows the molecules to organize in a
chain-like configuration, similar to a polymer, where the oxygen-bridged links can form linear struc-
tures (polyphosphates), ring structures (metaphosphates), or cage structures (ultraphosphates) with a
random distribution in an amorphous state or in an organized distribution in a crystalline state ( Pilliar
et al., 2001 ). CPP can be sintered at different temperatures to produce a range of crystalline phases,
with decreasing rates of degradation in order of CPP > a -CPP > b -CPP > g -CPP ( Qiu et al., 2006 ).
CPP has been proven to be osteoconductive and osteoinductive in vivo ( Kandel et al., 2006; Pilliar
et al., 2007; Shanjani et al., 2013 ) via release of calcium and phosphate ions, with the main resorption
mechanism being cell-mediated.
Aside from bioceramics, amorphous glasses, such as the bioactive glass, form a family of glass com-
posites used in biomedical applications for bone augmentation or replacement ( Rahaman et al., 2011 ).
In spite of their inherent brittleness, bioactive amorphous glass materials have been extensively investi-
gated, specifically the commercially available Bioglass® (45S5 glass) ( Rahaman et al., 2011; Gerhardt
and Boccaccini, 2010 ) due to their proven osteoconductivity and bonding to bone, as the material
forms a HA-like layer once implanted at the site, which encourages bonds with the surrounding bone
( Rahaman et al., 2011 ). Bioglass® is biocompatible; however, it has a limited resorption rate, mak-
ing it difficult to tune the resorption rate with the rate of new bone formation ( Bohner, 2010; Raha-
man et al., 2011 ). Borate bioactive glasses are also biocompatible and show faster degradation rates;
however, there are some concerns over the toxicity of the borate ions in vitro and in vivo ( Rahaman et
al., 2011 ). Phosphate bioactive glasses offer the possibility of tuning the resorption rate by changing the