ADDITIVE MANUFACTURING FOR BONE LOAD BEARING APPLICATIONS - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 239

FIGURE 11.5
Classification of additive manufacturing methods for fabricating porous bioceramic bone substitutes.
weight monomers that are capable of forming long chain polymers ( Yang et al., 2002 ). The family of
polymers is called photopolymers. Biophotopolymers are a type of photopolymers that can biologically
degrade by hydrolytic cleavage ( Liska et al., 2007 ). A photopolymerization approach is DLP, where an
array of micromirrors is used to direct light onto a photosensitive resin only at specific locations within
each layer ( Woesz et al., 2005; Skoog et al., 2014 ). SL is another photopolymerization method, where
a UV laser moves in the x - y plane to irradiate and solidify a layer of UV curable polymer at specific
locations ( Lee et al., 2008; Skoog et al., 2014 ). The most common polymers used in SL for load bearing
applications are poly(propylene fumarate) (PPF) (typically used with a diethylfumarate (DEF) cross-
linking agent) and poly(D, L-lactide) (PDLLA), as they are biocompatible and bioresorbable, with
attractive mechanical properties after cross-linking, specifically in producing bone substitutes for load
bearing applications ( Skoog et al., 2014 ). Skoog et al. ( Skoog et al., 2014 ) have provided an excellent
overview of the current state of the art on SL in tissue engineering, specifically for bone substitutes.
They have outlined the benefits of SL in having a high degree of control over the internal and external
architecture of the scaffolds, as well as the possibility of integrating bioactive ceramics fillers into the
polymer matrix to enhance the mechanical and biological properties of the composite, at the cost of
increasing the difficulty of SL processing, as the resulting resins are more viscous and more difficult
to work with. For instance, HA powder was used with PPF/DEF ( Lee et al., 2009 ) to produce intricate
scaffolds with increased bioactivity. The most common use of SL in fabricating bone substitutes via in-
direct fabrication, where a mold is produced via SL and then infiltrated with bioceramic slurries such as
b -TCP ( X. Li et al., 2007 ) or HA/ b -TCP ( Sánchez-Salcedo et al., 2008 ) followed by a postprocessing
protocol intended to remove the intermediate mold and to cure the bioceramics ( Skoog et al., 2014 ). In
general, photopolymerization is one of the most accurate AM techniques, with a high resolution, but it
is limited to using photopolymeric materials ( Stevens et al., 2008 ) and is generally used to manufacture
bioceramic bone substitutes via indirect manufacturing by producing a mold.