CRANIOFACIAL BONE - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 212

10.2.2 CRANIOFACIAL MICROSTRUCTURE
The craniofacial skeleton arises from intramembranous ossification while the long bones in the body
arise from endochondral ossification. The bones of the craniofacial skeleton are composed of a hard
cortical shell and an inner trabecular region. The cortical shell is composed of concentric lamellar
sheets about 5 m m in thickness, while the trabecular network within is made of struts about 150 m m in
diameter and pores around 500 m m in diameter ( Rho et al., 1998 ). Trabecular ultrastructure resembles
that of osteons; comprised of concentric lamellar sheets but without the central Haversian canals that
are found in cortical bone. This microarchitecture is thought to result from a mechanism in which la-
mellar sheets are deposited sequentially over time ( Cohen and Harris, 1958 ). Generally, TE scaffolds
for bone regeneration mimic the inherent porosity of trabecular bone. Modern printers can achieve very
finely controlled pore sizes mimicking the 500 m m diameter pores found in trabecular bone as well
as the strut networks reminiscent of the trabecular network. These struts may serve as nucleation sites
for cell-mediated mineral deposition and for mechanical support. For applications where compressive
strength is a priority, it should be noted that increased porosity comes at the expense of mechanical
integrity since the large voids within the scaffold structure are not load bearing. As a reference, average
maximum bite force which a scaffold within the mandible must withstand is around 1 kN ( Hidaka
et al., 1999 ) applied toward the posterior angle. As such, the organization of the strut network is im-
portant and the advantages of 3DP techniques in their ability to control porosity and pore structure as
functions of spatial location within the scaffolds become clear. One example later in this chapter will
explore the use of a perpendicular lattice network to create porosity while generating a “microtruss” for
the scaffold's overall mechanical integrity.
10.3 DIFFERENT 3D PRINTING TECHNIQUES AND THEIR APPLICATION TO
CRANIOFACIAL SCAFFOLDS
3DP approaches can meet the demands of constructing the complex shapes of the craniofacial skeleton
while finely controlling porous microarchitecture. There are several different 3DP techniques, each
with specific relative advantages. A more in-depth discussion on 3DP methods can be found in an
earlier chapter. Here we focus on a smaller number of common techniques and their advantages and
disadvantages when applied to constructing craniofacial scaffolds. A summary table of the techniques
discussed here can be found in Table 10.1 .
One early method of 3DP was the ink-jet/binder system, in which a nozzle dispenses a binder
solution onto a powder bed. Powder that contacts the binding solution is bound, while the other pow-
der remains free and serves as support. For bone, the powder is generally the mineral b -tricalcium
phosphate to mimic the mineral phase of bone and the binder is generally some type of acid, such as
citric acid ( Khalyfa et al., 2007 ) or phosphoric acid ( Inzana et al., 2014 ). While this method does not
require support structures, thereby allowing the printing of overhangs, the main drawback is that the
low viscosity of the acid binder solutions result in low resolution and residual acid trapped within
large scaffolds that can compromise cell viability. The mechanical integrity of scaffolds printed by
this method also depends highly on the interaction between the binder and the powder; in general,
scaffolds printed using this technique tend to be brittle. One key advantage of the ink-jet/binder
method, however, is the ability to print structures at room temperature, allowing for the incorpora-
tion of cells or growth factors. A demonstration of these considerations is included in Case Study 2
( Klammert et al., 2010 ).