3D Plotting of Bioceramic Scaffolds under Physiological Conditions for Bone Tissue Engineering - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 94

(a)

(b) (c)

500 µm 500 µm 1000 µm

FIGURE 4.6
Light microscope images of CPC scaffolds plotted with nozzle diameters of (a) 200 µm and (b)
410 µm. (c) A biphasic scaffold consisting of two parts with different porosities.

material (e.g., molecular weight of polymers, particle size of ceramic pow-
der, composition of composites, etc.) and on the properties of the respective
pastes prepared for plotting (such as viscosity and homogeneity). Critical
factors and parameters that affect the viscosity and therefore the resolution
of plotted scaffolds are the solid-to-liquid ratio and temperature (in case of
polymer melts). In our work, the cement particle size has been reduced by
optimization of the grinding procedure during preparation of the P-CPC
that resulted in a clear enhancement of the resolution achieved by applica-
tion of finer nozzles: CPC strands of less than 200 µm width can be plotted.
The microscopic images depicted in Figure 4.6 demonstrate the perfor-
mance of the new technique of CPC plotting for scaffold fabrication. Plotting
of the optimized P-CPC with nozzle diameters of 200 and 410 µm, respec-
tively, lead to a uniform pore size and consistent pore structure. Moreover,
CPC scaffolds possessing an anisotropic structure—one example is shown
in Figure 4.6c—can also be fabricated with this technique. In that scaffold, a
denser part with smaller pores and lower porosity for mimicking compact
bone and a less dense part with bigger pores and higher porosity mimicking
trabecular bone were designed and fabricated, leading to a distinct anisotro-
pic material. In addition, some studies have shown that scaffolds with small
pores can lead to hypoxic conditions and induce osteochondral tissue forma-
tion before osteogenesis, while large pores, that can be better vascularized,
lead to direct osteogenesis (Karageorgiou and Kaplan 2005). Therefore, such
anisotropic structures with gradients in pore size would be recommended
for the formation of multiple tissues and tissue interfaces such as biphasic
scaffolds for repair of osteochondral defects.
Scaffolds developed for tissue engineering as well as implants should be
able to mediate stability to the defect ideally possessing mechanical proper-
ties comparable to those of the host tissue. Plotted CPC scaffolds of different
porosity (generated by using different strand distances) were tested for com-
pressive strength and modulus. The data revealed that plotted CPC scaffolds
with completely interconnected pores have compressive strengths of 1 to 3
MPa, reciprocally proportional to the porosity, that is, the mechanical sta-
bility decreases with increase of porosity (Figure 4.7). For porous scaffolds,
the mechanical properties do not only depend on the strength of the bulk