Biomedical Engineering Reference
In-Depth Information
low and the pressure too high, strands are generated that are much wider
than the nozzle diameter; the reverse conditions (high speed and low pres-
sure) result in the production of inhomogeneous strands that can be dis-
rupted (Figure 4.4d).
Preparation parameters of the P-CPC such as powder-to-oil ratio and par-
ticle size have a strong influence on injectability of the P-CPC as well as on
the properties of the plotted scaffolds. For instance, a P-CPC with a lower
powder-to-oil mass ratio can be extruded through a finer nozzle with an
inner diameter of 150-250 µm, however, the material is not stiff enough to
support the whole weight of the plotted scaffold. This results in closure of
the pores that are oriented parallel to the building platform and deformation
of the scaffold, especially for those with a height in the centimeter range
that might be clinically relevant. To solve this problem, two strategies can
be considered. One is to increase the powder-to-oil ratio of the pastes, which
can enhance the stiffness of the P-CPC, but with the compromise to accept a
decrease in the extrusion ability and continuity by possible blocking of the
plotting nozzles, especially for fine nozzles (100-250 µm in diameter), which
are necessary for achieving scaffolds with high resolution
.
The other way is the application of a temporary material for filling the
space between the CPC strands and therefore supporting the structure dur-
ing the plotting process. This sacrificial material should be injectable to
enable plotting with the same device and water soluble so that it can be dis-
solved during setting and hardening of the CPC scaffold in water to unblock
its macroporous structure. We successfully adopted a 25 wt% polyvinyl alco-
hol (PVA) paste as temporary material since it fulfills these requirements
and obtained CPC scaffolds that kept their shape and whose pores oriented
parallel to the x- and y-axis were completely opened and uniform. In con-
trast, the pores of scaffolds fabricated from the same P-CPC material but
without temporary PVA strands were partially closed at the cross-section
direction (Figure 4.5). The stabilizing effect of the PVA supporting strands is
not only caused by simple filling of the space between two CPC strands, but
also due to the fact that the PVA paste contains water that permeates the CPC
strands and triggers the cement setting reaction. This phenomenon results in
a graded increase of stiffness of the layers from the bottom up as a function
of contact time between CPC strands and water from PVA sol.
The application of a sacrificial paste for plotting of supporting strands,
which was successfully used for 3D plotting of CPC scaffolds with completely
interconnected pores, might also be interesting for 3D plotting of other scaf-
folds such as silicon-based bioceramics, biopolymer hydrogels, or composites.
Deformation of the scaffolds including closure of the macropores in x-/y-
direction described earlier (Figure 4.5b) is caused by gravitational force,
which is effective especially in case of a considerable difference in the den-
sity of plotting material (P-CPC) and plotting medium (air). Therefore, an
alternative method might be plotting of P-CPC into water or cell culture
medium to minimize this difference in density. In this case, the CPC strands
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