3D Plotting of Bioceramic Scaffolds under Physiological Conditions for Bone Tissue Engineering - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 99

collagen type I). Due to its composite character, bone is perfectly adapted to
compression, tension, bending, and torsion.
The following sections describe two different types of CPC-alginate
composite scaffolds. Beside monophasic composite scaffolds prepared of
a CPC-alginate mixture paste, biphasic structures are fabricated through
simultaneous plotting of a CPC and an alginate paste demonstrating the
option to generate biphasic organic and inorganic scaffolds with a designed
structure adapted to special requirements of complex tissue defects.
4.3.1 Fabrication of Biphasic and Mixed CPC-Alginate Scaffolds
Alginate is a polysaccharide mostly isolated from brown algae. Its specific
properties make it interesting for biomedical applications: By dissolving
sodium alginate powder in water or aqueous solutions, an alginate sol can
be prepared whose transition into a gel is induced by multivalent ions such
as Ca 2+ , that is, under mild conditions. Alginate was selected as a polymer
phase for preparation of the composite scaffolds due to the favorable charac-
teristics of biocompatibility and biodegradability as well as the simple and
nontoxic cross-linking process, which can be performed simultaneously to
the CPC setting reaction.
In a first step, an alginate paste was developed whose consistency allows
its plotting in combination with the P-CPC (Section 4.2). A highly concen-
trated alginate-polyvinyl alcohol (PVA) paste with an alginate content of 16.7
wt% was prepared by mixing alginate powder with 6% w/v PVA solution in
a mass ratio of 1 to 5 until a homogeneous plotting paste was achieved. The
P-CPC and the alginate-PVA paste were transferred into two different car-
tridges and used for fabrication of biphasic CPC-alginate scaffolds by alter-
nate extrusion of CPC and alginate strands (Figure 4.10, left).
Compared to low concentrated alginate sols (1-4 wt%), which are nor-
mally used for the preparation of hydrogels and scaffolds, the highly con-
centrated alginate-PVA paste has significant advantages for 3D plotting of
scaffolds (Luo, Wu, et al. 2013). First, its consistency is comparable to those
of the P-CPC enabling simultaneous processing of both pastes in one and
the same scaffold. This alginate-PVA paste can support the whole structure
of the printed scaffold without deformation. Second, in contrast to the less
viscous low concentrated alginate sols, the viscosity of the highly concen-
trated alginate-PVA paste is high enough to allow plotting in air instead of a
CaCl 2 solution as it would be necessary otherwise to stabilize the structures
immediately after extrusion. This is an important prerequisite for plotting in
combination with the P-CPC and can avoid problems, caused by extrusion
into a CaCl 2 bath such as blocking of the nozzle during the manufacturing
process and inhomogeneous cross-linking of the scaffold with incomplete or
no bonding between the strands.
For fabrication of the monophasic composite scaffolds, a mixture paste
consisting of CPC precursor powder (Khairoun et al. 1997) and alginate was