3D BIOPRINTING TECHNIQUES - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 63

FIGURE 3.3
Pressure-assisted bioprinting using sodium alginate and calcium chloride as the cross-linking agent to fabricate
calcium alginate-based hydrogels. The hydrogels can encapsulate or immobilize any desired cell type within the
gel. The precursor solution of sodium alginate and water can be modified to include collagen or other peptides to
enhance cellular attachment to promote growth within the gel.
Multiple commercially available multinozzle systems that can be used to deposit biopolymers,
cells, and growth factors are currently being marketed by Envisiontec GmbH, Germany (3D Bioplot-
ter) and nScyrpt, Florida, USA. Other low-cost 3D printing systems, such as those available from Fab@
Home, are being modified to fabricate cellular constructs. Traditionally, pressure-assisted deposition
processes were being utilized for the fabrication of tissue-engineered scaffolds, since the controlled
pore architecture due to the CAD/CAM-controlled layer-by-layer approach was not achievable by tra-
ditional scaffold fabrication processes, such as salt leaching and solvent casting. Several studies have
focused on the design and optimization of scaffolds using polymers, such as PCL, PLGA, PLLA, PEG,
and their blends, and their composites with ceramics, such as hydroxyapatite (HA) and tricalcium phos-
phate (TCP), have been reported ( Vozzi et al., 2002; Wang et al., 2004; Xiong et al., 2001; Landers and
Mulhaupt, 2000; Park et al., 2011 ).
In recent years, with the emergence of the concept of organ printing, the focus has shifted toward
direct printing of cell-encapsulated hydrogels. ( Yan et al., 2005a ; Yan et al., 2005b ; Wang et al., 2006 ;
Cheng et al., 2008 ), and ( Xu et al., 2007 ) used pressure-assisted multisyringe deposition systems to
fabricate liver constructs by encapsulating rat hepatocytes in hydrogels including gelatin in conjunction
with chitosan, alginate, and fibrinogen. The initial structural support was achieved by thermal cross-
linking of gelatin extruded from a low-temperature syringe onto a warmer stage, and the constructs
were further strengthened by chemical cross-linking. Favorable cell viability and function results were
obtained based on liver tissue markers (urea and albumin production). Although there was difficulty in
stabilizing the 3D structure due to enzymatic degradation of the gelatin/chitosan constructs, the process
allowed simultaneous deposition of living cells within a biomaterial. Furthermore, ( Xu et al., 2009 ) and
( Li et al., 2009 ) fabricated biomimetic 3D constructs by simultaneous deposition of adipose-derived
stem cells and hepatocytes encapsulated in gelatin-based hydrogels. Similarly, Fedorovich et al. have
demonstrated the feasibility of multicellular bioprinted constructs incorporating goat multipotent stro-
mal cells (MPSCs), endothelial progenitor cells in Matrigel ® , alginate-based materials for bone grafts
( Fedorovich et al., 2001 ), human mesenchymal stem cells (MSCs), and articular chondrocytes for
osteochondral grafts ( Fedorovich et al., 2011 ).
Most recent approaches with pressure-assisted deposition have focused on creating multimaterial
multifunctional constructs. For example, ( Schuurman et al., 2011 ) have demonstrated fabrication of
hybrid constructs of polycaprolactone (PCL) and chondrocytes-laden alginate. The alginate was cross-
linked with calcium chloride postdeposition, but showed relatively good cell viability. Using the same
strategy, ( Shim et al., 2011 ) successfully fabricated hybrid constructs containing a PCL/PLGA blend as