Biomedical Engineering Reference
In-Depth Information
applied to fabricate gel/cell hybrid constructs [
189-
193
]. For each hydrogel material,
it is indicated whether or not cell encapsulation experiments were performed. In
addition, Table
9.5
comprises the more technical details, focussed on the disadvan-
tages specific for hydrogels. As shown, scaffolds produced by a 3D-Bioplotter have
limited resolution and mechanical strength. Material rigidity was shown to influence
cell spreading and migration speed, as demonstrated by Wong et al. [
197
] . Cells
displayed a preference for stiffer regions, and tended to migrate faster on surfaces
with lower compliance. In addition, the 3D-Bioplotter technology is a time consum-
ing technique due to the optimization of the plotting conditions for each different
material. Ang et al. [
179
] reported 3D chitosan and chitosan-HA scaffolds using the
RPBOD. Solutions of chitosan or chitosan-HA were extruded into a sodium hydrox-
ide and ethanol medium to induce precipitation of the chitosan component. The
concentration of sodium hydroxide was identified as important in controlling the
adhesion between the layers. The scaffolds were then hydrated, frozen and freeze-
dried. Prior to cell culturing with osteogenic cells, the scaffolds were seeded with
fibrin glue. Drawbacks of this technique largely follow those of the 3D-Bioplotter
and analogues. The inks used in the direct ink writing (DIW) have the disadvantage
that the used hydrogel systems must satisfy two important criteria. First, they must
exhibit a well-controlled viscoelastic response, i.e., they must flow through the
deposition nozzle and then “set” immediately to facilitate shape retention of the
deposited features even as it spans gaps in the underlying layer(s). Second, they
must contain a high colloid-volume fraction to minimize drying-induced shrinkage
after assembly is complete, i.e., the particle network is able to resist compressive
stresses arising from capillary tension [
187
] .
Khalil et al. [
164
] developed a multinozzle low-temperature deposition system
with four different micro-nozzles: pneumatic microvalve, piezoelectric nozzle,
solenoid valve and precision extrusion deposition (PED) nozzle. The system con-
sisted of an air pressure supply. Multiple pneumatic valves were simultaneously
operated for performing heterogeneous deposition in the development of the 3D
scaffold. With this technique, multi-layered cell-hydrogel composites can be fabri-
cated [
192
]. Hydrogels have also been processed with the PAM technique [
170,
171
] .
Of the 3D rapid prototyping micro-fabrication methods available for tissue engi-
neering, PAM has the highest lateral resolution. Recently, it has been demonstrated
that the performance of this method is comparable to that of soft lithography [
198
] .
However, capillaries with a very small diameter require careful handling to avoid
any tip breakage. In addition, pressures are needed to expel the material from a
small orifice. Robocasting relies on the rheology of the slurry and partial drying of
the deposited layers. This implies that a pure hydrogel composition cannot be pro-
cessed via this particular technique, being the most fundamental drawback of the
technique. A last nozzle-based system is the extruding/aspiration patterning system.
One of the advantages is that its set-up is favourable for cell encapsulation purposes
and the fact that cell patterns can be filled into another cell matrix. However, the
hydrogel materials require a small temperature hysteresis, so it has limited
applicability.
Concerning the nozzle-based systems in rapid prototyping of hydrogels, several
challenges need to be addressed. Looking at the limited range of materials, the fol-
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