Biomedical Engineering Reference
In-Depth Information
be obtained by deposition of a polymer solution over
a non-woven mesh of another polymer, and the sub-
sequent evaporation. The advantages of fiber meshes are
the large surface area for cell attachments and the rapid
diffusion of nutrients, but they are not suitable for the
fine control of porosity.
emerged, but they involve time-consuming sequential
writing processes with a narrow processing window and
relatively low resolution. The SFF fabrication technolo-
gies create 3-D structures in laminated fashion from
numerical models. Commercially available SFF technol-
ogies, such as fused deposition modeling, SLA, and se-
lective laser sintering have been utilized to fabricate
scaffolds. Computer-aided engineering has also emerged
to fabricate multifunctional scaffolds with control over
scaffold composition, porosity, macroarchitecture, and
mechanical properties based on optimization models. To
print tissue engineering scaffolds, inkjet solvent binder is
printed onto a powder bed of porogens and polymer
particles. The solvent will dissolve the polymer and
evaporate, and the polymer will re-precipitate to form
solid structures. The final porosity is achieved after par-
ticulate leaching and solvent removal.
Figure 7.2-12
shows a custom-designed fiber-deposition device
[6]
.
Hollister reviewed how integration of computational
topology design and SFF has made scaffolds with de-
signed characteristics possible
[7]
.
7.2.3.4 Gas foaming
The gas-foaming technique uses high-pressure CO
2
gas
processing. The porosity and pore structure depend on
the amount of gas dissolved in the polymer, the rate and
type of gas nucleation, and the diffusion rate of gas
molecules through the polymer to the pore nuclei.
However, this technique often produces a scaffold that is
too compact for the cells. This may be improved by as-
sociating the gas foaming with salt leaching. Another
technique derived from improvement of gas foaming
consists of the substitution of high-pressure CO
2
with
ammonium bicarbonate, which acts also as porogen. In
this case the porosity depends only on the amount of salt
particulates added, whereas the pore diameter is due to
the size of the salt crystals.
7.2.3.6 Electrospinning
7.2.3.5 Rapid prototyping
Electrospinning provides a mechanism to produce nano-
fibrous scaffolds from synthetic and natural polymers,
with high porosity, a wide distribution of pore diameter,
high surface-area-to-volume ratio, and morphological
similarities to natural collagen fibrils. Fiber diameters are
in the range from several micrometers down to less than
100 nm. The electrospinning process is based on a fiber
spinning technique driven by a high-voltage electrostatic
field using a polymeric solution or liquid.
Figure 7.2-13
represents an electrospinning apparatus
[8]
. The un-
derlying physics of this technique relies on the applica-
tion of an electrical force, especially when at the polymer
droplet surface it overcomes the surface tension force,
and a charged jet is ejected. As the solvent evaporates,
the charge density increases on the fibers, resulting in an
unstable jet, which stretches the fibers over about one
million fold. The variables controlling the behavior of the
electrified fluid jet during electrospinning can be divided
into fluid properties and operating parameters. The rel-
evant fluid properties are viscosity, conductivity, di-
electric constant, boiling point, and surface tension. The
operating parameters include flow rate, applied electric
potential, and distance between the tip and the collector
called ''air gap.''
The final product of electrospinning generally consists
of randomly interconnected webs of sub-micron size
fibers. Nanofibrous scaffolds formed by electro-spinning,
by virtue of structural similarity to natural ECM, may
represent promising structures for tissue engineering
applications.
Rapid prototyping (RP) or solid free-form (SFF) fabri-
cation refers to a group of technologies that build
a physical, 3-D object in a layer-by-layer fashion. The RP
is a subset of mechanical processing techniques which
allow highly complex structures to be built as a series of
thin two-dimensional (2-D) slices using computer-aided
design (CAD) and computer-aided manufacturing
(CAM) programs. These techniques essentially allow
researchers to predefine properties such as porosity,
interconnectivity, and pore size. The RP methodologies
include stereolithography (SLA), selective laser sintering,
ballistic particle manufacturing, and 3-D printing. Ex-
amples of RP applied to generate tissue engineering scaf-
folds include laser sintering to fabricate Nylon-6 scaffolds,
fused deposition molding of poly(3-caprolactone) (PCL)
scaffolds, and SLA of PU patterns. Direct and indirect
SLA have been used to make ceramic scaffolds and can-
cellous bone structure models. Few studies address the
generation of scaffolds by RP techniques for soft tissue
engineering such as 3-D hydrogel scaffolds. SLA, one of
the most common types of RP, operates by selectively
shining a laser beam onto a vat of liquid photopolymer.
This method has been employed in numerous biomedical
applications such as building models of biological struc-
tures, bone substrate scaffolds, and heart valve scaffolds.
Using an RP technique to pattern not only the scaffold
but also cells will accelerate and improve tissue assembly.
On the basis of this idea, various RP methods have
