Biomedical Engineering Reference
In-Depth Information
using moisture or using heat to merge sugar
particles or paraffin spheres. By merging the
porogen material, the probability of isolated
particles remaining in the polymer mixture is
significantly reduced. Moreover, cellular com-
munication and the exchange of nutrients and
waste should be improved with better intercon-
nectivity between pores
[81]
.
pores within the structure. Finally, the scaffold
is submerged into deionized water (or some
other solvent) to dissolve the porogen particle,
creating more pores and enhancing pore inter-
connectivity
[52]
.
Scaffolds produced using either the gas-foaming
method or the hybrid method have been shown
to support cellular functions critical for tissue
regeneration
[52]
. These scaffolds have also been
used for concomitant release of various biomol-
ecules and small molecules in addition to struc-
turally supporting tissue growth
[52]
.
7.2.1.2 Gas-Foaming Method
The
gas-foaming method
has been demonstrated
to produce effective bioscaffolds for tissue engi-
neering applications
[53, 79, 82, 83]
. This method
takes advantage of a gas at high pressure to pro-
duce a porous structure instead of the harsh sol-
vents often used in particle-leaching methods.
The gas-foaming method begins by placing a
material, typically polymeric, in a chamber with
a gas such as CO
2
and increasing the pressure
to the point where the gas becomes sufficiently
soluble in the polymeric solid phase. This effec-
tively saturates the polymer with the gas. The
pressure is then lowered to the ambient pres-
sure to induce thermodynamic instability of the
gaseous phase. The gas begins to phase separate
from the polymer, and in an effort to minimize
free energy, the gas molecules begin to cluster
and cause pore formation. The pores grow by
the gas molecules diffusing to the pore nuclei.
The resulting structure is highly porous, but is
primarily a closed-pore structure because of the
rapid depletion of the gas between pores
[52, 79]
.
The process is limited in its ability to produce
consistently repeatable results. Pore formation
does not occur in a predictable manner each
time the technique is used.
To create better interconnectivity between
pores in the gas-foaming method, this technique
has been combined with the salt-leaching
method. The hybrid technique first creates a
composite of the polymer with a porogen. The
composite is then placed with a gas in a high-
pressure environment to allow the polymer and
gas to mix. The pressure is then decreased to the
ambient pressure, and the gas molecules create
7.2.1.3 Textile Fiber Bonding
To produce fibrous 3D scaffolds with good
mechanical properties,
textile fiber bonding
may
be employed. Fiber-bonded structures have
been shown to perform better than other materi-
als in withstanding the
in-situ
stressors experi-
enced during tissue growth
[54]
.
Textile fiber was developed in 1993 by Mikos
and colleagues
[55]
. The process was initially
described in a series of steps that include the
formation of a composite material of nonbonded
fibers embedded in a polymer matrix, subse-
quent thermal treatment of the matrix, and
finally, selective dissolution of the matrix. The
nonbonded fibrous mesh is created by isolating
fibers from a thicker multi-lamellar mat. This mat
is then either submerged into another polymer
solution, thereby ensuring the fibers are immis-
cible in the second polymer solution, or the fibers
are placed into a mold and the other polymer
solution is allowed to fill the remaining mold
volume. After the solvent evaporates, the com-
posite is heated to a temperature above the melt-
ing point of the polymer that comprises the fiber
network to form welded points at the crosspoints
of the fiber mesh. Finally, the non-fiber polymer
is selectively dissolved using a solvent that is
immiscible with the fiber network. The resulting
fiber matrix is then vacuum-dried to completely
remove solvents
[55]
.
Two prominent drawbacks of the fiber-bonding
technique include the inability to control pore
