Tissue engineering - Biomedical Engineering Desk Reference - page 657

decreased because of the harsh chemical or thermal envi-
ronments used in some polymer processing techniques.
Using a novel phase separation technique, scaffolds loaded
with small hydrophilic and hydrophobic bioactive mole-
cules have been manufactured ( Lo et al., 1995 ). The
polymer is dissolved in a solvent such as molten phenol or
naphthalene, followed by dispersion of the bioactive mol-
ecule in this homogeneous solution. A liquid-liquid phase
separation is induced by lowering the solution tempera-
ture. The resulting bicontinuous polymer and solvent
phases are then quenched to create a two-phase solid.
Subsequent removal of the solidified solvent by sub-
limation leaves a porous polymer scaffold loaded with
bioactive molecules.
The fabricated PLA foams have pore sizes up to 500
m m with relatively uniform distributions. The properties
of the foams depends on the polymer type, molecular
weight, concentration, and solvent used. It has been
shown that proteins such as alkaline phosphatase retain
as much as 75% of their activity after scaffold fabrication
with the naphthalene system, but the activity is com-
pletely lost in the phenol system. Although phenol has
a lower melting temperature than naphthalene, it is
a more polar solvent and can interact with proteins and
weaken the hydrogen bonding within the protein struc-
ture, resulting in a loss of protein activity. The phenol
system may be useful for the entrapment of small drugs
or short peptides instead.

1998 ). The expansion and fusion of the polymer particles
lead to the formation of a continuous matrix with
entrapped salt particles, which are subsequently leached
out. The GF/PL process produces porous matrices with
predominately interconnected macropores (created by
leaching out salt) and smaller, closed pores (created by
the nucleation and growth of gas pores in the polymer
particles). The fabricated matrices have a more uniform
pore structure and higher mechanical strength than those
obtained with SC/PL.
For injectable scaffolds, a combination of ascorbic
acid, ammonium persulfate, and sodium bicarbonate has
been used at atmospheric conditions to form highly
porous hydrogel materials for bone tissue engineering
( Behravesh et al. , 2002 ). In this case, as the hydrogel is
cross-linked, carbon dioxide is produced, causing pore
formation. The ratio of the three components just listed
determines the final porosity (43-84%) and pore size
(50-200 m m) of the scaffolds ( Behravesh et al. , 2002 ). As
mentioned previously, these porous [P(PF- co -EG)]
foams supported rat-marrow stromal cell differentiation
and bone matrix production during in vitro culture
( Behravesh and Mikos, 2003 ).

Solid freeform fabrication

Solid freeform fabrication (SFF) refers to computer-aided
design, computer-aided manufacturing (CAD/CAM)
methodologies such as stereolithography, selective laser
sintering (SLS), ballistic particle manufacturing, and 3D
printing (3DP) for the creation of complex shapes directly
from CAD models. SFF techniques, although mainly in-
vestigated for industrial applications such as rapid proto-
typing, offer the possibility to fabricate polymer scaffolds
with well-defined architecture because local composition,
macrostructure, and microstructure can be specified and
controlled at high resolution in the interior of the com-
ponents. These methods build complex 3D objects by
material addition and fusion of cross-sectional layers (2D
slices decomposed from CAD models). In addition, they
allow the formation of multimaterial structures by selec-
tive deposition. Prefabricated structures can also be em-
bedded during material buildup. By carefully controlling
the processing conditions, cells, bioactive molecules, or
synthetic vasculature may be included directly into layers
of polymer scaffolds during fabrication.
An example of the use of stereolithography is the de-
velopment of a diethyl fumarate/PPF resin as a liquid base
material for a custom-designed apparatus using a com-
puter-controlled, ultraviolet laser and suitable photo-
initiator ( Cooke et al. ,2002 ). In this case, the machine
builds the desired structure from the bottom toward the
top, with the resin allowed to wash over the sample after
each layer is formed. This provides new base material to
be photo-cross-linked in the desired geometry for the next

Gas foaming

In one example of the gas foaming (GF) technique, solid
disks of PLGA prepared by either compression molding
or SC are exposed to high-pressure CO 2 (5.5 MPa,
25 C) environment to allow saturation of CO 2 in the
polymer ( Mooney et al. , 1996a ). A thermodynamic in-
stability is then created by reducing the CO 2 gas pressure
to ambient level, which results in nucleation and ex-
pansion of dissolved CO 2 pores in the polymer particles.
PLGA sponges with a porosity of up to 93% and a pore
size of about 100 m m have been fabricated. The porosity
and pore structure are dependent on the amount of CO 2
dissolved, the rate and type of gas nucleation, and the
rate of gas diffusion to the pore nuclei.
The major advantage of this technique is that it in-
volves no organic solvent or high temperature and
therefore is promising for incorporating tissue induction
factors in the polymer scaffolds. However, the effects of
high pressure on the retention of activity of proteins still
need to be assessed. In addition, this process yields
mostly nonporous surfaces and a closed pore structure
inside the polymer matrix, which is undesirable for cell
transplantation. In an improved method, a porogen such
as salt particles can be combined with the polymer to
form composite disks before gas foaming ( Harris et al.,