Biomedical Engineering Reference
In-Depth Information
For example, photopolymerization is a com-
monly used technique based on photopolymer
polymerization initiated by electromagnetic
radiation [
at the tissue defect site [
]. Liquid components
are injected into the desired site, and their
deformability allows for improved integration
into the host tissue. Furthermore, because this
method uses liquid components, it is less inva-
sive than the surgical procedures sometimes
necessary for prefabricated constructs. How-
ever, in situ fabrication does not allow for the
removal of harmful by-products, and the sur-
rounding tissue may therefore be exposed to
toxic components. As a result, the variety of
chemical components that can be used to form
the construct in situ is restricted.
33
]. The photopolymers used are
typically low-molecular-weight monomers that
react to form long-chain polymers when acti-
vated by a specifi c wavelength. In addition,
since scaffold formation occurs in response to
a signal, the polymer may be used as an inject-
able material and can form in situ when exposed
to the signal. However, the chemical reactions
necessary for cross linking are often associated
with unreacted components and reaction by-
products that may harm the surrounding
tissue.
98
6.2.3 Conventional Scaffold
Fabrication Methods
6.2.2 Polymer Assembly
Fabrication is the process of forming a cured or
curing polymer into a scaffold. Scaffold fabri-
cation can be performed by conventional or
rapid prototyping (solid free-form) practices
(Table
Polymer assembly may occur before or during
implantation of the scaffold into the body [
].
Prefabrication is the term applied to assembly
before implantation. The scaffold is formed
outside of the body, and any cytotoxic or non-
biocompatible by-products can be removed
prior to transplantation. Prefabrication also
allows for cell encapsulation and in vitro
culture before implantation. However, the
structure of the prefabricated construct gen-
erally may not fi t the host site precisely. An
imperfect match may lead to host immune
reactions such as fi brosis and thus to construct
failure. In situ fabrication techniques address
this concern and involve curing the construct
97
). A number of conventional tech-
niques are used to create porous scaffolds, fi ber
bonding, solvent-casting particulate leaching,
phase separation, melt molding, freeze drying,
and gas foaming.
6
.
1
6.2.3.1 Fiber Bonding
Fibers are commonly processed from semi-
crystalline polymers, including poly(glycolic
acid) (PGA). These fi bers can be used to create
Table 6.1.
Fabrication methods, associated characteristics, and synthetic polymers used in bone tissue-engineering
scaffolds
Fabrication Method
Scaffold Attributes
Polymers
References
Fiber bonding
High porosity
PGA, PCL
[31, 56, 64]
Solvent-casting particulate leaching
Controlled porosity
PLA, PLGA, PPF
[12, 67, 73, 87, 95]
Controlled pore size
Phase separation
Porous
PLLA, PLGA, PLA
[40, 69, 70, 101]
Biomolecule incorporation
Melt molding
Controlled porosity
PLGA
[90, 97]
Controlled pore size
Biomolecule incorporation
Freeze drying
Controlled pore size
PLGA
[39, 78, 94]
Gas foaming
Controlled porosity
PLLA, PLGA, PLA
[61, 71, 82]
Controlled pore structure
Sheet lamination
Porous
PLA, PLGA
[66]
Three-dimensional printing
Controlled mechanical strength
PCL, PEO, PLGA, PLA
[30, 47, 96]
Laser stereolithography
Biomolecule incorporation
PPF, PEGDA
[20, 60]
Fused deposition
Controlled pore size
PCL
[16, 43, 77, 100]
