Biomedical Engineering Reference
In-Depth Information
6.
Engineering Polymeric Scaffolds for
Bone Grafts
Martha W. Betz, Diana M. Yoon, and John P. Fisher
6.1 Introduction
6.2 Scaffold Formation
6.2.1 Curing Methods
Orthopedic injuries resulting from trauma or
improper development often require surgical
intervention to restore natural tissue function.
Currently, over one million operations are per-
formed annually for the surgical reconstruc-
tion of bone [
The method of curing requires knowledge of
polymer chain formation into bulk material
and the chemical nature of the polymer, spe-
cifi cally polymer length and functionality [
]. The well-known limitations
associated with autografts, allografts, and bone
cements have led to the investigation of syn-
thetic polymers as support matrices for bone
tissue engineering. Polymers are long-chain
molecules that are formed by linking repetitive
monomer units. They have been extensively
studied for tissue-engineering applications.
Constructs designed from these polymers can
act as a support matrix to deliver cell popula-
tions or induce surrounding tissue ingrowth.
The properties of scaffolds directly determine
their success in tissue engineering and must be
designed specifi cally for each application. A
successful scaffold provides initial support,
growth factors, and transitions through degra-
dation to allow tissue regeneration and return
of function. This chapter will discuss the fabri-
cation and properties of polymeric tissue-
engineering scaffolds, including curing
methods, polymer assembly, scaffold fabrica-
tion, surface properties, macrostructure,
mechanical properties, biodegradation, and
biocompatibility, as well as current synthetic
polymers under investigation.
50
].
Two major curing methods often used are
polymer entanglement and cross-linking.
Polymer entanglement is based on the prin-
ciple that many polymers associate with one
another in solution. This is common with long,
linear polymers as well as branched polymers.
The polymer is dissolved in an appropriate
solvent and placed in a mold. The solvent is
removed by evaporation, leaving the polymer
in the shape of the mold. This is accomplished
with the aid of pressure, temperature, or both
[
5
]. The process is relatively simple, but the
result may lack mechanical stability.
Cross-linking of individual polymers
through chemical bonds to form a bulk mate-
rial is another curing method. Individual
polymer chains can form hydrogen or ionic
bonds with one another through noncovalent
interactions [
53
]. For the formation of covalent
bonds, the polymer must contain a reactive site
for cross-linking, such as a carbon-carbon
double bond. Covalent cross-linking is gener-
ally induced by a free radical that is initiated
by heat, light, chemical accelerant, or time [
5
33
].
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