Biomedical Engineering Reference
In-Depth Information
4.4.2 Biocompatibility
4.4.4 Mechanical Properties
One of the most critical requirements biode-
gradable materials must meet is biocompatibil-
ity. Not only should scaffold materials avoid
eliciting infl ammatory and immunogenic
responses, but also degraded materials and
related chemicals should be biocompatible in
terms of both the local and the systemic
response [
The location of a skeletal defect often imposes
strict requirements for the mechanical pro-
perties of an implant [
]. For example, scaf-
folds for treating load-bearing bone defects
should be suffi ciently hard and stiff to sustain
normal loads during healing. Similarly, materi-
als for cartilage tissue engineering should
possess viscoelastic properties similar to
those of native tissue in order to withstand
both the frictional and the compressive forces
imparted within the joint. The mechanical
properties of implants directly after implanta-
tion are especially critical, since these materi-
als will be receiving the full load intended for
the native tissue. The decrease in strength
associated with material degradation should
be slow and predictable, leading to graded
load transfer to encourage growth of neotissue
with properties similar to those of native tissue
[
13
]. The biocompatibility of a
polymer depends on both its chemical struc-
ture and the processing method that produces
it. During a polymerization process, an initia-
tor, a monomer, and sometimes a catalyst are
needed, and these materials often remain in
preformed implants even after purifi cation.
Residual unreacted monomers or initiators are
also a particular concern for in situ forming
implants. Therefore, the toxicity and concen-
tration of these substances should be consid-
ered when assessing biocompatibility. Removal
of these potentially toxic components is usually
effected by prolonged rinsing in aqueous solu-
tion. Biocompatibility of the remaining mate-
rial is confi rmed in vitro by cytotoxicity assays
that use appropriate cells in contact with test
scaffolds and their degradable products. In
vivo observation of the infl ammatory response
after implantation in animal models is also an
important step before clinical application can
be considered [
11
,
27
].
Cells in scaffolds experience different
mechanical signals, depending on the mech-
anical properties of the scaffold or the ECM,
that result in altered cell function and protein
production [
2
,
28
]. For example, the load-
bearing and lubrication properties of cartilage
are attributed to the complex structure and
composition of its extracellular matrix
formed under unique biomechanical and
frictional infl uences [
2
,
103
11
,
96
].
]. Therefore, proper
modulation of scaffold mechanical properties
is extremely important, not only to provide
proper support to the surrounding tissue, but
also to engineer functional replacement
constructs.
103
4.4.3 Biological Functionality
Tissue-engineering applications often require
functional materials that induce cellular
healing responses rather than simply provide
biocompatible tissue replacements. This func-
tionality is achieved either by the addition of
soluble bioactive molecules such as growth
factors and cytokines or by chemical modifi ca-
tion of biomaterials for covalent attachment of
these molecules [
4.4.5 Processability: Sterility,
Reproducibility, and Ease
of Handling
]. For example, syn-
thetic hydrogels that contain covalently linked
peptide sequences that direct cellular attach-
ment and migration have been shown to possess
properties of natural materials, while still
maintaining the advantages of synthetic mate-
rials, such as mechanical properties. Like
natural materials, modifi ed hydrogels are
susceptible to degradation by enzymes [
55
,
81
,
87
As with other biomedical implants it must be
possible to sterilize biodegradable scaffolds
without affecting their chemical or physical
properties and to produce and package them
on a large scale for practical and economic
uses. Factors such as viscosity, curing time,
and implant shape should also be optimized
for injectable scaffolds to facilitate their use
during complex surgical procedures [
33
,
55
,
28
,
70
,
81
,
87
].
92
].
