Biomedical Engineering Reference
In-Depth Information
lack strong mechanical stability and are diffi -
cult to sterilize [
b o n e i n g r o w t h w i t h p r o p e r m e c h a n i c a l s u p p o r t .
In one the physical scaffold provides mechani-
cal support for the polymer/cell/tissue con-
struct from initial seeding to remodeling by the
host [
25
,
36
].
6.3.3 Mechanical Properties
]. The scaffold matrix must therefore
provide enough support to withstand in vivo
stresses and loading. The other strategy imposes
transitional support. Here the scaffold provides
mechanical support while the cells proliferate
and differentiate in vitro [
41
The ability of a scaffold to provide needed
mechanical support is a critical component of
the construct. However, the high mechanical
st reng t h of bone a s compa red w it h ot her t issues
makes the design of a structure with this feature
challenging. Compact bone is mechanically the
equivalent of a semibrittle, viscoelastic, and
orientation-dependent material [
]. Once implanted,
the scaffold is designed to degrade at the same
rate at which the cells produce the ECM for
support.
41
]. The longi-
tudinal strength of compact bone varies
between
7
6.3.4 Biodegradation
78
.
8
and
151
MPa for tension and
131
and
224
MPa for compression [
99
]. The elastic
modulus for compact bone is
GPa in
the longitudinal direction, with a shear modulus
of
17
.
0
to
20
.
0
The majority of scaffolds are designed to
degrade by the time the tissue is completely
formed. Synthetic polymers degrade primarily
by chemical hydrolysis of unstable polymer
backbones [
3
.
30
GPa and a structural density of
1
.
80
g/
cm
3
[
]. In contrast to compact bone, cancel-
lous bone is spongy and highly porous, with a
structural density of
99
]. The polymer can also be
designed to degrade enzymatically, relying on
body enzymes or catalysts embedded within
the scaffold. Degradation can alter the mechan-
ical properties of the construct; this in turn
infl uences the effectiveness of the implant.
Additionally, the degradation products can
modify the implant environment, depending
on their biocompatibility. Degradation prod-
ucts are a function of the structure, compo-
nents, and fabrication techniques of the
material and the rate of degradation. Degrada-
tion also depends on the location and geometry
of the implant, as well as the presence of cata-
lysts, impurities and other additives [
63
g/cm
3
. In general,
cancellous bone is oriented along the direc-
tions of the principal stresses imposed by the
external loading environment [
0
.
20
]. The strength
of cancellous bone is based upon its apparent
density; it ranges from
7
2
.
00
to
5
.
00
MPa and
from
90
.
0
to
400
MPa for strength and modulus,
respectively [
].
For proper tissue regeneration without sig-
nifi cant deformation, a scaffold should provide
a mechanical modulus of
76
10
to
1500
MPa for
hard tissues and
0
.
4
to
350
MPa for soft tissues
[
]. Mechanical requirements are therefore
very important for orthopedic hard tissues and
dictate the method of fabrication of the polymer.
For example, fabrication with particulate leach-
ing and gas foaming leads to a maximum com-
pressive modulus of
37
].
Hydrolysis of the polymer backbone occurs
in two stages [
63
]. First, water penetrates the
polymer, converting the long chains into
shorter water-soluble degradation products by
attacking the chemical bonds in the amorphous
phase. Next, the fragments are enzymatically
degraded, causing a rapid decrease in polymer
mass. These two phases are part of two overall
mechanisms of degradation.
Overall scaffold degradation has been
well described. Polymeric scaffolds undergo
bulk or surface degradation, or both. In bulk
degradation, erosion at the surface is slower
than in the interior [
63
MPa and therefore is
not appropriate for scaffolds to be used for
hard-tissue regeneration [
0
.
4
]. The lack of
mechanical stability associated with many of
the conventional fabrication techniques empha-
sizes the utility of rapid prototyping techniques
for engineered scaffolds. These more precise
methods of fabrication result in scaffolds with
signifi cant mechanical stability.
Finally, scaffolds should provide interim
support while the tissue regenerates. The scaf-
fold material should therefore not degrade
before the regenerated tissue provides suffi -
cient load-bearing support and stress dissipa-
tion. Two common scaffold designs support
37
]. Initially, the surface
begins to degrade when the construct is in
contact with water. Then, as water penetrates
the inside of the material, the bulk of the scaf-
fold begins to degrade. Bulk degradation is
63
