Biomedical Engineering Reference
In-Depth Information
7.2.4 Viscous and
Viscoelastic Properties
tosan, cellulose derivatives, silicone oils, bio-
degradable polyester copolymers, and a variety
of other biocompatible, biodegradable poly-
mers have been studied for this purpose [
37
,
46
,
Initially, an injectable scaffold material must
be a fl uid, stable liquid/solid dispersion or gel
that can be injected through a needle of the size
required by the specifi c application. Once the
material is injected, its state must change to an
elastic or viscoelastic solid in order for it to
remain intact at the defect site and eventually
be capable of supporting a load. There are
several ways of accomplishing this physical
change. One technique is to utilize a thermor-
eversible system that is a liquid solution (sol) at
injection temperature and a viscoelastic solid
or gel in situ at body temperature. Another pos-
sibility is to employ a thixotropic fl uid, paste,
or reversible gel that is suffi ciently shear-
thinned during injection to fl ow through the
required needle yet maintains suffi cient elas-
ticity in situ to be retained at the defect site.
Another technique is to chemically or ionically
crosslink the injectable fl uid during the place-
ment procedure. In all cases, the reaction time
must be short enough to set the material before
it fl ows from the placement site. The tempera-
ture change occurring during the change in
state must be small enough to avoid or mini-
mize damage to the surrounding tissue. If the
scaffold matrix is a charged, water-soluble
polymer, it may undergo a sol-gel transition in
response to a pH change. If appropriate, the
pH-temperature phase behavior may be uti-
lized as a mechanism for injection and harden-
ing of the scaffold [
71
,
90
]. Since their description in the
1970
s and
1980
s, numerous formulations of resorbable
calcium phosphate cements have been investi-
gated and commercialized [
].
Resorbable calcium phosphate-based com-
posite scaffolds generate bone trabeculae,
provided the rate of resorption of the calcium
phosphate is suffi ciently slow for osteoblasts to
be able to regenerate new bone [
13
,
26
,
39
8
]. Research
carried out in the late
1980
s and early
1990
s [
22
,
23
] has stimulated considerable interest
in the use of particulate calcium phosphate-
based ceramic mixtures for bone reconstruc-
tive surgery [
,
24
,
36
22
,
30
,
31
,
40
,
55
,
56
,
63
,
79
,
88
,
102
]. When their compositions are designed to
match as closely as possible the mineral com-
position of natural bone, their biological
responses closely mimic those of the inorganic
phase of natural bone.
With regard to the necessary properties of
ceramic biomaterial scaffolds in general, two of
the unique factors are porosity and bioresorb-
ability [
]. The bioceramic material provides an
osteoconductive scaffold for the growth of new
bone. For many applications, the particulate
phase consists of micron-sized porous particles
with a broad distribution of pore size. Pores of
less than about
8
m are important for biore-
sorbability, whereas pores in the range of
5
µ
400
to
m facilitate the infi ltration and differ-
entiation of osteogenic cells necessary for bone
reconstruction [
600
µ
37
].
]. The morphology of the
porous structure also infl uences that of the
regenerated bone, inasmuch as the penetrated
fi brovascular tissue moves in the direction of
the pore channels. An interconnecting pore
structure is thought to be superior to that of
isolated pores, because it better provides for
spatial continuity of the new bone [
8
7.3 Ceramic-Based
Injectable Scaffolds
Calcium phosphate ceramics, such as hydroxy-
apatite, tricalcium phosphate (TCP), biphasic
calcium phosphate (BCP), and bioactive glasses
(BG), in combination with a variety of biode-
gradable polymeric matrices, have been exten-
sively studied and used during the past decades
as alternatives to autogenous bone for repair,
substitution, or augmentation [
]. Since
macropores are required for bone reconstruc-
tion, the range of particle sizes required of a
scaffold is at least on the order of
8
,
79
m.
A scaffold material containing an appropriate
amount of particulate matter of these dimen-
sions is not easily injectable and therefore often
has to be surgically implanted. To provide for
both injectability and a more natural path for
fi brovascular tissue, particles with diameters
on the order of
100
to
500
µ
]. In the
injectable versions of scaffolds, these biomate-
rials are dispersed in water or polymeric solu-
tions that serve as modifi ers of rheological
properties or as binding agents. Alginates, chi-
20
,
39
m have been used
that allow the invading tissue to grow over and
10
to
100
µ
