Biomedical Engineering Reference
In-Depth Information
There are other linear aliphatic polyesters, such as poly(ε-caprolactone) (PCL), which are also
used in TE research.
45-47
PCL degrades at a signifi cantly slower rate than PLA, PGA, and PLGA.
The slow degradation makes PCL less attractive for general TE applications but more attractive
for long-term implants.
48
PCL-based copolymers have recently been synthesized to improve deg-
radation properties.
49,50
Other important synthetic biodegradable polymers include poly(propylene
fumarate) (PPF),
51
which degrades through hydrolysis of the ester bonds similar to glycolide and
lactide polymers
52
and segmented polyurethanes whose structural variations enable the production
of materials with a range of mechanical properties.
53
Synthetic polymers from the families of poly-
phosphoesters,
54
poly(ethylene oxide terephthalate)-poly(butylene terephthalate) (PEOT/PBT),
55
polyanhydrides
56
and poly(ortho esters)
57
are also under study for potential use in TE applications.
3.3.1.2
Natural Origin Polymers
Most of the interest in natural origin polymers stems from their biocompatibility, relative abun-
dance, ease of processing, and possibility of mimicking the microenvironment found
in vivo
.
Collagen is present in most of the connective tissues of the body. Although there are 16 types
of collagens, the most abundant one is collagen type I. The macromolecular i ber structure derived
from a triple helix arrangement confers this polymer resistance to pressure. As a result, collagen
acts as a structural matrix protein.
58,59
Collagen has been used in several TE applications, like
human cornea reconstruction,
60
cardiac regeneration,
61
skin,
62
cartilage,
63
and bone regeneration.
64
Alginate is composed of two repeating monosaccharides: l-guluronic acid and d-mannuronic
acid. Repeated strands of these units form linear water-soluble polysaccharides. Once these poly-
saccharides are exposed to calcium ions, a three dimensional (3-D) gel quickly forms.
65
Therefore
various drugs, growth factors, or cells can be encapsulated in the gel.
66
Alginate gels have been used
in a wide range of TE, such as the regeneration of cardiac tissue,
61
liver,
67
pancreas,
68
cartilage,
69
and bone,
66,70
alone or as a blend.
71,72
Among the wide range of available materials for TE applications, chitosan is considered to
be very promising because of some of its properties like biocompatibility, biodegradability, and
antibacterial activity.
73-78
Chitosan is a partially deacetylated derivative of chitin, found in arthro-
pod exoskeletons. Structurally, chitosan is a linear polysaccharide consisting of β(1
4) linked
d -glucosa m ine residues with a va r iable number of ra ndom ly located
N
-acetyl-glucosamine groups.
79
The presence of such chemical groups provides active sites for the grafting of relevant molecules for
the improvement of cell-material interactions.
80,81
This polymer is degraded by the action of several
enzymes,
82
but in an
in vivo
environment enzymatic degradation is mainly attributed to the action
of lysozyme.
83,84
This polymer has been used in several studies as potential material for pancreas,
85
bone,
86,87
cartilage,
88,89
and skin
90,91
TE.
Other natural origin polymers with good potential in TE applications are starch,
83,92-96
polyhy-
droxybutyrate,
97,98
hyaluronic acid,
99,100
or silk.
101,102
→
3.3.2 P
ROCESSING
T
ECHNIQUES
TO
C
ONTROL
THE
S
CAFFOLDS
' A
RCHITECTURE
Various techniques have been developed to produce scaffolds. The ultimate goal is to obtain highly
interconnected porous scaffolds with required internal architectures suitable for different tissues.
3.3.2.1 Solvent-Based Techniques
Solvent-evaporation/particulate leaching is a technique that has been widely used to fabricate scaf-
folds for TE applications. In this technique, a polymer solution mixed with a porogen is cast into
a mold fi lled with a porogen, usually a salt, such as sodium chloride. After the evaporation of the
solvent, the salt crystals are leached away with water leaving empty spaces that form the pores of
the scaffold.
103,104
Phase-separation techniques to produce porous scaffolds are based on the fact
that under certain conditions, a homogeneous multicomponent system becomes thermodynamically
