Biomedical Engineering Reference
In-Depth Information
4.5 Materials
osteoblasts showed increased alkaline phos-
phatase activity over
3
weeks [
18
,
73
].
Depending on the defect site and strategy to
be employed, certain orthopedic biomaterials
may be more suitable than others. These mate-
rials can either be obtained from natural
sources, with or without subsequent modifi ca-
tion, or synthesized. The following is an over-
view of natural and synthetic biodegradable
materials that are currently being investigated
for orthopedic applications.
4.5.1.2 Gelatin
Gelatin is a promising biomaterial prepared by
the thermal denaturation of collagen isolated
from animal skins and bone. It contains a
mixture of collagen strands along with their
oligomers and degradation products and thus
has the same primary composition as collagen
but is not as highly organized. Two types of
gelatin are produced, depending on whether or
not the preparation involves alkaline pretreat-
ment, which converts asparagine and gluta-
mine residues to their respective acids. Acidic
pretreatment of pig skin produces type A
gelatin, whereas alkaline pretreatment of cattle
hides and bones produces type B gelatin.
Gelatin is used mainly as a scaffold for regen-
eration of soft tissues or for delivery of bioac-
tive molecules [
4.5.1 Natural Materials
Many natural biomaterials are either currently
used or under development for tissue-
engineering applications. Natural materials
have the advantage over synthetic materials in
being similar to materials in the body and thus
may encourage tissue development by direct-
ing cell adhesion and function [
]. Gelatin has also been
investigated as an injectable scaffold for carti-
lage tissue engineering, because of its ease of
gelation in situ [
29
,
46
,
57
]. These
materials, however, are more likely to evoke an
immunogenic response or carry a risk of disease
transmission [
62
]. Other work has shown that
gelatin microparticles provide a promising
delivery system for various growth factors,
because their release is regulated by enzymatic
degradation of the microparticle carriers [
46
76
].
4.5.1.1 Collagen
Collagen is the most abundant natural polymer,
constituting more than a third of the protein
content in the body. Although several different
types of collagen exist in the tissues, the major
constituents of orthopedic tissues are the fi bril-
lar collagens (most predominantly types I and
II) [
40
,
57
].
4.5.1.3 Polysaccharides: Agarose, Alginate,
Chitosan, and Hyaluronic Acid
Agarose is prepared by extraction from
seaweed, such as agar or agar-bearing algae.
It is a linear polysaccharide composed of the
basic repeat unit, made up of alternating
]. These collagens possess a triple-
helix structure that results in fi brils with high
tensile strength [
36
,
76
]. Recently, many scientists
have investigated collagen scaffolds for tissue
engineering of soft orthopedic tissues, since
collagen is widely available and easily cross-
linked chemically (by glutaraldehyde, formal-
dehyde, or carbodiimide) or physically (by
ultraviolet light or heat). Thus, collagen has the
potential for a wide range of scaffolding appli-
cations [
59
β
-D-
galactose and
-L-galactose units.
In orthopedic tissue engineering, agarose is
mainly used in the form of a gel prepared by
cooling an agarose solution to allow cross-
linking of the network. The mechanical prop-
erties of agarose gels vary with the concentration
of agarose [
3
,
6
-anhydro-
α
]. Agarose-based materials
have been used in several studies for cartilage
regeneration and found to promote cell prolif-
eration, cell retention and chondrogenesis in
vivo and in vitro [
9
,
62
]. Collagen implants can be
fabricated for use as both preformed and inject-
able scaffolds and can be easily combined with
cells, growth factors, or both, thus further
enhancing their usefulness for orthopedic
tissue engineering. In vitro studies with anionic
collagen scaffolds prepared by a hydrolysis
treatment demonstrated that seeded bovine
60
,
73
,
78
].
Like agarose, alginate is linear polysac-
charide purifi ed from seaweed. It consists of
linear chains of
64
,
69
,
74
β
-D-mannuronic acid residues
and
α
-L-guluronic acid. Gelation occurs when
