Scope of tissue engineering - Biomedical Engineering Desk Reference - page 671

proline/hydroxyproline(OHP)-rich junction zones.
Higher levels of these pyrrolidines result in stronger gels.
Each of the three strands in the triple helix requires 25
residues to complete one turn; typically there would be
between one and two turns per junction zone. Gelatin
films containing greater triple-helix content swell less in
water and are consequently much stronger.
Gelatin has been used for a range of medical applica-
tions including adhesion prevention because of good
processability, transparency, and bioabsorbability. Aque-
ous solution of gelatin sets to a gel through hydrogen
bonding below room temperature and recovers to the sol
state upon raising the temperature to destroy the hy-
drogen bonding. This reversible sol-gel transition facili-
tates the molding of gelatin into definite shapes such as
block and microsphere, but chemical crosslinking is re-
quired when its dissolution in aqueous media at the body
temperature should be avoided. When applied as a scaf-
fold of cells or a carrier of growth factors, gelatin needs to
be permanently crosslinked. Glutaraldehyde has most
frequently been used for chemical crosslinking of gelatin
to link lysine to lysine, similar to other proteins.

employed for scaffold fabrication. Among them are
hyaluronic acid (HAc), chitin, chitosan, alginate, and
agarose. A prominent feature common to most of poly-
saccharides is the lack of cell-adhesive motifs in the
molecules. This makes these biopolymers suitable as
biomaterials for fabrication of a scaffold whose inter-
action with cells should be minimized. An exception is
chitosan which has basic NH 2 groups.

Hyaluronic acid
Industrially, HAc is obtained from animal tissues such as
umbilical cord, cock's comb, vitreous body, and synovial
fluid. Biotechnology also produces HAc on a large scale.
HAc is an only non-sulfated glycosaminoglycan (GAG)
that is present in all connective tissues as a major con-
stituent of ECM and plays pivotal roles in wound heal-
ing. As shown in Fig. 7.2-4 , this linear, non-adhesive
polysaccharide consists of repeating disaccharide units
(b-1,4- D -glucuronic acid and b-l,3- N -acetyl- D -glycosamine)
with weight-average MWs (Mw) up to 10,000 kDa. This
anionic polymer is also a major constituent of the vitreous
(0.1-0.4 mg/g), synovial joint fluid (3-4 mg/ml) and
hyaline cartilage, where it reaches approximately 1 mg/g
wet weight. SerumHAc levels range from 10 to 100 m g/l,
but are elevated during disease. Clearance of HAc from
the systemic circulation results in a half-life of 2.5-5.5
min in plasma.
In solution, HAc assumes a stiffened helical configu-
ration due to hydrogen bonding, and the ensuing coil
structure traps approximately 1000-fold weight of water.
The highly viscous aqueous solutions thus formed give
HAc unique physicochemical and biological properties
that make it possible to preserve tissue hydration, regu-
late tissue permeability through steric exclusion, and
permit joint lubrication. In the ECM of connective tis-
sues, HAc forms a natural scaffold for binding other large
GAGs and proteoglycans (aggrecans), which are main-
tained through specific HAc-protein interactions. Con-
sequently, HAc plays important roles in maintaining
tissue morphologic organization, preserving extracellular
space, and transporting ions, solutes, and nutrients. Along
with ECM proteins, HAc binds to specific cell surface
receptors such as CD44 and RHAMM. The resulting
activation of intracellular signaling events leads to carti-
lage ECM stabilization, regulates cell adhesion and mo-
bility, and promotes cell proliferation and differentiation.
The HAc signaling takes place also during morphogenesis
and embryonic development, modulation of inflammation,
and in the stimulation of wound healing. In correspon-
dence with these functions, HAc is a strong inducer of
angiogenesis, although its biological activity in tissues has
been shown to depend on the molecular size. High MW
native-HAc (n-HAc) has been shown to inhibit angio-
genesis, whereas degradation products of low MW
stimulate endothelial cell proliferation and migration.

Fibrin
Fibrin is a product of partial hydrolysis of fibrinogen by
the enzymatic action of thrombin. Upon crosslinking it
converts to gel. This is called ''fibrin glue'' or ''surgical
adhesive''. Human fibrin adhesives are approved and
available in most major geographical regions of the world.
Fibrin is applied to patients as a liquid and solidifies
shortly thereafter in situ. Furthermore, fibrin gel can be
readily infiltrated by cells, because most migrating cells
locally activate the fibrinolytic cascade.

Silk fibroin
Due to their high strength, native silk proteins from
silkworm have been used in the medical field as suture
material for centuries. Undesirable immunological prob-
lems attributed to the sericin protein of silk limited the
use of silk in the last two decades. However, purified silk
fibroin, which remains after removal of sericin, exhibits
low immunogenicity and retains many of the attributes of
native silk fibers. This has sparked a renewed interest in
the use of silk fibroin as a biomaterial. Silk fibroin has
unique properties that meet many of the demands for
scaffolds. Silk exhibits high strength and flexibility and
permeability to water and oxygen. In addition, silk fibroin
can be molded into fibers, sponges, or membranes,
making silk a good substrate for biomedical applications
such as implant biomaterials, cell culture scaffolds, and
cell carriers.

7.2.2.1.2 Polysaccharides
In addition to proteinous materials, naturally occurr-
ing polysaccharides and their derivatives have been