Biomedical Engineering Reference
In-Depth Information
COOH
COOH
O
O
OH
OH
H
H
H
COOH
HO
HO
OH
OH
H
H
H
O
H
O
H
H
H
H
β
-
D
-Mannuronic acid
α
-
L
-Guluronic acid
FIGURE 15.9
The M and G units of alginic acid.
and molecular weight determine the physical properties of the alginate.
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The solubility of
alginates and water retention capacity depends on the pH and precipitation occurs at about pH 3.5.
The molecular weight also affects water holding capacity, and calcium alginates with low molecular
weights or less than 500 residues exhibit higher water uptake and binding. Sodium alginate and
most other alginates from monovalent metals are soluble in water, forming solutions of considerable
viscosity. The rheological properties of alginate sols make them particularly useful as a thicken-
ing agent and have long been used in the pharmaceutical industry as thickening or gelling agents,
as colloidal stabilizers and as blood expanders. Alginates are used to take impressions of the oral
tissue due to their ability to undergo a transformation from the sol state to a gel state through
ionotropic gelation in the presence of many multivalent ions such as Ca
2
+
. The crosslinking can be
carried out under very mild conditions at low temperatures and high humidity. Alginates have been
widely investigated for cartilage and bone regeneration, either as a scaffold or as carriers for biologi-
cally active molecules and drug delivery.
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15.3.2 C
HITOSAN
Chitosan is a linear copolymer of glucosamine and
N
-acetyl glucosamine in a β-1- 4 linkage, usu-
ally obtained from chitin. Chitin is the second most abundant natural polymer in the world after
cellulose. Chitosan is produced by the deacetylation of chitin (Figure 15.10), which is the structural
element of the shells of crustaceans such as crab and shrimps. The degree of deacetylation (%DA)
and the molecular weight determine the properties of chitosan. The degree of deacetylation can be
determined from nuclear magnetic resonance (NMR) data and ranges between 60% and 100% in
commercial chitosan. Chitosan is positively charged and soluble in acidic to neutral solution with a
charge density dependent on pH and the %DA-value.
The DA, which signifi es the mole fraction of the N-acetylated units is a structural parameter and
infl uences charge density, crystallinity, solubility and proneness to enzymatic degradation. Chitosan
readily binds to negatively charged surfaces such as mucosal membranes and hence acts as a bioad-
hesive. It displays interesting properties such as biocompatibility, biodegradability, and its degrada-
tion products are nontoxic, nonimmunogenic and noncarcinogenic.
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As a polysaccharide of natural
origin, chitosan has many useful features such as nontoxicity, biocompatibility, biodegradability,
and antimicrobial properties.
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It exhibits excellent biological properties such as biodegradation in
the human body, immunological, antibacterial, wound-healing activity, and is used as microcapsule
implants for controlled release in drug delivery.
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Chitosan is one of the most promising biopolymers
for tissue engineering and possible orthopedic applications. In particular, the possibility to generate
structures with predictable pore sizes and degradation rates makes it a suitable material as a bone
graft alternative in orthopedic procedures.
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It has been recently approved for use in bandages as
it can cause rapid clotting. Chitosan has been found to be the material that supports gene delivery,
cell culture, and tissue engineering.
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It has also been proven to be safe for use as a pharmaceutical
excipient and has properties that make it suitable for drug delivery applications. As chitosan shows
great promise in controlled release, this material is being increasingly exploited for developing nano-
and microparticles for drug delivery, gene therapy, and regenerative medicine.
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