Page 344 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOPOLYMERS 321

Dextran is a simple water-soluble polysaccharide manufactured by Leuconostoc mesenteroides and
L. dextranicum (Lactobacteriaceae). Its structure is shown as a linear polymer, but some branching
occurs at the three remaining OH groups. The native form of dextran has a high molecular weight
8
near 5 × 10 g/mol. Dextran is depolymerized to yield a variety of molecular weights depending on
the application. Similar to polyvinyl pyrrolidinone, dextran solutions can be used as a blood plasma
extender for mass casualty situations. Dextran of between 50,000 and 100,000 g/mol is used for this
application. Like many of the water-soluble polymers, cross-linked dextran can be used as a drug
delivery matrix in whole or microsphere form. Dextran-coated magnetite (Fe O ) nanoparticles are
3
4
finding use as a magnetic resonance imaging (MRI) contrast agent. The dextran adsorbs onto the
particle surfaces and provides a steric barrier to prevent agglomeration of the nanoparticles.
Starch. Degradation: biodegradable.

OH
CH 2 CH 2 OH
O O
H H H H
H H
OH H OH H
O O
H OH H OH n
Amylose: Poly(1, 4'-α-D-glucopyranose)

Starch is the primary source of carbohydrate in the human diet. Starch is composed of two mono-
saccharides: amylose and amylopectin. Amylose is a linear polymer that varies in molecular weight
between 100,000 and 500,000 g/mol. Amylopectin is similar to amylose, having the same backbone
structure, but with 4 percent branching. Starch is insoluble in water, but can be made soluble by treat-
ing with dilute HCl. Soluble starch has similar properties to dextran and therefore has similar
applications.

13.3.2 Gelling Polymers
Gelling polymers are polymers in solution that transform into relatively rigid network structures with
a change in temperature or by addition of ionic cross-linking agents. This class of polymers is useful
because hydrogels can be formed at mild conditions. These polymers can therefore be used for cell
immobilization and for injectable materials that gel in vivo. They are also used as coatings for drug
tablets to control release in vivo.

Poloxamers. Degradation: bioinert.

CH 3
HO CH CH O CH CH O CH CH O H
2
2
2
2
2
a b c
Poloxamers consist of two polyethylene oxide (PEO) blocks attached on both sides of a polypropy-
lene oxide (PPO) block. The polymers are water soluble, but increasing the temperature or concen-
tration can lead to gel formation. The gelling properties are a function of the polypropylene content
and the block lengths. Figure 13.12 shows the viscosity as a function of temperature for poloxamer
407. For a given concentration of poloxamer, the viscosity increases by several orders of magnitude
at a transition temperature. The transition temperature decreases as polymer concentration increases.