Biomedical Engineering Reference
In-Depth Information
this process can be repeated many times. Both systems are derived from natural poly-
mers; as we have already noted, gelatin is extracted from collagen (protein), particularly
originating from mammals or
fish, whereas agarose is a polysaccharide extracted from
marine algae. At one level, these gel networks consist of long, thin interconnected
bres.
We analyse the mechanisms of network formation and the sol
gel transition. In both
cases gelation is not driven by ionic interactions and so differs from the systems
presented in Chapter 5 . We focus in particular on the contribution of the helical assem-
blies to the rigidity of the networks for types of helical structures.
-
Chapter 8 Gelation through phase transformation in synthetic and natural polymers
Network junctions can be created by physical means, when gels are associated with
a phase transformation (Keller, 1995 ), although the gels arising speci
cally from
phase transformations usually do not correspond to a stable state for these systems. At its
final stage, liquid
-
liquid phase separation generates two
fluid layers; in a liquid-to-crystal
phase transition the
final state is a crystal with macroscopic dimensions. The liquid-to-glass
transition in bulk polymers inhibits molecular diffusion and freezes the conformations.
In order to obtain a gel from such a polymer solution the
final stages of phase
transformation should not be attained; instead, the new structure needs to be arrested at
a certain stage just where the network is formed. Every case reported in this chapter
illustrates a different situation, although most of the examples examined involve syn-
thetic polymers. In each case phase transformation leads to network formation under
speci
c conditions. The systems described in this chapter include: poly(vinylchloride)
(PVC) gels with various solvents, atactic poly(styrene) (PS) in trans-decalin, atactic PS
in carbon disulphide, isotactic and syndiotactic poly(methylmethacrylate) (PMMA) gels
and,
finally, cryogels from poly(vinyl alcohol) (PVA) and from certain plant polysac-
charides in water such as the galactomannan locust bean gum (LBG).
Chapter 9 Colloidal gels from proteins and peptides
This class of physical gels encompasses a range of systems including those formed from
mildly denaturing globular proteins so as to generate partially folded but still essentially
globular entities. These are formed when the protein in aqueous (or electrolyte) solution
is altered by some physical (e.g. heating) or chemical treatment (e.g. changing pH).
Under these apposite conditions, some of the hydrophobic groups, which normally
remain buried in protein core, become exposed. This unstable condition leads to aggre-
gation, giving rise either to coarse-stranded amorphous particulate structures or to more
fine-stranded networks. The former are sometimes referred to as
'
particulate
'
or colloidal
gels. The latter, with a more uniform assembly of
ne strands, are denoted
'
'fibrillar',
'
,or
'
'
often now
, gels because of the similarities between their structures and those
found in disease conditions such as Alzheimer
amyloid
'
s. In this chapter we also discuss certain
gels formed from milk (casein), the speci
fibrous assemblies produced, for
example, by actin, as well as the very topical class of materials formed from the self-
assembly of specially synthesized peptides.
c ordered
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