Biomedical Engineering Reference
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Figure 11.3 Magnetic resonance imaging (MRI) of an alginate bead (1.8% w/v alginate in 50 mM CaCl 2 ).
Polymer concentration decreases from dark grey to light grey. Images obtained with Bruker DMX
200 spectrometer. Adapted from Thu et al.( 2000 ) with permission from John Wiley & Sons.
expectations, this does not disrupt gelation per se but simply produces a system where
intramolecular cross-links become more favoured, just as in the original microgel con-
cept. An alternative is, of course, to prepare the gels as emulsions, and this does have the
advantage of maintaining more homogeneous sizes and shapes. Norton and co-workers
(Norton et al., 1999 ; Adams et al., 2004 ) have exploited both approaches to produce
so-called
'
'fluid gels
'
. Here the gelling system is allowed to cool quite rapidly through the
disorder
-
order transition while being agitated (in their terms
'
sheared
'
, although during
the process analysis must be dif
cult, so other more complex deformation
fields are
probably involved). The gel particles produced lie typically in the range 5
m and
appear to be roughly ellipsoidal, although with many surface defects. The resultant
properties of these systems are not those of typical physical gels, but are described as
'
-
50
μ
. The exact properties are dependent upon the choice and concen-
tration of polymer, and on ionic composition if gelation is ion-mediated.
Adams et al.( 2004 ) have examined agar microgels produced by
'fluid or paste-like
'
first preparing a
classical water-in-oil emulsion and subsequently cooling it to gel the internal phase. The
gelled particles (generally close to spherical) were then separated and re-suspended in
water. Depending upon the initial agar concentration, particles sizes are typically of the
order of 25
μ
m. They then studied the rheological response of these high volume fraction
(>80%, ~
ϕ m , the maximum packing fraction) soft spherical particle suspensions. Above
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