Biomedical Engineering Reference
In-Depth Information
Complex micelles with mixed shells (CMMSs) can be prepared by
simultaneous self-assembly of two block copolymers either via hydrophobic
or electrostatic interaction in aqueous solution. Using a mixed shell generally
composed of neutral hydrophilic chains, e.g. PEG, and environment-sensitive
chains such as PNIPAM, P4VP, etc., a series of CMMSs has been developed
by Shi et al. for the delivery of drugs under different conditions. Generally, the
polymeric micelles have a common core surrounded by two different kinds of
polymer chains as a mixed shell. External stimuli-induced collapse of one kind
of polymer chain results in phase separation in the micelle shell. The collapsed
polymer chains form hydrophobic domains around the micelle core and block
mass exchange between the inner core and the outer milieu to some extent.
Another kind of shell-forming polymer chain, which is soluble and connects
the inner core and outer milieu, can act as channels for small molecules to pass
through. As an example (Figure 9.11), a novel kind of complex micelle with a
common PtBA core and a mixed and double-responsive P4VP/PNIPAM shell
is prepared by co-micellization of two diblock copolymers, poly(t-butyl
acrylate)-b-poly(N-isopropylacrylamide) (PtBA-b-PNIPAM) and poly(t-butyl
acrylate)-b-poly(4-vinylpyridine) (PtBA-b-P4VP), in aqueous solution.
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With
an increase in temperature or pH value, the core-shell micelles can be
converted into a CSC structure due to the collapse of PNIPAM or P4VP, while
the soluble chains stretch out from the core through the collapsed shell and
stabilize the micelles, resulting in the formation of P4VP or PNIPAM
channels, respectively, as indicated in Figure 9.11. The channels, which
connect the micelle core and the outer milieu, provide an effective way for mass
exchange between the inner and outer micelles. Different channels may endow
complex micelles with different functions in controlling mass exchange. The
size and permeability of the channels can be regulated by manipulating the
composition of the diblock copolymers or by changing the environmental
conditions. By using similar methods, complex micelles with PEG channels are
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Figure 9.11
Formation of complex micelles with double responsive channels from a
mixture of PtBA-b-PNIPAM and PtBA-b-P4VP.
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