  Microcapsules allow for the controlled containment, transport, and release of
cargoes ranging from pharmaceuticals to fragrances. Given the interest from a
variety of industries in microcapsules and other core-shell structures, a
multitude of fabrication strategies exist. Here, we report on a method relying
on a mixture of temperature-responsive microgel particles,
poly(N-isopropylacrylamide) (pNIPAM), and a polymer which undergo fluid-fluid
phase separation. At room temperature this mixture separates into colloid-rich
(liquid) and colloid-poor (gas) fluids. By heating the sample above a critical
temperature where the microgel particles shrink dramatically and develop a more
deeply attractive interparticle potential, the droplets of the colloid-rich
phase become gel-like. As the temperature is lowered back to room temperature,
these droplets of gelled colloidal particles reliquefy and phase separation
within the droplet occurs. This phase separation leads to colloid-poor droplets
within the colloid-rich droplets surrounded by a continuous colloid-poor phase.
The gas/liquid/gas all-aqueous double emulsion lasts only a few minutes before
a majority of the inner droplets escape. However, the colloid-rich shell of the
core-shell droplets can solidify with the addition of salt. That this method
creates a core-shell structures with a shell composed of stimuli-sensitive
microgel colloidal particles using only aqueous components makes it attractive
for encapsulating biological materials and making capsules that respond to
changes in, for example, temperature, salt concentration, or pH.
