A new type of microscopic particle that has a hard shell and a soft core that changes structure depending on the temperature has been developed by Walter Richtering and graduate student Ingo Berndt of the University of Aachen, Germany, and Jan Skov Pedersen of the University of Århus, Denmark, and their colleagues. The particles might have industrial and biomedical applications. For instance, they could be used for the controlled release of substances held within the shell.

The particles are polymeric microspheres with a core composed of poly-N-isopropylacrylamide and a poly-N-isopropylmethacrylamide shell. Both polymers swell in water, forming microgels, but because of the different polymer building blocks used in the shell and core, they differ in how much water they can absorb.

The microspheres are densely packed when prepared at 70 Celsius, but cooling to approximately 25 Celsius changes the structure so that the core and shell have the highest water content and the lowest density and dissolved molecules can pass through the shell into the core, where they disperse. Warming to body temperature (about 39 Celsius), however, causes the shell and not the core to swell. This expels the water, causing it to shrink and become denser than the core. Any substances dissolved in the core can no longer pass through the shell and so are locked inside.

Aside from their sensitivity to temperature, such polymeric microgels with core-shell architectures have other potential uses. By choosing appropriate components and reaction conditions, the researchers suggest they could construct particles with multiple shells that might be used to trap different compounds and then allow them to mix at a specific temperature; allowing them to be used as microscopic reaction vessels, for instance.

Angew. Chem. Int. Ed. 2006, 45, 1737-1741; http://dx.doi.org/10.1002/anie.200503888

http://www.ipc.rwth-aachen.de/richtering/

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