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Snowmen turn to snowballs © Angew. Chem. Int. Ed.

Like me, you’ve no doubt put this year’s hottest nanogadget – the nanocar – at the top of your Christmas list. But have you given any thought to how exactly you expect Santa to wrap it up? I didn’t think so. Luckily, someone else has.

Seung-Man Yang at KAIST in South Korea has been folding up polymer bilayers to make microcapsules that might just do the job (along with various other, much more practical jobs).

Yang and his team use a micro-origami approach (folding up tiny things to make tiny shapes) to create their capsules. Using photolithography, they build a planar template from two different polymer hydrogels, one atop the other. An applied stimulus then causes the material to spontaneously arrange itself into the desired shape.

So how does it work? In the bilayer material, the bottom layer – poly(2-hydroxyethyl methacrylate-co­-acrylic acid), if you’re interested – swells up in high pH conditions, while the top layer – just plain old poly(2-hydroxyethyl methacrylate) – remains unchanged. So, dial up the pH and the ensuing mechanical stress between the two layers causes the whole thing to curl (or fold – origami, remember?) into a closed, spherical capsule. Reduce the pH and the swelling goes down, opening the capsules again.

The capsules come in two flavours – a pair of hinged hemispheres, which the team endearingly term a snowman particle, and a six-segment design that they call a flower particle (but that, owing to its similarity to a certain confection and in a stubborn attempt to tie this post together with a Christmas theme, I’m calling a chocolate orange).

As you can see in the film, the snowmen neatly wrap themselves up into microcapsules (snowballs, presumably?) in a matter of moments, faster than even my own hasty present wrapping. They’re pretty good at keeping their secrets too: in one example, they securely hold on to some polystyrene particles. Their performance with bubble-wrap is as yet untested.

Festive flippancy aside, it’s an elegant example of microencapsulation with a variety of potential uses. It all depend on what you put in them – pharmaceuticals, inks, flavourings, cells, or (I’ve got a theme and I’m sticking with it) nanogadget gifts.

Philip Robinson

Ref: T S Shim et al, Angew. Chem. Int. Ed., 2011, 50, 1 (DOI: 10.1002/anie.201106723)

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