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January 15th, 2013

Self-oscillating gels, DARPA worms and spontaneous self-aggregation

Published on January 15th, 2013 | By: pwray@ceramics.org

This has little to do with ceramics or glass—but everything to do with the biggest “What in the world…” moment I have had in a long, long time.” I will try to keep this brief, but its nearly impossible to convey the weird (not meant to be pejorative) materials work of Anna C. Balazs’s team at the University of Pittsburgh.

This all started last week when I breezily was scrolling through a list of new papers published in the recent issue of PNAS. Something in the abstract of “Reconfigurable Assemblies of Active, Autochemotactic Gels” caught my eye. Maybe it was the word “autochemotactic,” which I had to look up. Or, maybe it was these two spooky sentences in the abstract,

“To the best of our knowledge, this is the closest system to the ultimate self-recombining material, which can be divided into separated parts and the parts move autonomously to assemble into a structure resembling the original, uncut sample.… Our findings pave the way for creating reconfigurable materials from self-propelled elements, which autonomously communicate with neighboring units and thereby actively participate in constructing the final structure.”

“Hmm,” I thought. “Wasn’t this the big gimmick in the second Terminator movie?”

Liquid metal or no liquid metal, Balazs had me seriously hooked.

It turns out that Balazs works with Belousov–Zhabotinsky (BZ) gels that are relatively simple and, most importantly, have the fascinating ability to “quiver” for extended periods (but not forever) in predictable patterns by means of a self-regenerating internal redox reaction. Watching how the waves spread through these gels is pretty astounding, but this is no one-trick pony. Balazs and her researchers learned quite a bit about how to manipulate the gels and the oscillations, based on things like shape and composition.

They also learned how to use light on one part of material to stimulate the oscillations to move through the gel from one end towards the other, or use two or more lighted areas to create even more complex oscillations. It turns out that if they made the gel into a cylinder shape, a precise use of the light could actually make the worm slowly move. And, the moves could be complex, with lots of twists and turns in three dimensions, kind of like steering a real worm with sticks, but in this case the sticks are just light beams. But, is this just a good bar trick? Not if you are, say, DARPA, and are looking for a soft, synthetic robot that could climb walls and follow complex routes.

And, Balazs’s group was just warming up. It turns out they also figured out how to make microcapsules of these gels that could emit—in a controllable manner, using light—nanoparticles that create gradients that act in philic or phobic fashions to help propel and steer the capsules, and attract other ones. This is where the self-propulsion, self-recombination and “train” functions starts to come into play. Their models (once they understood the chemistry, most of the group’s work was done through modeling, so lots of video snippets are available) indicated that snake-like assemblies of these capsules could selectively “attract” or drop off other capsules as might be needed.

Okay—I know I am not doing this work justice. But please do me (and yourself) a favor and set a side about 15-30 minutes to watch the above video. The first part features a fairly recent lecture by Balazs (with lots of delightful animations and videos) at Harvard/Radcliffe. You can save yourself some time by starting at about the 2:10 mark.


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