The physics of life

Gabriel Popkin in Nature:

GettyImages-537023587CMYKFirst, Zvonimir Dogic and his students took microtubules — threadlike proteins that make up part of the cell's internal 'cytoskeleton' — and mixed them with kinesins, motor proteins that travel along these threads like trains on a track. Then the researchers suspended droplets of this cocktail in oil and supplied it with the molecular fuel known as adenosine triphosphate (ATP). To the team's surprise and delight, the molecules organized themselves into large-scale patterns that swirled on each droplet's surface. Bundles of microtubules linked by the proteins moved together “like a person crowd-surfing at a concert”, says Dogic, a physicist at Brandeis University in Waltham, Massachusetts. With these experiments, published1 in 2012, Dogic's team created a new kind of liquid crystal. Unlike the molecules in standard liquid-crystal displays, which passively form patterns in response to electric fields, Dogic's components were active. They propelled themselves, taking energy from their environment — in this case, from ATP. And they formed patterns spontaneously, thanks to the collective behavior of thousands of units moving independently.

These are the hallmarks of systems that physicists call active matter, which have become a major subject of research in the past few years. Examples abound in the natural world — among them the leaderless but coherent flocking of birds and the flowing, structure-forming cytoskeletons of cells. They are increasingly being made in the laboratory: investigators have synthesized active matter using both biological building blocks such as microtubules, and synthetic components including micrometre-scale, light-sensitive plastic 'swimmers' that form structures when someone turns on a lamp. Production of peer-reviewed papers with 'active matter' in the title or abstract has increased from less than 10 per year a decade ago to almost 70 last year, and several international workshops have been held on the topic in the past year.

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