By Kim Krieger of Nature magazine
Katia Bertoldi is talking fast. She has only 12 minutes to present her work in the burgeoning field of 'extreme mechanics'. But first, the Harvard University engineer smiles at the physicists gathered in Boston at the March 2012 meeting of the American Physical Society. She has to show them what she found in a toy shop.
Projected onto the screen, the Hoberman Twist-O looks like a hollow football made of garishly colored plastic links. Twist it just so, however, and hinges between the links allow it to collapse into a ball a fraction of its original size. Twist it the other way, and it springs back open. Bertoldi explains that the Twist-O inspired her group to create a spherical device that collapses and re-expands, not with hinges but through mechanical instabilities: carefully designed weak spots that behave in a predictable way. Applications might include lightweight, self-assembling portable shelters or nanometer-scale drug-delivery capsules that would expand and release their cargo only after they had passed through the bloodstream and reached their target.
The challenge, Bertoldi says, is to figure out the exact instabilities a structure needs to achieve its desired behavior. She quickly describes the necessary geometry and runs down a list of constraints. There are just 25 shapes that satisfy all the requirements, she explains, glossing over the months of computation it took to solve the problem. Then she starts a video to show the assembled throng the design that her team has come up with.
An image of a rubbery chartreuse ball with 24 carefully spaced round dimples (pictured) materializes on the screen. The test begins and the ball slowly collapses, each dimple squeezing shut as the structure twists into a smaller version of itself. There is a moment of silence, then everyone in the room begins to clap.
Student engineers have always been taught that mechanical instabilities are a problem to avoid. Such instabilities can quickly lead to structural failures — the collapse of a weight-bearing pillar, the crumpling of a flat steel plate or the buckling of a metal shell. From failures come disasters, such as the Second World War Liberty Ships that broke up while at sea. And the devilishly complex mathematical analysis of buckling structures ground to a halt in the late nineteenth century, because it was unworkable with the methods then available.
During the past half-decade or so, however, a new generation of physicists and engineers has begun to embrace instability. These researchers have been inspired, in part, by advances in geometry and nonlinear mathematics that have allowed them to progress where their forebears could not. They have already, for example, devised a theory for why cabbage leaves and torn plastic rubbish bags ripple; calculated the patterns of wrinkles in fabric and crumples in paper; and accounted for the way coils and loops develop in the guts of vertebrate embryos.
On the practical side, one source of inspiration has been the widespread availability of flexible polymers and silicone materials, as epitomized by the vast selection of soft yet tough covers for smart phones. Such materials make it possible to imagine electronics, robots, tools and vehicles whose structures can radically deform yet still recover their original shapes.
The resulting extreme-mechanics movement has grown rapidly. The first three conference sessions to bear the name, totaling fewer than 40 presentations, were held during the March 2010 meeting of the American Physical Society (APS). Just two years later, Bertoldi's talk on collapsible spheres was one of 111 presentations on extreme mechanics spread across 8 sessions. Hundreds of researchers are now active in the field worldwide. In spring 2011, the US National Science Foundation announced an opportunity for substantial funding in the field: it would allot up to US$2 million over four years to projects in Origami Design for Integration of Self-assembling Systems for Engineering Innovation (ODISSEI). The foundation expects to announce the awards this month.