The Hoberman Twist-O is a commercial toy which comprises a rigid network of struts connected by rotating hinges and can easily collapse into a ball having a fraction of its original size. It inspired us, in collaboration with Pedro Reis (MIT), to design a spherical device, the Buckliball, that collapses and re-expands, not with hinges but through mechanical instabilities, opening avenues for a new class of active encapsulation systems. The geometry of the Buckliball comprises a spherical shell patterned with a regular array of circular voids.  When a syringe extracts the air out of the Buckliball, at a critical point the narrow ligaments between the voids suddenly buckle, leading to a cooperative buckling cascade of the skeleton of the ball. This ligament buckling leads to closure of the voids and a reduction of the total volume of the shell by up to 54%, while remaining spherical, thereby opening the possibility of encapsulation. Our combined experimental, numerical, and theoretical approach allows us to rationalize the underlying mechanical ingredients and yielded a series of simple design guidelines, including a master curve, for buckling-induced encapsulation. Finally, because the folding mechanism that characterizes the Buckliball exploits a mechanical instability that is general, our study raises the possibility for reversible, tunable, and controllable encapsulation, over a wide range of length scales.   The Buckliball provides the first example of a non-planar structure where buckling and soft materials in novel structural layouts are successfully combined to design an active device. We believe that many mechanical systems based on the same principles will follow, with exciting applications over a wide range of length scales.

 Sequence of progressively deformed shapes. (A) Hobermans Twist-o, a commercial toy, compressed by hand. (B) Buckliball, made of siliconebased rubber, pressurized by a motorized syringe pump.

Publications:

• J. Shim*, C. Perdigou, E.R. Chen, K. Bertoldi^$* and P.M. Reis^$* ($ corresponding author, * equal contribution). Buckling-induced encapsulation of structured elastic shells under pressure. Proceedings of the National Academy of Sciences of the United States of America, 109: 5978-5983, 2012.

Press coverage:

• K. Krieger. Extreme mechanics: Buckling down. Nature, 488: 146, 2012.