Scientists at the University of Massachusetts Amherst have appropriated a less-common technique of origami known as "collapse"-type, in which all folds are carried out more or less simultaneously, to create complex reversibly self-folded 3D structures around a millimeter in size. The new technique is expected to have applications in soft robotics, mechanical metamaterials, and biomimetic systems (synthetic systems that mimic systems from nature).

Origami long ago found its way into creative scientific practice, with recent examples including a self-folding robot that scuttles away under its own power and a solar array that unfolds to 10 times its launch size once deployed in outer space. But the UMass Amherst researchers note that these fall far short in complexity compared to what can be folded by hand using paper. The new technique promises a leap forward by enabling an approach that's extremely difficult in traditional origami yet comparatively straightforward for programmable matter.

"Collapse-type origami designs have not been thoroughly explored in the past because of the difficulty of actuating tens or hundreds of folds with human hands," explains lead author Ryan Hayward. "Our technique removes this restriction and we expect that with the actuation scalability provided by our technique, vastly more complex collapsible structures may now be readily explored."

It works by sandwiching a thin layer of a temperature-responsive hydrogel between two patterned films made of a rigid plastic. The patterns, which dictate where the "folds" occur, are created by a maskless lithographic technique that's based on a digital micromirror device (a kind of projector used in digital light processing). Traditional photolithography would suffice if they wanted just one material property, but the researchers wanted to easily pattern multiple layers of polymers with widely contrasting material properties – stiff in one place, flexible in another, with variations tuned on a microscopic scale.

This is important for producing sophisticated structures for deployment in fields such as bioengineering and biomedicine, where the new approach could lead to advanced implantable medical devices that self-deploy once inside the body – unfolding, essentially, when they get into position.

Future research in the area will explore ways to improve the technique, with smaller structures, thinner films, tighter curvatures, and better lithographic methods. The researchers expect that the study will not only find real-world applications in fields such as biomedicine, mechanical metamaterials and microrobotics, but also in the theoretical world where they and others are grappling with questions about the mechanics of self-folded structures.

A paper describing the study was published in the journal Advanced Materials.