A flapping bird and an octahedron-tetrahedron truss were made out of trilayers of polymers. Credit: UMass Amherst
A flapping bird and an octahedron-tetrahedron truss were made out of trilayers of polymers. Credit: UMass Amherst

A route to polymeric origami structures that spontaneously fold and unfold, in response to external stimuli, has been reported by US scientists. These small-scale 3D structures have a plethora of potential uses including in tiny robots and biomedical devices such as stents.

Taking inspiration from the Japanese art of paper-folding, over the past 15 years a growing number of polymeric, metallic and semi-conducting 3D structures have been made by the self-folding of 2D sheets. Most of the self-folded structures to date don’t match the complexity of the simple traditional paper origami pieces folded by hand, let alone the remarkable designs that are possible using modern algorithms and software, explains the lead researcher Ryan Hayward at the University of Massachusetts Amherst.

Hayward’s team developed a simple and fast approach to reversible self-folding polymeric 3D structures: UV photolithographic patterning of trilayers of cross-linkable polymers. A flapping bird and an octahedron-tetrahedron truss, both highly-complicated designs, were amongst the structures made using this method. This work is published in Advanced Materials [Na J.-K. et al, Adv. Mater. (2014) doi: 10.1002/adma.201403510].

The team’s trilayers are comprised of a soft hydrogel layer sandwiched between two very thin, rigid plastic sheets. The top and bottom layers contain photo-crosslinkable polymers, that crosslink when exposed to UV light (preventing them from dissolving in organic solvent). The design is ‘drawn’ onto these layers using a UV light beam: the areas not required to fold are crosslinked, and the fold lines are not (meaning these can be washed away using organic solvent).

To trigger the self-folding, the trilayer is placed in water. “The middle hydrogel layer soaks up water and expands in volume, while the top and bottom plastic layers do not,” explains Hayward.  Where there are no fold lines, the hydrogel expands in thickness without causing bending.  “However, wherever there is a gap in the top plastic layer, the difference in expansion between the hydrogel and the bottom plastic layer causes the sheet to bend into a so-called ‘mountain’ fold.  Likewise, wherever there is a gap in the bottom plastic layer, the sheet bends in the opposite direction to make a ‘valley’ fold. By changing the width of the gap, we can control the bend angle: narrow gaps only give rise to a small amount of bending, while wider gaps allow for bending by up to 180 degrees.” The hydrogel layer is temperature responsive, so that upon heating it will shrink and the sheet will become 2D again.

The reversibility is a particular highlight of this approach, explains Hayward.  “If a biomedical device were delivered into the body in a tightly folded form and deployed into its functional shape by partially unfolding, reversibility would allow it to be refolded later if it needed to be removed. In the case of a micro-scale robot that needed to change shape multiple times to accomplish its task, reversible actuation of folds could be very valuable.” 

The structures the team have made so far range from 0.1 to 1mm wide. “With a few improvements in our patterning methods, it should be possible to reduce these sizes to 10, or even 1μm,” says Hayward. “In addition to making smaller structures, we would like to understand how fast these structures can fold and unfold themselves, and to introduce multiple different responsive materials that would allow for folding of a single sheet into several different 3D shapes.”