Schematic of the self-folding approach and a cube structure formed in this way.
(a) Two types of basic elements were designed by combining permanently-deformed kirigami-based monolayers with an elastic layer to create bilayers (left). Stretching the bilayers results in self-folding elements that exhibit out-of-plane bending when the force is released (right). (b) The kirigami elements are made by laser cutting after which they are assembled with molded PDMS layers to create the basic elements.
Complex shape-shifting behavior for three different designs as predicted by finite element analysis models and observed in our experiments. Self-folding of a cube from a flat state (a); sequential shape-shifting from flat states to complex multi-storey shapes (b and c). All designs were realized using multi-stable (red) and plastically deforming (gray) kirigami elements.Inspired by the ancient Japanese art of origami, ‘self-folding’ flat materials that pop up into complex three-dimensional structures could prove useful for robotics, metamaterials, electronics, and biomaterials. To date, however, most approaches are limited to simple folding sequences, specific materials, and larger length scales.
Now researchers from Delft University of Technology in the Netherlands have come up with a simple alternative triggered mechanically that works with a wide range of materials, produces complex structures, and is suitable for microscale structures.
“We aimed to develop a technique allowing the programming of complex shape shifting in wide range of materials at smaller scales such that initially flat materials can be self-folded into multi-story objects of interest for many applications,” explains Teunis van Manen, first author of the study.
The key to the new approach is a combination of an elastic layer and a permanently deformed layer, which can be made from different materials including polymers and metals. Typically, the elastic layer is polydimethylsiloxane (PDMS) and the deformed layer is a kirigami-patterned thin film metal (e.g. titanium) or polymer (e.g. polyolefin). When the combined material is stretched both layers elongate but upon release the elastic layer tries to return to its original shape while the deformed layer does not. The conflict between two materials forces the flat layers to fold out into three-dimensional structures with angles up to 100 degrees.
The team used the approach to create a range of multi-story components such as cubes and more complex structures by combining arrays of the basic elements with stiff panels. Complicated architectures can be snapped into place by stretching the flat starting layers first in one direction and then in the orthogonal direction.
“One of the main advantages of such a self-folding approach compared with direct manufacturing of porous three-dimensional geometries (e.g. using 3D printing) is that we have full access to the flat surface of the material prior to folding,” points out van Manen.
This opens up the surface to all types of functionalization, two of which the researchers demonstrate – micropatterns and flexible electronics. In the latter case, by adding a micro-LED and coil-like copper connectors to the flat design, the researchers created a self-folding box with a light inside.
“Our self-folding technique is novel [because] there is no need for exotic stimuli-responsive materials, such as shape-memory polymers or hydrogels,” says van Manen. “Our approach means is also highly scalable, as mechanical forces work similarly across a wide range of length scales.”
He believes this approach could be particularly useful in the production of porous bioscaffolds for tissue regeneration, but also for other medical devices, micro-robotics, and smart implantable devices for monitoring or on-demand drug delivery.
Van Manen et al., Kirigami-enabled self-folding origami. Materials Today (2019), https://doi.org/10.1016/j.mattod.2019.08.001