Dutch researchers have demonstrated self-folding, biocompatible 3D structures
Thanks to the ancient Japanese art of origami, we all know that it’s possible to transform a single sheet of paper into a complex, three-dimensional structure, simply by folding it. So, its perhaps surprising that origami took so long to attract the attention of engineers. In recent decades, the art form has inspired the design of everything from stents and scaffolds, to shopping bags and solar sails. And now, a team of biomedical engineers from Delft University of Technology say it could be used to build a new generation of implantable devices and microrobots.
Writing in an upcoming issue of Materials Today, the researchers report on the use of origami, combined with a variation of it known as kirigami (which roughly translates to ‘cut paper’), to fabricate various shape-shifting 3D structures. These ranged from simple cubes to multi-storey polyhedral lattices, all made from polymer sheets and metal foils. Each of these structures are triggered to change shape through simple stretching. The authors say that their use of externally-applied tensile, rather than compressive, forces reduces the risk of buckling in the final 3D structure, making its behaviour easier to predict. This approach also lends itself to multi-step, sequential self-folding – an important step in fabricating complex structures.
Each of the flat surfaces were made using an elastic layer supported by a layer that exhibited permanent (plastic) deformation. When stretched, both layers elongate. When the force is released, the elastic layer attempts to contract, but the other, plastically-deformed layer opposes it. This mismatch produces a self-powered, out-of-plane deformation; a permanent fold that turns a flat sheet into a 3D object. The team relied on two forms of kirigami to make this possible – a series of cuts to form four rotating square elements, and parallel grooves that produced sharp corners. These could be combined to create multi-storey, self-folding structures from titanium and polyolefin polymers.
A key tool in this research was the use of finite element analysis – it simulated both the stretching and folding mechanisms. In all cases, the predicted behaviour was in excellent agreement with experimental observations, leading the authors to suggest that FEA “…could be used as a predictive tool for the rational design of complex assemblies of basic elements.” As a proof of concept, the researchers designed flexible copper connectors to successfully integrate a working micro-LED into their self-folding cubes. In addition, they say that is the first time that “…such self-folding 3D porous structures (have been) fabricated at this scale from biocompatible materials (e.g. titanium foils)”.
Taken together, these results point to a potentially new approach to fabricating smart, implantable medical devices. Starting with flat surfaces means that techniques like nanolithography can be used to pattern surfaces and embed new functions within them. And unlike some similar approaches, this one doesn’t rely on high-temperatures to activate the self-folding mechanism. It’ll be interesting to see how this develops.
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Teunis van Manen, Shahram Janbaz, Mahya Ganjian, and Amir A. Zadpoor. “Kirigami-enabled self-folding origami”, Materials Today 305 (2019). In press.
