3D printing technology is often combined with the principles of origami to fabricate things like face masks, robots, and self-folding electronics. Notably, researchers at Delft University of Technology (TU Delft) in the Netherlands used origami and 3D printing to develop self-folding medical implants last year, furthering their research efforts with a paper published last month on complex shape-shifting objects that fold according to a pre-planned sequence. Now TU Delft researchers are returning to the Japanese art of paper folding with a new research topic – one that mostly leaves out 3D printing.

They are actually working to develop an alternative to 3D printing technology that will give the final products far more functionalities than those produced with 3D printing, which could be used in devices with flexible electronics or medical implants.

The research team has managed to successfully apply its new technique to lattice structures, which have huge internal surface areas and are used to design metamaterials so they can achieve unusual biological, mechanical, and physical properties. They discuss the results in a paper, titled “Origami lattices with free-form surface ornaments,’ published in Science Advances; co-authors include Shahram Janbaz, Niels Noordzij, Dwisetya S. Widyaratih, Cornelis W. Hagen, Lidy E. Fratila-Apachitei, and Amir A. Zadpoor, a TU Delft professor with the Additive Manufacturing Laboratory in the Department of Biomechanical Engineering.

The abstract reads, “Lattice structures are used in the design of metamaterials to achieve unusual physical, mechanical, or biological properties. The properties of such metamaterials result from the topology of the lattice structures, which are usually three-dimensionally (3D) printed. To incorporate advanced functionalities into metamaterials, the surface of the lattice structures may need to be ornamented with functionality-inducing features, such as nanopatterns or electronic devices. Given our limited access to the internal surfaces of lattice structures, free-form ornamentation is currently impossible. We present lattice structures that are folded from initially flat states and show that they could bear arbitrarily complex surface ornaments at different scales. We identify three categories of space-filling polyhedra as the basic unit cells of the cellular structures and, for each of those, propose a folding pattern. We also demonstrate “sequential self-folding” of flat constructs to 3D lattices. Furthermore, we folded auxetic mechanical metamaterials from flat sheets and measured the deformation-driven change in their negative Poisson’s ratio. Finally, we show how free-form 3D ornaments could be applied on the surface of flat sheets with nanometer resolution. Together, these folding patterns and experimental techniques present a unique platform for the fabrication of metamaterials with unprecedented combination of physical properties and surface-driven functionalities.”

Self-folding of origami lattices.

Metamaterials get their varied properties, such as impact resistance, fluid-like qualities in solids, and light but ultra-stiff makeup, from the complex geometry of their lattice structures, not from their base materials’ properties. Typically, lattice structures can be made only with 3D printing, which, according to the researchers, limits their functionalities. But the TU Delft research team has, for the first time, managed to fold complex lattice structures from flat sheets.

The team knew that in order to give metamaterials advanced functionalities, the lattice structure surfaces would need to have special features (ornaments) incorporated, like electronic devices and surface nano-patterns. Starting from a flat shape, rather than a three-dimensional one, would allow these functionalities, which can typically only be applied to flat shapes, to be incorporated in this situation. The researchers were able to add free-form patterning on surfaces by using advanced micro- and nano-patterning techniques, like electron beam nanolithography.

Professor Zadpoor said, “However, they generally work only on flat surfaces. Moreover, our access to the internal surface areas of 3D-printed lattice structures is very limited.”

Folding kinematics.

Folding of the flat sheet, which forms the complex 3D structures, occurs when the folding mechanism is activated by a stimulus like temperature change.

“Combining free-form surface ornaments with lattice forms seemed therefore impossible. But, inspired by the Japanese art of paper folding (origami), we have found a way that does allow for that combination. We have proposed the unusual approach of ‘folding’ lattice structures from initially flat states. That approach provides us with full access to the entire surface of what will eventually become our lattice structure. We could then use the currently available techniques to ornament the surface. We have categorized lattices into three basic categories and, for each of those, have proposed a folding strategy. Self-folding mechanisms have been also incorporated into the flat material to allow for self-folding into the final lattice shape,” explained Professor Zadpoor.

“We show how free-form 3D ornaments could be applied on the surface of flat sheets with a resolution of a few nanometers.”

The TU Delft lattice structures could even bear, as the researchers put it, “arbitrarily complex surface ornaments at different scales.”

The team used an Ultimaker 2+ 3D printer to make polymeric models of the self-folding lattice structures out of PLA, while the metallic lattices were folded from a laser-cut aluminum sheet. There are several applications where the team’s approach could be used to make metamaterials with advanced functionalities, such as integrating flexible electronics into the design or developing meta-biomaterials which could stimulate tissue regeneration.

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[Source/Images: TU Delft]


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