I remember learning about Microsoft PowerPoint for the first time when I was in junior high, and since then I have used the program often; little did I know that a humble PowerPoint slide would someday be used to help make self-folding 3D origami structures out of photocurable liquid polymers. Researchers from the Georgia Institute of Technology (Georgia Tech) and Peking University developed the new technique, which also makes good use of an LED projector.
This isn’t the first time we’ve seen origami and 3D printing combine: last year, the two techniques came together to help produce self-folding medical implants. But the research team believes that this could be the first time that controlled volume shrinkage during patterned photopolymerization has been applied to create self-folding 3D origami structures.
The researchers from Peking University and Georgia Tech published a paper on their research, titled “Origami by frontal photopolymerization,” in the journal Science Advances; co-authors include Haosen Chen, Daining Fang, Xiaoming Mu, H. Jerry Qi, Jiangtao Wu, and Zeang Zhao. The research was supported by the National Science Foundation (NSF), the Chinese Scholarship Council, and the Air Force Office of Scientific Research (AFOSR).
The paper’s abstract reads: “Origami structures are of great interest in microelectronics, soft actuators, mechanical metamaterials, and biomedical devices. Current methods of fabricating origami structures still have several limitations, such as complex material systems or tedious processing steps. We present a simple approach for creating three-dimensional (3D) origami structures by the frontal photopolymerization method, which can be easily implemented by using a commercial projector. The concept of our method is based on the volume shrinkage during photopolymerization. By adding photoabsorbers into the polymer resin, an attenuated light field is created and leads to a nonuniform curing along the thickness direction. The layer directly exposed to light cures faster than the next layer; this nonuniform curing degree leads to nonuniform curing–induced volume shrinkage. This further introduces a nonuniform stress field, which drives the film to bend toward the newly formed side. The degree of bending can be controlled by adjusting the gray scale and the irradiation time, an easy approach for creating origami structures. The behavior is examined both experimentally and theoretically. Two methods are also proposed to create different types of 3D origami structures.”
The team’s technique projects a grayscale pattern of both dark and light shapes onto a thin layer, between two glass slides, of liquid acrylate polymer. For decades, scientists have been experimenting with hundreds of different polymer types, which react in special ways to light: some materials will harden, while others decompose when making contact with visible or ultraviolet light. In this new research, a solid film forms, about 200 microns thick, when a photoinitiator material that’s mixed into the polymer is struck by light from an LED projector for five to ten seconds, which causes a crosslinking reaction; the regulator for the projector light is a light-absorbing dye found in the polymer. The interaction between the photo curing volume shrinkage and the polymer network evolution is fairly complicated, so any areas of the polymer which get less light will show “more apparent bending behavior.”
“The basic idea of our method is to utilize the volume shrinkage phenomenon during photo-polymerization. During a specific type of photopolymerization, frontal photopolymerization, the liquid resin is cured continuously from the side under light irradiation toward the inner side. This creates a non-uniform stress field that drives the film to bend along the direction of light path,” Qi, a professor in the Woodruff School of Mechanical Engineering at Georgia Tech, explained.
“The areas that receive light become solid; the other parts of the pattern remain liquid, and the structure can then be removed from the liquid polymer. The technique is very simple.”
The self-folding begins when the film is removed from the original liquid polymer: the differential shrinkage creates stress in the film, which starts the folding process. The researchers learned that by shining light on each side, they can create more complex origami structures, about half an inch in size, like capsules, flowers, tiny tables and birds, and a traditional miura-ori fold. To fabricate shapes that bend in both directions, the patterned film is simply flipped over, in order to create crosslinking on the second side as well as the first.
“We have developed two types of fabrication processes. In the first one, you can just shine the light pattern towards a layer of liquid resin, and then you will get the origami structure,” said Zhao, a PhD student at both Peking University and Georgia Tech. “In the second one, you may need to flip the layer and shine a second pattern. This second process gives you much wider design freedom.”
What’s really interesting is that the process which creates the differential shrinkage that begins the folding process is actually, in different uses of the polymer, considered to be harmful.
Fang, a professor at Peking University during the research project who now works at the Beijing Institute of Technology, explained, “Volume shrinkage of polymer was always assumed to be detrimental in the fabrication of composites and in the conventional 3-D printing technology. Our work shows that with a change of perspective, this phenomenon can become quite useful.”
When a polymer film is continuously cured from one side in a thick liquid resin layer, the process is called frontal photopolymerization. Once it’s hit with light, the solidification front begins at the surface and, as the irradition time increases, grows toward the liquid side. This method, which has been used to synthesize microparticles and fabricate microfluidic devices, is able to be adjusted, simply by controlling the intensity of the light and the illumination time. While this technique should work with any number of photocurable polymers and dye colors, the researchers used poly(ethylene glycol) diacrylate and orange dye.
Zhao hand-crafted a PowerPoint pattern for the proof-of-principle; the system could also be connected to a CAD tool in order to scale the process up and create more precise patterns. The self-folding structures could potentially have applications in biomedical devices, soft robotics, and mechanical metamaterials, and Qi believes the team’s technique could fabricate structures up to an inch.
Qi said, “The self-folding requires relatively thin films which might not be possible in larger structures. We have developed a simple approach to fold a thin sheet of polymer into complicated three-dimensional origami structures. Our approach is not limited by specific materials, and the patterning is so simple that anybody with PowerPoint and a projector could do it.”
[Source: Georgia Tech]