Written by: Ritu Raman
Original Article: Zhao et al. 2017
The Gist of It:
At some point in your life (maybe right after your class in school read “Sadako and the Thousand Paper Cranes”), you’ve probably experimented with origami. This ancient art of paper folding has recently garnered a lot of interest in the engineering community, because it’s a really well-established way of turning a simple 2D material into a complex 3D structure. This could potentially have a lot of applications, ranging from the design of futuristic spacecraft to tiny implantable medical devices. A recent paper, published in the open-access journal Science Advances, demonstrated a really elegant approach for manufacturing millimeter-scale origami structures with complex 3D shapes. The technique relies on a phenomenon known as “photopolymerization”, using light to turn a liquid material into a solid material. The transition from liquid to solid causes the material to shrink in total volume. In this study, the team projected light onto a vat of liquid, and the surface of the liquid closest to the light turned solid first. As time went on, the liquid deeper in the vat started turning solid as well. We can think of this as two layers, where layer 1 turns solid first and layer 2 turns solid next. When layer 2 starts to shrink, it pulls against layer 1, causing the whole two-layer solid to bend inward (see figure below). This shrinkage-induced bending can be used as “hinges” in a flat 2D pattern to turn it into a 3D shape. The team was able to use this simple technique to turn flat sheets into cubes, flowers, and even cranes! The method presented in this paper could be used by other engineers in future to build complex 3D devices for a variety of real-world applications. I’m really excited to see the problems they solve!
The Nitty Gritty:
Zhao et al. formulated a photo-sensitive liquid resin by mixing the biocompatible hydrogel polymer poly (ethylene glycol) diacrylate (PEGDA) with the photoinitiator phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide (Irgacure 819). A photoabsorber, Sudan I, was used to regulate the depth of penetration of light into the liquid resin. This resin, similar to that used in a variety of papers on stereolithographic 3D printing, was then irradiated with an LED projecting ultraviolet light in the wavelength range at which Irgacure 819 is active. The team investigated the effect of light irradiation for different lengths of time on overall thickness of the polymerized material, as well as the resultant shrinkage-induced bending of the material. They conducted theoretical calculations to form a mathematical model for the internal stresses in the material as a resultant of light exposure, and matched their experimental data with great accuracy. With this predictive model, they were able to very precisely predict the folding behavior of printed structures, thus generating complex 3D shapes from 2D printed layers.
Original Research Article: Zhao, Z., Wu, J., Mu, X., Chen, H., Qi, H.J. and Fang, D., 2017. Origami by frontal photopolymerization. Science Advances, 3(4), p.e1602326.