Recently, cooperating with Prof. H. Jerry Qi’s group in Georgia Institute of Technology (US), Prof. Daining Fang’s group in the College of Engineering made progress in the self-folding origami of polymers. The work was published online in Science Advances on April 28 (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.).
Figure 1. Self-folding origami structures fabricated by frontal photopolymerization
During the past few years, self-folding origami, which refers to the deformation of a thin sheet from functional materials or specific stimulus, has attracted great interests from several fields. Because of its superior mechanical property and wide structural adaptability, self-folding origami structures show great potentials in wearable electronics, soft robots, biomedical engineering as well as mechanical metamaterials. However, current methods to fabricate self-folding origamis often rely on complex structure design, special materials systems or tedious fabrication process, and the real application of origami structure is quite limited.
In their research, Prof. Fang’s group found that without introducing special materials or extra stimulus, self-folding could be realized just by utilizing the volume shrinkage phenomenon during photopolymerization. The reaction-induced volume shrinkage causes shape distortion, and it is always assumed detrimental in the fabrication of composites and conventional 3D printings. The research of Prof. Fang’s group started from a different aspect. During frontal photopolymerization, polymer film is cured continuously from the side under illumination towards the other side, with the thickness increasing. Because of the shrinkage behavior, nonuniform residual stress field is created inside the material. Material cured at the very first is in compression state, while material cured afterwards is in tensile state, and this drives the film to bend along the light path.
Based on experiments and theoretical investigations, the self-bending behavior could be quantitatively controlled by changing the light intensity and illumination time during frontal photopolymerization. Inspired by this result, light pattern with nonuniform intensities created from grayscale image was projected to a layer of liquid resin. Polymer under high intensity remained flat while polymer under low intensity showed significant bending deformation, and a complex 3D origami structure was fabricated in this way. Interesting origami shapes the researchers created include capsules, flowers and cranes. This novel method opened a new direction in the research of self-folding, that is to regulate the intrinsic internal stress of materials. Prof. Fang’s group explored several interesting directions of self-folding origami, for example the reversible origami triggered by solutions (Zhao, Z., Wu, J., Mu, X., Chen, H., Qi, H.J. and Fang, D., 2016. Desolvation Induced Origami of Photocurable Polymers by Digit Light Processing. Macromolecular Rapid Communications.).
Figure 2. (A) Schematic figure of the self-folding from frontal photopolymerization. (B) Experiments and theoretical predictions of self-folding from frontal photopolymerization. (C) Development of residual stress in photopolymerized polymer films.
The first author of the above papers is Ze’ang Zhao, a PhD candidate in the College of Engineering. The corresponding authors are Prof. Daining Fang in the College of Engineering and Prof. H. Jerry Qi in Georgia Institute of Technology. The research was supported by the Chinese Scholarship Council and the National Science Foundation of the United States.
Links to the papers: