A team from the Georgia Institute of Technology and the Singapore University of Technology and Design has developed a 4D printing technology that allows for the production of complex self-folding structures based on components made from smart shape-memory materials that offer different responses to heat. Their study could help in the development of easily transported 3D structures that can sequentially fold themselves from components that had been flat or rolled into a tube, with potential applications in solar cells, space structures, biomedical devices and self-assembling robots.

The research, as featured in Scientific Reports [Mao et al. Sci. Rep. (2015) DOI: 10.1038/srep13616], used smart shape memory polymers (SMPs) that can remember a shape and then change to another programmed shape when uniform heat is applied. This ability comes from the printing of a number of materials that have different dynamic mechanical properties in prescribed patterns. On heating the components, each of the SMPs reacts at a different rate in change its shape – if these changes are specifically timed, 3D objects can be programmed to self-assemble. The components could also respond to stimuli including light, temperature and moisture.

Previous approaches to the problem had required differential heating at particular locations in the flat structure in order to stimulate the shape changes. However, as researcher Jerry Qi points out, they “exploited the ability of different materials to internally control their rate of shape change through their molecular design.” For instance, they were able to demonstrate a mechanism that could be switched from a flat strip into a locked configuration while one end bends controllably to thread through a keyhole, and also a flat sheet that can fold itself into a 3D box with interlocking flaps.

The work required the accurate manipulation of the folding sequence of different parts of the structure to prevent collisions among the components during folding. The avoidance of self-collisions is crucial, as if different parts of the folding structure come into contact they can block further folding. The metric used can predict any such collisions, and was combined with the reduced-order model to design self-folding structures that lock themselves into stable desired configurations.

The team expects there to be extensive applications for such technology, and are now looking to extend the concept of digital SMPs to allow for printing of SMPs with dynamic mechanical properties that can change continuously in 3D space.

"they exploited the ability of different materials to internally control their rate of shape change through their molecular design"Jerry Qi