Advanced morphing material able to take on any shape

“Today’s shapeshifting materials and structures can only transition between a few stable configurations, but we have shown how to create structural materials that have an arbitrary range of shape-morphing capabilities,” said Professor Lakshminarayanan Mahadeva, a renowned expert in the organisation of matter. “These structures allow for independent control of the geometry and mechanics, laying the foundation for engineering functional shapes using a new type of morphable unit cell.”

A major challenge in designing shape-morphing materials is balancing the seemingly contradictory needs of conformability and rigidity. Conformability enables transformation to new shapes, but going too far in this direction compromises the material’s ability to maintain its shape. Rigidity helps lock the material into place, but a very rigid material cannot take on new shapes.

The researchers, from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), began with a “neutrally stable” unit cell with two rigid elements – a strut and lever – and two stretchy springs.

In general, neutrally stable systems balance the energy of unit cells with a combination of rigid and elastic elements. The famous bouncing lamp seen at the beginning of Pixar films is a good example of a neutrally stable material; the lamp head is stable in any position, because its weight is always counteracted by springs that stretch and compress in a coordinated way to balance it in any configuration. This quality allows for transition between an infinite number of positions and orientations, maintaining stability throughout.

“By having a neutrally stable unit cell we can separate the geometry of the material from its mechanical response at both the individual and collective level,” said Dr Gaurav Chaudhary, co-first author of the study. “The geometry of the unit cell can be varied by changing both its overall size as well as the length of the single movable strut, while its elastic response can be changed by varying either the stiffness of the springs within the structure or the length of the struts and links.”

The researchers describe the unique assembly as “totimorphic materials” or “totimorphs”, thanks to its ability to transition into any stable shape.

They connected individual totimorphic cells with naturally stable joints to build 2D and 3D structures. Using mathematical modelling and practical experimentation, they demonstrated the material’s shapeshifting ability. A single sheet of connected totimorphic cells can be twisted into all sorts of shapes – including the shapes of faces – and even bear weight.

“We show that we can assemble these elements into structures that can take on any shape with heterogeneous mechanical responses,” said Dr S. Ganga Prasath, another co-first author. “Since these materials are grounded in geometry, they could be scaled down to be used as sensors in robotics or biotechnology or could be scaled up to be used at the architectural scale.”