Design of 4D Printed Large, Shape Transforming Auxetic Structures

3D printing of active materials, a.k.a. 4D printing, allows the manufacturing of structures that can undergo transformations in shape as a response to an external stimulus. In this study, we present a new class of 3D printed active metamaterials with architected complex microstructure. They exhibit negative Poisson’s ratios and are capable of large shape changes (up to 200% in area). We use a 3D printable shape memory polymer to actuate the metamaterials. This polymer can be deformed at high temperatures and fixed in the deformed shaped when cooled below a characteristic actuation temperature. When subjected to a heat stimulus, the polymer exhibits a shape memory effect and recovers the original shape.

The shape transformation of the active structures is simulated using finite element method. A thermo-viscoelastic material model is implemented, based on experimental characterization of the shape memory polymer. Simulation prediction is compared to experimentally measured shape memory cycles. For efficient simulation of the shape transformation of complex large structures, an efficient reduced beam modelling approach is developed.

The presented metamaterials serve as unit cells that can form highly complex active structures. This is illustrated by a structure shaped like the ETH logo composed of reentrant honeycomb unit cells. The structure is programmed into a small circular shape. Triggered by heat, the programmed shape transformation is initiated and the ETH lettering shape is recovered.