Four-Dimensional Multistep Vertical Printing for Hygroresponsive Shape Change with Nonplanar Rest-State Geometries.

Four-dimensional printing (4DP) via fused deposition modeling has been used to create hygromorphic biocomposite actuators through wood polymer composite (WPC) filaments. The shape-change transformation of the 4DP composite mechanism is preprogrammed by controlling the printing process parameters and the design of the print-path pattern. Until now, most 4DP approaches involving Wood Polymer Composite (WPCs) have focused on planar actuators featuring a bilayer structure composed of laminar layers with distinct material properties. These mechanisms show a laminar initial rest state, presenting as flat objects, and can only achieve a complex three-dimensional shape when subjected to the moisture variations stimulus. The presented research highlights the development of a multistage printing method that expands the capabilities of three-axis printers to enable the 4DP of mechanism with complex nonplanar rest-state geometries. The new technical capabilities of this method are demonstrated here through the creation and testing of novel nonlaminar 4DP mechanisms that harness their unique doubly curved rest-state geometry to achieve kinematic amplification. We expect that this approach can greatly improve the range and complexity of 4DP mechanisms that can be developed using the commonly available three-axis printers.