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 reststate 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.