Vertical Parallel Multi-Material 4D Printing (VPM-4DP): a novel strategy for achieving bio-inspired doubly curved non-planar hygroscopic actuators

Most 4D-printed hygromorphic mechanisms using fibre-polymer composite filaments are thin laminated bilayer composite structures, which are printed flat on the 3D printer bed and shape-change into single or double-curvature geometries in response to moisture uptake. The tendency towards thin laminated 2D-like bilayers might be traced to early research on shape-change laminates, such as bimetals, or to the vertical layering sequencing of most additive manufacturing methods. Recently, actuators with non-planar geometries have been achieved through complex rotational 3D printing, 5 or 6-axis printers, or multi-step repositioning of parts, but these methods require bespoke equipment or intensive post-processing, which greatly limits their application to the ubiquitous 3-axis gantry system. In this paper, we introduce a strategy for vertically printing non-planar shape-changing hygromorphic structures that requires only one print session and minimal post-processing. We demonstrate the novel capability of the presented strategy by creating actuators with double-curvature geometries based on the biological models of the Bauhinia variegata seed pod and the Lilium ‘Casa Blanca’ tepals. This novel approach reduces the complexity of manufacturing intricate 4D-printed actuators that have doubly curved geometry. The possibilities in 4D-printed shape parameters are expanded through this method, considering that dimensional curvature and form can now be more easily introduced to structures previously limited by planar printing.