Through their anisotropic cellular mesostructure and differential swelling and shrinking properties, hygroscopic plant structures move in response to changes in the environment without consuming metabolic energy. When the movement is choreographed in sequential time steps, either in individual structures or with a coordinated interplay of various structural elements, complex functionalities such as dispersal and protection of seeds are achieved. Inspired by the multi-phase motion in plant structures, this paper presents a method to physically program the timescale and the sequences of shape-change in 4D-printed hygromorphic structures. Using the FDM 3D-printing method, we have developed multi-layered, multi-material functional bilayers that combine highly hygroscopic active layers (printed with hygroscopic bio-composite materials) with hydrophobic restrictive and blocking layers (printed with PLA and TPC materials). The timescale of motion is programmed through the design of the mesostructured layers and 3D-printing process parameters, including thickness (number of printed active layers), porosity (filling ratio of the active layer), and water permeability (filling ratio of the blocking layer). Through a series of experiments, it is shown that the timescale of motion can be extended by increasing the thickness of the active layer, decreasing the porosity of the active layer, or increasing the filling ratio of the hydrophobic restrictive and blocking layers. Similarly, a lower thickness of the active layer and lower filling ratio of all layers result in a faster motion. As a proof of concept, we demonstrate several prototypes that exhibit sequential motion, including an aperture with overlapping elements where each completes its movement sequentially to avoid collision, and a self-locking mechanism where defined areas of the structure are choreographed to achieve a multi-step self-shaping and locking function. The presented method extends the programmability and the functional capabilities of hygromorphic 4D-printing, allowing for novel applications across fields such as robotics, smart actuators, and adaptive architecture.

This paper presents a material programming approach for designing 4D-printed self-shaping material systems based on biological role models. Plants have inspired numerous adaptive systems that move without using any operating energy; however, these systems are typically designed and fabricated in the form of simplified bilayers. This work introduces computational design methods for 4D-printing bio-inspired behaviors with compounded mechanisms. To emulate the anisotropic arrangement of motile plant structures, material systems are tailored at the mesoscale using extrusion-based 3D-printing. The methodology is demonstrated by transferring the principle of force generation by a twining plant (Dioscorea bulbifera) to the application of a self-tightening splint. Through the tensioning of its stem helix, D. bulbifera exhibits a squeezing force on its support to provide stability against gravity. The functional strategies of D. bulbifera are abstracted and translated to customized 4D-printed material systems. The squeezing forces of these bio-inspired motion mechanisms are then evaluated. Finally, the function of self-tightening is prototyped in a wrist-forearm splint-a common orthotic device for alignment. The presented approach enables the transfer of novel and expanded biomimetic design strategies to 4D-printed motion mechanisms, further opening the design space to new types of adaptive creations for wearable assistive technologies and beyond.

(1) Significance of geometry for bio-inspired hygroscopically actuated bilayer structures is well studied and can be used to fine-tune curvatures in many existent material systems. We developed a material design space to find new material combinations that takes into account unequal effective widths of the layers, as commonly used in fused filament fabrication, and deflections under self-weight. (2) For this purpose, we adapted Timoshenko’s model for the curvature of bilayer strips and used an established hygromorphic 4D-printed bilayer system to validate its ability to predict curvatures in various experiments. (3) The combination of curvature evaluation with simple, linear beam deflection calculations leads to an analytical solution space to study influences of Young’s moduli, swelling strains and densities on deflection under self-weight and curvature under hygroscopic swelling. It shows that the choice of the ratio of Young’s moduli can be crucial for achieving a solution that is stable against production errors. (4) Under the assumption of linear material behavior, the presented development of a material design space allows selection or design of a suited material combination for application-specific, bio-inspired bilayer systems with unequal layer widths.

Hygromorphic smart structures are advantageous as passively actuated systems for generating movement, with applications ranging from weather-responsive architectural building skins to adaptive wearables and microrobotics. Four-dimensional (4D) printing is a valuable method for multiscale fabrication and physical programming of such structures. However, material limitations in terms of printability, responsiveness, and mechanical properties are major bottlenecks in achieving reliable and repeatable humidity-responsive actuation. We propose a codesign method for 4D printing hygromorphic structures through fused filament fabrication, incorporating parallel development of (1) biobased cellulose-filled filaments with varying stiffness and hygroresponsiveness, and (2) designed mesoscale structuring in printed elements. We first describe the design of a pallet of filaments produced by compounding cellulose powder in mass ratios of 0–30% within two matrix polymers with high and low stiffness. We then present the design, fabrication, and testing of a series of 4D-printed prototypes tuned to change shape, that is, open and close, in response to relative humidity (RH). The structures can fully transform in conditions of 35–90% RH, which corresponds to naturally occurring shifts in RH in daily and seasonal weather cycles. Furthermore, their motion is fast (within the range of minutes), fully reversible, and repeatable in numerous cycles. These results open new opportunities for the utilization of 4D printing and natural resources for the development of functional humidity-responsive smart structures.

Because plants are considered immobile, they remain underrepresented as concept generators for soft robots and soft machines. However, plants show a great variety of movements exclusively based on elastic deformation of regions within their moving organs. The absence of gliding parts, as found in the joints of vertebrates and insects, prevents stress concentration and attrition. Since plants have no central control unit (brain), stimulus-sensing, decision-making and reaction usually take place noncentrally in the hierarchically structured materials systems of the moving organs, in what can be regarded as an example of physical intelligence. These characteristics make plants interesting models for a new group of soft robots and soft machines that differ fundamentally from those inspired by animals. The potential of such plant-inspired soft robots and machines is shown in six examples and is illustrated by examples applied in architecture and medicine. Graphical abstract