Pine cones have given lots of inspiration to the design and fabrication of artificial actuators by virtue of the hygroscopic movement of the scales. It is generally accepted that the opening of pine cones is caused by a larger shrinkage of the bast tissues (sclereids) than of the vascular bundle (VB) tissues in the scale. However, previous research was only focused on exploring the bending mechanisms ignoring the bending process, where the pine cone opens only in a long-term dry environment for the long-distance dispersal of seeds from the parent tree by wind and animals. Therefore, the hygroscopic movement of pine cones are restudied and new phenomena, mechanism and physical model are revealed for artificial actuators with unperceivable motion.
Phenomenon & Discovery
- The hygroscopic deformation of the pine cone is an ultra-slow process.
It takes a rather long time of around 24 h for the pine cone to complete its typical geometric reshaping (Fig. 1a). As summarized in Fig. 1b, with the deformation velocity normalized by thickness, the scales of the pine cone give the lowest value among plant organisms capable of hygroscopic deformation, which is consistent with their function for long-distance seed dispersal.
- The VB itself exhibits hygroscopic deformation.
As has been reported, the scale is mainly composed of two tissues, inner VBs (assembled as the skeleton) and outer sclereid tissue (figuratively called ‘skin’ hereafter). The VB itself enables hygroscopic geometric reshaping with an even larger deformability and motion velocity than that of the scale (Fig. 1c, d), indicating their crucial role in driving the humidity-responsive motion. While both the skin and the whole scale have a much lower motion velocity than the skeleton and VBs. Similarly, the waterlogged scale and skin exhibit higher water contents and slower dehydration velocity than the skeleton and VBs.
Therefore, it can be concluded that the VBs drive the hygroscopic deformation of the scales and the sclereids (skin) with good water retention slow down the deformation.
The underlying mechanism of the VB deformation
- Heterostructure of spring microtubes and square microtubes
To explore the bending mechanism of the VBs, the microstructures of the VB and their hygroscopic expansion performances are investigated. As shown in the cross-sectional SEM image, the VB shows a typical heterostructure comprising two kinds of tube-like constituent cell walls with a clear boundary (Fig. 2a-d). The reconstructed three-dimensional (3D) XCT image (Fig. 2e) and longitudinal cross-sectional SEM images of the microtubes (Fig. 2f, g) further verified two kinds of microtubes, one with a spring-like structure and the other with a square-tube-like structure, in a parallel arrangement.
- Different hygroscopic expansion of spring microtubes and square microtubes
In situ observation of the hygroscopic motion of a side-by-side spring/square microtube couple from the VB was conducted using environmental SEM (ESEM) under controllable RH (Fig. 2h). With increasing RH, the spring microtube extends and thus the spring/square microtube couple bends towards the square microtube with the inflection angle α at a certain point decreasing to α′ (from the solid tangent line to the dashed tangent line, Fig. 3c). On the contrary, the microtube couple bends towards the spring microtube side as RH decreases with an increased inflection angle.
According to the above results, a simplified model of one-dimensional heterostructured spring/square microtubes was used to address the hygroscopic geometric reshaping of the VB (Fig. 2i). The model is merely heterogeneous in structure, independent of the chemical nature of the materials. With increasing RH, the spring microtube stretches and its coil pitch increases, while the square microtube with its dense structure shows only very limited change. To accommodate the difference in the length of the two microtubes, the couple bends towards the square microtube side. On the contrary, with decreasing RH, the couple bends towards the spring microtube side.
Pine cone-mimicked actuators with unperceivable motion
Based on the simplified model, a couple of one-dimensional heterostructured spring/square (○/□) pillars by 3D printing. The spring pillar is an elastomer spring embedded in a hygroscopic material and the square pillar is an elastomer square tube filled with a hygroscopic material (Fig. 3a, b). The filled hygroscopic material endows the pillars with the capability of hygroscopic expansion and also works as the skin in the scale to increase the water diffusion path and decrease the expansion velocity. The fabricated spring/square (○/□) pillars exhibit a similar hygroscopic deformation performance as the pine cones (Fig. 3c). By virtue of the editability and compatibility of such a one-dimensional heterostructured spring/square (○/□) pillar couple with the 3D-printing technique, various elaborate shape transformations can be realized in a controllable manner by merely regulating the structures (Fig. 3d). As a proof of concept, we fabricated a table with a tabletop that moves reversibly underwater and in air to transport an object on it, using spring/square (○/□) pillars as legs (Fig. 3e, f). The table exhibits a stable motion with no disturbance to the surrounding water (Fig. 3f), as verified by the simulated result (Fig. 3g). With the artificial pillar couples as the holder, detectors also can achieve a largely increased monitoring scope with unperceivable motion (Fig. 3h).
This study offers deep insight into understanding the hygroscopic deformation of plant tissues, but also into developing responsive actuators that enable ultraslow motions.
This work has been highlighted by Prof. Cecilia Laschi ( National University of Singapore) and Prof. Barbara Mazzolai ( Istituto Italiano di Tecnologia) with a view article titled ‘Move imperceptibly’ in Nature Materials.
For more details, please check out our paper “Unperceivable motion mimicking hygroscopic geometric reshaping of pine cones”.
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