Actuated plant materials are a source of inspiration for the design of adaptive materials and structures that are responsive to specific external stimuli. Hydro-responsive, metabolismindependent plant movements are particularly fascinating, because the extracted concepts are more amenable to transfer into engineering than those dependent on cellular activity. Here we investigate the structural and compositional basis of a sophisticated plant movement mechanism-the hydration-dependent unfolding of ice plant seed capsules. This reversible origami-like folding pattern proceeds via a cooperative flexing-and-packing mechanism actuated by a swellable cellulose layer filling specialized plant cells. Swelling is translated into a bidirectional organ movement through simple geometric constraints embedded in the hierarchical architecture of the ice plant valves. Extracted principles from this reliable and reversible actuated movement have relevance to the emerging field of 'programmable matter' with applications as far-reaching as the design of satellites and artificial muscles.

Nanofibers were prepared by electrospinning from pure polyvinyl alcohol (PVA), polyhydroxybutyrate (PHB), and their blends. Miscibility and morphology of both polymers in the nanofiber blends were studied by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC), revealing that PVA and PHB were miscible with good compatibility. DSC also revealed suppression of crystallinity of PHB in the blend nanofibers with increasing proportion of PVA. The hydrolytic degradation of PHB was accelerated with increasing PVA fraction. Cell culture experiments with a human keratinocyte cell line (HaCaT) and dermal fibroblast on the electrospun PHB and PVA/PHB blend nanofibers showed maximum adhesion and proliferation on pure PHB. However, the addition of 5 wt % PVA to PHB inhibited growth of HaCaT cells but not of fibroblasts. On the contrary, adhesion and proliferation of HaCaT cells were promoted on PVA/PHB (50/50) fibers, which inhibited growth of fibroblasts.

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Unlike well-known plant hydro-actuation systems that respond to changes in relative humidity (RH) (e.g. wheat awns), ice plant seed capsules undergo a reversible origami-like unfolding and release their seeds only in response to exposure to liquid water. The engine for ice plant actuation was found to be the water uptake and swelling of a highly swellable cellulosic inner layer (CIL) inside the cell lumen of a hygroscopic tissue responsible for the unfolding movement. CIL was found to have an open structure with porous lamellae filling the gap between denser cellulosic mats. Thermogravimetric analysis of water–CIL interaction showed that the initial enthalpy-driven adsorption of water can only account for increasing the moisture content up to about 0·4 mg/mg, which is not sufficient to initiate the actuation. By applying a combined chemo-mechanical model, we could show that the entropic gain of the system through further water uptake (40–350 wt%) is sufficient to accomplish a full opening of the seed capsules through a sophisticated design at various hierarchical levels of the system. The principles behind this actuation mechanism may inspire the development of hydro-responsive devices that, although being highly hydrophilic, only respond to liquid water and not to changes in RH.