Hydro-actuation of ice plant seed capsules powered by water uptake

Abstract

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.

Pressurized honeycombs as soft-actuators: a theoretical study

“…This was shown in a recent study on the hydro-actuated unfolding of the ice plant (Delosperma nakurense) seed capsule [18,19]. The seed capsule contains five seed chambers, separated by septa (figure 1).…”

“…Hence, the swollen CIL exerts a pressure on the cells' walls, giving rise to an overall shape change of the keel tissue ( figure 1b,d). The physics of pressure generation by CIL's swelling and its interplay with structures of the seed capsule to generate this reversible motion has been recently presented [19]. Still, little is known about the role of cell architecture and cell shape on the swellability of honeycombs.…”

Bioinspired composites – next generation of materials and devices

“…The first contribution on bioinspired material actuation is from Razghandi's group from Swiss Federal Institute of Technology (ETH Zürich), Zurich, Switzerland on the mechanism of liquid water-actuated seed release in ice plants 1 . The presented chemomechanical model attributes the opening mechanism primarily to an entropy gain of the system due to water uptake.…”

Programing stimuli-responsiveness of gelatin with electron beams: basic effects and development of a hydration-controlled biocompatible demonstrator

“…Thereby, polymeric materials, especially synthetic materials 1 , are already investigated in order to construct active elements such as actuating objects and self-expanding stents 2 , 3 . Furthermore, the actuatoric potential of biological systems found in nature were investigated in more detail 4 . However, bio-polymers, especially collagenous materials such as gelatin, have a similarly high potential for applications as active elements but are not yet well studied.…”