Reversible actuation of the bilayer-cell upon wetting-drying cycles.

<p>Opening and closing of a bilayer-cell upon wetting and drying at room temperature (50%RH) is plotted as a function of time.</p

Ice plant hydro-actuation mechanism.

<p>Top: (A) The seed capsules of the ice plant, <i>Delosperma nakurense</i> are shown in dry (closed) and wet (open) state. The five seed containing compartments are closed and covered by a protective valve (each measuring ca. 3mm in length) in the dry state and open upon wetting. (B) Two hygroscopic keels halves responsible for unfolding/folding of the seed capsule; the keels are bent inward in the dry state and bend outward upon hydration. (C) Each keel consists of a network of ellipsoid/hexagonal shape cells (confocal microscopy images of the transverse cell cross-section -field diameter: ca. 0.4 mm). (D) A highly swellable cellulosic inner layer filling up the lumen of the cells is responsible for the unidirectional opening of the cells and the reversible anisotropic expansion/contraction of the cell-network upon wetting/drying cycles (FCA stained cells cross section with lignified cell wall stained in red and the cellulosic inner layer in blue- field diameter: ca. 100 μm). (E) Cryo-SEM micrograph of the cellulosic inner layer (field diameter: ca. 20μm). (redrawn after [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0163506#pone.0163506.ref008" target="_blank">8</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0163506#pone.0163506.ref020" target="_blank">20</a>]). Bottom: Abstraction of the actuation principles from lower to higher lengthscale; (5) volume change of a highly swellable material inside a circular confinement induces an isotropic volume change of the cell; (4) Tailoring the geometry of such cell enables an anisotropic deformation upon swelling/shrinkage cycles; (3) Through periodic arrangement of the cells, the cooperative anisotropic deformation of individual cells results in a unidirectional expansion/contraction of the cellular structure at a larger length scale,(2,1) which can be translated into bending of the whole honeycomb structure when the deformation is restricted at one side (re-sketched after [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0163506#pone.0163506.ref020" target="_blank">20</a>]).</p

Structure and deformation of the bilayer-cell upon actuation.

<p>(a) Spatial arrangement of spruce (active) and paper (passive) layers in the bilayer-cell, with the corresponding fibers’ orientation within each layer. (b) Dimensional changes of the bilayer-cells upon exposure from initial 50% RH to a 95% RH, with the corresponding sequential images of the hydro-actuated movement (t = cell width, l = cell length).</p

Input parameters for FE simulation of bilayer-cell actuation.

<p>Input parameters for FE simulation of bilayer-cell actuation.</p

Equilibrium curvature of the actuated bilayer and bilayer-cell upon exposure to 95% relative humidity and wetting with liquid water.

<p>Equilibrium curvature of the actuated bilayer and bilayer-cell upon exposure to 95% relative humidity and wetting with liquid water.</p

Influence of walls (G_W) and inclusions (G_I) rigidity on the honeycomb swelling deformations (dots: experiments; solid lines: analytical prediction).

<p>(a) Schematics to estimate the honeycomb wall curvature due to a deformation from straight (solid line) to S-shaped (dashed line). (b) Influence of increasingly rigid walls and constant stiffness inclusions (G_MI = 0.135 MPa) on the overall honeycomb expansion. Dots correspond to measured swelling of 3D printed honeycombs with soft inclusions (G_MI = 0.135 MPa) and increasingly stiff walls (respectively, G_MW1 = 0.459, G_MW2 = 2.089, G_MW3 = 96.5 and G_MW4 = 176.7 MPa). (c, d) For a fixed wall rigidity (c: G_W = G_MW4 = 176.7 MPa; d: G_W = 0.1*G_MW4) and increasingly softer inclusions, the (anisotropic) honeycomb expansion increases until a plateau is reached. Analytical prediction validated through experiments only for case b).</p

Passive hydro-actuation of bio-inspired bilayer-honeycomb device.

<p>Initial (left) and final actuated state (right) of the passive hydro-actuation upon changing the relative humidity from 50 to 95% are depicted at different levels of the design; (a) A bilayer made up of spruce veneer (active layer) glued to a thick paper (passive layer), bends upon anisotropic swelling of the spruce veneer in the direction perpendicular to the cellulose fibrils orientation. (b) Two of such bilayers attached together, constructs a cell-like structure that can open/closes upon changes in the relative humidity. (c) Scaling up the bilayer-cell concept into a hydro-actuated honeycomb prototype that expands up to 5 fold upon actuation (sequential images after 0, 2, 4 and 16 hours of exposure to 95%RH) [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0163506#pone.0163506.ref020" target="_blank">20</a>].</p

Finite element simulation of bilayer and bilayer cell.

<p>Left: single bilayer in initial (a) and actuated state (c); Right: Bilayer-cell in initial (b) and actuated (d) configuration.</p

Swelling-honeycombs model.

<p>Diamond-shaped honeycombs inspired by the microstructure of the ice plant keel’s tissue. The unit cell has a diamond shape with a width-to-height ratio of 4 for stiff walls (1.13 mm thickness) and round soft inclusions. The honeycombs were built as rectangular arrays of 5-by-10 cells. All dimensions are in mm.</p

Swelling of a honeycomb with stiff (MW4) walls and soft (MI) inclusions.

<p>The honeycomb’s unit cell geometry is reported with the kinematic variables (upper left) used in the analytical model and the actual dimensions (upper right). Swelling of a diamond-shaped honeycomb with”stiff” walls (G_walls = G_MW4 = 176.7 MPa) and soft cell inclusions (G_inclusions = G_MI = 0.135 MPa). Snapshots cover 4 days of swelling in isopropanol at room temperature (T = 22°).</p