The responsive mechanism of the Venus flytrap has captured the interest of scientists for centuries. Although a complete understanding of the mechanism controlling the Venus flytrap movement has yet to be determined, a recent publication by Forterre et al. [1] demonstrates the importance of geometry and material properties for this fast, stimuliresponsive movement. Specifically, the movement is attributed to a snapthrough elastic instability whose sensitivity is dictated by the length scale, geometry, and materials properties of the features.[2] Here, we use lessons from the Venus flytrap to design surfaces that dynamically modify their topography. We present a simple, robust, biomimetic responsive surface based on an array of microlens shells that snap from one curvature (e.g., concave) to another curvature (e.g., convex) ( Fig. 1) when a critical stress develops in the shell structure. This snap-transition is due to the onset of an elastic, snap-through instability similar to the capture mechanism of the Venus flytrap. The response rates can be over two orders of magnitude faster than the typical response of shape-memory polymers, and the sensitivity and rate of the response can be tuned with predictable geometric and/or material property changes. Based on materials choice, a wide variety of external stimuli can trigger this stress development, such as temperature, pH, solvent swelling, magnetism, electric current, and light. This strategy has great potential for the design of responsive surfaces, which will impact a variety of applications including: release-on-command coatings [3] and adhesives, [4][5][6][7] on-command frictional changes, [8,9] instant modification of optical properties at an interface, [10,11] rapid response drug delivery, [12][13][14] chemical sensing, [15][16][17] and antimicrobial devices. [18] To fabricate the active surface structures, we use the Euler buckling of plates to generate a controlled array of microlens shells under equibiaxial compressive stress (Fig. 2a). First, we pattern cylindrical posts of photoresist onto a silicon wafer and micromold poly(dimethyl siloxane) (PDMS) (Sylgard 184 TM ) elastomer onto it, creating an array of holes. This elastic, PDMS array of holes is then placed in equi-biaxial strain through a simple inflation procedure. A thin film of crosslinked PDMS (typically 15-60 lm in thickness) coated with a thin (∼ 1 lm) layer of uncured PDMS is placed on the surface of the strained holes. The assembly is heated to crosslink the uncured PDMS and bond the film to the array of holes while under equibiaxial tension. Releasing the tension causes an equibiaxial compressive strain to develop in the thin PDMS coating. The associated compressive stresses cause the circular plate of PDMS on the surface of each hole to buckle, thus creating an array of convex microlenses. This technique for microlens preparation is simple, robust, and should be scalable to much smaller length scales across a multitude of materials. COMMUNICATION

Cells are wrapped in inelastic membranes, yet they can sustain large mechanical strains by regulating their area. The area regulation in cells is achieved either by membrane folding or by membrane exo- and endocytosis. These processes involve complex morphological transformations of the cell membrane, i.e., invagination, vesicle fusion, and fission, whose precise mechanisms are still under debate. Here we provide mechanistic insights into the area regulation of cell membranes, based on the previously neglected role of membrane confinement, as well as on the strain-induced membrane tension. Commonly, the membranes of mammalian and plant cells are not isolated, but rather they are adhered to an extracellular matrix, the cytoskeleton, and to other cell membranes. Using a lipid bilayer, coupled to an elastic sheet, we are able to demonstrate that, upon straining, the confined membrane is able to regulate passively its area. In particular, by stretching the elastic support, the bilayer laterally expands without rupture by fusing adhered lipid vesicles; upon compression, lipid tubes grow out of the membrane plane, thus reducing its area. These transformations are reversible, as we show using cycles of expansion and compression, and closely reproduce membrane processes found in cells during area regulation. Moreover, we demonstrate a new mechanism for the formation of lipid tubes in cells, which is driven by the membrane lateral compression and may therefore explain the various membrane tubules observed in shrinking cells.

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A polymer film draping over a point of contact will wrinkle due to the strain imposed by the underlying substrate. The wrinkle wavelength is dictated by a balance of material properties and geometry; most directly the thickness of the draping film. At a critical strain, the stress in the film will localize, causing hundreds of wrinkles to collapse into several discrete folds. In this Letter, we examine the deformation of an axisymmetric sheet and quantify the force required to generate a fold. We observe that the energy of formation for a single fold scales nearly linearly with the film thickness. The onset of folding, in terms of a critical force or displacement, scales as the thickness to the four-ninth power, which we predict from the energy balance of the system. The folds increase the tension in the remainder of the film causing the radial stress to increase, thereby decreasing the wavelength of the remaining wrinkles.

We consider the dynamic snapping instability of elastic beams and shells. Using the Kirchhoff rod and Föppl-von Kármán plate equations, we study the stability, deformation modes, and snap-through dynamics of an elastic arch with clamped boundaries and subject to a concentrated load. For parameters typical of everyday and technological applications of snapping, we show that the stretchability of the arch plays a critical role in determining not only the postbuckling mode of deformation but also the timescale of snapping and the frequency of the arch's vibrations about its final equilibrium state. We show that the growth rate of the snap-through instability and its subsequent ringing frequency can both be interpreted physically as the result of a sound wave in the material propagating over a distance comparable to the length of the arch. Finally, we extend our analysis of the ringing frequency of indented arches to understand the 'pop' heard when everted shell structures snap-through to their stable state. Remarkably, we find that not only are the scaling laws for the ringing frequencies in these two scenarios identical but also the respective prefactors are numerically close; this allows us to develop a master curve for the frequency of ringing in snapping beams and shells. ULC 0.5 N load cell. p-2