Hierarchical anti-adhesive materials by mimicking insect traps

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We aim at elucidation of structure/property relations in hierarchically organised anti-adhesive trapping organs of carnivorous pitcher plants and at the design of bio-inspired prototypes. Structural characterisation of all three hierarchical levels (level 1: lunate cells; level 2: lower wax layer; level 3: upper wax layer) in pitchers of the model plant Nepenthes alata was completed, successive stages in the development of the complex wax coverage were determined, and the absence of the regeneration ability in the lower wax layer was shown. We examined adhesive properties of the complete hierarchical system on the global scale by probing insect attachment and demonstrated specific anisotropic properties of biological level 1. We performed physicochemical and mechanical characterisation of levels 2 and 3 on the local scale and developed an experimental approach for measuring adhesion forces on surfaces with very low adhesive capability. Level 1 lunate cells were mimicked by mechanically stable arrays of polymer spheres. The adhesion of the latter was one order of magnitude lower than that of flat controls, even without the anisotropy of biological level 1 lunate cells. Level 2 in both the biological model and bio-inspired systems does not reduce adhesion. For the biological model reduced adhesion is only found for combinations of lower and upper wax layers (levels 2 and 3). We will implement nanorod coatings as bio-inspired level 3 in artificial anti-adhesives that may allow for reliable one-time detachment of adhering surfaces, for example, in the presence of ice.We will evaluate the absorption ability of different wax layers in N. alata pitchers and the influence of the wax crystal size on adhesion at global and local scales. Microstructure and wetting properties of different functional pitcher surfaces in plants from other carnivorous genera will be characterized. To study the role of fluids at the contact interface of bio-inspired anti-adhesives, single bio-inspired hierarchical levels and combinations thereof containing continuous pore systems will be made by swelling-induced morphology reconstruction of block copolymers. The bio-inspired samples will be characterized by light microscopy, scanning electron microscopy, atomic force microscopy, white light interferometry, and contact angle measurements. Pull-off force tests will be carried out under dry conditions and in the presence of water at the contact interface. We will systematically elucidate the effect of (1) the bio-inspired level 3, (2) the interplay of the bio-inspired level 3 with other hierarchical levels, (3) the fluid at the contact zone, and (4) the polarity/hydrophilicity of the surface on anti-adhesive properties of bio-inspired samples under dry and wet conditions. Porous bio-inspired anti-adhesives will be systematically compared with their solid counterparts.