Biomimetic Super-Hydrophobic Surfaces Patterned via 3D Laser Lithography

In the last decade direct laser lithography (DLL), has disruptively emerged as a powerful tool for fabricating outstanding 3D microstructures, with features at the nanoscale, for optics, photonics, microfluidics, and life sciences. DLL has also found valid application in surface engineering since it allows the fabrication of micropatterns with remarkable reproducibility, precision and resolution. Recently, surface engineering has been constantly directing interest to biomimicry and, indeed, the employment of DLL in a biomimetic framework could represent a technological breakthrough, despite being a relatively underdeveloped approach. Plants and animals offer a great variety of functional surfaces to take inspiration from, thanks to their patterning at the micro- and nanoscale. At such dimensional scales, the biomimetic paradigm focuses on the deep connection between materials and morphology: nature has been able to achieve very complex functionalities with a relatively small number of structural materials, simply by exploiting proper superficial morphologies¹.<br/>We present an extension of the range of applications of DLL to multifunctional plant-inspired surfaces, whose complex hierarchical microstructures fabrication is prevented by inherent limitations of traditional techniques. We replicated at the microscale the wettability characteristics of the <i>Salvinia</i>-effect. <i>Salvinia Molesta</i> is a floating fern able to long-term air retention on the surface of its leaves when submerged in water, thanks to the peculiar superficial pattern and the related high hydrophobicity. The upper side of the leaves is covered with hairs composed of a stalk, up to 1.5 mm long, bearing on the tip four rounded filaments which are connected at the apex, thus forming a crown-like structure of about 500 μm in height. The overall multicellular hair structure, except for the apex, is covered with hydrophobic wax crystals, with low surface energy and nanometric roughness. The apex is made of dead cells, smooth and hydrophilic, which ensures an anchoring point for water droplets, contributing to stabilizing the trapped air layer².<br/>We demonstrated that downscaling the hairs allows to exploit the microscale physics and obtain wettability properties not achievable at bigger scales. Dimensions two orders of magnitude smaller than the natural model are required to obtain a surface with tunable hydrophobicity starting from a single hydrophilic material³. We reproduced the same pinning effect present in the <i>Salvinia</i> plant, chemically functionalizing the micropatterned surfaces, in order to resemble the same dual wettability of the natural counterpart: super-hydrophobic hairs with hydrophilic tips. To this purpose, we integrated DLL with micro-contact printing. Microstructures with geometries inspired by <i>Salvinia</i> patterns were reproduced in hydrophilic photoresist (IP-S, Nanoscribe) ten times smaller than the natural ones, and were coated with a nanometric hydrophobic layer of Teflon by deep-coating; finally, hydrophilic patches in IP-S were placed on the tips of the crown-like heads by micro-contact printing. We obtained three designs for three different superficial properties each in terms of contact angles: microstructures in IP-S, covered in Teflon, and <i>Salvinia</i>-like. Roll-off angle describes the capability of a surface to anchor the water: while it is low for surfaces covered with Teflon, it is virtually infinite for untreated hairs, which remain hydrophobic; <i>Salvinia</i>-like hairs show properties in between those with Teflon and those untreated, thus allowing the possibility to tune the desired wettability. Finally, air retention experiments reproduced the <i>Salvinia</i>-effect up to 4 atm of overpressure, confirming the effectiveness of the proposed approach.<br/>1 Bhushan, B., <i>Philos. Trans. R. Soc. Math. Phys. Eng. Sci.</i> <b>2009</b>, <i>367</i>, 1445-1486.<br/>2 Barthlott, W. et al., <i>Adv. Mater.</i> <b>2010</b>, <i>22</i>, 2325-2328.<br/>3 Tricinci, O. et al., <i>ACS Appl. Mater. Interfaces</i> <b>2015</b>, 7.46: 25560-25567<br/>This research received funding from the European Horizon 2020 Research and Innovation Programme (No. 899349 - 5D NanoPrinting).