Applying the Photonic Professional laser lithography system, a group of scientists led by PD. Dr. Hendrik Hölscher from Karlsruhe Institute of Technology (KIT), Germany, designed hierarchical gecko-type structures which match the Young’s modulus and the relevant length scale of the gecko’s setae very closely.
These setae, delicate hairs covering the gecko’s toes, originate the famous ability of geckos to climb effortlessly walls and ceilings. The setae are consisting of beta-keratine, a stiff material with Young’s modulus of around 1 - 4 GPa. Due to their hierarchical design, they are very compliant and can achieve a very high real contact area even to rough substrates. Thus, the van der Waals forces cause high adhesion that enables geckos to stick to nearly every surface. Mimicking the gecko-effect is, therefore, pursued by groups all over the world. However, such structures are mostly based on soft materials that usually have some drawbacks like degradation or rapid contamination.
Hence, Hölscher’s group used the laser lithography system to structure Nanoscribe’s photoresist IP-G 780, which Young’s modulus is approximately 4 GPa. Measuring the adhesional forces by AFM, the scientists examined systematically the influence of several design parameters like hierarchy, density, aspect ratio, and tip-shape on dry adhesion performance. In this way, the positive impact of mushroom-shaped tips could be demonstrated for stiff materials with lateral dimensions in the nanometer range. In addition, it was shown that the hierarchical structure of dry adhesives positively affects adhesion for appropriate preloads.
This result supports the longstanding hypothesis that adhesion to natural rough surfaces requires a hierarchical design to ensure intimate contact and therefore a high overall amount of van-der-Waals forces.
Several gecko-inspired arrays were fabricated with the Photonic Professional laser lithography system. The structures differ in their order of magnitude, amount of hierarchy levels, pitch, aspect ratio and tip shape (1).
(1) M. Röhrig , M. Thiel , M. Worgull , H. Hölscher, ”3D Direct Laser Writing of Nano- and Microstructured Hierarchical Gecko-Mimicking Surfaces”, Small 8, 3009 – 3015 (2012).