An experimental study on adhesive or antiadhesiveand strong bio-inspired nanomaterials

This experimental PhD thesis presents the results of research performed in five different facilities: in the Laboratory of Bio-inspired Nanomechanics “Giuseppe Maria Pugno” at the Politecnico of Torino, the “Nanofacility Piemonte” at the INRIM Institute in Torino, the Division of Dental Sciences and Biomaterials of the Department of Biomedicine at the University of Trieste, the Physics Department of the Politecnico of Torino, the Toscano- Buono Veterinary Surgery in Torino and the Department of Human and Animal Biology at the University of Torino. The adhesive abilities of insects, spiders and reptiles have inspired researchers for a long time. All these organisms present outstanding performance particularly for force, adhesion and climbing abilities, especially considering their size and weight. Scientists have focused attention on the gecko’s adhesive paw system and climbing abilities, and its adhesion mechanism has been an important topic of research for nearly 150 years. However, certain phenomena about geckos are still not completely understood and nowadays these still represent the main challenge of several scientific discussions which aim to better understand the gecko’s adhesive ability. This thesis deals first with the influence of surface roughness on the gecko’s adhesion on the inverted surface of Poly(methyl meth-acrylate) (PMMA) and glass in Chapter 1, of PMMA with different surface roughness in Chapter 2, while Chapter 3 deals with the gecko’s maximum normal adhesive force and Chapter 4 looks at the optimal adhesion angle at different hierarchical levels. The gecko’s moult (Appendix 1) is examined in a preliminary way. The Tokay gecko (Gekko gecko) is the most studied gecko among more than 1050 Gekkonid lizard species in the world, due to its strong adhesive ability. Because this thesis reports clear experimental measurements on two living Tokay geckos, it is comparable to scientific results reported in the literature. It is well known how small insects can carry many times their own weight and can walk quickly, but their most interesting ability is their extremely high adhesion. In recent decades, many scientists have studied a number of insects in order to understand and measure their adhesive abilities. Biological adhesion can be obtained through different adhesive mechanisms (e.g. claw, clamp, sucker, glue, friction). In particular, this thesis focuses on living specimens of the non-climbing cockroach (Blatta Orientalis Linnaeus) by evaluating its maximum shear safety factor on artificial surfaces using a centrifuge machine, see Chapter 5. In general, the adhesive structure and mechanism of an animal could be connected to the micro-structured roughness of natural substrata (e.g. plant surfaces), which animals usually find when they move around in the natural environment. In nature, plants show an extraordinary variety of morphologies and surface structures. Some plants possess two special properties; superhydrophobicity (or water-repellency) and self-cleaning (or dirt-freedom). These two related phenomena were observed for the first time by Aristotle more than 2,000 years ago but it was only in the 20th century that scientists examined them accurately on some natural leaves, e.g. the lotus (Nelumbo nucifera) on which “raindrops take a clear, spherical shape without spreading, which probably has to be ascribed to some kind of evaporated essence”, as Goethe described in 1817. Accordingly to scientific literature, a strong influence of surface roughness on wettability and self-cleaning behaviour clearly emerges. This well-defined problem was of particular interest (for the Indesit Company) since we wanted to find an industrial solution which would leave the internal sides of refrigerators clean from condensed water or dirt. For this reason, a collaborative project started. Two industrial processes, plasma and thermoforming treatments, were applied to polystyrene surfaces. The Indesit refrigerator box is made of polystyrene. The influence of these industrial treatments on the surface wettability were analysed, see Chapter 6. The focus of the Indesit Company was to understand the role of roughness and to produce a superhydrophobic and self-cleaning surface. Thus, this thesis reports the method which we developed in order to design an artificial biomimetic superhydrophobic polystyrene surface, copying the natural lotus leaf (Chapter 7). In addition, surface roughness implies a modification of the tribological and frictional properties, so it assumes a crucial role when designing two contacting surfaces, in particular at nano-scale (Chapter 8). The nanometer scale characterises this thesis and is involved in everything from gecko spatulae to the waxy nanotubules of the lotus leaf, to the fibroin protein materials which constitute spider silks. In general, spider silks display superior mechanical properties but, only in the last few decades, reserachers have studied various types of silks and have evaluated their very different mechanical properties. The fact that the mechanical behaviour of spider silks varies accordingly to their type is well-known, since silk properties have been demonstrated to be species-specific and are linked to silk-based peptide fibrils or protein aggregates, with different structural and mechanical properties. The dragline silk (or radial silk) and the flag silk (or circumferential silk) of orb weaving spiders have been characterized in scientific literature while, to our knowledge, few studies have been conducted on bundles, which connect the cocoons of Meta menardi to the ceiling of caves. These were tested to determine their mechanical properties in terms of stress, strain and toughness (Chapter 9).