Vol.:(0123456789) 1 3 Journal of Bio- and Tribo-Corrosion (2018) 4:43 https://doi.org/10.1007/s40735-018-0160-1 Natural Adhesion System Leads to Synthetic Adhesives Ashish K. Kasar 1  · Rahul Ramachandran 1  · Pradeep L. Menezes 1 Received: 21 December 2017 / Revised: 30 April 2018 / Accepted: 3 June 2018 © Springer International Publishing AG, part of Springer Nature 2018 Abstract Nature has developed multi-functional geometric structures, and surface textures with excellent tribological characteristics, such as feet of geckos. Geckos have extraordinary abilities to climb walls and even upside down on the ceiling. Studies have revealed that hierarchical structure of gecko’s feet can bear the weight of two humans and this strong adhesion force is mainly generated by weak van der Waals force. This paper reviews the mechanisms and the forces responsible for gecko’s adhesion, and the efect of humidity on adhesion against diferent hydrophobic/hydrophilic surfaces. The excellent adhesive and fric- tional properties of gecko adhesion system have inspired many researchers to develop gecko-inspired synthetic adhesives. In this paper, recent development of gecko-inspired synthetic adhesives has been presented in terms of various fabrication methods, diferent tip structures, and the efect of counter surface roughness as well as design criteria to avoid bunching of nano-structures. The application of synthetic adhesives is also discussed for wall climbing robots and novel applications in the feld of space, biomedical and sports accessories. Keywords Dry adhesion · Van der Waals force · Capillary force · Frictional force · Adhesion force · Nano fbers 1 Introduction Nature has always been an ideal model for developing new technologies. By understanding and utilizing nature’s various mechanism, researchers have engineered several technologies, such as body armor inspired by turtle [1], inchworm robots inspired by the caterpillar [2], strong-light- weight hexagonal structural inspired by honeycombs [3], etc. Development of these biologically inspired technologies is known as biomimetics. The term biomimetics came into existence in the late 1960s, and it is derived from Greek word: Bios meaning life and mimesis meaning to intimate [4]. The emerging feld of biomimetics is also important in tribology which is the study of interactive surfaces in relative motion. Several animals and plants have developed unique and multi-functional topographic tribological char- acteristics during their evolution. These characteristics in turn become the basis for new surface engineering designs. For example, superhydrophobic self-cleaning surface of lotus leaves inspired the creation of self-cleaning surfaces [5, 6]. The textured surface of shark skin inspired to design swimming cloth that reduces the frictional resistance under water [7]. Animals such as a beetle, mussels, frogs, and geckos have shown unique adhesive systems. The adhesive systems can be categorized into three types based on the adhesion mechanism involved: chemical, wet, and dry adhe- sion. Some examples of animals with adhesion mechanism are listed in Table 1. In chemical adhesion system, animals secrete a strong adhesive as shown in Fig. 1a. Mostly, these adhesives are composed of proteins. For example, in mus- sels the protein consists of a amino acid known as L-DOPA (L-3,4-dihydroxyphenylalanine). Mussels secrete this protein through 50–100 byssal threads which can generate a detach- ment force of 0.49 ± 0.19 N. It helps mussels to adhere to rocks against the crashing sea waves [8]. Similar proteins provide adhesion in other aquatic animals, such as caddisfy larva, barnacles and sandcastle worm [9]. In wet adhesion system, animals secret a meniscus fuid which develops cap- illarity between the counter surface and fbrous structure or toe pad of animals (Fig. 1b). Animals with wet adhesion are observed to have higher adhesion force against the hydro- philic surface compared to hydrophobic surface [10–13]. The adhesion force due to capillary and viscous forces can be computed using following equation for spherical geom- etry against a fat surface as shown in Fig. 2. The capillary * Pradeep L. Menezes pmenezes@unr.edu 1 Department of Mechanical Engineering, University of Nevada Reno, Reno, NV 89501, USA