10. Gecko Pads: A Force to Be Reckoned with

Gecko adhesion is strong, sufficient enough for geckos to scale vertical walls and stick to surfaces upside down. In fact, a gecko weighing about 40–50 g is capable of generating a force that is 100 times greater than its weight (Niewiarowski et al., PLoS One 3(5):e2192, 2008). One of the main features of gecko adhesion that scientists have been trying to replicate but still find elusive is its good performance over the course of many cycles of attachment and detachment (Lee et al., Nature 448:338–342, 2007). Though there were many hypotheses for the mechanism behind this ability, van der Waals is recognized as the main contributor to gecko adhesion, with capillary forces potentially at play when water vapor exists (Zhou et al., Friction 1(2):114–129, 2013). Geckos’ adhesive ability comes from the hierarchical morphology of their toe pads, which feature lamellae, setae, and spatulae (Bhushan, Gecko effect. In: Bhushan B (ed) Encyclopedia of nanotechnology. Springer, Netherlands, pp 943–951, 2012)—all of which are described in detail in this chapter. Geckos’ dry adhesion mechanism is made possible because of the splitting of setae into hundreds and even up to a thousand spatulae (Zhou et al., Friction 1(2):114–129, 2013). Furthermore, due to the flexibility and softness of lamellar skin, it can be seen as a spring with a weak spring constant as opposed to stiff skin. This weak springiness ensures adhesion to surfaces over a wider range of normal compression displacement by preventing large deformations among the setae (Tian et al., Sci Rep 3:1382, 2013). Successful gecko-inspired adhesives would boast an array of outstanding properties, such as the ability to adhere and detach repetitively without breakage. Uses of such adhesives include sports equipment and robots that climb walls, among a variety of others (Zhou et al., Friction 1(2):114–129, 2013). Two general fabrication methods—polymer-based and carbon nanotube (CNT)-based dry adhesives—have been developed to create gecko-inspired adhesives (Jeong and Suh, Nano Today 4(4):335–346, 2009).