Gecko toe and lamellar shear adhesion on macroscopic, engineered rough surfaces

Abstract

The role in adhesion of the toes and lamellae -intermediate-sized structures -found on the gecko foot remains unclear. Insight into the function of these structures can lead to a more general understanding of the hierarchical nature of the gecko adhesive system, but in particular how environmental topology may relate to gecko foot morphology. We sought to discern the mechanics of the toes and lamellae by examining gecko adhesion on controlled, macroscopically rough surfaces. We used live Tokay geckos, Gekko gecko, to observe the maximum shear force a gecko foot can attain on an engineered substrate constructed with sinusoidal patterns of varying amplitudes and wavelengths in sizes similar to the dimensions of the toes and lamellae structures (0.5 to 6 mm). We found shear adhesion was significantly decreased on surfaces that had amplitudes and wavelengths approaching the lamella length and inter-lamella spacing, losing 95% of shear adhesion over the range tested. We discovered that the toes are capable of adhering to surfaces with amplitudes much larger than their dimensions even without engaging claws, maintaining 60% of shear adhesion on surfaces with amplitudes of 3 mm. Gecko adhesion can be predicted by the ratio of the lamella dimensions to surface feature dimensions. In addition to setae, remarkable macroscopic-scale features of gecko toes and lamellae that include compliance and passive conformation are necessary to maintain contact, and consequently, generate shear adhesion on macroscopically rough surfaces. Findings on the larger scale structures in the hierarchy of gecko foot function could provide the biological inspiration to drive the design of more effective and versatile synthetic fibrillar adhesives.

Adhesive interactions of geckos with wet and dry fluoropolymer substrates

“…Gecko adhesion to rough substrates has been tested previously at a variety of size scales and substrates [19][20][21][22][23]. Generally, adhesion seems to be reduced on rough surfaces, especially those that scale on the size of the adhesive components, specifically the spatula (nanometre) and lamellae (millimetre) [19,20], though this is also likely for the setae (micrometre) [21] and also toe pad (centimetre). The substrates used in this experiment were relatively smooth, root mean square (RMS) roughness was between 4 and 42 nm on all three substrates (FEP 40 nm; ETFE 20 nm; PET 4 nm; electronic supplementary material, figure S4).…”

“…Roughness produced by self-assembling waxes (tens to hundreds of nanometres) on the surfaces of variously shaped epidermal cells (micrometres) provide the chemical and physical basis for leaf hydrophobicity including the lotus and salvinia effect [52]. In fact, the ubiquity of variably rough surfaces at what should be challenging length scales for the gecko system [19] highlight significant gaps in our understanding of how geckos are so adept at moving through their three-dimensional habitats with such ease. Our work suggests that geckos may be able to adhere to wet rough substrates as well as they adhere to wet smooth substrates.…”

Passively stuck: death does not affect gecko adhesion strength

“…The increasing weight of sand transmitted highly repeatable and linearly increasing pulling forces to the gecko foot, allowing us to assess whole-animal adhesion under extremely controlled conditions for the first time. Other approaches where geckos are pulled by hand [10,11] or with speed-driven motors [12,13] do not standardize the dynamic pulling force.…”

Simulation of synthetic gecko arrays shearing on rough surfaces

“…Furthermore, interfibre collision and adhesion is neglected. Although interfibre compaction likely becomes significant under high loads in the natural gecko [11,36], a previous simulation of a GSA which neglected interfibre collision showed reasonable agreement with experiment [20]. The simulation was run with an array of 250 hairs, because this number was large enough to make an individual hair contribution significantly small if all hairs were in contact (less than 1%).…”

“…However, an integrated approach should consider the relationship between the surface roughness, the size of the adhesive structures and their ability to conform at a variety of length scales [10,11]. Understanding the abilities and limitations of these structures on varying length scales of roughness is necessary to create an adhesive that is able to adhere to naturally rough surfaces.…”