Dynamic Friction Performance of Hierarchical Biomimetic Surface Pattern Inspired by Frog Toe‐Pad

Abstract

Hu et al. provided insight into the mushroom-shaped dry adhesives for future bioinspired dry adhesive designs. [4] Contrarily, researchers employed a bioinspired core-shell structure to exhibit enhanced antiwear and friction-reduction performance. [5] Li et al. used thermoplastic polyurethane (TPU) to fabricate bioinspired e-skin with excellent rheological behavior. [6] However, in all these studies, researchers mostly investigate dry adhesives. Anwer et al. developed textured composite surface with extremely high coefficient of friction (COF) on ice. [7] However, there is no emphasis on the presence of wet environment. Researchers could potentially employ the concept of biomimicry to influence technological fields where drainage of water is essential, such as tire treads, surgical grippers, etc. Researchers have identified the potential to remain stable on flooded surfaces in some tree frogs and torrent frogs with the help of their toe-pads. [8] The eighteen toes of frogs (four at each forelimb and five at each hindlimb) consist of disklike pads at the tip. [9] Their toe-pads consist of epithelial cells which are mostly hexagonal in shape (length ≈ 10 µm), [10] and separate from each other at the top. Figure 1 shows a schematic morphological structure of frog toe-pad and epithelial cells. Though small in quantity, there are also cells of pentagonal (23.1%) and heptagonal (16.1%) shape on the pad surface. [11,12] The existence of grooves (width ≈ 1 µm, height/ depth ≈ 10 µm) [10] over the pad distinguish these epithelial cells from other similar cells on the rest of the body. [9] Moreover, the surface of epithelial cells is covered with peg-like projections (diameter ≈ 0.2 µm), which are bordered by small channels (width ≈ 40 nm, height ≈ 0.2 µm). [3] The pores of mucous glands, scattered among these epithelial cells, produce the watery secretion that forms an essential part of the adhesive mechanism of each toe-pad. But in case of torrent frogs, this layer becomes demolished due to the presence of water. [9] From the studies [13,14] of ranid frogs (torrent living), it has been observed that the prismatic (epithelial) cells for most of the ranid species are elongated (deviated geometry from regular hexagonal shaped) in comparison to regular cells. Ratio of width to length is around 60% whereas it is 80-100% for normal prismatic cells which helped them to have more improved projection and thus make a larger contribution to provide better friction. [15,16] In this study, the dynamic friction performance of frog toe-pad inspired surface patterns is investigated in three folds. First, frog toe-pad morphology is mimicked, designed, and fabricated using 3D printing technology. Friction coefficients of the models are measured experimentally over a wet medium, with varying velocity, load, and sliding direction. Furthermore, time is recorded to reach a 5 mm height by the water flow through a steady model in another experimental setup. Second, numerical simulation is employed to study the contact area, sliding displacement...

Recent progress of tree frog toe pads inspired wet adhesive materials

“…In 2020, Tan et al. prepared the bionic surfaces with various surface morphologies (regular hexagon, extended hexagon and studded hexagon) using thermoplastic polyurethane (TPU) with rubbery characteristics similar to typical tire [51]. The TPU‐based bionic surface with studded hexagon showed the optimal wet adhesion performance than the other two.…”

“…photolithography [13, 29–35, 37, 38, 43–50, 55] and etching [52–54, 56]) and top‐down (e.g. 3D printing [51] and embossing [36–38, 49]) approaches.…”

“…Computer-controlled layer-by-layer curing of polymer resins can produce surfaces with complex micro-/nano structures. Taking advantage of 3D printing, TPU [51,57,58] with hierarchical structures can be obtained. They can be directly used as adhesive surfaces or as templates to replicate adhesive polymer materials.…”

“…In 2020, Tan et al fabricated three types of tree frog-inspired surfaces using TPU by 3D printing technology. Embossing as another important micro-/nano fabrication technology has been used to construct patterns of soft materials (Figure 3d) [51]. A predetermined micro-/nanostructure was first etched into a hard material (such as Si), and then the surface structure on the hard substance was further transferred to the soft material through embossing.…”

Numerical investigation of squeeze film lubrication on bioinspired hexagonal patterned surface

“…As for the microchannels, facilitating liquid to be expelled from the contact area is claimed to be the main function of the microchannels (Banik and Tan, 2020); the guiding function of microchannels for liquid flowing also attracts people's attention (Dai et al, 2021;Grützmacher et al, 2021;Grützmacher et al, 2018). However, a contrasting perspective of wet adhesion (Zhang et al, 2020) for frog's toe pad is also being widely accepted, which suggests that microchannels store mucus and inject them into the whole pad-substrate contact area to build up capillary bridges to generate adhesive forces.…”