Physical mechanisms behind the wet adhesion: From amphibian toe-pad to biomimetics
Graphical abstract
Introduction
The epidermal of organisms in nature which possess the extraordinary adhesions, such as walking on the ceiling or hang upside down on wet substrates, endowed by their near-surface architectures always impress us deeply. One such example is the fibrillar surface of gecko toe pads [1,2], which impose a strong but switchable role in the adhesion to almost any dry surface [3]. Experimental evidence has suggested that the gecko toe pad is composed of hundreds of thousands of keratinous hairs (called setae), which then branches into hundreds of even finer hairs (called spatula) [4,5]. With these hierarchically organized structures, the toe pads can maximize the van der Waals interaction to substrates by the intimately contacting [6,7]. Those adhesive features provide a great idea for novel surface designing [[8], [9], [10], [11]].
Apart from the fibrillar surface of geckos, nature also provide a number of biological organisms with intelligent structures for facilitating strong adhesion associated with high friction under wet conditions. Amphibians such as tree frogs, torrent frogs, and newts whose toe pads usually patterned with a polygonal topography of epidermal cells separated by mucus-filled channels give great talents of attaching to vertical and overhanging wet surfaces without slipping or falling (Fig. 1) [[12], [13], [14], [15]]. As detailly investigated in previous work, these amphibian toe pad surfaces are super-hydrophilic [16] and, thus, a very thin layer of fluid always comes between the toe pads and the adhering substrates [17], which means the adhesive mechanism is quite different from geckos’ case. It is generally assumed that the adhesive forces arise from the capillary forces and rate-dependent viscous forces, and that the of van der Waals forces has a probable contribution because of the several nanometers of film thickness, but to what extent is unclear [[18], [19], [20]]. The physical properties of toe pads, such as the surficial structures [21,22], elasticity modulus [23], hydrophilicity [16,24], etc., have been demonstrated to play a positive role for the living body wet attachments. For example, the array of specialized polygonal designs of cells is suggested to have a “draining effect” that the surplus water will be removed out of the pad contact area, allowing an intimate contact situation [17,25]. It favors producing a high friction force for the amphibian attachments when climbing on the incline slip surface [16,24,26]. In addition, other celebrated approaches to the wet adhesion such as the suction effect or the chemical interactions found on the marine organisms also gain much attentions, and have been widely reported [[27], [28], [29]].
Motivated by the wide range of technological application, such as climbing robot feet [10,30], novel gripper [31,32], pick-place system [33,34], etc., many researchers devote to fabricating the artificial structured surfaces for copying the super adhesive properties of living organisms, and more than one thousand articles have been published in the field over last two decades, especially for the gecko inspired fibrillary surfaces. Some of them are excellent reviews that have well discussed and systematically summarized the dry adhesion system including the underlying mechanism, the fabricating approaches, and the engineering applications, e.g., refs [9,11,35,36]. However, considering the possible case of artificially executing adhesions or bondings happening under a wet condition, the amphibians’ epidermal adhesive technology, similarly, apply many potential uses for us. Obvious examples of them are improved wet weather tires, nonslip footwear, plaster for surgery able to adhere to tissue, and holding devices for neurosurgery or MEMS device. As a result, it seems necessary to give a comprehensive overview of the physical principles underlying the toe-pad wet adhesions from biological models to artificial counterparts, directing and inspiring us to carry out wide engineering applications.
This article aims to review the current state of toe pad adhesive studying, with the certain perspective of fundamental physical principles from a detailed understanding of microstructures, to tunable strategy of capillary forces. We first provide a short overview of the fascinating ability of wet attachments tested on the amphibian toe-pads (section 2). Section 3 covers the various surficial structures for wet adhesion and friction behaviors from biological epidermis structures to their biomimetics, and discussing the physical mechanisms behind them. In section 4, we reported the bulk softness, the secreted mucus, the sandwiched film in the contacts of tree frog toe-pad, providing further insights into the wedged film stability, wetting case, the bulk viscoelastic dissipation for the interfacial adhesive contacts. Section 5 reports some novel approaches to tune capillary forces between two solids for a controllable wet adhesion. A summary and future perspectives conclude the paper in Section 6.
Section snippets
The ability of wet attachments of toe-pads and their physical forces
Experimental tests on the biological organisms (Fig. 1) not only reliably verify the attachment of amphibians but also show that abilities quantitatively in a scientific way. Barness’s group have a pioneering work on the measurements of the tree frog attachment ability. Their early work were performed on Osteopilus septentrionalis (tree frog) using a force platform, showing the largest measurable adhesive forces of toe pads is about 1.2 mN mm−2 [37]. The climb tests of tree frogs with different
Toe pads’ micro/nano structures and their function
The soft domed pads of above amphibians are usually characterized by additional surface features, which seem to contribute to their better capacity to attach to wet surfaces [14,15,18,19,40,45]. Appearing smooth at low magnification, the toe pads, in fact, evolved polygonal micro-structures of outermost layer separated by mucus-filled channels. The most striking is the regular hexagonal one, found in representatives of tree frogs; the diameter of single epidermal cell is approximately 10 μm
The physical properties of biological toe-pads (the structures underlying surface and the elastic modulus)
Present studies of anatomy showed the epithelium of toe pads of amphibians, such as the tree frog and torrent frog, were stratified, consisting of at least four layers (Fig. 5a–c) [45,46]. The outermost layer is keratinised and nonliving with a much larger thickness (about 15 μm for tree frog), which may have a function of resisting the injury and the abrasion during attaching on rough surfaces. This layer is stained darkly, whose outer region is separated by the interlinked channels [21]. The
Some ways to adjust the capillary adhesions
Turning focus back to the toe pads, the physical mechanism of wet adhesion to a variety of surfaces is quite complicated, but capillary forces and viscous forces generally accepted as the two main contributions for the normal adhesives. After examining the relationship between toe pad area and adhesive force in different tree frog species (Fig. 8a), Barnes suggested that the capillary force is the main component of wet adhesion rather than the viscosity one [19]. In nature, the amphibians seem
Conclusions and outlook
The toe pad wet adhesion and its biomimics have drawn much attention, and reached a high level of understanding over the last 10 years. The outstanding wet adhesion properties is primarily raised from the special designs of epidermis structures by which the capillary force, the friction force can be controlled and amplified on a wet substrate. Research work so far, though, have well revealed and confirmed some mechanisms behind the wet attachments, e.g., the draining effect of hexagonal
Declaration of Competing Interest
None.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) (grant no. 51675268), the Natural Science Research Fund of Higher Education of Anhui Province (KJ2019A0078), and the Research Fund for Young Teachers of Anhui University of Technology (QZ202009).
References (108)
- et al.
Bioinspired dry adhesive materials and their application in robotics: a review
J. Bionic Eng.
(2016) - et al.
Sticking/Climbing ability and morphology studies of the toe pads of chinese fire belly newt
J. Bionic Eng.
(2016) - et al.
Normal capillary forces
Adv. Colloid Interface Sci.
(2009) - et al.
Metallic glass nanotube arrays: preparation and surface characterizations
Mater. Today
(2018) - et al.
Key parameters of biomimetic patterned surface for wet adhesion
Int. J. Adhes. Adhes.
(2018) - et al.
Effect of wetting case and softness on adhesion of bioinspired micropatterned surfaces
J. Mech. Behav. Biomed. Mater.
(2018) - et al.
Combined dry and wet adhesion between a particle and an elastic substrate
J. Colloid Interface Sci.
(2016) - et al.
On the modified Tabor parameter for the JKR–DMT transition in the presence of a liquid meniscus
J. Colloid Interface Sci.
(2007) - et al.
Dynamic contact model based on Meniscus adhesion for wet bio-adhesive pads: simulation experiments
Tribol. Trans.
(2010) - et al.
The capillary force in micro- and nano-indentation with different indenter shapes
Int. J. Solids Struct.
(2008)
Meniscus and viscous forces during separation of hydrophilic and hydrophobic surfaces with liquid-mediated contacts
Mater. Sci. Eng. R Rep.
Adhesive force of a single gecko foot-hair
Nature
Mechanisms of adhesion in geckos
Integr. Comp. Biol.
Biological Micro- and Nanotribology
Gecko adhesion: structure, function, and applications
MRS Bull.
Frictional adhesion: a new angle on gecko attachment
J. Exp. Biol.
Adhesion and friction in gecko toe attachment and detachment
Proc. Natl. Acad. Sci. U. S. A.
Microfabricated adhesive mimicking gecko foot-hair
Nat. Mater.
Gecko-inspired surfaces: a path to strong and reversible dry adhesives
Adv Mater
Functional adhesive surfaces with “Gecko” effect: the concept of contact splitting
Adv. Eng. Mater.
Engineering micropatterned dry adhesives: from contact theory to handling applications
Adv. Funct. Mater.
Treefrog toe pads: comparative surface morphology using scanning electron microscopy
Can. J. Zool.
Toe pad morphology and mechanisms of sticking in frogs
Biol. J. Linn. Soc.
Sticking under wet conditions: the remarkable attachment abilities of the torrent frog, Staurois guttatus
PLoS One
Biomimetic design of elastomer surface pattern for friction control under wet conditions
Bioinspir. Biomim.
Insights into the adhesive mechanisms of tree frogs using artificial mimics
Adv. Funct. Mater.
Wet but not slippery: boundary friction in tree frog adhesive toe pads
J. R. Soc. Interface
Bionics and wet grip
Tire Technol. Int.
Tree frogs and tire technology
Tire Technol. Int.
Adhesion in Wet Environments: Frogs
Ultrastructure and physical properties of an adhesive surface, the toe pad epithelium of the tree frog, Litoria caerulea White
J. Exp. Biol.
Wet adhesion with application to tree frog adhesive toe pads and tires
J. Phys. Condens. Matter
Elastic modulus of tree frog adhesive toe pads
J. Comp. Physiol. A
Torrent frog-inspired adhesives: attachment to flooded surfaces
Adv. Funct. Mater.
Hexagonal surface micropattern for dry and wet friction
Adv. Mater.
Sticking like sticky tape: tree frogs use friction forces to enhance attachment on overhanging surfaces
J. R. Soc. Interface
The structure and adhesive mechanism of octopus suckers
Integr. Comp. Biol.
The morphology and adhesion mechanism of Octopus vulgaris suckers
PLoS One
A wet-tolerant adhesive patch inspired by protuberances in suction cups of octopi
Nature
Adhesion technologies of bio-inspired climbing robots: a survey
Int. J. Robot. Autom.
Controllable interfacial adhesion applied to transfer light and fragile objects by using gecko inspired mushroom-shaped pillar surface
ACS Appl. Mater. Interfaces
Switchable dry adhesion with step-like micropillars and controllable interfacial contact
ACS Appl. Mater. Interfaces
Switchable adhesion in vacuum using bio-inspired dry adhesives
ACS Appl. Mater. Interfaces
A novel bioinspired switchable adhesive with three distinct adhesive states
Adv. Funct. Mater.
Towards the next level of bioinspired dry adhesives: new designs and applications
Adv. Funct. Mater.
Recent developments in gecko-inspired dry adhesive surfaces from fabrication to application
Surf. Topogr. Metrol. Prop.
Adhesion and detachment of the toe pads of tree frogs
J. Exp. Biol.
Whole animal measurements of shear and adhesive forces in adult tree frogs: insights into underlying mechanisms of adhesion obtained from studying the effects of size and scale
J. Comp. Physiol. A
Application of peeling theory to tree frog adhesion, a biological system with biomimetic implications
Eur. Acad. Sci. E Newslett Sci. Technol.
Ueber die Fortbewegung der Thiere an senkrechten, glatten Flächen vermittelst eines secretes
Arch Ges Physiol
Cited by (17)
Texture design of microgrooves to improve the tribological properties of wiper blades
2023, Tribology InternationalResearch on laser preparation and grinding performance of hydrophilic structured grinding wheels
2023, Ceramics InternationalCitation Excerpt :Finally, the grinding experiment of alumina ceramics will be carried out, and the grinding performance of the hydrophilic structured grinding wheel will be evaluated with the reduction of grinding temperature and damage as indicators. The surface of the array regular hexagonal structure is distributed with grooves that communicate with each other, and the liquid can flow and diffuse rapidly in the grooves, and it has good hydrophilicity and wear resistance [23]. Assuming that the side length and depth of a single regular hexagonal structure were B and H, respectively, and the distance between adjacent regular hexagonal structures was D.
Synergistic lubrication mechanisms of AISI 4140 steel in dual lubrication systems of multi-solid coating and oil lubrication
2022, Tribology InternationalCitation Excerpt :The researchers found a variety of microstructures in tree frog's footpads, such as quadrilateral, pentagonal, and hexagonal (primary). In addition, there are channels several micrometers wide between the polygons [30,31]. When the toes are in contact with countpart, the liquid between the interfaces can be effectively guided into the channel to realize the direct contact between the contact surfaces.
Optimization of bionic textured parameter to improve the tribological performance of AISI 4140 self-lubricating composite through response surface methodology
2021, Tribology InternationalCitation Excerpt :A low-viscosity liquid secreted by the toe epithelial cells of tree frogs spreading on the palm surface through tiny channels inside the toe (Fig. 1b). Through the study of toe surface microstructure, a large number of polygon structures were observed, mainly hexagonal structures [26] (Fig. 1c). The special polygon structure shows excellent material transport effect and carrying capacity, so that the secreted mucus can be uniformly transmitted on the surface of the foot.