Sticking under wet conditions: the remarkable attachment abilities of the torrent frog, Staurois guttatus

Bioinspired surface structures for added shear stabilization in suction discs

Many aquatic organisms utilize suction-based organs to adhere to diverse substrates in unpredictable environments. For multiple fish species, these adhesive discs include a softer disc margin consisting of surface structures called papillae, which stabilize and seal on variable substrates. The size, arrangement, and density of these papillae are quite diverse among different species, generating complex disc patterns produced by these structures. Considering papillae arrangements in three fish species, the Northern Clingfish (Gobiesox maeandricus), Tidepool Snailfish (Liparis florae), and Chilean Clingfish (Sicyases sanguineus), we fabricated physical disc models that tested relevant surface pattern parameters under shear loading conditions. Parameters of interest included the area of papillae-like structures, the spacing between adjacent structures (channel spacing), and the percent coverage of elements relative to the total disc area. To create our models, a soft silicone elastomer was added to a stiff circular suction cup, which was then "stamped" using a laser-etched and thermoformed mold base to create the desired surface patterning. Discs were tested using a robotic arm equipped with a force sensor, which sheared them across smooth and rough surfaces at a fixed speed and distance. The arm was also used to vary the initial compression to test performance under both suction-dominant and friction-dominant preloads. For our designs, patterns with smaller papillae-like structures and channel spacing often produced higher peak forces than those with larger features. However, the design that withstood the highest shear load featured an intermediate pad size and channel spacing, potentially highlighting a balance between overall surface area and fluid channeling. Additionally, discs with surface patterns often outperformed the control discs (no pattern) on both smooth and rough surfaces, but performance was highly dependent on preload, with patterned discs exhibiting benefits with the higher "friction-dominant" preloads.

Enhanced wet grip with North American river otter paws

The semi-aquatic North American river otter (Lontra canadensis) has the unique challenge of navigating slippery algae-coated rocks. Unlike other river otter species, each rear paw of the North American river otter has a series of soft, circular, and keratinized plantar pads similar to the felt pads on the boots of fly fishermen. Surrounding these soft pads is a textured epidermal layer. In this combined experimental and numerical study, we investigate the influence of the plantar pads and surrounding skin on the otter's grip. We filmed an otter walking and performed materials testing and histology on preserved otter paws. We present experiments and numerical modeling of how the otter paw may help evacuate water when contacting the river bed. We hope this study will draw interest into natural amphibious grip mechanisms for use in sports and the military.

Holding in the stream: convergent evolution of suckermouth structures in Loricariidae (Siluriformes)

Suckermouth armoured catfish (Loricariidae) are a highly speciose and diverse freshwater fish family, which bear upper and lower lips forming an oral disc. Its hierarchical organisation allows the attachment to various natural surfaces. The discs can possess papillae of different shapes, which are supplemented, in many taxa, by small horny projections, i.e. unculi. Although these attachment structures and their working mechanisms, which include adhesion and interlocking, are rather well investigated in some selected species, the loricariid oral disc is unfortunately understudied in the majority of species, especially with regard to comparative aspects of the diverse oral structures and their relationship to the ecology of different species. In the present paper, we investigated the papilla and unculi morphologies in 67 loricariid species, which inhabit different currents and substrates. We determined four papilla types and eight unculi types differing by forms and sizes. Ancestral state reconstructions strongly suggest convergent evolution of traits. There is no obvious correlation between habitat shifts and the evolution of specific character states. From handling the structures and from drying artefacts we could infer some information about their material properties. This, together with their shape, enabled us to carefully propose hypotheses about mechanisms of interactions of oral disc structures with natural substrates typical for respective fish species.

Stiff skin, soft core: soft backings enhance the conformability and friction of fibre-reinforced adhesives

Biomimetic adhesives with a stiff fibre-reinforced base layer generate strong attachment, even without bioinspired micropatterning of the contact surface. However, current fibre-reinforced adhesive designs are still less versatile with respect to substrate variability than their biological counterparts. In this study, we enhance the comformability of a fibre-reinforced adhesive on curved substrates by adding bioinspired soft backings. We designed and fabricated soft backing variations (polyurethane foams and silicone hydroskeletons) with varying compressive stiffnesses that mimic the soft viscoelastic structures in the adhesive appendages of tree frogs, geckos and other animals. The backings were mounted on a smooth silicone layer enforced with a polyester mesh, and we experimentally investigated the contact area and friction performance of these adhesives on a curved substrate. The results show that the contact area and friction created by a fibre-reinforced adhesive with a soft backing in contact with a non-flat substrate scale inversely with backing stiffness. The integration of stiff fibre-reinforcement with a compressible backing represents an important step in bringing bioinspired adhesives out of the laboratory and into the real world, for example in soft robotic grippers. Moreover, our findings stimulate further research into the role of soft tissues in biological adhesive systems.

Influence of surface free energy of the substrate and flooded water on the attachment performance of stick insects (Phasmatodea) with different adhesive surface microstructures

Stick and leaf insects (Phasmatodea) are exclusively herbivores. As they settle in a broad range of habitats, they need to attach to and walk on a wide variety of plant substrates, which can vary in their surface free energy (SFE). The adhesive microstructures (AMs) on the euplantulae of phasmids are assumed to be adapted to such substrate properties. Moreover, the natural substrates can often be covered with water as a result of high relative humidity or rain. Although considerable experimental research has been carried out on different aspects of stick insect attachment, the adaptations to cope with the influence of flooded water on attachment performance remain unclear. To elucidate the role of AMs in this context, we here measured attachment forces in three species of stick insects with different AMs. The results show that attachment forces of the three species studied were influenced by the SFE and the presence of water: they all showed higher pull-off (vertical) and traction (horizontal) forces on dry surfaces, compared with when the surfaces were covered with a water film. However, the extent to which the surface properties influenced attachment differed depending on the species and its AMs. All three species showed approximately the same attachment performance on dry surfaces with different surface free energy but maintained attachment underwater to different extents.

Studying Stickiness: Methods, Trade-Offs, and Perspectives in Measuring Reversible Biological Adhesion and Friction

Controlled, reversible attachment is widely spread throughout the animal kingdom: from ticks to tree frogs, whose weights span from 2 mg to 200 g, and from geckos to mosquitoes, who stick under vastly different situations, such as quickly climbing trees and stealthily landing on human hosts. A fascinating and complex interplay of adhesive and frictional forces forms the foundation of attachment of these highly diverse systems to various substrates. In this review, we present an overview of the techniques used to quantify the adhesion and friction of terrestrial animals, with the aim of informing future studies on the fundamentals of bioadhesion, and motivating the development and adoption of new or alternative measurement techniques. We classify existing methods with respect to the forces they measure, including magnitude and source, i.e., generated by the whole body, single limbs, or by sub-structures. Additionally, we compare their versatility, specifically what parameters can be measured, controlled, and varied. This approach reveals critical trade-offs of bioadhesion measurement techniques. Beyond stimulating future studies on evolutionary and physicochemical aspects of bioadhesion, understanding the fundamentals of biological attachment is key to the development of biomimetic technologies, from soft robotic grippers to gentle surgical tools.

The Lesser-Known Transitions: Organismal Form and Function Across Abiotic Gradients

From minute-to-minute changes, or across daily, seasonal, or geological timescales, animals are forced to navigate dynamic surroundings. Their abiotic environment is continually changing. These changes could include alterations to the substrates animals locomote on, flow dynamics of the microhabitats they feed in, or even altitudinal shifts over migration routes. The only constancy in any organism's day-to-day existence is the heterogeneity of the habitats they move through and the gradients in the physical media (e.g., air, water) they live in. We explored a broad range of organismal transitions across abiotic gradients and investigated how these organisms modify their form, function, and behavior to accommodate their surrounding media. We asked the following questions: (1) What are some challenges common to animals in changing media or moving between media? (2) What are common solutions to these recurring problems? (3) How often are these common solutions instances of either convergence or parallelism? Our symposium speakers explored these questions through critical analysis of numerous datasets spanning multiple taxa, timescales, and levels of analysis. After discussions with our speakers, we suggest that the role of physical principles (e.g., drag, gravity, buoyancy, viscosity) in constraining morphology and shaping the realized niche has been underappreciated. We recommend that investigations of these transitions and corresponding adaptations should include comparisons at multiple levels of biological organization and timescale. Relatedly, studies of organisms that undergo habitat and substrate changes over ontogeny would be worthwhile to include in comparisons. Future researchers should ideally complement lab-based morphological and kinematic studies with observational and experimental approaches in the field. Synthesis of the findings of our speakers across multiple study systems, timescales, and transitional habitats suggests that behavioral modification and exaptation of morphology play key roles in modulating novel transitions between substrates.

Are they arboreal? Climbing abilities and mechanics in the red-backed salamander (Plethodon cinereus)

While red-backed salamanders (Plethodon cinereus) are most often observed in terrestrial forested areas, several studies report arboreal substrate use and climbing behavior. However, salamanders do not have any of the anatomical features commonly observed in specialized climbing species (e.g., claws, setae, suction cups). Instead, salamanders cling to surfaces using the shear and adhesive properties of their mucous layer. In this study, we explore the capabilities and spatiotemporal gait patterns of arboreal locomotion in the red-backed salamander as they move across twelve substrate conditions ranging in diameter, orientation, and roughness. On arboreal substrates, red-backed salamanders decreased locomotor speed, stride frequency, phase and stride length, and increased duty factor and stride duration. Such responses have been observed in other non-salamander species and are posited to increase arboreal stability. Furthermore, these findings indicate that amphibian locomotion, or at least the locomotor behavior of the red-backed salamander, is not stereotyped and that some locomotor plasticity is possible in response to the demands of the external environment. However, red-backed salamanders were unable to locomote on any small-diameter or vertically-oriented coarse substrates. This finding provides strong evidence that the climbing abilities of red-backed salamanders are attributable solely to mucous adhesion and that this species is unable to produce the necessary external "gripping" forces to achieve fine-branch arboreal locomotion or scale substrates where adhesion is not possible. The red-backed salamander provides an ideal model for arboreal locomotor performance of anatomically-unspecialized amphibians and offers insight into transitionary stages in the evolution of arborealism in this lineage.

Sticking to it: testing passive pull-off forces in waterfall-climbing fishes across challenging substrates

The pelvic sucker of Hawaiian waterfall climbing gobies allows these fishes to attach to substrates while climbing waterfalls tens to hundreds of meters tall. Climbing ability varies by species and may be further modulated by the physical characteristics of the waterfall substrate. In this study, we investigated the influence of surface wettability (hydrophobic versus hydrophilic surface charges) and substrate roughness on the passive adhesive system of four species of gobies with different climbing abilities. Overall, passive adhesive performance varied by species and substrate, with the strongest climbers showing the highest shear pull-off forces, particularly on rough surfaces. Thus, differences in passive adhesive performance may help to explain the ability of some species to migrate further upstream than others and contribute to their ability to invade new habitats.

Physical mechanisms behind the wet adhesion: From amphibian toe-pad to biomimetics

Some amphibians, such as tree frogs, torrent frogs, newts, are able to climb or attach to wet slippery smooth surfaces, even in a vertical or overhanging state, by their reliable reversible adhesions developed on the epidermal of toe pads. It is widely believed that such outstanding function originates from the possible factors of the specialized evolutions of surficial micro/nanostructures, the chemical components of secreted mucus, the solid-liquid behavior of epidermal and the bulk softness of toe pads. In this review, we summarize the main physical mechanisms of these factors behaving underlying the wet adhesion of toe pads from the researches on biological models to artificial counterparts. The discussion of the organism attachments, the interfacial physical forces and the switchable strategies for artificial wet adhesion are also included. The paper gives a deeply, comprehensively understanding of the characters of wet adhesives on amphibians, which performs necessarily for the new strategies of exploring artificial adhesive surfaces.

Toward Bioinspired Wet Adhesives: Lessons from Assessing Surface Structures of the Suction Disc of Intertidal Clingfish

The clingfish attaches to rough surfaces with considerable strength using an intricate suction disc, which displays complex surface geometries from structures called papillae. However, the exact role of these structures in adhesion is poorly understood. To investigate the relationship between papillae geometry and adhesive performance, we developed an image processing tool that analyzed the surface and structural complexity of papillae, which we then used to model hydrodynamic adhesion. Our tool allowed for the automated analysis of thousands of papillae in specimens across a range of body sizes. The results led us to identify spatial trends in papillae across the complex geometry of the suction disc and to establish fundamental structure-function relationships used in hydrodynamic adhesion. We found that the surface area of papillae changed within a suction disc and with fish size, but that the aspect ratios and channel width between papillae did not. Using a mathematical model, we found that the surface structures can adhere considerably when subjected to disturbances of moderate to high velocities. We concluded that a predominant role of the papillae is to leverage hydrodynamic adhesion and wet friction to reinforce the seal of the suction disc. Overall, the trends in papillae characteristics provided insights into bioinspired designs of surface microstructures for future applications in which adhesion is necessary to attach to diverse surfaces (in terrestrial or aquatic environments), even when subjected to disturbance forces of randomized directionality.

The Effects of Roughness and Wetness on Salamander Cling Performance

Animals clinging to natural surfaces have to generate attachment across a range of surface roughnesses in both dry and wet conditions. Plethodontid salamanders can be aquatic, semi-aquatic, terrestrial, arboreal, troglodytic, saxicolous, and fossorial and therefore may need to climb on and over rocks, tree trunks, plant leaves, and stems, as well as move through soil and water. Sixteen species of salamanders were tested to determine the effects of substrate roughness and wetness on maximum cling angle. Substrate roughness had a significant effect on maximum cling angle, an effect that varied among species. Substrates of intermediate roughness (asperity size 100-350 µm) resulted in the poorest attachment performance for all species. Small species performed best on smooth substrates, while large species showed significant improvement on the roughest substrates (asperity size 1000-4000 µm), possibly switching from mucus adhesion on a smooth substrate to an interlocking attachment on rough substrates. Water, in the form of a misted substrate coating and a flowing stream, decreased cling performance in salamanders on smooth substrates. However, small salamanders significantly increased maximum cling angle on wetted substrates of intermediate roughness, compared with the dry condition. Study of cling performance and its relationship to surface properties may cast light onto how this group of salamanders has radiated into the most speciose family of salamanders that occupies diverse habitats across an enormous geographical range.

Design of Tree-Frog-Inspired Adhesives

The adhesive toe pads of tree frogs have inspired the design of various so-called ‘smooth’ synthetic adhesives for wet environments. However, these adhesives do not reach the attachment performance of their biological models in terms of contact formation, maintenance of attachment, and detachment. In tree frogs, attachment is facilitated by an interconnected ensemble of superficial and internal morphological components, which together form a functional unit. To help bridging the gap between biological and bioinspired adhesives, in this review, we (1) provide an overview of the functional components of tree frog toe pads, (2) investigate which of these components (and attachment mechanisms implemented therein) have already been transferred into synthetic adhesives, and (3) highlight functional analogies between existing synthetic adhesives and tree frogs regarding the fundamental mechanisms of attachment. We found that most existing tree-frog-inspired adhesives mimic the micropatterned surface of the ventral epidermis of frog pads. Geometrical and material properties differ between these synthetic adhesives and their biological model, which indicates similarity in appearance rather than function. Important internal functional components such as fiber-reinforcement and muscle fibers for attachment control have not been considered in the design of tree-frog-inspired adhesives. Experimental work on tree-frog-inspired adhesives suggests that the micropatterning of adhesives with low-aspect-ratio pillars enables crack arresting and the drainage of interstitial liquids, which both facilitate the generation of van der Waals forces. Our analysis of experimental work on tree-frog-inspired adhesives indicates that interstitial liquids such as the mucus secreted by tree frogs play a role in detachment. Based on these findings, we provide suggestions for the future design of biomimetic adhesives. Specifically, we propose to implement internal fiber-reinforcements inspired by the fibrous structures in frog pads to create mechanically reinforced soft adhesives for high-load applications. Contractile components may stimulate the design of actuated synthetic adhesives with fine-tunable control of attachment strength. An integrative approach is needed for the design of tree-frog-inspired adhesives that are functionally analogous with their biological paradigm.

Cling performance and surface area of attachment in plethodontid salamanders

Plethodontid salamanders inhabit terrestrial, scansorial, arboreal and troglodytic habitats in which clinging and climbing allow them to access additional food and shelter as well as escape from unfavorable temperature and moisture conditions and ground-dwelling predators. Although salamanders lack claws and toe pads found in other taxa, they successfully cling to and climb on inclined, vertical and inverted substrates in nature. Maximum cling angle was tested on smooth acrylic, and the relationship between cling angle, body mass and surface area of attachment (contact area) was investigated. This study found that many salamander species can cling fully inverted using only a portion of their ventral surface area to attach. Salamanders fall into three functional groups based on mass and maximum cling angle: (1) high-performing, very small salamanders, (2) moderately high performing small and medium-sized salamanders and (3) low-performing large salamanders. They show significant differences in maximum cling angle, even between species of similar mass. In species of similar mass experiencing significantly different detachment stress (resulting from significantly different contact area), differences in morphology or behavior affect how much body surface is attached to the substrate. High performance in some species, such as Desmognathus quadramaculatus, is attributable to large contact area; low performance in a similarly sized species, Ensatina eschscholtzii, is due to behavior that negatively impacts contact area. There was no clear evidence of scaling of adhesive strength with increasing body size. Salamander maximum cling angle is the result of morphology and behavior impacting the detachment stresses experienced during clinging.

Nature inspired solid-liquid phase amphibious adhesive

Here we report a new class of bio-inspired solid-liquid adhesive, obtained by simple mechanical dispersion of PVDF (polyvinylidene fluoride) (solid spheres) into PDMS (polydimethylsiloxane) (liquid). The adhesive behavior arises from strong solid-liquid interactions. This is a chemical reaction free adhesive (no curing time) that can be repeatedly used and is capable of instantaneously joining a large number of diverse materials (metals, ceramic, and polymer) in air and underwater. The current work is a significant advance in the development of amphibious multifunctional adhesives and presents potential applications in a range of sealing applications, including medical ones.

Evolution of the Unique Anuran Pelvic and Hind limb Skeleton in Relation to Microhabitat, Locomotor Mode, and Jump Performance

Anurans (frogs and toads) have a unique pelvic and hind limb skeleton among tetrapods. Although their distinct body plan is primarily associated with saltation, anuran species vary in their primary locomotor mode (e.g., walkers, hoppers, jumpers, and swimmers) and are found in a wide array of microhabitats (e.g., burrowing, terrestrial, arboreal, and aquatic) with varying functional demands. Given their largely conserved body plan, morphological adaptation to these diverse niches likely results from more fine-scale morphological change. Our study determines how shape differences in Anura's unique pelvic and hind limb skeletal structures vary with microhabitat, locomotor mode, and jumping ability. Using microCT scans of preserved specimens from museum collections, we added 3D landmarks to the pelvic and hind limb skeleton of 230 anuran species. In addition, we compiled microhabitat and locomotor data from the literature for these species that span 52 of the 55 families of frogs and ∼210 million years of anuran evolution. Using this robust dataset, we examine the relationship between pelvic and hind limb morphology and phylogenetic history, allometry, microhabitat, and locomotor mode. We find pelvic and hind limb changes associated with shifts in microhabitat ("ecomorphs") and locomotor mode ("locomorphs") and directly relate those morphological changes to the jumping ability of individual species. We also reveal how individual bones vary in evolutionary rate and their association with phylogeny, body size, microhabitat, and locomotor mode. Our findings uncover previously undocumented morphological variation related to anuran ecological and locomotor diversification and link that variation to differences in jumping ability among species.

Mechanical Ecology-Taking Biomechanics to the Field

Synopsis Interdisciplinary research can have strong and surprising synergistic effects, leading to rapid knowledge gains. Equally important, it can help to reintegrate fragmented fields across increasingly isolated specialist sub-disciplines. However, the lack of a common identifier for research "in between fields" can make it difficult to find relevant research outputs and network effectively. We illustrate and address this issue for the emerging interdisciplinary hotspot of "mechanical ecology," which we define here as the intersection of quantitative biomechanics and field ecology at the organism level. We show that an integrative approach crucially advances our understanding in both disciplines by (1) putting biomechanical mechanisms into a biologically meaningful ecological context and (2) addressing the largely neglected influence of mechanical factors in organismal and behavioral ecology. We call for the foundation of knowledge exchange platforms such as meeting symposia, special issues in journals, and focus groups dedicated to mechanical ecology.

Tree frog adhesion biomimetics: opportunities for the development of new, smart adhesives that adhere under wet conditions

Enlarged adhesive toe pads on the tip of each digit allow tree frogs to climb smooth vertical and overhanging surfaces, and are effective in generating reversible adhesion under both dry and wet conditions. In this review, we discuss the complexities of the structure of tree frog toe pads in relation to their function and review their biomimetic potential. Of particular importance are the (largely) hexagonal epithelial cells surrounded by deep channels that cover the surface of each toe pad and the array of nanopillars on their surface. Fluid secreted by the pads covers the surface of each pad, so the pads adhere by wet adhesion, involving both capillarity and viscosity-dependent forces. The fabrication and testing of toe pad mimics are challenging, but valuable both for testing hypotheses concerning tree frog toe pad function and for developing toe pad mimics. Initial mimics involved the fabrication of hexagonal pillars mimicking the toe pad epithelial structure. More recent ones additionally replicate the nanostructures on their surface. Finally we describe some of the biomimetic applications that have been developed from toe pad mimics, which include both bioinspired adhesives and friction-generating devices. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology (part 2)'.

Convergence to the tiniest detail: vocal sac structure in torrent-dwelling frogs

Cascades and fast-flowing streams impose severe restrictions on acoustic communication, with loud broadband background noise hampering signal detection and recognition. In this context, diverse behavioural features, such as ultrasound production and visual displays, have arisen in the evolutionary history of torrent-dwelling amphibians. The importance of the vocal sac in multimodal communication is being increasingly recognized, and recently a new vocal sac visual display has been discovered: unilateral inflation of paired vocal sacs. In the diurnal stream-breeding Hylodidae from the Atlantic forest, where it was first described, this behaviour is likely to be enabled by a unique anatomical configuration of the vocal sacs. To assess whether other taxa share this exceptional structure, we surveyed torrent-dwelling species with paired vocal sacs across the anuran tree of life and examined the vocal sac anatomy of exemplar species across 18 families. We found striking anatomical convergence among hylodids and species of the distantly related basal ranid genera Staurois, Huia, Meristogenys and Amolops. Ancestral character state reconstruction identified three new synapomorphies for Ranidae. Furthermore, we surveyed the vocal sac configuration of other anuran species that perform visual displays and report observations on what appears to be unilateral inflation of paired vocal sacs, in Staurois guttatus – an extremely rare behaviour in anurans.

Humidity-tolerant rate-dependent capillary viscous adhesion of bee-collected pollen fluids

We report a two-phase adhesive fluid recovered from pollen, which displays remarkable rate tunability and humidity stabilization at microscopic and macroscopic scales. These natural materials provide a previously-unknown model for bioinspired humidity-stable and dynamically-tunable adhesive materials. In particular, two immiscible liquid phases are identified in bioadhesive fluid extracted from dandelion pollen taken from honey bees: a sugary adhesive aqueous phase similar to bee nectar and an oily phase consistent with plant pollenkitt. Here we show that the aqueous phase exhibits a rate-dependent capillary adhesion attributed to hydrodynamic forces above a critical separation rate. However, the performance of this adhesive phase alone is very sensitive to humidity due to water loss or uptake. Interestingly, the oily phase contributes scarcely to the wet adhesion. Rather, it spreads over the aqueous phase and functions as a barrier to water vapor that tempers the effects of humidity changes and stabilizes the capillary adhesion.

Estimating the maximum attachment performance of tree frogs on rough substrates

Tree frogs can attach to smooth and rough substrates using their adhesive toe pads. We present the results of an experimental investigation of tree frog attachment to rough substrates, and of the role of mechanical interlocking between superficial toe pad structures and substrate asperities in the tree frog species Litoria caerulea and Hyla cinerea. Using a rotation platform setup, we quantified the adhesive and frictional attachment performance of whole frogs clinging to smooth, micro-, and macrorough substrates. The transparent substrates enabled quantification of the instantaneous contact area during detachment by using frustrated total internal reflection. A linear mixed-effects model shows that the adhesive performance of the pads does not differ significantly with roughness (for nominal roughness levels of 0-15 µm) in both species. This indicates that mechanical interlocking does not contribute to the attachment of whole animals. Our results show that the adhesion performance of tree frogs is higher than reported previously, emphasising the biomimetic potential of tree frog attachment. Overall, our findings contribute to a better understanding of the complex interplay of attachment mechanisms in the toe pads of tree frogs, which may promote future designs of tree-frog-inspired adhesives.

Adhesion and running speed of a tropical arboreal ant (Cephalotes atratus) on wet substrates

In the tropical forest canopy, wingless worker ants must cling to and run along diverse vegetative surfaces with little protection from sun, wind and rain. Ants rely in part on their tiny adhesive tarsal pads to maintain sufficient contact with substrates to prevent falls under these varied conditions. Here, we examined the effects of substrate wettability and surface water on the tarsal pad adhesive performance of a common tropical arboreal ant. Ant adhesion was consistently higher on an intermediately wetting substrate (static water contact angle ca 90°) when resisting both perpendicular (normal) force and parallel (shear) force. Normal adhesion was maintained on intermediately wetting and hydrophobic substrates following the addition of rain-mimicking water droplets, whereas shear adhesion declined on all substrate types tested after wetting. Ant running speed was slower on wet substrates. On wood substrates, normal and shear adhesion declined with increasing wetness from dry, to misted, to water-soaked. These differences probably contributed to lower ant running speed on wet wood. The results of this study provide the first quantitative assessment of tropical arboreal ant adhesive performance under substrate conditions that are commonly encountered in the rainforest canopy.

Tree frog attachment: mechanisms, challenges, and perspectives

Tree frogs have the remarkable ability to attach to smooth, rough, dry, and wet surfaces using their versatile toe pads. Tree frog attachment involves the secretion of mucus into the pad-substrate gap, requiring adaptations towards mucus drainage and pad lubrication. Here, we present an overview of tree frog attachment, with focus on (i) the morphology and material of the toe pad; (ii) the functional demands on the toe pad arising from ecology, lifestyle, and phylogenetics; (iii) experimental data of attachment performance such as adhesion and friction forces; and (iv) potential perspectives on future developments in the field. By revisiting reported data and observations, we discuss the involved mechanisms of attachment and propose new hypotheses for further research. Among others, we address the following questions: Do capillary and hydrodynamic forces explain the strong friction of the toe pads directly, or indirectly by promoting dry attachment mechanisms? If friction primarily relies on van der Waals (vdW) forces instead, how much do these forces contribute to adhesion in the wet environment tree frogs live in and what role does the mucus play? We show that both pad morphology and measured attachment performance suggest the coaction of several attachment mechanisms (e.g. capillary and hydrodynamic adhesion, mechanical interlocking, and vdW forces) with situation-dependent relative importance. Current analytical models of capillary and hydrodynamic adhesion, caused by the secreted mucus and by environmental liquids, do not capture the contributions of these mechanisms in a comprehensive and accurate way. We argue that the soft pad material and a hierarchical surface pattern on the ventral pad surface enhance the effective contact area and facilitate gap-closure by macro- to nanoscopic drainage of interstitial liquids, which may give rise to a significant contribution of vdW interactions to tree frog attachment. Increasing the comprehension of the complex mechanism of tree frog attachment contributes to a better understanding of other biological attachment systems (e.g. in geckos and insects) and is expected to stimulate the development of a wide array of bioinspired adhesive applications.

The biomechanics of tree frogs climbing curved surfaces: a gripping problem

The adhesive mechanisms of climbing animals have become an important research topic because of their biomimetic implications. We examined the climbing abilities of hylid tree frogs on vertical cylinders of differing diameter and surface roughness to investigate the relative roles of adduction forces (gripping) and adhesion. Tree frogs adhere using their toe pads and subarticular tubercles, the adhesive joint being fluid-filled. Our hypothesis was that on an effectively flat surface (adduction forces on the largest 120 mm diameter cylinder were insufficient to allow climbing), adhesion would effectively be the only means by which tree frogs could climb, but on the 44 and 13 mm diameter cylinders, frogs could additionally utilise adduction forces by gripping the cylinder either with their limbs outstretched or by grasping around the cylinder with their digits, respectively. The frogs' performance would also depend on whether the surfaces were smooth (easy to adhere to) or rough (relatively non-adhesive). Our findings showed that climbing performance was highest on the narrowest smooth cylinder. Frogs climbed faster, frequently using a 'walking trot' gait rather than the 'lateral sequence walk' used on other cylinders. Using an optical technique to visualise substrate contact during climbing on smooth surfaces, we also observed an increasing engagement of the subarticular tubercles on the narrower cylinders. Finally, on the rough substrate, frogs were unable to climb the largest diameter cylinder, but were able to climb the narrowest one slowly. These results support our hypotheses and have relevance for the design of climbing robots.

The use of clamping grips and friction pads by tree frogs for climbing curved surfaces

Most studies on the adhesive mechanisms of climbing animals have addressed attachment against flat surfaces, yet many animals can climb highly curved surfaces, like twigs and small branches. Here we investigated whether tree frogs use a clamping grip by recording the ground reaction forces on a cylindrical object with either a smooth or anti-adhesive, rough surface. Furthermore, we measured the contact area of fore and hindlimbs against differently sized transparent cylinders and the forces of individual pads and subarticular tubercles in restrained animals. Our study revealed that frogs use friction and normal forces of roughly a similar magnitude for holding on to cylindrical objects. When challenged with climbing a non-adhesive surface, the compressive forces between opposite legs nearly doubled, indicating a stronger clamping grip. In contrast to climbing flat surfaces, frogs increased the contact area on all limbs by engaging not just adhesive pads but also subarticular tubercles on curved surfaces. Our force measurements showed that tubercles can withstand larger shear stresses than pads. SEM images of tubercles revealed a similar structure to that of toe pads including the presence of nanopillars, though channels surrounding epithelial cells were less pronounced. The tubercles' smaller size, proximal location on the toes and shallow cells make them probably less prone to buckling and thus ideal for gripping curved surfaces.

Soft Matter Lubrication: Does Solid Viscoelasticity Matter?

Classical lubrication theory is unable to explain a variety of phenomena and experimental observations involving soft viscoelastic materials, which are ubiquitous and increasingly used in e.g. engineering and biomedical applications. These include unexpected ruptures of the lubricating film and a friction-speed dependence, which cannot be elucidated by means of conventional models, based on time-independent stress-strain constitutive laws for the lubricated solids. A new modeling framework, corroborated through experimental measurements enabled via an interferometric technique, is proposed to address these issues: Solid/fluid interactions are captured thanks to a coupling strategy that makes it possible to study the effect that solid viscoelasticity has on fluid film lubrication. It is shown that a newly defined visco-elasto-hydrodynamic lubrication (VEHL) regime can be experienced depending on the degree of coupling between the fluid flow and the solid hysteretic response. Pressure distributions show a marked asymmetry with a peak at the flow inlet, and correspondingly, the film thickness reveals a pronounced shrinkage at the flow outlet; friction is heavily influenced by the viscoelastic hysteresis which is experienced in addition to the viscous losses. These features show significant differences with respect to the classical elasto-hydrodynamic lubrication (EHL) regime response that would be predicted when solid viscoelasticity is neglected. A simple yet powerful criterion to assess the importance of viscoelastic solid contributions to soft matter lubrication is finally proposed.

A micropatterned elastomeric surface with enhanced frictional properties under wet conditions and its application

Engineered surfaces that have high friction under wet or lubricated conditions are important in many practical applications. However, it is not easy to achieve stable high friction under wet conditions because a layer of fluid prevents direct solid-solid contact. Here, we report a micropatterned elastomeric surface with superior wet friction. The surface has unique arch-shaped microstructures arrayed in a circle on the surface to provide high friction on wet or flooded surfaces. The arch-shaped micropatterned surface exhibits remarkably enhanced and stable friction under wet conditions, surpassing even the performance of the hexagonal patterns of tree frogs, owing to the large contact surface and the optimal shape of drainage channels. Robotic substrate transportation systems equipped with the micropatterned surfaces can manipulate a delicate wet substrate without any sliding in a highly stable and reproducible manner, demonstrating the superior frictional capabilities of the surface under wet conditions.

Hybrid Surface Patterns Mimicking the Design of the Adhesive Toe Pad of Tree Frog

Biological materials achieve directional reinforcement with oriented assemblies of anisotropic building blocks. One such example is the nanocomposite structure of keratinized epithelium on the toe pad of tree frogs, in which hexagonal arrays of (soft) epithelial cells are crossed by densely packed and oriented (hard) keratin nanofibrils. Here, a method is established to fabricate arrays of tree-frog-inspired composite micropatterns composed of polydimethylsiloxane (PDMS) micropillars embedded with polystyrene (PS) nanopillars. Adhesive and frictional studies of these synthetic materials reveal a benefit of the hierarchical and anisotropic design for both adhesion and friction, in particular, at high matrix-fiber interfacial strengths. The presence of PS nanopillars alters the stress distribution at the contact interface of micropillars and therefore enhances the adhesion and friction of the composite micropattern. The results suggest a design principle for bioinspired structural adhesives, especially for wet environments.

Effective Elastic Modulus of Structured Adhesives: From Biology to Biomimetics

Micro- and nano-hierarchical structures (lamellae, setae, branches, and spatulae) on the toe pads of many animals play key roles for generating strong but reversible adhesion for locomotion. The hierarchical structure possesses significantly reduced, effective elastic modulus (Eeff), as compared to the inherent elastic modulus (Einh) of the corresponding biological material (and therefore contributes to a better compliance with the counterpart surface). Learning from nature, three types of hierarchical structures (namely self-similar pillar structure, lamella⁻pillar hybrid structure, and porous structure) have been developed and investigated.

When the going gets rough - studying the effect of surface roughness on the adhesive abilities of tree frogs

Tree frogs need to adhere to surfaces of various roughnesses in their natural habitats; these include bark, leaves and rocks. Rough surfaces can alter the effectiveness of their toe pads, due to factors such as a change of real contact area and abrasion of the pad epithelium. Here, we tested the effect of surface roughness on the attachment abilities of the tree frog Litoria caerulea. This was done by testing shear and adhesive forces on artificial surfaces with controlled roughness, both on single toe pads and whole animal scales. It was shown that frogs can stick 2-3 times better on small scale roughnesses (3-6 µm asperities), producing higher adhesive and frictional forces, but relatively poorly on the larger scale roughnesses tested (58.5-562.5 µm asperities). Our experiments suggested that, on such surfaces, the pads secrete insufficient fluid to fill the space under the pad, leaving air pockets that would significantly reduce the Laplace pressure component of capillarity. Therefore, we measured how well the adhesive toe pad would conform to spherical asperities of known sizes using interference reflection microscopy. Based on experiments where the conformation of the pad to individual asperities was examined microscopically, our calculations indicate that the pad epithelium has a low elastic modulus, making it highly deformable.

Soft matter dynamics: Accelerated fluid squeeze-out during slip

Using a Leonardo da Vinci experimental setup (constant driving force), we study the dependency of lubricated rubber friction on the time of stationary contact and on the sliding distance. We slide rectangular rubber blocks on smooth polymer surfaces lubricated by glycerol or by a grease. We observe a remarkable effect: during stationary contact the lubricant is only very slowly removed from the rubber-polymer interface, while during slip it is very rapidly removed resulting (for the grease lubricated surface) in complete stop of motion after a short time period, corresponding to a slip distance typically of order only a few times the length of the rubber block in the sliding direction. For an elastically stiff material, poly(methyl methacrylate), we observe the opposite effect: the sliding speed increases with time (acceleration), and the lubricant film thickness appears to increase. We propose an explanation for the observed effect based on transient elastohydrodynamics, which may be relevant also for other soft contacts.

Nanoscale friction and adhesion of tree frog toe pads

Tree frogs have become an object of interest in biomimetics due to their ability to cling to wet and slippery surfaces. In this study, we have investigated the adhesion and friction behavior of toe pads of White's tree frog (Litoria caerulea) using atomic force microscopy (AFM) in an aqueous medium. Facilitating special types of AFM probes with radii of ∼400 nm and ∼13 μm, we were able to sense the frictional response without damaging the delicate nanopillar structures of the epithelial cells. While we observed no significant adhesion between both types of probes and toe pads in wet conditions, frictional forces under such conditions were very pronounced and friction coefficients amounted between 0.3 and 1.1 for the sliding friction between probes and the epithelial cell surfaces.

Extreme positive allometry of animal adhesive pads and the size limits of adhesion-based climbing

Organismal functions are size-dependent whenever body surfaces supply body volumes. Larger organisms can develop strongly folded internal surfaces for enhanced diffusion, but in many cases areas cannot be folded so that their enlargement is constrained by anatomy, presenting a problem for larger animals. Here, we study the allometry of adhesive pad area in 225 climbing animal species, covering more than seven orders of magnitude in weight. Across all taxa, adhesive pad area showed extreme positive allometry and scaled with weight, implying a 200-fold increase of relative pad area from mites to geckos. However, allometric scaling coefficients for pad area systematically decreased with taxonomic level and were close to isometry when evolutionary history was accounted for, indicating that the substantial anatomical changes required to achieve this increase in relative pad area are limited by phylogenetic constraints. Using a comparative phylogenetic approach, we found that the departure from isometry is almost exclusively caused by large differences in size-corrected pad area between arthropods and vertebrates. To mitigate the expected decrease of weight-specific adhesion within closely related taxa where pad area scaled close to isometry, data for several taxa suggest that the pads' adhesive strength increased for larger animals. The combination of adjustments in relative pad area for distantly related taxa and changes in adhesive strength for closely related groups helps explain how climbing with adhesive pads has evolved in animals varying over seven orders of magnitude in body weight. Our results illustrate the size limits of adhesion-based climbing, with profound implications for large-scale bio-inspired adhesives.

Gecko Adhesion on Wet and Dry Patterned Substrates

Perhaps one of the most astounding characteristics of the gecko adhesive system is its versatility. Geckos can locomote across complex substrates in a variety of conditions with apparent ease. In contrast, many of our synthetic pressure sensitive adhesives fail on substrates that are dirty, wet or rough. Although many studies have investigated the effect of environmental challenges on performance, the interaction of multiple, potentially compromising variables is studied less often. Here we focus on substrate structure and surface water, both of which are highly relevant to the biological system and to synthetic design. To do this we utilized a highly controlled, patterned substrate (Sharklet^®, by Sharklet^® Technologies Inc.). This allowed us to test independently and jointly the effects of reduced surface area substrates, with a defined pattern, on adhesion in both air and water. Our results show that adhesion is not significantly impaired in air, whereas surface area and pattern significantly affect adhesion in water. These findings highlight the need to study multiple parameters that are relevant to the gecko adhesive system to further improve our understanding of the biological system and to design better, more versatile synthetics.

Bioinspired Surface for Surgical Graspers Based on the Strong Wet Friction of Tree Frog Toe Pads

Soft tissue damage is often at risk during the use of a surgical grasper, because of the strong holding force required to prevent slipping of the soft tissue in wet surgical environments. Improvement of wet friction properties at the interface between the surgical grasper and soft tissue can greatly reduce the holding force required and, thus, the soft tissue damage. To design and fabricate a biomimetic microscale surface with strong wet friction, the wet attachment mechanism of tree frog toe pads was investigated by observing their epithelial cell structure and the directionally dependent friction on their toe pads. Using these observations as inspiration, novel surface micropatterns were proposed for the surface of surgical graspers. The wet friction of biomimetic surfaces with various types of polygon pillar patterns involving quadrangular pillars, triangular pillars, rhomboid pillars, and varied hexagonal pillars were tested. The hexagonal pillar pattern exhibited improved wet frictional performance over the modern surgical grasper jaw pattern, which has conventional macroscale teeth. Moreover, the deformation of soft tissue in the bioinspired surgical grasper with a hexagonal pillar pattern is decreased, compared with the conventional surgical grasper.

Coupled effects of applied load and surface structure on the viscous forces during peeling

Tree frogs are able to take advantage of an array of epithelial cells in their toe pads to modulate their adhesion to surfaces under dry, wet, and flooded environments. It has been hypothesized that the interconnected channels separating the epithelial cells could reduce the hydrodynamic repulsion to facilitate contact under a completely submerged environment (flooded conditions). Using a custom-built apparatus we investigate the interplay between surface structure and loading conditions on the peeling force. By combining a normal approach and detachment by peeling we can isolate the effects of surface structure from the loading conditions. We investigate three surfaces: two rigid structured surfaces that consist of arrays of cylindrical posts and a flat surface as a control. We observe three regimes in the work required to separate the structured surface that depend on the fluid film thickness prior to pull out. These three regimes are based on hydrodynamics and our experimental results are compared with a simple scaling argument that relates the surface features to the different regimes observed. Overall we find that the work of separation of a structured surface is always less than or equal to that for a smooth surface when considering purely viscous contributions.

Morphological studies of the toe pads of the rock frog, Staurois parvus (family: Ranidae) and their relevance to the development of new biomimetically inspired reversible adhesives

The morphology of the toe epithelium of the rock frog, Staurois parvus (Family Ranidae), was investigated using a variety of microscopical techniques. The toe pad epithelium is stratified (four to five cell layers), the apical parts of the cells of the outermost layer being separated by fluid-filled channels. The surface of these cells is covered by a dense array of nanopillars, which also cover the surface of subarticular tubercles and unspecialized ventral epithelium of the toes, but not the dorsal epithelium. The apical portions of the outer two layers contain fibrils that originate from the nanopillars and are oriented approximately normal to the surface. This structure is similar to the pad structure of tree frogs of the families Hylidae and Rhacophoridae, indicating evolutionary convergence and a common evolutionary design for reversible attachment in climbing frogs. The main adaptation to the torrent habitat seems to be the straightness of the channels crossing the toe pad, which will assist in drainage of excess water. The presence of nanopillar arrays on all ventral surfaces of the toes resembles that on clingfish suckers and may be a specific adaptation for underwater adhesion and friction. The relevance of these findings to the development of new biomimetically inspired reversible adhesives is discussed.

Aquatic versus terrestrial attachment: Water makes a difference

Animal attachment to a substrate is very different in terrestrial and aquatic environments. We discuss variations in both the forces acting to detach animals and forces of attachment. While in a terrestrial environment gravity is commonly understood as the most important detachment force, under submerged conditions gravity is nearly balanced out by buoyancy and therefore matters little. In contrast, flow forces such as drag and lift are of higher importance in an aquatic environment. Depending on the flow conditions, flow forces can reach much higher values than gravity and vary in magnitude and direction. For many of the attachment mechanisms (adhesion including glue, friction, suction and mechanical principles such as hook, lock, clamp and spacer) significant differences have to be considered under water. For example, the main principles of dry adhesion, van der Waals forces and chemical bonding, which make a gecko stick to the ceiling, are weak under submerged conditions. Capillary forces are very important for wet adhesion, e.g., in terrestrial beetles or flies, but usually do not occur under water. Viscous forces are likely an important contributor to adhesion under water in some mobile animals such as torrent frogs and mayflies, but there are still many open questions to be answered. Glue is the dominant attachment mechanism of sessile aquatic animals and the aquatic realm presents many challenges to this mode of attachment. Viscous forces and the lack of surface tension under submerged conditions also affect frictional interactions in the aquatic environment. Moreover, the limitation of suction to the pressure difference at vacuum conditions can be ameliorated under water, due to the increasing pressure with water depth.

Use of biomimetic hexagonal surface texture in friction against lubricated skin

Smooth contact pads that evolved in insects, amphibians and mammals to enhance the attachment abilities of the animals' feet are often dressed with surface micropatterns of different shapes that act in the presence of a fluid secretion. One of the most striking surface patterns observed in contact pads of these animals is based on a hexagonal texture, which is recognized as a friction-oriented feature capable of suppressing both stick-slip and hydroplaning while enabling friction tuning. Here, we compare this design of natural friction surfaces to textures developed for working in similar conditions in disposable safety razors. When slid against lubricated human skin, the hexagonal surface texture is capable of generating about twice the friction of its technical competitors, which is related to it being much more effective at channelling of the lubricant fluid out of the contact zone. The draining channel shape and contact area fraction are found to be the most important geometrical parameters governing the fluid drainage rate.