Toe Pads Research Articles

Developing biomimetic PVA/PAA hydrogels with cellulose nanocrystals inspired by tree frog structures for superior wearable sensor functionality

This study developed a wearable sensor device featuring biomimetic structures inspired by tree frog toe pads to measure shoulder joint movements effectively. A PVA/PAA double-network hydrogel was formulated by mixing PVA and PAA in a 3:7 ratio, resulting in a Young's modulus of 7.2 kPa, closely matching human skin's mechanical properties. To enhance moisture retention, the hydrogel was supplemented with 20 mL glycerin, increasing its weight retention rate to 95 % after 28 days, a 5.6-fold improvement over glycerin-free hydrogels. Additionally, the incorporation of 0.2 wt% cellulose nanocrystals (CNC) increased the cross-linking density, reducing the swelling ratio from 124 % to 106 % and minimizing the impact of swelling cycles on mechanical properties from 22 % to 4 %. Treefrog-inspired microstructures were fabricated on the sensor surface using laser cutting and casting techniques, significantly enhancing adhesion under humid conditions, with normal, lateral, and peel-off adhesion forces improving by 500, 700, and 700 %, respectively. The sensor demonstrated accurate measurement of shoulder joint movements with an error margin of 6.3 % for polar angles (45°-135°) and 7.8 % for azimuth angles (0°-45°). The accompanying smartphone application provided real-time feedback, helping to prevent exercise injuries and improve user awareness and control of shoulder movements, making it suitable for both rehabilitation and athletic training.

Effect of elastic deformation on squeezing film lubrication properties of soft tribocontacts with microstructured surface

PurposeInspired by the high-friction performance of the soft toe pads of tree frogs, this study aims to investigate the effect of elastic deformation on the lubrication properties of squeezing films inside soft tribocontacts with microstructured surface under wet conditions.Design/methodology/approachA one-dimensional hydrodynamic extrusion model was used to study the film lubrication characteristics of conformal contact. The lubrication characteristics of the extruded film, including load-carrying capacity, liquid flow and surface elastic deformation, were obtained through the simultaneously iterative solution of the fluid-governing and deformation equations.FindingsThe results show that the hydrodynamic pressure is approximating parabolically and symmetrically distributed in the contact area, and the peak value appears in the center of the extrusion surface. Elastic deformation increases the thickness of the liquid film, weakens the bearing capacity and homogenizes the liquid flow rate of inside soft friction contact. The magnitude of this effect greatly increases as the initial liquid film thickness decreases. Moreover, the elastic deformation directly affects the average film thickness of the extrusion contact. Narrow and shallow microchannels are found to result in a more prominent elastic deformation on the microstructured soft surface.Originality/valueThese results present a design for soft tribocontacts suitable for submerged or wet environments involving high friction, such as wiper blades, in situ flexible electrons and underwater robots.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-02-2024-0049/

A soft bioinspired suction cup with tunable adhesion force using shape memory alloy

Abstract Suction cups has been widely utilized to grasp objects, but they typically encounter challenges with sealing failure and non-adjustable adhesion force. In this study, a bioinspired suction cup integrated with an shape memory alloy actuated module was proposed to solve these problems. The actuating performance under different input current was firstly investigated to ensure the effectiveness of the module. Then, inspired by the surface structures of the tree frog’s toe pad, the synthetic bioinspired suction cups with hexagonal microstructures at the rims were designed. The regular cup with soft and smooth rim was also fabricated for comparison study. Furthermore, the adhesion performance and surface adaptability of different two cups were studied in both dry and water conditions on substrates with various roughness levels. The results indicated that the proposed active bioinspired suction cup exhibited higher pull-off strength and better sealing on less rough substrates. The proposed bioinspired suction cup possessed the advantages of compactness and lightweight, thus demonstrating potential for integration into arrayed suction grippers.

Two-step printing bionic self-lubricating composite surface fabricated by selective laser melting ink-printed metal nanoparticles

The combination of self-lubricating materials and surface texture was a promising strategy to improve tribology performance. The designed parameters and manufacturing methods of the textured composite surface were pivotal in fully harnessing this synergistic effect. Inspired by natural structures such as the scapharca subcrenata shell, trimeresurus stejnegeri snakeskin, crocodile, and tree frog toe pad surface, bionic self-lubricating composite surfaces (BSCSs) with different textures were prepared on AISI 4140 steel substrate to enhance tribological properties. These BSCSs combined self-lubricating composite material (SLCM) with copper (Cu) material in bionic patterns using the selective laser melting ink-printing metal nanoparticles manufacturing method. Tribological properties and worn surfaces of the BSCSs with various design parameters were analyzed under dry sliding conditions. Results indicated that the synergy effects of printed Cu and SLCM texture effectively reduced friction coefficient and improved wear resistance of the BSCSs. The SLCM texture density and the size of array element in the BSCSs were found to influence the types of lubricating film formed during the sliding process, thereby affecting the tribological performance. This investigation provided a potential surface design strategy and a more flexible manufacturing method for surface engineering.

Toe pad morphology and adhesion in the miniaturized gecko, Chatogekko amazonicus (Gekkota: Sphaerodactylidae).

Chatogekko amazonicus is a miniaturized gecko from northern South America and is among the smallest of toe pad bearing lizards. The toe pads of C. amazonicus are miniscule, between 18% and 27% of the plantar surface area. We aimed to investigate the relationship between adhesive toe pad morphology, body size, and adhesive capabilities. Using scanning electron microscopy, we determine that the adhesive pads of C. amazonicus exhibit branched setae similar to those of other geckos, but that are generally much smaller. When compared with other gecko taxa, we show that C. amazonicus setae occupy a similar range of seta length: snout-vent length ratio and aspect ratio as other gekkonoid species (i.e. Gekkonidae, Phyllodactylidae, and Sphaerodactylidae). We demonstrate that C. amazonicus-even with its relatively small toe pads-is capable of climbing a smooth glass surface at a nearly vertical angle. We suggest that sphaerodactylids like C. amazonicus offer an excellent system for studying toe pad morphology and function in relation to miniaturization.

Developmental Patterns Underlying Variation in Form And Function Exhibited by House Gecko Toe Pads.

Adhesive toe pads have evolved numerous times over lizard evolutionary history, most notably in geckos. Despite significant variation in adult toe pad morphology across independent origins of toe pads, early developmental patterns of toe pad morphogenesis are similar among distantly related species. In these distant phylogenetic comparisons, toe pad variation is achieved during the later stages of development. We aimed to understand how toe pad variation is generated among species sharing a single evolutionary origin of toe pads (house geckos-Hemidactylus). We investigated toe pad functional variation and developmental patterns in three species of Hemidactylus, ranging from highly scansorial (H. platyurus), to less scansorial (H. turcicus), to fully terrestrial (H. imbricatus). We found that H. platyurus generated significantly greater frictional adhesive force and exhibited much larger toe pad area relative to the other two species. Furthermore, differences in the offset of toe pad extension phase during embryonic development results in the variable morphologies seen in adults. Taken together, we demonstrate how morphological variation is generated in a complex structure during development and how that variation relates in important functional outcomes.

Proximal toe wrap-around: a coverage technique for circumferential skin defects of the fingers

Circumferential skin defects of the fingers are a technical challenge. Although rare, their management should respect tissue organization and functional abilities. We report two cases of circumferential skin defect. Management used individually tailored “wrap-around” flaps taken from the hallux. The sample concerned the proximal cutaneous sheath of the first toe and the neurovascular pedicle of the first inter-metatarsal space. Nail and toe pad were spared. Both cases had complex circumferential skin defect of the finger, involving the neuro-vascular pedicle. Postoperative results were favorable, without functional limitation. The wrap-around technique provided skin coverage and also neurovascular pedicle reconstruction. Donor site damage was limited, with no functional consequences. This technique is a valuable option for management of circumferential skin defect of the finger.

Picking food by robot hand with tree-frog like pad in various wet conditions

Achieving stability with less squeeze in picking up wet-soft objects is still challenging for robots. To accomplish this challenge, preventing slippage between robotic grippers and an object is crucial. We used micropatterned pads on robotic grippers to enhance wet adhesion when picking up food items. This paper examines the role of micropattern interfaces in preventing slippage by experimental evaluations, in which soft robotic grippers picked up and released food samples such as tofu, quail egg, coffee jelly, konjac, and jelly under various wet conditions. A micropatterned pad, inspired by the toe pad of a tree-frog, comprises a large number of squared cells that are separated by channels. Normal pads without any micropattern were also made for comparison. Experimental results showed the micropatterned pad required less squeeze force than that of the normal pads, resulting in less deformation of a grasped object such as a piece of tofu. The potential of the micropatterned pad to prevent slippage between a robotic gripper and a fragile deformable object in various wet conditions without a complicated control method was demonstrated, thereby promising wider robotic applications in the food, service, and medical industries.

Nature-Inspired Wet Drug Delivery Platforms.

Nature has created various organisms with unique chemical components and multi-scale structures (e.g., foot proteins, toe pads, suckers, setose gill lamellae) to achieve wet adhesion functions to adapt to their complex living environments. These organisms can provide inspirations for designing wet adhesives with mediated drug release behaviors in target locations of biological surfaces. They exhibit conformal and enhanced wet adhesion, addressing the bottleneck of weaker tissue interface adhesion in the presence of body fluids. Herein, it is focused on the research progress of different wet adhesion and bioinspired fabrications, including adhesive protein-based adhesion and inspired adhesives (e.g., mussel adhesion); capillarity and Stefan adhesion and inspired adhesive surfaces (e.g., tree frog adhesion); suction-based adhesion and inspired suckers (e.g., octopus' adhesion); interlocking and friction-based adhesion and potential inspirations (e.g., mayfly larva and teleost adhesion). Other secreted protein-induced wet adhesion is also reviewed and various suckers for other organisms and their inspirations. Notably, one representative application scenario of these bioinspired wet adhesives is highlighted, where they function as efficient drug delivery platforms on target tissues and/or organs with requirements of both controllable wet adhesion and optimized drug release. Finally, the challenges of these bioinspired wet drug delivery platforms in the future is presented.

Early burst of parallel evolution describes the diversification of gecko toe pads

IntroductionSimilar traits appearing in distantly related organisms have intrigued scientists for generations. While anole lizards of the Caribbean are often touted as a classic example of repeated evolution, the adhesive toe pads of gecko lizards are an equally striking yet underappreciated example of relatedly evolved traits. The strikingly diverse toe pads of gecko lizards (Gekkota) have been gained and lost multiple times throughout the clade’s evolutionary history. In addition, distantly related genera have repeatedly evolved remarkably similar morphologies. This complicated combination of divergent and repeated evolution represents a useful system for understanding the evolution of complex structures, including repeated adaptation.MethodsUsing geometric morphometrics, we evaluated parallel morphological differences across families and expanded existing approaches fitting models of trait evolution to use geometric morphometric data. Adapting the use of phylogenetic independent contrasts for shape data, we conducted a node height test to investigate how toe pad shape has evolved across geckos.ResultsWe found multiple examples of significant parallel differences in toe pad morphology and support for a model of early burst morphological evolution.DiscussionOur results suggest the diversification of Gekkotan toe pads included repeated parallel changes from padless ancestral morphologies to derived pad bearing morphologies. This morphological diversification occurred rapidly early in the diversification of gecko families and genera and slowed more recently, during diversification within genera.

Actin-templated Structures: Nature's Way to Hierarchical Surface Patterns (Gecko's Setae as Case Study).

The hierarchical design of the toe pad surface in geckos and its reversible adhesiveness have inspired material scientists for many years. Micro- and nano-patterned surfaces with impressive adhesive performance have been developed to mimic gecko's properties. While the adhesive performance achieved in some examples has surpassed living counterparts, the durability of the fabricated surfaces is limited and the capability to self-renew and restore function-inherent to biological systems-is unimaginable. Here the morphogenesis of gecko setae using skin samples from the Bibron´sgecko (Chondrodactylus bibronii) is studied. Gecko setae develop as specialized apical differentiation structures at a distinct cell-cell layer interface within the skin epidermis. A primary role for F-actin and microtubules as templating structural elements is necessary for the development of setae's hierarchical morphology, and a stabilization role of keratins and corneus beta proteins is identified.Setae grow fromsingle cells in a bottom layer protruding into four neighboring cells in the upper layer. The resulting multicellular junction can play a role during shedding by facilitating fracture of the cell-cell interface and release of the high aspect ratio setae. The results contribute to the understanding of setae regeneration and may inspire future concepts to bioengineer self-renewable patterned adhesive surfaces.

A Reversible, Versatile Skin‐Attached Haptic Interface Platform with Bioinspired Interconnection Architectures Capable of Resisting Sweat and Vibration

AbstractA stable conformal interface technology for rough and sweaty complex skin is essential for a haptic interface capable of delivering sophisticated mechanical stimuli. However, conventional polymeric/hydrogel‐based skin adhesives cannot maintain adequate adhesion interaction performance at the haptic interface due to repetitive vibrations or sweaty skin. This study reports a reversible, versatile skin‐attached haptic interface platform, which embeds the hybrid architecture of a water‐drainable hexagonal array of frog toe pads and the energy‐dissipation matrix of snail pedal muscles with interconnected structures. The hybrid frog–snail‐inspired adhesive patch exhibits remarkable adhesion in pulling and shear directions under both dry and sweaty conditions. Furthermore, the microchannels between the hexagonal array can effectively drain liquid under sweaty conditions while also enhancing skin‐conformal contacts. The adhesion force enhanced by energy‐dissipation is analyzed considering a simple theory based on the adhesion, elastic, dissipation energies, and geometric features, resulting in the vibration‐resistant characteristics against diverse dry and wet vibration environments (vibrational frequency: 1–150 Hz). Bioinspired integrated skin‐attached haptic interface platform demonstrates the versatility of being reversibly applicable to various skin surfaces such as fingers, arms, and legs, yielding the feasibility of dynamic handling a basketball with multiple contacts and impacts in virtual reality (VR).

Bio-inspired anti-slip and anti-adhesion surface with hemispherical microstructures for wafer handling

Natural climbers leverage strong friction and minimal adhesion for effective multidirectional movement. Toe pads of species like tree frogs display distinct microstructures with protrusions. Such insights have spurred the formulation of a biomimetic microstructure array for wafer handling applications. The curvature of these protrusions was meticulously analyzed to decipher contact mechanics. Moreover, the intricate balance between strong-friction and weak-adhesion mechanisms was evaluated, considering design variations across surface roughness levels. This culminated in a crafted microstructure array optimized for wafer-handling, highlighting the practicality of bioinspired friction enhancement in contemporary engineering domains.

Multi-objective design and optimization of high cushioning bionic shoe midsole under limited thickness of forefoot

To enhance the shock-absorbing performance of the midsole in conditions where thickness is limited, particularly in the forefoot region, this paper presents a novel design inspired by the structure and material assembly characteristics of ostrich metatarsophalangeal joints and toe pads. The shock-absorbing performance of the bionic unit was validated through finite element simulations and experimental testing using 3D-printed samples. In order to unravel the underlying mechanisms that enhance the cushioning performance, optimization factors for the forefoot bionic cushioning unit were determined based on the cushioning mechanism. The influence of the cantilever angle (La), cantilever taper (Lo), cantilever spacing (L3), thickness of the elastic energy-absorbing component (L4), and selected materials on the cushioning performance was discussed. The results demonstrate that with the configuration of La:Lo:L3:L4 = 60°:0.6:9 mm:3 mm and the outer frame material to elastic energy-absorbing component material ratio of T-4:T-2, compared to the non-optimized unit, the peak negative acceleration of the forefoot bionic unit has been reduced by 32.64%.Based on the optimization results, the production of the bionic cushioning sports shoes and the control shoes was completed. Finally, human wear experiments have shown that the bionic shoes exhibit a 23.7% to 29.8% increase in impact absorption compared to control shoes.

Micro-adhesive structure inspired by tree frog toe pads fabricated by femtosecond laser processing of PVA sponge

Tree frogs of the species Zhangixalus arboreus are known to generate high adhesive force in wet environments due to the microstructure of their toe pads. Inspired by this toe pads, we fabricated a micro-adhesive structure with hexagonal channels (500, 375, 250, 188, and 125 μm per side) on the surface of a polyvinyl alcohol sponge. Femtosecond laser processing was used to create fine grooves on the surface of the sponge. When the sponge is pressed against the object, the liquid in the sponge is released onto the contact surface. In wet conditions, it is important to maintain the proper thickness of the liquid phase between the microstructure and the object, and this is achieved by the sponge. The characteristics of friction (shear force) between soft and hard objects differ from those between hard materials. When the liquid present on the contact surface is very small, the surface tension of the liquid phase causes the formation of numerous microcapillary bridges, which generate shear forces. The shear force was evaluated by soaking the sponge with water, pressing (300, 400, 500 μm) a flat or uneven surface against the sponge, and then sliding (20, 40 mm/s) the object. The maximum shear force was 0.22 N for the flat surface and 0.34 N for the uneven surface. It was found that the shear force became smaller when the structure became too fine. This phenomenon is due to the agglomeration of the microstructures.

The first hadrosaurid trackway from the horseshoe canyon formation (campanian/maastrichtian) of Alberta, Canada

The Horseshoe Canyon Formation has provided a wealth of vertebrate skeletal fossils over the past 100 years of collection. Vertebrate trace fossils such as footprints and trackways, however, have been markedly underrepresented. Trackways can provide a wealth of information regarding behaviour and soft tissues reflective of autopodial anatomy. Discovery of several giant, high-fidelity ichnites in series from the Morrin Member represents the first definitive dinosaur trackway reported from the Horseshoe Canyon Formation. The trackway is composed of three footprint casts in series, alongside 22 associated ichnites. The ichnites were formed in soft sediment molds along a non-uniform walking surface. The deep substrates allowed for the preservation of both the plantar surface and dorsal components of the trackmakers’ feet. Some ichnites show soft tissue morphologies like interdigital webbing, toe pads, and skin impressions. Presence of these features and small kidney-shaped forelimb prints allow referral to Hadrosauropodus langstoni. The preservation of the trackway demonstrates asymmetries useful for comparison of the relief and impression surfaces of prints using both discrete and morphometric characteristics, improving confidence in identifying partial or isolated Hadrosauropodus prints of various modes of formation.

Bionic Structure Inspired by Tree Frogs to Enhance Damping Performance.

Magnetic fluid shock absorbers (MFSAs) have been successfully utilized to eliminate microvibrations of flexible spacecraft structures. The method of enhancing the damping efficiency of MFSAs has always been a critical issue. To address this, we drew inspiration from the tree frog's toe pads, which exhibit strong friction due to their unique surface structure. Using 3D printing, we integrated bionic textures copied from tree frog's toe pad surfaces onto MFSAs, which is the first time to combine bionic design and MFSAs. Additionally, this is also the first time that surface textures have been applied to MFSAs. However, we also had to consider practical engineering applications and manufacturing convenience, so we modified the shape of bionic textures. To do so, we used an edge extraction algorithm for image processing and obtained recognition results. After thorough consideration, we chose hexagon as the shape of surface textures instead of bionic textures. For theoretical analysis, a magnetic field-flow field coupling dynamic model for MFSAs was built for the first time to simulate the magnetic fluid (MF) flow in one oscillation cycle. Using this model, the flow rate contours of the MF were obtained. It was observed that textures cause vortexes to form in the MF layer, which produced an additional velocity field. This increased the shear rate, ultimately leading to an increase in flow resistance. Finally, we conducted vibration reduction experiments and estimated damping characteristics of the proposed MFSAs to prove the effectiveness of both bionic texture and hexagon surface textures. Fortunately, we concluded that hexagon surface textures not only improve the damping efficiency of MFSAs but also require less MF mass.

Reappraising the evolutionary history of the largest known gecko, the presumably extinct Hoplodactylus delcourti, via high-throughput sequencing of archival DNA

Hoplodactylus delcourti is a presumably extinct species of diplodactylid gecko known only from a single specimen of unknown provenance. It is by far the largest known gekkotan, approximately 50% longer than the next largest-known species. It has been considered a member of the New Zealand endemic genus Hoplodactylus based on external morphological features including shared toe pad structure. We obtained DNA from a bone sample of the only known specimen to generate high-throughput sequence data suitable for phylogenetic analysis of its evolutionary history. Complementary sequence data were obtained from a broad sample of diplodactylid geckos. Our results indicate that the species is not most closely related to extant Hoplodactylus or any other New Zealand gecko. Instead, it is a member of a clade whose living species are endemic to New Caledonia. Phylogenetic comparative analyses indicate that the New Caledonian diplodactylid clade has evolved significantly more disparate body sizes than either the Australian or New Zealand clades. Toe pad structure has changed repeatedly across diplodactylids, including multiple times in the New Caledonia clade, partially explaining the convergence in form between H. delcourti and New Zealand Hoplodactylus. Based on the phylogenetic results, we place H. delcourti in a new genus.

Shedding skin renews gecko's grip

Grazed knees are a fact of life for most kids, but human skin heals gradually and scabs are soon gone. Geckos, on the other hand, shed their skin all at once, repairing scrapes and tears at the same time. So when Rishab Pillai from James Cook University, Australia, noticed damage on the toe pads of velvet geckos, he was curious to find out how breaks in the adhesive skin on gecko soles affects their ability to cling to surfaces and whether sloughing their skin and rejuvenating adhesive structures restores their sticking powers. Pillai housed 27 geckos – including northern velvet geckos (Oedura castelnaui), ocellated velvet geckos (Oedura monilis) and dowdier Kristin's spiny tailed geckos (Strophurus krisalys) – in plastic enclosures that would be gentle on their feet. Then, he photographed the reptiles’ right feet – front and back – with help from Jendrian Riedel (Bielefeld University, Germany) and Lin Schwarzkopf (James Cook University) to calculate each gecko's sticky surface area. Next, he periodically checked the adhesive strength of the reptiles’ feet over 30 days as the animals walked on horizontal glass by giving them a gentle tug backwards – resulting in wear and tear to the soles of their feet – until the geckos eventually discarded their aged skin. Sure enough, the geckos’ ability to cling on declined after each tug and Pillai could see damaged areas that were no longer sticky on the animals’ toes.Once the reptiles had renewed their skin, Pillai allowed them to scamper around their enclosures for several weeks to find out how normal wear and tear affected their adhesive strength and, sure enough, it took less of a toll than tugging repeatedly had. Finally, Pillai checked the geckos’ adhesion within a few days of them replacing their skin again and this time the reptiles’ stickiness recovered almost perfectly.So, shedding their skin replaces damaged sticking structures that allow geckos to get a grip on any surface, and Pillai and his colleagues are keen to find out how clambering on more rugged surfaces might impact the reptiles’ extraordinary stickability in the run up to getting a new skin.

Osteological and histological comparison of the development of the interphalangeal intercalary skeletal element between hyloid and ranoid anurans.

Some frog species have a unique skeletal element, referred to as the intercalary element (IE), in the joints between the terminal and subterminal phalanges of all digits. IEs are composed of cartilage or connective tissue and have a markedly differ shape than the phalanges. IEs are highly related to the arboreal lifestyle and toe pads. The IE is found only in neobatrachian frogs among anurans, suggesting that it is a novelty of Neobatrachia. IEs are widely distributed among multiple neobatrachian lineages and are found in the suborders Hyloides and Ranoides (the two major clades in Neobatrachia). However, it is unclear whether the IEs found in multiple linages resulted from convergent evolution. Therefore, in this study, we aimed to examine how similar or different the developmental trajectories of the IEs are between Hyloides and Ranoides. To that end, we compared the osteological and histological developmental processes of the IEs of the hyloid frog Dryophytes japonicus and the ranoid frog Zhangixalus schlegelii. Both species shared the same IE-initiation site and level of tissue differentiation around the IE when it began to form in tadpoles, although the IE developments initiated at different stages which were determined by external criteria. These results suggest that similar mechanisms drive IE formation in the digits of both species, supporting the hypothesis that the IEs did not evolve convergently. This article is protected by copyright. All rights reserved.