Advances in adhesion of microneedles for bioengineering

Microneedles have provided promising platforms in various fields thanks to their safety, painlessness, minimal invasiveness and ease of operation. The excellent adhesion of microneedles is the key characteristic to achieve long-term and comfortable treatment. However, a complex environment, such as the roughness of skin, various bodily fluids in vivo, and the movement of the body, presents great challenges to the adhesion characteristics of microneedles. This review mainly reports the remarkable adhesion properties of microneedles based on interlocking by shape effects, chemical bonds, and suction forces. Firstly, the main mechanisms of adhesion and various types of microneedles are introduced, with an emphasis on the progress in adhesive microneedles. Combined with the preparation and application of microneedles, the challenges and future trends of adhesive microneedles are discussed.

A Biomimetic Adhesive Disc for Robotic Adhesion Sliding Inspired by the Net-Winged Midge Larva

Net-winged midge larvae (Blephariceridae) are known for their remarkable ability to adhere to and crawl on the slippery surfaces of rocks in fast-flowing and turbulent alpine streams, waterfalls, and rivers. This remarkable performance can be attributed to the larvae's powerful ventral suckers. In this article, we first develop a theoretical model of the piston-driven sucker that considers the lubricated state of the contact area. We then implement a piston-driven robotic sucker featuring a V-shaped notch to explore the adhesion-sliding mechanism. Each biomimetic larval sucker has the unique feature of an anterior-facing V-shaped notch on its soft disc rim; it slides along the shear direction while the entire disc surface maintains powerful adhesion on the benthic substrate, just like the biological counterpart. We found that this biomimetic sucker can reversibly transit between "high friction" (4.26 ± 0.34 kPa) and "low friction" (0.41 ± 0.02 kPa) states due to the piston movement, resulting in a frictional enhancement of up to 93.9%. We also elucidate the frictional anisotropy (forward/backward force ratio: 0.81) caused by the V-shaped notch. To demonstrate the robotic application of this adhesion-sliding mechanism, we designed an underwater crawling robot Adhesion Sliding Robot-1 (ASR-1) equipped with two biomimetic ventral suckers. This robot can successfully crawl on a variety of substrates such as curved surfaces, sidewalls, and overhangs and against turbulent water currents with a flow speed of 2.4 m/s. In addition, we implemented a fixed-wing aircraft Adhesion Sliding Robot-2 (ASR-2) featuring midge larva-inspired suckers, enabling transit from rapid water surface gliding to adhesion sliding in an aquatic environment. This adhesion-sliding mechanism inspired by net-winged midge larvae may pave the way for future robots with long-term observation, monitoring, and tracking capabilities in a wide variety of aerial and aquatic environments.

Nature-inspired adhesive systems

Many organisms in nature thrive in intricate habitats through their unique bio-adhesive surfaces, facilitating tasks such as capturing prey and reproduction. It's important to note that the remarkable adhesion properties found in these natural biological surfaces primarily arise from their distinct micro- and nanostructures and/or chemical compositions. To create artificial surfaces with superior adhesion capabilities, researchers delve deeper into the underlying mechanisms of these captivating adhesion phenomena to draw inspiration. This article provides a systematic overview of various biological surfaces with different adhesion mechanisms, focusing on surface micro- and nanostructures and/or chemistry, offering design principles for their artificial counterparts. Here, the basic interactions and adhesion models of natural biological surfaces are introduced first. This will be followed by an exploration of research advancements in natural and artificial adhesive surfaces including both dry adhesive surfaces and wet/underwater adhesive surfaces, along with relevant adhesion characterization techniques. Special attention is paid to stimulus-responsive smart artificial adhesive surfaces with tunable adhesive properties. The goal is to spotlight recent advancements, identify common themes, and explore fundamental distinctions to pinpoint the present challenges and prospects in this field.

Stickiness in shear: stiffness, shape, and sealing in bioinspired suction cups affect shear performance on diverse surfaces

Aquatic organisms utilizing attachment often contend with unpredictable environments that can dislodge them from substrates. To counter these forces, many organisms (e.g. fish, cephalopods) have evolved suction-based organs for adhesion. Morphology is diverse, with some disc shapes deviating from a circle to more ovate designs. Inspired by the diversity of multiple aquatic species, we investigated how bioinspired cups with different disc shapes performed in shear loading conditions. These experiments highlighted pertinent physical characteristics found in biological discs (regions of stiffness, flattened margins, a sealing rim), as well as ecologically relevant shearing conditions. Disc shapes of fabricated cups included a standard circle, ellipses, and other bioinspired designs. To consider the effects of sealing, these stiff silicone cups were produced with and without a soft rim. Cups were tested using a force-sensing robotic arm, which directionally sheared them across surfaces of varying roughness and compliance in wet conditions while measuring force. In multiple surface and shearing conditions, elliptical and teardrop shapes outperformed the circle, which suggests that disc shape and distribution of stiffness may play an important role in resisting shear. Additionally, incorporating a soft rim increased cup performance on rougher substrates, highlighting interactions between the cup materials and surfaces asperities. To better understand how these cup designs may resist shear, we also utilized a visualization technique (frustrated total internal reflection; FTIR) to quantify how contact area evolves as the cup is sheared.

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.

Adhesion Behavior in Fish: From Structures to Applications

In nature, some fish can adhere tightly to the surface of stones, aquatic plants, and even other fish bodies. This adhesion behavior allows these fish to fix, eat, hide, and migrate in complex and variable aquatic environments. The adhesion function is realized by the special mouth and sucker tissue of fish. Inspired by adhesion fish, extensive research has recently been carried out. Therefore, this paper presents a brief overview to better explore underwater adhesion mechanisms and provide bionic applications. Firstly, the adhesion organs and structures of biological prototypes (e.g., clingfish, remora, Garra, suckermouth catfish, hill stream loach, and goby) are presented separately, and the underwater adhesion mechanisms are analyzed. Then, based on bionics, it is explained that the adhesion structures and components are designed and created for applications (e.g., flexible gripping adhesive discs and adhesive motion devices). Furthermore, we offer our perspectives on the limitations and future directions.

Uncover rock-climbing fish's secret of balancing tight adhesion and fast sliding for bioinspired robots

The underlying principle of the unique dynamic adaptive adhesion capability of a rock-climbing fish (Beaufortia kweichowensis) that can resist a pull-off force of 1000 times its weight while achieving simultaneous fast sliding (7.83 body lengths per second (BL/S)) remains a mystery in the literature. This adhesion-sliding ability has long been sought for underwater robots. However, strong surface adhesion and fast sliding appear to contradict each other due to the need for high surface contact stress. The skillfully balanced mechanism of the tight surface adhesion and fast sliding of the rock-climbing fish is disclosed in this work. The Stefan force (0.1 mN/mm2) generated by micro-setae on pectoral fins and ventral fins leads to a 70 N/m2 adhesion force by conforming the overall body of the fish to a surface to form a sealing chamber. The pull-off force is neutralized simultaneously due to the negative pressure caused by the volumetric change of the chamber. The rock-climbing fish's micro-setae hydrodynamic interaction and sealing suction cup work cohesively to contribute to low friction and high pull-off-force resistance and can therefore slide rapidly while clinging to the surface. Inspired by this unique mechanism, an underwater robot is developed with incorporated structures that mimic the functionality of the rock-climbing fish via a micro-setae array attached to a soft self-adaptive chamber, a setup which demonstrates superiority over conventional structures in terms of balancing tight underwater adhesion and fast sliding.

Sticky, stickier and stickiest - a comparison of adhesive performance in clingfish, lumpsuckers and snailfish

The coastal waters of the North Pacific are home to the northern clingfish (Gobiesox maeandricus), Pacific spiny lumpsucker (Eumicrotremus orbis) and marbled snailfish (Liparis dennyi) - three fishes that have evolved ventral adhesive discs. Clingfish adhesive performance has been studied extensively, but relatively little is known about the performance of other sticky fishes. Here, we compared the peak adhesive forces and work to detachment of clingfish, lumpsuckers and snailfish on surfaces of varying roughness and over ontogeny. We also investigated the morphology of their adhesive discs through micro-computed tomography scanning and scanning electron microscopy. We found evidence that adhesive performance is tied to the intensity and variability of flow regimes in the fishes' habitats. The northern clingfish generates the highest adhesive forces and lives in the rocky intertidal zone where it must resist exposure to crashing waves. Lumpsuckers and snailfish both generate only a fraction of the clingfish's adhesive force, but live more subtidal where currents are slower and less variable. However, lumpsuckers generate more adhesive force relative to their body weight than snailfish, which we attribute to their higher-drag body shape and frequent bouts into the intertidal zone. Even so, the performance and morphology data suggest that snailfish adhesive discs are stiffer and built more efficiently than lumpsucker discs. Future studies should focus on sampling additional diversity and designing more ecologically relevant experiments when investigating differences in adhesive performance.

Sucker Shapes, Skeletons and Bioinspiration: How Hard and Soft Tissue Morphology Generates Adhesive Performance in Waterfall Climbing Goby Fishes

Many teleost fishes, such as gobies, have fused their paired pelvic fins into an adhesive disc. Gobies can use their pelvic suckers to generate passive adhesive forces (as in engineered suction cups) and different species exhibit a range of adhesive performance, with some even able to climb waterfalls. Previous studies have documented that, in the Hawaiian Islands, species capable of climbing higher waterfalls produce the highest passive pull-off forces, and species found at higher elevation sites are likely to have more rounded suction discs than those found in the lowest stream segments. Morphology of the pelvic girdle also varies between species, with more robust skeletons in taxa with superior passive adhesion. To investigate what factors impact the passive adhesive performance of waterfall climbing gobies, we tested biomimetic suction cups designed with a range of shapes and embedded bioinspired "skeletons" based on micro-CT scans of goby pelvic girdles. We found that while the presence of an internal skeleton may provide some support against failure, the performance of suction cups may be more strongly affected by their external shape. Nonetheless, factors besides external shape and skeletal morphology may still have a stronger influence on sucker tenacity. Our results suggest that the relationship between suction disc morphology and adhesive performance may be influenced by a variety of physical factors, and live animal performance likely is further complicated by muscle activation and climbing behavior. These results have implications for the evolution of suction disc shape in adhesive fishes and for improving the design of biomimetic suction cups.

Locomotor challenges of waterfall-climbing gobies during transitions between media

An amphidromous goby, Sicyopterus japonicus, migrates from the ocean to upstream regions of many streams and rivers in the Pacific coasts of Japan and Taiwan. Using its mouth and fused pelvic fins (pelvic sucker), this gobiid species exhibits a rock-climbing behavior and surmounts sizable waterfalls, which block the upstream movement of many of its competitors and predators. When gobies emerge from the water to commence their climbing behavior, the change in effective density (i.e., lack of buoyancy) that occurs in this transition substantially increases the force required for adhesion. Consequently, these fish must exert adhesive suction strong enough to support their body weight against gravity during climbing on the rock surface. Suction performance for adhesion and modulatory capacity of S. japonicus were evaluated with two different sets of experimental conditions: climbing on the vertical surface with no water flow, versus climbing on a 60o-inclined surface with 2 L/min flow. Individuals of S. japonicus showed 50.7% greater mean safety factor (suction force for adhesion/gravitational force) and 56.6% shorter time to reach maximum pressure differential during climbing on the 60o-inclined surface with water rushing over their bodies than during climbing on the vertical surface with no water flow. These results indicate that when climbing with drag force from flowing water, greater functional demands are imposed and therefore, S. japonicus is required to increase neuromuscular stimulation of the pelvic muscles to elevate suction performance. In addition, S. japonicus individuals at different ontogenetic stages modulate their climbing behaviors and strategies to accommodate changing functional demands as they make transitions between different inclines, as well as media, while ascending waterfalls.

Adhesive force and endurance of the pelvic sucker across different modes of waterfall-climbing in gobiid fishes: Contrasting climbing mechanisms share aspects of ontogenetic change

Waterfall-climbing gobiids from oceanic islands use a suction-based adhesive mechanism formed by fused pelvic fins (pelvic sucker) and exhibit rock-climbing behavior during upstream migration. Although adhesion is a common feature of locomotion in these fishes, two distinct climbing styles - powerburst climbing and inching - have evolved. We compared the performance of the pelvic sucker during climbing across a range of body sizes between two species that use these different styles, collecting new data from the powerburst climber Lentipes concolor, and comparing these to published data for the inching climber Sicyopterus japonicus. Suction force for adhesion generated during continuous climbing did not differ between the species, with similar mean safety factors of 2.5-3.0. However, L. concolor engaged its pelvic sucker for a significantly longer duration of time (approximately 34 % longer per climbing cycle) than S. japonicus during continuous climbing. During sustained adhesion, both species exhibited non-linear scaling of fatigue time, with intermediate-sized individuals (e.g., large juveniles to small adults) showing the greatest endurance. However, the two species exhibited strikingly different maxima and variability in the endurance of their pelvic suckers. Maximum time to fatigue in L. concolor was less than half that of S. japonicus, but L. concolor showed more than double the variability of S. japonicus in time to fatigue. Our comparisons of these species reveal that despite differences in several aspects of their adhesive performance, some features of sucker function remain similar across climbing styles, including several related to how performance changes through ontogeny.

Bendy to the bone: Links between vertebral morphology and waterfall climbing in amphidromous gobioid fishes

Locomotor force production imposes strong demands on organismal form. Thus, the evolution of novel locomotor modes is often associated with morphological adaptations that help to meet those demands. In the goby lineage of fishes, most species are marine and use their fused pelvic fins to facilitate station holding in wave-swept environments. However, several groups of gobies have evolved an amphidromous lifecycle, in which larvae develop in the ocean but juveniles migrate to freshwater for their adult phase. In many of these species, the pelvic fins have been co-opted to aid in climbing waterfalls during upstream migrations to adult habitats. During horizontal swimming, forces are produced by axial musculature pulling on the vertebral column. However, during vertical climbing, gravity also exerts forces along the length of the vertebral column. In this study, we searched for novel aspects of vertebral column form that might be associated with the distinctive locomotor strategies of climbing gobies. We predicted that stiffness would vary along the length of the vertebral column due to competing demands for stability of the suction disk anteriorly and flexibility for axial thrust production posteriorly. We also predicted that derived, climbing goby species would require stiffer backbones to aid in vertical thrust production compared to non-climbing species. To test these predictions, we used microcomputed tomography scans to compare vertebral anatomy (centrum length, centrum width, and intervertebral space) along the vertebral column for five gobioid species that differ in climbing ability. Our results support our second prediction, that gobies are more flexible in the posterior portion of the body. However, the main variation in vertebral column form associated with climbing ability was the presence of larger intervertebral spaces in Sicyopterus stimpsoni, a species that uses a distinctive inching behavior to climb. These results build on past kinematic studies of goby climbing performance and lend insights into how the underlying vertebral structure of these fishes may enable their novel locomotion.

Suction feeding by elephants

Despite having a trunk that weighs over 100 kg, elephants mainly feed on lightweight vegetation. How do elephants manipulate such small items? In this experimental and theoretical investigation, we filmed elephants at Zoo Atlanta showing that they can use suction to grab food, performing a behaviour that was previously thought to be restricted to fishes. We use a mathematical model to show that an elephant's nostril size and lung capacity enables them to grab items using comparable pressures as the human lung. Ultrasonographic imaging of the elephant sucking viscous fluids show that the elephant's nostrils dilate up to [Formula: see text] in radius, which increases the nasal volume by [Formula: see text]. Based on the pressures applied, we estimate that the elephants can inhale at speeds of over 150 m s^-1, nearly 30 times the speed of a human sneeze. These high air speeds enable the elephant to vacuum up piles of rutabaga cubes as well as fragile tortilla chips. We hope these findings inspire further work in suction-based manipulation in both animals and robots.

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.

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.

Detachment of the remora suckerfish disc: kinematics and a bio-inspired robotic model

Remora suckerfish can attach to a wide diversity of marine hosts, however, their detachment mechanism remains poorly understood. Through analyzing high-speed videos, we found that the detachment of the live remora (Echeneis naucrates) is a rapid behavior that can happen within 240 ms. We separate this remarkable behavior into three stages: 1) lamellae folding down and soft lip curling, 2) disc raising and 3) complete detachment. To quantitatively investigate the detachment behavior, we fabricated a multi-material biomimetic disc and utilized it to study each stage of the detachment process. In stage one, we found that folding down lamellae is essential for decreasing the detachment resistance (vertical interfacial force and friction force) of the suction disc. Also, curling up the soft lip to breaking the adhesive seal reduced the vertical pull-off force up to 94 times. During disc raising in stage 2, we found that the partially flexible base (Young's modulus: ∼3 MPa) of the disc can lead to a 30% power-use reduction compared to a rigid base (Young's modulus: ∼3 GPa). After completing full detachment in stage 3, the corresponding drag wake flow decreased by 44% compared to an attached state due to lamellae folding and the entire soft lip uncurling. We developed a bio-inspired remora suckerfish robot propelled by a water jet to demonstrate a complete detachment which covers all three stages within 200 ms. We also demonstrated that an ROV has both hitchhiking and pick-and-place capabilities by integrating remora-inspired discs at appropriate locations. This study may shed light on future research in bio-inspired adhesives and lay a foundation for developing an untethered, multimodal, underwater hitchhiking robot.

Functional correlations of axial muscle fiber type proportions in the waterfall-climbing Hawaiian stream fish Sicyopterus stimpsoni

Assessing the factors that contribute to successful locomotor performance can provide critical insight into how animals survive in challenging habitats. Locomotion is powered by muscles, so that differences in the relative proportions of red (slow-oxidative) vs. white (fast-glycolytic) fibers can have significant implications for locomotor performance. We compared the relative proportions of axial red muscle fibers between groups of juveniles of the amphidromous gobiid fish, Sicyopterus stimpsoni, from the Hawaiian Islands. Juveniles of this species migrate from the ocean into freshwater streams, navigating through a gauntlet of predators that require rapid escape responses, before reaching waterfalls which must be climbed (using a slow, inching behavior) to reach adult breeding habitats. We found that fish from Kaua'i have a smaller proportion of red fibers in their tail muscles than fish from Hawai'i, matching expectations based on the longer pre-waterfall stream reaches of Kaua'i that could increase exposure to predators, making reduction of red muscle and increases in white muscle advantageous. However, no difference in red muscle proportions was identified between fish that were either successful or unsuccessful in scaling model waterfalls during laboratory climbing trials, suggesting that proportions of red muscle are near a localized fitness peak among Hawaiian individuals.

Migratory flexibility in native Hawai'ian amphidromous fishes

We assessed the prevalence of life history variation across four of the five native amphidromous Hawai'ian gobioids to determine whether some or all exhibit evidence of partial migration. Analysis of otolith Sr.: Ca concentrations affirmed that all are amphidromous and revealed evidence of partial migration in three of the four species. We found that 25% of Lentipes concolor (n = 8), 40% of Eleotris sandwicensis (n = 20) and 29% of Stenogobius hawaiiensis (n = 24) did not exhibit a migratory life-history. In contrast, all individuals of Sicyopterus stimpsoni (n = 55) included in the study went to sea as larvae. Lentipes concolor exhibited the shortest mean larval duration (LD) at 87 days, successively followed by E. sandwicensis (mean LD = 102 days), S. hawaiiensis (mean LD = 114 days) and S. stimpsoni (mean LD = 120 days). These findings offer a fresh perspective on migratory life histories that can help improve efforts to conserve and protect all of these and other at-risk amphidromous species that are subject to escalating anthropogenic pressures in both freshwater and marine environments.

Morphology of powerful suction organs from blepharicerid larvae living in raging torrents

Background

Suction organs provide powerful yet dynamic attachments for many aquatic animals, including octopus, squid, remora, and clingfish. While the functional morphology of suction organs from some cephalopods and fishes has been investigated in detail, there are only few studies on such attachment devices in insects. Here we characterise the morphology and ultrastructure of the suction attachment organs of net-winged midge larvae (genus Liponeura; Diptera: Blephariceridae) – aquatic insects that live on rocks in rapid alpine waterways where flow speeds can reach 3 m s− 1 – using scanning electron microscopy, confocal laser scanning microscopy, and X-ray computed micro-tomography (micro-CT). Furthermore, we study the function of these organs in vivo using interference reflection microscopy.

Results

We identified structural adaptations important for the function of the suction attachment organs in L. cinerascens and L. cordata. First, a dense array of spine-like microtrichia covering each suction disc comes into contact with the substrate upon attachment, analogous to hairy structures on suction organs from octopus, clingfish, and remora fish. These spine-like microtrichia may contribute to the seal and provide increased shear force resistance in high-drag environments. Second, specialised rim microtrichia at the suction disc periphery were found to form a continuous ring in close contact and may serve as a seal on a variety of surfaces. Third, a V-shaped cut on the suction disc (“V-notch“) is actively opened via two cuticular apodemes inserting on its flanks. The apodemes are attached to dedicated V-notch opening muscles, thereby providing a unique detachment mechanism. The complex cuticular design of the suction organs, along with specialised muscles that attach to them, allows blepharicerid larvae to generate powerful attachments which can withstand strong hydrodynamic forces and quickly detach for locomotion.

Conclusion

The suction organs from Liponeura are underwater attachment devices specialised for resisting extremely fast flows. Structural adaptations from these suction organs could translate into future bioinspired attachment systems that perform well on a wide range of surfaces.

Functional Diversity of Evolutionary Novelties: Insights from Waterfall-Climbing Kinematics and Performance of Juvenile Gobiid Fishes

The evolution of novel functional traits can contribute substantially to the diversification of lineages. Older functional traits might show greater variation than more recently evolved novelties, due to the accrual of evolutionary changes through time. However, functional complexity and many-to-one mapping of structure to function could complicate such expectations. In this context, we compared kinematics and performance across juveniles from multiple species for two styles of waterfall-climbing that are novel to gobiid fishes: ancestral “powerburst” climbing, and more recently evolved “inching”, which has been confirmed only among species of a single genus that is nested within the clade of powerburst climbers. Similar net climbing speeds across inching species seem, at first, to indicate that this more recently evolved mode of climbing exhibits less functional diversity. However, these similar net speeds arise through different pathways: Sicyopterus stimpsoni from Hawai’i move more slowly than S. lagocephalus from La Réunion, but may also spend more time moving. The production of similar performance between multiple functional pathways reflects a situation that resembles the phenomenon of many-to-one mapping of structure to function. Such similarity has the potential to mask appropriate interpretations of relative functional diversity between lineages, unless the mechanisms underlying performance are explored. More specifically, similarity in net performance between “powerburst” and “inching” styles indicates that selection on climbing performance was likely a limited factor in promoting the evolution of inching as a new mode of climbing. In this context, other processes (e.g., exaptation) might be implicated in the origin of this functional novelty.

An adhesive locomotion model for the rock-climbing fish, Beaufortia kweichowensis

The rock-climbing fish (Beaufortia kweichowensis) adheres to slippery, fouled surfaces and crawls both forward and backward in torrential streams. During locomotion, two suckers can be distinguished. Here, the general skeletal structure of the rock-climbing fish was determined using microtomography. Friction and adhesion were positively correlated, as were friction and fin ray angle. The unique adhesive locomotion system used by the rock-climbing fish was observed with a high speed camera. This system comprised two anisotropic suckers bearing two paired fins and two girdle muscles. A locomotion model was established based on these results. In this model, the fin states controlled the direction of motion using anisotropic friction, and alternate contractions of the girdle muscles provided propulsion during bidirectional crawling. This adhesive locomotion system was compared with other biological locomotion mechanisms. Based on these comparisons, we hypothesized that this novel system might represent an energy-saving solution for undulatory underwater vertical movement without detaching from the substrate.

Learning from Northern clingfish (Gobiesox maeandricus): bioinspired suction cups attach to rough surfaces

While artificial suction cups only attach well to smooth surfaces, the Northern clingfish can attach to surfaces ranging from nanoscale smooth to rough stone. This ability is highly desirable for technical applications. The morphology of the fish's suction disc and its ability to attach to rough and slimy surfaces have been described before, and here we aim to close gaps in the biomechanical understanding, and transfer the biomechanical principles to technical suction cups. We demonstrate that the margin of the suction disc is the critical feature enabling attachment to rough surfaces. Second, friction measurements show that friction of the disc rim is increased on rough substrates and contributes to high tenacity. Increased friction causes a delay in failure of the suction cup and increases the attachment force. We were able to implement these concepts to develop the first suction cups bioinspired by Northern clingfish. These cups attach with tenacities up to 70 kPa on surfaces as rough as 270 µm grain size. The application of this technology is promising in fields such as surgery, industrial production processes and whale tagging. This article is part of the theme issue 'Transdisciplinary approaches to the study of adhesion and adhesives in biological systems'.

Bioinspired remora adhesive disc offers insight into evolution

Remoras are a family of fishes that can attach to other swimming organisms via an adhesive disc evolved from dorsal fin elements. However, the factors driving the evolution of remora disc morphology are poorly understood. It is not possible to link selective pressure for attachment to a specific host surface because all known hosts evolved before remoras themselves. Fortunately, the fundamental physics of suction and friction are mechanically conserved. Therefore, a morphologically relevant bioinspired model can be used to examine performance of hypothetical evolutionary intermediates. Using a bioinspired remora disc, we experimentally investigated the performance of increased lamellar number on shear adhesion. Herein, we translated fundamental biological principles into engineering design rules and show that a passive model system can autonomously achieve adhesive forces measured in live remoras in any environment. Our experimental results show that an increase in lamellar number resulted in an increase in shear adhesive performance, supporting the phylogenetic trend observed in extant remoras. The greatest pull-off forces measured for our model were on surface roughness on the order of shark skin and exceeded those measured for live remoras attached to shark skin by almost 60%. Overall, relative to fossil remoras and their closest ancestor, extant remoras exhibit a morphology indicative of selection for enhanced shear adhesive performance.

Adhesive force and endurance during waterfall climbing in an amphidromous gobiid, Sicyopterus japonicus (Teleostei: Gobiidae): Ontogenetic scaling of novel locomotor performance

An amphidromous sicydiine goby, Sicyopterus japonicus, exhibits rock-climbing behavior during upstream migration along rivers and streams. Using a pelvic sucker formed by fused pelvic fins, S. japonicus generates suction adhesion on the climbing surface. By measuring performance variables that correlate with successful rock-climbing capability, we evaluated scaling relationships of adhesive suction force generated by the pelvic sucker and fatigue during climbing in S. japonicus during ontogeny. In continuous climbing on the experimental 60°-inclined surface, the pelvic sucker of S. japonicus exhibited strong positive allometry in generating suction force for adhesion during ontogeny. In contrast, fatigue time of the pelvic sucker muscles for sustained adhesion scaled non-linearly with body mass during ontogeny. In addition, fatigue time and body mass showed the best fit to a quadratic regression, which predicted intermediate-sized individuals (large juveniles to small adults) to have better performance in adhesive endurance than smaller or larger individuals. Our experimental results indicate that different sizes of waterfall-climbing gobies have different performance capacities for rock climbing perhaps because of physiological differences in their pelvic muscles. In addition, our data from S. japonicus indicates that selection pressures on the locomotor capacities of waterfall-climbing gobiids vary during ontogeny.

Anchoring like octopus: biologically inspired soft artificial sucker

This paper presents a robotic anchoring module, a sensorized mechanism for attachment to the environment that can be integrated into robots to enable or enhance various functions such as robot mobility, remaining on location or its ability to manipulate objects. The body of the anchoring module consists of two portions with a mechanical stiffness transition from hard to soft. The hard portion is capable of containing vacuum pressure used for actuation while the soft portion is highly conformable to create a seal to contact surfaces. The module is integrated with a single sensory unit which exploits a fibre-optic sensing principle to seamlessly measure proximity and tactile information for use in robot motion planning as well as measuring the state of firmness of its anchor. In an experiment, a variable set of physical loads representing the weights of potential robot bodies were attached to the module and its ability to maintain the anchor was quantified under constant and variable vacuum pressure signals. The experiment shows the effectiveness of the module in quantifying the state of firmness of the anchor and discriminating between different amounts of physical loads attached to it. The proposed anchoring module can enable many industrial and medical applications where attachment to environment is of crucial importance for robot control.

Functional morphology of suction discs and attachment performance of the Mediterranean medicinal leech (Hirudo verbana Carena)

Medicinal leeches use their suction discs for locomotion, adhesion to the host and, in the case of the anterior disc, also for blood ingestion. The biomechanics of their suction-based adhesion systems has been little understood until now. We investigated the functional morphology of the anterior and posterior suckers ofH irudo verbena by using light and scanning electron microscopy. Furthermore, we analysed the adhesion qualitatively and quantitatively by conducting behavioural and mechanical experiments. Our high-speed video analyses provide new insights into the attachment and detachment processes and we present a detailed description of the leech locomotion cycle. Pull-off force measurements of the anterior and posterior suction organs on seven different substrates under both aerial and water-submersed conditions reveal a significant influence of the surrounding medium, the substrate surface roughness and the tested organ on attachment forces and tenacities.

The Adhesive System and Anisotropic Shear Force of Guizhou Gastromyzontidae

The Guizhou gastromyzontidae (Beaufortia kweichowensis) can adhere to slippery and fouled surfaces in torrential streams. A unique adhesive system utilized by the fish was observed by microscope and CLSM as an attachment disc sealed by a round belt of micro bubbles. The system is effective in wet or underwater environments and can resist a normal pulling force up to 1000 times the fish’s weight. Moreover, a mechanism for passive anisotropic shear force was observed. The shear forces of the fish under different conditions were measured, showing that passive shear force plays an important role in wet environments. The adhesive system of the fish was compared with other biological adhesion principles, from which we obtained potential values for the system that refer to the unique micro sealing and enhanced adhesion in a wet environment.

Comparative lever analysis and ontogenetic scaling in esocid fishes: Functional demands and constraints in feeding biomechanics

When animals grow, the functional demands that they experience often change as a consequence of their increasing body size. In this study, we examined the feeding biomechanics in esocid species that represent different size classes (small, Esox americanus; intermediate, Esox niger; large, Esox lucius), and how their bite forces and associated functional variables change as they grow. In order to evaluate bite performance through ontogeny, we dissected and measured dimensions of the feeding apparatus and the adductor mandibulae muscle complex with its segmentum facialis subdivisions such as the ricto-malaris, stegalis and endoricto-malaris across a wide range of body sizes. The collected morphological data was used as input variables for a published anatomical model to simulate jaw function in these fish species. Maximum bite forces for both anterior bite and posterior bite increased in isometry in E. americanus and E. niger. The posterior bite of E. lucius also increases in isometry, however, the anterior bite increases in positive allometry. Intraspecific comparison within E. lucius indicated the increase of bite forces in more developed individuals accelerated after the fish grew out of fingerling stage. In addition, our analysis indicated functional differentiation between subdivisions of the adductor mandibulae segmentum facialis, as well as interspecific differences in the pattern of contribution to the bite performance by these subdivisions. Our study provides insights into not only the musculoskeletal basis of the jaw function of esocid species, but also the feeding capacity of this species in relation to the functional demands it faces as one of the top predators in lake and river systems. J. Morphol. 277:1447-1458, 2016. © 2016 Wiley Periodicals, Inc.

Understanding Surface Adhesion in Nature: A Peeling Model

Nature often exhibits various interesting and unique adhesive surfaces. The attempt to understand the natural adhesion phenomena can continuously guide the design of artificial adhesive surfaces by proposing simplified models of surface adhesion. Among those models, a peeling model can often effectively reflect the adhesive property between two surfaces during their attachment and detachment processes. In the context, this review summarizes the recent advances about the peeling model in understanding unique adhesive properties on natural and artificial surfaces. It mainly includes four parts: a brief introduction to natural surface adhesion, the theoretical basis and progress of the peeling model, application of the peeling model, and finally, conclusions. It is believed that this review is helpful to various fields, such as surface engineering, biomedicine, microelectronics, and so on.

Flowing water affects fish fast-starts: escape performance of the Hawaiian stream goby, Sicyopterus stimpsoni

Experimental measurements of escape performance in fishes have typically been conducted in still water; however, many fishes inhabit environments with flow that could impact escape behavior. We examined the influences of flow and predator attack direction on the escape behavior of fish, using juveniles of the amphidromous Hawaiian goby Sicyopterus stimpsoni. In nature, these fish must escape ambush predation while moving through streams with high-velocity flow. We measured the escape performance of juvenile gobies while exposing them to a range of water velocities encountered in natural streams and stimulating fish from three different directions. Frequency of response failure across treatments indicated strong effects of flow conditions and attack direction. Juvenile S. stimpsoni had uniformly high response rates for attacks from a caudal direction (opposite flow); however, response rates for attacks from a cranial direction (matching flow) decreased dramatically as flow speed increased. Mechanical stimuli produced by predators attacking in the same direction as flow might be masked by the flow environment, impairing the ability of prey to detect attacks. Thus, the likelihood of successful escape performance in fishes can depend critically on environmental context.

Remora fish suction pad attachment is enhanced by spinule friction

The remora fishes are capable of adhering to a wide variety of natural and artificial marine substrates using a dorsal suction pad. The pad is made of serial parallel pectinated lamellae, which are homologous to the dorsal fin elements of other fishes. Small tooth-like projections of mineralized tissue from the dorsal pad lamella, known as spinules, are thought to increase the remora's resistance to slippage and thereby enhance friction to maintain attachment to a moving host. In this work, the geometry of the spinules and host topology as determined by micro-computed tomography and confocal microscope data, respectively, are combined in a friction model to estimate the spinule contribution to shear resistance. Model results are validated with natural and artificially created spinules and compared with previous remora pull-off experiments. It was found that spinule geometry plays an essential role in friction enhancement, especially at short spatial wavelengths in the host surface, and that spinule tip geometry is not correlated with lamellar position. Furthermore, comparisons with pull-off experiments suggest that spinules are primarily responsible for friction enhancement on rough host topologies such as shark skin.

Attachment to challenging substrates--fouling, roughness and limits of adhesion in the northern clingfish (Gobiesox maeandricus)

Northern clingfish use a ventral suction disc to stick to rough substrates in the intertidal zone. Bacteria, algae and invertebrates grow on these surfaces (fouling) and change the surface properties of the primary substrate, and therefore the attachment conditions for benthic organisms. In this study, we investigate the influence of fouling and surface roughness on the adhesive strength of northern clingfish, Gobiesox maeandricus. We measured clingfish tenacity on unfouled and fouled substrates over four surface roughnesses. We exposed surfaces for 6 weeks in the Pacific Ocean, until they were covered with periphyton. Clingfish tenacity is equivalent on both fouled and unfouled smooth substrates; however, tenacity on fouled rough surfaces is less compared with tenacity on unfouled ones. We hypothesize that parts of biofilm may act as a lubricant and decrease friction of the disc margin, thereby making disc margins slip inwards and fail at lower tenacities. Nevertheless, even on fouled surfaces the adhesive forces are approximately 150 times the body weight of the fish. To identify the upper threshold of surface roughness the fish can cling to, we tested seven unfouled substrates of increasing surface roughness. The threshold roughness at which northern clingfish failed increased with specimen size. We hypothesize that because of the elastic properties of the disc margin, a larger disc can adapt to larger surface irregularities. The largest specimens (length 10-12 cm) were able to cling to surfaces with 2-4 mm grain size. The fish can attach to surfaces with roughness between 2 and 9% of the suction disc width.

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.

Stairway to heaven: evaluating levels of biological organization correlated with the successful ascent of natural waterfalls in the Hawaiian stream goby Sicyopterus stimpsoni

Selective pressures generated by locomotor challenges act at the level of the individual. However, phenotypic variation among individuals that might convey a selective advantage may occur across any of multiple levels of biological organization. In this study, we test for differences in external morphology, muscle mechanical advantage, muscle fiber type and protein expression among individuals of the waterfall climbing Hawaiian fish Sicyopterus stimpsoni collected from sequential pools increasing in elevation within a single freshwater stream. Despite predictions from previous laboratory studies of morphological selection, few directional morphometric changes in body shape were observed at successively higher elevations. Similarly, lever arm ratios associated with the main pelvic sucker, central to climbing ability in this species, did not differ between elevations. However, among climbing muscles, the adductor pelvicus complex (largely responsible for generating pelvic suction during climbing) contained a significantly greater red muscle fiber content at upstream sites. A proteomic analysis of the adductor pelvicus revealed two-fold increases in expression levels for two respiratory chain proteins (NADH:ubiquinone reductase and cytochrome b) that are essential for aerobic respiration among individuals from successively higher elevations. Assessed collectively, these evaluations reveal phenotypic differences at some, but not all levels of biological organization that are likely the result of selective pressures experienced during climbing.

Evolutionary novelty versus exaptation: oral kinematics in feeding versus climbing in the waterfall-climbing Hawaiian Goby Sicyopterus stimpsoni

Species exposed to extreme environments often exhibit distinctive traits that help meet the demands of such habitats. Such traits could evolve independently, but under intense selective pressures of extreme environments some existing structures or behaviors might be coopted to meet specialized demands, evolving via the process of exaptation. We evaluated the potential for exaptation to have operated in the evolution of novel behaviors of the waterfall-climbing gobiid fish genus Sicyopterus. These fish use an “inching” behavior to climb waterfalls, in which an oral sucker is cyclically protruded and attached to the climbing surface. They also exhibit a distinctive feeding behavior, in which the premaxilla is cyclically protruded to scrape diatoms from the substrate. Given the similarity of these patterns, we hypothesized that one might have been coopted from the other. To evaluate this, we filmed climbing and feeding in Sicyopterus stimpsoni from Hawai’i, and measured oral kinematics for two comparisons. First, we compared feeding kinematics of S. stimpsoni with those for two suction feeding gobiids (Awaous guamensis and Lentipes concolor), assessing what novel jaw movements were required for algal grazing. Second, we quantified the similarity of oral kinematics between feeding and climbing in S. stimpsoni, evaluating the potential for either to represent an exaptation from the other. Premaxillary movements showed the greatest differences between scraping and suction feeding taxa. Between feeding and climbing, overall profiles of oral kinematics matched closely for most variables in S. stimpsoni, with only a few showing significant differences in maximum values. Although current data cannot resolve whether oral movements for climbing were coopted from feeding, or feeding movements coopted from climbing, similarities between feeding and climbing kinematics in S. stimpsoni are consistent with evidence of exaptation, with modifications, between these behaviors. Such comparisons can provide insight into the evolutionary mechanisms facilitating exploitation of extreme habitats.