Repeated Hyposalinity Pulses Immediately And Persistently Impair The Sea Urchin Adhesive System.

Climate change will increase the frequency and intensity of extreme climatic events (e.g., storms) that result in repeated pulses of hyposalinity in nearshore ecosystems. Sea urchins inhabit these ecosystems and are stenohaline (restricted to salinity levels ∼ 32 ‰), thus are particularly susceptible to hyposalinity events. As key benthic omnivores, sea urchins use hydrostatic adhesive tube feet for numerous functions, including attachment to and locomotion on the substratum as they graze for food. Hyposalinity severely impacts sea urchin locomotor and adhesive performance but several ecologically-relevant and climate change-related questions remain. First, does sea urchin locomotion and adhesion acclimate to repeated pulses of hyposalinity? Second, how do tube feet respond to tensile forces during single and repeated hyposalinity events? Third, do the negative effects of hyposalinity exposure persist following return to normal salinity levels? To answer these questions, we repeatedly exposed green sea urchins (Strongylocentrotus droebachiensis) to pulses of three different salinities (control: 32 ‰, moderate hyposalinity: 22 ‰, severe hyposalinity: 16 ‰) over the course of two months and measured locomotor performance, adhesive performance, and tube foot tensile behavior. We also measured these parameters 20 hours after sea urchins returned to normal salinity levels. We found no evidence that tube feet performance and properties acclimate to repeated pulses of hyposalinity, at least over the timescale examined in this study. In contrast, hyposalinity has severe consequences on locomotion, adhesion, and tube foot tensile behavior and these impacts are not limited to the hyposalinity exposure. Our results suggest both moderate and severe hyposalinity events have the potential to increase sea urchin dislodgment and reduce movement, which may impact sea urchin distribution and their role in marine communities.

Hyposalinity reduces coordination and adhesion of sea urchin tube feet.

Climate change will increase the frequency and intensity of low-salinity (hyposalinity) events in coastal marine habitats. Sea urchins are dominant herbivores in these habitats and are generally intolerant of salinity fluctuations. Their adhesive tube feet are essential for survival, effecting secure attachment and locomotion in high wave energy habitats, yet little is known about how hyposalinity impacts their function. We exposed green sea urchins (Strongylocentrotus droebachiensis) to salinities ranging from ambient (32‰) to severe (14‰) and assessed tube feet coordination (righting response, locomotion) and adhesion [disc tenacity (force per unit area)]. Righting response, locomotion and disc tenacity decreased in response to hyposalinity. Severe reductions in coordinated tube foot activities occurred at higher salinities than those that affected adhesion. The results of this study suggest moderate hyposalinities (24-28‰) have little effect on S. droebachiensis dislodgement risk and survival post-dislodgment, while severe hyposalinity (below 24‰) likely reduces movement and prevents recovery from dislodgment.

The bumpy road ahead: the role of substrate roughness on animal walking and a proposed comparative metric.

Outside laboratory conditions and human-made structures, animals rarely encounter flat surfaces. Instead, natural substrates are uneven surfaces with height variation that ranges from the microscopic scale to the macroscopic scale. For walking animals (which we define as encompassing any form of legged movement across the ground, such as walking, running, galloping, etc.), such substrate 'roughness' influences locomotion in a multitude of ways across scales, from roughness that influences how each toe or foot contacts the ground, to larger obstacles that animals must move over or navigate around. Historically, the unpredictability and variability of natural environments has limited the ability to collect data on animal walking biomechanics. However, recent technical advances, such as more sensitive and portable cameras, biologgers, laboratory tools to fabricate rough terrain, as well as the ability to efficiently store and analyze large variable datasets, have expanded the opportunity to study how animals move under naturalistic conditions. As more researchers endeavor to assess walking over rough terrain, we lack a consistent approach to quantifying roughness and contextualizing these findings. This Review summarizes existing literature that examines non-human animals walking on rough terrain and presents a metric for characterizing the relative substrate roughness compared with animal size. This framework can be applied across terrain and body scales, facilitating direct comparisons of walking over rough surfaces in animals ranging in size from ants to elephants.

An investigation of gecko attachment on wet and rough substrates leads to the application of surface roughness power spectral density analysis.

The roughness and wettability of surfaces exploited by free-ranging geckos can be highly variable and attachment to these substrates is context dependent (e.g., presence or absence of surface water). Although previous studies focus on the effect of these variables on attachment independently, geckos encounter a variety of conditions in their natural environment simultaneously. Here, we measured maximum shear load of geckos in air and when their toes were submerged underwater on substrates that varied in both surface roughness and wettability. Gecko attachment was greater in water than in air on smooth and rough hydrophobic substrates, and attachment to rough hydrophilic substrates did not differ when tested in air or water. Attachment varied considerably with surface roughness and characterization revealed that routine measurements of root mean square height can misrepresent the complexity of roughness, especially when measured with single instruments. We used surface roughness power spectra to characterize substrate surface roughness and examined the relationship between gecko attachment performance across the power spectra. This comparison suggests that roughness wavelengths less than 70 nm predominantly dictate gecko attachment. This study highlights the complexity of attachment in natural conditions and the need for comprehensive surface characterization when studying biological adhesive system performance.

Plasticity in fluctuating hydrodynamic conditions: tube foot regeneration in sea urchins.

Regenerating structures critical for survival provide excellent model systems for the study of phenotypic plasticity. These body components must regenerate their morphology and functionality quickly while subjected to different environmental stressors. Sea urchins live in high-energy environments where hydrodynamic conditions pose significant challenges. Adhesive tube feet provide secure attachment to the substratum but can be amputated by predation and hydrodynamic forces. Tube feet display functional and morphological plasticity in response to environmental conditions, but regeneration to their pre-amputation status has not been achieved under quiescent laboratory settings. In this study, we assessed the effect of turbulent water movement, periodic emersion and quiescent conditions on the regeneration process of tube foot morphology (length, disc area) and functionality (maximum disc tenacity, stem breaking force). Disc area showed significant plasticity in response to the treatments; when exposed to emersion and turbulent water movement, disc area was larger than that of tube feet regenerated in quiescent conditions. However, no treatment stimulated regeneration to pre-amputation sizes. Tube foot length was unaffected by treatments and remained shorter than non-amputated tube feet. Stem breaking force for amputated and non-amputated treatments increased in all cases when compared with pre-amputation values. Maximum tenacity (force per unit area) was similar among tube feet subjected to simulated field conditions and amputation treatments. Our results suggest a role of active plasticity of tube foot functional morphology in response to field-like conditions and demonstrate the plastic response of invertebrates to laboratory conditions.

Direct evidence of acid-base interactions in gecko adhesion.

While it is generally accepted that van der Waals (vdW) forces govern gecko adhesion, several studies indicate contributions from non-vdW forces and highlight the importance of understanding the adhesive contact interface. Previous work hypothesized that the surface of gecko setae is hydrophobic, with nonpolar lipid tails exposed on the surface. However, direct experimental evidence supporting this hypothesis and its implications on the adhesion mechanism is lacking. Here, we investigate the sapphire-setae contact interface using interface-sensitive spectroscopy and provide direct evidence of the involvement of acid-base interactions between polar lipid headgroups exposed on the setal surface and sapphire. During detachment, a layer of unbound lipids is left as a footprint due to cohesive failure within the lipid layer, which, in turn, reduces wear to setae during high stress sliding. The absence of this lipid layer enhances adhesion, despite a small setal-substrate contact area. Our results show that gecko adhesion is not exclusively a vdW-based, residue-free system.

The effect of variable temperature, humidity, and substrate wettability on Gecko (Gekko gecko) locomotor performance and behavior.

Adhesive and locomotor performances of geckos are inherently linked by specialized morphological and biomechanical features. As such, we predict that conditions that lead to poor adhesive performance (i.e., low resistance to applied force while clinging) also lead to poor locomotor performance and behavior (i.e., slowed running speed, increased frequency and duration of stops, more failed or incomplete runs). In this study, we test the prediction that running speed changes as a function of adhesive performance in variable temperature (12 and 32°C), humidity (30, 55, 70, 80% relative humidity), and substrate wettability (hydrophilic glass, intermediately wetting plexiglass). We also expect other locomotor performance traits and behaviors, such as stopping and avoiding treatment conditions, to change as a function of adhesive performance. The results of this study do not fully support our prediction: gecko locomotor performance does not change as a function of humidity or substrate wettability, unlike adhesive performance. As an anticipated result of ectothermy, geckos run significantly slower and stop more frequently and longer at 12°C than 32°C. At high temperature, geckos required significantly more running attempts on hydrophilic glass than plexiglass to complete the experimental procedure, suggesting that this treatment condition is unfavorable. The results of this study highlight the robust locomotive response of geckos to variation in adhesive performance and environmental conditions, and have significant implications for predictions about habitat use and behavior in their natural environment.

The effect of substrate wettability and modulus on gecko and gecko-inspired synthetic adhesion in variable temperature and humidity.

Gecko adhesive performance increases as relative humidity increases. Two primary mechanisms can explain this result: capillary adhesion and increased contact area via material softening. Both hypotheses consider variable relative humidity, but neither fully explains the interactive effects of temperature and relative humidity on live gecko adhesion. In this study, we used live tokay geckos (Gekko gecko) and a gecko-inspired synthetic adhesive to investigate the roles of capillary adhesion and material softening on gecko adhesive performance. The results of our study suggest that both capillary adhesion and material softening contribute to overall gecko adhesion, but the relative contribution of each depends on the environmental context. Specifically, capillary adhesion dominates on hydrophilic substrates, and material softening dominates on hydrophobic substrates. At low temperature (12 °C), both capillary adhesion and material softening likely produce high adhesion across a range of relative humidity values. At high temperature (32 °C), material softening plays a dominant role in adhesive performance at an intermediate relative humidity (i.e., 70% RH).

Adhesive plasticity among populations of purple sea urchin (Strongylocentrotus purpuratus).

Sea urchins native to the nearshore open coast experience periods of high, repeated wave forces that can result in dislodgement. To remain attached while clinging and locomoting across rocky substrates, sea urchins use adhesive tube feet. Purple sea urchins (Strongylocentrotus purpuratus) adhere to a variety of rock substrates (e.g. sandstone, mudstone, granite), and display morphological plasticity (skeletal morphology) to native substrate. We tested the hypothesis that their adhesive system is also plastic and varies as a function of native population and substrate. The results of our study support our hypothesis. Sea urchins from sandstone adhere less strongly to most substrates than those native to mudstone and granite rock. Sandstone produced the lowest whole animal adhesive force values across all populations, suggesting that this rock type is particularly challenging for sea urchins to adhere to. The number of adhesive tube feet that failed during experimental trials and the area used by sea urchins to attach, matches closely with whole animal adhesive force values: higher forces resulted in more tube foot failure and larger attachment area. On artificial substrates (glass and Plexiglass), differences in adhesion among populations was consistent with differences in adhesion on rock substrates except on glass, where sea urchins native to sandstone adhered more strongly to glass than any other substrate tested. To our knowledge, this study is the first to describe population-level plasticity in a biological adhesive system related to native substrate, and has significant implications for sea urchin ecology, behavior and functional morphology.

Adhesion and Running Speed of a Tropical Arboreal Ant (Cephalotes atratus) on Rough, Narrow, and Inclined Substrates.

Arboreal ants must navigate variably sized and inclined linear structures across a range of substrate roughness when foraging tens of meters above the ground. To achieve this, arboreal ants use specialized adhesive pads and claws to maintain effective attachment to canopy substrates. Here, we explored the effect of substrate structure, including small and large-scale substrate roughness, substrate diameter, and substrate orientation (inclination), on adhesion and running speed of workers of one common, intermediately-sized, arboreal ant species. Normal (orthogonal) and shear (parallel) adhesive performance varied on sandpaper and natural leaf substrates, particularly at small size scales, but running speed on these substrates remained relatively constant. Running speed also varied minimally when running up and down inclined substrates, except when the substrate was positioned completely vertical. On vertical surfaces, ants ran significantly faster down than up. Ant running speed was slower on relatively narrow substrates. The results of this study show that variation in the physical properties of tree surfaces differentially affects arboreal ant adhesive and locomotor performance. Specifically, locomotor performance was much more robust to surface roughness than was adhesive performance. The results provide a basis for understanding how performance correlates of functional morphology contribute to determining local ant distributions and foraging decisions in the tropical rainforest canopy.

The Integrative Biology of Gecko Adhesion: Historical Review, Current Understanding, and Grand Challenges.

Geckos are remarkable in their ability to reversibly adhere to smooth vertical, and even inverted surfaces. However, unraveling the precise mechanisms by which geckos do this has been a long process, involving various approaches over the last two centuries. Our understanding of the principles by which gecko adhesion operates has advanced rapidly over the past 20 years and, with this knowledge, material scientists have attempted to mimic the system to create artificial adhesives. From a biological perspective, recent studies have examined the diversity in morphology, performance, and real-world use of the adhesive apparatus. However, the lack of multidisciplinarity is likely a key roadblock to gaining new insights. Our goals in this paper are to 1) present a historical review of gecko adhesion research, 2) discuss the mechanisms and morphology of the adhesive apparatus, 3) discuss the origin and performance of the system in real-world contexts, 4) discuss advancement in bio-inspired design, and 5) present grand challenges in gecko adhesion research. To continue to improve our understanding, and to more effectively employ the principles of gecko adhesion for human applications, greater intensity and scope of interdisciplinary research are necessary.

Stick or Slip: Adhesive Performance of Geckos and Gecko-Inspired Synthetics in Wet Environments.

The gecko adhesive system has inspired hundreds of synthetic mimics principally focused on replicating the strong, reversible, and versatile properties of the natural system. For geckos native to the tropics, versatility includes the need to remain attached to substrates that become wet from high humidity and frequent rain. Paradoxically, van der Waals forces, the principal mechanism responsible for gecko adhesion, reduce to zero when two contacting surfaces separate even slightly by entrapped water layers. A series of laboratory studies show that instead of slipping, geckos maintain and even improve their adhesive performance in many wet conditions (i.e., on wet hydrophobic substrates, on humid substrates held at low temperatures). The mechanism for this is not fully clarified, and likely ranges in scale from the chemical and material properties of the gecko's contact structures called setae (e.g., setae soften and change surface confirmation when exposed to water), to their locomotor biomechanics and decision-making behavior when encountering water on a substrate in their natural environment (e.g., some geckos tend to run faster and stop more frequently on misted substrates than dry). Current work has also focused on applying results from the natural system to gecko-inspired synthetic adhesives, improving their performance in wet conditions. Gecko-inspired synthetic adhesives have also provided a unique opportunity to test hypotheses about the natural system in semi-natural conditions replicated in the laboratory. Despite many detailed studies focused on the role of water and humidity on gecko and gecko-inspired synthetic adhesion, there remains several outstanding questions: (1) what, if any, role does capillary or capillary-like adhesion play on overall adhesive performance of geckos and gecko-inspired synthetics, (2) how do chemical and material changes at the surface and in the bulk of gecko setae and synthetic fibrils change when exposed to water, and what does this mean for adhesive performance, and (3) how much water do geckos encounter in their native environment, and what is their corresponding behavioral response? This review will detail what we know about gecko adhesion in wet environments, and outline the necessary next steps in biological and synthetic system investigations.

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.

Adhesive performance of tropical arboreal ants varies with substrate temperature.

The surface temperature of tree branches in the tropical rainforest canopy can reach up to 55°C. Ants and other small cursorial organisms must maintain adequate attachment in this extreme microenvironment to forage effectively and avoid falling. Ant adhesion depends on liquid secretions that should become less viscous at high temperatures, causing ants to slip. However, tropical arboreal ants have high thermal tolerance and actively forage on hot canopy surfaces, suggesting that these ants can maintain adhesion on hot substrates. We measured tarsal pad shear adhesion of 580 workers (representing 11 species and four subfamilies) of tropical arboreal ants at temperatures spanning the range observed in the field (23-55°C). Adhesive performance among species showed three general trends: (1) a linear decrease with increasing temperature, (2) a non-linear relationship with peak adhesive performance at ca. 30-40°C, and (3) no relationship with temperature. The mechanism responsible for these large interspecific differences remains to be determined, but likely reflects variation in the composition of the secreted adhesive fluid. Understanding such differences will reveal the diverse ways that ants cope with highly variable, and often unpredictable, thermal conditions in the forest canopy.

Out on a limb: Thermal microenvironments in the tropical forest canopy and their relevance to ants.

Small, cursorial ectotherms like ants often are immersed in the superheated air layers that develop millimeters above exposed, insolated surfaces (i.e., the thermal boundary layer). We quantified the thermal microenvironments around tree branches in the tropical rainforest canopy, and explored the effects of substrate color on the internal body temperature and species composition of arboreal ants. Branch temperatures during the day (09:00-16:00) were hottest (often > 50°C) and most variable on the upper surface, while the lowest and least variable temperatures occurred on the underside. Temperatures on black substrates declined with increasing distance above the surface in both the field and the laboratory. By contrast, a micro-scale temperature inversion occurred above white substrates. Wind events (ca. 2ms-1) eliminated these patterns. Internal temperatures of bodies of Cephalotes atratus workers experimentally heated in the laboratory were 6°C warmer on white vs. black substrates, and 6°C cooler than ambient in windy conditions. The composition of ant species foraging at baits differed between black-painted and unpainted tree branches, with a tendency for smaller ants to avoid the significantly hotter black surfaces. Collectively, these outcomes show that ants traversing canopy branches experience very heterogeneous thermal microenvironments that are partly influenced in predictable ways by branch surface coloration and breezy conditions.

Thermal constraints on foraging of tropical canopy ants.

Small cursorial ectotherms risk overheating when foraging in the tropical forest canopy, where the surfaces of unshaded tree branches commonly exceed 50 °C. We quantified the heating and subsequent cooling rates of 11 common canopy ant species from Panama and tested the hypothesis that ant workers stop foraging at temperatures consistent with the prevention of overheating. We created hot experimental "sunflecks" on existing foraging trails of four ant species from different clades and spanning a broad range of body size, heating rate, and critical thermal maxima (CTmax). Different ant species exhibited very different heating rates in the lab, and these differences did not follow trends predicted by body size alone. Experiments with ant models showed that heating rates are strongly affected by color in addition to body size. Foraging workers of all species showed strong responses to heating and consistently abandoned focal sites between 36 and 44 °C. Atta colombica and Azteca trigona workers resumed foraging shortly after heat was removed, but Cephalotes atratus and Dolichoderus bispinosus workers continued to avoid the heated patch even after >5 min of cooling. Large foraging ants (C. atratus) responded slowly to developing thermal extremes, whereas small ants (A. trigona) evacuated sunflecks relatively quickly, and at lower estimated body temperatures than when revisiting previously heated patches. The results of this study provide the first field-based insight into how foraging ants respond behaviorally to the heterogeneous thermal landscape of the tropical forest canopy.

The effect of temperature and humidity on adhesion of a gecko-inspired adhesive: implications for the natural system.

The adhesive system of geckos has inspired hundreds of synthetic adhesives. While this system has been used relentlessly as a source of inspiration, less work has been done in reverse, where synthetics are used to test questions and hypotheses about the natural system. Here we take such an approach. We tested shear adhesion of a mushroom-tipped synthetic gecko adhesive under conditions that produced perplexing results in the natural adhesive system. Synthetic samples were tested at two temperatures (12 °C and 32 °C) and four different humidity levels (30%, 55%, 70%, and 80% RH). Surprisingly, adhesive performance of the synthetic samples matched that of living geckos, suggesting that uncontrolled parameters in the natural system, such as surface chemistry and material changes, may not be as influential in whole-animal performance as previously thought. There was one difference, however, when comparing natural and synthetic adhesive performance. At 12 °C and 80% RH, adhesion of the synthetic structures was lower than expected based on the natural system's performance. Our approach highlights a unique opportunity for both biologists and material scientists, where new questions and hypotheses can be fueled by joint comparisons of the natural and synthetic systems, ultimately improving knowledge of both.

Superhydrophobicity of the gecko toe pad: biological optimization versus laboratory maximization.

While many gecko-inspired hierarchically structured surfaces perform as well as or better than the natural adhesive system, these designs often fail to function across a variety of contexts. For example, the gecko can adhere to rough, wet and dirty surfaces; however, most synthetic mimics cannot maintain function when faced with a similar situation. The solution to this problem lies in a more thorough investigation of the natural system. Here, we review the adhesive system of the gecko toe pad, as well as the far less-well-studied anti-adhesive system that results from the chemistry and structure of the toe pad (superhydrophobicity). This paradoxical relationship serves as motivation to study functional optimization at the system level. As an example, we experimentally investigate the role of surface lipids in adhesion and anti-adhesion, and find a clear performance trade-off related to shear adhesion in air on a hydrophilic surface. This represents the first direct investigation of the role of surface lipids in gecko adhesion and anti-adhesion, and supports the argument that a system-level approach is necessary to elucidate optimization in biological systems. Without such an approach, bioinspired designs will be limited in functionality and context, especially compared to the natural systems they mimic.This article is part of the themed issue 'Bioinspired hierarchically structured surfaces for green science'.

Sticking to the story: outstanding challenges in gecko-inspired adhesives.

The natural clinging ability of geckos has inspired hundreds of studies seeking design principles that could be applied to creating synthetic adhesives with the same performance capabilities as the gecko: adhesives that use no glue, are self-cleaning and reusable, and are insensitive to a wide range of surface chemistries and roughness. Important progress has been made, and the basic mechanics of how 'hairy' adhesives work have been faithfully reproduced, advancing theory in surface science and portending diverse practical applications. However, after 15 years, no synthetic mimic can yet perform as well as a gecko and simultaneously meet of all the criteria listed above. Moreover, processes for the production of inexpensive and scalable products are still not clearly in view. Here, we discuss our perspective on some of the gaps in understanding that still remain; these gaps in our knowledge should stimulate us to turn to deeper study of the way in which free-ranging geckos stick to the variety of surfaces found in their natural environments and to a more complete analysis of the materials composing the gecko toe pads.

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.