Robert J. Full

Adhesive force of a single gecko foot-hair

Geckos are exceptional in their ability to climb rapidly up smooth vertical surfaces. Microscopy has shown that a geckos foot has nearly five hundred thousand keratinous hairs or setae. Each 30–130 µm long seta is only one-tenth the diameter of a human hair and contains hundreds of projections terminating in 0.2–0.5 µm spatula-shaped structures. After nearly a century of anatomical description, here we report the first direct measurements of single setal force by using a two-dimensional micro-electro-mechanical systems force sensor and a wire as a force gauge. Measurements revealed that a seta is ten times more effective at adhesion than predicted from maximal estimates on whole animals. Adhesive force values support the hypothesis that individual seta operate by van der Waals forces. The geckos peculiar behaviour of toe uncurling and peeling led us to discover two aspects of setal function which increase their effectiveness. A unique macroscopic orientation and preloading of the seta increased attachment force 600-fold above that of frictional measurements of the material. Suitably orientated setae reduced the forces necessary to peel the toe by simply detaching above a critical angle with the substratum.

Evidence for van der Waals adhesion in gecko setae

Geckos have evolved one of the most versatile and effective adhesives known. The mechanism of dry adhesion in the millions of setae on the toes of geckos has been the focus of scientific study for over a century. We provide the first direct experimental evidence for dry adhesion of gecko setae by van der Waals forces, and reject the use of mechanisms relying on high surface polarity, including capillary adhesion. The toes of live Tokay geckos were highly hydrophobic, and adhered equally well to strongly hydrophobic and strongly hydrophilic, polarizable surfaces. Adhesion of a single isolated gecko seta was equally effective on the hydrophobic and hydrophilic surfaces of a microelectro-mechanical systems force sensor. A van der Waals mechanism implies that the remarkable adhesive properties of gecko setae are merely a result of the size and shape of the tips, and are not strongly affected by surface chemistry. Theory predicts greater adhesive forces simply from subdividing setae to increase surface density, and suggests a possible design principle underlying the repeated, convergent evolution of dry adhesive microstructures in gecko, anoles, skinks, and insects. Estimates using a standard adhesion model and our measured forces come remarkably close to predicting the tip size of Tokay gecko seta. We verified the dependence on size and not surface type by using physical models of setal tips nanofabricated from two different materials. Both artificial setal tips stuck as predicted and provide a path to manufacturing the first dry, adhesive microstructures.

Similarity in multilegged locomotion: Bouncing like a monopode

Despite impressive variation in leg number, length, position and type of skeleton, similarities of legged, pedestrian locomotion exist in energetics, gait, stride frequency and ground-reaction force. Analysis of data available in the literature showed that a bouncing, spring-mass, monopode model can approximate the energetics and dynamics of trotting, running, and hopping in animals as diverse as cockroaches, quail and kangaroos. From an animals mechanical-energy fluctuation and ground-reaction force, we calculated the compression of a virtual monopodes leg and its stiffness. Comparison of dimensionless parameters revealed that locomotor dynamics depend on gait and leg number and not on body mass. Relative stiffness per leg was similar for all animals and appears to be a very conservative quantity in the design of legged locomotor systems. Differences in the general dynamics of gait are based largely on the number of legs acting simultaneously to determine the total stiffness of the system. Four- and six-legged trotters had a greater whole body stiffness than two-legged runners operating their systems at about the same relative speed. The greater whole body stiffness in trotters resulted in a smaller compression of the virtual leg and a higher natural frequency and stride frequency.

RHex: A Biologically Inspired Hexapod Runner

RHex is an untethered, compliant leg hexapod robot that travels at better than one body length per second over terrain few other robots can negotiate at all. Inspired by biomechanics insights into arthropod locomotion, RHex uses a clock excited alternating tripod gait to walk and run in a highly maneuverable and robust manner. We present empirical data establishing that RHex exhibits a dynamical (“bouncing”) gait—its mass center moves in a manner well approximated by trajectories from a Spring Loaded Inverted Pendulum (SLIP)—characteristic of a large and diverse group of running animals, when its central clock, body mass, and leg stiffnesses are appropriately tuned. The SLIP template can function as a useful control guide in developing more complex autonomous locomotion behaviors such as registration via visual servoing, local exploration via visual odometry, obstacle avoidance, and, eventually, global mapping and localization.

Dynamics of geckos running vertically

SUMMARY Geckos with adhesive toe pads rapidly climb even smooth vertical surfaces. We challenged geckos (Hemidactylus garnotii) to climb up a smooth vertical track that contained a force platform. Geckos climbed vertically at up to 77 cm s-1 with a stride frequency of 15 Hz using a trotting gait. During each step, whole body fore–aft, lateral and normal forces all decreased to zero when the animal attached or detached its toe pads. Peak fore–aft force was twice body weight at mid-step. Geckos climbed at a constant average velocity without generating decelerating forces on their center of mass in the direction of motion. Although mass-specific mechanical power to climb was ten times the value expected for level running, the total mechanical energy of climbing was only 5–11% greater than the potential energy change. Fore- and hindlegs both pulled toward the midline, possibly loading the attachment mechanisms. Attachment and detachment of feet occupied 13% and 37% of stance time, respectively. As climbing speed increased, the absolute time required to attach and detach did not decrease, suggesting that the period of fore–aft force production might be constrained. During ascent, the forelegs pulled toward, while hindlegs pushed away from the vertical surface, generating a net pitching moment toward the surface to counterbalance pitch-back away from the surface. Differential leg function appears essential for effective vertical as well as horizontal locomotion.

Fast and Robust

Robots to date lack the robustness and performance of even the simplest animals when operating in unstructured environments. This observation has prompted an interest in biomimetic robots that take design inspiration from biology. However, even biomimetic designs are compromised by the complexity and fragility that result from using traditional engineering materials and manufacturing methods. We argue that biomimetic design must be combined with structures that mimic the way biological structures are composed, with embedded actuators and sensors and spatially-varied materials. This proposition is made possible by a layered-manufacturing technology called shape deposition manufacturing (SDM). We present a family of hexapedal robots whose functional biomimetic design is made possible by SDMs unique capabilities and whose fast (over four body-lengths per second) and robust (traversal over hip-height obstacles) performance begins to compare to that seen in nature. We describe the design and fabrication of the robots and we present the results of experiments that focus on their performance and locomotion dynamics.

An Integrative Study of Insect Adhesion: Mechanics and Wet Adhesion of Pretarsal Pads in Ants

Abstract Many animals that locomote by legs possess adhesive pads. Such organs are rapidly releasable and adhesive forces can be controlled during walking and running. This capacity results from the interaction of adhesive with complex mechanical systems. Here we present an integrative study of the mechanics and adhesion of smooth attachment pads (arolia) in Asian Weaver ants (Oecophylla smaragdina). Arolia can be unfolded and folded back with each step. They are extended either actively by contraction of the claw flexor muscle or passively when legs are pulled toward the body. Regulation of arolium use and surface attachment includes purely mechanical control inherent in the arrangement of the claw flexor system. Predictions derived from a ‘wet’ adhesion mechanism were tested by measuring attachment forces on a smooth surface using a centrifuge technique. Consistent with the behavior of a viscid secretion, frictional forces per unit contact area linearly increased with sliding velocity and the increment strongly decreased with temperature. We studied the nature and dimensions of the adhesive liquid film using Interference Reflection Microscopy (IRM). Analysis of ‘footprint’ droplets showed that they are hydrophobic and form low contact angles. In vivo IRM of insect pads in contact with glass, however, revealed that the adhesive liquid film not only consists of a hydrophobic fluid, but also of a volatile, hydrophilic phase. IRM allows estimation of the height of the liquid film and its viscosity. Preliminary data indicate that the adhesive secretion alone is insufficient to explain the observed friction and that rubbery deformation of the pad cuticle is involved.

Quantifying dynamic stability and maneuverability in legged locomotion.

Abstract Animals can swerve, dodge, dive, climb, turn and stop abruptly. Their stability and maneuverability are remarkable, but a challenge to quantify. Formal stability analysis can allow for quantitative comparisons within and among species. Stability analysis used in concert with a template (a simple, general model that serves as a guide for control) can lead to testable hypotheses of function. Neural control models postulated without knowledge of the animals mechanical (musculo-skeletal) system can be counterproductive and even destabilizing. Perturbations actively corrected by reflex feedback in one direction can result in perturbations in other directions because the system is coupled dynamically. The passive rate of recovery from a perturbation in one direction differs from rates in other directions. We hypothesize that animals might exert less neural control in directions that rapidly recover via passive dynamics (e.g., in body orientation and rotation). By contrast, animals are likely to exert more neural control in directions that recover slowly or not at all via passive dynamics (e.g., forward velocity and heading). Neural control best enhances stability when it works with the natural, passive dynamics of the mechanical system. Measuring maneuverability is more challenging and new, general metrics are needed. Templates reveal that simple analyses of summed forces and quantification of the center of pressure can lead to valuable hypotheses, whereas kinematic descriptions may be inadequate. The study of stability and maneuverability has direct relevance to the behavior and ecology of animals, but is also critical if animal design is to be understood. Animals appear to be grossly over-built for steady-state, straight-ahead locomotion, as they appear to possess too many neurons, muscles, joints and even too many appendages. The next step in animal locomotion is to subject animals to perturbations and reveal the function of all their parts.

Tail-assisted pitch control in lizards, robots and dinosaurs

In 1969, a palaeontologist proposed that theropod dinosaurs used their tails as dynamic stabilizers during rapid or irregular movements, contributing to their depiction as active and agile predators. Since then the inertia of swinging appendages has been implicated in stabilizing human walking, aiding acrobatic manoeuvres by primates and rodents, and enabling cats to balance on branches. Recent studies on geckos suggest that active tail stabilization occurs during climbing, righting and gliding. By contrast, studies on the effect of lizard tail loss show evidence of a decrease, an increase or no change in performance. Application of a control-theoretic framework could advance our general understanding of inertial appendage use in locomotion. Here we report that lizards control the swing of their tails in a measured manner to redirect angular momentum from their bodies to their tails, stabilizing body attitude in the sagittal plane. We video-recorded Red-Headed Agama lizards (Agama agama) leaping towards a vertical surface by first vaulting onto an obstacle with variable traction to induce a range of perturbations in body angular momentum. To examine a known controlled tail response, we built a lizard-sized robot with an active tail that used sensory feedback to stabilize pitch as it drove off a ramp. Our dynamics model revealed that a body swinging its tail experienced less rotation than a body with a rigid tail, a passively compliant tail or no tail. To compare a range of tails, we calculated tail effectiveness as the amount of tailless body rotation a tail could stabilize. A model Velociraptor mongoliensis supported the initial tail stabilization hypothesis, showing as it did a greater tail effectiveness than the Agama lizards. Leaping lizards show that inertial control of body attitude can advance our understanding of appendage evolution and provide biological inspiration for the next generation of manoeuvrable search-and-rescue robots.

Dynamics and stability of legged locomotion in the horizontal plane: a test case using insects

Abstract. Motivated by experimental studies of insects, we propose a model for legged locomotion in the horizontal plane. A three-degree-of freedom, energetically conservative, rigid-body model with a pair of compliant virtual legs in intermittent contact with the ground allows us to study how dynamics depends on parameters such as mass, moment of inertia, leg stiffness, and length. We find periodic gaits, and show that mechanics alone can confer asymptotic stability of relative heading and body angular velocity. We discuss the relevance of our idealized models to experiments and simulations on insect running, showing that their gait and force characteristics match observations reasonably well. We perform parameter studies and suggest that our model is relevant to the understanding of locomotion dynamics across species.