Adhesive Contact in Animal： Morphology, Mechanism and Bio-Inspired Application

Many animals possess adhesive pads on their feet, which are able tO attach to various substrates while controlling adhesive forces during locomotion. This review article studies the morphology of adhesive devices in animals, and the physical mechanisms of wet adhesion and dry adhesion. The adhesive pads are either ＇smooth＇ or densely covered with special adhesive setae. Smooth pads adhere by wet adhesion, which is facilitated by fluid secreted from the pads, whereas hairy pads can adhere by dry adhesion or wet adhesion. Contact area, distance between pad and substrate, viscosity and surface tension of the liquid filling the gap between pad and substrate are the most important factors which determine the wet adhesion. Dry adhesion was found only in hairy pads, which occurs in geckos and spiders. It was demonstrated that van der Waals interaction is the dominant adhesive force in geckos＇ adhesion. The bio-inspired applications derived from adhesive pads are also reviewed.

Development of Wheel-Legged Biped Robots:A Review

The wheel-legged biped robot is a typical ground-based mobile robot that can combine the high velocity and high efficiency pertaining to wheeled motion and the strong,obstacle-crossing performance associated with legged motion.These robots have gradually exhibited satisfactory application potential in various harsh scenarios such as rubble rescue,military operations,and wilderness exploration.Wheel-legged biped robots are divided into four categories according to the open–close chain structure forms and operation task modes,and the latest technology research status is summarized in this paper.The hardware control system,control method,and application are analyzed,and the dynamic balance control for the two-wheel,biomimetic jumping control for the legs and whole-body control for integrating the wheels and legs are analyzed.In summary,it is observed that the current research exhibits problems,such as the insufficient application of novel materials and a rigid–flexible coupling design;the limited application of the advanced,intelligent control methods;the inadequate understanding of the bionic jumping mechanisms in robot legs;and the insufficient coordination ability of the multi-modal motion,which do not exhibit practical application for the wheel-legged biped robots.Finally,this study discusses the key research directions and development trends for the wheel-legged biped robots.

Wheel-legged In-pipe Robot with a Bioinspired Hook and Dry Adhesive Attachment Device

In-pipe robots have been widely used in pipes-with smooth inner walls.However,current in-pipe robots face challenges in terms of moving past obstacles and climbing in marine-vessel pipeline systems,which are affected by marine biofouling and electrochemical corrosion.This paper takes inspiration from the dual-hook structure of Trypoxylus dichotomus’s feet and gecko‑like dry adhesives,proposing an in-pipe robot that is capable of climbing on rough and smooth pipe inwalls.The combination of the bioinspired hook and dry adhesives allows the robot to stably attach to rough or smooth pipe inwalls,while the wheel-leg hybrid mechanism provides better conditions for obstacle traversal.The paper explores the attachment and obstacle-surmounting mechanisms of the robot.Moreover,motion strategies for the robot are devised based on different pipe structural features.The experiments showed that this robot can adapt to both smooth and rough pipe environments simultaneously,and its motion performance is superior to conventional driving mechanisms.The robot’s active turning actuators also enable it to navigate through horizontally or vertically oriented 90°bends.

Detachment Behavior of Gecko Toe in Functional Strategies for Bionic Toe

Geckos can efficiently navigate complex terrains due to their multi-level adhesive system that is present on their toes.The setae are responsible for the gecko’s extraordinary adhesion and have garnered wide attention from the scientific community.The majority of the reported works in the literature that have dealt with the peeling models mainly focus on the gecko hierarchical adhesive system,with limited attention given to investigating the influence of gecko toe structure on the detachment.Along these lines,to gain a deeper understanding of the rapid and effortless detachment abilities of gecko toes,the peeling behavior of gecko toes on vertical surfaces was primarily investigated in this work.More specifically,the detachment time of a single toe on a smooth acrylic plate was measured to be 0.41±0.21 s.Moreover,it was observed that the toe assumed a"U"-shaped structure upon complete detachment.Additionally,Finite Element Analysis(FEA)models for three different types of gecko toes were developed to simulate both the displacement-peel and the moment-peel modes.Increasing the segmentation of the adhesive layer led to a gradual decrease in the resultant force,as well as the normal and tangential components.Lastly,a gecko-inspired toe model was constructed and powered by Shape Memory Alloy(SMA).A systematic comparison between the vertical drag separation and the outward flip separation was also conducted.From our analysis,it was clearly demonstrated that outward peel separation significantly necessitated the reduction of the peeling force,thus confirming the advantageous nature of the outward motion in gecko toe detachment.Our data not only contribute to a deeper understanding of the gecko detachment behavior but also offer valuable insights for the advancement of the wall-climbing robot feet.

An Aerial–Wall Robotic Insect That Can Land, Climb, and Take Off from Vertical Surfaces

Insects that can perform flapping-wing flight,climb on a wall,and switch smoothly between the 2 locomotion regimes provide us with excellent biomimetic models.However,very few biomimetic robots can perform complex locomotion tasks that combine the 2 abilities of climbing and flying.Here,we describe an aerial–wall amphibious robot that is self-contained for flying and climbing,and that can seamlessly move between the air and wall.It adopts a flapping/rotor hybrid power layout,which realizes not only efficient and controllable flight in the air but also attachment to,and climbing on,the vertical wall through a synergistic combination of the aerodynamic negative pressure adsorption of the rotor power and a climbing mechanism with bionic adhesion performance.On the basis of the attachment mechanism of insect foot pads,the prepared biomimetic adhesive materials of the robot can be applied to various types of wall surfaces to achieve stable climbing.The longitudinal axis layout design of the rotor dynamics and control strategy realize a unique cross-domain movement during the flying–climbing transition,which has important implications in understanding the takeoff and landing of insects.Moreover,it enables the robot to cross the air–wall boundary in 0.4 s(landing),and cross the wall–air boundary in 0.7 s(taking off).The aerial–wall amphibious robot expands the working space of traditional flying and climbing robots,which can pave the way for future robots that can perform autonomous visual monitoring,human search and rescue,and tracking tasks in complex air–wall environments.

Design of an Active Flexible Spine for Wall Climbing Robot Using Pneumatic Soft Actuators

Wall climbing robots can be used to undertake missions in many unstructured environments.However,current wall climbing robots have mobility difficulties such as in the turning or accelarating.One of the main reasons for the limitations is the poor flexibility of the spines.Soft robotic technology can actively enable structure deformation and stiffness varations,which provides a solution for the design of active flexible spines.This research utilizes pneumatic soft actuators to design a flexible spine with the abilities of actively bending and twisting by each joint.Using bending and torsion moment equilibriums,respectively,from air pressure to material deformations,the bending and twisting models for a single actuator with respect to different pressure are obtained.The theoretical models are verified by finite-element method simulations and experimental tests.In addition,the bending and twisiting motions of single joint and whole spine are analytically modeled.The results show that the bionic spine can perform desired deformations in accordance with the applied pressure on specified chambers.The variations of the stiffness are also numerically assessed.Finally,the effectiveness of the bionic flexible spine for actively producing sequenced motions as biological spine is experimentally validated.This work demonstrated that the peneumatic spine is potential to improve the spine flexibility of wall climbing robot.

Design and Theoretical Research on Aerial-Aquatic Vehicles:A Review

With the rapid development of unmanned aerial and underwater vehicles,various tasks,such as biodiversity monitoring,surveying,and mapping,as well as,search and rescue can now be completed in a single medium,either underwater or in the air.By systematically examining the water–air cross-medium locomotion of organisms,there has been growing interest in the development of aerial-aquatic vehicles.The goal of this review is to provide a detailed outline of the design and cross-medium theoretical research of the existing aerial-aquatic vehicles based on the research on the organisms capable of transiting between water and air.Although these designs and theoretical frameworks have been validated in many aerial-aquatic vehicles,there are still many problems that need to be addressed,such as inflexible underwater motion and unstable medium conversion.As a result,supplementation of the existing cross-medium biomimetic research,vehicle design,power selection,and cross-medium theory is urgently required to optimize the key technologies in detail.Therefore,by summarizing the existing designs and theoretical approaches on aerial-aquatic vehicles,including biomimetic research on water–air cross-medium locomotion in nature,different power selections,and cross-medium theoretical research,the relative problems and development trends on aerial-aquatic vehicles were thoroughly explored,providing significant help for the subsequent research process.

A Bio-inspired Climbing Robot with Flexible Pads and Claws

Many animals exhibit strong mechanical interlocking in order to achieve efficient climbing against rough surfaces by using their claws in the pads. To maximally use the mechanical interlocking, an innovative robot which utilizes flexible pad with claws is designed. The mechanism for attachments of the claws against rough surfaces is further revealed according to the theoretical analysis. Moreover, the effects of the key parameters on the performances of the climbing robots are obtained. It indicates that decreasing the size of the tip of the claws while maintaining its stiffness unchanged can effectively improve the attachment ability. Furthermore, the structure of robot body and two foot trajectories are proposed and the new robot is presented. Using experimental tests, it demonstrates that this robot has high stability and adaptability while climbing on vertical rough surfaces up to a speed of 4.6 cm.s^-1.

The Mechanics and Trajectory Control in Locust Jumping

Locusts （Locusta migratoria manilensis） are characterised by their flying ability and abiding jump ability. Research on the jumping mechanics and behavior of locusts plays an important role in elucidating the mechanism of hexapod locomotion. The jump gestures of locusts were observed using high-speed video camera at 250 fps. The reaction forces of the hindlegs were measured using two three-dimensional sensors, in case the two hindlegs attached on separated sensor plates. The jump gestures and reaction forces were used to illustrate the locust jumping mechanism. Results show that the trajectory control is achieved by rapid rolling and yawing movements of the locust body, caused by the forelegs, midlegs and hindlegs in different jumping phases. The final jump trajectory was not determined until hind tarsi left platform. The horizontal co-impulse between two hindlegs might play a key role in jump stability and accuracy. Besides, the angle between two hindlegs affects the control of jump trajectory but has a little effect on the elevation angle of a jump, which is controlled mechanically by the initial position of the hindlegs. This research lays the groundwork for the probable design and development ofbiomimetic robotics.

Contribution of friction and adhesion to the reliable attachment of a gecko to smooth inclines

Geckos' ability to move on steep surfaces depends on their excellent adhesive structure, timely adjustments on locomotor behaviors, and elaborates control on reaction forces. However, it is still unclear how they can generate a sufficient driving force that is necessary for locomotion, while ensuring reliable adhesion on steep inclines. We measured the forces acting on each foot and recorded the contact states between feet and substrates when geckos encountered smooth inclination challenges ranging from 0° to 180°. The critical angles of the resultant force vectors of the front and hind-feet increased with respect to the incline angles. When the incline angle became greater than 120°, the critical angles of the front- and hind-feet were similar, and the averages of the critical angles of the front - and hind-feet were both smaller than 120°, indicating that the complicated and accurate synergy among toes endows gecko's foot an obvious characteristic of "frictional adhesion" during locomotion. Additionally, we established a contact mechanical model for gecko's foot in order to quantify the contribution of the frictional forces generated by the heel, and the adhesion forces generated by the toes on various inclines. The synergy between multiple contact mechanisms(friction or adhesion) is critical for the reliable attachment on an inclined surface, which is impossible to achieve by using a single-contact mechanism, thereby increasing the animal's ability to adapt to its environment.

Variation in spatial and temporal kinematics of level, vertical and inverted locomotion on a stinkbug Erthesina fullo

In the learning of locomotion behavior of a stinkbug Erthesina fullo, the seeked principle of its locomotion can be an important inspiration on the design of six-legged robot. To achieve this goal, in this paper, locomotion behavior of stinkbugs on glass and plastic foam are recorded. Hereby, variation in spatial and temporal kinematics of level, vertical and inverted locomotion is analyzed. Differential leg function and adhesive mechanism as well as the advantage of non-isometric legs of insects are presented. With increasing stride frequency, the speed of level, vertical and inverted locomotion can be higher without adjusting stride length. Variation in gait characteristics between level and vertical locomotion is very little, but lower speed and larger duty factor of inverted locomotion can be occurred while climbing on glass. On the surface of vertical and inverted plastic foam, stinkbugs cannot walk steady and agilely due to its adhesive mechanism.

Effects of Morphological Integrity of Secondary Feather on Their Drag Reduction in Pigeons

Flight feathers of birds interact with the air flow during flight.How the observed low drag and high lift values at wind speeds from 9.0 to 19.8 m/s can be achieved due to the feather aerodynamics remains unknown.In the present paper,we tested and compared morphological changes,drag reduction and flow visualization results of intact,damaged,and artificial feathers at different wind speeds in a wind tunnel.Through the analysis of the drag force and resultant force angle,we proved that the integrity of feathers,whose barbs are usually closely interconnected,played an important role in the drag,which potentially triggers excellent drag reduction performance.The wind tunnel tests indicated that intact secondary feathers had a surprisingly high maximum drag reduction property at v?=?9 m/s compared with the feathers,where the integrity of barbs was damaged.The hook cascades facilitated elasticity under pressure and suitable permeability in an intact feather,when the hooks were interlocked.It was indicated that the suitable permeability of intact feathers would prevent flow separation and reduce drag force at low wind speed;at high wind speed,elasticity under pressure and suitable permeability in an intact feather would facilitate strong squeezing effect,helping feathers withstand larger aerodynamic forces to which they might be subjected during flight.It was revealed that the intact secondary feather is a compromise between strong lift generation and drag reduction,which has a great significance for the bird’s flight.

The Unique Strategies of Flight Initiation Adopted by Butterflies on Vertical Surfaces

As the basis of flight behavior,the initiation process of insect flight is accompanied by a transition from crawling mode to flight mode,and is clearly important and complex.Insects take flight from a vertical surface,which is more difficult than takeoff from a horizontal plane,but greatly expands the space of activity and provides us with an excellent bionic model.In this study,the entire process of a butterfly alighting from a vertical surface was captured by a high-speed camera system,and the movements of its body and wings were accurately measured for the first time.After analyzing the movement of the center of mass,it was found that before initiation,the acceleration perpendicular to the wall was much greater than the acceleration parallel to the wall,reflecting the positive effects of the legs during the initiation process.However,the angular velocity of the body showed that this process was unstable,and was further destabilized as the flight speed increased.Comparing the angles between the body and the vertical direction before and after leaving the wall,a significant change in body posture was found,evidencing the action of aerodynamic forces on the body.The movement of the wings was further analyzed to obtain the laws of the three Euler angles,thus revealing the locomotory mechanism of the butterfly taking off from the vertical surface.

Effect of Dual-Tasks Walking on Human Gait Patterns

A human gait pattern conformed to the law of the inverted pendulum and could be described by the movement of its center of mass(COM).A sinusoidal function based on an inverted pendulum was used to plan the COM trajectory of humanoid robots for gait control.Therefore,studying the human dual-tasks gait pattern is essential for improving the gait adaptability of humanoid robots.Many well-known humanoid robot prototypes can complete stable walking,running,jumping,and other movements closely mimicking human beings.This study provides a theoretical reference for the gait planning and design of humanoid robots by comparing differences in the stride time,cadence,step length,step width,stride length,walking speed and COM movement during single-and dual-tasks walking.Results showed no significant difference in the time parameters.In dual-tasks walking,the walking speeds decreased(P=0.001)and the COM sidewise movement increased,indicating an enhancement in the automatic motion rhythm in cognitive tasks.Humans can easily select new different speeds to better adapt to the competition of attention resources.To enhance the adaptability of humanoid robots to the COM offset,the independent motion of the hip joint separated from the trunk and lower limbs must be considered in the robot design.

Corrigendum to ＂The Mechanics and Trajectory Control in Locust Jumping＂ [Journal of Bionic Engineering, 2013, 10, 194-200]

This communication gives the corrigendum to the paper ＂The Mechanics and Trajectory Control in Locust Jumping＂, Journal of Bionic Engineering, 2013, 10, 194-200. doi： 10.1016/S1672-6529（13）60215-2