Table of contents

One of the major challenges in robotics and engineering is to develop efficient technological solutions that are able to cope with complex environments and unpredictable constraints. Taking inspiration from natural organisms is a well-known approach to tackling these issues. Climbing plants are an important, yet innovative, source of inspiration due to their ability to adapt to diverse habitats, and can be used as a model for developing robots and smart devices for exploration and monitoring, as well as for search and rescue operations. This review reports the main methodologies and approaches used by scientists to investigate and extract the features of climbing plants that are relevant to the artificial world in terms of adaptation, movement, and behaviour, and it summarizes the current available climbing plant-inspired engineering solutions.

Dramatic changes in electricity generation, use and storage are needed to keep pace with increasing demand while reducing carbon dioxide emissions. There is great potential for application of bioengineering in this area. We have the tools to re-engineer biological molecules and systems, and a significant amount of research and development is being carried out on technologies such as biophotovoltaics, biocapacitors, biofuel cells and biobatteries. However, there does not seem to be a satisfactory overarching term to describe this area, and I propose a new word—'electrosynbionics'. This is to be defined as: the creation of engineered devices that use components derived from or inspired by biology to perform a useful electrical function. Here, the phrase 'electrical function' is taken to mean the generation, use and storage of electricity, where the primary charge carriers may be either electrons or ions. 'Electrosynbionics' is distinct from 'bioelectronics', which normally relates to applications in sensing, computing or electroceuticals. Electrosynbionic devices have the potential to solve challenges in electricity generation, use and storage by exploiting or mimicking some of the desirable attributes of biological systems, including high efficiency, benign operating conditions and intricate molecular structures.

Propulsion of swimming robots at the surface and underwater is largely dominated by rotary propellers due to high thrust, but at the cost of low efficiency. Due to their inherently high speed turning motion, sharp propeller blades and generated noise, they also present a disturbance to maritime ecosystems. Our work presents a bio-inspired approach to efficient and eco-friendly swimming with moderate to high thrust. This paper describes the concept, development and experimental validation of the novel anguilliform robot MAR. With 15 elements making up the 0.5 m long propulsive section and driven by a single, speed-controlled brushless DC motor (BLDC), the robot creates a smooth continuous traveling wave for propulsion. Steering and autonomy are realized by an actuated head with integrated batteries that serves as a front-rudder. Almost neutral buoyancy paired with individually actuated pectoral fins furthermore enable submerged swimming and diving maneuvers. MAR accomplished high thrusts at a moderate power consumption in first performance tests. The achieved maximum velocity and the speed related efficiency (defined as the achieved speed over the power consumption m Ws⁻¹) did not fulfill the expectations in the first tests (in comparison to commercial rotary thrusters), which can be largely attributed to the spatial limitations and an imperfect test setup. Nevertheless, the potential towards highly efficient and high thrust propulsion is visible and will be further investigated in future efforts.

Sea lions swim using primarily their foreflippers, which is uncommon among aquatic mammals. While a significant body of literature exists which investigates the hydrodynamics of body-caudal swimming, relatively little research has looked at sea lion propulsion. In this work, particle imaging velocimetry is used to observe the flow around a robotic model sea lion flipper. The model flipper was cast in silicone from a high-resolution scan of a sample sea lion foreflipper. The model flipper was actuated at the root, and its motion was controlled by a programmable servomotor. It was observed that the thrust-producing clapping motion of the flipper entrained significant fluid momentum on the suction side of the flipper, which developed into a shed vortex and contributed to downstream momentum (and therefore thrust). Rotating the robotic flipper more quickly produced greater downstream jet velocities, but at a lower conversion of rotational velocity, suggesting that this mechanism of propulsion can be optimized based on the system needs.

We demonstrate that shape-changing or morphing fins provide a new paradigm for improving the ability of vehicles to maneuver and move rapidly underwater. An ingenuous solution is employed by fish to accommodate both the need for stability of locomotion and the ability to perform tight maneuvers: Retractable fins can alter the stability properties of a vehicle to suit their particular goals. Tunas, for example, are large fish that are fast swimmers and yet they need rapid turning agility to track the smaller fish they pursue; they have perfected the use of their dorsal and ventral fins to ensure stability when retracted and rapid turning when erected. Although fish employ unsteady propulsors rather than propellers, we show that engineering rigid-hull underwater vehicles can also exploit similar solutions. We explore the basic flow mechanisms and design considerations of employing morphing fins to alter the stability and maneuvering qualities of vehicles and apply unsteady forces and moments under active control. We also show results from maneuvering simulations and experiments on a model of an underwater vehicle.

Soft robots take advantage of rich nonlinear dynamics and large degrees of freedom to perform actions often by novel means beyond the capability of conventional rigid robots. Nevertheless, there are considerable challenges in analysis, design, and optimization of soft robots due to their complex behaviors. This is especially true for soft robotic swimmers whose dynamics are determined by highly nonlinear fluid-structure interactions. We present a holistic computational framework that employs a multi-objective evolutionary method to optimize feedback controllers for maneuvers of a soft robotic fish under artificial muscle actuation. The resultant fluid-structure interactions are fully solved by using a novel fictitious domain/active strain method. In particular, we consider a two-dimensional elastic plate with finite thickness, subjected to active contractile strains on both sides of the body. Compared to the conventional approaches that require specifying the entire-body curvature variation, we demonstrate that imposing contractile active strains locally can produce various swimming gaits, such as forwarding swimming and turning, using far fewer control parameters. The parameters of a pair of proportional-integral-derivative (PID) controllers, used to control the amplitude and the bias of the active strains, respectively, are optimized for tracking a moving target involving different trajectories and Reynolds numbers, with three objectives, tracking error, cost of transport, and elastic strain energy. The resulting Pareto fronts of the multi-objective optimization problem reveal the correlation and trade-off among the objectives and offer key insight into the design and control of soft swimmers.

Theoretical guarantees of capture become complicated in the case of a swimming fish or fish robot because of the oscillatory nature of the fish heading. Building on the connection between a swimming fish and the canonical Chaplygin sleigh, a novel state feedback control law is shown to result in closed-loop dynamics that exhibit a limit cycle resulting in steady forward-swimming motion in a desired heading. Analysis of this limit cycle reveals boundaries on the size of the oscillations around the desired heading, which are then used to specify under what conditions (e.g. prey speed, predator speed, control gains) capture is guaranteed. By changing the desired swimming direction in response to prey movements, the control law is shown to be capable of pure pursuit, deviated pure pursuit, intercept, and parallel navigation in simulation. An experimental demonstration of pure pursuit by a flexible fish-inspired robot actuated with an internal reaction wheel is described.

Organisms like the octopus or the clingfish are a precious source of inspiration for the design of innovative adhesive systems based on suction cups, but a complete mechanical description of their attachment process is still lacking. In this paper, we exploit the recent discovery of the presence of hairs in the acetabulum roof of octopus suction cups to revise the current model for its adhesion to the acetabulum wall. We show how this additional feature, which can be considered an example of a hierarchical structure, can lead to an increase of adhesive strength, based on the analysis of the cases of a simple tape and an axisymmetrical membrane adhering to a substrate. Using peeling theory, we discuss in both cases the influence of hierarchical structure and the resulting variation of geometry on the adhesive energy, highlighting how an increase in number of hierarchical levels contributes to its increment, with a corresponding improvement in functionality for the octopus suckers.

Tunas of the genus Thunnus are a group of high-performance pelagic fishes with many locomotor traits that are convergently shared with other high-performance fish groups. Because of their swimming abilities, tunas continue to be an inspiration for both comparative biomechanics and the design of biomimetic autonomous underwater vehicles (AUVs). Despite the strong history of studies in tuna physiology and current interest in tuna biomechanics and bio-inspired design, we lack quantitative data on the function of many features of tunas. Here we present data on the morphology, behavior, and function of tunas, focusing especially on experimentally examining the function of tuna lateral keels, finlets, and pectoral fins by using simple physical models. We find that both triangular lateral keels and flexible finlets decrease power requirements during swimming, likely by reducing lateral forces and yaw torques (compared to models either without keels or with rectangular keels, and models with stiff finlets or strip fins of equal area, respectively). However, both triangular keels and flexible finlets generate less thrust than other models either without these features or with modified keels or finlets, leading to a tradeoff between power consumption and thrust. In addition, we use micro computed tomography (µCT) to show that the flexible lateral keels possess a lateral line canal, suggesting these keels have a sensory function. The curved and fully-attached base of tuna pectoral fins provides high lift-to-drag ratio at low angles of attack, and generates the highest torques across speeds and angles of attack. Therefore, curved, fully-attached pectoral fins grant both better gliding and maneuvering performance compared to flat or curved, partially-attached designs. We provide both 3D models of tuna morphology derived from µCT scans and conclusions about the performance effects of tuna-like features as a resource for future biological and engineering work for next-generation tuna-inspired AUV designs.

Weakly electric fish polarize the nearby environment with a stereotyped electric field and gain information by detecting the changes imposed by objects with tuned sensors. Here we focus on polarization strategies as paradigmatic bioinspiring mechanisms for sensing devices. We begin this research developing a toy model that describes three polarization strategies exhibited by three different groups of fish. We then report an experimental analysis which confirmed predictions of the model and in turn predicted functional consequences that were explored in behavioral experiments in the pulse fish Gymnotus omarorum. In the experiments, polarization was evaluated by estimating the object's stamp (i.e. the electric source that produces the same electric image as the object) as a function of object impedance, orientation, and position. Signal detection and discrimination was explored in G. omarorum by provoking novelty responses, which are known to reflect the increment in the electric image provoked by a change in nearby impedance. To achieve this, we stepped the longitudinal impedance of a cylindrical object between two impedances (either capacitive or resistive). Object polarization and novelty responses indicate that G. omarorum has two functional regions in the electrosensory field. At the front of the fish, there is a foveal field where object position and orientation are encoded in signal intensity, while the qualia associated with impedance is encoded in signal time course. On the side of the fish there is a peripheral field where the complexity of the polarizing field facilitates detection of objects oriented in any angle with respect to the fish´s longitudinal axis. These findings emphasize the importance of articulating field generation, sensor tuning and the repertoire of exploratory movements to optimize performance of artificial active electrosensory systems.

There are many reports on the special wettability of hierarchical surface structures in nature. Snail shells with three types of roughness of 10, 100, and 500 µm have a unique wetting behavior. In the present study, we investigate the influence of the surface structure on the water wettability using snail shells with different surface roughness. The wettability of a water droplet on the samples was evaluated. The three types of roughness on the surface structure of snail shell had higher water droplet spreading properties than the two types of roughness 500 µm and, 10 or 100 µm. Surface structures of snail shells with different surface roughness were simulated using epoxy resins to clarify the mechanism for the dynamics wetting behavior. The contact angle with a hydrophobic nature, of the epoxy resin with the three types of roughness decreased with increasing time, indicating a hydrophilic nature. The base diameter of the epoxy resins with the three types of roughness increased with increasing time. This was larger than that for a flat epoxy resin with hydrophilicity. Other epoxy resins with shell texture containing 100 and 500 or 10 and 500 µm roughness showed almost no change in the contact angle and diameter of the droplet base. The three types of roughness on the sample surface contributed to development of the water droplet spreading. The 10 µm roughness of the sample surface influenced the dynamic contact angles.

Some recent achievements in microfabrication have demonstrated ultrasound-actuated artificial micro-swimmers for medical applications. However, the theoretical model of actuation and swimming is still lacking. Here we report a theoretical study of an acoustically actuated sperm-like artificial micro-swimmer which consists of a rigid head and a flexible flagellum. We provide the quantitative relation between head oscillation amplitude and acoustic pressure and frequency, and the theoretical account of how the flagellum is whipped, which brings about propulsion. The resistive force theory is employed in our model to relate the dynamic response of a flagellum and the motility of the swimmer. In order to make our theoretical model applicable in a realistic design of sperm-like micro-swimmer, we have involved the inertia term and material damping in the governing equation and considered the variable cross-section of a flagellum. The numerical results reveal that the micro-swimmer actuated by ultrasound can achieve a perceptible velocity, especially at resonance. Influences of non-dimensional parameters, such as the resonance index, sperm number, and material damping coefficient, are discussed and a comparison with experimental results demonstrates the validity of the proposed model.

Particulate air pollution has an adverse effect on cardiovascular and respiratory health. Air filtration systems are therefore essential in closed indoor environments. While mechanical filtration is described as an efficient technology, particle filters may act as a source of pollution if not correctly installed and frequently maintained. The sandfish lizard, a sand swimmer that spends nearly its whole life in fine desert sand, inspired us to rethink traditional filtering systems due to its unique ability of filtering sand from its nasal cavity. During a slow, prolonged inhalation, strong cross-flow velocities develop in a certain region of the upper respiratory tract; these cross-flows enhance gravitational settling and force inhaled sand grains towards the wall where they adhere to mucus, which covers the walls in this region. During an intense, cough-like exhalation the particles are blasted out. In this work, the sandfish's aerodynamic filtering system was analyzed experimentally and by computational fluid dynamics simulations to study the flow profile and particle trajectories. Based on these findings, we discuss the development of a biomimetic filtering system, which could have the following advantages: due to the absence of a membrane, total pressure losses can be reduced. The mucus-covered surface would be mimicked by a specifically treated surface to trap particulate matter. Also, the device would contain a self-cleaning mechanism that simulates the lizard's exhalation. This biomimetic filtering system would therefore have an enhanced life-time and it would be low-maintenance and therefore economical and sustainable.

Balaenid whales, as continuous ram filter feeders, can efficiently separate prey from water by baleen. The feeding process of balaenid whales is extremely complex, in which the flow distribution and pressure drop in the oral cavity play a significant role. In this paper, a theoretical model coupled with oral cavity velocity and pressure in balaenid whales is established based on mass conservation, momentum conservation and pressure drop equations, considering both the inertial and the friction terms. A discrete method with section-by-section calculation is adopted to solve the theoretical model. The effects of four crucial parameters, i.e. the ratio of filtration area to inlet area (S), the Reynolds number of entrance (Re_in), the ratio of thickness to permeability of the porous media formed by the fringe layer (ϕ) and the width ratio of the anteroposterior canal within the mouth along the tongue (APT channel) to that along the lip (APL channel) (H) are discussed. The results show that, for a given case, the flow distribution and the pressure drop both show increasing trends with the flow direction. For different cases, when S is small, Re_in is small and ϕ is large, a good flow pattern emerges with a smoother flow speed near the oropharynx, better drainage, better shunting and filtration, and higher energy efficiency. However, for smaller values of H, some energy efficiency is sacrificed to achieve additional average transverse flow in order to produce better shunting and filtration. The research in this paper provides a reference for the design of high-efficiency bionic filters.

Numerous nature inspired algorithms have been suggested to enable robotic swarms, mobile sensor networks and other multi-agent systems to exhibit various self-organized behaviors. Swarm intelligence and swarm robotics research have been underway for a few decades and have produced many such algorithms based on natural self-organizing systems. While a large body of research exists for variations and modifications in swarm intelligence algorithms, there have been few attempts to unify the underlying agent level design of these widely varying behaviors. In this work, a design paradigm for a swarm of agents is presented which can exhibit a wide range of collective behaviors at swarm level while using minimalistic single-bit communication at the agent level. The communication in the proposed paradigm is based on waves of 'ping'-signals inspired by strategies for communication and self organization of slime mold (Dictyostelium discoideum) and fireflies (lampyridae). The unification of common collective behaviors through this Wave Oriented Swarm Paradigm (WOSP) enables the control of swarms with minimalistic communication and yet allowing the emergence of diverse complex behaviors. It is demonstrated both in simulation and using real robotic experiments that even a single-bit communication channel between agents suffices for the design of a substantial set of behaviors. Ultimately, the reader will be enabled to combine different behaviours based on the paradigm to develop a control scheme for individual swarms.

The glass sponge is a porous lightweight structure in the deep sea. It has high toughness, high strength, and high stability. In this work, a super-depth-of-field microscope was employed to observe the microstructure of the glass sponge. Based on its morphological characteristics, two novel bio-inspired lightweight structures were proposed, and the finite-element analyses (FEA) of the structures were carried out under compression, torsion, and bending loads, respectively. The structure samples were fabricated using stereolithography 3D-printing technology, and the dimension sizes of the samples were equal to those of the corresponding FEA models. Mechanical tests were performed on an electronic universal testing machine, and the results were used to demonstrate the reliability of the FEA. Additionally, lightweight numbers (LWN) were proposed to evaluate the lightweight efficiency, and a honeycomb structure was selected as the reference structure. The results indicate that the lightweight numbers of the novel bio-inspired structures are higher than those of the honeycomb structure, respectively. Finally, the proposed structures were optimized by the response surface, BP (Back Propagation) and GA-BP (Genetic Algorithm optimized Back Propagation) method. The results show that the GA-BP model after training has a high accuracy. These results can provide significant guidance for the design of tube-shaped, thin-walled structures in the engineering.

Dynamics of drop impact on soft surfaces has drawn a lot of attention for its applications and is motivated by natural examples like raindrop impact on a leaf. Previous studies have focused on categorizing the bending motion observed, using cantilever beam theory, but the complex dynamic response shown by a leaf involving other degrees of motions like torsion about the petiole, remains yet to be understood. In this study, we demonstrated that the complex response of a superhydrophobic Katsura leaf upon raindrop impact can be decomposed into simple single degree-of-freedom linear modes of bending and torsion, modeled as damped harmonic oscillators. Our theoretical estimates were in good agreement with experimental measurements of the frequency and maximum amplitude of bending and torsional modes. We also illustrated the energy transfer from the raindrop to these modes as a function of the impact location, which may shed light on the design of potential raindrop energy harvesting devices mimicking a leaf's structure. Finally, we concluded with a brief description of an unresolved mode (i.e. flapping) and the limitations of our approach.

Inspired by a scallop's strong underwater propulsion mechanism, we designed and prototyped a scallop robot capable of clapping and swimming. In this work, an artificial velum was used to work as a check valve to stimulate the robot's swimming. A couple of supporting plates were fixed on the robot shells to achieve the modulation of clapping process of the shells. The scallop robot can move at a maximum average and instantaneous speed of 3.4 and 4.65 body lengths per second, respectively. The effect of the supporting plates, the artificial velum, as well as the clapping frequency and amplitude on the swimming performance of the scallop robot was also experimentally evaluated. By tuning the sizes of the jet apertures, the scallop robot is capable of achieving high mobility actions such as turning. We also obtained the aperture ratio with the corresponding turning radius. This scallop robot provides a new propulsion mechanism in underwater bionic robots; it is also of help to understand the swimming principle of scallops in terms of jet propulsion and clapping motion.

Nowadays, the focus on the development of assistive devices just for people with mobility disorders has shifted towards enhancing physical abilities of able-bodied humans. As a result, the interest in the design of cheap and soft wearable exoskeletons (called exosuits) is distinctly growing. In this paper, a passive lower limb exosuit with two biarticular variable stiffness elements is introduced. These elements are in parallel to the hamstring muscles of the leg and controlled based on a new version of the FMCH (force modulated compliant hip) control framework in which the force feedback is replaced by the length feedback (called LMCH). The main insight to employ leg length feedback is to develop a passive exosuit. Fortunately, similar to FMCH, the LMCH method also predicts human-like balance control behaviours, such as the VPP (virtual pivot point) phenomenon, observed in human walking. Our simulation results, using a neuromuscular model of human walking, demonstrate that this method could reduce the metabolic cost of human walking by 10%. Furthermore, to validate the design and simulation results, a preliminary version of this exosuit comprised of springs with constant stiffness was built. An experiment with eight healthy subjects was performed. We made a comparison between the walking experiments while the exosuit is worn but the springs were slack and those when the appropriate springs were contributing. It shows that passive biarticular elasticity can result in a metabolic reduction of 14.7 4.27%. More importantly, compared to unassisted walking (when exosuit is not worn), such a passive device can reduce walking metabolic cost by 4.68 4.24%.

We have previously suggested a biologically-inspired natural dynamic controller for biped locomotion, which applies torque pulses to the different joints at particular phases of an internal phase variable. The parameters of the controller, including the timing and magnitude of the torque pulses and the dynamics of the phase variable, can be kept constant in open loop or adapted to the environment in closed loop.

Here we demonstrate the implementation of this approach to a mono-ped robot and the optimization of the controller parameters to enhance robustness via policy gradient. Policy gradient was applied in simulations rather than the actual robot due to safety and hardware considerations. A grounded action transformation (GAT) was learned and used to facilitate the transfer of the learned policy from simulation to hardware. We demonstrate how GAT improves the match between simulations and experiments and how learning enhances the performance and robustness of the mono-ped robot.

Pneumatic artificial muscles (PAMs) have a wide range of robotics applications, especially in soft robots, for their ability to generate linear force and displacement with the soft, lightweight, compact, and safe characteristics as well as high power densities. However, the compressibility of the air causes a spring-like behavior of PAMs, resulting in several common issues of limited stroke, load-dependent stroke lengths, difficulty in maintaining their length against disturbance, and necessity of accurate pressure control system. To address these issues, this study borrows inspiration from a biological soft linear actuator, a muscle, and proposes a ratchet-integrated pneumatic actuator (RIPA). Utilizing two pawls integrated at both ends of a McKibben muscle and a flexible rack inserted in the middle of the muscle, the RIPA achieves a large stroke length by accumulating displacements from multiple small strokes of the McKibben muscle by repeating the cycle of pressurization and depressurization. This cycle mimics the cross-bridge model of a sarcomere, a basic unit of a skeletal muscle, in which a muscle accumulates nanoscale strokes of myosin head motors to generate large strokes. The synergy between a PAM and the inspiration from a sarcomere enabled a large-stroke soft linear actuator that can generate independent strokes from loads. The proposed actuator is not only capable of maintaining its length against unexpected mechanical disturbances but also controllable with a relatively simple system. In this paper, we describe the design of the RIPA and provide analytical models to predict the stroke length and the period per cycle for actuation. We also present experimental results for characterization and comparison with model predictions.

Biological systems have adapted to environmental constraints and limited resource availability. In the present study, we evaluate the algorithm underlying leaf venation (LV) deployment using graph theory. We compare the traffic balance, travel and cost efficiency of simply-connected LV networks to those of the fan tree and of the spanning tree. We use a Pareto front to show that the total length of leaf venations (LVs) is close to optimal. Then we apply the LV algorithm to design transportation networks in the city of Atlanta. Results show that leaf-inspired models can perform similarly or better than computer-intensive optimization algorithms in terms of network cost and service performance, which could facilitate the design of engineering transportation networks.

Bipedal animals have diverse morphologies and advanced locomotion abilities. Terrestrial birds, in particular, display agile, efficient, and robust running motion, in which they exploit the interplay between the body segment masses and moment of inertias. On the other hand, most legged robots are not able to generate such versatile and energy-efficient motion and often disregard trunk movements as a means to enhance their locomotion capabilities. Recent research investigated how trunk motions affect the gait characteristics of humans, but there is a lack of analysis across different bipedal morphologies. To address this issue, we analyze avian running based on a spring-loaded inverted pendulum model with a pronograde (horizontal) trunk. We use a virtual point based control scheme and modify the alignment of the ground reaction forces to assess how our control strategy influences the trunk pitch oscillations and energetics of the locomotion. We derive three potential key strategies to leverage trunk pitch motions that minimize either the energy fluctuations of the center of mass or the work performed by the hip and leg. We suggest how these strategies could be used in legged robotics.

The amazing multi-modal locomotion of flying squid helps to achieve fast-speed migration and predator-escape behavior. Observation of flying squid has been rarely reported in recent years, since it is challenging to clearly record the flying squid's aquatic-aerial locomotion in a marine environment. The existing reports of squid-flying events are rare and merely record the in-air motion. Therefore, the water-air locomotor transition of flying squid is still unknown. This paper proposes the idea of using CFD to simulate the process of the flying squid (Sthenoteuthis oualaniensis (S. oualaniensis)) launching from water into air. The results for the first time reveal the flow field information of squid in launching phase and show the kinematic parameters of flying squid in quantification. Both a trailing jet and pinch-off vortex rings are formed to generate launching thrust, and the formation number L_ω/D_ω is 5.22, demonstrating that the jet strategy is to produce greater time-averaged thrust rather than higher propulsion efficiency. The results also indicate that the maximum flying speed negatively correlates with the launch angle, indicating that a lower launch angle could result in a larger flying speed for the flying squid to escape. These findings explore the multi-modal locomotion of flying squid from a new perspective, helping to explain the trade-off strategy of water-to-air transition, and further enhance the performance of aquatic-aerial vehicles.

This work provides comparative modeling approaches to determine the velocities and natural frequencies of plunge-diving bird and bioinspired drone systems when entering water. These systems are chosen to further explain the survival of diving birds as they impact water and to provide insight into the design geometry and material choice of bioinspired diving drones. A nonlinear reduced-order model is developed and utilized to analyze the dive at impact considering both Timoshenko and Euler–Bernoulli beam theories. Using Hamilton's principle, the equations of motion are first derived. Then, static and dynamic buckling analyses are conducted. For this study, a geometrically simplified cone-beam system is considered, where the cone represents the head and the beam represents both the neck and body of the plunge-diving systems. The first study is to analyze the effects different diving drone materials and cone dimensions play on the sensitivity of the system. The second study applies geometric parameters to the cone-beam system representative of a plunge-diving bird (Northern gannet) and a surface-diving bird (Double-crested cormorant). The results show that choosing a material with a higher Young's modulus and a cone with a smaller half angle increase the velocity at which buckling occurs. The buckling velocities of the predicted Northern gannet model appear to be much greater than the average recorded diving speeds, suggesting that the bird is capable of plunge-diving at more extreme conditions. The natural frequencies are found for the aforementioned plunge-diving systems to predict failure if any external frequencies are known to act on the system while on a mission, such as conditions dependent on the climate or environment. It is shown in all buckling studies that the Euler-Bernoulli beam theory consistently overestimates the responses when compared with the Timoshenko beam theory. In the dynamic responses, Euler–Bernoulli beam theory overestimates for the pre-buckling region, then underestimates at the start of the post-buckling region until a point where the two theories cross paths. The amount of error with Euler–Bernoulli beam theory depends heavily on the slenderness ratio of the beam due to the theory being a simplification of the Timoshenko beam theory. It is noted that as the development of a more realistic bird model improves, the thickness will become significant and the use of Euler–Bernoulli beam theory at the point of impact will no longer be a valid assumption.

Transpiration cooling is considered to be one of the most effective cooling methods for protecting components from ablation in extremely high temperature environments, so improving transpiration cooling efficiency is quite useful in practical applications. Living creatures always have the optimal properties for cooling after long-term evolution. This study proposes a novel transpiration cooling concept using a biomimetic non-smooth surface inspired by the earthworm's rough skin. The transpiration cooling efficiencies of porous plates with three different bio-inspired non-smooth surfaces - isosceles-trapezoid, right-angled-trapezoid and parallelogram grooves -are numerically investigated. The numerical model is validated by experimental data. The structure of the non-smooth surface dramatically affects the film thickness and surface heat convection intensity of transpiration cooling. The cooling efficiency is significantly improved by the parallelogram style non-smooth surface. The bio-inspired non-smooth surface successfully thickens the protective film and achieves a significantly better cooling performance. The protective film of transpiration cooling is thickened 22.7% while the transpiration cooling efficiency is significantly increased by 12% with the assistance of the bio-inspired non-smooth surface.