Table of contents

The field encompassing biomimetics, bioinspiration and nature inspiration in engineering science is growing steadily, pushed by exogenous factors like the search for potentially sustainable engineering solutions that might already exist in nature. With the help of information provided by a bibliometric database and further processed with a dynamic network and semantic analysis tool, we provide insight at two scales into the corpus of nature-inspired engineering field and its dynamics. At the macroscale, the Web of Science^® (WoS) categories, countries and institutions are ranked and ordered by thematic clusters and country networks, highlighting the leading countries and institutions and how they focus on specific topics. Such an insight provides an overview at the macroscale that can be valuable to orient scientific strategy at the country level. At the mesoscale, where science is incarnated by collaborative networks of authors and institutions that run across countries, we identify six semantic clusters and subclusters within them, and their dynamics. We also pinpoint leading academic collaborative networks and their activity in relation to the six semantic clusters. Trends and prospective are also discussed. Typically, one observes that the field is becoming mature since, starting by imitating nature, it proceeded with mimicking more complex natural structures and functions and now it investigates ways used in nature in response to changes in the environment and implements them in innovative and adaptive artefacts. The sophistication of devices, methods and tools has been increasing over the years as well as their functionalities and adaptability, whereas the size of devices has decreased at the same time.

Of all mammalian vibrissae, those of certain species of pinnipeds are exceptional. Researchers believe that their curious undulating form evolved for hydrodynamic detection. Our understanding of how these whiskers work depends on a geometrical model that captures the crucial pertinent features of the natural vibrissae including its tapering and curvature. It should also account for the form of the whisker when it flexes under external loading. We introduce and study a normal skeleton of a two-dimensional projection of a harbour seal whisker. The normal skeleton is a complete shape descriptor that involves reduction to the centreline equipped with a thickness function of the orthogonal cross-section. The contours of the whisker shape are extracted from a 2D greyscale scan. Our analysis reveals correspondence between the undulations of the width and oscillations of the centreline curvature as functions of arc length. We discuss two possible explanations for that remarkable feature: one based on consideration of growth and the other of plastic deformation. For the latter we employ a mechanical model to demonstrate appearance of curvature oscillations caused by extensive deflection of the undulating whisker due to external loading.

Remora suckerfish (Echeneis naucrates) can perform skimming and sliding motions on the surfaces of moving hosts to optimize adhesion positions. We found that remora achieve skimming and sliding motions through coordinated movement of the suction disc's lamellae and lip locomotion through live animal observations. We implemented an integrated biomimetic remora suction disc based on morphological and kinematic data of biological remoras. With soft actuators enabling 'compression–rotation' and 'compression–extension', the biomimetic disc controls the disc lip and lamellar movement under driving with only one degree of freedom, and can switch freely between three states: zero, low-friction, and robust adhesion. Then we investigate the effects of the biomimetic suction-disc soft-lip material, preload, and lamellar movement on the tangential friction force (both forward and backward) under different adhesion states. This biomimetic suction disc with a low-modulus soft lip can adhere to a smooth surface under 0.1 N preload and achieve normal adhesion-force and tangential frictional-force control ranges spanning ∼10⁻¹ to ∼10² N and ∼10⁻¹ to ∼10¹ N, respectively. The results reveal how remora disc achieved fast, tunable adhesion for skimming and sliding on surfaces. Furthermore, we demonstrate a bio-inspired robot capable of attachment, detachment, skimming, and sliding motions with the aiding of simple biomimetic pectoral-fin flapping. This study lays a foundation for future integrated applications of underwater adhesion robots and related biomechanical exploration.

Biological soft interfaces often exhibit complex microscale interlocking geometries to ensure sturdy and flexible connections. If needed, the interlocking can rapidly be released on demand leading to an abrupt decrease of interfacial adhesion. Here, inspired by lizard tail autotomy where such apparently tunable interfacial fracture behavior can be observed, we hypothesized an interlocking mechanism between the tail and body based on the muscle-actuated mushroom-shaped microinterlocks along the fracture planes. To mimic the fracture behavior of the lizard tail, we developed a soft bilayer patch that consisted of a dense array of soft hemispherical microstructures in the upper layer acting as mechanical interlocks with the counter body part. The bottom control layer contained a microchannel that allowed to deflect the upper layer when applying the negative pressure, thus mimicking muscle contraction. In the microinterlocked condition, the biomimetic tail demonstrated a 2.7-fold and a three-fold increase in adhesion strength and toughness, respectively, compared to the pneumatically released microinterlocks. Furthermore, as per the computational analysis, the subsurface microchannel in the control layer enabled augmented adhesion by rendering the interface more compliant as a dissipative matrix, decreasing contact opening and strain energy dissipation by 50%. The contrasting features between the microinterlocked and released cases demonstrated a highly tunable adhesion of our biomimetic soft patch. The potential applications of our study are expected in soft robotics and prosthetics.

In this paper, we present an open-source software library that can be used to numerically simulate the advection and diffusion of a chemical concentration or heat density in a viscous fluid where a moving, elastic boundary drives the fluid and acts as a source or sink. The fully-coupled fluid-structure interaction problem of an elastic boundary in a viscous fluid is solved using Peskin's immersed boundary method. The addition or removal of the concentration or heat density from the boundary is solved using an immersed boundary-like approach in which the concentration is spread from the immersed boundary to the fluid using a regularized delta function. The concentration or density over time is then described by the advection-diffusion equation and numerically solved. This functionality has been added to our software library, IB2d, which provides an easy-to-use immersed boundary method in two dimensions with full implementations in MATLAB and Python. We provide four examples that illustrate the usefulness of the method. A simple rubber band that resists stretching and absorbs and releases a chemical concentration is simulated as a first example. Complete convergence results are presented for this benchmark case. Three more biological examples are presented: (1) an oscillating row of cylinders, representative of an idealized appendage used for filter-feeding or sniffing, (2) an oscillating plate in a background flow is considered to study the case of heat dissipation in a vibrating leaf, and (3) a simplified model of a pulsing soft coral where carbon dioxide is taken up and oxygen is released as a byproduct from the moving tentacles. This method is applicable to a broad range of problems in the life sciences, including chemical sensing by antennae, heat dissipation in plants and other structures, the advection-diffusion of morphogens during development, filter-feeding by marine organisms, and the release of waste products from organisms in flows.

Current soft robotic arms commonly follow traditional—i.e. hard—robot design conventions. Omnidirectional soft arms most commonly consist of segments that contain three parallel longitudinal actuators, or actuator groups, of which one or two are activated to induce bending. This design uses the minimum number of actuators required for omnidirectional bending, which is consistent with traditional robot design but unnecessarily constrains the soft arm design space. The cephalopods that frequently inspire soft robot arms have tens of bundles of longitudinal muscle fibers, and these bundles are distributed around the limb circumference rather than grouped. This article analyzes fluid-driven soft arm architectures with up to 12 actuators, which mimics the redundant and distributed nature of cephalopod arms and tentacles, in order to investigate the performance implications of an arm morphology closer to nature. Overactuation (i.e. activating more than two actuators) was considered, which is possible because soft robots—unlike traditional robots—can tolerate misaligned or partially opposed actuation without jamming. A previously-developed generalizable model was used to simulate design performance, and a subset of the examined architectures were constructed and tested. Many-actuator soft arms achieve high strokes under equivalent load and equivalent actuation pressures, without sacrificing no-load reach. These arms are also able to achieve quasi-omnidirectionality, as well as to execute near constant-curvature turns using a simple actuation pattern and a single pressure signal. The introduced framework also enables non-circular cross sections that are not possible with minimalist arm designs, allowing the load capacity and reach to be tuned in multiple directions. Several elliptical cross section arms were analyzed to explore the potential of non-circular cross sections. Overactuated, many-actuator arms significantly expand the design space of soft arms, improving performance in some cases and introducing options to tailor behavior that are infeasible in structurally minimal arms.

Rigid suckers commonly used in surgical procedures often cause absorption damage, while their soft counterparts are difficult to handle due to their weak anchoring. Alternatively, the octopus sucker is both soft and has strong suction power. Further observation revealed that its structure is self-sealing and that the tissues are layered in hardness. Inspired by said structure and the characteristics of associated materials, a bionic soft sucker with stiffness gradient and acetabular roof structure was proposed, made of silicone with varying hardness including structures such as acetabular roof and circle muscles. The automatic tensile force measurement system was used to experimentally analyze the adsorption performance of the suckers to the soft curved contact surface. Both dry and wet conditions were tested, along with practical tests on organisms. The bionic sucker adsorption force was increased by 25.1% and 34.6% on the cylindrical surface, and 45.2% and 7.3% on the spherical surface for dry and wet conditions, respectively. During the experiment, the bionic suckers did not cause notable suction damage to the contact surfaces. Thus, this type of bionic sucker shows good application prospects in the field of surgery.

Biologically inspired strategies have long been adapted to swarm robotic systems, including biased random walks, reaction to chemotactic cues and long-range coordination. In this paper we apply analysis tools developed for modeling biological systems, such as continuum descriptions, to the efficient quantitative characterization of robot swarms. As an illustration, both Brownian and Lévy strategies with a characteristic long-range movement are discussed. As a result we obtain computationally fast methods for the optimization of robot movement laws to achieve a prescribed collective behavior. We show how to compute performance metrics like coverage and hitting times, and illustrate the accuracy and efficiency of our approach for area coverage and search problems. Comparisons between the continuum model and robotic simulations confirm the quantitative agreement and speed up by a factor of over 100 of our approach. Results confirm and quantify the advantage of Lévy strategies over Brownian motion for search and area coverage problems in swarm robotics.

Textile technology is a traditional approach to additive manufacturing based on one-dimensional yarn. Printing solid rods onto pre-stretched textiles creates internal stresses upon relaxation of the pre-stretch, which leads to buckling-induced out-of-plane deformation of the textile. Similar behaviours are well known to occur also in biological systems where differential growth leads to internal stresses that are responsible for the folding or wrinkling of leaves, for example. Our goal was to get a quantitative understanding of this wrinkling by a systematic experimental and numerical investigation of parallel rods printed onto a pre-stretched textile. We vary rod thickness and spacing to obtain wavelength and phase coherence of the wrinkles as a function of these parameters. We also derive a simple analytical description to rationalize these observations. The result is a simple analytical estimate for the phase diagram of behaviours that may be used for design purposes or to describe wrinkling phenomena in biological or bioinspired systems.

Today's gecko-inspired robots have shown the ability of omnidirectional climbing on slopes with a low centre of mass. However, such an ability cannot efficiently cope with bumpy terrains or terrains with obstacles. In this study, we developed a gecko-inspired robot (Nyxbot) with an adaptable body height to overcome this limitation. Based on an analysis of the skeletal system and kinematics of real geckos, the adhesive mechanism and leg structure design of the robot were designed to endow it with adhesion and adjustable body height capabilities. Neural control with exteroceptive sensory feedback is utilised to realise body height adaptability while climbing on a slope. The locomotion performance and body adaptability of the robot were tested by conducting slope climbing and obstacle crossing experiments. The gecko robot can climb a 30° slope with spontaneous obstacle crossing (maximum obstacle height of 38% of the body height) and can climb even steeper slopes (up to 60°) without an obstacle or bump. Using 3D force measuring platforms for ground reaction force analysis of geckos and the robot, we show that the motions of the developed robot driven by neural control and the motions of geckos are dynamically comparable. To this end, this study provides a basis for developing climbing robots with adaptive bump/obstacle crossing on slopes towards more agile and versatile gecko-like locomotion.

Terrestrial locomotion requires generating appropriate ground reaction forces which depend on substrate geometry and physical properties. The richness of positions and orientations of terrain features in the 3D world gives limbless animals like snakes that can bend their body versatility to generate forces from different contact areas for propulsion. Despite many previous studies of how snakes use lateral body bending for propulsion on relatively flat surfaces with lateral contact points, little is known about whether and how much snakes use vertical body bending in combination with lateral bending in 3D terrain. This lack had contributed to snake robots being inferior to animals in stability, efficiency, and versatility when traversing complex 3D environments. Here, to begin to elucidate this, we studied how the generalist corn snake traversed an uneven arena of blocks of random height variation five times its body height. The animal traversed the uneven terrain with perfect stability by propagating 3D bending down its body with little transverse motion (11° slip angle). Although the animal preferred moving through valleys with higher neighboring blocks, it did not prefer lateral bending. Among body-terrain contact regions that potentially provide propulsion, 52% were formed by vertical body bending and 48% by lateral bending. The combination of vertical and lateral bending may dramatically expand the sources of propulsive forces available to limbless locomotors by utilizing various asperities available in 3D terrain. Direct measurements of contact forces are necessary to further understand how snakes coordinate 3D bending along the entire body via sensory feedback to propel through 3D terrain. These studies will open a path to new propulsive mechanisms for snake robots, potentially increasing the performance and versatility in 3D terrain.

Inspired by the albatross, this paper presents the construction of a dynamic soaring simulation system with distributed pressure sensors. The advantage of our system lies in harvesting energy from the wind shear layer and estimating the wind information using a pressure-based sensor system. Specifically, the dynamic soaring simulation system contains an offline training stage and an online estimation and control stage. In the offline training stage, computational fluid dynamics simulations are conducted and used as the data source. A surrogate model is established to correlate the local flow conditions and the surface pressure at optimal sensor positions. In the online estimation and control stage, through sensing the pressure information, the real-time wind velocity and wind gradient are estimated by the surrogate model trained in the offline stage. Moreover, wind information is adopted in the simulation of dynamic soaring control. In this study, the simulation system was applied to linear and circular path-following tasks. It was found that the dynamic soaring simulation system with distributed pressure sensors provides an acceptable estimation of wind velocity and wind gradient with a certain time delay caused by numerical differentiation.

The paper aims to examine the effects of mechanical losses on the performance of a bioinspired flapping-wing micro aerial vehicle (FWMAV) and ways to mitigate them by introducing a novel electromechanical model. The mathematical model captures the effect of a DC gear motor, slider-crank, flapping-wings aerodynamics, and frictional losses. The aerodynamic loads are obtained using a quasi-steady flow model. The parameters of the flight mechanism are estimated using published experimental data which are also used to validate the mathematical model. Incorporating the flapping mechanism friction losses into the mathematical model enables capturing the physics of the problem with higher accuracy, which is not possible with simpler models. It also makes it possible to estimate the aerodynamic energetic requirements. Moreover, the model enabled evaluations of the effects of adding bioinspired elastic elements on the efficiency of the system. Although it is established through experimental studies that the addition of a bioinspired elastic element can improve system efficiency and increase lift generation, the existing mathematical models fail to model and predict such effects. It has been demonstrated that the addition of an elastic element can reduce friction losses in the system by decreasing the internal forces. Optimised parameters for a FWMAV incorporating elastic elements are also obtained.

By adopting bioinspired morphing fins, we demonstrate how to achieve good directional stability, exceptional maneuverability, and minimal adverse response to turbulent flow, properties that are highly desirable for rigid hull AUVs, but are presently difficult to achieve because they impose contradictory requirements. We outline the theory and design for switching between operating with sufficient stability that ensures a steady course in the presence of disturbances, with low corrective control action; reverting to high maneuverability to execute very rapid course and depth changes, improving turning rate by 25% up to 50%; and ensuring at all times that angular responses to external turbulence are minimized. We then demonstrate the developments through tests on a 1 m long autonomous underwater vehicle, named Morpheus. The vehicle is capable of dynamically changing its stability-maneuverability qualities by using tuna-inspired morphing fins, which can be deployed, deflected and retracted, as needed. A series of free-swimming experiments and maneuvering simulations, combined with mathematical analysis, led to the design of optimal retractable morphing fins.

The adhesive performance of biomimetic controllable adhesive based on wedge-shaped microstructures is affected by some relevant control parameters in the process of loading and unloading. An appropriate selection of these control parameters is of great significance for its effective application. However, little research has concentratively and comprehensively explored these control parameters. In order to make up for the shortcoming, this study systematically explored the macroscopic adhesive performance of the self-developed wedge-shaped microstructures under different loading and unloading control parameters. The results show that preloading depth and tangential dragging distance have a positive effect on the adhesive performance, while preloading angle and peeling angle have a negative effect on the adhesive performance. Specifically, a low preloading angle can weaken the normal preloading force under the same preloading depth, thereby improving the preloading benefit; the application of tangential dragging distance can also induce the normal preloading force generated in the preloading stage to change the adhesion, so as to stimulate more adhesion. Based on the interactive analysis of these control parameters, it can be sure that applying a moderate normal preloading force and a larger tangential dragging distance to the wedge-shaped microstructures at low preloading angle not only makes the wedge-shaped microstructures possess better adhesive capacity, but also can obtain a good preloading benefit. In addition, the promotion effect of a low peeling angle on the adhesive performance also implies that a higher peeling angle should be used to realize the easy detachment of the adhesive interface. The first concentrative and comprehensive investigation of the relevant control parameters of wedge-shaped microstructures lays the foundation for designing a climbing robot or adhesive gripper based on the wedge-shaped microstructures, and also provides guidance for formulating the corresponding control strategies.