Segmentations in fins enable large morphing amplitudes combined with high flexural stiffness for fish-inspired robotic materials

Design and mechanical behavior of hyperbolic weaves with naturally curved ribbons

One important technique for morphological transition is weaving, structure weaves with in-plane curved ribbons have smoother structural configurations and less topological flaws. The design and production of a hyperbolic woven structure (negative Gaussian curvature) is not covered in the aforementioned paper, and we think that a hyperbolic woven structure with smooth geometric features has to have unique mechanical qualities. The new hyperbolic structure created by weaving in-plane curved ribbons was suggested in this study. The ribbon was manufactured using 3D print technology based on geometric design. Mechanical Behavior of hyperbolic weave structure with naturally in-plane curved ribbons under vertical compression was investigated through tests and finite elements. It was found that the failure modes of hyperbolic weave structure under vertical loading was the "ridge" buckling of the structure. A finite element model, including detailed modeling of ribbon joint connections, was established using bending modeling method. It was demonstrated that, the aspect ratios, ribbon width and thickness have a great effect on the buckling behavior of hyperbolic weaves. The high load capacity and strong stability of hyperbolic weaves with thicker or wider ribbons were exhibited by test and FEM. In addition, through theoretical analysis, we established the buckling theoretical model of hyperbolic woven structure, and the exact buckling load formula was proposed.

Active Fabrics With Controllable Stiffness for Robotic Assistive Interfaces

Assistive interfaces enable collaborative interactions between humans and robots. In contrast to traditional rigid devices, conformable fabrics with tunable mechanical properties have emerged as compelling alternatives. However, existing assistive fabrics actuated by fluidic or thermal stimuli struggle to adapt to complex body contours and are hindered by challenges such as large volumes after actuation and slow response rates. To overcome these limitations, inspiration is drawn from biological protective organisms combining hard and soft phases, and active assistive fabrics consisting of architectured rigid tiles interconnected with flexible actuated fibers are proposed. Through programmable tessellation of target body shapes into architectured tiles and controlling their interactions by the actuated fibers, the active fabrics can rapidly transition between soft compliant configurations and rigid states conformable to the body (>350 times stiffness change) while minimizing the device volume after actuation. The versatility of these active fabrics is demonstrated as exosuits for tremor suppression and lifting assistance, as body armors for impact mitigation, and integration with electrothermal actuators for smart actuation with convenient folding capabilities. This work offers a practical framework for designing customizable active fabrics with shape adaptivity and controllable stiffness, suitable for applications in wearable exosuits, haptic devices, and medical rehabilitation systems.

Scale-inspired programmable robotic structures with concurrent shape morphing and stiffness variation

Biological organisms often have remarkable multifunctionality through intricate structures, such as concurrent shape morphing and stiffness variation in the octopus. Soft robots, which are inspired by natural creatures, usually require the integration of separate modules to achieve these various functions. As a result, the whole structure is cumbersome, and the control system is complex, often involving multiple control loops to finish a required task. Here, inspired by the scales that cover creatures like pangolins and fish, we developed a robotic structure that can vary its stiffness and change shape simultaneously in a highly integrated, compact body. The scale-inspired layered structure (SAILS) was enabled by the inversely designed programmable surface patterns of the scales. After fabrication, SAILS was inherently soft and flexible. When sealed in an elastic envelope and subjected to negative confining pressure, it transitioned to its designated shape and concurrently became stiff. SAILS could be actuated at frequencies as high as 5 hertz and achieved an apparent bending modulus change of up to 53 times between its soft and stiff states. We further demonstrated both the versatility of SAILS by developing a soft robot that is amphibious and adaptive and tunable landing systems for drones with the capacity to accommodate different loads.

Stiff morphing composite beams inspired from fish fins

Morphing materials are typically either very compliant to achieve large shape changes or very stiff but with small shape changes that require large actuation forces. Interestingly, fish fins overcome these limitations: fish fins do not contain muscles, yet they can change the shape of their fins with high precision and speed while producing large hydrodynamic forces without collapsing. Here, we present a 'stiff' morphing beam inspired from the individual rays in natural fish fins. These synthetic rays are made of acrylic (PMMA) outer beams ('hemitrichs') connected with rubber ligaments which are 3-4 orders of magnitude more compliant. Combinations of experiments and models of these synthetic rays show strong nonlinear geometrical effects: the ligaments are 'mechanically invisible' at small deformations, but they delay buckling and improve the stability of the ray at large deformations. We use the models and experiments to explore designs with variable ligament densities, and we generate design guidelines for optimum morphing shape (captured using the first moment of curvature), that capture the trade-offs between morphing compliance (ease of morphing the structure) and flexural stiffness. The design guidelines proposed here can help the development of stiff morphing bioinspired structures for a variety of applications in aerospace, biomedicine or robotics.

Gradients of properties increase the morphing and stiffening performance of bioinspired synthetic fin rays

State-of-the-art morphing materials are either very compliant to achieve large shape changes (flexible metamaterials, compliant mechanisms, hydrogels), or very stiff but with infinitesimal changes in shape that require large actuation forces (metallic or composite panels with piezoelectric actuation). Morphing efficiency and structural stiffness are therefore mutually exclusive properties in current engineering morphing materials, which limits the range of their applicability. Interestingly, natural fish fins do not contain muscles, yet they can morph to large amplitudes with minimal muscular actuation forces from the base while producing large hydrodynamic forces without collapsing. This sophisticated mechanical response has already inspired several synthetic fin rays with various applications. However, most 'synthetic' fin rays have only considered uniform properties and structures along the rays while in natural fin rays, gradients of properties are prominent. In this study, we designed, modeled, fabricated and tested synthetic fin rays with bioinspired gradients of properties. The rays were composed of two hemitrichs made of a stiff polymer, joined by a much softer core region made of elastomeric ligaments. Using combinations of experiments and nonlinear mechanical models, we found that gradients in both the core region and hemitrichs can increase the morphing and stiffening response of individual rays. Introducing a positive gradient of ligament density in the core region (the density of ligament increases towards the tip of the ray) decreased the actuation force required for morphing and increased overall flexural stiffness. Introducing a gradient of property in the hemitrichs, by tapering them, produced morphing deformations that were distributed over long distances along the length of the ray. These new insights on the interplay between material architecture and properties in nonlinear regimes of deformation can improve the designs of morphing structures that combine high morphing efficiency and high stiffness from external forces, with potential applications in aerospace or robotics.

Intelligent Fish-Inspired Foraging of Swarm Robots with Sub-Group Behaviors Based on Neurodynamic Models

This paper proposes a novel intelligent approach to swarm robotics, drawing inspiration from the collective foraging behavior exhibited by fish schools. A bio-inspired neural network (BINN) and a self-organizing map (SOM) algorithm are used to enable the swarm to emulate fish-like behaviors such as collision-free navigation and dynamic sub-group formation. The swarm robots are designed to adaptively reconfigure their movements in response to environmental changes, mimicking the flexibility and robustness of fish foraging patterns. The simulation results show that the proposed approach demonstrates improved cooperation, efficiency, and adaptability in various scenarios. The proposed approach shows significant strides in the field of swarm robotics by successfully implementing fish-inspired foraging strategies. The integration of neurodynamic models with swarm intelligence not only enhances the autonomous capabilities of individual robots, but also improves the collective efficiency of the swarm robots.

Complex Modal Characteristic Analysis of a Tensegrity Robotic Fish's Body Waves

A bionic robotic fish based on compliant structure can excite the natural modes of vibration, thereby mimicking the body waves of real fish to generate thrust and realize undulate propulsion. The fish body wave is a result of the fish body's mechanical characteristics interacting with the surrounding fluid. Thoroughly analyzing the complex modal characteristics in such robotic fish contributes to a better understanding of the locomotion behavior, consequently enhancing the swimming performance. Therefore, the complex orthogonal decomposition (COD) method is used in this article. The traveling index is used to quantitatively describe the difference between the real and imaginary modes of the fish body wave. It is defined as the reciprocal of the condition number between the real and imaginary components. After introducing the BCF (body and/or caudal fin) the fish's body wave curves and the COD method, the structural design and parameter configuration of the tensegrity robotic fish are introduced. The complex modal characteristics of the tensegrity robotic fish and real fish are analyzed. The results show that their traveling indexes are close, with two similar complex mode shapes. Subsequently, the relationship between the traveling index and swimming performance is expressed using indicators reflecting linear correlation (correlation coefficient (Rc) and p value). Based on this correlation, a preliminary optimization strategy for the traveling index is proposed, with the potential to improve the swimming performance of the robotic fish.

Nonlinear Multimaterial Architecture for Greater Soft Material's Toughness and Delaying Damage Propagation

Designing soft robots that have greater toughness and better resistance to damage propagation while at the same time retaining their properties of compliance is fundamentally important for soft robotics applications. This study's main contribution is proposing a framework for nonlinear multimaterial architectural design of soft structures to increase their toughness and delay damage propagation. What are the limits when combining significantly different materials in one structure that will delay crack propagation while significantly maintaining postdamage toughness? Through this study, we observed that there is a very dynamic interplay when combining significantly different materials in one structure; this interplay could weaken or strengthen the multimaterial structure's toughness. In biological evolutionary terms, the Pangolin, Seashell, and Arapaima have found their answer for deflecting the crack and maintaining strength in their bodies. How does nature put these multimaterial structures together? Our research led us to find that the multimaterial toughness limits depend largely on the following parameters: components' relative morphology, architecture, spatial distribution, surface areas, and Young's Modulus. We found that a linear geometry, when it comes to morphology and/or architecture relative to surface area in multimaterial design, significantly reduces total toughness and fails to delay crack propagation. In contrast, incorporating geometric nonlinearities in both morphology and architecture significantly maintains higher total toughness even after damage, and significantly delays crack propagation. We believe that this study can open the door to further research and ultimately to promising and wide applications in soft robotics.

Mechanics and properties of fish fin rays in nonlinear regimes of large deformations

Fins from ray-finned fishes do not contain muscles, yet fish can change the shape of their fins with high precision and speed, while producing large hydrodynamics forces without collapsing. This remarkable performance has been intriguing researchers for decades, but experiments have so far focused on homogenized properties, and models were developed only for small deformations and small rotations. Here we present fully instrumented micromechanical tests on individual rays from Rainbow trout in both morphing and flexural deflection mode and at large deflections. We then present a nonlinear mechanical model of the ray that captures the key structural elements controlling the mechanical behavior of rays under large deformations, which we successfully fit onto the experiments for property identification. We found that the flexural stiffness of the mineralized layers in the rays (hemitrichs) is 5-6 times lower than their axial stiffness, an advantageous combination to produce stiff morphing. In addition, the collagenous core region can be modeled with spring elements which are 3-4 orders of magnitude more compliant than the hemitrichs. This fibrillar structure provides negligible resistance to shearing from the initial position, but it prevents buckling and collapse of the structure at large deformations. These insights from the experiments and nonlinear models can serve as new guidelines for the design of efficient bioinspired stiff morphing materials and structures at large deformations. STATEMENT OF SIGNIFICANCE: Fins from ray-finned fishes do not contain muscles, yet fish can change the shape of their fins with high precision and speed, while producing large hydrodynamics forces without collapsing. Experiments have so far focused on homogenized properties, and models were developed only for small deformations and small rotations providing limited insight into the rich nonlinear mechanics of natural rays. We present micromechanical tests in both morphing and flexural deflection mode on individual rays, a nonlinear model of the ray that captures the mechanical behavior of rays under large deformations and combine microCT measurements to generate new insights into the nonlinear mechanics of rays. These insights can serve as new guidelines for the design of efficient bioinspired stiff morphing materials and structures at large deformations.

Research on effects of different internal structures on the grasping performance of Fin Ray soft grippers

Fin Ray soft grippers, as a notable passive compliant structures, can be easily actuated by external devices to adapt their shape to conform to a grasped object. Their unique ability is aided by their V-shaped structure and morphable material utilized by the Fin Ray finger. Thus, when the internal structure changes, the adaptability and grasping abilities also change. However, related works focus on the effects of changing key parameters on the grasping performance based on the Festo structure, and few works have explored the effects of changing the internal structure. To close the research gap, four different Fin Ray structures are presented in this article, and a parameter determination process was carried out by maximizing their adaptability by investigating the key parameters of each structure through finite element analysis. Then, the force responses of four selected Fin Ray structures are analyzed and experimentally validated. The results show that the No Internal Filling structure obtained by omitting the crossbeams is ideal for grasping delicate targets with the best adaptability and the minimum resultant force. The cross structure attained by adding vertical beams connected to crossbeams decreases the adaptability of the Fin Ray finger but significantly increases the contact force. The unsymmetric design of the branched structure significantly enhances the final contact force while improving the passive adaptation to objects. Thus, the application of the Fin Ray finger ranges from adaptive delicate grasping tasks to high-force manipulation tasks.

A novel wave-like crawling robot has excellent swimming capabilities

Multiple animals ranging from micro-meter scale bacteria to meter scale vertebrates rely on undulatory motion to propel themselves on land and in the water. This type of locomotion also appears in amphibious animals such as sea snakes and salamanders. While undulatory motion can be used for both crawling and swimming, it requires the coordination of multiple joints so that only a few robots have the ability to mimic this motion. Here, we report a new minimalistic method for both crawling and swimming based on producing a wave motion in the sagittal (vertical) plane. A robotic prototype AmphiSAW was developed to demonstrate this methodology in a variety of scenarios. AmphiSAW (using its wave mechanism only) crawled at 1.5 B s^-1and swam at 0.74 B s^-1. The robot can be fitted with legs or wheels at the front, which can further increase its performance especially when crawling on uneven terrains. In addition to its high speeds, the robot has the lowest cost of transport among all amphibious robots reported in literature.

Manta Ray Inspired Soft Robot Fish with Tough Hydrogels as Structural Elements

The design of soft robots capable of navigation underwater has received tremendous research interest due to the robots' versatile applications in marine explorations. Inspired by marine animals such as jellyfish, scientists have developed various soft robotic fishes by using elastomers as the major material. However, elastomers have a hydrophobic network without embedded water, which is different from the gel-state body of the prototypes and results in high contrast to the surrounding environment and thus poor acoustic stealth. Here, we demonstrate a manta ray-inspired soft robot fish with tailored swimming motions by using tough and stiff hydrogels as the structural elements, as well as a dielectric elastomer as the actuating unit. The switching between actuated and relaxed states of this unit under wired power leads to the flapping of the pectoral fins and swimming of the gel fish. This robot fish has good stability and swims with a fast speed (∼10 cm/s) in freshwater and seawater over a wide temperature range (4-50 °C). The high water content (i.e., ∼70 wt %) of the robot fish affords good optical and acoustic stealth properties under water. The excellent mechanical properties of the gels also enable easy integration of other functional units/systems with the robot fish. As proof-of-concept examples, a temperature sensing system and a soft gripper are assembled, allowing the robot fish to monitor the local temperature, raise warning signals by lighting, and grab and transport an object on demand. Such a robot fish should find applications in environmental detection and execution tasks under water. This work should also be informative for the design of other soft actuators and robots with tough hydrogels as the building blocks.