McKibben artificial muscles (AMs) are known as a prominent class of pneumatic actuators in soft robotics and biomechanical engineering, due to their unique structure and multifunctionality. This paper presents a comprehensive review of recent advancements in McKibben AMs, focusing on their performance, structural variations, and operational principles. A systematic literature search on Scopus identified 146 relevant articles, which were analysed for both performance metrics and design characteristics. Inspired by natural muscle behaviour, McKibben AMs enable complex motions such as bending, linear extension, and twisting. These actuators can be organised as individual or bundled systems: individual units are typically arranged in linear or circular patterns, while bundled systems occur in serial, parallel, braided, convergent, or pennate configurations. Recent innovations in smart actuation methods, braided sleeves and internal bladders have expanded their capabilities, enabling embedded sensing, environmental adaptability and untethered operation. Additionally, alternative manufacturing methods offer promising solutions for developing McKibben muscles with enhanced functionality and tailored properties.

Biomimetics replicates biological functions and helps investigate mechanisms and movements. This study focuses on mimicking canine limb structures, particularly the flexible forelimb connection. Due to vestigial clavicles, dogs’ forelimbs attach to the trunk via muscles, creating a suspension system, unlike the skeletal hip joint connection of hindlimbs. Zoological studies highlight this structure's role in movement, but robotics has largely overlooked it. This research aims to clarify its function in locomotion by constructing a musculoskeletal canine‐limb robot using thin McKibben muscles, known for their lightweight, flexibility, and contraction ability even when bent. Experiments demonstrated that adjusting pneumatic pressure in the shoulder muscles influenced stroke length, confirming that the hammock‐like structure enhances movement. Increased pressure resulted in longer strokes, contributing to efficient locomotion. Walking experiments verified successful straight‐line progression using a walking gait, demonstrating the feasibility of this biomimetic approach. These findings suggest that incorporating flexible shoulder structures in quadrupedal robots could improve mobility and adaptability. Future research could further optimize muscle control and gait patterns to enhance robotic performance. By better understanding and implementing biological musculoskeletal features, this study contributes to both zoological biomechanics and robotic advancements, bridging the gap between biology and engineering for more efficient locomotion.

Shape adaptability to conform to surrounding objects in various settings is required for robotic arms performing earthwork in complex and rapidly changing environments such as disaster sites. We therefore focused on soft robotic arms, which possess this characteristic. However, soft robotic arms are mainly focused on flexible motion, and their load capacities are insufficient for practical use in earthwork. Therefore, this letter proposes an elephant-trunk-inspired power soft robotic arm, designed for high load capacity, inspired by the musculature of an elephant's trunk. The arm mainly comprises Pneumatic Power Bags (PPBs), which expand cylindrically with pneumatic pressure, and Hydraulic Artificial Muscles (HAMs). We hypothesized that high load capacity is achieved as PPBs support compressive stress and HAMs support tensile stress under load. We prototyped the arm and modeling and experimental studies demonstrate that the arm's stiffness can be tuned by varying pneumatic and hydraulic pressures. Additionally, the arm successfully lifted a 294 N weight and it was shown that the arm has capacity to passively deform along an object simply by applying pressure, without active control.

Fecal incontinence, arising from a myriad of pathogenic mechanisms, has attracted considerable global attention. Despite its significance, the replication of the defecatory system for studying fecal incontinence mechanisms remains limited largely due to social stigma and taboos. Inspired by the rectum's functionalities, we have developed a soft robotic system, encompassing a power supply, pressure sensing, data acquisition systems, a flushing mechanism, a stage, and a rectal module. The innovative soft rectal module includes actuators inspired by sphincter muscles, both soft and rigid covers, and soft rectum mold. The rectal mold, fabricated from materials that closely mimic human rectal tissue, is produced using the mold replication fabrication method. Both the soft and rigid components of the mold are realized through the application of 3D-printing technology. The sphincter muscles-inspired actuators featuring double-layer pouch structures are modeled and optimized based on multilayer perceptron methods aiming to obtain high contractions ratios (100 high generated pressure (9.8 kPa), and small recovery time (3 s). Upon assembly, this defecation robot is capable of smoothly expelling liquid faeces, performing controlled solid fecal cutting, and defecating extremely solid long faeces, thus closely replicating the human rectum and anal canal's functions. This defecation robot has the potential to assist humans in understanding the complex defecation system and contribute to the development of well-being devices related to defecation.

Various soft actuators have been developed in the past, achieving flexible motion. However, soft actuators that combine high force and high work have not yet been realized. The ratchet movement between actin and myosin, which causes muscle contraction in living organisms, could potentially lead to such high-efficiency soft actuators. In this paper, we describe a thread-based soft linear actuator inspired by this biological principle. The mechanism of this actuator utilizes the interaction between threads and objects to generate motion. Multiple units could be stacked to produce high force when used as a contracting actuator. To verify the principle of the proposed mechanism, we conducted transport experiments. The object placed on the threads was successfully transported at a speed of 2.2 mm/min.

The McKibben muscle, widely employed in antagonistic drive robots, exhibits approximately 20% contraction under pneumatic pressure but lacks crucial passive extensibility. Previous attempts to achieve passive extensibility resulted in reduced overall contraction ratios due to the series connection of elongation and contraction sections. This study proposes a novel artificial muscle design that achieves extension through external force by arranging extensible and contractile components in parallel. The proposed design incorporates a composite thread consisting of an elastic thread and an inextensible thread connected in series, arranged parallel to a conventional McKibben muscle. This configuration yields an artificial muscle capable of approximately 16% contraction and over 40% extension under external force when relaxed. The simplicity of this design facilitates straightforward integration into robotic systems. To demonstrate the efficacy of the proposed artificial muscle, we applied it to a tensegrity robot, exemplifying an antagonistic drive system. This application showcases the potential of our design to enhance the performance and versatility of soft robotic systems.

We are developing a vine-Like, power soft gripper based on Euler’s belt theory to achieve high load capacity for grasping irregularly shaped heavy objects at disaster sites. This gripper consists of a fire hose with rubber sheets adhered to both sides and a spiral constant-force spring inserted inside. Initially coiled in a helical shape, it extends while increasing its radius of curvature when air pressure is applied. In this state, it approaches the target object and wraps around it when depressurized. Therefore, the inner diameter of the gripper in its initial state determines the minimum diameter of the object that can be grasped. Additionally, at present, the radius of curvature is small when pressurized, limiting its range of motion and restricting the objects it can grasp and its use in confined spaces. Hence, in this study, we have fabricated grippers with varying design parameters and experimentally verified the inner diameter in the initial state and the radius of curvature when pressurized. We fabricated grippers with varying stiffness of rubber sheets and constant-force springs, which are components of the gripper, and experimentally verified their shape. The results show that the radius of curvature increases with increasing stiffness of the inner rubber sheet and decreasing stiffness of the constant-force spring. Additionally, it has been confirmed that the effect of the outer rubber sheet stiffness is sufficiently small.

The use of thin artificial muscles and soft materials will facilitate the advancement of robotics, enabling the development of robots that mimic the intricate structural characteristics observed in animal bodies. It is anticipated that this will result in superior locomotion performance, including flexible movements that emulate those of a living creature, adaptability to the environment, shock absorption, and intelligent movements based on the physical characteristics of the body. The crucial technology is a flexible material that is capable of active movement. This presentation will introduce our latest research on active flexible materials, as well as the field of deep biomimetic robotics and its medical applications.

The myocardial contracting ratio is approximately 20%, whereas ejection fraction exceeds 60%. Understanding the structure and kinetic mechanisms of the heart that enable this high ejection fraction is crucial in both basic and clinical medicine. However, these mechanisms remain incompletely elucidated. The authors have developed a functional model based on the unique myocardial band theory, which posits that the ventricle is formed by a single myocardial band winding into a spiral. According to this theory, a muscle band, which incorporated thin McKibben artificial muscles embedded within a soft elastomer, was formed, and it was subsequently rolled to replicate the ventricle's structure. Thin McKibben muscles are well-suited for mimicking cardiac muscles due to their longitudinal contraction, radial expansion, and ability to operate in a curved position. In general, animal hearts exhibit approximately 20% myocardial contracting ratio, a 1.2-fold change in myocardial band thickness, and an ejection fraction in the range 50-70%. In comparison, soft robotic hearts demonstrated values of 17.3%, a 1.28-fold thickness change, and a 47.8% ejection fraction, respectively, which closely approximated those of real hearts. Water ejection experiments conducted using a soft robotic heart revealed that the maximum pressure during contraction reached 200 mmHg, generating a pressure-volume loop similar to that observed in the human heart. Thus, soft robotic hearts hold the potential for a wide range of clinical applications, including the elucidation of heart failure pathophysiology and the development of surgical treatments.

McKibben artificial muscles are capable of contracting when pneumatic pressure is applied. However, they face difficulties in passively elongating from their natural length due to external forces when not pressurized. This limitation poses challenges in systems where artificial muscles interact, such as in antagonistic drive configurations. To address this issue, we have so far developed a novel type of McKibben artificial muscle called the back-stretchable McKibben muscle (BSM). The BSM consists of two primary sections: a contraction section and an elongation section. An inlet tube is inserted between these two sections to restrict airflow. This design enables the elongation section to activate prior to the contraction motion of the contraction section. While this sequential operation allows the BSM to be used in antagonistic drive systems, a new challenge emerged: the restricted airflow resulted in slower response times of the BSM. To address this issue, this paper proposes a mechanical valve called the "Time-lag generation mechanical valve (TLV)," which generates a time lag in air inflow to the sections without using an inlet tube. Experimental results demonstrate that incorporating the TLV into the BSM significantly enhances its time response: by approximately 300 times during contraction and approximately 230 times during pressure release. Furthermore, the integration of TLV-equipped BSMs enabled the successful implementation of object throwing in an antagonistic drive robotic arm, a feat previously unattainable with conventional BSMs.

Flexible/stretchable sensors comprising soft structures that do not interfere with the softness of the bodies of soft robots are essential for achieving soft robots with superior operational performance. These sensors are expected to be applicable as sensing skin for humanoid robots and as interfaces for human-robot interactions. Herein, we refer to flexible/stretchable sensors investigated for soft robotics applications as “soft sensors” and review recent research trends. Specifically, we discuss optical, resistive, capacitive, and inductive soft sensors with emphasis on their materials and structures.

Silicone rubber is widely used in various soft robots, sensors, and actuators. One of the most popular fabrication processes for silicone rubber is molding, which can easily duplicate three-dimensional (3D) shapes using a mold. Among the molding techniques, vacuum molding is a useful approach for preventing defects in molded parts. However, during the vacuum molding process, handling objects in a vacuum is challenging, requiring the mold to be carefully designed to prevent air bubble entrapment without manipulating the silicone rubber pouring point or the mold. In this study, to address these challenges, we propose an in-vacuum silicone rubber forming process. This process allows the silicone rubber pouring point to be controlled in a vacuum using a five-bar parallel link manipulator. This manipulator can be easily installed with only two rotational rods, typically employed as mixing rods within the chamber, facilitating efficient molding processes such as multi-part fabrication with minimal silicone rubber waste without the need for special mold designs. We validate the effectiveness of this process by molding cylindrical structures with diameters ranging from 1.2 to 5 mm and three types of liquid silicone rubbers with viscosities from 3 to 20 Pa·s . Compared to conventional methods—standard molding and vacuum molding, our proposed approach could successfully mold all cylinders without voids or air bubbles. Additionally, the applicability of this method to soft robotics was demonstrated through two applications using major soft actuators, pneumatic actuator and dielectric elastomer actuator (Non-standard abbreviation: DEA—Dielectric Elastomer Actuator), for soft robotics.

Animal muscles have complex, three-dimensional structures with fibers oriented in various directions. The tongue, in particular, features a highly intricate muscular system composed of four intrinsic muscles and several types of extrinsic muscles, enabling flexible and diverse movements essential for feeding, swallowing, and speech production. Replicating these structures could lead to the development of multifunctional manipulators and advanced platforms for studying muscle-motion relationships. In this study, we developed a pig tongue soft robot that focuses on replicating the intrinsic muscles using thin McKibben artificial muscles, silicone rubber, and gel. We began by performing three-dimensional scans and sectional observations in the coronal and sagittal planes to examine the arrangement and orientation of the intrinsic muscles in the actual pig tongue. Additionally, we used the diffusible iodine-based contrast-enhanced computed tomography (Dice-CT) technique to observe the three-dimensional flow of muscle pathways. Based on these observations, we constructed a three-dimensional model and molded the pig tongue shape with silicone rubber and gel, embedding artificial muscles into the robot body. We conducted experiments to assess both the motion of the tongue robot’s tip and its stiffness during muscle contractions. The results confirmed characteristic tongue motions, such as tip extension, flexion, and lateral bending, as well as stiffness changes during actuation, suggesting the potential for this soft robot to serve as a platform for academic and engineering studies.

The McKibben muscle can produce a high force-to-mass ratio, beneficial for various applications in the soft mechatronics field. The thin McKibben muscle, which has a small diameter, has the advantage of a high force-to-mass ratio and sufficient flexibility for use in a bent state. This flexibility permits the realization of flexible mechatronics. However, the thin McKibben muscle is easily broken in a bent state while it is very durable in a straight state. Over repetitive operations, the fibers within the sleeve gradually shift, causing the rubber tube inside to protrude and ultimately leading to cracking. This study investigates improvements in the durability of artificial muscles using adhesives to prevent this fiber-to-fiber misalignment. The durability test showed that the adhesive could provide a durability of up to 10,000-times greater than that of a normal thin artificial muscle in the maximum case. Using the thin McKibben muscle with the proposed method, tensegrity modules were fabricated. The durability test revealed a 500-fold increase under an applied pressure of 0.5 MPa. Furthermore, the durability of the adhesive-applied artificial muscles was also confirmed to be enhanced during the dynamic movements of a soft tensegrity robot that throws a ball with 0.7 MPa.

Tensegrity robotic joints, inspired by the musculoskeletal systems of vertebrate animals, have gained interest due to their unique interplay of tension and compression forces, offering a high strength‐to‐weight ratio and inherent adaptability. However, their use in legged and grasping applications remains challenging. A key challenge is striking the right balance between high compliance, which can undermine stability and control, and high stiffness, which can restrict essential movement and demand more powerful actuators. This study presents a tensegrity‐inspired spine‐like joint that integrates thin McKibben pneumatic artificial muscles and rubber cords directly into its tensile network. The artificial muscles enable active joint bending while also serving as tensile elements. The rubber cords counteract these forces, storing elastic energy to facilitate smooth recovery to the original position. The joint's topology design is primarily influenced by the artificial muscles’ limited 20% contraction. To optimize the balance between range of motion, payload capacity, and structural stability, the joint's geometry is refined through an evolutionary algorithmic form‐finding process. Two generated joints are integrated into two functional robotic applications: a crawling robot and a gripper, showcasing their adaptability for diverse robotic applications.

In this study, we have fabricated and evaluated a flow control valve for oil-hydraulic soft actuators. To install oil-hydraulic actuators on a robot to realize actuation with high-power, there is a demand for compact and lightweight control valve. Based on particle excitation type valve driven by piezoelectric vibrators, we have designed a fluidic control valve for hydraulic actuators. In this valve, the particle excitation type valve is used as a pilot valve to control the spool of main valve. The fabricated valve has been evaluated for a control of the supplied fluid at a pressure of 5 MPa. As a result, we have succeeded in controlling a hydraulic artificial muscle.

A tensegrity-based system is a promising approach for dynamic exploration of uneven and unpredictable environments, particularly, space exploration. However, implementing such systems presents challenges in terms of intelligent aspects: state recognition, wireless monitoring, human interaction, and smart analyzing and advising function. Here, we introduce a 6-strut tensegrity integrate with 24 multimodal strain sensors by leveraging both deep learning model and large language models to realize smart tensegrity. Using conductive flexible tendons assisted by long short-term memory model, the tensegrity achieves the self-shape reconstruction without extern sensors. Through integrating the flask server and gpt-3.5-turbo model, the tensegrity autonomously enables to send data to iPhone for wireless monitoring and provides data analysis, explanation, prediction, and suggestions to human for decision making. Finally, human interaction system of the tensegrity helps human obtain necessary information of tensegrity from the aspect of human language. Overall, this intelligent tensegrity-based system with self-sensing tendons showcases potential for future exploration, making it a versatile tool for real-world applications.