Applications In Soft Robotics Research Articles

A minimally designed soft crawling robot for robust locomotion in unstructured pipes

Soft robots have attracted increasing attention due to their excellent versatility and broad applications. In this article, we present a minimally designed soft crawling robot (SCR) capable of robust locomotion in unstructured pipes with various geometric/material properties and surface topology. In particular, the SCR can squeeze through narrow pipes smaller than its cross section and propel robustly in spiked pipes. The gait pattern and locomotion mechanism of this robot are experimentally investigated and analysed by the finite element analysis, revealing that the resultant forward frictional force is generated due to the asymmetric mechanical properties along the length direction of the robot. The proposed simple yet working SCR could inspire novel designs and applications of soft robots in unstructured narrow canals such as large intestines or industrial pipelines.

Electroactive Polymer-Based Composites for Artificial Muscle-like Actuators: A Review.

Unlike traditional actuators, such as piezoelectric ceramic or metallic actuators, polymer actuators are currently attracting more interest in biomedicine due to their unique properties, such as light weight, easy processing, biodegradability, fast response, large active strains, and good mechanical properties. They can be actuated under external stimuli, such as chemical (pH changes), electric, humidity, light, temperature, and magnetic field. Electroactive polymers (EAPs), called ‘artificial muscles’, can be activated by an electric stimulus, and fixed into a temporary shape. Restoring their permanent shape after the release of an electrical field, electroactive polymer is considered the most attractive actuator type because of its high suitability for prosthetics and soft robotics applications. However, robust control, modeling non-linear behavior, and scalable fabrication are considered the most critical challenges for applying the soft robotic systems in real conditions. Researchers from around the world investigate the scientific and engineering foundations of polymer actuators, especially the principles of their work, for the purpose of a better control of their capability and durability. The activation method of actuators and the realization of required mechanical properties are the main restrictions on using actuators in real applications. The latest highlights, operating principles, perspectives, and challenges of electroactive materials (EAPs) such as dielectric EAPs, ferroelectric polymers, electrostrictive graft elastomers, liquid crystal elastomers, ionic gels, and ionic polymer–metal composites are reviewed in this article.

Dynamic response of Maxwell fluid in an elastic cylindrical tube

In the present study, the dynamic response of Maxwell fluid in an elastic cylindrical tube is considered. Focusing on the viscoelastic flow through a thin-walled slender elastic cylindrical shell and neglecting inertia in the liquid and solid, a non-homogeneous linear diffusion equation controlling the coupled viscous–elastic system is obtained. The fluid pressure and deformation fields are obtained by numerical Laplace inversion. The results show that the relaxation time of Maxwell fluid has a significant effect on the flow and deformation fields of the viscoelastic system. This research can be used for the design and control of complex time-varying deformation fields and has a certain value for the applications of soft actuators, micro-autonomous systems, and soft robotics.

Simulation and optimization of magnetoelastic thin shells

Magnetoelastic thin shells exhibit great potential in realizing versatile functionalities through a broad range of combination of material stiffness, remnant magnetization intensity, and external magnetic stimuli. In this paper, we propose a novel computational method for forward simulation and inverse design of magnetoelastic thin shells. Our system consists of two key components of forward simulation and backward optimization. On the simulation side, we have developed a new continuum mechanics model based on the Kirchhoff-Love thin-shell model to characterize the behaviors of a megnetolelastic thin shell under external magnetic stimuli. Based on this model, we proposed an implicit numerical simulator facilitated by the magnetic energy Hessian to treat the elastic and magnetic stresses within a unified framework, which is versatile to incorporation with other thin shell models. On the optimization side, we have devised a new differentiable simulation framework equipped with an efficient adjoint formula to accommodate various PDE-constraint, inverse design problems of magnetoelastic thin-shell structures, in both static and dynamic settings. It also encompasses applications of magnetoelastic soft robots, functional Origami, artworks, and meta-material designs. We demonstrate the efficacy of our framework by designing and simulating a broad array of magnetoelastic thin-shell objects that manifest complicated interactions between magnetic fields, materials, and control policies.

A Ball Joint With Continuously Adjustable Load Capacity Based on Positive Pressure Method

Research on the variable load capacity method is crucial in promoting the application of soft robots. However, the effects on the performance of current structures are still limited. This letter proposes a ball joint with continuously adjustable load capacity based on the principle of friction with positive pressure. The proposed ball joint can rotate freely under the atmosphere. Once the ball joint is actuated to the reference position, the inner air chamber expands with high pressure, locking the ball joint with friction torque and achieving high load capacity. Since the friction torque is proportional to the input air pressure, the load capacity of the ball joint can be continuously adjusted along with the change of air pressure. The theoretical model is verified via the test of the ball joints. Experimental studies show that the ball joint has a significant advantage in maximum driving torque than other conventional variable stiffness ball joints of similar size. A six-module flexible arm is built for applications. The experiments show excellent flexibility and load capacity of the arm.

Micropipette‐Based Fabrication of Free‐Standing, Conducting Polymer Bilayer Actuators

AbstractElectrically driven actuators have found widespread applications in biomimetics and soft robotics. Among different actuation materials, conducting polymers (CPs) have stood out due to their unique doping‐based actuation mechanism. Fabricating actuators at the microscale is particularly important, not only in the manufacturing of delicate biomimetic/robotic devices but also in advanced microphysiological studies. However, the choice for microfabrication techniques is limited, with the reported CP microactuators being mainly planar. To overcome this issue, a new micropipette‐based method is developed for the fabrication of free‐standing 3D CP actuators. The two‐layer actuator consists of a layer of PPy:CF3SO3, fabricated by localized electropolymerization, and a layer of PEDOT:PSS, fabricated by a “direct writing” technique. Due to the opposite contraction and expansion behavior of these two CPs, determined by the size of dopants, the electrically driven bending actuators have been demonstrated. This fabrication approach provides unprecedented capability for fabricating high aspect ratio microactuators: the 360° bending orientation of the actuators can be controlled by the relative position of the two layers. As a proof‐of‐principle, we demonstrate CP “micro‐tweezers” and the manipulation of a PDMS microsphere. The technique developed in this work opens exciting opportunities to manufacture advanced artificial muscles and sophisticated soft microrobotics.

Heat- and shear-reversible networks in food: A review.

While nature behaves like an irreversible network with respect to entropy and time, certain systems in nature exist that are, to some extent, reversible. The property of reversibility imparts unique benefits to systems that possess them, making them suitable for designing self-healing, stimuli-responsive, and smart materials that can be used in widely divergent fields. Reversible networks are currently being exploited for applications in tissue engineering, drug delivery, and soft robotics. They are also being utilized as low-calorie fat mimetics with melt-in-your-mouth textures, as well as being explored as potential scaffolds for three-dimensional (3D) printable food, among other applications. This review aims to gather representative examples of heat- and shear-reversible networks in the food science literature from the last 30 or so years, in other words, reversible food gels made either from linear biopolymers or from colloidal, particulate dispersions, including those that have been modified specifically to induce reversibility. An overview of the network mechanisms involved that impart reversibility, including a discussion of the strength and range of forces involved, will be highlighted. A model that explains why certain networks are thermoreversible while others are shear-reversible, and why others are both, will also be proposed. A fundamental understanding of these mechanisms will prove invaluable when designing reversible networks in the future, making possible the precise control of their properties, thus fostering innovative applications within the food industry and beyond.

Recent Advances in Flexible Sensors and Their Applications.

Flexible sensors are low cost, wearable, and lightweight, as well as having a simple structure as per the requirements of engineering applications. Furthermore, for many potential applications, such as human health monitoring, robotics, wearable electronics, and artificial intelligence, flexible sensors require high sensitivity and stretchability. Herein, this paper systematically summarizes the latest progress in the development of flexible sensors. The review briefly presents the state of the art in flexible sensors, including the materials involved, sensing mechanisms, manufacturing methods, and the latest development of flexible sensors in health monitoring and soft robotic applications. Moreover, this paper provides perspectives on the challenges in this field and the prospect of flexible sensors.

Out-of-Plane Soft Lithography for Soft Pneumatic Microactuator Arrays.

Elastic pneumatic actuators are fueling new devices and applications in soft robotics. Actuator miniaturization is critical to enable soft microsystems for applications in microfluidics and micromanipulation. This work proposes a fabrication technique to make out-of-plane bending microactuators entirely by soft lithography. The only bonding step required is to seal the embedded fluidic channels, assuring the structural integrity of the microactuators. The process consists of fabricating two SU8 mold halves using different lithographic layers. Polydimethilsiloxane is poured on the bottom mold, which is subsequently aligned and assembled with the top mold. The process allows for out-of-plane actuators with a diameter of 300 μm and for fabricating arrays of up to 36 actuators that are row addressable. These active micropillars have an aspect ratio of 1:1.5 and, when pressurized at 1 bar, show a bending angle of ∼30°.

Dual-Mode Flexible Capacitive Sensor for Proximity-Tactile Interface and Wireless Perception

Flexible sensors with proximity and tactile sensing have attracted great interest in electronic skins (E-skin), human-machine interfaces (HMI), and soft robotics. However, it remains challenging to have both proximity and haptic sensing ability in one flexible device, yet enable wireless sensing. Here, based on the edge electric field and piezo-capacitive effect, we successfully present a combination of approaching non-contact sensation with pressure perception in one flexible dual-mode capacitive sensor (FDCS) which possesses both proximity-tactile interface interaction and wireless sensing. Large scale and low-cost methods of the screen printing and the laser engraving are selectively used to fabricate flexible electrodes and cone-pattern mold, respectively. Thanks to the stable conductivity of the electrodes and the micro-cone-like dielectric structure, the FDCS exhibits a detection distance of up to 20 cm in proximity mode and an extended operating range of 200 kPa, a low detection limit (10.2 Pa), fast response time (90 ms), and long endurance (>10,000 cycles) in pressure mode. A 64-unit pressure matrix successfully interacted with the coupling of the proximity finger and the actual pressure in two different modes. In addition, by assembling the FDCS with a planar spiral coil to form an LC (inductance-capacitance) resonator, rather than connecting directly to an LCR device, the device was able to detect external pressure wirelessly. The development of this dual-mode sensor and its demonstration paves the way for potential flexible sensors for applications in HMI and soft robots.

Large-Area and Low-Cost Force/Tactile Capacitive Sensor for Soft Robotic Applications.

This paper presents a novel design and development of a low-cost and multi-touch sensor based on capacitive variations. This new sensor is very flexible and easy to fabricate, making it an appropriate choice for soft robot applications. Materials (conductive ink, silicone, and control boards) used in this sensor are inexpensive and easily found in the market. The proposed sensor is made of a wafer of different layers, silicone layers with electrically conductive ink, and a pressure-sensitive conductive paper sheet. Previous approaches like e-skin can measure the contact point or pressure of conductive objects like the human body or finger, while the proposed design enables the sensor to detect the object’s contact point and the applied force without considering the material conductivity of the object. The sensor can detect five multi-touch points at the same time. A neural network architecture is used to calibrate the applied force with acceptable accuracy in the presence of noise, variation in gains, and non-linearity. The force measured in real time by a commercial precise force sensor (ATI) is mapped with the produced voltage obtained by changing the layers’ capacitance between two electrode layers. Finally, the soft robot gripper embedding the suggested tactile sensor is utilized to grasp an object with position and force feedback signals.

A cyclic freezing-thawing approach to layered Janus hydrogel tapes with single-sided adhesiveness for wearable strain sensors

Hydrogel strain sensors, which have potential applications in wearable devices, soft robots and human–machine interfaces, etc., are increasingly investigated. But it remains challenging to fabricate hydrogel strain sensors with integrated wearability and high sensing performances. To address this issue, we herein propose a cyclic freezing-thawing approach to layered Janus hydrogel tapes for wearable strain sensors. The hydrogel tapes are composed of an adhesive poly(vinyl alcohol)/phytic acid (PVA/PA) layer and a non-adhesive poly(vinyl alcohol)/polyaniline (PVA/PANI) layer and exhibit controllable flexibility and approximate elastic modulus to soft tissues. A robust interface forms between the two layers and the interfacial peeling force reaches 114.6 N/m. Strain sensors assembled from the Janus hydrogel tapes achieve low response time (loading: 60 ms, unloading: 62 ms), reversible response and high sensitivity (gauge factor: 3.4). Interestingly, such strain sensors can be further integrated with a Bluetooth system to fabricate wireless wearable sensors for convenient monitoring of physiological activities such as walking, squatting, stair climbing, etc. Overall, this work will provide not only a new approach to Janus hydrogels, but also a new clue for developing functional flexible devices.

Modeling and Control of IPMC-Based Artificial Eukaryotic Flagellum Swimming Robot: Distributed Actuation

Ionic polymer-metal composites (IPMCs) are electrically driven materials that undergo bending deformations in the presence of relatively low external voltages, exhibiting a great potential as actuators in applications in soft robotics, microrobotics, and bioengineering, among others. This paper presents an artificial eukaryotic flagellum (AEF) swimming robot made up of IPMC segments for the study of planar wave generation for robot propulsion by single and distributed actuation, i.e., considering the first flagellum link as an actuator or all of them, respectively. The robot comprises three independent and electrically isolated actuators, manufactured over the same 10 mm long IPMC sheet. For control purposes, a dynamic model of the robot is firstly obtained through its frequency response, acquired by experimentally measuring the flagellum tip deflection thanks to an optical laser meter. In particular, two structures are considered for such a model, consisting of a non-integer order integrator in series with a resonant system of both non-integer and integer order. Secondly, the identified models are analyzed and it is concluded that the tip displacement of each actuator or any IPMC point is characterized by the same dynamics, which remains unchanged through the link with mere variations of the gain for low-frequency applications. Based on these results, a controller robust to gain variations is tuned to control link deflection regardless of link length and enabling the implementation of a distributed actuation with the same controller design. Finally, the deflection of each link is analyzed to determine whether an AEF swimming robot based on IPMC is capable of generating a planar wave motion by distributed actuation.

Intentional Blocking Based Photoelectric Soft Pressure Sensor with High Sensitivity and Stability.

Soft pressure sensors have recently attracted considerable attention because of their applications in human-machine interface, soft robotics, and prosthetics. However, there remain some challenges in achieving satisfactory performance (e.g., high sensitivity, wide sensing range, high stability) for soft pressure sensors. This article reports an intentional blocking based photoelectric pressure sensor. Two different blocking methods are investigated: the single-row-pyramid blocking and the double-row-pyramid blocking. The sensor has a simple structure, which is made of a light-emitting diode, photosensitive element, and silicone sensor shell. Experiments demonstrate that the sensor has a high sensitivity (the maximum sensitivity is 48.07 kPa-1, and the minimum measurement pressure is 0.8 Pa), large pressure-sensing range (the sensing range is up to 120 kPa), superior stability (a drift about 0.4% over 12,130 repetitive cycles at 0-80 kPa), low drift (< ±0.2% in different 3-day testing), negligible hysteresis, and high signal-to-noise ratio (over 55 dB). By mounting the pressure sensor at the end of a robotic arm, the robot can detect subtle collisions (such as touching a balloon through a pinpoint). In addition, this article fabricates a tactile glove based on the proposed pressure sensor and shows the application of this glove for music playing and object weighing. This study provides a new structure for photoelectric sensors to increase sensitivity and also provides a more convenient way to fabricate photoelectric pressure sensors.

Evaluation of two complementary modeling approaches for fiber-reinforced soft actuators

Although robots are increasingly found in a wide range of applications, their use in proximity to humans is still fraught with challenges, primarily due to safety concerns. Roboticists have been seeking to address this situation in recent years through the use of soft robots. Unfortunately, identifying appropriate models for the complete analysis and investigation of soft robots for design and control purposes can be problematic. This paper seeks to address this challenge by proposing two complementary modeling techniques for a particular type of soft robotic actuator known as a Fiber-Reinforced Elastomeric Enclosure (FREE). We propose that researchers can leverage multiple models to fill gaps in the understanding of the behavior of soft robots. We present and evaluate both a dynamic, lumped-parameter model and a finite element model to extend understanding of the practicability of FREEs in soft robotic applications. The results of experimental simulations using a lumped-parameter model show that at low pressures FREE winding angle and radius change no more than 2%. This observation provided confidence that a linearized, dynamic, lumped-mass model could be successfully used for FREE controller development. Results with the lumped-parameter model demonstrate that it predicts the actual rotational motion of a FREE with at most 4% error when a closed-loop controller is embedded in the system. Additionally, finite element analysis was used to study FREE design parameters as well as the workspace achieved with a module comprised of multiple FREEs. Our finite element results indicate that variations in the material properties of the elastic enclosure of a FREE are more significant than variations in fiber properties (primarily because the fibers are essentially inextensible in comparison to the elastic enclosure). Our finite element analysis confirms the results obtained by previous researchers for the impact of variations in winding angle on FREE rotation, and we extend these results to include an analysis of the effect of winding angle on FREE force and moment generation. Finally, finite element results show that a 30^{circ } difference in winding angle dramatically alters the shape of the workspace generated by four FREEs assembled into a module. Concludingly, comments are made about the relative advantages and limitations of lumped-parameter and finite element models of FREEs and FREE modules in providing useful insights into their behavior.

Soft Multi-Directional Force Sensor for Underwater Robotic Application.

Tactile information is crucial for recognizing physical interactions, manipulation of an object, and motion planning for a robotic gripper; however, concurrent tactile technologies have certain limitations over directional force sensing. In particular, they are expensive, difficult to fabricate, and mostly unsuitable for underwater use. Here, we present a facile and cost-effective synthesis technique of a flexible multi-directional force sensing system, which is also favorable to be utilized in underwater environments. We made use of four flex sensors within a silicone-made hemispherical shell structure. Each sensor was placed 90 apart and aligned with the curve of the hemispherical shape. If the force is applied on the top of the hemisphere, all the flex sensors would bend uniformly and yield nearly identical readings. When force is applied from a different direction, a set of flex sensors would characterize distinctive output patterns to localize the point of contact as well as the direction and magnitude of the force. The deformation of the fabricated soft sensor due to applied force was simulated numerically and compared with the experimental results. The fabricated sensor was experimentally calibrated and tested for characterization including an underwater demonstration. This study would widen the scope of identification of multi-directional force sensing, especially for underwater soft robotic applications.

Beating of a Spherical Liquid Crystal Elastomer Balloon under Periodic Illumination.

Periodic excitation is a relatively simple and common active control mode. Owing to the advantages of direct access to environmental energy and controllability under periodic illumination, it enjoys broad prospects for application in soft robotics and opto-mechanical energy conversion systems. More new oscillating systems need to be excavated to meet the various application requirements. A spherical liquid crystal elastomer (LCE) balloon model driven by periodic illumination is proposed and its periodic beating is studied theoretically. Based on the existing dynamic LCE model and the ideal gas model, the governing equation of motion for the LCE balloon is established. The numerical calculations show that periodic illumination can cause periodic beating of the LCE balloon, and the beating period of the LCE balloon depends on the illumination period. For the maximum steady-state amplitude of the beating, there exists an optimum illumination period and illumination time rate. The optimal illumination period is proved to be equivalent to the natural period of balloon oscillation. The effect of system parameters on beating amplitude are also studied. The amplitude is mainly affected by light intensity, contraction coefficient, amount of gaseous substance, volume of LCE balloon, mass density, external pressure, and damping coefficient, but not the initial velocity. It is expected that the beating LCE balloon will be suitable for the design of light-powered machines including engines, prosthetic blood pumps, aircraft, and swimmers.

Microspines in tropical climbing plants: a small-scale fix for life in an obstacle course.

Many climbing plants have microspines on their stems, which facilitate attachment and prevent slipping and falling from host plant supports. Extending via growth through complex environments and anchoring stems to substrates with minimal contact forces are key benefits for climbing plants. Microspines are also highly desirable features for new technologies and applications in soft robotics. Using a novel sled-like device, we investigated static and sliding attachment forces generated by stems in 10 species of tropical climber from French Guiana differing in size and climbing habit. Eight species showed higher static and sliding forces when their stems were pulled in the basal direction against a standard surface than in the apical direction. This anisotropic behaviour suggests that tropical climbers have evolved different ratchet-like mechanisms that allow easy sliding forwards but are resistant to slipping downwards. The presence of a downwards 'stick-and-slip' phenomenon, where static attachment is not significantly stronger than maximal sliding attachment, was present in most species apart from three showing relatively weak attachment by microspines. This indicates that diverse microspine attachment strategies exist in climbing plants. This diversity of functional properties offers a range of potential design specifications for climbing strategies on different substrates for artificial climbing artefacts.

Actuation Mechanisms of a Semicrystalline Elastomer-Based Polymer Artificial Muscle with High Actuation Strain

Semicrystalline elastomer-based artificial muscles (sePAMs) are of high actuation strain generally above 30% and are promising for applications of soft robots, flexible electronics, etc. However, a comprehensive and in-depth understanding of the actuation behavior and mechanisms of the sePAMs is lacking. This obstructs the future design and development of the sePAMs. In this paper, we reported a sePAM with a high actuation strain above 40% under 1.0 MPa. Furthermore, we presented an in-depth analysis of the underlying molecular and thermodynamic mechanisms related to this high actuation strain. It is found that both the entropy-elastic actuation and the crystallization-induced elongation (CIE)/melting-induced contraction (MIC) play important roles in controlling the actuation behavior. More importantly, it is revealed that the primary and direct cause for the CIE is the further elongation of the amorphous section in the stretched network during oriented crystallization. The oriented crystallization induces an entropy increase of the amorphous section, causing the stress of the network to be partly relaxed and the network to be further stretched by the external stress. This entropy increase was confirmed by theoretical calculation. The theory also predicts that an oriented crystallization of about 10% can induce a large elongation of the network with a strain increment of about 80%, which agrees well with the experiments reported in this paper as well as in the literature. This work greatly advances our understanding of the actuation behavior and mechanisms of the sePAMs, opening up new strategies for the future design and development of high-performance polymer actuators.

Martensitic transformation controlled by electromagnetic field: From experimental evidence to wireless actuator applications

Mechanical actuators based on shape memory alloys (SMA) are becoming a key component in the development of novel soft robotic applications and surgically implantable devices. Their working principle relies in the temperature induced martensitic transformation (MT), which is responsible of the actuation mechanism. In this work, we found experimental evidence to show that the martensitic transformation can be controlled by electromagnetic field (EF) by a wireless process in ferromagnetic shape memory alloys. It is shown that the martensitic transformation can be driven by an external EF (frequency 45 kHz) while the specific absorption rate (SAR), which was determined through real-time dynamic magnetization measurements, allows the instantaneous monitoring of the transformation evolution. On the basis of the obtained results, we propose a strategy to achieve a battery-free wireless SMA actuator that can be remotely controlled. This concept can be applicable to other SMA material that exhibit a similar magneto-structural phase transition.