Soft Robotics Research Articles

Performance of Organic Electrochemical Transistors with Ionic Liquid Crystal Elastomers as Solid Electrolytes.

Organic electrochemical transistors (OECTs) have emerged as attractive devices for bioelectronics, wearable electronics, soft robotics, and energy storage devices. The electrolyte, being a fundamental component of OECTs, plays a crucial role in their performance. Recently, it has been demonstrated that ionic liquid crystal elastomers (iLCEs) can be used as a solid electrolyte for OECTs. Their capabilities, however, have only been shown for relatively large size substrate-free OECTs. Here, we study the influence of the different alignments of iLCEs on steady state and transient behavior of OECTs using a lateral geometry with source, drain, and gate in the same plane. We achieve excellent electrical response with an ON/OFF switching ratio of >105 and minimal leakage current. The normalized maximum transconductance gm/w of the most sensitive iLCE was found to be 33 S m-1, which is one of the highest among all solid-state-based OECTs reported so far. Additionally, iLCEs show high stability and can be removed and reattached multiple times to the same OECT device without decreasing performance.

Shape reconstruction of soft continuum robots via the fusion of local strains and global poses

Shape reconstruction of soft continuum robots via the fusion of local strains and global poses

On the Piezomagnetism of Magnetoactive Elastomeric Cylinders in Uniform Magnetic Fields: Height Modulation in the Vicinity of an Operating Point by Time-Harmonic Fields.

Soft magnetoactive elastomers (MAEs) are currently considered to be promising materials for actuators in soft robotics. Magnetically controlled actuators often operate in the vicinity of a bias point. Their dynamic properties can be characterized by the piezomagnetic strain coefficient, which is a ratio of the time-harmonic strain amplitude to the corresponding magnetic field strength. Herein, the dynamic strain response of a family of MAE cylinders to the time-harmonic (frequency of 0.1-2.5 Hz) magnetic fields of varying amplitude (12.5 kA/m-62.5 kA/m), superimposed on different bias magnetic fields (25-127 kA/m), is systematically investigated for the first time. Strain measurements are based on optical imaging with sub-pixel resolution. It is found that the dynamic strain response of MAEs is considerably different from that in conventional magnetostrictive polymer composites (MPCs), and it cannot be described by the effective piezomagnetic constant from the quasi-static measurements. The obtained maximum values of the piezomagnetic strain coefficient (∼102 nm/A) are one to two orders of magnitude higher than in conventional MPCs, but there is a significant phase lag (35-60°) in the magnetostrictive response with respect to an alternating magnetic field. The experimental dependencies of the characteristics of the alternating strain on the amplitude of the alternating field, bias field, oscillation frequency, and aspect ratio of cylinders are given for several representative examples. It is hypothesized that the main cause of observed peculiarities is the non-linear viscoelasticity of these composite materials.

Vision-based reinforcement learning control of soft robot manipulators

Purpose This study aims to tackle control challenges in soft robots by proposing a visually-guided reinforcement learning approach. Precise tip trajectory tracking is achieved for a soft arm manipulator. Design/methodology/approach A closed-loop control strategy uses deep learning-powered perception and model-free reinforcement learning. Visual feedback detects the arm’s tip while efficient policy search is conducted via interactive sample collection. Findings Physical experiments demonstrate a soft arm successfully transporting objects by learning coordinated actuation policies guided by visual observations, without analytical models. Research limitations/implications Constraints potentially include simulator gaps and dynamical variations. Future work will focus on enhancing adaptation capabilities. Practical implications By eliminating assumptions on precise analytical models or instrumentation requirements, the proposed data-driven framework offers a practical solution for real-world control challenges in soft systems. Originality/value This research provides an effective methodology integrating robust machine perception and learning for intelligent autonomous control of soft robots with complex morphologies.

Sensor‐Embedded Muscle for Closed‐Loop Controllable Actuation in Proprioceptive Biohybrid Robots

Biohybrid robots are soft robots that exploit unique characteristics of biological cells and tissues for motion generation. Skeletal muscle tissue‐based bioactuators respond to externally applied stimuli, such as electrical fields. However, current bioactuation systems rely on open‐loop control strategies that lack knowledge of the actuator's state. The regulation of output force and position of biohybrid robots requires self‐sensing control systems that combine bioactuators with sensors and control paradigms. Herein, a soft, fiber‐shaped mechanical sensor based on a piezoresistive composite is proposed that efficiently integrates with engineered skeletal muscle tissue and senses its contracting states in a cell culture environment in the presence of applied electrical fields. After testing the sensor's insulation and biocompatibility, its sensitivity for typical strains (<1%) is characterized, and its ability is proven to detect motions from contractile skeletal muscle tissue constructs. Finally, it is shown that the sensor response can feed an autonomous control system, thus demonstrating the first proprioceptive biohybrid robot that senses and responds to its contraction state. In addition to inspiring implantable systems, biomedical models, and other bioelectronic devices, the proposed technology will confer biohybrids with decisional autonomy, thus driving the paradigm shift between bioactuators and intelligent biohybrid robots.

Programming Hydrogen Bonds for Reversible Elastic‐Plastic Phase Transition in a Conductive Stretchable Hydrogel Actuator with Rapid Ultra‐High‐Density Energy Conversion and Multiple Sensory Properties

AbstractSmart hydrogels have recently garnered significant attention in the fields of actuators, human‐machine interaction, and soft robotics. However, when constructing large‐scale actuated systems, they usually exhibit limited actuation forces (≈2 kPa) and actuation speeds. Drawing inspiration from hairspring energy conversion mechanism, an elasticity‐plasticity‐controllable composite hydrogel (PCTA) with robust contraction capabilities is developed. By precisely manipulating intermolecular and intramolecular hydrogen‐bonding interactions, the material's elasticity and plasticity can be programmed to facilitate efficient energy storage and release. The proposed mechanism enables rapid generation of high contraction forces (900 kPa) at ultra‐high working densities (0.96 MJ m−3). Molecular dynamics simulations reveal that modifications in the number and nature of hydrogen bonds lead to a distinct elastic‐plastic transition in hydrogels. Furthermore, the conductive PCTA hydrogel exhibits multimodal sensing capabilities including stretchable strain sensing with a wide sensing range (1–200%), fast response time (180 ms), and excellent linearity of the output signal. Moreover, it demonstrates exceptional temperature and humidity sensing capabilities with high detection accuracy. The strong actuation power and real‐time sensory feedback from the composite hydrogels are expected to inspire novel flexible driving materials and intelligent sensing systems.

Soft Micropneumatic Touchpad

Soft tactile sensors outputting fluidic signals have many potential applications in soft microfluidic devices and soft robots. However, existing systems have been limited to a single or a few sensors in parallel, so they are not comparable to the state‐of‐the‐art electrical resistive and capacitive touchpads, which can detect rich tactile information, including touch location, pressure, area, and even multiple touches simultaneously. This work reports a soft micropneumatic touchpad. The touchpad consists of 32 pneumatic channels inside soft elastomer, with 16 channels aligned row‐wise and 16 column‐wise. The flow resistance of each channel is measured using a pressure divider. When the pad is touched, the cross‐sectional area of the channels close to the contact location deforms, which changes the flow resistance of those channels. With 32 sensing channels, the location, depth, area of the contact, and even two simultaneous touches can be detected. Letters hand‐written on the touchpad can be reconstructed from the measured data. With the assumption of sparsity, a tactile pressure map, with a value at each 16 × 16 grid point, can also be reconstructed. This work opens a path to replace electronic tactile sensors in soft devices with all‐fluidic alternatives.

Stimuli-Responsive Polymer Actuator for Soft Robotics.

Polymer actuators are promising, as they are widely used in various fields, such as sensors and soft robotics, for their unique properties, such as their ability to form high-quality films, sensitivity, and flexibility. In recent years, advances in structural and fabrication processes have significantly improved the reliability of polymer sensing-based actuators. Polymer actuators have attracted considerable attention for use in artificial or biohybrid systems, as they have the potential to operate under diverse conditions with high durability. This review briefly describes different types of polymer actuators and provides an understanding of their working mechanisms. It focuses on actuation modes controlled by diverse or multiple stimuli. Furthermore, it discusses the fabrication processes of polymer actuators; the fabrication process is an important consideration in the development of high-quality actuators with sensing properties for a wide range of applications in soft robotics. Additionally, the high potential of polymer actuators for use in sensing technology is examined, and the latest developments in the field of polymer actuators, such as the development of biohybrid polymers and the use of polymer actuators in 4D printing, are briefly described.

Humidity-responsive fiber actuators assembled from cellulose nanofibrils

Fiber actuators, particularly valuable in soft robotics and environmental sensing, are at the forefront of “smart” materials and materials innovation. In this realm, torsional and tensile biofiber actuators, notable for their cost-effectiveness and biodegradability, mark a critical gap in the development of next-generation functional systems and devices. To address this gap, this study showcased moisture-responsive fiber actuators made from cellulose nanofibrils (CNFs). The initial focus of this contribution was on an innovative torsional actuator, which capitalized on the hydrophilic nature of the CNFs filaments produced through wet-spinning processes. These robust filaments, with a mechanical strength of (237.0 ± 10.7) MPa, were twisted to form the torsional actuator. This actuator demonstrated a rapid rotational response, achieving up to 1180 rpm within merely 10 s of exposure to moisture, and maintained high durability over multiple cycles. Building upon this platform, the study continued and aimed to build up a tensile actuator, which ingeniously integrated a supercoiled nylon fiber core within a moisture-sensitive CNFs sheath. This design enhances the structural support and functionality of the actuator. The pursued transition from torsional to tensile actuator demonstrates an iterative and innovative approach in actuator technology, underscoring the versatility and potential of CNFs in the realm of “smart” actuation materials.

High-Voltage Power Supply for Four-Quadrant Dielectric Elastomer Actuators.

Dielectric elastomer actuators (DEAs) are emerging as promising candidates for various applications in robotics and optical devices due to their lightweight, miniaturization potential, high energy density, simple structure, and low power consumption. However, their effective actuation always demands sophisticated high-voltage driving circuits that are compact and responsive. DEAs need to be capable of generating intricate high-voltage waveforms or simultaneously controlling multiple quadrants with distinct high-voltage levels. This paper proposes a high-voltage power supply for DEAs, featuring a four-quadrant high-voltage driving circuit. The circuit is capable of independently generating high-voltage signals ranging from 100 V to 6000 V and producing arbitrary waveforms with adjustable frequencies. The independent operation of the quadrants without crosstalk showcases the system's integration and potential for cross-disciplinary applications.

Optimizing soft robot design and tracking with and without evolutionary computation: an intensive survey

Optimizing soft robot design and tracking with and without evolutionary computation: an intensive survey

Mechanochromic Suction Cups for Local Stress Detection in Soft Robotics

Advancements in smart soft materials are enhancing the capabilities of robotic manipulators in object interactions and complex tasks. Mechanochromic materials, acting as lightweight sensors, offer easily interpretable visual feedback for localized stress detection, structural health monitoring, and energy‐efficient robotic skins. Herein, an innovative mechanochromic soft end‐effector capable of discerning local contact stresses during mechanical interactions with objects is presented and their relative position is ascertained. This system utilizes a reversible force‐induced color switch in a thin layer of spiropyran‐functionalized polydimethylsiloxane, which coats a silicone‐made suction cup. The mechanochromic suction cup is integrated with a 3D‐printed compact load‐transferring system and electronic color‐changing detection elements. The assembly may serve as a synthetic receptor for robotic actuators, discerning localized interaction forces down to 3 N. The system's resilience to varying environmental factors, including illumination, tilting, and interaction with objects of various shapes is verified. The results indicate potential for exteroceptive solutions in reconfigurable manipulation tasks without compromising the overall softness of the manipulator.

Advanced Materials for Energy Harvesting and Soft Robotics: Emerging Frontiers to Enhance Piezoelectric Performance and Functionality.

Piezoelectric energy harvesting captures mechanical energy from a number of sources, such as vibrations, the movement of objects and bodies, impact events, and fluid flow to generate electric power. Such power can be employed to support wireless communication, electronic components, ocean monitoring, tissue engineering, and biomedical devices. A variety of self-powered piezoelectric sensors, transducers, and actuators have been produced for these applications, however approaches to enhance the piezoelectric properties of materials to increase device performance remain a challenging frontier of materials research. In this regard, the intrinsic polarization and properties of materials can be designed or deliberately engineered to enhance the piezo-generated power. This review provides insights into the mechanisms of piezoelectricity in advanced materials, including perovskites, active polymers, and natural biomaterials, with a focus on the chemical and physical strategies employed to enhance the piezo-response and facilitate their integration into complex electronic systems. Applications in energy harvesting and soft robotics are overviewed by highlighting the primary performance figures of merits, the actuation mechanisms, and relevant applications. Key breakthroughs and valuable strategies to further improve both materials and device performance are discussed, together with a critical assessment of the requirements of next-generation piezoelectric systems, and future scientific and technological solutions.

Using a Guidance Virtual Fixture on a Soft Robot to Improve Ureteroscopy Procedures in a Phantom

Manipulating a flexible ureteroscope is difficult, due to its bendable body and hand–eye coordination problems, especially when exploring the lower pole of the kidney. Though robotic interventions have been adopted in various clinical scenarios, they are rarely used in ureteroscopy. This study proposes a teleoperation system consists of a soft robotic endoscope together with a Guidance Virtual Fixture (GVF) to help users explore the kidney’s lower pole. The soft robotic arm was a cable-driven, 3D-printed design with a helicoid structure. GVF was dynamically constructed using video streams from an endoscopic camera. With a haptic controller, GVF can provide haptic feedback to guide the users in following a trajectory. In the user study, participants were asked to follow trajectories when the soft robotic arm was in a retroflex posture. The results suggest that the GVF can reduce errors in the trajectory tracking tasks when the users receive the proper training and gain more experience. Based on the NASA Task Load Index questionnaires, most participants preferred having the GVF when manipulating the robotic arm. In conclusion, the results demonstrate the benefits and potential of using a robotic arm with a GVF. More research is needed to investigate the effectiveness of the GVFs and the robotic endoscope in ureteroscopic procedures.

Low‐Temperature Flexibility of Chiral Organic Crystals with Highly Efficient Second‐Harmonic Generation

Flexible crystals with unique mechanical properties have presented enormous applications in optoelectronics, soft robotics and sensors. However, there have been no reports of low‐temperature‐resistant flexible crystals with second‐order nonlinear optical properties (NLO). Here, we report the flexible chiral Schiff‐base crystals capable of efficient second harmonic generation (SHG). Both enantiomers and racemic modifications of these crystals are mechanically flexible in two directions at both room temperature and at ‐196 °C, although their mechanical responses differ. The enantiomers display SHG with an intensity of up to 12 times that of potassium dihydrogenphosphate (KDP) when pumped at 980 nm, and they also have high laser‐induced damage thresholds (LDT). Even when bent, the crystals retain strong second harmonic generation, although with a different intensity distribution depending on the polarization, compared to when they are straight. This work describes the first instance of flexible organic crystal with NLO properties and lays the foundation for the development of mechanically flexible organic NLO materials.

Magnetic elastomer composites with tunable magnetization behaviors for flexible magnetic transducers

AbstractMagnetic elastomer composites are extending the field of soft robotics by integrating the flexibility of elastomers with the versatile performance of magnetic materials. In this work, a series of magnetic elastomers (BaCoxTixFe12–2xO19/PDMS) with tunable magnetic response have been successfully synthesized. In this composite, the magnetic component consists of Co‐Ti co‐doped barium hexaferrite (BaCoxTixFe12–2xO19, BCTF) nano powders, and the matrix is made of polydimethylsiloxane (PDMS). The coercivity (Hc) of BCTF can be reduced from 3950 Oe to 70 Oe, which bestow on the magnetic elastomer different magnetic response behavior. These magnetic elastomer composites obtained by blending have low Young's modulus, approximately 1.5 MPa. Furthermore, when applying the same external magnetic field, BaCo1.2Ti1.2Fe9.6O19/PDMS the magnetic deformation Angle can be as high as 60°. These magnetic elastomers exhibit different magnetic‐induced deformation owing to the adjustable permeability of the magnetic nanoparticles. This research significantly advances the design of applications with specific magnetic characteristics, such as sensors and transducers, thus opening new horizons for innovation in soft robotics and related fields.Highlights All elastomer composite exhibit a low modulus of approximately 1.5 MPa. These composites exhibit different magnetic response behaviors. This research provides a solution for the innovation of transducers.

Flexible multimaterial fibers in modern biomedical applications.

Biomedical devices are indispensable in modern healthcare, significantly enhancing patients' quality of life. Recently, there has been a drastic increase in innovations for the fabrication of biomedical devices. Amongst these fabrication methods, the thermal drawing process has emerged as a versatile and scalable process for the development of advanced biomedical devices. By thermally drawing a macroscopic preform, which is meticulously designed and integrated with functional materials, hundreds of meters of multifunctional fibers are produced. These scalable flexible multifunctional fibers are embedded with functionalities such as electrochemical sensing, drug delivery, light delivery, temperature sensing, chemical sensing, pressure sensing, etc. In this review, we summarize the fabrication method of thermally drawn multifunctional fibers and highlight recent developments in thermally drawn fibers for modern biomedical application, including neural interfacing, chemical sensing, tissue engineering, cancer treatment, soft robotics and smart wearables. Finally, we discuss the existing challenges and future directions of this rapidly growing field.

Augmenting perceived stickiness of physical objects through tactile feedback after finger lift-off.

Haptic Augmented Reality (HAR) is a method that actively modulates the perceived haptics of physical objects by presenting additional haptic feedback using a haptic display. However, most of the proposed HAR research focuses on modifying the hardness, softness, roughness, smoothness, friction, and surface shape of physical objects. In this paper, we propose an approach to augment the perceived stickiness of a physical object by presenting additional tactile feedback at a particular time after the finger lifts off from the physical object using a thin and soft tactile display suitable for HAR. To demonstrate this concept, we constructed a thin and soft tactile display using a Dielectric Elastomer Actuator suitable for HAR. We then conducted two experiments to validate the effectiveness of the proposed approach. In Experiment 1, we showed that the developed tactile display can augment the perceived stickiness of physical objects by presenting additional tactile feedback at appropriate times. In Experiment 2, we investigated the stickiness experience obtained by our proposed approach and showed that the realism of the stickiness experience and the harmony between the physical object and the additional tactile feedback are affected by the frequency and presentation timing of the tactile feedback. Our proposed approach is expected to contribute to the development of new applications not only in HAR, but also in Virtual Reality, Mixed Reality, and other domains using haptic displays.

A robust low-friction triple network hydrogel based on multiple synergistic enhancement mechanisms

Hydrogels exhibit promising applications, particularly due to their high water content and excellent biocompatibility. Despite notable progress in hydrogel technology, the concurrent enhancement of water content, mechanical strength, and low friction poses substantial challenges to practical utilization. In this study, employing molecular and network design guided based on multiple synergistic enhancement mechanisms, we have developed a robust polyvinyl alcohol (PVA)–polyacrylic acid (PAA)–polyacrylamide (PAAm) three-network (TN) hydrogel exhibiting high water content, enhanced strength, low friction, and fatigue resistance. The hydrogel manifests a water content of 63.7%, compression strength of 6.3 MPa, compression modulus of 2.68 MPa, tensile strength reaching 7.3 MPa, and a tensile modulus of 10.27 MPa. Remarkably, even after one million cycles of dynamic loading, the hydrogel exhibits no signs of fatigue failure, with a minimal strain difference of only 1.15%. Furthermore, it boasts a low sliding coefficient of friction (COF) of 0.043 and excellent biocompatibility. This advancement extends the applications of hydrogels in emerging fields within biomedicine and soft bio-devices, including load-bearing artificial tissues, artificial blood vessels, tissue scaffolds, robust hydrogel coatings for medical devices, and joint parts of soft robots.

Advancements in Castor Oil‐Derived Smart Materials: A Comprehensive Mini‐Review

AbstractSmart materials encompass a class of materials capable of reorienting themselves in response to external stimuli (heat, light, and chemical), and generate smart properties such as self‐healing (SH) and shape‐memory (SM), etc. Recently, castor oil has emerged as a bio‐renewable feedstock facilitating the synthesis of smart polymers such as polyurethanes and epoxidized castor oil (CO)‐based polymers. Furthermore, fillers and nanomaterials have been added to prepare responsive materials (SH and SM) and drug delivery systems. This review delves into the recent developments in CO‐based smart materials since 2015 under thermal, photo, and chemical stimuli and their applications in coating, adhesive, soft robotics, and drug delivery. Additionally, the challenges and future scopes for the development of castor oil‐based advanced smart materials have been discussed.