Applications In Soft Robotics Research Articles

Bending sensor using ionic liquid crystal elastomers as solid electrolyte of organic electrochemical transistors

ABSTRACT Organic Electrochemical Transistors (OECTs) have the potential to enable fully flexible, low-cost electronics. Here, we demonstrate that OECTs can be combined with ionic liquid crystal elastomers (iLCEs) to realise highly sensitive bending sensors. The transfer curve of the iLCE-based OECT shows opposite variation with upward and downward bending representing a directional sensitive strain sensor. The change in the drain current is 4 orders of magnitude larger than the flexo-ionic current. This high sensitivity is due to the OECT that amplifies the flexo-ionic current. iLCE-based OECTs therefore present a new class of curvature sensors that may find applications in soft robotics and biosensing.

Multi-pole magnetization of NdFeB magnetic elastomers via a programmable magnetic stamp inspired by movable type printing

Multi-pole magnetization is crucial for enhancing the magnetic field strength of magnetic films. Programmable multi-pole magnetization with adjustable pole pitch and covering thicknesses from submillimeter to millimeter are challenging tasks. This paper introduces a programmable magnetic stamp method inspired by movable printing. Demagnetization curves at various temperatures illustrate that NdFeB has lower coercivity when heated, making it easier to magnetize. SmCo magnets as high temperature resistant magnetic stamps provide enough external magnetic field. Heating transfers the magnetic pole distribution of magnetic stamps to the NdFeB film. Using this method, we magnetize the NdFeB magnetic film with a checkerboard pattern (interlaced N and S poles). The minimum width of the pole pitch is 1.3 mm. Comparing the measured magnetic field results with the simulation values of ANSYS Maxwell, the trends are relatively close. Furthermore, we investigate the magnetic field component (Bz) related to height and compare the differences between direct and reverse magnetization. Additionally, five magnetic patterns are customized using arranged SmCo magnet arrays. Notably, this study accomplishes multi-pole magnetization with up to 100 poles, covering a magnetic film thickness ranging from 100 µm to 1 mm. The proposed approach provides submillimeter/millimeter-thick NdFeB magnetic films that work as powerful flux sources and exhibit promising applications in magnetic tactile sensors and soft robots.

Dual-vat photopolymerization 3D printing of vitrimers

Herein, the processing of multi-material vitrimers with heterogenous properties along their x-,y- and z-axis is demonstrated using dual-vat digital light processing (DLP) 3D printing. The printer is based on a conventional vat exchange set-up containing two vats and a cleaning station, which are positioned on a linearly moving platform, and are exchanged automatically during the layer-by-layer build-up of the object. Two thiol-acrylate resins are selected bearing ample -OH and ester moieties, which undergo thermo-activated transesterification in the presence of an organic phosphate as catalyst. Based on the functionality of the acrylate monomers, printing of multi-material structures with flexible (σ = 1.1 MPa, ε = 14.0%) and rigid (σ = 33.3 MPa, ε = 4.4%) domains is feasible. A good interlayer adhesion between the soft and hard domains is evidenced by uniaxial tensile tests, whilst optical microscopy is used to study the interface. Dynamic mechanical analysis further shows that the stiffness of the photopolymers varies over several orders of magnitude (E’23 °C = 209 kPa - 507 MPa). Jacobs working curves of the two different resins reveal comparable printing parameters at higher exposure doses, which do not only facilitate the printing of multiple materials between individual layers (z-heterogeneity) but also within the same layer (x,y-heterogeneity). This is confirmed by the printing of a multi-material gripper as “proof of concept” demonstrator, which contains soft inner teeth and a stiff outer core. Gripping and releasing of objects is shown by exploiting the glass transition-based shape memory properties of the stiff domains. Moreover, due to the dynamic nature of the covalent bonds, damages inserted into the gripper are intrinsically repairable due to a thermo-activated macroscopic reflow. Shape memory experiments further confirmed that the multi-material gripper fully regains its function after the thermal mending process. Thus, the results clearly show that objects with a high degree of functionality can be realized by combining multi-material 3D printing with the chemistry of vitrimers. This is of particular interest for future soft robotic applications and for mimicking biological composite structures.

Active compliant mechanisms for optimized actuation by LCE-based artificial muscles

Compliant mechanisms leverage deformation of materials for precise, lightweight solution to transfer force and motion. By incorporating smart materials, active compliant mechanisms are formed with self-embedded actuation source. Here, we propose lightweight active compliant mechanisms that mimic biological joints, which are capable of transmitting displacements and forces from the linear contraction of artificial muscles into desired magnitudes and directions using a compact structural design. The artificial muscles are braided from liquid crystal elastomer (LCE) fibers and heating threads, competent of generating large force and stroke. Meanwhile, the compliant mechanisms are designed by topology optimization to imitate the functions of biological joints. Combining the braided artificial muscles and optimized compliant mechanisms, these active compliant mechanisms effectively achieve transmission of centralized linear actuation into multiple and various output motions, which is verified by both numerical simulations and experiments. Furthermore, we demonstrate that the active compliant mechanisms can be used for potential surgical procedures such as in-body grasping, cavity dilation, drug delivery, and suturing. The demonstrated biomimetic motions of the active compliant mechanisms hold significant potential for applications in medical devices and soft robots.

Liquid Metal Memory.

Storage systems are vital components of electronic devices, while significant challenges persist in achieving flexible memory due to the limitations of existing storage methodologies. Inspired by the polarization and depolarization mechanisms in the human brain, here a novel class of storage principles is proposed and achieve a fully flexible memory through introducing the oxidation and deoxidation behaviors of liquid metals. Specifically, reversible electrochemical oxidation is utilized to modulate the overall conductivity of the target liquid metals, creating a substantial 11-order resistance difference for binary data storage. To obtain the best storage performance, systematic optimizations of multiple parameters are conducted. Conceptual experiments demonstrate the memory's stability under extreme deformations (100% stretching, 180° bending, 360° twisting). Further tests reveal that the memory performs better when its unit size gets smaller, warranting superior integrability. Finally, a complete storage system achieves remarkable performance metrics, including rapid storage speed (>33Hz), long data retention capacity (>43200s), and stable repeatable operation (>3500 cycles). This groundbreaking method not only overcomes the inherent rigidity limitations of existing electronic storage units but also opens new possibilities for innovating neuromorphic devices, offering fundamental and practical avenues for future applications in soft robotics, wearable electronics, and bio-inspired artificial intelligence systems.

Graphite- and Graphene-Based Polymer Nanocomposites for Flexible Sensors and Actuators in Health Care and Soft Robotics Applications

Flexible sensors and actuators have broad applications in the fields of wearable electronics for health, sports, functional textiles, robotics and cobot applications. Graphene-or graphite-based polymer nanocomposites are promising materials for the development of soft sensors and actuators. This study investigates strain sensing properties of silicon rubber with various graphene filler concentrations (8wt%-12wt%). Current-voltage characteristics have been measured under various strains. We obtain that the sensor’s electrical resistance, for a given voltage, can be approximated by a linear fit of the logarithmic resistance as function of the extension ratio of the sensor. The obtained mechanically induced logarithmic resistor behavior of the polymer nanocomposite is highly promising for the development of electronic sensing and control. Furthermore, thin film graphite layers were investigated on highly stretchable silicone membranes.

Air-Working Electrochromic Artificial Muscles.

Artificial muscles are indispensable components for next-generation robotics to mimic the sophisticated movements of living systems and provide higher output energies when compared with real muscles. However, artificial muscles actuated by electrochemical ion injection have problems with single actuation properties and difficulties in stable operation in air. Here, air-working electrochromic artificial muscles (EAMs) with both color-changing and actuation functions are reported, which are constructed based on vanadium pentoxide nanowires and carbon tube yarn. Each EAM can generate a contractile stroke of ≈12% during stable operation in the air with multiple color changes (yellow-green-gray) under ±4V actuation voltages. The reflectance contrast is as high as 51%, demonstrating the excellent versatility of the EAMs. In addition, a torroidal EAM arrangement with fast response and high resilience is constructed. The EAM's contractile stroke can be displayed through visual color changes, which provides new ideas for future artificial muscle applications in soft robots and artificial limbs.

Magneto-active composites with locally tailored stiffness produced by laser powder bed fusion

Additive manufacturing technologies enable the production of complex and bioinspired shapes using magneto-responsive materials, which find diverse applications in soft robotics. Particularly, the development of composites with controlled gradients in mechanical properties offers new prospects for advancements in magneto-active materials. However, achieving such composites with gradients typically involves complex multi-material printing procedures. In this study, a single-step laser powder bed fusion (LPBF) process is proposed that enables precise local adjustments of the mechanical stiffness within magneto-active composites. By utilizing distinct laser parameters in specific regions of a composite containing thermoplastic polyurethane and atomized magnetic powder derived from hard magnetic Nd-Fe-B, the stiffness of the composite can be modified within the range of 2–22 MPa. Various magneto-responsive actuators with locally tailored stiffness are fabricated and their magnetic performance is investigated. The enhanced response exhibited by actuators with locally adjusted mechanical properties in comparison to their homogeneous counterparts with identical geometries is shown. As a demonstration of a biomedical application, a magnetically responsive stent with localized adjustment is presented with the ability to meet specific requirements in terms of geometry and local stiffness based on an individual's anatomy and disease condition. The proposed method presents an approach for creating functionally graded materials using LPBF, not only for magneto-active materials but also for several other structural and functional materials.

Self-Rotation of Electrothermally Responsive Liquid Crystal Elastomer-Based Turntable in Steady-State Circuits.

Self-excited motions, characterized by their ability to harness energy from a consistent environment and self-regulate, exhibit significant potential in micro-devices, autonomous robotics, sensor technology, and energy generation. This study introduces an innovative turntable system based on an electrothermally responsive liquid crystal elastomer (LCE). This system facilitates self-rotation within a steady-state circuit. Employing an electrothermal LCE model, we have modeled and numerically analyzed the nonlinear dynamics of an LCE-rope within steady-state circuits, utilizing the four-order Runge-Kutta method for calculations. The numerical results reveal the emergence of two distinct motion patterns in the turntable system under steady-state conditions: a self-rotation pattern and a static pattern. The self-rotation is initiated when the system's absorbed energy surpasses the energy lost due to damping effects. Furthermore, this paper delves into the critical conditions necessary for initiating self-rotation and examines the influence of various key dimensionless parameters on the system's rotation amplitude and frequency. These parameters include gravitational acceleration, the initial position of the mass ball, elastic stiffness of the LCE and spring, limiting temperature, heating zone angle, thermal shrinkage coefficient, and damping factor. Our computational findings establish that these parameters exert a modulatory impact on the rotation amplitude and period. This research enhances the understanding of self-excited motions and offers promising avenues for applications in energy harvesting, monitoring, soft robotics, medical devices, and micro- and nano-devices.

Electrospun (K,Na)NbO3 Piezoceramic Fibers for Self‐Powered Tactile Sensing Application

This study investigates the electrical properties of electrospun ceramic fiber mats based on potassium sodium niobate ((K,Na)NbO3;KNN) for potential applications in soft‐robotic tactile sensors. The rheological studies of electrospinning solutions exhibit shear thinning behavior, with a viscosity plateau between shear rates of 1–100 s−1. The thermal decomposition behavior of the KNN samples is analyzed using thermogravimetric analysis; the samples show multistep decomposition processes leading into a phase pure perovskite structure. For the electrospun KNN fiber mats, the electrical characterization reveals ferroelectric hysteresis loops with a high leakage current of ≈270 nA at 20 kV cm−1 and switching current behavior. The ferroelectric results indicate that a full polarization of KNN‐fiber mats is not possible due to their electrical breakdown at high electrical fields. The integration of KNN short fibers into a soft‐bending actuator demonstrates successful tactile sensing capabilities. The experimental findings reported here highlight the potential of electrospun KNN fiber mats for flexible and autonomous device applications in soft robotics.

Squid-Inspired Powerful Untethered Soft Pumps via Magnetically Induced Phase Transitions.

Soft robots possess unique deformability and hence result in great adaptability to various unconstructive environments; meanwhile, untethered soft actuation techniques are critical in fully exploiting their potential for practical applications. However, restricted by the material's softness and structural compliance, most untethered actuation systems were incapable of achieving fully soft construction with a powerful output. While in Nature, with a fully soft body, a squid can burst high-pressure jet flow from a cavity that drives the squid to swim swiftly. Here, inspired by such a unique actuation strategy of squids, an entirely soft pump capable of high-pressure output, fast jetting, and untethered control is presented, and it helps a bionic soft robotic squid to achieve a high-efficient untethered motion in water. The soft pump is designed by a reversible liquid-gas phase transition of an inductive heating magnetic liquid metal composite that acts as an adjustable power source with high heat efficiency. In particular, being purely soft, the pump can yet lift ∼20 times its weight and achieve ∼3 times the specific pressure of the previous record. It may promote the application of soft robots with independent actuation, high output power, and embodied energy supply.

Mechanics of hard-magnetic soft materials: A review

Hard-magnetic soft materials are a class of magnetically responsive composites obtained by embedding hard-magnetic particles into a soft polymeric matrix. They have found widespread applications in shape-morphing systems, soft robotics, biomedical devices, and active metamaterials due to their ability to feature complex, untethered, reversible, and rapid deformations in response to magnetic loads. To guide the rational design of these functional applications, extensive efforts have been devoted to studying the mechanical behavior of hard-magnetic soft materials. In this paper, we review the recent progress in the mechanics of hard-magnetic soft materials. First, we introduce existing constitutive models capable of describing the coupled magneto-elastic deformations of hard-magnetic soft materials. Then, we discuss the mechanical response of structures made of hard-magnetic soft materials, including rods, beams, plates, and shells, under mechanical and magnetic loading. Subsequently, we introduce the design and behavior of magneto-mechanical metamaterials with tunable properties enabled by hard-magnetic soft materials. In addition, optimization-guided inverse design strategies for hard-magnetic soft materials to achieve predefined properties or deformations are also briefly reviewed. Finally, we provide our views on the potential future directions in the field of mechanics of hard-magnetic soft materials. We expect the current review to guide researchers to better understand different theoretical and computational frameworks of mechanics of hard-magnetic soft materials and thus aid with designing functional systems using these materials for various applications.

Kinematic analysis of a finger-mimicking pneumatic network actuator consisting of parallelogram-shaped chambers

Fast pneumatic network (fast Pneu-Net, fPN) actuators made of silicone materials have aroused increasing attention these years for their promising applications in soft robotics. To enrich current research, we present a novel fPN actuator that consists of parallelogram-shaped chambers and aluminum endoskeleton that takes the shape of human finger. The proposed actuator can bend at multi-positions when inflated, bringing about more degrees of freedom controllability. Additionally, the performed finite element analysis indicates that the parallelogram-shaped chamber that tilts 30° towards the distal end exhibits an increase in deformation for about 11 % compared to the rectangle-shaped counterpart, offering an improved bending performance. An analytical model that based on the pressurized hyperelastic membrane theory is developed to estimate the actuator’s bending angles/profiles at varied pressures. Geometric relation of the chamber interactions upon deformation is discussed depending on whether the lateral walls are contact or not. The stress-strain relation of the elastomer is described using 3-term Yeoh constitutive model. Experimental validations on the theoretical calculations are carried out, from which the coefficient of proportionality that takes into account the effect on deformation caused by the embedded endoskeleton is determined. Furthermore, the Modified Denavit-Hartenberg (MDH) notation is employed to establish the forward kinematics of the actuator, and the numerical calculations are compared with the physical actuator.

High-Stroke, High-Output-Force, Fabric-Lattice Artificial Muscles for Soft Robots.

Artificial muscles, providing safe and close interaction between humans and machines, are essential in soft robotics. However, their insufficient deformation, output force, or configurability usually limits their applications. Herein, this work presents a class of lightweight fabric-lattice artificial muscles (FAMs) that are pneumatically actuated with large contraction ratios (up to 87.5%) and considerable output forces (up to a load of 20kg, force-to-weight ratio of over 250). The developed FAMs consist of a group of active air chambers that are zigzag connected into a lattice through passive connecting layers. The geometry of these fabric components is programmable to convert the in-plane lattice of FAMs into out-of-plane configurations (e.g., arched and cylindrical) capable of linear/radial contraction. This work further demonstrates that FAMs can be configured for various soft robotic applications, including the powerful robotic elbow with large motion range and high load capability, the well-fitting assistive shoulder exosuit that can reduce muscle activity during abduction, and the adaptive soft gripper that can grasp irregular objects. These results show the unique features and broad potential of FAMs for high-performance soft robots.

Multi-modal deformation and temperature sensing for context-sensitive machines

Owing to the remarkable properties of the somatosensory system, human skin compactly perceives myriad forms of physical stimuli with high precision. Machines, conversely, are often equipped with sensory suites constituted of dozens of unique sensors, each made for detecting limited stimuli. Emerging high degree-of-freedom human-robot interfaces and soft robot applications are delimited by the lack of simple, cohesive, and information-dense sensing technologies. Stepping toward biological levels of proprioception, we present a sensing technology capable of decoding omnidirectional bending, compression, stretch, binary changes in temperature, and combinations thereof. This multi-modal deformation and temperature sensor harnesses chromaticity and intensity of light as it travels through patterned elastomer doped with functional dyes. Deformations and temperature shifts augment the light chromaticity and intensity, resulting in a one-to-one mapping between stimulus modes that are sequentially combined and the sensor output. We study the working principle of the sensor via a comprehensive opto-thermo-mechanical assay, and find that the information density provided by a single sensing element permits deciphering rich and diverse human-robot and robot-environmental interactions.

3D knit pneumatic actuators for wearable haptic displays

Weft knit textiles are growing in popularity as materials for soft robotic applications due to their scalability and near infinite mechanical tunability. We leverage the benefits of 3D weft knitting to develop a novel method of manufacturing small scale (5-8 mm) pneumatic actuators that can be embedded across a distributed textile surface. Weft knitting (i.e., intarsia) enables seamless transitions between contrasting yarns and textile structures, which increases manufacturing resolution for small-scale actuators. Additionally, weft knitting enables mechanical property tunability beyond what is available off the shelf, enabling customizable actuation behavior. We demonstrate that 3D knit textile actuation properties, such as blocking force, free displacement, and bending stiffness, can be tuned by manipulating actuator design parameters, including (1) yarn tensile stiffness, (2) textile structural strain, (3) local textile geometry, and (4) input pressure. To demonstrate the manufacturability and scalability within a 3D wearable form factor, we build a haptic glove prototype with surface embedded actuation to mimic the sensation of touching and grasping virtual objects.

Quality analysis of material jetted silicone material for soft robotics application

Silicone and silicone-based elastomers find their applications in micro-electromechanical systems (MEMS) and soft robotics. Mechanical properties of such materials upon processing them with additive manufacturing are challenging and need significant improvement for enhanced performance in biomedical applications. With extrusion-based additive manufacturing, there is an inherent porosity inside the printed volume which negatively influences the quality and mechanical properties of printed parts. In this study, material jetting a drop-on-demand additive manufacturing technique is used to 3D print silicone samples of shore hardness 60A. These samples are analysed for tensile strength, surface roughness, porosity, and pores characterization for assessing the quality and performance of silicone 60A and the results suggest that silicone 60A material used in material jetting printing process is a potential material for low force application in soft robotics.

High-performance electroionic artificial muscles boosted by superior ion transport with Ti3C2Tx MXene/Cellulose nanocomposites for advanced 3D-motion actuation

Low-voltage driven electro-ionic soft actuators with large deformation strain are of promising candidate for high-performance artificial muscles, with practical applications in flexible electronics, biomedical assistive devices and soft robotics. However, enhancing the actuation response and long-term stability along with excellent bending properties simultaneously still remain significant challenges. Herein, an elaborate ionically crosslinked nanocomposite membrane, assembled with 1D bacterial cellulous (BC) nanofibers and 2D Ti3C2Tx MXene nanomaterials, is reported to improve constrained ion transport for high-performance electrically activated ionic actuator. The 3D interpenetrating frameworks, arising from the synergistic effects of such combination, not only serve to enhance ion selectivity through the presence of intrinsically negatively charged functional groups but also expand the initially confined pathways, thus diminishing the energy barrier to ion transport. As a result, the integrated superior functionalities of the soft actuator exhibit large bending strain of 0.88 % with the tip displacement of 10.8 mm, ultra-stable cycling performance (>99 % retention for 5 h), and ultrabroad material response range up to 18 Hz under an ultralow voltage exertion of 0.5 V. Furthermore, the demonstration of the prototype soft actuators as actuating tracking beams of 3D-adaptive motion platform for plate deformable mirror shows their potential applications of soft robotics, thereby offering a dependable solution for driving the next generation of sophisticated intelligent systems.

Mesoscopic Supramolecular Assembly of Stereolithographically Printed Microgels

AbstractMicrogels that assemble upon issuing of a trigger open up miscellaneous applications in soft robotics, drug delivery, and life‐like materials, due to their unique physical and chemical properties. In this work, a novel hydrogel photoresist system is formulated, consisting of 2‐hydroxyethyl acrylate, ethylene glycol diacrylate, and phenyl bis(2,4,6‐trimethylbenzoyl) phosphine oxide, as well as redox‐sensitive supramolecular host‐guest association motifs. Complex and multi‐compartmented geometries with precisely located recognition motifs are printed in high resolution, resulting in multifunctional structured microgel building blocks for mesoscopic self‐assembly. An enzymatic feedback system is employed to dissipatively assemble and disassemble the redox‐active microgel building blocks in a temporally and spatially controlled pathway. The self‐assembling photoresist system is characterized in terms of the lower critical solution temperature, swelling behavior, and printability. The present work provides new perspectives for the design of tailor‐made mesoscopic microgels with customizable functionality and high applicability for various technological applications.

Morphing matter: from mechanical principles to robotic applications

The adaptability of natural organisms in altering body shapes in response to the environment has inspired the development of artificial morphing matter. These materials encode the ability to transform their geometrical configurations in response to specific stimuli and have diverse applications in soft robotics, wearable electronics, and biomedical devices. However, achieving the morphing of intricate three-dimensional shapes from a two-dimensional flat state is challenging, as it requires manipulations of surface curvature in a controlled manner. In this review, we first summarize the mechanical principles extensively explored for realizing morphing matter, both at the material and structural levels. We then highlight its applications in the soft robotics field. Moreover, we offer insights into the open challenges and opportunities that this rapidly growing field faces. This review aims to inspire researchers to uncover innovative working principles and create multifunctional morphing matter for various engineering fields.