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 20 kg, 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.

Guoying Gu.

Guoying Gu designs bioinspired soft robots, with applications including soft wearable rehabilitation robots, bioinspired robotic systems for grasping and manipulation, and soft sensing systems for human-robot interaction. He discusses the inherently multi-disciplinary nature of soft robotics research and particularly the deep back-and-forth connection with neuroscience.

X-crossing pneumatic artificial muscles.

Artificial muscles are promising in soft exoskeletons, locomotion robots, and operation machines. However, their performance in contraction ratio, output force, and dynamic response is often imbalanced and limited by materials, structures, or actuation principles. We present lightweight, high-contraction ratio, high-output force, and positive pressure-driven X-crossing pneumatic artificial muscles (X-PAMs). Unlike PAMs, our X-PAMs harness the X-crossing mechanism to directly convert linear motion along the actuator axis, achieving an unprecedented 92.9% contraction ratio and an output force of 207.9 Newtons per kilogram per kilopascal with excellent dynamic properties, such as strain rate (1603.0% per second), specific power (5.7 kilowatts per kilogram), and work density (842.9 kilojoules per meter cubed). These properties can overcome the slow actuation of conventional PAMs, providing robotic elbow, jumping robot, and lightweight gripper with fast, powerful performance. The robust design of X-PAMs withstands extreme environments, including high-temperature, underwater, and long-duration actuation, while being scalable to parallel, asymmetric, and ring-shaped configurations for potential applications.

Dexterous electrical-driven soft robots with reconfigurable chiral-lattice foot design.

Dexterous locomotion, such as immediate direction change during fast movement or shape reconfiguration to perform diverse tasks, are essential animal survival strategies which have not been achieved in existing soft robots. Here, we present a kind of small-scale dexterous soft robot, consisting of an active dielectric elastomer artificial muscle and reconfigurable chiral-lattice foot, that enables immediate and reversible forward, backward and circular direction changes during fast movement under single voltage input. Our electric-driven soft robot with the structural design can be combined with smart materials to realize multimodal functions via shape reconfigurations under the external stimulus. We experimentally demonstrate that our dexterous soft robots can reach arbitrary points in a plane, form complex trajectories, or lower the height to pass through a narrow tunnel. The proposed structural design and shape reconfigurability may pave the way for next-generation autonomous soft robots with dexterous locomotion.

3D printable high-performance conducting polymer hydrogel for all-hydrogel bioelectronic interfaces.

Owing to the unique combination of electrical conductivity and tissue-like mechanical properties, conducting polymer hydrogels have emerged as a promising candidate for bioelectronic interfacing with biological systems. However, despite the recent advances, the development of hydrogels with both excellent electrical and mechanical properties in physiological environments is still challenging. Here we report a bi-continuous conducting polymer hydrogel that simultaneously achieves high electrical conductivity (over 11 S cm-1), stretchability (over 400%) and fracture toughness (over 3,300 J m-2) in physiological environments and is readily applicable to advanced fabrication methods including 3D printing. Enabled by these properties, we further demonstrate multi-material 3D printing of monolithic all-hydrogel bioelectronic interfaces for long-term electrophysiological recording and stimulation of various organs in rat models.

Soft Robotics Enables Neuroprosthetic Hand Design.

Development and implementation of neuroprosthetic hands is a multidisciplinary field at the interface between humans and artificial robotic systems, which aims at replacing the sensorimotor function of the upper-limb amputees as their own. Although prosthetic hand devices with myoelectric control can be dated back to more than 70 years ago, their applications with anthropomorphic robotic mechanisms and sensory feedback functions are still at a relatively preliminary and laboratory stage. Nevertheless, a recent series of proof-of-concept studies suggest that soft robotics technology may be promising and useful in alleviating the design complexity of the dexterous mechanism and integration difficulty of multifunctional artificial skins, in particular, in the context of personalized applications. Here, we review the evolution of neuroprosthetic hands with the emerging and cutting-edge soft robotics, covering the soft and anthropomorphic prosthetic hand design and relating bidirectional neural interactions with myoelectric control and sensory feedback. We further discuss future opportunities on revolutionized mechanisms, high-performance soft sensors, and compliant neural-interaction interfaces for the next generation of neuroprosthetic hands.

A soft neuroprosthetic hand providing simultaneous myoelectric control and tactile feedback.

Neuroprosthetic hands are typically heavy (over 400 g) and expensive (more than US$10,000), and lack the compliance and tactile feedback of human hands. Here, we report the design, fabrication and performance of a soft, low-cost and lightweight (292 g) neuroprosthetic hand that provides simultaneous myoelectric control and tactile feedback. The neuroprosthesis has six active degrees of freedom under pneumatic actuation, can be controlled through the input from four electromyography sensors that measure surface signals from residual forearm muscles, and integrates five elastomeric capacitive sensors on the fingertips to measure touch pressure so as to enable tactile feedback by eliciting electrical stimulation on the skin of the residual limb. In a set of standardized tests performed by two individuals with transradial amputations, we show that the soft neuroprosthetic hand outperforms a conventional rigid neuroprosthetic hand in speed and dexterity. We also show that one individual with a transradial amputation wearing the soft neuroprosthetic hand can regain primitive touch sensation and real-time closed-loop control.

High-Stretchability, Ultralow-Hysteresis ConductingPolymer Hydrogel Strain Sensors for Soft Machines.

Highly stretchable strain sensors based on conducting polymer hydrogel are rapidly emerging as a promising candidate toward diverse wearable skins and sensing devices for soft machines. However, due to the intrinsic limitations of low stretchability and large hysteresis, existing strain sensors cannot fully exploit their potential when used in wearable or robotic systems. Here, a conducting polymer hydrogel strain sensor exhibiting both ultimate strain (300%) and negligible hysteresis (<1.5%) is presented. This is achieved through a unique microphase semiseparated network design by compositing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) nanofibers with poly(vinyl alcohol) (PVA) and facile fabrication by combining 3D printing and successive freeze-thawing. The overall superior performances of the strain sensor including stretchability, linearity, cyclic stability, and robustness against mechanical twisting and pressing are systematically characterized. The integration and application of such strain sensor with electronic skins are further demonstrated to measure various physiological signals, identify hand gestures, enable a soft gripper for objection recognition, and remote control of an industrial robot. This work may offer both promising conducting polymer hydrogels with enhanced sensing functionalities and technical platforms toward stretchable electronic skins and intelligent robotic systems.

Muscle-fiber array inspired, multiple-mode, pneumatic artificial muscles through planar design and one-step rolling fabrication.

Advances in development of artificial muscles have enabled creation of soft robots with biological dexterity and self-adaption in unstructured environments; however, production of scalable artificial muscles with multiple-mode actuations remains elusive. Inspired by muscle-fiber arrays in muscular hydrostats, we present a class of versatile artificial muscles called MAIPAMs (muscle-fiber array inspired pneumatic artificial muscles), capable of multiple-mode actuations (such as parallel elongation-bending-spiraling actuations, 10 parallel bending actuations and cascaded elongation-bending-spiraling actuations). Our MAIPAMs consist of active 3D elastomer-balloon arrays reinforced by a passive elastomer membrane, achieved through a planar design and one-step rolling fabrication approach. We introduce prototypical designs for the MAIPAMs and demonstrate their muscle-mimic structures and versatility, as well as their scalable ability to integrate flexible but non-stretchable layers for contraction and twisting actuation modes and compliant electrodes for self-sensing. We further demonstrate that this class of artificial muscles shows potential for versatile robotic applications, such as carrying a camera for recording videos, gripping or manipulating objects, and climbing a pipe-line.

Cutaneous Ionogel Mechanoreceptors for Soft Machines, Physiological Sensing, and Amputee Prostheses.

Touch sensing has a central role in robotic grasping and emerging human-machine interfaces for robot-assisted prosthetics. Although advancements in soft conductive polymers have promoted the creation of diverse pressure sensors, these sensors are difficult to be employed as touch skins for robotics and prostheses due to their limited sensitivity, narrow pressure range, and complex structure and fabrication process. Here, a highly sensitive and robust soft touch skin is presented with ultracapacitive sensing that combines ionic hydrogels with commercially available conductive fabrics. Prototypical designs of the capacitive sensors through facile manufacturing methods are introduced and a high sensitivity up to 1.5 kPa-1 (≈44 times higher than conventional parallel-plate capacitive counterparts), a broad pressure detection range of over four orders of magnitudes (≈35 Pa to 330 kPa), ultrahigh baseline of capacitance, fast response time (≈18 ms), and good repeatability are demonstrated. Ionogel skins composed of an array of cutaneous mechanoreceptors capable of monitoring various physiological signals and shape detection are further developed. The touch skin can be integrated within a soft bionic hand and provide an industrial robot and an amputee with robust tactile feedback when handling delicate objects, illustrating its potential applications in next-generation human-in-the-loop robotic systems with tactile sensing.

Analytical Modeling and Design of Generalized Pneu-Net Soft Actuators with Three-Dimensional Deformations.

Pneu-net soft actuators, consisting of pneumatic networks of small chambers embedded in elastomeric structures, are particularly promising candidates in the society of soft robotics. However, there are few studies on the analytical modeling of pneu-net soft actuators, especially in the three-dimensional space. In this article, based on the minimum potential energy method and the continuum rod theory, we propose an analytical model and corresponding design approach for a class of generalized pneu-net soft actuators (gPNSAs) with both bending and twisting deformations by combining the geometric complexity and material elasticity. We experimentally verify our modeling approach and finally investigate the effects of geometric parameters, material properties, and external force on the deformations of gPNSAs, which can be used as a tool for the design of gPNSAs. We further demonstrate that our developed model can predict the deformations of gPNSAs made of multiple materials.

Biomimetic Impact Protective Supramolecular Polymeric Materials Enabled by Quadruple H-Bonding.

Nature has been inspiring scientists to fabricate impact protective materials for applications in various aspects. However, it is still challenging to integrate flexible, stiffness-changeable, and protective properties into a single polymer, although these merits are of great interest in many burgeoning areas. Herein, we report an impact-protective supramolecular polymeric material (SPM) with unique impact-hardening and reversible stiffness-switching characteristics by mimicking sea cucumber dermis. The emergence of softness-stiffness switchability and subsequent protective properties relies on the dynamic aggregation of the nanoscale hard segments in soft transient polymeric networks modulated by quadruple H-bonding. As such, we demonstrate that our SPM could efficiently reduce the impact force and increase the buffer time of the impact. Importantly, we elucidate the underlying mechanism behind the impact hardening and energy dissipation in our SPM. Based on these findings, we fabricate impact- and puncture-resistant demos to show the potential of our SPM for protective applications.

Stimuli-responsive functional materials for soft robotics.

Functional materials have spurred the advancement of soft robotics with the potential to perform safe interactions and adaptative functions in unstructured environments. The responses of functional materials under external stimuli lend themselves to programmable actuation and sensing, opening up new possibilities of robot design with built-in mechanical intelligence and unlocking new applications. Here, we review the development of stimuli-responsive functional materials particularly used for soft robotic systems. This review covers five representative types of soft stimuli-responsive functional materials, namely (i) dielectric elastomers, (ii) hydrogels, (iii) shape memory polymers, (iv) liquid crystal elastomers, and (v) magnetic materials, with focuses on their inherent material properties, working mechanisms, and design strategies for actuation and sensing. We also highlight the state-of-the-art applications of soft stimuli-responsive functional materials in locomotion robots, grippers and sensors. Finally, we summarize the current challenges and map out future trends for engineering next-generation functional materials for soft robotics.

Adaptive Variable Stiffness Particle Phalange for Robust and Durable Robotic Grasping.

Grasping is an important characteristic of robots in interacting with humans and the environment. Due to the inherent compliance of soft grippers, they can easily adapt to novel objects and operate safely in a human-centered environment. However, soft hands suffer from poor grasping robustness and operation durability, especially for heavy objects or objects with sharp spikes, mainly due to their fragile material and low structural stiffness of the soft actuators. Thus, the widespread use of soft hands in daily applications is still limited. Existing works have shown a promising direction to enhance grasping performance by solving the contradiction between inherent compliance/adaptability and loading capacity. It is known that the stiffness of the robotic phalange is highly related to the performance of robotic hands. In this article, we propose a novel variable stiffness particle phalange, called VSPP here. The proposed VSPP exhibits variable stiffness characteristics without the need for dedicated actuation by utilizing passive particle jamming resulted from forces in interacting with the environment. The VSPP can cooperate with any kind of actuators, soft or rigid, to function as a compliant and robust robotic hand. A prototype robotic hand based on VSPP could maintain reliable grasping even when pierced by sharp objects such as a needle, a cactus, and a durian. This durability is effective both in air and underwater, thus presents new possibilities for the soft robotic hand to work in a harsh environment. The inherent multidirectional compliance of the VSPP makes safety in human/robot interaction guaranteed. The design and modeling presented in this research will provide useful guidance in VSPP applications. A prototype gripper, VSPP-3, composed of three 2-segments VSPP fingers and pneumatic joints, has been built for demonstrations in reliable and robust grasping of daily objects. The sample grasping has shown that the proposed VSPP has great potential for a robust and durable soft robotic hand or gripper design.

Design of 3D Printed Programmable Horseshoe Lattice Structures Based on a Phase-Evolution Model.

By 3D printing lattice structure with active materials, the structures can exhibit shape and functional changes under external stimulus. However, the programmable shape changes of the 3D printed lattice structures are limited due to the complex geometries, nonlinear behaviors of the active materials, and the diverse external stimuli. In this work, we propose a design framework combining experiments, theoretical modeling, and finite element simulations for the controllable shape changes of the 3D printed horseshoe under thermal stimulus. The theoretical model is based on a phase evolution model that combines the geometrical nonlinearity and the material nonlinearity. Results show that the shapes with positive or negative Poisson's ratio and bending intermediate shapes can be programmed by tuning the geometrical parameters and the temperature distribution. This work provides a method to aid the design of 3D printed functional lattice structures and have potential applications in soft robotics, biomedicine, and energy absorbing fields.

Design, Modeling, and Evaluation of Fabric-Based Pneumatic Actuators for Soft Wearable Assistive Gloves.

Textile fabrics are compliant, lightweight, and inherently anisotropic, making them promising for the design of soft pneumatic actuators. In this article, we present the design, modeling, and evaluation of a class of soft fabric-based pneumatic actuators (SFPAs) for soft wearable assistive gloves that can simultaneously assist the thumb abduction and finger flexion and extension motions for brachial plexus injury patients. We investigate the mechanical behaviors of various woven fabrics and rib weft-knitted fabric structures, guiding us to design a thumb-abduction SFPA, a finger-flexion SFPA, and a finger-extension SFPA. We further develop a mathematical model to evaluate the influence of the geometric parameters on the blocked tip forces of the finger-flexion SFPAs and extension torques of the finger-extension SFPAs, which are also verified by the experimental results. We then integrate our SFPAs into a soft wearable assistive glove with a portable control system. The glove is finally tested on a healthy volunteer and a brachial plexus injury patient. The clinical evaluation results demonstrate the effectiveness of our designed glove in assisting hand motions and grasping tasks.

Trimethyltriazine-derived olefin-linked covalent organic framework with ultralong nanofibers.

Two-dimensional (2D) olefin-linked covalent organic frameworks (COFs) with excellent π-electron communication and high stability are emerging as promising crystalline polymeric materials. However, because of the limited species of COFs, their characteristics, processability and potential applications have not been completely understood and explored. In this work, we prepared two novel olefin-linked 2D COFs through Knoevenagel condensation of 2,4,6-trimethyl-1,3,5-triazine with tritopic triazine-cored aldehydes. The resulting COFs exhibit highly crystalline honeycomb-like structures stacked from hexagonal-latticed polymeric layers and display well-defined nanofibrillar morphologies with the uniform diameters of ca. 80 nm and ultra-lengths up to several micrometers. Such COF nanofibers can be readily composited with carbon nanotubes into high-quality continuous thin films, which are further compacted by a typical hot-pressing process to enhance their densities and mechanical strength without changing their fibrous microstructures. Such film-fabricated interdigital microelectrodes and the ionogel electrolyte are assembled into planar micro-supercapacitors (MSCs), which exhibit an outstanding areal capacitance of 44.3 mF cm-2, large operating voltage window of 2.5 V, high volumetric energy density of 38.5 mWh cm-3 as well as excellent cycling stability.

Mechanical Effect on Gene Transfection Based on Dielectric Elastomer Actuator.

Gene transfection has been widely applied in genome function and gene therapy. Although many efforts have been focused on designing carrier materials and transfection methods, the influence of mechanical stimulation on gene transfection efficiency has rarely been studied. Herein, dielectric elastomer actuator (DEA)-based stimulation bioreactors are designed to generate tensile and contractile stress on cells simultaneously. With the example of the EGFP transfection, cells with high membrane tension in the stretching stimulation regions had lower transfection efficiency, while the transfection efficiency of cells in the compressing regions tended to increase. Besides, the duty cycle and loading frequency of the applied stress on cells were also important factors that affect gene transfection efficiency. Furthermore, the pathways of cell endocytosis with the effect of mechanical stimulation were explored on the mechanism for the change of EGFP transfection efficiency. This design of the DEA-based bioreactor, as a strategy to study gene transfection efficiency, could be helpful for developing efficient transfection methods.

Stretchable elastic synaptic transistors for neurologically integrated soft engineering systems.

Artificial synaptic devices that can be stretched similar to those appearing in soft-bodied animals, such as earthworms, could be seamlessly integrated onto soft machines toward enabled neurological functions. Here, we report a stretchable synaptic transistor fully based on elastomeric electronic materials, which exhibits a full set of synaptic characteristics. These characteristics retained even the rubbery synapse that is stretched by 50%. By implementing stretchable synaptic transistor with mechanoreceptor in an array format, we developed a deformable sensory skin, where the mechanoreceptors interface the external stimulations and generate presynaptic pulses and then the synaptic transistors render postsynaptic potentials. Furthermore, we demonstrated a soft adaptive neurorobot that is able to perform adaptive locomotion based on robotic memory in a programmable manner upon physically tapping the skin. Our rubbery synaptic transistor and neurologically integrated devices pave the way toward enabled neurological functions in soft machines and other applications.