Reconfigurable origami with variable stiffness joints for adaptive robotic locomotion and grasping

With its compactness and foldability, origami has recently been applied to robotic systems to enable versatile robots and mechanisms while maintaining a low weight and compact form. This work investigates how to generate different motions and shapes for origami by tuning its creases' stiffness on the fly. The stiffness tuning is realized by a composite material made by sandwiching a thermoplastic layer between two shape memory polymer layers. This enables the composite to act as a living hinge, whose stiffness can be actively controlled through Joule heating. To demonstrate our concept, we fabricate an origami module with four variable stiffness joints (VSJs), allowing it to have freely controlled crease stiffnesses across its surface. We characterize the origami module's versatile motion when heating different VSJs with different temperatures. We further use two origami modules to build a two-legged robot that can locomote on the ground with different locomotion gaits. The same robot is also used as an adaptive gripper for grasping tasks. Our work can potentially enable more versatile robotic systems made from origami as well as other mechanical systems with programmable properties (e.g. mechanical metamaterials).This article is part of the theme issue 'Origami/Kirigami-inspired structures: from fundamentals to applications'.

Stiffness Change for Reconfiguration of Inflated Beam Robots

Abstract Active control of the shape of soft robots is challenging. Despite having an infinite number of passive degrees of freedom (DOFs), soft robots typically only have a few actively controllable DOFs, limited by the number of degrees of actuation (DOAs). The complexity of actuators restricts the number of DOAs that can be incorporated into soft robots. Active shape control is further complicated by the buckling of soft robots under compressive forces; this is particularly challenging for compliant continuum robots due to their long aspect ratios. In this study, we show how variable stiffness enables shape control of soft robots by addressing these challenges. Dynamically changing the stiffness of sections along a compliant continuum robot selectively "activates" discrete joints. By changing which joints are activated, the output of a single actuator can be reconfigured to actively control many different joints, thus decoupling the number of controllable DOFs from the number of DOAs. We demonstrate embedded positive pressure layer jamming as a simple method for stiffness change in inflated beam robots, its compatibility with growing robots, and its use as an "activating" technology. We experimentally characterize the stiffness change in a growing inflated beam robot and present finite element models that serve as guides for robot design and fabrication. We fabricate a multisegment everting inflated beam robot and demonstrate how stiffness change is compatible with growth through tip eversion, enables an increase in workspace, and achieves new actuation patterns not possible without stiffening.

Active Materials for Functional Origami

In recent decades, origami has been explored to aid in the design of engineering structures. These structures span multiple scales and have been demonstrated to be used toward various areas such as aerospace, metamaterial, biomedical, robotics, and architectural applications. Conventionally, origami or deployable structures have been actuated by hands, motors, or pneumatic actuators, which can result in heavy or bulky structures. On the other hand, active materials, which reconfigure in response to external stimulus, eliminate the need for external mechanical loads and bulky actuation systems. Thus, in recent years, active materials incorporated with deployable structures have shown promise for remote actuation of light weight, programmable origami. In this review, active materials such as shape memory polymers (SMPs) and alloys (SMAs), hydrogels, liquid crystal elastomers (LCEs), magnetic soft materials (MSMs), and covalent adaptable network (CAN) polymers, their actuation mechanisms, as well as how they have been utilized for active origami and where these structures are applicable is discussed. Additionally, the state-of-the-art fabrication methods to construct active origami are highlighted. The existing structural modeling strategies for origami, the constitutive models used to describe active materials, and the largest challenges and future directions for active origami research are summarized.

Bioinspiration and Biomimetic Art in Robotic Grippers

The autonomous manipulation of objects by robotic grippers has made significant strides in enhancing both human daily life and various industries. Within a brief span, a multitude of research endeavours and gripper designs have emerged, drawing inspiration primarily from biological mechanisms. It is within this context that our study takes centre stage, with the aim of conducting a meticulous review of bioinspired grippers. This exploration involved a nuanced classification framework encompassing a range of parameters, including operating principles, material compositions, actuation methods, design intricacies, fabrication techniques, and the multifaceted applications into which these grippers seamlessly integrate. Our comprehensive investigation unveiled gripper designs that brim with a depth of intricacy, rendering them indispensable across a spectrum of real-world scenarios. These bioinspired grippers with a predominant emphasis on animal-inspired solutions have become pivotal tools that not only mirror nature's genius but also significantly enrich various domains through their versatility.

Stiffness Variable Polymer for Soft Actuators with Sharp Stiffness Switch and Fast Response

Stiffness variable polymers are an essential family of materials that have aroused considerable attention in soft actuators. Although lots of strategies have been proposed to achieve variable stiffness, it remains a formidable challenge to achieve a polymer with a wide stiffness range and fast stiffness change. Herein, a series of variable stiffness polymers with a fast stiffness change and wide stiffness range were successfully synthesized, and the formulas were optimized via Pearson correlation tests. The rigid/soft stiffness ratio of the designed polymer samples can reach up to 1376-folds. Impressively, owing to the phase-changing side chains, the narrow endothermic peak can be observed with full width at half-maximum within 5 °C. Moreover, the shape memory properties of the shape fixity (R_f) and shape recovery ratio (R_r) values of the shape memory properties could reach up to 99.3 and 99.2%, respectively. Then, the obtained polymer was introduced into a kind of designed 3D printing soft actuator. The soft actuator can achieve sharp heating-cooling cycle of 19 s under a 1.2 A current with 4 °C water as coolant and can lift a 200 g weight at the actuating state. Moreover, the stiffness of the soft actuator can reach up to 718 mN/mm. The soft actuator exhibits an outstanding actuate behavior and stiffness switchable capability. We expect our design strategy and obtained variable stiffness polymers to be potentially applied in soft actuators and other devices.

Micro-Leakage Image Recognition Method for Internal Detection in Small, Buried Gas Pipelines

In order to resolve the problem that the sample of image for internal detection of DN100 buried gas pipeline microleakage is single and difficult to identify, a recognition method of microleakage image of the pipeline internal detection robot is proposed. First, nongenerative data augmentation is used to expand the microleakage images of gas pipelines. Secondly, a generative data augmentation network, Deep Convolutional Wasserstein Generative Adversarial Networks (DCWGANs), is designed to generate microleakage images with different features for detection in the pipeline of gas pipelines to achieve sample diversity of microleakage images of gas pipelines. Then, a bi-directional feature pyramid network (BiFPN) is introduced into You Only Look Once (YOLOv5) to retain more deep feature information by adding cross-scale connecting lines in the feature fusion structure; finally, a small target detection layer is constructed in YOLOv5 so that more shallow feature information can be retained to achieve small-scale leak point recognition. The experimental results show that the precision of this method for microleak identification is 95.04%, the recall rate is 94.86%, the mAP value is 96.31%, and the minimum size of identifiable leaks is 1 mm.

Magnetically induced stiffening for soft robotics

Soft robots are well-suited for human-centric applications, but the compliance that gives soft robots this advantage must also be paired with adequate stiffness modulation such that soft robots can achieve more rigidity when needed. For this reason, variable stiffening mechanisms are often a necessary component of soft robot design. Many techniques have been explored to introduce variable stiffness structures into soft robots, such as pneumatically-controlled jamming and thermally-controlled phase change materials. Despite fast response time, jamming methods often require a bulkier pneumatic pressure line which limits portability; and while portable via electronic control, thermally-induced methods require compatibility with high temperatures and often suffer from slow response time. In this paper, we present a magnetically-controlled stiffening approach that combines jamming-based stiffening principles with magnetorheological fluid to create a hybrid mechanical and materials approach. In doing so, we combine the advantages of fast response time from pneumatically-based jamming with the portability of thermally-induced phase change methods. We explore the influence of magnetic field strength on the stiffening of our magnetorheological jamming beam samples in two ways: by exploiting the increase in yield stress of magnetorheological fluid, and by additionally using the clamping force between permanent magnets to further stiffen the samples via a clutch effect. We introduce an analytical model to predict the stiffness of our samples as a function of the magnetic field. Finally, we demonstrate electronic control of the stiffness using electropermanent magnets. In this way, we present an important step towards a new electronically-driven stiffening mechanism for soft robots that interact safely in close contact with humans, such as in wearable devices.

Reprogrammable allosteric metamaterials from disordered networks

Prior works on disordered mechanical metamaterial networks-consisting of fixed nodes connected by discrete bonds-have shown that auxetic and allosteric responses can be achieved by pruning a specific set of the bonds from an originally random network. However, bond pruning is irreversible and yields a single bulk response. Using material stiffness as a tunable design parameter, we create metamaterial networks where allosteric responses are achieved without bond removal. Such systems are experimentally realized through variable stiffness bonds that can strengthen and weaken on-demand. In a disordered mechanical network with variable stiffness bonds, different subsets of bonds can be strategically softened to achieve different bulk responses, enabling a multiplicity of reprogrammable input/output allosteric responses.

Stretchable and conformable variable stiffness device through an electrorheological fluid

Stiffness variations extend creatures' functions and capabilities to deal with complex environments. In this study, we proposed an electrorheological fluid-based variable stiffness device, named VSERF, made up of soft materials. Our device is soft, thin, and stretchable so that it can conform to surfaces with complex morphologies. The stiffness of the VSERF device can be continuously, independently, and reversibly adjusted by applying an electric field. It achieves 14.8-times compressive stiffness variation and 3.5-times tangential stiffness variation when the electric field intensity increases from 0 V mm^-1 to 750 V mm^-1. The VSERF device is able to return to its initial shape after removing the external force and electric field, allowing it to be reused. The effects of stretching and bending on the device's capability of stiffness variations are investigated experimentally and the results show that the stiffness variation is unaffected by a stretching strain of up to 20% and a bending curvature of up to 50 m^-1. Finally, we show that the VSERF device is capable of conforming to complex surfaces (coral stones, pencils, and 3D printed cubes) in its inactive state, hanging on them with a weight of up to 80 g (19 times of its own weight) in its active state, and detaching when the electric field is removed. The device's short-term and long-term stabilities are experimentally investigated as well. The demonstration of the VSERF's attaching and detaching ability shows that the stiffness-variation device's adaptability to complex environments can be improved.

Towards enduring autonomous robots via embodied energy

Autonomous robots comprise actuation, energy, sensory and control systems built from materials and structures that are not necessarily designed and integrated for multifunctionality. Yet, animals and other organisms that robots strive to emulate contain highly sophisticated and interconnected systems at all organizational levels, which allow multiple functions to be performed simultaneously. Herein, we examine how system integration and multifunctionality in nature inspires a new paradigm for autonomous robots that we call Embodied Energy. Whereas most untethered robots use batteries to store energy and power their operation, recent advancements in energy-storage techniques enable chemical or electrical energy sources to be embodied directly within the structures and materials used to create robots, rather than requiring separate battery packs. This perspective highlights emerging examples of Embodied Energy in the context of developing autonomous robots.

Shape memory polymer/graphene nanocomposites: State-of-the-art

Graphene is one of most exceptional type of nanocarbon. It is a two-dimensional, one atom thick, nanosheet of sp2 hybridized carbon atoms. Graphene has been employed as nanofiller for shape memory polymeric nanocomposites due to outstanding electrical conductivity, mechanical strength, flexibility, and thermal stability characteristics. Consequently, graphene nanostructures have been reinforced in the polymer matrices to attain superior structural, physical, and shape recovery properties. This review basically addresses the important class of shape memory polymer (SMP)/graphene nanocomposites. This assessment is revolutionary to portray the scientific development and advancement in the field of polymer and graphene-based shape memory nanocomposites. In SMP/graphene nanocomposites, polymer shape has been fixed at above transition temperature and then converted to memorized shape through desired external stimuli. Presence of graphene has caused fast switching of temporary shape to original shape in polymer/graphene nanocomposites. In this regard, better graphene dispersion, interactions between matrix-nanofiller, and well-matched interface formation leading to high performance stimuli-responsive graphene derived nanocomposites, have been described. Incidentally, the fabrication, properties, actuation ways, and relevance of the SMP/graphene nanocomposite have been discussed here. The potential applications of these materials have been perceived for the aerospace/automotive components, self-healing nanocomposites, textiles, civil engineering, and biomaterials.

Shape memory of a polymer grating surface fabricated by two-beam interference lithography

Switchable and reversible optical elements have potential applications in self-adaptive optics. Shape-memory polymer devices with adaptive properties could be easily switched under environment or field stimuli. Here, the laser beam interference technique was used to realize the periodic grating structures of the shape-memory polymer, and memory and recovery of the grating structures were performed. A one-dimensional grating structure was fabricated from dual-beam interference lithography of a nanosecond laser and underwent pressure in a condition of 195°C. The vertical height of the grating was reduced, and the diffraction light was weakened. When the sample was cooled down to room temperature, the morphology of the grating could be kept. After raising the ambient temperature of the sample to 120°C, the morphology of the grating was recovered to the original state, which realized the shape-memory function.

Transient Electrically Driven Stiffness-Changing Materials from Liquid Metal Polymer Composites

Stiffness-changing materials (SCMs) have received lots of interests due to their reversible transition between their soft and rigid states for modern applications. However, the irreversible stiffness transition, slow response, and sustained external stimuli strictly hinder the broad utilizations of SCMs. Here, this work reports electrically driven SCMs based on supercooled liquid metals (LMs). A small voltage (5 V) can successfully initiate the stable and reversible stiffness change of the SCMs in electrolyte solution. Surprisingly, the LM-based SCMs (LM-SCMs) exhibited a significant change in 1000 times difference of moduli (65 kPa versus 79 MPa). Moreover, such a stiffness transition of the LM-SCM was ultrarapidly completed in a few seconds (<30 s). Importantly, after transient stimulation of LM nucleation, the rigidity of the LM-SCM could be maintained when the external stimulus (voltage) was removed, highly different from previously reported SCMs that require sustained energy to maintain their mechanical states. Based on the unique features of LM-SCMs, advanced robotics like smart valves and mechanical paws in seawater were successfully fabricated.

Materials with Electroprogrammable Stiffness

Stiffness is a mechanical property of vital importance to any material system and is typically considered a static quantity. Recent work, however, has shown that novel materials with programmable stiffness can enhance the performance and simplify the design of engineered systems, such as morphing wings, robotic grippers, and wearable exoskeletons. For many of these applications, the ability to program stiffness with electrical activation is advantageous because of the natural compatibility with electrical sensing, control, and power networks ubiquitous in autonomous machines and robots. The numerous applications for materials with electrically driven stiffness modulation has driven a rapid increase in the number of publications in this field. Here, a comprehensive review of the available materials that realize electroprogrammable stiffness is provided, showing that all current approaches can be categorized as using electrostatics or electrically activated phase changes, and summarizing the advantages, limitations, and applications of these materials. Finally, a perspective identifies state-of-the-art trends and an outlook of future opportunities for the development and use of materials with electroprogrammable stiffness.

Printed miniature robotic actuators with curvature-induced stiffness control inspired by the insect wing

Stimuli-responsive actuating materials offer a promising way to power insect-scale robots, but a vast majority of these material systems are too soft for load bearing in different applications. While strategies for active stiffness control have been developed for humanoid-scale robots, for insect-scale counterparts for which compactness and functional complexity are essential requirements, these strategies are too bulky to be applicable. Here, we introduce a method whereby the same actuating material serves not only as the artificial muscles to power an insect-scale robot for load bearing, but also to increase the robot stiffness on-demand, by bending it to increase the second moment of area. This concept is biomimetically inspired by how insect wings stiffen themselves, and is realized here with manganese dioxide as a high-performing electrochemical actuating material printed on metallized polycarbonate films as the robot bodies. Using an open-electrodeposition printing method, the robots can be rapidly fabricated in one single step in ∼15 minutes, and they can be electrochemically actuated by a potential of ∼1 V to produce large bending of ∼500° in less than 5 s. With the stiffness enhancement method, fast (∼5 s) and reversible stiffness tuning with a theoretical increment by ∼4000 times is achieved in a micro-robotic arm at ultra-low potential input of ∼1 V, resulting in an improvement in load-bearing capability by about 4 times from ∼10μN to ∼41μN.

A Human-Inspired Soft Finger with Dual-Mode Morphing Enabled by Variable Stiffness Mechanism

Elevating stiffness without compromising compliance and agility is a key problem for soft finger applications, especially for articulate ones. Inspired by human finger, a multijoint soft finger with dual morphing through active/passive variable rigidity is proposed. The fabricated soft finger weighs 27.4 g. Conductive thermoplastic starch polymers (CTPSs) are embedded in a U-shape-joint pneumatic soft actuator segmentally like biological phalanges. Their stiffness can be independently adjusted utilizing customized thermomechanical property. Yoshimura origami imitating ligaments can passively match deformation and stiffness of the joints. Through electrothemal activation of CTPSs, the finger can realize dual independent articulate morphing: stiffened phalanges (mode 1) for dexterous manipulation and heavy load, softened phalanges (mode 2) for large deformation contact and light load. Comparative experiments of bending angle, output force, and stiffness are carried out between the active and passive stiffness adjustment of mode 1 and mode 2. The results show that the output force and stiffness of the finger adopting mode 1 can be improved more than two times and five times, and its compliance using mode 2 is almost similar, compared with the pure soft one. To further demonstrate performances of dual-mode morphing, a three-fingered gripper is assembled for grasping and manipulating targets with different shapes, sizes, rigidity, and weight, including playing card, unshelled raw egg, grapes, and unscrewing the bottle cap. It can successfully lift a dumbbell weighing 1460 g with a 7.6 load/weight ratio through a two-mode switch.

Versatile Soft Robot Gripper Enabled by Stiffness and Adhesion Tuning via Thermoplastic Composite

Within the field of robotics, stiffness tuning technologies have potential for a variety of applications-perhaps most notably for robotic grasping. Many stiffness tuning grippers have been developed that can grasp fragile or irregularly shaped objects without causing damage and while still accommodating large loads. In addition to limiting gripper deformation when lifting an object, increasing gripper stiffness after contact formation improves load sharing at the interface and enhances adhesion. In this study, we present a novel stiffness and adhesion tuning gripper, enabled by the thermally induced phase change of a thermoplastic composite material embedded within a silicone contact pad. The gripper operates by bringing the pad into contact with an object while in its heated, soft state, and then allowing the pad to cool and stiffen to form a strong adhesive bond before lifting the object. Pull-off tests conducted using the gripper show that transitioning from a soft to stiff state during grasping enables up to 6 × increase in adhesion strength. Additionally, a finite element model is developed to simulate the behavior of the gripper. Finally, pick-and-place demonstrations are performed, which highlight the gripper's ability to delicately grasp objects of various shapes, sizes, and weights.

Tendon-Driven Jamming Mechanism for Configurable Variable Stiffness

Stiffness transition of a soft continuum body is an essential feature for dexterous interaction with an unstructured environment. Softness ensures safe interaction, whereas rigidness generates high force for movement or manipulation. Vacuum-based granular jamming is a widely used technique for on-line stiffness transition because of its high reconfigurability and intuitive driving method. However, vacuum driving method produces limited force levels, and the heavy weight and bulky size are unfavorable for portable applications. In this work, we propose a tendon-driven jamming mechanism for configurable variable stiffness. Compared with a vacuum system, an electric motor-tendon drive system has the benefits of force, bandwidth, size, and weight, but has different force characteristics for distribution, directionality, and transmissibility. In this study, a long snake-like shape is chosen instead of a lump shape for compatibility with tendon-drive characteristics. The snake-like shape is likely to cause buckling under the tendon force as the length increases, making the system extremely unstable. Implanting skeletal disk nodes in the structure is our solution to the buckling phenomenon by maintaining the tendon path in the desired position and for distributing the force evenly, thereby achieving stable stiffness transition capabilities for long free-curved shapes. As a proof of concept, a soft wearable device for wrist support is presented using the proposed variable stiffness mechanism. The weight of the device is 184 g, including the actuators, and it can support 2 kgf. Furthermore, the stiffness transition is completed within 2 s, thus achieving quick responses.

Phase Changing Materials-Based Variable-Stiffness Tensegrity Structures

Soft robots leverage deformable bodies to achieve different types of locomotion, improve transportability, and safely navigate cluttered environments. In this context, variable-stiffness structures provide soft robots with additional properties, such as the ability to increase forces transmitted to the environment, to lock into different body configurations, and to reduce the number of actuators required for morphological change. Tensegrity structures have been recently proposed as a biologically inspired design principle for soft robots. However, the few examples of tensegrity structures with variable stiffness displayed relatively small stiffness change (i.e., by a factor of 3) or resorted to multiple and bulky actuators. In this article, we describe a novel design approach to variable-stiffness tensegrity structures (VSTSs) that relies on the use of variable-stiffness cables (VSCs). As an example, we describe the design and implementation of a three-strut tensegrity structure with VSCs made of low melting point alloys. The resulting VSTS displays unprecedented stiffness changes by a factor of 28 in compression and 13 in bending. We show the capabilities of the proposed VSTS in three validation scenarios with different tensegrity architectures: (1) a beam with tunable load-bearing capability, (2) a structure that can self-deploy and lock its shape in both deployed and undeployed states, and (3) a joint with underactuated shape deformations.

Multimodal pipe-climbing robot with origami clutches and soft modular legs

In nature, climbing trees and pipes of varying diameters or even navigating inside of hollow pipes and tree holes is easy for some climbing animals and insects. However, today's pipe-climbing robots, which are important for automatically conducting periodic inspections and maintenance of pipelines to save time and keep humans away from hazardous environments, are designed mainly for a specific task, limiting their adaptability to different working scenarios and further implementation in real-life. In this paper, we propose a pipe-climbing robot with a soft linear actuator for bioinspired propulsion, two origami clutches to realize multi-degrees-of-freedom (DoF) motion and two pairs of soft modular legs for multimodal climbing. Design, modeling and experimental validation of the origami clutch are introduced in detail. Preliminary experimental results show that we can achieve a stroke of up to 289.6% and a maximum 45° bending angle on the soft linear actuator by regulating the air pressure inside the soft actuator and origami clutches. Additionally, by choosing the leg-type, three climbing modes, including out-pipe versatile mode, out-pipe high-force mode and in-pipe mode can be realized for particular working scenarios. A prototype climbing robot demonstrates that in out-pipe versatile mode, the robot can climb on the exterior of pipes made of various materials including PVC, rubber and metal with diameters ranging from 105 to 117 mm. In the out-pipe high-force mode, the climber can navigate along a specific pipe carrying maximum 675 g external load at the top or 200 g hanging from the bottom, as well as keeping functional without failure under static loads as high as 1968 g. In the in-pipe mode, the robot is able to travel inside pipes. This research might bridge the design gap between in-pipe and out-pipe climbing robots while offering an alternative option for soft robots to execute multi-DoF motion.

Soft Rehabilitation and Nursing-Care Robots: A Review and Future Outlook

Rehabilitation and nursing-care robots have become one of the prevalent methods for assistant treatment of motor disorder patients in the field of medical rehabilitation. Traditional rehabilitation robots are mostly made of rigid materials, which significantly limits their application for medical rehabilitation and nursing-care. Soft robots show great potential in the field of rehabilitation robots because of their inherent compliance and safety when they interact with humans. In this paper, we conduct a systematic summary and discussion on the soft rehabilitation and nursing-care robots. This study reviews typical mechanical structures, modeling methods, and control strategies of soft rehabilitation and nursing-care robots in recent years. We classify soft rehabilitation and nursing-care robots into two categories according to their actuation technology, one is based on tendon-driven actuation and the other is based on soft intelligent material actuation. Finally, we analyze and discuss the future directions and work about soft rehabilitation and nursing-care robots, which can provide useful guidance and help on the development of advanced soft rehabilitation and nursing-care robots.

Soft Robotic Grippers

Advances in soft robotics, materials science, and stretchable electronics have enabled rapid progress in soft grippers. Here, a critical overview of soft robotic grippers is presented, covering different material sets, physical principles, and device architectures. Soft gripping can be categorized into three technologies, enabling grasping by: a) actuation, b) controlled stiffness, and c) controlled adhesion. A comprehensive review of each type is presented. Compared to rigid grippers, end-effectors fabricated from flexible and soft components can often grasp or manipulate a larger variety of objects. Such grippers are an example of morphological computation, where control complexity is greatly reduced by material softness and mechanical compliance. Advanced materials and soft components, in particular silicone elastomers, shape memory materials, and active polymers and gels, are increasingly investigated for the design of lighter, simpler, and more universal grippers, using the inherent functionality of the materials. Embedding stretchable distributed sensors in or on soft grippers greatly enhances the ways in which the grippers interact with objects. Challenges for soft grippers include miniaturization, robustness, speed, integration of sensing, and control. Improved materials, processing methods, and sensing play an important role in future research.