Functional Fibers and Fabrics for Soft Robotics, Wearables, and Human-Robot Interface.

Open AccessCCS ChemistryMINI REVIEWS7 Jul 2022Autonomous Chemistry Enabling Environment-Adaptive Electrochemical Energy Storage Devices Zhisheng Lv, Wenlong Li, Jiaqi Wei, Fanny Ho, Jie Cao and Xiaodong Chen Zhisheng Lv Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 138634 Singapore , Wenlong Li Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 138634 Singapore , Jiaqi Wei Innovative Centre for Flexible Devices (iFLEX), Max Planck—NTU Joint Lab for Artificial Senses, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore , Fanny Ho Innovative Centre for Flexible Devices (iFLEX), Max Planck—NTU Joint Lab for Artificial Senses, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore , Jie Cao Innovative Centre for Flexible Devices (iFLEX), Max Planck—NTU Joint Lab for Artificial Senses, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore and Xiaodong Chen *Corresponding author: E-mail Address: [email protected] Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 138634 Singapore Innovative Centre for Flexible Devices (iFLEX), Max Planck—NTU Joint Lab for Artificial Senses, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore https://doi.org/10.31635/ccschem.022.202202153 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Next-generation electronics that are fused into the human body can play a key role in future intelligent communication, smart healthcare, and human enhancement applications. As a promising energy supply component for smart biointegrated electronics, environment-adaptive electrochemical energy storage (EES) devices with complementary adaptability and functions have garnered huge interest in the past decade. Owing to the advancements in autonomous chemistry, which regulate the constitutional dynamic networks in materials, EES devices have witnessed higher freedom of autonomous adaptability in terms of mechano-adaptable, biocompatibility, and stimuli-response properties for biointegrated and smart applications. In this mini-review, we summarize the recent progress in emerging environment-adaptive EES devices enabled by the constitutional dynamic network of mechanical adaptable materials, biocompatible materials, and stimuli-responsive supramolecular polymer materials. Finally, the challenges and perspectives of autonomous chemistry on the environment-adaptive EES devices are discussed. Download figure Download PowerPoint Introduction Advancements made in the merging of human consciousness and machine capabilities have driven the emergence of smart electronics.1–6 The fusion of electronics into human beings to form biointegrated systems has motivated the development of electronics with a higher degree of adaptability in ever-changing environments. To intimately integrate electronics with the human body, electronics are becoming more flexible and stretchable to accommodate the arbitrary shapes of the human body and mechanical deformation during human motion. Alongside mechanical adaptability, biointegrated electronics are also required to be biocompatible so as to operate in physiological environments, such as near-body, on-skin, and in vivo environments. Moreover, by combining with other smart functions, such as biometric sensing and stimuli-responsive actuating, smart biointegrated electronics can allow humans to share their senses and engage in collaborative work with machines to extend the limitation of the human body and machines. Progress in such adaptability of electronics and emerging symbiosis between humans and machines has unlocked exciting opportunities in fabricating biointegrated electronics for smart healthcare and human enhancement applications. To seamlessly power the emerging environment-adaptative electronics, in the last decade, the indispensable electrochemical energy storage (EES) devices, mainly supercapacitors and batteries, have witnessed complementary changes in mechanical adaptability, biocompatibility, and smart functionalities for biointegrated applications.7–9 The environment-adaptive transformations in the EES devices would not be realized without the participation of autonomous chemistry. Autonomous chemistry is an adaptive and self-evolved chemical system that manipulates the covalent and noncovalent constitutional dynamic networks to autonomously adapt and respond to internal and external stimuli, such as light, temperature, pH, and biological enzymes (Figure 1). The molecular chemistry governing the dynamic covalent bonds (Figure 1a) and supramolecular chemistry implementing the intermolecular interactions (Figure 1b) create building blocks of constitutional dynamic chemistry networks.10–14 These dynamic chemistry networks extend and combine with molecules of various sizes to allow for variation and adaptation through autonomous component selection and response to internal and external environmental stimuli (Figure 1c). The introduction of autonomous chemistry into materials science opens up new perspectives on self-adaptive materials. These autonomously adaptive materials integrated with device technologies provide great opportunities for the development of environment-adaptive EES devices. Figure 1 | The emergence of autonomous chemistry toward adaptive and self-evolved chemical systems. (a) Schematic representation of the dynamic reversible and irreversible molecular interactions for the formation of the crystalline and amorphous covalent organic framework. Adapted with permission from ref 14. Copyright 2019 American Chemical Society. (b) Schematic representation of the noncovalent constitutional dynamic network for supramolecular interactions. (c) Autonomous chemistry with covalent and noncovalent constitutional dynamic networks to construct adaptive and self-evolved chemical systems. Download figure Download PowerPoint Autonomous chemistry allows for regulating the functions of environment-adaptive EES devices. By using autonomously adaptive materials, autonomously operating chemical systems in devices can adapt and respond to environmental changes (Figure 2a–f, h). Since the first transformation from the original rigid prototype into flexible devices in 2007, environment-adaptive EES devices have ushered in a new era.7,15 With enhanced flexibility of electrode materials via flexible and stretchable molecular and supramolecular design, intrinsically flexible and stretchable electrodes can endow the mechano-adaptable EES devices with adaptability to arbitrary shapes of the human body.16–22 Accompanying the enhanced mechanical adaptability, the molecular designs of biodegradable and bioresorbable materials enable EES devices to be autonomously biocompatible with the human body for skin-mounted, ingestible, and implantable applications.23–26 Furthermore, the incorporation of stimuli-responsive alloys and supramolecular polymers has spurred EES devices towards smart transformations (e.g., self-healing, shape memory, thermal protection, and electrochromism), providing environment-adaptive EES devices with self-responsive and self-protective functions under external mechanical, thermal, and/or electrical stimuli.27–30 The design and synthesis of autonomous molecular and supramolecular networks with autonomous operation in mechano-adaptable materials, biocompatible materials, and stimuli-responsive supramolecular polymer materials have enabled the development of EES devices with programmable environment-adaptive functions, offering new opportunities to power the next-generation smart and biointegrated electronics.31,32 Figure 2 | The transformation of traditional EES devices into environment-adaptive EES devices enabled by autonomous chemistry. Autonomous chemistry modulates the constitutional dynamic (a) rigid and elastic networks, (b) biological regulatory networks, and (c) the stimulus-response networks for environment-adaptive EES with (d–f) desired functionalities. (a) Chemical structure and illustration of stretchable and adhesive water-dispersible PUs with both soft and hard segments. Reproduced with permission from ref 22. Copyright 2021 Wiley-VCH. (b) Colorimetric detection of glucose using cascade enzyme reaction systems with glucose oxidase and horseradish peroxidase. Adaptable with permission from ref 26. Copyright 2019 Springer Nature Limited. (c) Stimuli-responsive polymers designed with different kinds of topological structures and morphologies. (d) The representative mechanical deformations. (e) The biofriendly interfaces with the human body that require biocompatible properties. (f) The stimulus-responsive smart functions. (g) Schematic device structure of conventional EES devices. (h) The schematic device structure of the environment-adaptive EES devices. Download figure Download PowerPoint Herein, we review recent important advances in environment-adaptive EES devices that are enabled by autonomous chemistry. Specifically, electrodes, electrolytes, and devices associated with constitutional dynamic networks of mechano-adaptable materials, biocompatible materials, and stimuli-responsive supramolecular polymers will be discussed. The emerging mechano-adaptable, biocompatible, and smart EES devices for environment-adaptive applications will also be emphasized. Finally, this review concludes with an outlook on the challenges and opportunities in this fast-developing field. Constitutional Dynamic Networks for Mechano-Adaptable EES Devices The basic mechanical deformations of EES devices involve bending and stretching to cover the curved surface of human skin and soft tissues. Conventional EES devices with packaging consist of two electrodes separated by an electrically insulating separator immersed in a liquid electrolyte (Figure 2g). However, the maximum elastic strains these components in conventional EES devices, including electrodes, separators, and packaging, are typically less than 1%, which cannot match the stretchability (>50%) required for biointegrated applications.33,34 Clearly, improving the tensile strains of these components is essential to designing mechano-adaptable EES devices. To make the components in EES devices stretchable, two strategies have been exploited, including directly replacing inelastic materials with elastic alternatives and constructing structural nanomaterial networks for stretchable electrodes. The inextensible components—electrolytes, separators, and packaging—can be directly replaced by their stretchable alternatives. For example, silicone rubbers such as polydimethylsiloxane and Ecoflex are frequently adopted as stretchable packaging materials, and gel electrolytes with stretchable molecular design can serve as both electrolytes and stretchable separators to eliminate the necessity of poorly elastic separators and possible leakage of liquid electrolyte. As for electrodes, the available inelastic materials can be reinforced with conductive nanomaterials (e.g., graphene, carbon nanotubes (CNTs), and conductive polymers) that have higher tensile strains. However, the tensile strain of composite electrodes is still less than 10%.35,36 In this regard, directly replacing electrode materials with elastic alternatives is not enough. As a complementary method, a stretchable structural design has been developed to improve the stretchability of the electrode. Based on thin and porous nanomaterials, more bendable electrodes cooperate with elastic polymer substrates and proper electrode layouts, such as wave-like wrinkled structures, coiled and braided structures, and kirigami structures, which are highly stretchable and can accommodate tensile strain greater than 100%.16,20,37–42 Instead of using structural designs, the molecular design of intrinsically stretchable polymers is an effective way to realize flexible and stretchable electrodes and electrolytes for mechano-adaptable EES devices. Common conjugated polymers, including polypyrrole (PPy), polyaniline (PANI), and poly(3,4-ethylenedioxythiophene) (PEDOT), when doped into a conducting state, are promising pseudocapacitive materials for flexible supercapacitors.21,43,44 However, the doped conductive polymers usually possess a close-packed and laminated structure that is not capable of releasing mechanical stresses induced by ion exchange in the charging/discharging process and mechanical deformation. This leads to the deterioration of both electrochemical and mechanical performances of as-fabricated devices during mechanical deformation.21 To address this issue, partially doping anion into a conductive polymer matrix has been applied to prevent the overstacking of polymer chains. Taking the electropolymerization of PPy as an example, a porous PPy/black phosphorus oxide (BPO) composite electrode was prepared through a two-step electropolymerization method by utilizing partially oxidized black phosphorus (BP) as dopant.21 The BPO surface was partially doped into the PPy matrix, while the unoxidized BP surface helped to form micropores during the deposition of PPy/BPO composites on the CNT films. As a comparison, the doped PPy composite electrode showed densely packed structures when the anion dopants (SO42− and graphene oxides (GO)) with fully covered oxygen functional groups were employed for the electropolymerization process. Owing to the porous structures to buffer the strain under deformation, the as-prepared stretchable supercapacitors with the porous PPy/BPO-CNT electrode exhibited 97% capacitance retention when being stretched to 2400%, which is superior to that of dense stacking PPy/SO42− and PPy/GO electrode-enabled stretchable supercapacitors (78% and 87% respectively). Besides tuning the dopant in conductive polymers, the additional template and solvent precursors also affect the polymerization process, thus affecting the resultant morphology and stretchability of the conductive polymer-based electrodes. With the template of ice crystals, anisotropic polyvinyl alcohol (PVA)/PANI hybrid hydrogels were synthesized through a cryopolymerization strategy (Figure 3a).43 During the freezing process, the 3D-ordered honeycomb structure of PVA was formed along the growing direction of vertically aligned ice crystals. Followed by cryopolymerization, the polymerization of PANI nanofiber was confined within the boundaries between PVA cell walls and ice crystals. The honeycomb structures with the PVA and PANI interpenetrating networks enabled isotropic PVA/PANI hybrid hydrogels with maximum elongation up to 416%. The as-fabricated supercapacitors delivered 85% capacitance under 200% tensile strain, demonstrating an exciting avenue to synthesize conductive polymer hybrid hydrogels as intrinsically stretchable electrodes for mechano-adaptable EES devices. Additionally, adding dimethyl sulfoxide (DMSO) into an aqueous PEDOT:polystyrene sulfonate (PSS) solution for dry annealing and rehydration processes can lead to a well-controlled phase separation to form interconnected networks of PEDOT:PSS nanofibrils in the as-prepared pure PEDOT:PSS hydrogels. In contrast, fragmented PEDOT:PSS microgel was obtained from the PEDOT:PSS aqueous solution without DMSO due to the separation between the soft PSS-rich domain and the rigid PEDOT-rich domain under the same drying and swelling process.44 The pure PEDOT:PSS hydrogels (20 vol % DMSO in the preparation) can be used as intrinsically stretchable electrodes for supercapacitors and reach a maximum stretchability up to 35% in phosphate-buffered saline (PBS) solution, which closely matches the stretchability of biological tissues (∼20% for neural tissues and ∼50% for skin).35 Figure 3 | Molecular design of flexible and stretchable polymers for mechano-adaptable EES devices. (a) The fabrication of the anisotropic hybrid porous PVA/PANI hydrogels for stretchable supercapacitors with diverse shapes under stretching and compression. Reproduced with permission from ref 43. Copyright 2019 Springer Nature Limited. (b) The synthesis process of VSNPs-PAM electrolytes for stretchable and compressible supercapacitors by cross-linking the VSNPs from vinyltriethoxysilane with acrylamide monomers at the presence of the ammonium persulfate initiator and phosphoric acid. Reproduced with permission from ref 45. Copyright 2017 Wiley-VCH. (c) Illustration of the VSNPs cross-linking PAM network for superstretchability and high compressibility. Reproduced with permission from ref 45. Copyright 2017 Wiley-VCH. (d) The chemical structure of the lithium-ion conductor (LIC) and diagram showing the LIC polymer electrolyte upon stretching. The orange squares represent hydrogen-bonding UPy moieties, black wires are poly(propylene glycol)-pol(ethylene glycol)-poly(propylene glycol) (PPG-PEG-PPG) chains, and the blue circles are lithium ions. Reproduced with permission from ref 46. Copyright 2019 Springer Nature Limited. (e) The application of the supramolecular LIC for stretchable LIBs. Reproduced with permission from ref 46. Copyright 2019 Springer Nature Limited. Download figure Download PowerPoint Apart from the stretchable electrodes, the molecular design of supramolecular dynamic polymers with both mechanically robust and ionically conductive segments is also used to fabricate intrinsically stretchable electrolytes for mechano-adaptable EES devices.45 Pure polyacrylamide (PAM) hydrogels with weak hydrogen bonds are hard to make ultrastretchable, which limits their applications as stretchable electrolytes for stretchable supercapacitors. To enhance the toughness and stretchability of the PAM hydrogel, the hydrogel skeleton was reinforced by the strong covalent bonding between the PAM chains and vinyl hybrid silica nanoparticles (VSNPs) (Figure 3b).45 The VSNPs-PAM hydrogels with ionically conductive PAM polyelectrolyte matrix and stress-buffering of VSNP cross-linkers enabled stretchable supercapacitors to possess intrinsic superstretchability (up to 1000% strain) and compressibility (up to 50% strain) without degradation of their initial capacitance (Figure 3c).45 Similarly, the crosslinking of hairy nanoparticles helped strengthen the stretchable hydrogel electrolytes. The stretchable supramolecular lithium-ion conductor with mechanically reinforced hydrogen bonds was designed to serve as the polymer electrolyte for stretchable lithium-ion batteries (LIBs) (Figure 3d).46 To decouple mechanical robustness from ionic conductivity in low-Tg (the glass transition temperature) polymer electrolytes, a copolymer was created by introducing a dynamic bonded ureido-pyrimidinone (UPy) backbone into low-Tg polyether backbone, wherein the low-Tg polyether backbone provided the polymer electrolyte with high ionic conductivity (1.2 ± 0.21 × 10−4 S cm−1), and the Upy group enhanced the mechanical toughness of the polymer electrolyte (29.3 ± 0.21 × 10−4 MJ m−3, three times higher than reported polymer electrolytes) by dynamic hydrogen bonds. The as-prepared polymer electrolyte-based stretchable LIBs, with a capacity density of 1.1 mAh cm−2 function well to power light-emitting diodes even when stretched up to 70%, suggesting the promising application of tough ion-conducting polymers for conformable EES devices (Figure 3e). Constitutional Dynamic Networks for Biocompatible EES Devices The boom in environment-adaptive electronics for real-time in vivo health monitoring and diagnosis has stimulated the development of EES devices with not only mechano-adaptability but also biocompatibility, so as to adapt to biological environments and intimately integrate onto/into essential organs of the human body.47 However, developing biocompatible EES devices that avoid the likelihood of infection remains a great challenge, especially in the exploration of nontoxic and biodegradable devices. To safely use biocompatible EES devices for in vivo applications, the original corrosive and toxic materials in EES devices should be replaced with nontoxic and biodegradable alternatives that can be autonomously dissolved and resorbed or disposed of by the body through biochemical processes like metabolization and bioabsorption. Such biodegradable materials include inorganic materials and organic polymers. Inorganic metals (e.g., Mg, Mo, and Li) that can react and dissolve in aqueous solutions are suitable materials to fabricate biodegradable electrodes for EES devices. Likewise, biodegradable polymers that can undergo chemical or enzymatical hydrolysis and/or oxidation are suitable as well.48–52 A representative biodegradable battery system is shown in Figure 4a, wherein the primary Mg–Mo battery is packaged with polyanhydride materials to provide a constant current density of 0.1 mA cm−2 at a voltage of 1.6 V for around 6 h.49 Because all the constituent materials in the battery are water-soluble, the Mg–Mo battery is fully degradable after 11 days in PBS at 37 °C followed by another 8 days in PBS at 85 °C. The biodegradation of polyanhydride in the Mg–Mo battery stems from the fact that its ester bonds are susceptible to hydrolysis. Other hydrolytically degradable moieties as shown in Figure 4b, like the amide, thioester, and imine, can serve as the synthetic polymer backbone for eco-friendly degradation.48 Complementary to hydrolysis, oxidation is another way to biologically degrade polymers. Polymers designed with oxidizable moieties, such as ethers, alcohols, and phenols, are susceptible to oxidative cleavage (Figure 4b).53 Currently, previous reports about the degradation of biocompatible EES devices are mainly limited to hydrolysis of electrode materials in physiological conditions, and the mechanism for the oxidation of polymers in biodegradable EES devices has yet to be thoroughly explored. Figure 4 | Biodegradable and biocompatible materials for biocompatible EES devices. (a) The dissolution of the biodegradable Mg–Mo battery with degradable inorganic metallic electrodes and an organic polyanhydride spacer. Reproduced with permission from ref 49. Copyright 2014 Wiley-VCH. (b) The chemical structures of moieties tend to be hydrolyzed and oxidized, and the red marks indicate the hydrolyzation and oxidation sites. Images adapted with permission from ref 48. Copyright 2018 American Chemical Society. (c) The biophilized graphene oxide (bGO)-Mb electrode for biocompatible supercapacitors. Upper: the synthesis of bGO with the negative charge by absorbing cationized bovine serum albumin (cBSA) ion of cBSA onto GO sheet. Down: dose-dependent toxicity in (COS-7) and MEF cells coincubated with GO and bGO/Mb as measured by the intracellular dehydrogenases activity. Reproduced with permission from ref 56. Copyright 2017 Wiley-VCH. (d) An implantable NAD/BQ/CNT yarn supercapacitor. Top: Fabrication scheme of NAD/BQ/CNT yarn electrode and the reversible redox reaction of NAD with the assistance of BQs (oxidized and reduced forms of NAD abbreviated as NAD+ and reduced nicotinamide adenine dinucleotide [NADH], respectively). Down: Implantation of the yarn supercapacitors into the abdominal cavity of a mouse and the capacitance retention of the implantable supercapacitors on the day of surgery, 3 and 14 days after implantation. Reproduced with permission from ref 58. Copyright 2021 Wiley-VCH. Download figure Download PowerPoint As the EES devices become incorporated into the digestive system, biocompatible EES devices that can be ingested by individuals also emerge, with emphasis on biodegradable, bioresorbable, and noncytotoxic characteristics of the devices. Conventional active materials, such as MnO2, which is a constituent material of supercapacitors, pose a threat to the human body (causing abdominal pain and nausea) when ingested. Moreover, the toxicity of many nanomaterials is still unknown.24,54 In this case, naturally derived materials (e.g., biochar, cellulose, silk, and collagen) provide alternatives to fabricate biocompatible and edible devices due to their intrinsic nontoxicity and enzymatic degradability.48,55 Edible supercapacitors with naturally-derived food materials, including active charcoal as the electrode material, egg whites as the edible binder, and high-purity gold leaf as the current collector, can be connected in to power a red and dissolved in the This that naturally derived materials are promising nontoxic materials to biocompatible EES As for the implantable EES devices, of the to degradable devices has only degradation in or stimulated such as PBS that the of in vivo degradation is different from that of in degradation due to the between electrode materials and biological components (e.g., and To improve the properties and the for implantable applications, the used active materials for supercapacitors, such as CNT and have been with functional and conductive A biocompatible with aligned by oxygen was to operate in biological such as and The has that the synthesized with oxygen including and and for cell and than biophilized reduced graphene oxide as the electrode for nontoxic supercapacitors (Figure toxicity of materials (up to high of to mouse and cell was in the biocompatible The maximum nontoxic was times higher than that of GO electrodes. Besides surface and the conductive composites also for implantation. A of CNT electrodes, was into the abdominal cavity of a infection was during the implantation. Moreover, the implantable capacitance retention after 8 days of implantation. In energy the redox like the nicotinamide adenine dinucleotide to the energy storage by this redox system, an implantable CNT yarn was by CNT electrodes with NAD and Figure The biocompatible NAD/BQ/CNT yarn electrodes into the abdominal cavity of a exhibited the in vivo electrical of a even after 14 days (Figure Constitutional Dynamic Networks for EES Devices With the of electronics into soft and smart EES devices with and to respond to external stimuli have as a new of biointegrated devices. In kinds of smart EES devices have been including self-healing, thermal and EES devices. These emerging smart EES devices cannot be realized

Endowed with the expected visions for future surgery, minimally invasive surgery (MIS) has become one of the most rapid developing areas in modern surgery. Soft robotics, which originates from interdisciplinary advances in materials, fabrication, and electronics, featuring better adaptability and safer interaction, holds great promises in addressing current technical challenges in MIS, which are difficult to be solved with current rigid robotic technologies. For the first time, herein, the expected characteristics of next‐generation MIS from the surgeons’ perspectives are analyzed and the recent progress of soft surgical instruments from three different aspects is comprehensively summarized: engineering design, fabrication techniques, and human–robot interaction. Perspectives of next‐generation soft surgical robots are then discussed, where some exciting possibilities are emphasized. It is believed that further developments of intelligent soft robotics enable the next‐generation MIS to agilely navigate to the target and conduct dexterous diagnostic or therapeutic procedures without any trade‐offs in invasiveness and ultimately be a propitious solution for future surgery.

Fast-growing interests in safe and effective robot-environment interactions stimulated global investigations on soft robotics. The inherent compliance of soft robots ensures promising safety features but drastically reduces force capability, thereby complicating system modeling and control. To tackle these limitations, a soft robotic joint with enhanced strength, servo performance, and impact behavior shaping is proposed in this paper, based on novel three-dimensional-printed soft origami rotary actuators. The complete workflow is presented from the concept of origami design and analytical modeling, joint design, fabrication, control, and validation experiments. The proposed approach facilitates a fully customizable joint design towards the desired force capability and motion range. Validation results from models and experiments using multiple fabricated prototypes proved the excellent performance linearity and superior force capability, with 18.5-N·m maximum torque under 180 kPa, and 300-g self-weight. The behavior shaping capability is achieved by a low-level joint-angle servo and a high-level variable-stiffness regulation; this significantly reduces the impact torque by 53% and ensures powerful and safe interactions. The comprehensive guidelines provide insightful references for soft robotic design for wider robotic applications.

BackgroundSoft materials, with their compliant properties, enable conformity and safe interaction with human body. With the advance in actuation and sensing of soft materials, new paradigm in robotics called “soft robotics” emerges. Soft robotics has become a new approach in designing medical devices such as wearable robotic gloves and exoskeleton. However, application of soft robotics in surgical instrument inside human body is still in its infancy.AimsIn this paper, current application and design of soft robots specifically applied for endoscopy are reviewed.Materials & MethodsDifferent aspects in the implementation of soft robotics in endoscope design were reviewed. The key studies about MIS and NOTES were reviewed to establish the clinical background and extract the limitations of current endoscopic device in the last decade.Results and discussionIn this review study, the implementation of soft robotics concepts in endoscopic application, with highlights on different features of several soft endoscopes, were evaluated. The progress in different aspects of soft robotics endoscope, current state, and future perspectives were also discussed.ConclusionBased on the survey on the structural specification, actuation, sensing, and stiffening the future soft surgical endoscopes are recommended to fulfil the following specifications: safe especially from pressure leakage, fully biocompatible materials, MR‐compatible, capable for large bending in at least two antagonistic directions, modularity, adjustable stiffness.

FEM-based trajectory tracking control of a soft trunk robot

Proxy-based torque control of motor-driven exoskeletons for safe and compliant human-exoskeleton interaction

A design of Human and overhead Robot Interaction (HoRI) framework for cooperative robotic applications in copper industry

This paper reports on the development of a novel soft robotic system for remote ultrasound applications. Direct contact of the ultrasound probe with the patient's body represents a safety risk and therefore control of the probe's positioning and applied force is a crucial task. The proposed robot uses a passive control system that provides safe interaction between the robot and the patient by leveraging soft robotics technology. The soft robot's structure can be considered as a nonlinear spring which can be designed to exert a safe force within the robot's workspace to guarantee the safety of human-robot interaction. The literature suggests that effective ultrasound imaging of both the heart and abdomen requires six degrees-of-freedom. These degrees-of-freedom consist of three translational motions, which are achieved using a novel hybrid soft cable-driven parallel robot, and three wrist motions, which is based on a universal joint design. The experimental results show that the robot can achieve all these six degrees-of-freedom, and its blocking force can be engineered to generate a uniform force within the workspace.

Liquid metals—that is, metals with low melting points—have fascinating and unique properties. Gallium is one of the most popular liquid metals because it melts near room temperature, has low toxicity, effectively zero vapor pressure, and can be combined with other metals to form alloys. The term "liquid metal" can be more broadly defined to include metals that melt at temperatures that are easy to access under common laboratory conditions. Room-temperature liquid metals open unique possibilities to leverage the soft, fluidic characteristics of liquids while simultaneously taking advantage of properties like high surface energy and thin oxide layers, high electrical and thermal conductivity, and other metallic characteristics that are typically reserved for rigid, solid materials. This liquid-metallic combination has resulted in remarkable properties, unusual behaviors, and new materials and devices that cannot be created with typical fluids or common metals alone. Interest in liquid metals has grown over the last decade due to the interest in applications that take advantage of their properties. For example, liquid metals can be used as conductors and components in soft and stretchable electronics. Likewise, they can be used for soft biomedical devices, energy harvesting devices, and e-textiles. Liquid metals can help catalyze reactions in unique ways and have interfacial properties that can be exploited for a variety of applications, such as actuators, batteries, and substrates for the deposition of thin films. The versatility of liquid metals presents many opportunities for future materials, devices, and systems, and represents an exciting paradigm for both scientific exploration and the creation of novel technologies. The special issue on Liquid Metals for Functional Materials consists of 49 manuscripts with 37 research articles and 12 reviews across a wide breadth of the liquid metal field. The work focuses on the latest advancements from fundamental properties to applied concepts, as broadly classified as Energy and Electrochemistry, Biomedical and Healthcare, Catalysts and Reactions, Composites and Multiphase Materials, Electronics and Conductors, Fabrication and Patterning, and Interfacial Properties. This collection highlights the multidisciplinary nature of liquid metal research and provides a platform to showcase our current understanding and outline future directions. We sincerely thank Dr. Joseph Krumpfer and Dr. Esther Levy, whose dedication to this special issue energized this effort and led to the exciting collection of papers presented in this special issue. Below, we will briefly highlight the contents of the special issue through the lens of several of the key subtopics in the field. Liquid metals offer promising potential in future energy storage and harvesting. Their electrical conductivity, deformability, compatibility with electrochemistry, lithium dissolution capability, and the possibility of creating porous composites make them ideal materials for efficient ion and electron exchanges. The addition of small metallic components can significantly alter the electrochemical behavior of liquid metals. In a work published in this special issue, Tang and co-workers showed the modulation of electrochemical properties by studying fractals that are formed from liquid metal alloys (adfm.202301348). In another work, Li and co-workers presented mixtures of MXene with EGaIn for energy-harvesting (adfm.202307830). Additionally, Boley and co-workers created a galvanic cell by mechanically rupturing surface oxide of liquid metals to convert chemical energy to electrical energy (adfm.202309177). Quality of electrical conductivity and stretchability are important factors in textile-based electronics with embedded supercapacitors, which are carefully investigated in another work by So and co-workers (adfm.202310318). An interesting paper by Deng and co-workers presented the application of liquid metals for forming elastomer seals in stretchable lithium-ion storage units (adfm.202309861). Finally, the special issue includes a review by Yan and co-workers on advances in energy storage based on liquid metal systems (adfm.202309706). The low toxicity of gallium-based liquid metals makes them suitable for medical applications. The use of liquid metals in these areas has been growing in popularity, and this issue features several papers that review recent biomedicine-related liquid metal works. For example, one paper by Seo and co-workers summarized the use of low-melting-point metals for biomedical applications (adfm.202307708). One paper by Park and co-workers reviewed the usage of gallium-based liquid metals for bioelectronics (adfm.202307990) while another paper by Lim and co-workers reviewed healthcare-related applications of liquid metals in wearables and soft robotics (adfm.202309989). Additionally, Wei and co-workers reviewed liquid metal-based biosensors (adfm.202308173). In more specific applications, Truong and co-workers created silver-gallium amalgamated particles that show antibacterial properties and show promise as a spray-coating on implantable devices (adfm.202310539). In another work by Markvicka and co-workers, composites of elastomers and liquid metals were developed with acoustic properties that improve the image quality of wearable ultrasound devices for long-term patient monitoring (adfm.202308954). Additionally, liquid-metal electrodes and particles were used by Kim and co-workers to create soft biosensors capable of detecting ascorbic and uric acid, dopamine, and glucose (adfm.202311696). Currently, a significant portion of global greenhouse emissions and energy consumption stems from industrial-scale chemical reactions used in the production of ammonia, fuel, hydrogen, polymers, and other chemicals. Progress in enhancing catalysis and reaction rates using solid materials has been limited. Exploring the untapped properties of liquid metals holds great promise for introducing new paradigms in these chemical processes. Researchers offered several interesting works in this special issue on catalysts and reactions using liquid metals. Here, Daeneke and co-workers showed a delicate liquid metal system with incorporated copper for electrocatalytic oxidation of ethanol (adfm.202304248). Kalantar-Zadeh and co-workers demonstrated that liquid metal can be used as the reservoir for zinc metal for sourcing it into the synthesis metal–organic frameworks (adfm.202300969). While another study put forward by Yarema and co-workers presented an approach to use liquid metal to create Pd-Zn nanocrystals (adfm.202309018). In another seminal manuscript, Sitti and co-workers utilized liquid metal initiated polymerization to create hydrogel composites (adfm.202308238). A liquid metal reaction media was also used by Zavabeti and co-workers for the creation of atomically thin bismuth oxides that enabled strong piezoelectric systems (adfm.202307348). O'Mullane and co-workers showed that liquid metals have a potential use in plasma-assisted carbon dioxide reduction by incorporating liquid metal droplets (adfm.202307846). The capabilities of liquid metals are not just limited to inorganic systems. Miyako and co-workers showed that liquid metal catalysts can also be effectively used in immunostimulants (adfm.202305886). Liquid metal can be combined with diverse components such as polymers, metals, carbon-based materials, or other organic or inorganic materials to create composites and multiphase systems. This can create novel composites with enhanced functional or mechanical properties relative to solid-based inclusions or other phases can be added into liquid metal to provide new properties not native to liquid metal. These concepts were well captured in the special issue. One review by Kramer-Bottiglio and co-workers focused on multiphase composites containing liquid metal and other (x) fillers for unique combinations of properties (adfm.202309529) while another review by Lee and co-workers showed how adding particles into liquid metal can create magnetic liquid metal (adfm.202311153). A third review by Tee and co-workers discussed liquid metal composites for wearable electronics (adfm.202400284). Additionally, a set of papers presented liquid-metal polymer composites with different functionalities. One work by Bartlett and co-workers showed how liquid-metal droplets in elastomers could create electrically conductive reversible adhesives for soft electronics (adfm.202304101), while another by Li and co-workers showed hydrogel composites with high toughness and conductivity (adfm.202308113). This polymer composite architecture also enabled thermally stable soft materials for high-temperature applications as presented by Majidi and co-workers (adfm.202309725) while Zhou and co-workers used liquid metal droplets as cross-links to enable recyclable conductive composites (adfm.202308032). The high deformability of liquid metal wires was also used in conjunction with a magnetic soft composite by Park and co-workers to create artificial muscles (adfm.202302895). Like solid metals, liquid metals have a high electrical conductivity, making them suitable as conductors in electronics, but with the added properties of liquids, such as discretization into droplets. One review by Hussain and co-workers examined liquid-metal droplets and their use for electronics such as sensors, switches, transistors, actuators, and more (adfm.202308116). Liquid metals are also especially suitable for soft, flexible, and stretchable electronics. Toward this, Zhang and co-workers created a soft, lightweight composite with networks of liquid-metal fibers (adfm.202308128) while Zhang and co-workers developed liquid-metal composite materials that show increased conductivity with applied strain (adfm.202310225). In another contribution, Zheng and co-workers demonstrated a permeable and stretchable liquid-metal fibers for sensors and other electronics (adfm.202308120). Bartlett and co-workers presented a liquid metal-based conductive adhesive for integration of soft electronics and rigid devices (adfm.202313567) while Hjort and co-workers presented a laser engraving methodology for creating liquid metal-based interconnects (adfm.202309707). Other papers in this issue describe the use of liquid metals in specific electronic devices. One review presented by Park and co-workers examined liquid-metal systems that respond to a variety of stimuli, and that can be used for electronics such as wearable sensors (adfm.202308703). The compliant nature of liquid metal was used by Lacour and co-workers to make sensors that can measure softness (adfm.202308698). A liquid-metal inductive sensor was created by Jeong and co-workers that is capable of distinguishing between various stimuli on a finger (adfm.202305776), and a wearable liquid-metal antenna was directly printed by Hashimoto and co-workers (adfm.202311219). Liquid metals can be patterned into useful shapes such as circuits, antennas, and wires to create soft and stretchable analogs to existing electronic devices. Relative to conventional metals, such as copper or aluminum, the fluidic nature of liquid metal allows it to be patterned in unique ways, including injection and 3D printing. In many cases, patterning is facilitated by the thin, solid oxide layer that forms rapidly on the surface of liquid metals in the presence of oxygen. This solid oxide "skin" allows the liquid metal to retain shapes that would normally be impossible with liquids due to surface tension. This special issue offers several interesting works on the fabrication and patterning of liquid metals. Liquid metals were 3D printed in ceramics by Yang and co-workers for use in microwave absorption (adfm.202307499). In another example, Syed and co-workers separated the oxide layer from the liquid metal to enable the fabrication of gas sensors (adfm.202309342). Park and co-workers also took advantage of the ability to use liquid metal particles to pattern stretchable electronics (adfm.202309660). The particles can be used to make thermally conductive composites as demonstrated by Lee and co-workers (adfm.202306698) or be cast as a film and subsequently merged together to form conductive traces as presented by Dickey and co-workers (adfm.202308574). Liquid metals have remarkable interfacial properties. For example, they have the largest interfacial tension of any liquid at room temperature, with values nearly an order of magnitude larger than that of water. Yet, the surface tension effectively can be lowered to near zero by using electrochemical oxidation. This phenomenon, as well as several others, can be used to control the flow and shape of liquid metal, as reviewed by Wang and co-workers in this special issue (adfm.202309614). In addition, studies by Daniels and co-workers provide new insights into this electrochemical phenomenon by carefully measuring the tension as a function of electrical potential (adfm.202311501) or by confining the metal to porous tubes as demonstrated by Khan and co-workers (adfm.202307919). Another interesting property of liquid metals is that they can react to form surface oxides on their surface. Herein, Tabor and co-workers enhance the mechanical strength of the skin by depositing thin silica layers on the oxide (adfm.202308167). Further, Elbourne and co-workers evaluated the structure of the oxide on liquid metal droplets (adfm.202310147) while Koo and co-workers utilized the oxide-coated liquid metal to form more stable solar cells (adfm.202311597). We are grateful for all the authors, reviewers, and editors who made this special issue possible. We hope this special issue will help highlight the challenges and exciting opportunities for the development and utilization of liquid metal in diverse applications. M.D.B., M.D.D., A.T.O., and K.K-Z. contributed equally to this work. The authors declare no conflict of interest Michael D. Bartlett is an associate professor and John R. Jones III Faculty Fellow of Mechanical Engineering at Virginia Tech. He received his B.S.E. from the University of Michigan, Ph.D. from the University of Massachusetts Amherst, and was a postdoctoral fellow at Carnegie Mellon University. Michael leads the Soft Materials and Structures Lab, which investigates multifunctional soft materials and composites with highly controllable mechanical and functional properties for the creation of soft electronics and robotics based on liquid metal, switchable and intelligent adhesives, and adaptive materials. Michael Dickey is the Camille and Henry Dreyfus Professor in the Department of Chemical & Biomolecular Engineering at NC State University. He received a B.S. in chemical engineering from Georgia Institute of Technology (1999) and a Ph.D. from the University of Texas (2006) under the guidance of Professor Grant Willson. From 2006 to 2008, he was a post-doctoral fellow in the lab of Professor George Whitesides at Harvard University. He completed a sabbatical at Microsoft in 2016 and EPFL in 2023. Michael's research interests include soft matter (liquid metals, gels, polymers) for soft and stretchable devices (electronics, energy harvesters, and soft robotics). Aaron Ohta is a professor in the Department of Electrical and Computer Engineering at the University of Hawaii at Manoa. He received his B.S. degree from the University of Hawaii at Manoa, his M.S. degree from the University of California, Los Angeles, and his Ph.D. degree from the University of California, Berkeley, all in electrical engineering. Aaron's research interests include reconfigurable circuits and systems using liquid metals and other materials, microfluidics, and microelectromechanical systems (MEMS). Kourosh Kalantar-Zadeh is a professor and Head of School of Chemical and Biomolecular Engineering at the University of Sydney. He is involved in research in the fields of analytical chemistry, materials sciences, gastroenterology, electronics, and sensors. Professor Kalantar-Zadeh is best known for his works on ingestible sensors, liquid metals, and 2D semiconductors. He led his group to the invention of an ingestible chemical sensor: human gas sensing capsule, one of the breakthroughs in the field of medical devices. He has received several international awards for his scientific contributions including the 2017 IEEE Sensor Council Achievement, and 2020 Robert Boyle Prize of RSC.

As autonomous systems begin to operate amongst humans, methods for safe interaction must be investigated. We consider an example of a small autonomous vehicle in a pedestrian zone that must safely maneuver around people in a free-form fashion. We investigate two key questions: How can we effectively integrate pedestrian intent estimation into our autonomous stack? Can we develop an online monitoring framework to give rigorous assurances on the safety of such human-robot interactions? We present a pedestrian intent estimation framework that can accurately predict future pedestrian trajectories given multiple possible goal locations. We integrate this into a reachability-based online monitoring and decision making scheme that formally assesses the safety of these interactions with nearly real-time performance (approximately 0. 1s). These techniques are both tested in simulation and integrated on a test vehicle with a complete in-house autonomous stack, demonstrating safe interaction in real-world experiments.

Skin-inspired textile-based tactile sensors enable multifunctional sensing of wearables and soft robots

Soft pneumatic actuators (SPAs) are commonly used in various applications because of their structural compliance, low cost, ease of manufacture, high adaptability, and safe human-robot interaction. The traditional approach for achieving proportional control of soft pneumatic robots requires the use of industrial proportional valves or syringe drivers, which are not only rigid and bulky but also hard to be integrated into the body of soft robots. In our previous research, we developed a Magnetorheological elastomer (MRE)-based soft valve that showed advantages for controlling SPAs due to its compliance, compactness, robustness, and compatibility for continuous pressure modulation. Modern soft robots with multiple chambers require more MRE valves onboard for their control. However, merely packing more MRE valves for soft robots can cause problems like magnetic interference, flow rate deviation, and overheating. Therefore, in this study, we proposed a two-dimensional MRE valve array design to solve issues of magnetic interference and overheating when expanding from a single MRE proportional valve into an integrated array. The magnetic interference and the overheating problem were investigated through multiphysics simulation, bringing the optimal choice of valve spacing (1.2 times the single valve diameter), magnetic coil pole arrangement (same pole), and the cooling system design (internal cooling chamber with flowing water). Physical experiments showed that our MRE valve array maintained its original flowrate performance with low magnetic interference (0.89 mT) and low coil temperature (under 73.9°C for 5 min).

Fluid actuated soft robots, or fluidic elastomer actuators, have shown great potential in robotic applications where large compliance and safe interaction are dominant concerns. They have been widely studied in wearable robotics, prosthetics, and rehabilitations in recent years. However, such soft robots and actuators are tethered to a bulky pump and controlled by various valves, limiting their applications to a small confined space. In this study, we report a new and effective approach to fluidic power actuation that is untethered, easy to design, fabricate, control, and allows various modes of actuation. In the proposed approach, a sealed elastic tube filled with fluid (gas or liquid) is segmented by adaptors. When twisting a segment, two major effects could be observed: (1) the twisted segment exhibits a contraction force and (2) other segments inflate or deform according to their constraint patterns. Utilizing such effects, various actuation modes could be realized. In this research, four modes of actuation are illustrated: (1) soft actuator and pump actuation, (2) serial actuation, (3) parallel actuation, and (4) agonist and antagonist actuation. Theoretic analysis and experimental studies for the basic actuation principle have been conducted. A case study on an anthropomorphic forearm based on the proposed twisting tube actuation has been developed to showcase the effectiveness of the actuation modes. The studies suggest that the proposed approach has a great potential in both soft and compliant robotics.

Fluid-driven and smart material-driven research for soft body robots

Soft robots equipped with multifunctionalities have been increasingly needed for secure, adaptive, and autonomous functioning in unknown and unpredictable environments. Robotic stacking is a promising solution to increase the functional diversity of soft robots, which are required for safe human-machine interactions and adapting in unstructured environments. However, most existing multifunctional soft robots have a limited number of functions or have not fully shown the superiority of the robotic stacking method. In this study, we present a novel robotic stacking strategy, Netting-Rolling-Splicing (NRS) stacking, based on a dimensional raising method via 2D-to-3D rolling-and-splicing of netted stackable pneumatic artificial muscles to quickly and efficiently fabricate multifunctional soft robots based on the same, simple, and cost-effective elements. To demonstrate it, we developed a TriUnit robot that can crawl 0.46 ± 0.022 body length per second (BL/s) and climb 0.11 BL/s, and can carry a 3 kg payload while climbing. Also, the TriUnit can be used to achieve novel omnidirectional pipe climbing including rotating climbing, and conduct bionic swallowing-and-regurgitating, multi-degree-of-freedom manipulation based on their multimodal combinations. Apart from these, steady rolling, with a speed of 0.19 BL/s, can be achieved by using a pentagon unit. Furthermore, we applied the TriUnit pipe climbing robot in panoramic shooting and cargo transferring to demonstrate the robot's adaptability for different tasks. The NRS stacking-driven soft robot here has demonstrated the best overall performance among existing stackable soft robots, representing a new and effective way for building multifunctional and multimodal soft robots in a cost-effective and efficient way.