Touch-Responsive Hydrogel for Biomimetic Flytrap-Like Soft Actuator

Disturbance-Triggered Instant Crystallization Activating Bioinspired Emissive Gels

Many marine organisms feature sensitive sensory-perceptual systems to sense the surrounding environment and respond to disturbance with intense bioluminescence. However, it remains a great challenge to develop artificial materials that can sense external disturbance and simultaneously activate intense luminescence, although such materials are attractive for visual sensing and intelligent displays. Herein, we present a new class of bioinspired smart gels constructed by integrating hydrophilic polymeric networks, metastable supersaturated salt and fluorophores containing heterogenic atoms. Upon external disturbance, the composite gels undergo an instant and reversible soft-rigid state transition, simultaneously turning on intense fluorescence and activating ultralong afterglow emission with a maximum lifetime of 877.15 ms. The experimental results and molecular dynamics simulations reveal that the disturbance-induced luminescence mainly results from the geometrical confinement of aggregated fluorophores and enhanced molecular interactions to immensely suppress the non-radiative dissipation. Given their versatile and sensitive disturbance-responsiveness, dynamic interactive painting and 3D smart optical displays are demonstrated. This study paves a new avenue to achieve disturbance-sensing soft materials and promotes the development of smart visual sensors and interactive optical displays.

Hydrogel-Based Continuum Soft Robots

This paper comprehensively reviews the latest advances in hydrogel-based continuum soft robots. Hydrogels exhibit exceptional flexibility and adaptability compared to traditional robots reliant on rigid structures, making them ideal as biomimetic robotic skins and platforms for constructing highly accurate, real-time responsive sensory interfaces. The article systematically summarizes recent research developments across several key dimensions, including application domains, fabrication methods, actuator technologies, and sensing mechanisms. From an application perspective, developments span healthcare, manufacturing, and agriculture. Regarding fabrication techniques, the paper extensively explores crosslinking methods, additive manufacturing, microfluidics, and other related processes. Additionally, the article categorizes and thoroughly discusses various hydrogel-based actuators responsive to solute/solvent variations, pH, chemical reactions, temperature, light, magnetic fields, electric fields, hydraulic/electro-osmotic stimuli, and humidity. It also details the strategies for designing and implementing diverse sensors, including strain, pressure, humidity, conductive, magnetic, thermal, gas, optical, and multimodal sensors. Finally, the paper offers an in-depth discussion of the prospective applications of hydrogel-based continuum soft robots, particularly emphasizing their potential in medical and industrial fields. Concluding remarks include a forward-looking outlook highlighting future challenges and promising research directions.

A highly stretchable, self-healing, self-adhesive polyacrylic acid/chitosan multifunctional composite hydrogel for flexible strain sensors

Conductive hydrogels have emerged as excellent candidates for the design and construction of flexible wearable sensors and have attracted great attention in the field of wearable sensors. However, there are still serious challenges to integrating high stretchability, self-healing, self-adhesion, excellent sensing properties, and good biocompatibility into hydrogel wearable devices through easy and green strategies. In this paper, multifunctional conductive hydrogels (PCGB) with good biocompatibility, high tensile (1694 % strain), self-adhesive, and self-healing properties were fabricated by incorporating boric acid (BA) and glucose (Glu) simultaneously into polyacrylic acid (PAA) and chitosan (CS) polymer networks using a simple one-pot polymerization method. Furthermore, the hydrogel strain sensor constructed from the PCGB assembly had great sensing property including high sensitivity (GF = 5.7), durability and stability (5000 cycles). The hydrogel strain sensor was applied to the detection of human motion, which exhibited accurate detection behavior for both large-scale motions and small activities. A strategy to design and fabricate multifunctional conductive hydrogels integrating high stretchability, self-healing, self-adhesion and good biocompatibility was provided, and the multifunctional conductive hydrogels broadened the application of hydrogel-based wearable sensor.

Environmentally Friendly, Dual-Responsive Actuator Based on Nafion, Carboxylated Multiwalled Carbon Nanotubes, and Polyethylene

As a type of smart material, flexible stimulus-responsive actuators have become a hot research topic nowadays. However, flexible actuators responding to a single stimulus source are susceptible to external perturbations, which may lead to an unstable function or even failure. Therefore, in this paper, we proposed a bilayer actuator that can be driven by both humidity and light by combining the humidity-sensitive Nafion, carboxylated multiwalled carbon nanotubes (cMWCNT) with excellent photothermal conversion properties, and commercial polyethylene (PE) tape with good humidity insensitivity and thermal expansion. First, the cMWCNT-Nafion film was prepared by a solution casting method and bonded together with PE tape to obtain a bilayer actuator. Then, the effects of the cMWCNT mass fraction and film thickness on the humidity and light response performance of the bilayer actuator were investigated. The optimal ratios of raw materials were obtained for different stimulation sources, respectively. Furthermore, the performance of the bilayer actuator with the optimal ratios was tested; it was verified that the proposed dual-responsive actuator can realize different degrees of bending deformation under different relative humidity (RH) and ultraviolet (UV) light intensity with good stability. Finally, the application potential in multiple scenarios was further verified by applying the prepared cMWCNT-Nafion/PE bilayer actuator to a smart window, crawling robot, and flexible gripper. This paper will provide a meaningful reference for the development and performance optimization of high-performance dual-responsive actuators.

Curving-Stretching Induced Alignment in Hydrogel Actuators for Enhanced Grip Strength and Rapid Response

Natural tissues, like ligaments and tendons, display not just robust mechanical performance but also complex anisotropic structures extending beyond one-directional arrangements. However, fabricating hydrogel actuators with biomimetic three-dimensional anisotropy remains challenging. Herein, a simple strategy involving curving-stretching induced alignment is proposed to prepare anisotropic Fe3+-cross-linked poly(acrylic acid)-poly(acrylamide) hydrogel actuators. These hydrogels exhibit exceptional mechanical properties, boasting a fracture stress of 7.1 MPa and a superior modulus of 33.2 MPa when prestretched to 200% strain, which are 2.3 times and 4.9 times higher than their unstretched counterparts. The stretched anisotropic hydrogel gripper, stronger than its unstretched counterpart, can lift heavy objects while also achieving rapid responsiveness to stimuli. This work introduces a novel and effective method for fabricating anisotropic hydrogels, highlighting their broad applicability in fields such as soft robotics, biomedical devices, and beyond.

Crystallization-Induced Liquid Gate for Tunable Gas Flow Control

Gas flow control is essential in multifarious fields, such as chemical engineering, environmental governance, and biomedical science. More precise regulation, especially tunable gas flow rates, will spark further applications in smart valves, microreactors, and drug delivery. Here, we propose a crystallization-induced liquid gate (CILG) comprising a supersaturated gating liquid confined within a solid framework capable of tunable gas flow rates under steady-state pressure in a simple and compact manner. When ultrasound is employed to stimulate the crystallization, the CILG exhibits different gas transport behaviors due to the adjustable pore sizes modulated by crystal morphologies under varied ultrasound intensities. Additionally, the exothermic crystallization process allows CILG with variable gas permeability to be observable via infrared imaging. Moreover, we demonstrate the potential applications of CILG in infrared-monitored flow-regulating valves and gas-involved chemical reactors.

Bioinspired H-Bonding Connected Gradient Nanostructure Actuators Based on Cellulose Nanofibrils and Graphene

The construction of flexible actuators with ultra-fast actuation and robust mechanical properties is crucial for soft robotics and smart devices, but still remains a challenge. Inspired by the unique mechanism of pinecones dispersing seeds in nature, a hygroscopic actuator with interlayer network-bonding connected gradient structure is fabricated. Unlike most conventional bilayer actuator designs, the strategy leverages biobased polyphenols to construct strong interfacial H-bonding networks between 1D cellulose nanofibers and 2D graphene oxide, endowing the materials with high tensile strength (172 MPa) and excellent toughness (6.64 MJ m-3). Furthermore, the significant difference in hydrophilicity between GO and rGO, along with the dense interlayer H-bonding, enables ultra-fast water exchange during water absorption and desorption processes. The resulted actuator exhibits ultra-fast driving speed (154° s-1), excellent pressure-resistant and cyclic stability. Taking advantages of these benefits, the actuator can be fabricated into smart devices (such as smart grippers, humidity control switches) with significant potential for practical applications. The presented approach to constructing interlayer H-bonding in gradient structures is instructive for achieving high performance and functionalization of biomass nanomaterials and the complex of 1D/2D nanomaterials.

Robotic flytrap with an ultra-sensitive 'trichome' and fast-response 'lobes'

Nature abounds with examples of ultra-sensitive perception and agile body transformation for highly efficient predation as well as extraordinary adaptation to complex environments. Flytraps, as a representative example, could effectively detect the most minute physical stimulation of insects and respond instantly, inspiring numerous robotic designs and applications. However, current robotic flytraps face challenges in reproducing the ultra-sensitive insect-touch perception. In addition, fast and fully-covered capture of live insects with robotic flytraps remains elusive. Here we report a novel design of a robotic flytrap with an ultra-sensitive 'trichome' and bistable fast-response 'lobes'. Our results show that the 'trichome' of the proposed robotic flytrap could detect and respond to both the external stimulation of 0.45 mN and a tiny touch of a flying bee with a weight of 0.12 g. Besides, once the 'trichome' is triggered, the bistable 'lobes' could instantly close themselves in 0.2 s to form a fully-covered cage to trap the bees, and reopen to set them free after the tests. We introduce the design, modeling, optimization, and verification of the robotic flytrap, and envision broader applications of this technology in ultra-sensitive perception, fast-response grasping, and biomedical engineering studies.

A Cautionary Perspective on Hydrogel-Induced Concentration Gradient Generation for Studying Chemotaxis

The achievement of consistent and static chemical gradients is critically important in the study of diffusion and chemotaxis at the micro- and nanoscales. In this context, a number of groups have reported on hydrogel-based systems for generating concentration gradients. Here, we analyze the behavior of agarose and gelatin-based hydrogels in hybridization chambers of different heights. Our focus is on the issues that are caused by the presence of robust bulk fluid flows in such systems due to the solutes present in the hydrogel and/or the surrounding fluid. We describe the key insights derived from these experiments, offering practical guidelines for establishing gradients using hydrogel-based systems and make the community aware of different variables that can make the experiments nonreproducible and prone to misinterpretations.

A Gradient Stiffness-Programmed Circuit Board by Spatially Controlled Phase-Transition of Supercooled Hydrogel for Stretchable Electronics Integration

Due to emerging demands in soft electronics, there is an increasing need for material architectures that support robust interfacing between soft substrates, stretchable electrical interconnects, and embedded rigid microelectronics chips. Though researchers have adopted rigid-island structures to solve the issue, this approach merely shifts stress concentrations from chip-conductor interfaces to rigid-island-soft region interfaces in the substrate. Here, a gradient stiffness-programmed circuit board (GS-PCB) that possesses high stretchability and stability with surface mounted chips is introduced. The board comprises a stiffness-programmed hydrogel substrate and a laser-patterned liquid metal conductor. The hydrogel simultaneously obtains a large stiffness disparity and robust interfaces between rigid-islands and soft regions. These seemingly contradictory conditions are accomplished by adopting a gradient stiffness structure at the interfaces, enabled by combining polymers with different interaction energies and a supercooled sodium acetate solution. By integrating the gel with laser-patterned liquid metal with exceptional properties, GS-PCB exhibits higher electromechanical stability than other rigid-island research. To highlight the practicality of this approach, a finger-sensor device that successfully distinguishes objects by direct physical contact is fabricated, demonstrating its stability under various mechanical disturbances.

Dopamine-Modified Chitosan Patterning Hydrogel with Dynamic Information Storage Ability

The storage of dynamic information in hydrogels has aroused considerable interest regarding the multiple responsiveness of soft matter. Herein, we propose an electrical writing methodology to prepare dopamine (DA)-modified chitosan hydrogels with a dynamic information storage ability. A pH-responsive chitosan hydrogel medium was patterned by cathodic writing to in situ generate OH- in the writing area, at which dopamine underwent an auto-oxidation reaction in the locally alkaline environment to generate a dark color. The patterned information on the hydrogel can be encoded simply by electrical signals. The speed of information retrieval is positively correlated with the charge transfer during the electrical writing process, and the hiding of information is negatively correlated with the environmental stimulus (i.e., dopamine concentration, pH value, etc.). To showcase the versatility of this medium for information storage and the precision of the pattern, a quick response (QR) code is electronically written on dopamine-modified chitosan hydrogel and can be recognized programmably by a standard mobile phone. The results show that electrical regulation is a novel means to program the information storage process of hydrogels, which inspires future research on structural and functional information storage using stimulus-responsive hydrogels.

An Electrochemically Initiated Self-Limiting Hydrogel Electrolyte for Dendrite-Free Zinc Anode

The zinc dendrite growth generally relies upon a "positive-feedback" mode, where the fast-grown tips receive higher current densities and ion fluxes. In this study, a self-limiting polyacrylamide (PAM) hydrogel that presents negative feedback to dendrite growth is developed. The monomers are purposefully polymerized at the dendrite tips, then the hydrogel reduces the local current density and ion flux by limiting zinc ion diffusion with abundant functional groups. As a consequence, the accumulation at the dendrite tips is restricted, and the (002) facets-oriented deposition is achieved. Moreover, the refined porous structure of the gel enhances Coulombic Efficiency by reducing water activity. Due to the synergistic effects, the zinc anodes perform an ultralong lifetime of 5100 h at 0.5 mA cm-2 and 1500 h at 5 mA cm-2, which are among the best records for PAM-based gel electrolytes. Further, the hydrogel significantly prolongs the lifespan of zinc-ion batteries and capacitors by dozens of times. The developed in situ hydrogel presents a feasible and cost-effective way to commercialize zinc anodes and provides inspiration for future research on dendrite suppression using the negative-feedback mechanism.

Hydrogel-Based Sensors for Human-Machine Interaction

In the past decades, remarkable progress has been made in the field of human-machine interaction. The need for accurate sensing devices with satisfactory user experiences has propelled the development of flexible, stretchable, biocompatible, and imperceptible hydrogel-based interfaces. These innovative interfaces facilitate direct interactions between humans and machines while receiving detected input signals from sensors and giving output commands to controllers, thus motivating accurate real-time responsiveness. This Perspective discusses the sensing mechanisms for the two categories of hydrogel-based sensors and summarizes the recent progress in the development of different representations of human-machine interactions, including intelligent identification, information secrecy, interactive control, and virtual reality and augmented reality technologies. The advantages of hydrogel-based systems over conventionally used rigid electrical components are explicitly discussed. The conclusion provides a perspective on current challenges and outlines a future roadmap for the realization of state-of-the-art hydrogel-based smart systems.

Bionic ordered structured hydrogels: structure types, design strategies, optimization mechanism of mechanical properties and applications

Natural organisms, such as lobsters, lotus, and humans, exhibit exceptional mechanical properties due to their ordered structures. However, traditional hydrogels have limitations in their mechanical and physical properties due to their disordered molecular structures when compared with natural organisms. Therefore, inspired by nature and the properties of hydrogels similar to those of biological soft tissues, researchers are increasingly focusing on how to investigate bionic ordered structured hydrogels and render them as bioengineering soft materials with unique mechanical properties. In this paper, we systematically introduce the various structure types, design strategies, and optimization mechanisms used to enhance the strength, toughness, and anti-fatigue properties of bionic ordered structured hydrogels in recent years. We further review the potential applications of bionic ordered structured hydrogels in various fields, including sensors, bioremediation materials, actuators, and impact-resistant materials. Finally, we summarize the challenges and future development prospects of bionic ordered structured hydrogels in preparation and applications.

A Programmable Actuator as Synthetic Earthworm

Natural earthworm with the ability to loosen soils that favors sustainable agriculture has inspired worldwide interest in the design of intelligent actuators. Given the inability to carry heavy loads and uncontrolled deformation, the vast majority of actuators can only perform simple tasks by bending, contraction, or elongation. Herein, a degradable actuator with the ability to deform in desired ways is presented, which successfully mimics the burrowing activities of earthworms to loosen soils with increased soil porosity by digging, grabbing, and lifting the soil when it receives rains. Such a scarifying actuator is made of degradable cellulose acetate and uncrosslinked polyacrylamide via the swelling-photopolymerizing method. The water absorption of polyacrylamide in moisture conditions causes rapid and remarkable bending. Such mechanical bending can be controlled in specific areas of the cellulose acetate film if polyacrylamide is polymerized in a patterned way, so as to generate complicated deformations of the whole cellulose acetate. Patterning polyacrylamide within cellulose acetate is achieved based on reversible surface protection by means of pen writing, rather than the traditional masking techniques. The water-induced deformation of programmable cellulose-based actuators is well preserved in soil, which is appropriate for promoting rain diffusion as well as root breath.

Environmentally adaptive polysaccharide-based hydrogels and their applications in extreme conditions: A review

Polysaccharide hydrogels are one of the most promising hydrogel materials due to their inherent characteristics, including biocompatibility, biodegradability, renewability, and easy modification, and their structure and functional designs have been widely researched to adapt to different application scenarios as well as to broaden their application fields. As typical wet-soft materials, the high water content and water-absorbing ability of polysaccharide-based hydrogels (PHs) are conducive to their wide biomedical applications, such as wound healing, tissue repair, and drug delivery. In addition, along with technological progress, PHs have shown potential application prospects in some high-tech fields, including human-computer interaction, intelligent driving, smart dressing, flexible sensors, etc. However, in practical applications, due to the poor ability of PHs to resist freezing below zero, dehydration at high temperature, and acid-base/swelling-induced deformation in a solution environment, they are prone to lose their wet-soft peculiarities, including structural integrity, injectability, flexibility, transparency, conductivity and other inherent characteristics, which greatly limit their high-tech applications. Hence, reducing their freezing point, enhancing their high-temperature dehydration resistance, and improving their extreme solution tolerance are powerful approaches to endow PHs with multienvironmental adaptability, broadening their application areas. This report systematically reviews the study advances of environmentally adaptive polysaccharide-based hydrogels (EAPHs), comprising anti-icing hydrogels, high temperature/dehydration resistant hydrogels, and acid/base/swelling deformation resistant hydrogels in recent years. First, the construction methods of EAPHs are presented, and the mechanisms and properties of freeze-resistant, high temperature/dehydration-resistant, and acid/base/swelling deformation-resistant adaptations are simply demonstrated. Meanwhile, the features of different strategies to prepare EAPHs as well as the strategies of simultaneously attaining multienvironmental adaptability are reviewed. Then, the applications of extreme EAPHs are summarized, and some meaningful works are well introduced. Finally, the issues and future outlooks of PH environment adaptation research are elucidated.

Smart Triboelectric Nanogenerators Based on Stimulus-Response Materials: From Intelligent Applications to Self-Powered Systems

Smart responsive materials can react to external stimuli via a reversible mechanism and can be directly combined with a triboelectric nanogenerator (TENG) to deliver various intelligent applications, such as sensors, actuators, robots, artificial muscles, and controlled drug delivery. Not only that, mechanical energy in the reversible response of innovative materials can be scavenged and transformed into decipherable electrical signals. Because of the high dependence of amplitude and frequency on environmental stimuli, self-powered intelligent systems may be thus built and present an immediate response to stress, electrical current, temperature, magnetic field, or even chemical compounds. This review summarizes the recent research progress of smart TENGs based on stimulus-response materials. After briefly introducing the working principle of TENG, we discuss the implementation of smart materials in TENGs with a classification of several sub-groups: shape-memory alloy, piezoelectric materials, magneto-rheological, and electro-rheological materials. While we focus on their design strategy and function collaboration, applications in robots, clinical treatment, and sensors are described in detail to show the versatility and promising future of smart TNEGs. In the end, challenges and outlooks in this field are highlighted, with an aim to promote the integration of varied advanced intelligent technologies into compact, diverse functional packages in a self-powered mode.

Humidity-Responsive Shape Memory Polyurea with a High Energy Output Based on Reversible Cross-Linked Networks

High-performance shape memory polymers with multifunctions are essential in sensors, wearable flexible electronics, artificial muscle actuators, and reversible morphing structures. In this work, a transparent and humidity-responsive shape memory polyurea featuring a high tensile strength (51 MPa), a high recovery stress (12 MPa) with an high energy output (0.98 J/g), and tolerance to extreme environments (retains great malleability at -196 °C) is prepared through constructing a bioinspired hard-soft nanophase structure and through hierarchical hydrogen bonding in the molecular network. The hard segment of a strong hydrogen bonding region is in charge of humidity-responsive behavior, and the soft segment of a weak bonding region provides the flexibility of the molecular chain. Furthermore, the periodicity of the phase-separated domains is 12 nm as characterized by small-angle X-ray scattering. The hydrogen bonding cross-linked network can be opened under the action of stress and re-bonded by heating, just like a zipper structure of reversible linking property. This unique molecular structure contributes to the humidity-responsive behavior of polyurea rolling up 160° in 20 s on the palm, as well as a high energy output lifting a 100 g weight exceeding 1631 times its own mass to 60 mm. The molecular structure of the hard-soft nanophase and the hierarchical hydrogen bonding offer an effective approach toward achieving a high-performance shape memory polymer with humidity-sensitive functions.