Autonomous, adaptable, and multimodal locomotion capabilities, which are crucial for the advanced intelligence of biological systems. A prominent focus of investigations in the domain of bionic soft robotics pertains to the emulation of autonomous motion, as observed in natural organisms. This research endeavor faces the challenge of enabling spontaneous and sustained motion in soft robots without relying on external stimuli. Considerable progress has been made in the development of autonomous bionic soft robots that utilize smart polymer materials, particularly in the realms of material design, microfabrication technology, and operational mechanisms. Nonetheless, there remains a conspicuous deficiency in the literature concerning a thorough review of this subject matter. This study aims to provide a comprehensive review of autonomous soft robots that have been developed based on self-regulation strategies that encompass self-propulsion, self-oscillation, multi-stimulus response, and topological constraint structures. Furthermore, this review engages in an in-depth discussion regarding their tunable self-sustaining motion and recovery capabilities, while also contemplating the future development of autonomous soft robotic systems and their potential applications in fields such as biomechanics.

Achieving continuous motions typically requires dynamic external stimuli for cyclic deformation, or crafted geometries with intricate modules to form a self-regulated feedback loop upon static stimulation. It is still a grand challenge to realize self-sustained motion in soft robots subject to unchanging environment, without complex geometry or a control module. In this work, we report soft robots based on an anisotropic cylindrical hydrogel showing self-regulated, continuous rolling motions under constant light irradiation. The robots are animated by mirror-symmetry-breaking induced by photothermal strain gradient. The self-sustained motion is attributed to the fast and reversible deformation of the gel and the autonomous refresh of the irradiated region during the rolling motion. The hydrogel robots can reach a rolling speed of 1.27 mm·s⁻¹ on a horizonal surface and even climb a ramp of 18° at a speed of 0.57 mm·s⁻¹ in an aqueous environment. Furthermore, the hydrogel robots can overcome an obstacle, with rolling direction controllable through irradiation angle of the light and local irradiation on selective regions. This work suggests a facile strategy to develop hydrogel robots and may provide unforeseen inspirations for the design of self-regulated soft robots by using other intelligent materials.

Fibers with deformation-triggered responses are essential for smart textiles and wearable electronics. Here, smart core-shell elastomer fibers with a conductive core and a liquid crystal elastomer shell showing simultaneous resistance and color responses are designed and prepared. The conductive core is consisted of interconnected liquid metal nanodroplets dispersed in a polymer matrix and the elastomer shell is made of cholesteric liquid crystals. When stretched, the fiber resistance increases as the interconnected pathways of liquid metal nanodroplets along the fiber axis become narrower, and the selective reflection color from the fiber surface blueshifts since the cholesteric pitch decreases. The smart elastomer fibers could be woven into smart textiles and respond to various mechanical deformations, including stretching, bending, compression and twisting. The average resistance change is 51% under 100% strain and its variation is smaller than 4% over 500 cycles, showing remarkable fatigue resistance. The simultaneous resistance and color responses to mechanical deformations make the fibers attractive for broad applications, such as flexible electronics.

Stimuli-responsive shape-changing materials, particularly hydrogel and liquid crystal elastomer (LCE), have demonstrated significant potential for applications across various fields. Although intricate deformation and actuation behaviors have been obtained in either hydrogels or LCEs, they typically undergo reversible shape change only once (e.g., one expansion plus one contraction) during one heating/cooling cycle. Herein, we report a study of a novel liquid crystalline hydrogel (LCH) and the achievement of dual actuation in a single heating/cooling cycle by integrating the characteristics of thermoresponsive hydrogel and LCE. The dual actuation behavior arises from the reversible volume phase transition of poly(N-isopropylacrylamide) (PNIPAM) and the reversible order-disorder phase transition of LC mesogens in the LCH. Due to a temperature window separating the two transitions belonging to PNIPAM and LCE, LCH actuator can sequentially execute their respective actuation, thus deforming reversibly twice, during a heating/cooling cycle. The relative actuation degree of the two mechanisms is influenced by the mass ratio of PNIPAM to LCE in the LCH. Moreover, the initial shape of a bilayer actuator made with an active LCH layer and a passive polymer layer can be altered through hydration or dehydration of PNIPAM, which further modifies the dual actuation induced deformation. This work provides an example that shows the interest of developing LCH actuators.

Smart actuators and wearable and implantable devices have attracted much attention in healthcare and environmental sensing. Flexible electronic and ionic materials are the two main approaches used to construct these devices. Among them, hydrogel-based ionic materials offer unique advantages, such as biocompatibility and adaptable mechanical properties. However, ionic hydrogels encounter challenges in achieving wirelessly powered and noncontact sensing. To address this, we introduce MXene nanosheets to construct ionotronic hydrogels. Leveraging the rich surface charges and electronic conductivity of MXene nanosheets, ionotronic hydrogels can harvest vibrational and electromagnetic waves as electrical energy and enable noncontact sensing. Under ultrasound, it can continuously generate voltages up to 85 V and light up light-emitting diodes, promising wireless charging of implanted devices. In addition, it achieves an absorption coefficient of 0.2 for 915 MHz electromagnetic waves, enabling noncontact sensing through radio frequency identification. Notably, the physically crosslinked network of the MXene-based hydrogels maintained structural and performance stability under ultrasonic stimulation and exhibited self-healing properties. Even when cut into two halves, the self-healing hydrogel fully regenerates its original performance. This study provides insight into the development of ionotronic hydrogels for wirelessly powered and noncontact sensing in smart actuators and wearable and implantable applications.

Polyaniline (PANi) hydrogels have a wide range of applications in artificial skin, flexible robotics, and movement monitoring. Nevertheless, limited by the modulus mismatch between rigid PANi and the soft hydrogel matrix, the high strength and toughness of the PANi hydrogel are mutually exclusive. Although the introduction of sacrificial bonds into the hydrogel network can alleviate this contradiction to a certain extent, it always causes pronounced energy hysteresis during hydrogel deformation. Inspired by the energy storage and release of macroscopic springs, in this work, we propose a molecular entanglement approach for the fabrication of PANi hydrogels featuring high toughness and low hysteresis, where flexible poly(ethylene glycol) (PEG) is entangled with chemically cross-linked poly(acrylic acid) (PAA) as a hydrogel matrix, and rigid PANi as a conductive filler. The resultant PAA/PEG/PANi hydrogel exhibited high mechanical properties (fracture strength of 0.75 MPa and toughness of 4.81 MJ·m⁻³) and a low energy dissipation ratio (28.2% when stretching to 300%). Moreover, the PAA/PEG/PANi hydrogel possesses a good electrical response to external forces and can be employed as a strain sensor to monitor human joint movements by producing specific electrical signals. This work provides a straightforward strategy for preparing tough conductive PANi hydrogels with low hysteresis, showing potential for the development of healthcare devices.

Soft robots have shown great advantages with simple structure, high degree of freedom, continuous deformation, and benign human-machine interaction. In the past decades, a variety of soft robots, including crawling, jumping, swimming, and climbing robots, have been developed inspired by living creatures. However, most of the reported bionic soft robots have only a single mode of motion, which limits their practical application. Herein, we report a fully 3D printed crawling and flipping soft robot using liquid metal incorporated liquid crystal elastomer (LM-LCE) composite as the actuator. With the application of voltage, liquid metal works as the conductive Joule heating material to induce the contraction of the LCE layer. The bending angle of the LM-LCE composite actuator highly depends on the applied voltage. We further demonstrate that the soft robot can exhibit distinct moving behaviors, such as crawling or flipping, by applying different voltages. The fully 3D printed LM-LCE composite structure provides a strategy for the fast construction of soft robots with diverse motion modes.

Photonic fibrous soft actuators that can modulate light and produce responsive deformation would have broad technological implications in areas, ranging from smart textiles and intelligent artificial muscles to medical devices. However, creating such multifunctional soft actuators has proved tremendously challenging. Here, we report novel cholesteric liquid crystal elastomer (CLCE) based photonic fibrous soft actuators (PFSAs). CLCE can serve as chiral photonic soft active material and allow for multiresponse in shapes and colors. We leveraged a tubular-mold-based processing technology to prepare fibrous CLCE actuators, and the prepared actuators exhibit the capabilities to dynamically switch structural colors and geometrical shapes by mechanical, temperature, or light stimuli. CLCE-based PFSAs demonstrate diverse functionalities, including visual weight feedback, optically driven object manipulation, and light driven locomotion. It is anticipated that our PFSAs would offer many new possibilities for developing advanced soft actuators.

Liquid crystal elastomers (LCEs) exhibit exceptional reversible deformation and unique physical properties owing to their order-disorder phase transition under external stimuli. Among these deformations, helical structures have attracted attention owing to their distinctive configurations and promising applications in biomimetics and microelectronics. However, the helical deformation behavior of fiber actuators is critically influenced by their morphologies and alignments; yet, the underlying mechanisms are not fully understood. Through a two-step aza-Michael addition reaction and direct ink writing (DIW) 4D printing technology, fiber-based LCE actuators with a core-sheath alignment structure were fabricated and exhibited reversible helical deformation upon heating. By adjusting the printing parameters, the filament number, width, thickness, and core-sheath structure of the fiber actuators can be precisely controlled, resulting in deformation behaviors, such as contraction, bending, and helical twisting. Finite element simulations were performed to investigate the deformation behaviors of the fiber actuators, providing insights into the variations in stress and strain during the shape-changing process, which can be used to explain the shape-morphing mechanism. These findings demonstrate that the precise tuning of printing parameters enables the controllable construction of LCE actuator morphology and customization of their functional properties, paving the way for advanced applications in smart fabrics, biomedical engineering, and flexible electronics.

With the rise in environmental awareness, the development of smart polymer materials is gradually becoming environmentally friendly and sustainable. Fluorescent liquid crystal elastomers (LCE) can change their shape or optical properties in response to external stimuli, showing great potential for applications in sensing, information storage, and encryption. However, their life cycle is often unsustainable and not in line with the circular economy model. Based on the principle of green chemistry, a fluorescent LCE was developed through the co-polymerization of multiple monomers with 1,2-dithiolane end groups, which exhibited excellent self-healing, reprocessing, and closed-loop recyclability. In addition, by tailoring the phase transition temperature of the LCE, the transparency and fluorescence intensity of the resulting material can change at a low temperature of 8.0 °C. By further integrating light or acid/base-triggered fluorescence information, a proof-of-concept for temperature monitoring during short-time vaccine transportation using the reusable fluorescent LCE film is demonstrated. This study establishes a new environmentally friendly manufacturing strategy for multifunctional LCE materials.

Flexible phase change materials (PCMs) have become increasingly critical to address the demand for thermal management in electronic technologies and energy conversion. However, their application remains challenging because of their rigidity, liquid leakage, and insufficient thermal conductivity. Herein, flexible glutamic acid@natural rubber/paraffin wax (PW)/carbon nanotubes-graphene nanoplatelets (GNR/PW/CGNP) phase change composites with high thermal conductivity, excellent shape stability, and recyclability were reported. Zn²⁺-based dynamic crosslinking was constructed through the reaction of zinc acetate and carboxyl groups on glutamic acid@natural rubber (GNR), which was used as a flexible matrix to physically blend with paraffin wax/carbon nanotubes/graphene nanoplatelets (PW/CGNP) to achieve uniform dispersion of PW/CGNP, continuous thermal conductivity networks, and good encapsulation of PW. The GNR/PW/CGNP composites showed excellent mechanical strength, flexibility, and recycling ability, and effective encapsulation prevented the outflow of melted PW during the phase transition. Also, the phase change enthalpy could attain 111.1 J/g with a higher thermal conductivity of 1.055 W/mK, 428% higher than that of pure PW owing to the formation of efficient thermal conductive pathways, which exhibited outstanding thermal management performance and superior temperature control behavior in electronic devices. The developed flexible composite PCMs may open new possibilities for next-generation flexible thermal management electronics.

High catalytic efficiencies in ring opening polymerization (ROP) of a large ring-sized macrolactone, ω-pentadecalactone (PDL), by using transition metal Fe(II)-based catalysts were achieved for the first time in this study. Benefited from the bulky nature of the ligated α-diimine ligands, as evidenced from single-crystal structures, as well as the weakly oxophilic nature of the metal centers, chain transesterification reactions could be partially suppressed, allowing the polymerization proceed in a living-like and semi-controllable manner, i.e. good linear dependence of propagation rates on catalyst concentration and PDL concentration as observed in the detailed kinetics studies. The whole polymerization proceeds via a “coordination-insertion” mechanism, and with the aid of density functional theory (DFT) calculation studies, a “slow insertion → fast elimination” manner was demonstrated for the monomer propagation step, suggesting the insertion of Fe-OR into the carbonyl group C＝O as the rate-determining step. The present catalytic system also showed fast chain transfer reactions to alcohol compounds, affording quasi-immortal characteristics. DFT calculations showed that such a transfer reaction only required an energy barrier of 6.4 kcal/mol, performing a good consistency with the fast chain transfer rates.

The strain-induced crystallization behaviors of ultrasonic micro-injection molded poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) samples from an amorphous state were investigated by stress-strain relations and in situ wide angle X-ray diffraction (WAXD) measurements. The formation of direct strain-induced stereocomplex (SC) was evident. In samples molded at 50 and 80 °C, this phenomenon can be attributed to the acceleration of the ordered structures due to the existence of a large number of SC nuclei. The SC nuclei are assumed to serve as the transient physical cross-links to initiate the strain-induced crystallization. The onset of strain-induced crystallization is analogous to the heating induced structural reorganization. Consequently, the observed strain-induced SC process can be considered a pseudo process, which is actually thermally induced. Upon further stretching, the actual strain-induced crystallization occurs with the exclusive formation of the homocrystallite (HC), while the preceding formed SC crystals undergo slight fragmentation during subsequent tensile deformation. At 120 °C, due to the reduced number of SC nuclei within the sample, the occurrence of cold crystallization during stretching plays a more significant role than SC nuclei with respect to the strain-induced SC process, as demonstrated by in situ WAXD measurements upon annealing in both the static and stretched states.

Diphenylalanine and its analogs cause many concerns owing to their perfect self-assembly properties in the fields of biology, medicine, and nanotechnology. Experimental research has shown that diphenylalanine-based analogs with ethylenediamine linkers (PA, P = phenylalanine, and A = analog) can self-assemble into spherical assemblies, which can serve as novel anticancer drug carriers. In this work, to understand the assembly pathways, drug loading behavior, and formation mechanism of PA aggregates at the molecular level, we carried out dissipative particle dynamics (DPD) simulations of PA molecule systems. Our simulation results demonstrate that PA molecules spontaneously assemble into nanospheres and can self-assemble into drug-loaded nanospheres upon addition of the cancer chemotherapeutic agent doxorubicin (DOX). We also found that the hydrophobic side chain beads of PA molecules exhibited a unique onion-like distribution inside the nanospheres, which was not observed in the experiment. The onion-like nanospheres were verified by calculating the radial distribution function (RDF) of the DPD beads. Furthermore, based on the analysis of the percentages of different interaction components in the total nonbonded energies, main chain-side chain interactions between PA molecules may be important in the formation of onion-like nanospheres, and the synergistic effects of main chain-side chain, main chain-drug, side chain-drug, and main chain-solvent interactions are significant in the formation of drug-loaded nanospheres. These findings provide new insights into the structure and self-assembly pathway of PA assemblies, which may be helpful for the design of efficient and effective drug delivery systems.