In nature, there are some amazing superfast actuations (Venus flytrap) and large-curvature helical deformations (the awn of Erodium). Although many bionic actuators have been made (electrothermal, hygroscopic, photoinduced), most of their actuations are slow and small, not comparable to the wonderful ones in nature. Here, we report an ultrafast photothermal actuator with large-curvature curling based on an ultrathin graphene oxide (GO) and biaxially oriented polyethylene (BOPE) bilayer film (thickness ∼11 μm). By virtue of the fast temperature changing rate (peak: 900 °C s–1 during infrared heating and −1200 °C s–1 during cooling) and the great difference in the coefficient of thermal expansion of GO and BOPE layers, the actuator deforms rapidly and greatly. The maximum bending speed and curvature can reach 5300° s–1 and 22 cm–1, respectively, which are comparable to those of wonderful natural actuators and far exceed the performances of the reported artificial actuators. Different from ordinary helical actuators made of uniaxial anisotropic materials, our actuator is based on a typical biaxial anisotropic material of BOPE. However, the morphing behaviors of this type of actuator have not been reported before. So for the first time, we systematically studied this problem through experiments and simulations using the GO-BOPE actuator as a prototype and have drawn clear conclusions. In addition, functional GO-BOPE actuators capable of winding around and manipulating tiny objects were also designed and developed. We think this ultrafast large-curvature photothermal actuator will have wide application prospects in bionic actuations and dexterous robots.
A flexible actuator, which can convert external stimuli to mechanical motion, is an essential component of every soft robot and determines its performance. As a novel two-dimensional material, MXene has been used to fabricate flexible actuators due to its excellent physical properties. Although MXene-based actuators exhibit excellent actuation performance, their bending deformation is solely due to the in-plane isotropy of the MXene film, and an MXene torsional actuator has not been reported. This study presents a flexible torsional actuator based on an MXene−carbon nanotube (CNT)−paraffin wax (PW) film. In this actuator, the MXene thin film acts as a light absorption layer with wavelength selectivity, superaligned CNT provides structural anisotropy for the composite film, and PW acts as the active layer. The chirality and helical structure of the actuator could be tuned by the orientation of the CNT film. Such an actuator delivers excellent actuation performance, including high work density (∼1.2 J/cm 3 ), low triggering power (77 mW/cm 2 ), high rotational speed (320°/s), long lifetime (30,000 cycles), and wavelength selectivity. Inspired by vines, we used the torsional actuator as a spiral grabber, which lifted an object that weighs 20 times more than the actuator. The high-performance torsional actuator would be potentially used as a noncontact sensor, rotary motor, and grabbing tool in the soft robot system.
Artificial muscle is a kind of soft actuators that can mimic biological muscles to realize contraction, torsion, and other action modes. Since the artificial muscles based on stimuli-responsive materials can drive the robots to achieve bionic motions under different external stimuli, they are expected to be used in various scenarios. Due to high requirements for the component fibrous materials, the artificial muscles capable of responding to two or more forms of stimuli and integrating multiple functions are rarely reported. Although some valuable attempts (host-guest, sheath-core, hybrid spinning methods) have been made to realize multi-responsive and functional artificial muscles, very complicated and demanding preparation processes are usually needed. Here, we used a direct and effective method to develop dual-responsive artificial muscles without excessive requirements on materials: by plying the viscose and silver-coated nylon coiled yarns (in response to water and electric heating, respectively) into double-helix structure, or tying them into single-strand serial structure, the contractile and torsional muscles can be fabricated readily and conveniently. Under concerted or alternate stimulation of water and electric heating, the dual-responsive muscles can exhibit significant performance improvements (in contractile stroke, output force, durability, etc.) and achieve new actuation mode (bidirectional torsion) through the interaction and cooperation of the two component yarns. The moisture detectors based on the dual-responsive muscles were also demonstrated. This work provides a feasible way to prepare dual-/multi-responsive muscles using ordinary stimulus-responsive materials. Besides, the multiple stimuli and the interactions between the muscle components can be fully utilized to improve the muscle performances and extend new functions. These methods and concepts will facilitate the development of multi-responsive and functional artificial muscles, and promote their wide applications.
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