A new function for thermal phase transition-based polymer actuators: autonomous motion on a surface of constant temperature

Repeatable Actuations of Organic Single Crystal Fibers Driven by Thermosalient-Phase-Transition-Induced Buckling

Thermosalient crystals are molecular solids that exhibit explosive motions, such as sudden breaks and jumps, due to temperature-induced structural phase transitions between two polymorphs. Therefore, the development of molecular actuators with superior speed and power by deriving mechanical work from explosive motion is a fascinating concept. However, thermosalient transitions often cause crystal disintegration, which hampers repeatable phase transitions between the polymorphs. Here, it is reported that single crystal nano/microfibers of 1, 2, 4, 5-tetrabromobenzene (TBB), whose bulk crystals exhibit thermosalient behavior at ≈40 °C, can repeatedly transform between the low and high temperature polymorphs without disintegration. The structural tolerance against phase transition is attributed to the high flexibility of the nano/microfibers. It is observed that a structure consisting of a TBB fiber with both ends pinned to the substrate repeatedly buckles and straightens when the temperature is varied between 30 and 40 °C. It is demonstrated that buckling can lead to large displacement actuation as compared to a simple length change of the fiber. Moreover, the force generated by the buckling fiber is estimated and it is found that it can generate a force large enough to flick an object ≈10⁴ times heavier than the fiber itself into the air against gravity.

Isomer Selective Thermosalience and Luminescence Switching in Organic Crystals

Organic crystals that respond to external stimuli are interesting for the design of smart materials. Here, we show that molecular engineering can transform simple naphthalidenimine-boron complexes─known for their exciting photophysical properties─into functional materials that exhibit thermosalience and thermal-luminescence switching. Detailed crystallographic and spectroscopic investigations revealed the role of subtle molecular parameters in deciphering charge-transfer interactions, which in turn imparted dynamic properties to the crystals. The simultaneous observation of thermally induced jumping and luminescence switching makes these crystals ideal for optoelectronic applications.

Pressure-Perceptive Actuators for Tactile Soft Robots and Visual Logic Devices

Soft actuators with sensing capabilities are important in intelligent robots and human-computer interactions. However, present perceptive actuating systems rely on the integration of multiple functional units with complex circuit design. Here, a new-type pressure-perceptive actuator is reported, which integrates functions of sensing, actuating, and decision making at material level without complex combination. The actuator is composed of an actuating unit and a pressure-sensing unit, both of which are fabricated by carbon nanotube (CNT), silk, and polymer composite. On the one hand, the actuating unit can be driven by low voltages (<13 V), owing to a Joule-heating effect. On the other hand, the current passing the pressure-sensing unit can be controlled by tactile pressure. In the integrated actuator, it is able to control the deformation amplitude of actuating unit by applying different pressures on the pressure-sensing unit. A portable tactile-activated gripper is fabricated to operate an object through pressure control, demonstrating its application in tactile soft robots. Finally, three visual logic gates (AND, OR, and NOT) are proposed, which convert "tactile" inputs into "visible" deformation outputs, using the CNT-silk-based material for sensing and actuating in the decision-making process. This study provides a new path for intelligent soft robots and new-generation logic devices.

Rapidly Reversible Organic Crystalline Switch for Conversion of Heat into Mechanical Energy

Solid-state thermoelastic behavior-a sudden exertion of an expansive or contractive physical force following a temperature change and phase transition in a solid-state compound-is rare in organic crystals, few are reversible systems, and most of these are limited to a dozen or so cycles before the crystal degrades or they reverse slowly over the course of many minutes or even hours. Comparable to thermosalience, wherein crystal phase changes induce energetic jumping, thermomorphism produces physical work via consistent and near-instantaneous predictable directional force. In this work, we show a fully reversible thermomorphic actuator that is stable at room temperature for multiple years and is capable of actuation for more than 200 cycles at near-ambient temperature. Specifically, the crystals shrink to 90% of their original length instantaneously upon heating beyond 45 °C and expand back to their original length upon cooling below 35 °C. Furthermore, the phase transition occurs instantaneously, with little obvious hysteresis, allowing us to create real-time actuating thermal fuses that cycle between on and off rapidly.

Wettability-Controlled Directional Actuating Strategy Based on Bilayer Photonic Crystals

Although the water-triggered bending behavior of bilayer films has been a wide concerned, there are few reports on wettability-controlled directional actuators with visible color changes. Using photonic crystals as carriers, bilayer directional bending structural color actuators were prepared based on the hydrophilic difference. Top inverse opal with strong hydrophilicity can promote water penetration and strengthen the effect of swelling. While, bottom inverse opal with weak hydrophilicity can inhibit water penetration and weaken the effect of swelling. When the bilayer structure is immersed in water, its wettability differences will produce different optical responses for visualization and will bring different swelling performances, resulting in directional bending. Infiltration differences are visualized as structural color red shifts or transparency. The mechanism of the design involves optical diffractions in the fabricated periodic nanostructures, differences in the surface wettability and swelling rate, uses the infiltration and capillary evaporation of water to realize the spectral diversity of reflectance, and the enhancement of bending by gradient infiltration. This work deeply analyzes the improvement of the photonic crystal structure on the optical and bending performance of the wettability-controlled actuator, provides a basic model for the design of bionic components, and opens an idea for the combination of bilayer photonic crystals and actuators.

Shape-adaptable biodevices for wearable and implantable applications

Emerging wearable and implantable biodevices have been significantly revolutionizing the diagnosis and treatment of disease. However, the geometrical mismatch between tissues and biodevices remains a great challenge for achieving optimal performances and functionalities for biodevices. Shape-adaptable biodevices enabling active compliance with human body tissues offer promising opportunities for addressing the challenge through programming their geometries on demand. This article reviews the design principles and control strategies for shape-adaptable biodevices with programmable shapes and actively compliant capabilities, which have offered innovative diagnostic/therapeutic tools and facilitated a variety of wearable and implantable applications. The state-of-the-art progress in applications of shape-adaptable biodevices in the fields of smart textiles, wound care, healthcare monitoring, drug and cell delivery, tissue repair and regeneration, nerve stimulation and recording, and biopsy and surgery, is highlighted. Despite the remarkable advances already made, shape-adaptable biodevices still confront many challenges on the road toward the clinic, such as enhanced intelligence for actively sensing and operating in response to physiological environments. Next-generation paradigms will shed light on future directions for extending the breadth and performance of shape-adaptable biodevices for wearable and implantable applications.

Light-Driven Flipping of Azobenzene Assemblies-Sparse Crystal Structures and Responsive Behaviour to Polarised Light

For creation of autonomous microrobots, which are able to move under conditions of a constant environment and a constant energy supply, a mechanism for maintenance of mechanical motion with a capacity for self-control is required. This requirement, known as self-organisation, represents the ability of a system to evade equilibrium through formation of a spatio-temporal pattern. Following our previous finding of a self-oscillatory flipping motion of an azobenzene-containing co-crystal, we present here regulation of the flipping motion by a light-receiving sensor molecule in relation to the alignment and role of azobenzene molecules in crystals. In the anisotropic structure, a specific azobenzene molecule acts as a reaction centre for the conversion of light to a mechanical function process, whereas the other molecules act as modulators for spatio-pattern regulation. The present results demonstrate that autonomously drivable molecular materials can exhibit information-responsive, self-sustainable motion by incorporating stimulus-responsive sensors.

Design principles for non-reciprocal photomechanical actuation

Non-reciprocal motions are a sequence of movements exhibiting time-reversal asymmetry. Such movements are common among various natural species, being adopted as a typical strategy for achieving efficient locomotion. Generally, the realization of non-reciprocal motions in man-made robotic devices requires synchronous control of at least two individual actuators, hence posing challenges to soft micro-robotics where the miniaturization limits integration of different mechanical components and the possibility of using onboard batteries. Here, we introduce general concepts for achieving non-reciprocal movements in wirelessly controlled soft actuators made of photomechanically responsive liquid crystal networks. The monolithic actuators are composed of two segments that can be actuated photochemically and photothermally, and the non-reciprocal motion is obtained by a control sequence that temporally modulates light sources of different wavelengths. Through proper selection of photoactive compounds, the number of modulated light sources can be decreased, from three to two, and eventually to one. Finally, we demonstrate non-reciprocal self-oscillation by self-shadowing effect in a flexible strip under a constant light field with no temporal modulation. This study provides general guidelines to light-controlled non-reciprocal actuation, offering new strategies for the control of wireless soft micro-robotics.

Synergetic Combination of Interfacial Engineering and Shape-Changing Modulation for Biomimetic Soft Robotic Devices

Robotics is a frontal interdisciplinary subject across the fields of mechanical engineering, chemical and materials engineering, artificial intelligence, and nanotechnology. Robotic devices with a variety of frameworks, functionalities, and actuation modes have been developed and employed in the manufacture of advanced materials and devices with improved efficiency and automation. In recent years, soft robots have attracted a significant amount of interest among scientific researchers and technological engineers because they can offer the desired safety, adaptability, sensibility, and dexterity that conventional robotics cannot deliver. To date, emulating living creatures in nature has been a promising approach to design soft robots. For living creatures, both body deformation and their surface characteristic are essential for them to function in dynamic ecological environments. Body deformation offers athletic ability while surface characteristics provide extraordinary adaptable interactions with the environment. In this article, we discuss the recent progress of emulating the body deformation of living creatures such as shrinking/expanding, bending, and twisting and programmable deformations based on the manipulation of shape-changing behaviors of liquid-crystal polymeric materials (LCPs) and the interfacial technologies to build up various microstructures similar to the interface of living creatures. We further review the pioneering work that integrates interfacial engineering and the shape-changing modulation of LCPs to develop biomimetic soft robotic devices. We also provide an outlook for opportunities and challenges in the design and fabrication of advanced biomimetic soft robots based on the synergetic combination of interfacial engineering and shape-changing modulation.

Two-Way and Multiple-Way Shape Memory Polymers for Soft Robotics: An Overview

Shape memory polymers (SMPs) are smart materials capable of changing their shapes in a predefined manner under a proper applied stimulus and have gained considerable interest in several application fields. Particularly, two-way and multiple-way SMPs offer unique opportunities to realize untethered soft robots with programmable morphology and/or properties, repeatable actuation, and advanced multi-functionalities. This review presents the recent progress of soft robots based on two-way and multiple-way thermo-responsive SMPs. All the building blocks important for the design of such robots, i.e., the base materials, manufacturing processes, working mechanisms, and modeling and simulation tools, are covered. Moreover, examples of real-world applications of soft robots and related actuators, challenges, and future directions are discussed.

Light-fuelled freestyle self-oscillators

Self-oscillation is a phenomenon where an object sustains periodic motion upon non-periodic stimulus. It occurs commonly in nature, a few examples being heartbeat, sea waves and fluttering of leaves. Stimuli-responsive materials allow creating synthetic self-oscillators fuelled by different forms of energy, e.g. heat, light and chemicals, showing great potential for applications in power generation, autonomous mass transport, and self-propelled micro-robotics. However, most of the self-oscillators are based on bending deformation, thereby limiting their possibilities of being implemented in practical applications. Here, we report light-fuelled self-oscillators based on liquid crystal network actuators that can exhibit three basic oscillation modes: bending, twisting and contraction-expansion. We show that a time delay in material response dictates the self-oscillation dynamics, and realize a freestyle self-oscillator that combines numerous oscillation modes simultaneously by adjusting the excitation beam position. The results provide new insights into understanding of self-oscillation phenomenon and offer new designs for future self-propelling micro-robots.

Electro and Light-Active Actuators Based on Reversible Shape-Memory Polymer Composites with Segregated Conductive Networks

Reversible shape-memory polymers (RSMPs) show great potential in actuating applications because of its repeatability among many other advantages. Indeed, in many cases, multiresponsive RSMPs are more expected, and the strategy to introduce functional fillers without deteriorating the reversible deformation performance is of great importance. Here, a facile strategy to balance the electro, photothermal performance, and molecular chain mobility is reported. Segregated conductive networks of carbon nanotubes (S-CNTs) are constructed in the poly(ethylene-co-octene) (POE) matrix at a relatively low filler loading, which renders the composite good electrical, photothermal, and actuating properties. A low percolation threshold of 0.25 vol % is achieved. The electrical conductivity is up to 0.046 S·cm^-1 for the POE/S-CNT composites with 2 vol % CNT, and the absorption of light (760 nm) is above 90%. These characteristics guarantee that the actuator can be driven at low voltage (≤36 V) and suitable light intensity (250 mW·cm^-2) with a good actuating performance. An electric gripper and a light-active crawling robot demonstrate the potential applications in multiresponsive robots. This work introduces a facile strategy to fabricate multiresponsive RSMPs by designing CNT network structures in polymer composites and holds great potential to enlarge the applications of RSMPs in many areas including artificial muscles and bionic robots.

A self-sustained soft actuator able to rock and roll

Liquid crystalline networks of specific geometry are observed to undergo thermally triggered chaotic continual rocking motion and light triggered rolling.

Three-dimensional displacement measurement of self-oscillating gel using digital holographic microscopy

To measure the 3D microdisplacement of a self-oscillating polymer gel driven by the Belousov-Zhabotinsky reaction, we propose, to the best of our knowledge, a new particle detection and tracking method based on a phase image/volume template matching using digital holographic microscopy. We demonstrate the precision of the proposed method and compare it with conventional approaches. The method is applied to 3D measurement of the motions of particles attached to the surface of an oscillating gel. Measurement results show that the local area of the gel oscillates periodically, and the motion propagates throughout the gel. Our method can measure rapid and complex 3D microdisplacement change.

Renewable boronic acid affiliated glycerol nano-adsorbents for recycling enzymatic catalyst in biodiesel fuel production

Nano-adsorbents composed of poly(4-vinylphenyl boronic acid) shell and silica core efficiently remove residual glycerol from a lipase catalyst layer in biodiesel fuel production, resulting in excellent lipase recycling. The subsequent CO2-acidolysis endows the adsorbents with sustainable renewability.