Smart Muscle-Driven Self-Cleaning of Biomimetic Microstructures from Liquid Crystal Elastomers

A bionic mimosa soft robot based on a multi-responsive PNIPAM-PEGDA hydrogel composition

Deformation plays a vital role in the survival of natural organisms. One example is that plants deform themselves to face the sun for sufficient sunlight exposure, which allows them to produce nutrients through photosynthesis. Drawing inspiration from nature, researchers have been exploring the development of 3D deformable materials. However, the traditional approach to manufacturing deformable hydrogels relies on complex technology, which limits their potential applications. In this study, we simulate the stress variations observed in the plant tissue to create a 3D structure from a 2D material. Using UV curing technology, we create a single-layer poly(N-isopropylacrylamide) hydrogel sheet with microchannels that exhibit distinct swelling rates when subjected to stimulation. After a two-step curing process, we produce a poly(N-isopropylacrylamide)-polyethylene glycol diacrylatedouble-layer structure that can be manipulated to change its shape by controlling the light and solvent content. Based on the double-layer structure, we fabricate a dual-response driven bionic mimosa robot that can perform a variety of functions. This soft robot can not only reversibly change its shape but also maintain a specific shape without continuous stimulation. Its capacity for reversible deformation, resulting from internal stress, presents promising application prospects in the biomedical and soft robotics domain. This study delivers an insightful framework for the development of programmable soft materials.

Light-driven soft microrobots based on hydrogels and LCEs: development and prospects

In the daily life of mankind, microrobots can respond to stimulations received and perform different functions, which can be used to complete repetitive or dangerous tasks. Magnetic driving works well in robots that are tens or hundreds of microns in size, but there are big challenges in driving microrobots that are just a few microns in size. Therefore, it is impossible to guarantee the precise drive of microrobots to perform tasks. Acoustic driven micro-nano robot can achieve non-invasive and on-demand movement, and the drive has good biological compatibility, but the drive mode has low resolution and requires expensive experimental equipment. Light-driven robots move by converting light energy into other forms of energy. Light is a renewable, powerful energy source that can be used to transmit energy. Due to the gradual maturity of beam modulation and optical microscope technology, the application of light-driven microrobots has gradually become widespread. Light as a kind of electromagnetic wave, we can change the energy of light by controlling the wavelength and intensity of light. Therefore, the light-driven robot has the advantages of programmable, wireless, high resolution and accurate spatio-temporal control. According to the types of robots, light-driven robots are subdivided into three categories, namely light-driven soft microrobots, photochemical microrobots and 3D printed hard polymer microrobots. In this paper, the driving materials, driving mechanisms and application scenarios of light-driven soft microrobots are reviewed, and their advantages and limitations are discussed. Finally, we prospected the field, pointed out the challenges faced by light-driven soft micro robots and proposed corresponding solutions.

Recent Advances in 4D Printing of Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are renowned for their large, reversible, and anisotropic shape change in response to various external stimuli due to their lightly cross-linked polymer networks with an oriented mesogen direction, thus showing great potential for applications in robotics, bio-medics, electronics, optics, and energy. To fully take advantage of the anisotropic stimuli-responsive behaviors of LCEs, it is preferable to achieve a locally controlled mesogen alignment into monodomain orientations. In recent years, the application of 4D printing to LCEs opens new doors for simultaneously programming the mesogen alignment and the 3D geometry, offering more opportunities and higher feasibility for the fabrication of 4D-printed LCE objects with desirable stimuli-responsive properties. Here, the state-of-the-art advances in 4D printing of LCEs are reviewed, with emphasis on both the mechanisms and potential applications. First, the fundamental properties of LCEs and the working principles of the representative 4D printing techniques are briefly introduced. Then, the fabrication of LCEs by 4D printing techniques and the advantages over conventional manufacturing methods are demonstrated. Finally, perspectives on the current challenges and potential development trends toward the 4D printing of LCEs are discussed, which may shed light on future research directions in this new field.

Stimuli-Triggered Multishape, Multimode, and Multistep Deformations Designed by Microfluidic 3D Droplet Printing

Elastomers generally possess low Young's modulus and high failure strain, which are widely used in soft robots and intelligent actuators. However, elastomers generally lack diverse functionalities, such as stimulated shape morphing, and a general strategy to implement these functionalities into elastomers is still challenging. Here, a microfluidic 3D droplet printing platform is developed to design composite elastomers architected with arrays of functional droplets. Functional droplets with controlled size, composition, position, and pattern are designed and implemented in the composite elastomers, imparting functional performances to the systems. The composited elastomers are sensitive to stimuli, such as solvent, temperature, and light, and are able to demonstrate multishape (bow- and S-shaped), multimode (gradual and sudden), and multistep (one- and two-step) deformations. Based on the unique properties of droplet-embedded composite elastomers, a variety of stimuli-responsive systems are developed, including designable numbers, biomimetic flowers, and soft robots, and a series of functional performances are achieved, presenting a facile platform to impart diverse functionalities into composite elastomers by microfluidic 3D droplet printing.

Thermally Driven Continuous Rolling of a Thick-Walled Cylindrical Rod

Self-sustained motion can take advantage of direct energy extraction from a steady external environment to maintain its own motion, and has potential applications in energy harvesting, robotic motion, and transportation. Recent experiments have found that a thermally responsive rod can perform self-sustained rolling on a flat hot plate with an angular velocity determined by the competition between the thermal driving moment and the friction moment. A rod with a hollow cross section tends to greatly reduce the frictional resistance, while promising improvements in thermal conversion efficiency. In this paper, through deriving the equilibrium equations for steady-state self-sustained rolling of the thick-walled cylindrical rod, estimating the temperature field on the rod cross-section, and solving the analytical solution of the thermally induced driving moment, the dynamic behavior of the thermally driven self-sustained rolling of the thick-walled cylindrical rod is theoretically investigated. In addition, we investigate in detail the effects of radius ratio, heat transfer coefficient, heat flux, contact angle, thermal expansion coefficient, and sliding friction coefficient on the angular velocity of the self-sustained rolling of the thick-walled cylindrical rod to obtain the optimal ratio of internal and external radius. The results are instructive for the application of thick-walled cylindrical rods in the fields of waste heat harvesters and soft robotics.

Plasticized liquid crystal networks and chemical motors for the active control of power transmission in mechanical devices

The miniaturization of mechanical devices poses new challenges in powering, actuation, and control since traditional approaches cannot be used due to inherent size limitations. This is particularly challenging in untethered small-scale machines where independent actuation of multicomponent and multifunctional complex systems is required. This work showcases the integration of self-powered chemical motors and liquid crystal networks into a powertrain transmission device to achieve orthogonal untethered actuation for power and control. Driving gears with a protein-based chemical motor were used to power the transmission system with Marangoni propulsive forces, while photothermal liquid crystal networks were used as a photoresponsive clutch to engage/disengage the gear system. Liquid crystal networks were plasticized for optimized photothermal bending actuation to break the surface tension of water and achieve reversible immersion/resurfacing at the air-water interface. This concept is demonstrated in a milliscale transmission gear system and offers potential solutions for aquatic soft robots whose powering and control mechanisms must be necessarily decoupled.

Investigation on the Anisotropic Wetting Properties of Water Droplets on Bio-Inspired Groove Structures Fabricated by 3D Printing and Surface Modifications

The self-driving structure to orientate the water movement has attracted considerable attention. Inspired by the wedgelike structures of biological materials in nature, such as spider silks and cactus spines, anisotropic spreading can be realized by combining Laplace pressure gradient and hydrophilic surface. In this study, a series of groove patterns were fabricated by a combination of 3D printing and surface modification. PLA pattern was modified by the atmospheric pressure plasma, followed by grafting with hydrolyzed APTES. This work reports the anisotropic transport of water droplets on a series of designed dart-shaped groove patterns with specific angles in the main arrow and tail regions. This structure can induce capillary force to regulate droplets from the main cone to two wedgelike, whereas the droplets are hindered toward the opposite side is oat the vicinity of the groove's tail. By means of the experiment, the mechanism of water transport in this pattern was revealed. This study can contribute a potential approach to manipulate and apply anisotropic wetting in many fields.

Actuation performance of a liquid crystalline elastomer composite reinforced by eiderdown fibers

Liquid crystalline elastomer (LCE) materials have been developed and investigated for several decades. One important obstacle, which impedes the practical industrial application of LCE materials, is their modest robustness as actuator materials. In this work, we developed a LCE composite which was fabricated by incorporating eiderdown fibers into a polysiloxane-based main-chain LCE matrix. The eiderdown fibers were used as the flexible reinforcement phase suitable for the shape-morphing performance of LCE materials upon being stimulated. Due to the long fiber property, specific structure and surface characteristics of the eiderdown fibers, they constructed a reinforcement network in the LCE matrix and formed tight interfacial adhesion with the matrix. The LCE composite demonstrated enhanced actuation mechanical properties and robust actuation performance. Its actuation blocking stress and modulus were increased due to the reinforcement effect of the eiderdown fibers. The tensile strength and the performance of anti-fatigue failure under repeated actuation cycles and high loadings were greatly improved due to the crack-resisting effect and bridging effect of the eiderdown fibers. While other properties, such as the liquid crystalline phase structure, the stimulus deformation ratio, phase transition temperature of the LCE matrix, etc., did not deteriorate or change due to the high flexibility, thermal stability and chemical stability of the eiderdown fibers.

Shape-programmable liquid crystal elastomer structures with arbitrary three-dimensional director fields and geometries

Liquid crystal elastomers exhibit large reversible strain and programmable shape transformations, enabling various applications in soft robotics, dynamic optics, and programmable origami and kirigami. The morphing modes of these materials depend on both their geometries and director fields. In two dimensions, a pixel-by-pixel design has been accomplished to attain more flexibility over the spatial resolution of the liquid crystal response. Here we generalize this idea in two steps. First, we create independent, cubic light-responsive voxels, each with a predefined director field orientation. Second, these voxels are in turn assembled to form lines, grids, or skeletal structures that would be rather difficult to obtain from an initially connected material sample. In this way, the orientation of the director fields can be made to vary at voxel resolution to allow for programmable optically- or thermally-triggered anisotropic or heterogeneous material responses and morphology changes in three dimensions that would be impossible or hard to implement otherwise.

Liquid-Crystal-Elastomer-Actuated Reconfigurable Microscale Kirigami Metastructures

Programmable actuation of metastructures with predesigned geometrical configurations has recently drawn significant attention in many applications, such as smart structures, medical devices, soft robotics, prosthetics, and wearable devices. Despite remarkable progress in this field, achieving wireless miniaturized reconfigurable metastructures remains a challenge due to the difficult nature of the fabrication and actuation processes at the micrometer scale. Herein, microscale thermo-responsive reconfigurable metasurfaces using stimuli-responsive liquid crystal elastomers (LCEs) is fabricated as an artificial muscle for reconfiguring the 2D microscale kirigami structures. Such structures are fabricated via two-photon polymerization with sub-micrometer precision. Through rationally designed experiments guided by simulations, the optimal formulation of the LCE artificial muscle is explored and the relationship between shape transformation behaviors and geometrical parameters of the kirigami structures is build. As a proof of concept demonstration, the constructs for temperature-dependent switching and information encryption is applied. Such reconfigurable kirigami metastructures have significant potential for boosting the fundamental small-scale metastructure research and the design and fabrication of wireless functional devices, wearables, and soft robots at the microscale as well.

Self-sustained eversion or inversion of a thermally responsive torus

When a cylindrical rod is placed on a flat and hot surface with a constant temperature, it can reach a steady state after certain time. In the steady state, though the temperature field inside the rod is inhomogeneous, it does not change with time. The inhomogeneous temperature change in the rod may induce inhomogeneous thermal expansion in it. Recent experiments have determined that if the rod is slightly curved, the inhomogeneous thermal expansion in the rod can drive its continuous and self-sustained rolling on a hot surface. It has been further shown that if the rod is bent to a closed torus and placed on a hot surface, the torus everts or inverts continuously due to the cross-coupling between the thermal field and the cyclic rotation. Such cyclic eversion or inversion of a torus can be regarded as a zero-elastic-energy mode because both the elastic energy and the shape of the torus remain unchanged during the rotation. In this article, we develop a coupled mechanics theory to model the continuous self-sustained eversion or inversion of a viscoelastic torus on a hot surface. We hope our modeling will inspire more novel designs of elastic motors being capable of zero-energy mode motion and help to quantitatively predict their performance.

3D-Printed Biomimetic Systems with Synergetic Color and Shape Responses Based on Oblate Cholesteric Liquid Crystal Droplets

Living organisms in nature have amazing control over their color, shape, and morphology in response to environmental stimuli for camouflage, communication, or reproduction. Inspired by the camouflage of the octopus via the elongation or contraction of its pigment cells, oblate cholesteric liquid crystal droplets are dispersed in a polymer matrix, serving as the role of pigment cells and showing structural color due to selective Bragg reflection by their periodic helical structure. The color of 3D-printed biomimetic systems can be tuned by changing the helical pitch via the chiral dopant concentration or temperature. When the oblate liquid crystal droplets are heated up to isotropic, the opaque and colored biomimetic systems become transparent and colorless. Meanwhile, the isotropic liquid crystal droplets tend to become spherical, causing volume contraction along the film plane and volume dilation in the perpendicular direction. The internal strain combined with the gradient distribution of the oblate isotropic liquid crystal droplets result in corresponding shape transformations. The camouflage of a biomimetic octopus and the blossom of a biomimetic flower, both of which show synergetic color and shape responses, are demonstrated to inspire the design of functional materials and intelligent devices.

Magnetic-Responsive Superhydrophobic Surface of Magnetorheological Elastomers Mimicking from Lotus Leaves to Rose Petals

In nature, many plants have evolved various wettability surfaces to survive and thrive in diverse environments. For example, the superhydrophobic surface of lotus can keep itself clean, while the rose petals can retain droplets for a long time. The former is referred to the "lotus effect," and the latter is known as the "rose petal effect." This research proposes a method to fabricate magnetic-responsive superhydrophobic magnetorheological elastomers (MREs) which could reversibly and instantly transition their surface wetting state between the "lotus effect" and the "rose petal effect." These surfaces with controllable wettability could find applications in the manipulation of liquids in biological and chemical systems. The MREs are cured by applying a uniform magnetic field to form "mountain-like" microstructures on their surfaces. This initial surface is rough and exhibits the lotus leaf effect. Because of the nonuniform magnetically induced deformation, the surface micromorphology and roughness can be altered by an applied magnetic field. The state of water droplets on its surface is changed from the Wenzel state to the Cassie-Baxter (CB) state. Therefore, the proposed MRE surface could switch their dynamic wetting features between the "rose petals" and "lotus leaves" via a magnetic field. An experimental platform for the wetting features of MRE surfaces is established to characterize the dynamic behaviors of water drops on MREs under a magnetic field. A magneto-mechanic coupled model is proposed to interpret how the magnetic field influences the MRE surface as well as the droplet movement.

Light-Fueled Nanoscale Surface Waving in Chiral Liquid Crystal Networks

Nano- and micro-actuating systems are promising for application in microfluidics, haptics, tunable optics, and soft robotics. Surfaces capable to change their topography at the nano- and microscale on demand would allow control over wettability, friction, and surface-driven particle motility. Here, we show that light-responsive cholesteric liquid crystal (LC) networks undergo a waving motion of their surface topography upon irradiation with light. These dynamic surfaces are fabricated with a maskless one-step procedure, relying on the liquid crystal alignment in periodic structures upon application of a weak electric field. The geometrical features of the surfaces are controlled by tuning the pitch of the liquid crystal. Pitch control by confinement allows engineering one-dimensional (1D) and two-dimensional (2D) structures that wave upon light exposure. This work demonstrates the potential that self-organizing systems might have for engineering dynamic materials, and harnessing the functionality of molecules to form dynamic surfaces, with nanoscale precision over their waving motion.

Bioinspired photocontrollable microstructured transport device

Geckos, which can walk upside down on vertical and underneath horizontal surfaces, owe this ability to the hierarchical structures under their toes. These structures are responsible for substantial adhesion and, at the same time, for quick detachment by mechanical stimulus through leg movements. Inspired by such stimuli-responsive systems in nature, we developed an artificial, photocontrollable microstructured transport device. Through tunable ultraviolet light illumination, the adhesive ability of this bioinspired transport device is reduced up to a factor of 2.7 in terms of adhesive forces and is quickly recovered when the light stimulus ceases. This bioinspired photocontrollable device has been used in a pick-up and drop-down system for transporting planar and three-dimensional solid objects.

Processing advances in liquid crystal elastomers provide a path to biomedical applications

Liquid crystal elastomers (LCEs) are a class of stimuli-responsive polymers that undergo reversible shape-change in response to environmental changes. The shape change of LCEs can be programmed during processing by orienting the liquid crystal phase prior to crosslinking. The suite of processing techniques that has been developed has resulted in a myriad of LCEs with different shape-changing behavior and mechanical properties. Aligning LCEs via mechanical straining yields large uniaxial actuators capable of a moderate force output. Magnetic fields are utilized to control the alignment within LCE microstructures. The generation of out-of-plane deformations such as bending, twisting, and coning is enabled by surface alignment techniques within thin films. 4D printing processes have emerged that enable the fabrication of centimeter-scale, 3D LCE structures with a complex alignment. The processing technique also determines, to a large extent, the potential applications of the LCE. For example, 4D printing enables the fabrication of LCE actuators capable of replicating the forces generated by human muscles. Employing surface alignment techniques, LCE films can be designed for use as coatings or as substrates for stretchable electronics. The growth of new processes and strategies opens and strengthens the path for LCEs to be applicable within biomedical device designs.

3D Microstructures of Liquid Crystal Networks with Programmed Voxelated Director Fields

The shape-shifting behavior of liquid crystal networks (LCNs) and elastomers (LCEs) is a result of an interplay between their initial geometrical shape and their molecular alignment. For years, reliance on either one-step in situ or two-step film processing techniques has limited the shape-change transformations from 2D to 3D geometries. The combination of various fabrication techniques, alignment methods, and chemical formulations developed in recent years has introduced new opportunities to achieve 3D-to-3D shape-transformations in large scales, albeit the precise control of local molecular alignment in microscale 3D constructs remains a challenge. Here, the voxel-by-voxel encoding of nematic alignment in 3D microstructures of LCNs produced by two-photon polymerization using high-resolution topographical features is demonstrated. 3D LCN microstructures (suspended films, coils, and rings) with designable 2D and 3D director fields with a resolution of 5 µm are achieved. Different shape transformations of LCN microstructures with the same geometry but dissimilar molecular alignments upon actuation are elicited. This strategy offers higher freedom in the shape-change programming of 3D LCN microstructures and expands their applicability in emerging technologies, such as small-scale soft robots and devices and responsive surfaces.

Three-dimensional printing of functionally graded liquid crystal elastomer

As a promising actuating material, liquid crystal elastomer (LCE) has been intensively explored in building diverse active structures and devices. Recently, direct ink writing technique has been developed to print LCE structures with various geometries and actuation behaviors. Despite the advancement in printing LCE, it remains challenging to print three-dimensional (3D) LCE structures with graded properties. Here, we report a facile method to tailor both the actuation behavior and mechanical properties of printed LCE filaments by varying printing parameters. On the basis of the comprehensive processing-structure-property relationship, we propose a simple strategy to print functionally graded LCEs, which greatly increases the design space for creating active morphing structures. We further demonstrate mitigation of stress concentration near the interface between an actuatable LCE tube and a rigid glass plate through gradient printing. The strategy developed here will facilitate potential applications of LCEs in different fields.

Electroplasticization of Liquid Crystal Polymer Networks

Shape-shifting liquid crystal networks (LCNs) can transform their morphology and properties in response to external stimuli. These active and adaptive polymer materials can have impact in a diversity of fields, including haptic displays, energy harvesting, biomedicine, and soft robotics. Electrically driven transformations in LCN coatings are particularly promising for application in electronic devices, in which electrodes are easily integrated and allow for patterning of the functional response. The morphing of these coatings, which are glassy in the absence of an electric field, relies on a complex interplay between polymer viscoelasticity, liquid crystal order, and electric field properties. Morphological transformations require the material to undergo a glass transition that plasticizes the polymer sufficiently to enable volumetric and shape changes. Understanding how an alternating current can plasticize very stiff, densely cross-linked networks remains an unresolved challenge. Here, we use a nanoscale strain detection method to elucidate this electric-field-induced devitrification of LCNs. We find how a high-frequency alternating field gives rise to pronounced nanomechanical changes at a critical frequency, which signals the electrical glass transition. Across this transition, collective motion of the liquid crystal molecules causes the network to yield from within, leading to network weakening and subsequent nonlinear expansion. These results unambiguously prove the existence of electroplasticization. Fine-tuning the induced emergence of plasticity will not only enhance the surface functionality but also enable more efficient conversion of electrical energy into mechanical work.

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.

Bioinspired underwater locomotion of light-driven liquid crystal gels

Soft-bodied aquatic invertebrates, such as sea slugs and snails, are capable of diverse locomotion modes under water. Recapitulation of such multimodal aquatic locomotion in small-scale soft robots is challenging, due to difficulties in precise spatiotemporal control of deformations and inefficient underwater actuation of existing stimuli-responsive materials. Solving this challenge and devising efficient untethered manipulation of soft stimuli-responsive materials in the aquatic environment would significantly broaden their application potential in biomedical devices. We mimic locomotion modes common to sea invertebrates using monolithic liquid crystal gels (LCGs) with inherent light responsiveness and molecular anisotropy. We elicit diverse underwater locomotion modes, such as crawling, walking, jumping, and swimming, by local deformations induced by selective spatiotemporal light illumination. Our results underpin the pivotal role of the physicomechanical properties of LCGs in the realization of diverse modes of light-driven robotic underwater locomotion. We envisage that our results will introduce a toolbox for designing efficient untethered soft robots for fluidic environments.

Preparation, photo-induced deformation behavior and application of hydrogen-bonded crosslinked liquid crystalline elastomers based on α-cyanostilbene

A series of high-performance α-cyanostilbene-based photoresponsive LCEs were prepared, and their application in information encryption materials was also explored.

Programmable 3D Shape-Change Liquid Crystalline Elastomer Based on a Vertically Aligned Monodomain with Cross-link Gradient

A liquid crystalline elastomer (LCE) as a kind of stimuli-responsive materials, which can be fabricated to present the three-dimensional (3D) change in shape, shows a wide range of applications. Herein, we propose a simple and robust way to prepare a 3D shape-change actuator based on gradient cross-linking of the vertically aligned monodomain of liquid crystals (LCs). First, gold nanoparticles grafted by liquid crystalline polymers (LCPs) are used to induce the homeotropic orientation of the LC monomer and cross-linkers. Then, photopolymerization under UV irradiation is carried out, which can result in the LCE film with a cross-link gradient. Different from the typical LCEs with homogenous alignment that usually show the shape change of extension/contraction, the obtained vertically aligned LCE film exhibits excellent bendability under a thermal stimulus. The nanoindentation experiment demonstrates that the deformation of LCE films comes from the difference in Young's modulus on two sides of the thin film. Simply scissoring the thin film can prepare the samples with different bending angles under the fixed length. Moreover, using a photomask to pattern the film during photopolymerization can realize the complex 3D deformation, such as bend, fold, and buckling. Further, the patterned LCE film doped with multiwalled carbon nanotubes modified by LCPs (CNT-PDB) can act as a light-fueled microwalker with fast crawl behavior.

Reversible Adhesion via Light-Regulated Conformations of Rubber Chains

Bio-inspired reversible adhesives have attracted great attention because of their promising applications in the electronic, biomedical, and robotic fields. Here, to achieve in situ reversible adhesion, a new concept is demonstrated by modulating the conformations of polydimethylsiloxane (PDMS) chains. The new adhesive, termed B_GP_P, is composed of the graphene/PDMS composite (GP) as the backing layer and PDMS as the micropillar array. The photothermal effect of graphene under UV irradiation heats up the micropillars, resulting in an increase in the chain conformations of PDMS and thus the contact points with the counterpart surface. The more contact points together with the alignment of PDMS chains during the shearing result in an adhesion much higher than that without UV irradiation. The adhesion switching thus does not rely on the changing of the contact area, and so the macroscopic deformation of structures is avoided. The results suggest a new design principle for light-controllable structured adhesive, which could be conceptualized into other rubbery materials.

4D Printing of a Liquid Crystal Elastomer with a Controllable Orientation Gradient

Liquid crystal elastomers (LCEs), a class of soft materials capable of a large and reversible change in the shape under the trigger of external stimuli, can be fabricated into diverse architectures with complicated deformation modes through four-dimensional (4D) printing. However, the printable LCE ink is only in the form of monomeric precursors and the deformation mode is limited to contraction/extension deformation. Herein, we report a novel approach to break through these limitations. We achieved 4D printing of a single-component liquid crystal polymer ink in its isotropy state through direct ink writing (DIW) technology. The drawing force imposed by the movement of nozzle in the extruded printing process was able to align the mesogen units along the specific printing path. An orientation gradient perpendicular to the printing direction was obtained due to the existence of a temperature gradient between the two sides of printed samples and could be further fixed by post-photo-cross-linking treatment through the dimerizable groups in the LCE, realizing a new actuation mode in the field of extrusion-based printing of LCE actuators. The printed film was able to change reversibly from a strip to a tightly hollow cylinder and could reversibly lift up an object with roughly 600 times its own weight. The orientation gradient can be patterned through liquid-assistant printing or programmed structure design to integrate both bending and contraction actuation modes on the same printed sample, leading to complex deformation and two-dimensional (2D) planar porous structure to three-dimensional (3D) porous cylinder transition. This study opens up a new prospect to directly print a wide variety of LCE actuators with versatile actuation modes.

Photothermal Actuation of Diverse Liquids on an Fe3O4-Doped Slippery Surface for Electric Switching and Cell Culture

The photoinduced manipulation of liquids on a slippery lubricant-infused porous surface (SLIPS) has attracted a tremendous amount of attention because of its merits of contactless stimulation and excellent spatial and temporal control. However, tedious fabrication methods by a combination of template transfer and fluorination for a photothermal-material-doped SLIPS and the lack of deeper systematically quantitative analysis with respect to droplet hydrokinetics are greatly perplexing in both academic research and industrial applications. Here we demonstrate a kind of Fe₃O₄-doped SLIPS by one-step femtosecond laser cross-scanning, which can readily steer diverse liquids toward arbitrary directions with a fast velocity of up to 1.15 mm/s in the presence of a unilateral NIR stimulus. The underlying mechanism is that the wettability gradient force (F_wet-grad) induced by the temperature gradient arising from asymmetric near-infrared-irradiation (NIR) loading would be generated within 1 s to actuate a targeted droplet's sliding behavior. Through tuning the NIR irradiating sites, we can slide a targeted droplet with controllable directions and routes. On the basis of fundamental physics, we have quantitatively analyzed the relationship among Fe₃O₄-doped content, lubricant rheological performance, droplet wettability variations, F_wet-grad, and the sliding velocity for diverse liquid species. Accordingly, we can remotely steer liquid droplets to realize the on-off state of an electrical circuit on demand, the droplet fusion of a microfluidic reactor, and the culture/inhibition of biological cells.

Morphing of liquid crystal surfaces by emergent collectivity

Liquid crystal surfaces can undergo topographical morphing in response to external cues. These shape-shifting coatings promise a revolution in various applications, from haptic feedback in soft robotics or displays to self-cleaning solar panels. The changes in surface topography can be controlled by tailoring the molecular architecture and mechanics of the liquid crystal network. However, the nanoscopic mechanisms that drive morphological transitions remain unclear. Here, we introduce a frequency-resolved nanostrain imaging method to elucidate the emergent dynamics underlying field-induced shape-shifting. We show how surface morphing occurs in three distinct stages: (i) the molecular dipoles oscillate with the alternating field (10-100 ms), (ii) this leads to collective plasticization of the glassy network (~1 s), (iii) culminating in actuation of the topography (10-100 s). The first stage appears universal and governed by dielectric coupling. By contrast, yielding and deformation rely on a delicate balance between liquid crystal order, field properties and network viscoelasticity.

Gecko toe pads inspired in situ switchable superhydrophobic shape memory adhesive film

Recently, smart adhesive superhydrophobic surfaces have attracted much attention. However, it is still a challenge to obtain a superhydrophobic surface with shape memory adhesive performance. Herein, inspired by the special back-scrolling/unfolding ability of gecko toe pads and corresponding tunable adhesion, we report such a film produced by sticking a layer of superhydrophobic pillar structured polyurethane (s-PU) onto a shape memory polyurethane-cellulose nanofiber (PU-CNF) substrate to mimic the hair-like skin structure and underlying muscle of the gecko toe pads, respectively. Similar to the muscle of the gecko toe pads, the excellent shape memory effect of the PU-CNF substrate can help the obtained film to memorize and repeatedly display different shapes and solid/water contact models. Thus reversible switching between multiple states from the low-adhesive rolling performance to the high-adhesive pinning performance can be realized. Meanwhile, based on its smart wetting performance, not only the traditional in situ capture/release of one microdroplet, but also the step-by-step release of different droplets can be realized on our film. This work reports a new superhydrophobic shape memory adhesive film, which offers a novel strategy for surface adhesion control and meanwhile opens a new road for applications in controlled droplet manipulation.

Direct shape programming of liquid crystal elastomers

Liquid crystal elastomers (LCEs) are shape morphing materials promising for many applications including soft robotics, actuators, and biomedical devices, but current LCE synthesis techniques lack a simple method to program new and arbitrary shape changes. Here, we demonstrate a straightforward method to directly program complex, reversible, non-planar shape changes in nematic LCEs. We utilize a double network synthesis process that results in a competitive double network LCE. By optimizing the crosslink densities of the first and second network we can mechanically program non-planar shapes with strains between 4-100%. This enables us to directly program LCEs using mechanical deformations that impart low or high strains in the LCE including stamping, curling, stretching and embossing methods. The resulting LCEs reversibly shape-shift between the initial and programmed shape. This work widens the potential application of LCEs in biomedical devices, soft-robotics and micro-fluidics where arbitrary and easily programmed shapes are needed.

Light or Thermally Powered Autonomous Rolling of an Elastomer Rod

Specially arranged external stimuli or carefully designed geometry are often essential for realizing continuous autonomous motion of active structures without self-carried power. As a consequence, it is usually very challenging to further integrate those structures as active building blocks into a system with a complex form and multiple functions. In this letter, we report an autonomous motion of a surprisingly simple setup: a cylindrical elastomer rod on a flat hot surface or under homogeneous illumination of visible light. We further show that the rod can roll continuously without any sign of a pause after 6 h, if an obstacle is put in front of it. We demonstrate that such nonintuitive autonomous rolling results from a combination of large thermal actuation of the elastomer and heat transfer between the rod and its surroundings. Quantitative predictions of the rolling speed from the developed thermomechanics model agree reasonably well with experimental measurements. Using the autonomous rolling rods as main building blocks, we further design and fabricate a light-powered vehicle and a thermally powered conveyor for mass transport in both air and water environments.

Reversible solvent-sensitive actuator with continuous bending/debending process from liquid crystal elastomer-colloidal material

A reversible solvent-sensitive actuator with a continuous bending/debending process is fabricated by over-infiltration of liquid crystal monomers into a colloidal template and subsequent photopolymerization. The fabricated actuator exhibits a maximum bending angle of 1080° in 1.58 s in dichloromethane, accompanied with successive debending in 0.32 s. The behavior of the actuator can be modulated by changing the solvent type, film thickness/length and molar ratio of A6OCB/C6M. This study will provide an important experimental and theoretical basis for the development of novel actuators.

Self-Sensing Paper Actuators Based on Graphite-Carbon Nanotube Hybrid Films

Soft actuators have demonstrated potential in a range of applications, including soft robotics, artificial muscles, and biomimetic devices. However, the majority of current soft actuators suffer from the lack of real-time sensory feedback, prohibiting their effective sensing and multitask function. Here, a promising strategy is reported to design bilayer electrothermal actuators capable of simultaneous actuation and sensation (i.e., self-sensing actuators), merely through two input electric terminals. Decoupled electrothermal stimulation and strain sensation is achieved by the optimal combination of graphite microparticles and carbon nanotubes (CNTs) in the form of hybrid films. By finely tuning the charge transport properties of hybrid films, the signal-to-noise ratio (SNR) of self-sensing actuators is remarkably enhanced to over 66. As a result, self-sensing actuators can actively track their displacement and distinguish the touch of soft and hard objects.

Liquid crystal polymers with motile surfaces

In analogy with developments in soft robotics it is anticipated that soft robotic functions at surfaces of objects may have a large impact on human life with respect to comfort, health, medical care and energy. In this review, we demonstrate the possibilities and versatilities of liquid crystal networks and elastomers being explored for soft robotics, with an emphasis on motile surface properties, such as topographical dynamics. Typically the surfaces reversibly transfer from a flat state to a pre-designed corrugated state under various stimuli. But also reversible conversion between different corrugated states is feasible. Generally, the driving triggers are heat, light, electricity or contact with pH changing media. Also, the macroscopic effects of those dynamic topographies, such as altering the friction, wettability and/or performing work are illustrated. The review concludes with the existing challenges as well as outlook opportunities.

Bioinspired Stimuli-Responsive Color-Changing Systems

Stimuli-responsive colors are a unique characteristic of certain animals, evolved as either a method to hide from enemies and prey or to communicate their presence to rivals or mates. From a material science perspective, the solutions developed by Mother Nature to achieve these effects are a source of inspiration to scientists for decades. Here, an updated overview of the literature on bioinspired stimuli-responsive color-changing systems is provided. Starting from natural systems, which are the source of inspiration, a classification of the different solutions proposed is given, based on the stimuli used to trigger the color-changing effect.

New insights into the nature of semi-soft elasticity and "mechanical-Fréedericksz transitions" in liquid crystal elastomers

The mechanical properties of an all-acrylate liquid crystal elastomer (LCE) with a glass transition of 14 ± 1 °C are reported. The highly nonlinear load curve has a characteristic shape associated with semi-soft elasticity (SSE). Conversely, measurements of the director orientation throughout tensile loading instead indicate a "mechanical-Fréedericksz" transition (MFT). Values of the step length anisotropy, r, are independently calculated from the theories of SSE (r = 3.2 ± 0.4), MFT (9.3 < r < 30.0) and thermally-induced length change (r = 3.8 ± 0.5). From simultaneously recorded polarising microscopy textures, the consequences of the above discrepancies are considered. Further, a mechanically-induced negative order parameter is observed. Results show the tensile load curve shape cannot solely be used to determine the underlying physics. Consequently, the LCE properties cannot be fully described by theories of SSE or MFTs alone. This suggests that the theory of LCEs is not yet complete. The conclusions suggest that both the LC order parameter and r must be functions of the mechanical deformation.

A self-healing photoinduced-deformable material fabricated by liquid crystalline elastomers using multivalent hydrogen bonds as cross-linkers

Liquid crystalline elastomers (LCEs) using multivalent hydrogen bonds as cross-linkers were successfully fabricated, which showed both self-healing and photoinduced-deformable properties. More interestingly, this LCE could be readily molded into different shapes through a versatile and efficient procedure, and the fibrous and filmy samples showed different photoinduced-deformable behavior originating from the difference in molecular orientations.

Directed Self-Assembly of Liquid-Crystalline Molecular Building Blocks for Sub-5 nm Nanopatterning

The thin-film directed self-assembly of molecular building blocks into oriented nanostructure arrays enables next-generation lithography at the sub-5 nm scale. Currently, the fabrication of inorganic arrays from molecular building blocks is restricted by the limited long-range order and orientation of the materials, as well as suitable methodologies for creating lithographic templates at sub-5 nm dimensions. In recent years, higher-order liquid crystals have emerged as functional thin films for organic electronics, nanoporous membranes, and templated synthesis, which provide opportunities for their use as lithographic templates. By choosing examples from these fields, recent progress toward the design of molecular building blocks is highlighted, with an emphasis on liquid crystals, to access sub-5 nm features, their directed self-assembly into oriented thin films, and, importantly, the fabrication of inorganic arrays. Finally, future challenges regarding sub-5 nm patterning with liquid crystals are discussed.

Liquid Crystalline Networks toward Regenerative Medicine and Tissue Repair

The communication reports the use of liquid crystalline networks (LCNs) for engineering tissue cultures with human cells. Their ability as cell scaffolds for different cell lines is demonstrated. Preliminary assessments of the material biocompatibility are performed on human dermal fibroblasts and murine muscle cells (C2C12), demonstrating that coatings or other treatments are not needed to use the acrylate-based materials as support. Moreover, it is found that adherent C2C12 cells undergo differentiation, forming multinucleated myotubes, which show the typical elongated shape, and contain bundles of stress fibers. Once biocompatibility is demonstrated, the same LCN films are used as a substrate for culturing human induced pluripotent stem cell-derived cardiomyocites (hiPSC-CMs) proving that LCNs are capable to develop adult-like dimensions and a more mature cell function in a short period of culture in respect to standard supports. The demonstrated biocompatibility together with the extraordinary features of LCNs opens to preparation of complex cell scaffolds, both patterned and stimulated, for dynamic cell culturing. The ability of these materials to improve cell maturation and differentiation will be developed toward engineered heart and skeletal muscular tissues exploring regenerative medicine toward bioartificial muscles for injured sites replacement.

Smart biomimetic micro/nanostructures based on liquid crystal elastomers and networks

A plethora of living organisms are equipped with smart functionalities that are usually rooted in their surface micro/nanostructures or underlying muscle tissues. Inspired by nature, extensive research efforts have been devoted to the development of novel biomimetic functional micro/nanostructured systems. Despite all the accomplishments, the emulation of biological adaptation and stimuli responsive actuation has been a longstanding challenge. The use of liquid crystal elastomers (LCEs) and networks (LCNs) for the fabrication of smart biomimetic micro/nanostructures has recently drawn extensive scientific attention and has become a growing field of research with promising prospects for emerging technologies. In this study, we review the recent progress in this field and portray the current challenges as well as the outlook of this field of research.

Reactive oligo(dimethylsiloxane) mesogens and their nanostructured thin films

Oligo(dimethylsiloxane)-based reactive mesogens were prepared and shown to form room-temperature smectic phases which were 'frozen-in' by photopolymerization. Homeotropically aligned, nanostructured thin films were obtained by spincoating, and micropatterning was demonstrated. These hybrid reactive mesogens are suitable for the preparation of aligned nanostructured polymer thin films with potential applications ranging from stimuli responsive coatings to nanoporous membranes.

Highly sensitive and selective liquid crystal optical sensor for detection of ammonia

Ammonia detection technologies are very important in environment monitoring. However, most existing technologies are complex and expensive, which limit the useful range of real-time application. Here, we propose a highly sensitive and selective optical sensor for detection of ammonia (NH₃) based on liquid crystals (LCs). This optical sensor is realized through the competitive binding between ammonia and liquid crystals on chitosan-Cu²⁺ that decorated on glass substrate. We achieve a broad detection range of ammonia from 50 ppm to 1250 ppm, with a low detection limit of 16.6 ppm. This sensor is low-cost, simple, fast, and highly sensitive and selective for detection of ammonia. The proposal LC sensing method can be a sensitive detection platform for other molecule monitors such as proteins, DNAs and other heavy metal ions by modifying sensing molecules.

Bioinspired, Mechano-Regulated Interfaces for Rationally Designed, Dynamically Controlled Collection of Oil Spills from Water

This study describes the fabrication of bioinspired mechano-regulated interfaces (MRI) for the separation and collection of oil spills from water. The MRI consists of 3D-interconnected, microporous structures of sponges made of ultrasoft elastomers (Ecoflex). To validate the MRI strategy, ecoflex sponges are first fabricated with a low-cost sugar-leaching method. This study then systematically investigates the absorption capacity (up to 1280% for chloroform) of the sponges to different oils and organic solvents. More importantly, the oil flux through the as-made sponges is controlled by mechanical deformation, which increases up to ≈33-fold by tensile strain applied to the sponge from 0 to 400%. On the basis of MRI, this study further demonstrates the application of ecoflex sponges in oil skimmers for selective collecting oil from water with high efficiency and durable recyclability. The as-developed MRI strategy has opened a new path to allow rational design and dynamical control toward developing high performance devices for oil permeation and selective collection of oil spills from water.

Light activated non-reciprocal motion in liquid crystalline networks by designed microactuator architecture

The cross-linking density controls light induced deformation dynamics in liquid crystalline networks. A double stripe system featuring different crosslinking densities affords non reciprocal and asymmetric millisecond movements at the microscale.

Thermally Active Liquid Crystal Network Gripper Mimicking the Self-Peeling of Gecko Toe Pads

Self-peeling of gecko toes is mimicked by integration of film-terminated fibrillar adhesives to hybrid nematic liquid crystal network (LCN) cantilevers. A soft gripper is developed based on the gecko-inspired attachment/detachment mechanism. Performance of the fabricated gripper for transportation of thin delicate objects is evaluated by the optimum mechanical strength of the LCN and the maximum size of the adhesive patch.

A color-changing plasmonic actuator based on silver nanoparticle array/liquid crystalline elastomer nanocomposites

The color-changing Ag NPs/LCE actuators can be used for smart environmental responsive devices by coupling the LSPR of Ag NPs with the deformation of the LCE.