Liquid Crystal Soft Actuators and Robots toward Mixed Reality

Liquid crystals (LCs) are soft but smart materials that can adjust its chemical or physical properties in response to various external stimuli. Using these materials to construct soft actuators and robots, referred as LC actuators and robots, is expected to replace current machinery part, obtaining lighter and smaller equipment with adjustable and complex functions. Especially, combining these LC actuator and robots with existing virtual reality and augmented reality technologies will produce a new world of mixed reality (MR) with the visual, auditory, and somatosensory interaction. In this review, the recent work on responsive LC actuators and robots is introduced, emphasizing on their potentials in haptic use. By discussing their programmable control via suitable stimuli, the LC actuators and robots are summarized for mechanical outputs, environmental mimic, and fine-tuning of surface texture and roughness. It is anticipated that the continuous development on LC actuators and robots will accelerate the MR technology toward practical application.     

Bio-Inspired Stimuli-Responsive Ti3C2Tx/PNIPAM Anisotropic Hydrogels for High-Performance Actuators

Near-infrared (NIR) light-responsive hydrogels have the advantages of high precision, remote control and excellent biocompatibility, which are widely used in soft biomimetic actuators. The process by which water molecules diffuse can directly affect the deformation of hydrogel. Therefore, it remains a serious challenge to improve the response speed of hydrogel actuator. Herein, an anisotropic photo-responsive conductive hydrogel is designed by a directional freezing method. Due to the anisotropy of the MXene-based PNIPAM/MXene directional (PMD) hydrogel, its mechanical properties and conductivity are enhanced in a specific direction. At the same time, with the presence of the internal directional channels and the assistance of capillary force, the PMD hydrogel can achieve a volume deswelling of 70% in 2 s under light irradiation, further building a hydrogel actuator with a fast response performance. Additionally, the hydrogel actuator can lift an object 40 times its weight by a distance of 6 mm, realizing the advantages of both rapid responsiveness and high driving strength, which makes the hydrogel actuator have important application significance in remote control, microflow valve, and soft robot.     

Dynamic characteristics of planar bending actuator embedded with shape memory alloy

Planar bending actuators embedded with a shape memory alloy (SMA) are a novel type of actuator capable of outputting high bending angle and bending moment. This actuator has a simple structure and high power–weight ratio, which enable it to have wide-ranging applications in the field of biomimetic robots, including biomimetic fish fins and soft robots. However, the existing mechanical model of planar bending actuators embedded with SMA is still quasi-static. Thus, the dynamic characteristics of this kind of actuator cannot be effectively described. Therefore, actuators are difficult to control during the deflection process. To effectively describe this process, a dynamic model of the actuator must be built. This paper aims to identify the dynamic characteristics of the actuator. First, a dynamic model for planar bending actuators embedded with a single SMA wire is established. Second, before SMA wire application, the constitutive characteristics are identified to obtain the necessary parameters for simulation. Third, stable planar bending actuators are fabricated for experiments on characteristics, and then the process flow chart of the planar bending actuator is developed to ensure consistency of the properties of all actuators. Finally, the actuator characteristics obtained experimentally are presented in comparison with those obtained by theoretical simulation. Analysis of the experimental data and simulation results indicates that the planar bending actuator performs better in pulse current heating mode, and that bending angle can be accurately predicted.     

Fast‐Response,Stiffness‐Tunable Soft Actuator by Hybrid Multimaterial 3D Printing

Soft robots have the appealing advantages of being highly flexible and adaptive to complex environments. However, the low‐stiffness nature of the constituent materials makes soft robotic systems incompetent in tasks requiring relatively high load capacity. Despite recent attempts to develop stiffness‐tunable soft actuators by employing variable stiffness materials and structures, the reported stiffness‐tunable actuators generally suffer from limitations including slow responses, small deformations, and difficulties in fabrication with microfeatures. This work presents a paradigm to design and manufacture fast‐response, stiffness‐tunable (FRST) soft actuators via hybrid multimaterial 3D printing. The integration of a shape memory polymer layer into the fully printed actuator body enhances its stiffness by up to 120 times without sacrificing flexibility and adaptivity. The printed Joule‐heating circuit and fluidic cooling microchannel enable fast heating and cooling rates and allow the FRST actuator to complete a softening–stiffening cycle within 32 s. Numerical simulations are used to optimize the load capacity and thermal rates. The high load capacity and shape adaptivity of the FRST actuator are finally demonstrated by a robotic gripper with three FRST actuators that can grasp and lift objects with arbitrary shapes and various weights spanning from less than 10 g to up to 1.5 kg.     

Stimuli-Responsive Peptide Self-Assembly to Construct Hydrogels with Actuation and Shape Memory Behaviors

Hydrogel actuators, capable of generating reversible deformation in response to external stimulus, are widely considered as new emerging intelligent materials for applications in soft robots, smart sensors, artificial muscles, and so on. Peptide self-assembly is widely applied in the construction of intelligent hydrogel materials due to their excellent stimulus response. However, hydrogel actuators based on peptide self-assembly are rarely reported and explored. In this study, a pH-responsive peptide (MA-FIID) is designed and introduced into a poly(N-isopropyl acrylamide) backbone (PNIPAM) to construct bilayer and heterogeneous hydrogel actuators based on the assembly and disassembly of peptide molecules under different pH conditions. These peptide-containing hydrogel actuators can perform controllable bending, bucking, and complex deformation under pH stimulation. Meanwhile, the Hofmeister effect of PNIPAM hydrogels endows these peptide-containing hydrogels with enhanced mechanical strength, ionic stimulus response (CaCl₂), and excellent shape-memory property. This work broadens the application of supramolecular self-assembly in the construction of intelligent hydrogels, and also provides new inspirations for peptide self-assembly to construct smart materials.     

Rapid 3D Printing of Electrohydraulic (HASEL) Tentacle Actuators

A comprehensive material system is introduced for the additive manufacturing of electrohydraulic (HASEL) tentacle actuators. This material system consists of a photo‐curable, elastomeric silicone‐urethane with relatively strong dielectric properties (ε_r ≈ 8.8 at 1 kHz) in combination with ionically‐conductive hydrogel and silver paint electrodes that displace a vegetable‐based liquid dielectric under the application of an electric field. The electronic properties of the silicone material as well as the mechanical properties of the constitutive silicone and hydrogel materials are investigated. The hydraulic pressure exerted on the dielectric working fluid in these capacitive actuators is measured in order to characterize their quasi‐static behavior. Various design features enabled by 3D printing influence this behavior—decreasing the voltage at which actuation begins or increasing the force density in the system. Using a capacitance change of >35% across the actuators while powered, a demonstration of self‐sensing inherent to HASELs is shown. Antagonistic pairs of the 3D printed actuators are shown to exert a blocked force of over 400 mN. An electrohydraulic tentacle actuator is then fabricated to demonstrate the use of this material and actuation system in a synthetic hydrostat. This tentacle actuator is shown to achieve motion in a multi‐dimensional space.     

Biomimetic 4D-Printed Breathing Hydrogel Actuators by Nanothylakoid and Thermoresponsive Polymer Networks

Shape-morphing actuators, which can breathe with the accompany of morphology changes to mimic botanical events, are challenging to fabricate with soft hydrogel materials. Herein, 4D printed-smart hydrogel actuators are reported that can not only dynamically deform but also generate oxygen (O₂) upon external stimulations. The printed breathing actuators featured with spinach leaf-derived thylakoid membrane (nanothylakoid) for photothermal conversion and catalytical O₂ evolution, a poly(N-isopropylacrylamide) (PNIPA) thermoresponsive polymer network for generating deformation forces by swelling/shrinkage (rehydration/dehydration), and an asymmetric bilayer poly(N-isopropylacrylamide)/polyacrylamide (PNIPA/PAA) structure to amplify the mechanical motions. Upon thermal stimulation or laser irradiation, the actuator can reversibly bend/unbend because of the photothermal conversion of nanothylakoid and the printed thermoresponsive asymmetric bilayer structure. Additionally, the catalase-like property of nanothylakoid imparts the actuator with O₂ evolution capability to breathe for further mimicking botanical systems. Notably, 4D printing can greatly facilitate and simplify the actuator fabrication process, including adjusting the size and layer compositions. This artificial breathing actuator with photothermal and catalytical properties provides a strategy in designing intelligent hydrogel systems and proves to be a highly promising material candidates in the fields of 3D/4D printing, automated robotics, and smart biomedical devices.     

Laser-Induced Graphene Tapes as Origami and Stick-On Labels for Photothermal Manipulation via Marangoni Effect

Direct light-to-work conversion enables remote actuation through a non-contact manner, among which the photothermal Marangoni effect is significant for developing light-driven robots because of the diversity of applicable photothermal materials and light sources, as well as the high energy conversion efficiency. However, the lack of nanotechnologies that enable flexible integration of advanced photothermal materials with actuators of complex configurations significantly restricts their practical applications. In this paper, laser-induced graphene (LIG) tape is reported as stick-on photothermal labels for developing light-driven actuators based on the Marangoni effect. With the help of direct laser writing technology, graphene patterns with superior photothermal properties are prepared on the PI tape. The patterned LIG tape can be stuck on any desired objects and generates an asymmetric photothermal field under light irradiation, forming a photothermal Marangoni actuator. Additionally, the PI tape with LIG patterns can be folded into 3D origami actuators that permit photothermal Marangoni actuation including both translation and rotation. The graphene-based photothermal Marangoni actuators feature biocompatibility, which is confirmed by MDA-MB-231 cells proliferation experiments. Owing to the excellent photothermal property of LIG patterns, the as-produced photothermal actuators can be manipulated by a variety of light sources, holding great promise for developing light-driven soft robots.     

Biomimetic Color Changing Anisotropic Soft Actuators with Integrated Metal Nanowire Percolation Network Transparent Heaters for Soft Robotics

To add more functionalities and overcome the limitation in conventional soft robots, highly anisotropic soft actuators with color shifting function during actuation is demonstrated for the first time. The electrothermally operating soft actuators with installed transparent metal nanowire percolation network heater allow easy programming of their actuation direction and instantaneous visualization of temperature changes through color change. Due to the unique direction dependent coefficient of thermal expansion mismatch, the suggested actuator demonstrates a highly anisotropic and reversible behavior with very large bending curvature (2.5 cm^?1) at considerably low temperature (≈40 °C) compared to the previously reported electrothermal soft actuators. The mild operating heat condition required for the maximum curvature enables the superior long‐term stability during more than 10 000 operating cycles. Also, the optical transparency of the polymer bilayer and metal nanowire percolation network heater allow the incorporation of the thermochromic pigments to fabricate color‐shifting actuators. As a proof‐of‐concept, various color‐shifting biomimetic soft robots such as color‐shifting blooming flower, fluttering butterfly, and color‐shifting twining tendril are demonstrated. The developed color‐shifting anisotropic soft actuator is expected to open new application fields and functionalities overcoming the limitation of current soft robots.     

Actuation of Three-Dimensional-Printed Nanocolloidal Hydrogel with Structural Anisotropy

Polymer hydrogels exhibit actuation properties that result in reversible shape transformations and have promising applications in soft robotics, drug delivery systems, sensors, and microfluidic devices. Actuation occurs due to differential hydrogel swelling and is generally achieved by modulating hydrogel composition. Here a different approach to hydrogel actuation that originates solely from its structural anisotropy is reported. For 3D-printed single-layer hydrogels formed by cellulose nanocrystals (CNCs) and gelatin methacryloyl it is shown that shear-induced orientation of CNCs results in anisotropic mechanical and swelling properties of the hydrogel. Upon swelling in water, planar hydrogels acquire multiple complex 3D shapes that are achieved by i) varying CNC orientation with respect to the shape on the hydrogel sheet and ii) patterning the hydrogel with the regions of shear-mediated and random CNC orientation. This study shows the capability to generate multiple shapes from the same hydrogel actuator based on the degree of its structural anisotropy. In addition, it introduces a biocompatible nanocolloidal ink with shear-thinning and self-healing properties for additive manufacturing of hydrogel actuators.     

Robotic Locomotion and Piezo1 Activity Controlled with Novel Liquid Marble-based Soft Actuators

A novel soft actuator is designed, fabricated, and optimized for applied use in soft robotics and biomedical applications. The soft actuator is powered by the expansion and contraction of a graphene-containing and encased liquid marble using the photothermal effect. Unfortunately, conventional liquid marbles are found to be too fragile and prone to cracking and failure for such applications. After experimentation, it is possible to remedy this problem by synthesizing liquid marbles encased with polymeric shells–polymerized in situ–for added mechanical strength and robustness. These marbles are shown to have intrinsic photothermal activity. They are then situated in bimorph-type soft actuators where one side of the actuator has a dramatically different Young's modulus than the other, leading to directional actuation which is successfully demonstrated in multistep walking soft robots. The soft actuators are shown to successfully activate the mechanosensitive Piezo protein in a transfected human cell line with high effectiveness and no toxicity. Overall, the liquid marble-powered soft actuators described here represent a new soft actuation methodology and a novel tool for mechanobiological studies, such as stem cell fate and organoid differentiation.     

Stable Walking Pattern for an SMA-Actuated Biped

In this paper, a walking pattern filter for shape-memory-alloy (SMA)-actuated biped robots is presented. SMAs are known for their high power-to-mass ratio as well as slow response. When used as actuators, the SMA speed limitation can potentially lead to stability problems for biped robots. The presented filter adapts the human motion such that an SMA biped robot maintains a stable walking pattern. The zero moment point (ZMP) is used as the main criterion of the filter to guarantee the stability of the motion. The SMA actuators are designed based on the dynamics and kinematics of the motion. The response time of each SMA actuator is modeled in order to estimate the behavior of the actuator in realizing the given trajectory. After applying the delay times to the motion, the new trajectories are generated and evaluated by the filter for the ZMP criterion. Using simulations, it is shown that the filter can generate smooth trajectories for the SMA-actuated biped robots. The filter furthermore guarantees the stability of a robot mimicking the human walking motion.     

Design and calibration of 3D printed soft deformation sensors for soft actuator control

Soft actuators made from compliant materials are superior to conventional rigid robots in terms of flexibility, adaptability and safety. However, an inherent drawback of soft actuator is the low actuation precision. Implementing closed loop control is a possible solution, but the soft actuator shape can hardly be measured directly by commercially available sensors, which either are too stiff for integration or cause performance degradation of the actuator. Although 3D printing has been applied to print bendable sensors from conductive materials, they either have larger stiffness than the soft actuator or are made from specially designed materials that are difficult to reproduce. In this study, easily accessible commercial soft conductive material is applied to directly 3D print soft sensors on soft actuators. Different configurations of the printed sensors are studied to investigate how the sensor design affects the performance. The best sensor configuration is selected to provide shape feedback using its changing resistance during deformation. Compared with a commercial flexible bending sensor, the printed sensor has less influences on the soft actuator performance and enjoys higher shape estimation accuracy. Closed loop shape control of the actuator using feedback from the 3D printed sensor is then designed, implemented and compared with the control results using image feedback. A gripper consisting of three individually controlled soft actuators demonstrates the applications of the soft sensor.     

Electrical and Mechanical Self‐Healing in High‐Performance Dielectric Elastomer Actuator Materials

Dielectric elastomers are of interest for actuator applications due to their large actuation strain, high bandwidth, high energy density, and their flexible nature. If future dielectric elastomers are to be used reliably in applications that include soft robotics, medical devices, artificial muscles, and electronic skins, there is a need to design devices that are tolerant to electrical and mechanical damage. In this paper, the first report of self‐healing of both electrical breakdown and mechanical damage in dielectric actuators using a thermoplastic methyl thioglycolate–modified styrene–butadiene–styrene (MGSBS) elastomer is provided. The self‐healing functions are examined from the material to device level by detailed examination of the healing process, and characterization of electrical properties and actuator response before and after healing. It is demonstrated that after dielectric breakdown, the initial dielectric strength can be recovered by up to 67%, and after mechanical damage, a 39% recovery can be achieved with no degradation of the strain–voltage response of the actuators. The elastomer can also heal a combination of mechanical and electrical failures. This work provides a route to create robust and damage tolerant dielectric elastomers for soft robotic and other applications related to actuator and energy‐harvesting systems.     

Starfish-Inspired Cut-Resistant Hydrogel with Self-Growing Armor: Where Softness Meets Toughness

Many soft animals like starfish have developed armors to protect their soft bodies in order to survive in harsh environment. Inspired by this fact, special hydrogels with self-growing protective armors are developed by allowing sodium acetate to crystallize on hydrogel surface. Poly(acrylic acid) chains limit the crystalline region to the surface of the hydrogel by decreasing the pH value and limiting the movement of ions, and then creating a set of tough armor. This armor-protected hydrogel is able to withstand a high pressure (> 78 MPa) under cutting and prevent the penetration of sharp objects. Interestingly, the unique stimulated-precipitation mechanism allows the armor to repair itself after damage. Besides, the surface of hydrogel changes from “sticky” to “slippery”, which also helps to improve its protective ability. Moreover, the armor helps to retrain water in hydrogel network, and significantly improve the mechanical properties of hydrogels (maximum compressive stress >18 MPa, compressive strain > 90%). In addition, the hydrogel can keep soft and work durably at extreme temperatures (−50 °C and 80 °C) due to the high salt concentration. This study provides an innovative approach for designing armor-protected hydrogels with great potential in engineering applications such as actuators and sensors for harsh environments.     

Continuum analysis of a soft bending actuator dynamics

Soft fiber-reinforced bending actuators are extensively used in different applications such as gripping mechanisms and rehabilitation robots. Modeling their dynamics is challenging due to their inherent nonlinearity that rises from both their material behavior and their geometrical features. This paper presents a continuum nonlinear model of the dynamic response of this actuator to inflation. The model is developed by considering the deformation to be hyperelastic and taking Rayleigh’s dissipative function into account. The deflection functions are obtained by solving the resulting differential equations using semi-inverse techniques. After fabricating a prototype and verifying the model experimentally in both time and frequency domains, it is further studied using bifurcation analysis concerning changes in the actuator’s geometrical parameters. Moreover, by analyzing the model in the frequency domain, a hardening behavior is observed in the system’s frequency response. In addition, the system’s response amplitude is inspected to decrease dramatically in higher frequencies, meaning that the actuator does not go back to its reference state owing to high actuation speed.     

Reconfigurable and Latchable Shape‐Morphing Dielectric Elastomers Based on Local Stiffness Modulation

Programmable soft materials exhibiting dynamically reconfigurable, reversible, fast, and latchable shape transformation are key for applications ranging from wearable tactile actuators to deployable soft robots. Multimorph soft actuator sheets with high load‐bearing capacity are reported, capable of bending on multiple axis, made by combining a single dielectric elastomer actuator (DEA) with two layers of shape memory polymers (SMPs) fibers and an array of stretchable heaters. The rigidity of the SMP fibers can be reduced by two orders of magnitude by Joule heating, thus allowing the orientation and location of soft and hard regions to be dynamically defined by addressing the heaters. When the DEA is then actuated, it bends preferentially along the soft axis, enabling the device to morph into multiple distinct configurations. Cooling down the SMPs locks these shape changes into place. A tip deflection angle of over 300° at 5 kV is achieved with a blocking force of over 27 mN. Devices using two antagonistic DEAs are also reported that attain more complex shapes. Multimorphing is demonstrated by gripping objects with different shapes. An analytical model is developed to determine the design parameters that offers the best trade‐off between large actuation and high holding forces.     

3D Knitting for Pneumatic Soft Robotics

Soft robots adapt passively to complex environments due to their inherent compliance, allowing them to interact safely with fragile or irregular objects and traverse uneven terrain. The vast tunability and ubiquity of textiles has enabled new soft robotic capabilities, especially in the field of wearable robots, but existing textile processing techniques (e.g., cut-and-sew, thermal bonding) are limited in terms of rapid, additive, accessible, and waste-free manufacturing. While 3D knitting has the potential to address these limitations, an incomplete understanding of the impact of structure and material on knit-scale mechanical properties and macro-scale device performance has precluded the widespread adoption of knitted robots. In this work, the roles of knit structure and yarn material properties on textile mechanics spanning three regimes–unfolding, geometric rearrangement, and yarn stretching–are elucidated and shown to be tailorable across unique knit architectures and yarn materials. Based on this understanding, 3D knit soft actuators for extension, contraction, and bending are constructed. Combining these actuation primitives enables the monolithic fabrication of entire soft grippers and robots in a single-step additive manufacturing procedure suitable for a variety of applications. This approach represents a first step in seamlessly “printing” conformal, low-cost, customizable textile-based soft robots on-demand.     

3D Printed Cartilage‐Like Tissue Constructs with Spatially Controlled Mechanical Properties

Developing biomimetic cartilaginous tissues that support locomotion while maintaining chondrogenic behavior is a major challenge in the tissue engineering field. Specifically, while locomotive forces demand tissues with strong mechanical properties, chondrogenesis requires a soft microenvironment. To address this challenge, 3D cartilage‐like tissue is fabricated using two biomaterials with different mechanical properties: a hard biomaterial to reflect the macromechanical properties of native cartilage, and a soft biomaterial to create a chondrogenic microenvironment. To this end, a bath composed of an interpenetrating polymer network (IPN) of polyethylene glycol (PEG) and alginate hydrogel (MPa order compressive modulus) is developed as an extracellular matrix (ECM) with self‐healing properties. Within this bath supplemented with thrombin, human mesenchymal stem cell (hMSC) spheroids embedded in fibrinogen are 3D bioprinted, creating a soft microenvironment composed of fibrin (kPa order compressive modulus) that simulate cartilage's pericellular matrix and allow a fast diffusion of nutrients. The bioprinted hMSC spheroids present high viability and chondrogenic‐like behavior without adversely affecting the macromechanical properties of the tissue. Therefore, the ability to locally bioprint a soft and cell stimulating biomaterial inside of a mechanically robust hydrogel is demonstrated, thereby uncoupling the micro‐ and macromechanical properties of the 3D printed tissues such as cartilage.     

Light-Driven Self-Oscillating Behavior of Liquid-Crystalline Networks Triggered by Dynamic Isomerization of Molecular Motors

The self-sustainable dynamic movement of soft actuators represents a continuous motion upon constant stimulus to achieve its great potential in emerging photoresponsive applications from self-propelling machines, artificial robots to advanced biomimetic devices. Conversion of dynamic isomerization of molecular motors and switches into macroscopic self-oscillation of soft materials is highly attractive but challenging. In this study, an overcrowded alkene motor with trifunctional acrylate groups is designed and synthesized, and its photoisomerization can be achieved in the liquid-crystalline networks. Furthermore, the photodynamic storage modulus can be mainly modulated by the dynamic stable–unstable isomerization of the molecular motor upon UV exposure. Thus, the light-driven self-oscillating behavior from chaotic to regular movement can be performed based on the photodynamic mechanical balance of the micro-oscillating modulus of the polymer network triggered by the dynamic reconfiguration of the motor. The results pave the way for inspirations in the development of advanced photoactive functional architectures and biomimetic actuators.