Ferromagnetic Liquid Metal Putty-Like Material with Transformed Shape and Reconfigurable Polarity

It is remarkably desirable and challenging to design reconfigurable ferromagnetic materials with high electrical conductivity. This has attracted great attention due to promising applications in many fields such as emerging flexible electronics and soft robotics. However, the shape and magnetic polarity of existing ferromagnetic materials with low conductivity are both hard to be reconfigured, and the magnetization of insulative ferrofluids is easily lost once the external magnetic field is removed. A novel reconfigurable ferromagnetic liquid metal (LM) putty-like material (FM-LMP) with high electrical conductivity and transformed shape, which is prepared through homogenously mixing neodymium–iron–boron microparticles into the gallium-based LM matrix, and turning this liquid-like suspension into the solid-like putty-like material by magnetization, is reported to achieve this. The induction magnetic field of FM-LMP is mainly attributed to the magnetic alignment of the dispersed ferromagnetic microparticles, which can be conveniently demagnetized by mechanical disordering and reversibly reconfigured through microparticle realignment by applying a weak magnetic field. FM-LMP with a low fraction of microparticles can be used as printable conductive ink for paper electronics, which are further exploited for applications including magnetic switching, flexible erasable magnetic recording paper, and self-sensing paper-based soft robotics using magnetic actuation.     

Universal Soft Robotic Microgripper

Here, a soft robotic microgripper is presented that consists of a smart actuated microgel connected to a spatially photopatterned multifunctional base. When pressed onto a target object, the microgel component conforms to its shape, thus providing a simple and adaptive solution for versatile micromanipulation. Without the need for active visual or force feedback, objects of widely varying mechanical and surface properties are reliably gripped through a combination of geometrical interlocking mechanisms instantiated by reversible shape‐memory and thermal responsive swelling of the microgel. The gripper applies holding forces exceeding 400 µN, which is high enough to lift loads 1000 times heavier than the microgel. An untethered version of the gripper is developed by remotely controlling the position using magnetic actuation and the contractile state of the microgel using plasmonic absorption. Gentle yet stable robotic manipulation of biological samples under physiological conditions opens up possibilities for high‐throughput interrogation and minimally invasive interventions.     

Slit Tubes for Semisoft Pneumatic Actuators

This article describes a new principle for designing soft or ‘semisoft’ pneumatic actuators: SLiT (for SLit‐in‐Tube) actuators. Inflating an elastomeric balloon, when enclosed by an external shell (a material with higher Young's modulus) containing slits of different directions and lengths, produces a variety of motions, including bending, twisting, contraction, and elongation. The requisite pressure for actuation depends on the length of the slits, and this dependence allows sequential actuation by controlling the applied pressure. Different actuators can also be controlled using external “sliders” that act as reprogrammable “on‐off” switches. A pneumatic arm and a walker constructed from SLiT actuators demonstrate their ease of fabrication and the range of motions they can achieve.     

Synergistic effect of siloxane modified aluminum nanopowders and carbon fiber on electrothermal efficiency of polymeric shape memory nanocomposite

The present study reports an effective approach of significantly enhancing electrothermal efficiency and shape recovery performance of shape memory polymer (SMP) nanocomposite, of which shape recovery was induced by electrically resistive heating. Metallic aluminum (Al) nanopowders synthesized from Al³⁺ solution were chemically grafted onto carbon fiber. Siloxane groups were grafted onto surfaces of the Al nanopowders to enhance the interfacial bonding between the carbon fiber and SMP matrix via van der Waals force and covalent bond, respectively. The siloxane modified Al surfaces could improve both the electrically induced shape recovery performance and electrothermal efficiency through facilitating the electrically resistive heating from carbon fiber into the SMP matrix. Effectiveness of the synergistic effect between siloxane modified Al surface and carbon fiber was demonstrated to achieve the electrical actuation for SMP nanocomposites at a low electrical voltage below 4.0 V.     

‘Two way’ shape memory composites based on electroactive polymer and thermoplastic membrane

A practical and facile strategy was proposed to fabricate composites that not only use the properties of individual components (commercial electroactive polymer and thermoplastic resin) to their advantage, but also produce synergy effect of ‘two way’ shape memory properties. In this design, electroactive polymer is treated as soft segment which provides actuation force via converting electrical energy to dynamic energy. Thermoplastic material serves as ‘hard segment’ to help with fixation of temporary shape thanks to its re-structuring and stiffness/modulus changing abilities through the reversible transitional temperature. Compared with traditional one way and two way shape memory materials, this composite material has the capability of changing shape without pre-programming. High shape recover property (99 ± 0.3%.) has been obtained due to the rubber elasticity of electroactive polymer matrix. Many features could be brought up based on this design, such as accurate control over deformation by changing strength of applied electric field as well as tailorable stimulus temperature and mechanical properties.     

Magnetically Addressable Shape‐Memory and Stiffening in a Composite Elastomer

With a specific stimulus, shape‐memory materials can assume a temporary shape and subsequently recover their original shape, a functionality that renders them relevant for applications in fields such as biomedicine, aerospace, and wearable electronics. Shape‐memory in polymers and composites is usually achieved by exploiting a thermal transition to program a temporary shape and subsequently recover the original shape. This may be problematic for heat‐sensitive environments, and when rapid and uniform heating is required. In this work, a soft magnetic shape‐memory composite is produced by encasing liquid droplets of magneto‐rheological fluid into a poly(dimethylsiloxane) matrix. Under the influence of a magnetic field, this material undergoes an exceptional stiffening transition, with an almost 30‐fold increase in shear modulus. Exploiting this transition, fast and fully reversible magnetic shape‐memory is demonstrated in three ways, by embossing, by simple shear, and by unconstrained 3D deformation. Using advanced synchrotron X‐ray tomography techniques, the internal structure of the material is revealed, which can be correlated with the composite stiffening and shape‐memory mechanism. This material concept, based on a simple emulsion process, can be extended to different fluids and elastomers, and can be manufactured with a wide range of methods.     

Two-Way 4D Printing: A Review on the Reversibility of 3D-Printed Shape Memory Materials

The rapid development of additive manufacturing and advances in shape memory materials have fueled the progress of four-dimensional (4D) printing. With the right external stimulus, the need for human interaction, sensors, and batteries will be eliminated, and by using additive manufacturing, more complex devices and parts can be produced. With the current understanding of shape memory mechanisms and with improved design for additive manufacturing, reversibility in 4D printing has recently been proven to be feasible. Conventional one-way 4D printing requires human interaction in the programming (or shape-setting) phase, but reversible 4D printing, or two-way 4D printing, will fully eliminate the need for human interference, as the programming stage is replaced with another stimulus. This allows reversible 4D printed parts to be fully dependent on external stimuli; parts can also be potentially reused after every recovery, or even used in continuous cycles—an aspect that carries industrial appeal. This paper presents a review on the mechanisms of shape memory materials that have led to 4D printing, current findings regarding 4D printing in alloys and polymers, and their respective limitations. The reversibility of shape memory materials and their feasibility to be fabricated using three-dimensional (3D) printing are summarized and critically analyzed. For reversible 4D printing, the methods of 3D printing, mechanisms used for actuation, and strategies to achieve reversibility are also highlighted. Finally, prospective future research directions in reversible 4D printing are suggested.     

Remotely Light‐Powered Soft Fluidic Actuators Based on Plasmonic‐Driven Phase Transitions in Elastic Constraint

Materials capable of actuation through remote stimuli are crucial for untethering soft robotic systems from hardware for powering and control. Fluidic actuation is one of the most applied and versatile actuation strategies in soft robotics. Here, the first macroscale soft fluidic actuator is derived that operates remotely powered and controlled by light through a plasmonically induced phase transition in an elastomeric constraint. A multiphase assembly of a liquid layer of concentrated gold nanoparticles in a silicone or styrene–ethylene–butylene–styrene elastic pocket forms the actuator. Upon laser excitation, the nanoparticles convert light of specific wavelength into heat and initiate a liquid‐to‐gas phase transition. The related pressure increase inflates the elastomers in response to laser wavelength, intensity, direction, and on–off pulses. During laser‐off periods, heating halts and condensation of the gas phase renders the actuation reversible. The versatile multiphase materials actuate—like soft “steam engines”—a variety of soft robotic structures (soft valve, pnue‐net structure, crawling robot, pump) and are capable of operating in different environments (air, water, biological tissue) in a single configuration. Tailored toward the near‐infrared window of biological tissue, the structures actuate also through animal tissue for potential medical soft robotic applications.     

Multifunctional picoliter droplet manipulation platform and its application in single cell analysis

We developed an automated and multifunctional microfluidic platform based on DropLab to perform flexible generation and complex manipulations of picoliter-scale droplets. Multiple manipulations including precise droplet generation, sequential reagent merging, and multistep solid-phase extraction for picoliter-scale droplets could be achieved in the present platform. The system precision in generating picoliter-scale droplets was significantly improved by minimizing the thermo-induced fluctuation of flow rate. A novel droplet fusion technique based on the difference of droplet interfacial tensions was developed without the need of special microchannel networks or external devices. It enabled sequential addition of reagents to droplets on demand for multistep reactions. We also developed an effective picoliter-scale droplet splitting technique with magnetic actuation. The difficulty in phase separation of magnetic beads from picoliter-scale droplets due to the high interfacial tension was overcome using ferromagnetic particles to carry the magnetic beads to pass through the phase interface. With this technique, multistep solid-phase extraction was achieved among picoliter-scale droplets. The present platform had the ability to perform complex multistep manipulations to picoliter-scale droplets, which is particularly required for single cell analysis. Its utility and potentials in single cell analysis were preliminarily demonstrated in achieving high-efficiency single-cell encapsulation, enzyme activity assay at the single cell level, and especially, single cell DNA purification based on solid-phase extraction.     

Controlling Au electrode patterns for simultaneously monitoring electrical actuation and shape recovery in shape memory polymer

This paper presents an effective approach to achieve efficient electrical actuation and monitoring of shape recovery based on patterned Au electrodes on shape memory polymer (SMP). The electrically responsive shape recovery behavior was characterized and monitored by the evolution change in electrical resistance of patterned Au electrode. Both electrical actuation and temperature distribution in the SMP have been improved by optimizing the Au electrode patterns. The electrically actuated shape recovery behavior and temperature evolution during the actuation were monitored and characterized. The resistance changes could be used to detect beginning/finishing points of the shape recovery. Therefore, the Au electrode not only significantly enhances the electrical actuation performance to achieve a fast electrical actuation, but also enables the resistance signal to detect the free recovery process.     

Magnetic Domains and Twin Boundary Movement of NiMnGa Magnetic Shape Memory Crystals

Time‐resolved metallographic optical microscopy techniques are used together with magnetic domain imaging to clarify the interaction between magnetic domains and twin boundary (TB) motion in magnetic shape memory NiMnGa single crystals. The magnetic field and stress induced magnetic domain formation is imaged by a magneto‐optical indicator film technique. Reversible TB motion is visualized up to high actuation speeds. From domain observation at adjacent crystal surfaces the fundamental volume magnetic processes during strain and field induced TB motions are derived. For magnetic field induced structural reorientations a concurrent absence of magnetic domain wall motion is found. In contrast, for strain induced reorientations processes, a complete rearrangement of the magnetic domain structure by the moving TB is observed. Dynamic actuation experiments on TB motion reveal non‐linear time effects on TB mobility. In addition to training effects, the maximum field induced strain increases with actuation speed. Both effects can be interpreted as the interaction of moving twin boundaries with local non‐fixed defects. The summarized results provide key information for the understanding of the connection of magnetic and crystallographic domains in magnetic shape memory alloys and for the optimization of devices for future technical applications.     

Effect of particle size distribution on the magnetic properties γ-Fe2O3 nanoparticles

The magnetic response of nanocomposites formed by non-interacting well dispersed γ-Fe₂O₃ nanoparticles in a polymer matrix is presented. Various low loading fraction of particles in polymer leads to an observation of similar values of blocking temperatures and coercive fields. ac response confirms that particles are non-interacting and follow Neel–Brown model. Effect of particle size distribution on hysteresis behavior and saturation magnetization as a function of temperature is discussed. Since particles have a size distribution, the experimental results of magnetic response are compared with simulations based on Stoner–Wohlfarth model of single size particles. We have devised a measurement method in which a constant magnetic field was applied while the thermal energy is varied by sequentially heating and cooling the sample below the blocking temperature. Nanoparticle–polymer composites show reversible magnetization behavior for sequential heating/cooling cycles. However, simulation based on single size particle system shows irreversible magnetization behavior during the heating and cooling cycles. These observations are qualitatively explained in terms of different behavior of magnetization as a function of temperature for smaller superparamagnetic particles and larger blocked particles below overall blocking temperature of the composite.     

热致形状记忆聚合物非接触式驱动的研究进展

非接触式驱动是将能量直接作用于热致形状记忆聚合物上,驱动其回复初始形状的驱动方法。从电驱动、激光驱动和磁场驱动等方面概述了非接触式驱动的实现方法、原理,综述了近年来热致形状记忆聚合物非接触式驱动的研究进展,并展望了非接触式驱动的可能应用领域与前景。     

Shape Memory Alloys: A Material and a Technology

Shape memory alloys offer attractive potentials such as: superelastic behaviour, reversible strains of several percent during heating or cooling over a limited temperature range, generation of high recovery stresses, and a work output with a high power/weight ratio. This paper describes the origin of those properties and gives an overview of the shape memory functions and shape memory applications.     

MOFBOTS: Metal–Organic‐Framework‐Based Biomedical Microrobots

Motile metal?organic frameworks (MOFs) are potential candidates to serve as small‐scale robotic platforms for applications in environmental remediation, targeted drug delivery, or nanosurgery. Here, magnetic helical microstructures coated with a kind of zinc‐based MOF, zeolitic imidazole framework‐8 (ZIF‐8), with biocompatibility characteristics and pH‐responsive features, are successfully fabricated. Moreover, it is shown that this highly integrated multifunctional device can swim along predesigned tracks under the control of weak rotational magnetic fields. The proposed systems can achieve single‐cell targeting in a cell culture media and a controlled delivery of cargo payloads inside a complex microfluidic channel network. This new approach toward the fabrication of integrated multifunctional systems will open new avenues in soft microrobotics beyond current applications.     

Shape Memory Alloy (SMA)‐Based Microscale Actuators with 60% Deformation Rate and 1.6 kHz Actuation Speed

Shape memory alloys (SMAs) are widely utilized as an actuation source in microscale devices, since they have a simple actuation mechanism and high‐power density. However, they have limitations in terms of strain range and actuation speed. High‐speed microscale SMA actuators are developed having diamond‐shaped frame structures with a diameter of 25 µm. These structures allow for a large elongation range compared with bulk SMA materials, with the aid of spring‐like behavior under tensile deformation. These actuators are validated in terms of their applicability as an artificial muscle in microscale by investigating their behavior under mechanical deformation and changes in thermal conditions. The shape memory effect is triggered by delivering thermal energy with a laser. The fast heating and cooling phenomenon caused by the scale effect allows high‐speed actuation up to 1600 Hz. It is expected that the proposed actuators will contribute to the development of soft robots and biomedical devices.     

NiMn‐Based Alloys and Composites for Magnetically Controlled Dampers and Actuators

As a new class of magnetically driven active materials, magnetic shape memory (MSM) alloys show large strains of up to 10% in Ni₂MnGa single crystal with a modulated martensitic structure. The magnetic field induced twin boundary motion can be exploited for actuators, sensors, and as it is an energy dissipating process, also for vibration damping devices. However, single crystals of Ni₂MnGa are difficult to prepare and are inherently very brittle. One solution is the use of aligned MSM particles suspended in a stiffness‐matched polymer matrix. In this review, we focus on the preparation and characterization of textured NiMn‐based MSM composite. Essential issues, e.g., the criterion of polymer selection and the interface of MSM particles and polymer are discussed. Second aspect of the paper is to review recent developments in metamagnetic shape memory Ni? Mn‐(In,Sn,Sb) systems which exhibit field‐driven multifunctional properties such as magnetocaloric effect, magnetoresistant change, magnetoelectricity, and thermoelectricity.     

Laser-Induced Graphene for Electrothermally Controlled,Mechanically Guided, 3D Assembly and Human–Soft Actuators Interaction

Mechanically guided, 3D assembly has attracted broad interests, owing to its compatibility with planar fabrication techniques and applicability to a diversity of geometries and length scales. Its further development requires the capability of on-demand reversible shape reconfigurations, desirable for many emerging applications (e.g., responsive metamaterials, soft robotics). Here, the design, fabrication, and modeling of soft electrothermal actuators based on laser-induced graphene (LIG) are reported and their applications in mechanically guided 3D assembly and human–soft actuators interaction are explored. Over 20 complex 3D architectures are fabricated, including reconfigurable structures that can reshape among three distinct geometries. Also, the structures capable of maintaining 3D shapes at room temperature without the need for any actuation are realized by fabricating LIG actuators at an elevated temperature. Finite element analysis can quantitatively capture key aspects that govern electrothermally controlled shape transformations, thereby providing a reliable tool for rapid design optimization. Furthermore, their applications are explored in human–soft actuators interaction, including elastic metamaterials with human gesture-controlled bandgap behaviors and soft robotic fingers which can measure electrocardiogram from humans in an on-demand fashion. Other demonstrations include artificial muscles, which can lift masses that are about 110 times of their weights and biomimetic frog tongues which can prey insects.     

Liquid Crystalline Elastomer for Separate or Collective Sensing and Actuation Functions

A porous liquid crystalline elastomer actuator filled with an ionic liquid (PLCE-IL) is shown to exhibit the functions of two classes of materials: electrically responsive, deformable materials for sensing and soft active materials for stimuli-triggered actuation. On one hand, upon the order–disorder phase transition of aligned mesogens, PLCE-IL behaves like a typical actuator capable of reversible shape change and can be used to assemble light-fuelled soft robot. On the other hand, at temperatures below the phase transition, PLCE-IL is an elastomer that can sustain and sense large deformations of various modes as well as environmental condition changes by reporting the corresponding electrical resistance variation. The two distinguished functions can also be used collectively with PLCE-IL integrated in one device. This intelligent feature is demonstrated with an artificial arm. When the arm is manually powered to fold and unfold, the PLCE-IL strip serves as a deformation sensor; while when the manual power is not available, the role of the PLCE-IL strip is switched to an actuator that enables light-driven folding and unfolding of the arm. This study shows that electrically responsive LCEs are a potential materials platform that offers possibilities for merging deformable electronic and actuation applications.