Magnetic Elastomers Research Articles

Reduced theory for hard magnetic rods with dipole–dipole interactions

Hard magnetic elastomers are composites of soft elastic foundations and magnetic particles with high coercivity. We formulate a theoretical framework to predict the large deformation of a hard magnetic elastomeric rod. In the previous work, the magnetic Kirchhoff rod equations, which constitute a framework for analyzing instabilities for hard magnetic rods, have been developed and validated experimentally for negligible dipole–dipole interactions. Building on previous studies, we derive the magnetic Kirchhoff rod equations with dipole–dipole interactions. The derived equations are integro-differential equations, representing the force and moment balance along the rod centerline that include long-ranged dipole-magnetic force and torque. On the basis of its discrete numerical simulation, we systematically study the effect of the the dipole–dipole interactions strength on the large deformation of hard magnetic rods. In addition, we find that our theory can predict previous experimental results without any adjustable parameters.

A theory of magneto-elastic nanorods obtained through rigorous dimension reduction

Starting from a two-dimensional theory of magneto-elasticity for fiber-reinforced magnetic elastomers we carry out a rigorous dimension reduction to derive a rod model that describes a thin magneto-elastic strip undergoing planar deformations. The main features of the theory are the following: a magneto-elastic interaction energy that manifests itself through a distributed torque; a penalization term that prevents local interpenetration of matter; a regularization term that depends on the second gradient of the deformation and models microstructure-induced size effects. As an application, we consider a problem involving magnetically-induced buckling and we study how the intensity of the field at the onset of the instability increases if the length of the rod is decreased. Finally, we assess the accuracy of the deduced model by performing numerical simulations where we compare the two-dimensional and the one-dimensional theories in some special cases, and we observe excellent agreement.

Voids induce wide-range modulation of elasticity for magnetic elastomers.

The relationship between the magnetorheological effect and the void ratio for a polyurethane magnetic elastomer with voids was investigated using a dynamic viscoelastic measurement under a magnetic field of 500 mT. The magnetic elastomer contains iron particles with a diameter of 235 μm at a concentration of 85 wt% (volume fraction: 0.43). The void ratio defined using the volume of vacancies in the non-filled volume of magnetic particles was increased by reducing the amount of polyurethane up to a maximum void ratio of 0.56. The storage modulus of the magnetic elastomer without voids was 1.5 × 105 Pa at 0 mT and 3.1 × 105 Pa at 500 mT, respectively; that is, no significant change in the modulus was observed. The storage modulus at 0 mT for the magnetic elastomer was independent of the void ratio, while the storage modulus at 500 mT increased in proportion to the void ratio. At a void ratio of 0.56, the storage modulus for the magnetic elastomer was 5.6 × 105 Pa at 0 mT and 6.1 × 106 Pa at 500 mT, respectively; that is, the magnetic elastomer demonstrated a significant change in the storage modulus on the order of MPa. This strongly indicates that production of voids enables movement of magnetic particles in the elastomer. Under both strains of 10-4 and 1, a significant and reversible response of storage modulus was observed after the first application of magnetic field even though the magnetic field was applied for 20 cycles, meaning that the change in the modulus is perfectly reversible although the elastomer contains many voids. SEM/EDX observations revealed that the area composed of carbon decreased with the increasing void ratio while the area composed of iron remained unchanged.

Magneto-elastic coupling as a key to microstructural response of magnetic elastomers with flake-like particles.

Using the combination of experiment and molecular dynamics simulations, we investigate structural transformations in magnetic elastomers with NdFeB flake-like particles, caused by applied moderate magnetic fields. We explain why and how those transformations depend on whether or not the samples are initially cured by a short-time exposure to a strong field. We find that in a cured sample, a moderate magnetic field leads mainly to in-place flake rotations that are fully reversed once the applied field is switched off. In contrast, in an initially non-cured sample the flakes perform both translation and rotations under the influence of a moderate applied field that lead to the formation of chain-like structures that remain such even if the field is switched off.

A magnetic responsive composite surface for high-performance droplet and bubble manipulation.

Here, a magnetic responsive composite surface (MRCS) was prepared by injecting a magnetic elastomer into ZnO nanoarrays for intelligent control of droplet/bubble transport. This non-pollution, non-contact operation method has shown great potential in micro-fluids, micro-chemical reactors, chip laboratory environments and other related applications.

Effects of Infill Orientation and Percentage on the Magnetoactive Properties of 3d Printed Magnetic Elastomer Structures

Effects of Infill Orientation and Percentage on the Magnetoactive Properties of 3d Printed Magnetic Elastomer Structures

Magnetically Steerable Serial and Parallel Structures by Mold‐Free Origami Templating and Domain Setting

AbstractMagnetically responsive elastomers generate continuous motions and locomotion through changes in external magnetic fields. The magnetic particles in an elastomer achieve shape‐morphing, but overall structural integrity can be limited by the compliance of elastomers. To overcome this limitation, magnetic elastomers are enhanced with programmable template creases to generate deployable structures as a library. Current methods to program magnetic domains into actuatable segments use sequential or continuous rasterized magnetization, which is gradual and requires dedicated equipment. To achieve faster magnetic domain programming, template folding for the magnetization process is explored. The crease pattern can reconfigure facets to attain the desired magnetization direction and act as discrete mechanical joints. Crease patterns from the foldable templates allow more variations in robotic kinematics and discretize the deformations. The approach using magnetic elastomers and folding templates as a library can simultaneously program a wide range of deployable mechanisms with inherent crease foldability. Additionally, magnetic responsive robots can encompass bistable actuation imparted from the crease design. Functionalization of magnetic origami structures is demonstrated by imbuing the template material with catalytic properties. These robots have the potential to be deployed from a compact folded form to confined environments, enabling a broader spectrum of minimally invasive procedures.

Stimuli-Responsive Polymers for Soft Robotics

This article reviews recent progress in the use of stimuli-responsive polymers for soft robotics. First, we introduce different types of representative stimuli-responsive polymers, which include liquid crystal polymers and elastomers, hydrogels, shape memory polymers, magnetic elastomers, electroactive polymers, and thermal expansion actuators. We focus on the mechanisms of actuation and the evaluation of performance and discuss strategies for improvements. We then present examples of soft robotic applications based on stimuli-responsive polymers for bending, grasping, walking, swimming, flying, and sensing control. Finally, we discuss current opportunities and challenges of stimuli-responsive soft robots for future study.

Anisotropic magnetic liquid metal film for wearable wireless electromagnetic sensing and smart electromagnetic interference shielding

Smart and wearable electromagnetic (EM) functional materials that can reasonably use EM energy and prevent EM pollution are of great significance to the future information society, but their design and preparation are still challenging. Here, a multifunctional EM response film is prepared based on a magnetic liquid metal droplet-filled elastomer with magnetic field-induced anisotropic structure, which can reversibly construct conductive network and tune heterogeneous interface during cyclic stretching. This film can withstand up to 600% tensile strain and shows stretching-enhanced electrical conductivity. Its EM interference shielding effectiveness (SE) significantly increases with stretching, and the relative SE change (ΔSE/SE0) shows a linear response to the tensile strain. The specific shielding effectiveness (an ultrahigh value of 404 dB mm−1 at 400% strain) and strain sensitivity of the film outperform other stretchable EM functional materials. More interestingly, this film exhibits rapid switching between EM wave transmission and shielding through continuous stretching, which enables an off/on switchable EM interference shielding function. Furthermore, it can be used in a wearable EM sensor to wirelessly monitor human movements by recording changes of EM wave intensity.

Highly motile nanoscale magnetic artificial cilia

Among the many complex bioactuators functioning at different scales, the organelle cilium represents a fundamental actuating unit in cellular biology. Producing motions at submicrometer scales, dominated by viscous forces, cilia drive a number of crucial bioprocesses in all vertebrate and many invertebrate organisms before and after their birth. Artificially mimicking motile cilia has been a long-standing challenge while inspiring the development of new materials and methods. The use of magnetic materials has been an effective approach for realizing microscopic artificial cilia; however, the physical and magnetic properties of the magnetic material constituents and fabrication processes utilized have almost exclusively only enabled the realization of highly motile artificial cilia with dimensions orders of magnitude larger than their biological counterparts. This has hindered the development and study of model systems and devices with inherent size-dependent aspects, as well as their application at submicrometer scales. In this work, we report a magnetic elastomer preparation process coupled with a tailored molding process for the successful fabrication of artificial cilia with submicrometer dimensions showing unprecedented deflection capabilities, enabling the design of artificial cilia with high motility and at sizes equal to those of their smallest biological counterparts. The reported work crosses the barrier of nanoscale motile cilia fabrication, paving the way for maximum control and manipulation of structures and processes at micro- and nanoscales.

Improvement of Machining Accuracy Through Support Method Using Magnetic Elastomer

Many mechanical parts used for various purposes, including medicine and information communication, have complicated and thin shapes owing to their functions and designs. To machine thin parts with high accuracy, it is necessary to reduce the cutting force induced on the workpiece or clamp the workpiece in an optimal manner. In this study, a support method capable of supporting the strength by using a magnetic elastomer is proposed. To test the effectiveness of the proposed support method, the use of the approach when applying a magnetic elastomer was compared with a method using a core, the low-melting point alloy, the low-melting point alloy and the elastomer. The effectiveness of the proposed method was clarified experimentally.

A Fast Autonomous Healing Magnetic Elastomer for Instantly Recoverable, Modularly Programmable, and Thermorecyclable Soft Robots

AbstractIntrinsically self‐healing stretchable polymers have been intensively explored for soft robotic applications due to their mechanical compliance and damage resilience. However, their prevalent use in real‐world robotic applications is currently hindered by various limitations such as low mechanical strength, long healing time, and external energy input requirements. Here, a self‐healing supramolecular magnetic elastomer (SHSME), featuring a hierarchical dynamic polymer network with abundant reversible bonds, is introduced. The SHSME exhibits high mechanical strength (Young's modulus of 1.2 MPa, similar to silicone rubber) and fast self‐healing capability (300% stretch strain after 5 s autonomous repair at ambient temperature). A few SHSME‐based robotic demonstrations, namely, rapid amphibious function recovery, modular‐assembling‐prototyping soft robots with complex geometries and diverse functionalities, as well as a dismembering–navigation–assembly strategy for robotic tasking in confined spaces are showcased. Notably, the SHSME framework supports circular material design, as it is thermoreformable for recycling, demonstrates autorepair for extended lifespan, and is modularizable for customized constructs and functions.

Magnetorheological Elastomers–Material Properties and Actuation Capabilities

Magnetorheological elastomers (MRE) are composite materials, which consist of a soft elastomer matrix filled with soft-magnetic particles. Mechanical and magnetic properties of silicone-based MRE with iron particles in different concentrations as well as their alteration in a magnetic field with variable strength are described in this paper. By applying a magnetic field already during the preparation of MRE bodies, magnetic elastomer materials with anisotropic properties are generated. The mechanical, magnetic and electric properties of such anisotropic MRE materials were studied as well. Beside the exploitation of MRE for applications of vibration reduction, these materials can also be utilized for actuation purposes. The working principles of linear and radial MRE actuators are described. Moreover, some possible applications of linear MRE actuators for haptic feedback and precisely controllable pumps as well as the possible use of radial MRE actuators for proportional valves and for locking devices are outlined in the paper as well.

Magnetically induced micropillar arrays for an ultrasensitive flexible sensor with a wireless recharging system

Significant efforts have been devoted to enhancing the sensitivity and working range of flexible pressure sensors to improve the precise measurement of subtle variations in pressure over a wide detection spectrum. However, achieving sensitivities exceeding 1000 kPa−1 while maintaining a pressure working range over 100 kPa is still challenging because of the limited intrinsic properties of soft matrix materials. Here, we report a magnetic field-induced porous elastomer with micropillar arrays (MPAs) as sensing materials and a well-patterned nickel fabric as an electrode. The developed sensor exhibits an ultrahigh sensitivity of 10,268 kPa−1 (0.6–170 kPa) with a minimum detection pressure of 0.25 Pa and a fast response time of 3 ms because of the unique structure of the MPAs and the textured morphology of the electrode. The porous elastomer provides an extended working range of up to 500 kPa with long-time durability. The sophisticated sensor system coupled with an integrated wireless recharging system comprising a flexible supercapacitor and inductive coils for transmission achieves excellent performance. Thus, a diverse range of practical applications requiring a low-to-high pressure range sensing can be developed. Our strategy, which combines a microstructured high-performance sensor device with a wireless recharging system, provides a basis for creating next-generation flexible electronics.

Magnetic flexible tactile sensor via direct ink writing

The development of flexible tactile sensors is an emerging field due to their wide applications ranging from human-robot interactions to wearable electronics. This paper proposes a flexible tactile sensor based on the magnetic field for detecting both forces and positions, which exhibited a linear relationship. The direct ink writing technology was used to fabricate magnetic elastomers for the quick test of variable patterns. The introduced folding magnetization allows for a higher magnetic flux density in the designated position which made it possible to get rid of magnets. A pattern was designed and printed. The linear relationship was found to be linear (R2 > 0.98) between the magnetic field signal and the magnitude and position of the load force. Moreover, the untethered sensor was proven to work well in the sealed box, which is expected to serve as the electrical skin of robots or the tactile sensor in extreme environments.

Programmable shape transformation of 3D printed magnetic hydrogel composite for hyperthermia cancer therapy

Recent advances in soft magnetic nanocomposites have enabled myriads of magnetic robots with programmable shape transformation, shedding light on various biomedical applications. However, the complex shape transformation of magnetic hydrogels remains a challenge because of their ultralow magnetization and simple geometry. Here we demonstrate an approach to meet this challenge by fabricating composite structures of magnetic hydrogels and elastomers with extrusion-based 3D printing. Under an alternating magnetic field, magneto-thermo-sensitive hydrogels — poly(N-isopropylacrylamide) (PNIPAm) embedding Fe3O4 nanoparticles, can undergo abrupt volume collapse due to magnetothermal effect. The mismatch in the responsiveness of magnetic hydrogels and elastomers enables the shape transformation of the composite structure. We have printed magnetic hydrogels with various geometries and achieved complex shape transformation of the composite structure. The shape transformative structure can simultaneously encase and kill cancer cells (human malignant melanoma cells) through magnetic hyperthermia. ∼50% of cancer cells can be killed by the heated magnetic hydrogel during deformation. This approach may open opportunities for applications in medicine and bioengineering.

Magnetic polydomain liquid crystal elastomers – synthesis and characterisation

ABSTRACT Although liquid crystal elastomers (LCE) are a fascinating class of materials with interesting thermomechanical properties of their own, the aim is to enhance their performance or add new features, e.g. response to external stimuli. The generally weak response of organic materials can be significantly intensified by mixing nano-sized magnetic particles into the host polymer matrix. An alternative approach is chemically coupling the nanoparticles to the elastomer. We achieved this by bonding functionalised magnetic nanoplatelets to the backbone of a main-chain LCE and obtained polydomain magnetic liquid crystal elastomers. We measured the magnetisation curves in samples, which were exposed to either small or large magnetic fields – their response being a consequence of partial particle reorientation or magnetic moment flipping. In contrast to the samples, which were exposed to small magnetic field and in which the remanent magnetisation can be reset to zero by heating the sample, the samples with flipped magnetisation within the platelets cannot be reversed into the original state. Coupling of magnetic and mechanical properties shows a slight magneto-elastic response at elevated temperatures and a significant inverse magneto-elastic effect: the magnetisation caused by mechanical stretching is almost equal to the magnetisation caused by an external magnetic field.

Controlling magnetic properties of 3D-printed magnetic elastomer structures via fused deposition modeling

Several methods have been used to optimize performance of magnetic elastomers by controlling the microstructure, such as magnetic annealing. Another way to introduce anisotropy is Fused Deposition Modeling (FDM), which has been shown to manipulate the magnetic anisotropy of rigid printed parts. However, the use of flexible composite materials has not yet been explored due to additional processing challenges. The primary goal of this study is to demonstrate tunable anisotropy of these materials via 3D printed structures without post-processing as a viable means to tune the performance of magnetic elastomer materials. Here, FDM structures were printed with thermoplastic polyurethane (TPU) polymer and either iron, carbonyl iron, or magnetite particulate. In order to determine the relative effect of different parameters on the magnetic properties, a series of samples were printed combining each material type with different aspect ratios, infill percentages, and infill orientations. A Vibrating Sample Magnetometer (VSM) was used to obtain magnetic hysteresis loops in order to compare the magnetic susceptibility between samples. Results demonstrated that FDM provides a method of achieving the directional signature of magnetic annealing without requiring any post-processing; instead, this manifests through the anisotropy of the part’s internal structure. As such, this concept is referred to as infill magnetic annealing (IMA). These variables were found to form a continuum of tunable magnetic responses. Additionally, the chosen particulate transfers its magnetic signature to the composite material. Overall, the highly customizable and nuanced characteristics of 3D-printed magnetic elastomer structures will allow for its application in a broad range of emerging magneto-mechanical applications such as magnetic actuation and soft robotics.

Mesostructure of Composite Materials Based on Segmented Poly(Urethane Imide) Containing Ferrite Nanoparticles

The mesostructure of segmented poly(urethane imide) containing 4–10 wt % CoFe2O4 or Al3Fe5O12 nanoparticles was experimentally studied by small-angle polarized neutron scattering, which can analyze the nuclear and magnetic subsystems of the obtained composites. The transformation of the domain structure of the magnetic elastomers based on poly(urethane imide) was investigated at various concentrations, chemical compositions, and localizations of ferrite nanoparticles in polymer chains. The dynamics of the change in the segmental mobility of soft blocks during heating of samples was characterized by magnetic–nuclear interference scattering. The experiments detected changes in the supramolecular structure of the multiblock copolymer, which depended on the percentage and type of nanoparticles. The correlation radii were determined in the magnetic subsystem of ferrite nanoparticles embedded in hard aromatic blocks of poly(urethane imide) as nodes of intermolecular crosslinkers and chain extenders. The temperature dependence was analyzed for the intensity of small-angle nuclear scattering by poly(urethane imide) and magnetic elastomers based on it, under the conditions of the transition of soft aliphatic segments from the glassy to the highly elastic state.

Magnetic Elastomer Sensor for Dynamic Torque and Speed Measurements

In this paper is proposed a dynamic torque, rotational speed, and shaft position sensor. It is built of magnetic elastomer coating directly applied over a rotating shaft. The sensor is used for precise measurements of changes in torque and speed, and it is usable at high rotational speeds, directly on the device shaft. The sensor is based on magnetic elastomer material deformation and the corresponding change in magnetic field amplitude and direction. The proposed sensor design is simple and can acquire reliable readings for a wide range of rotational speeds. Sensor design consists of the following: magnetic elastomer coating with nanoparticles, in which, elastomer is used for a sensing convertor; magneto-resistive linear field sensor; and microprocessor unit for calibration and control. Numerical and experimental test results are demonstrated and analyzed. Sensor implementation aims to meet magnetic mechatronic systems’ specific requirements.