A Highly Tunable Silicone-Based Magnetic Elastomer with Nanoscale Homogeneity

Multifunctional Self-Assembled Block Copolymer/Iron Oxide Nanocomposite Hydrogels Formed from Wormlike Micelles

This article reports the preparation of multifunctional magnetic nanocomposite hydrogels formed from wormlike micelles. Specifically, iron oxide nanoparticles were incorporated into a temperature responsive block copolymer, poly(glycerol monomethacrylate)-b-poly(2-hydroxypropyl methacrylate) (PGMA-b-PHPMA), and graphene oxide (GO) dispersion at a low temperature (∼2 °C) through high-speed mixing and returning the mixture to room temperature, resulting in the formation of nanocomposite gels. The optimal concentrations of iron oxide and GO enhanced the gel strength of the nanocomposite gels, which exhibited a strong magnetic response when a magnetic field was applied. These materials retained the thermoresponsiveness of the PGMA-PHPMA wormlike micelles allowing for a solid-to-liquid transition to occur when the temperature was reduced. The mechanical and rheological properties and performance of the nanocomposite gels were demonstrated to be adjustable, making them suitable for a wide range of potential applications. These nanocomposite worm gels were demonstrated to be relatively adhesive and to act as strain and temperature sensors, with the measured electrical resistance of the nanocomposite gels changing with applied strain and temperature sweeps. The nanocomposite gels were found to recover efficiently after the application of high shear with approximately 100% healing efficiency within seconds. Additionally, these nanocomposite worm gels were injectable, and the addition of GO and iron oxide nanomaterials seemed to have no significant adverse impact on the biocompatibility of the copolymer gels, making them suitable not only for 3D printing in nanocomposite engineering but also for potential utilization in various biomedical applications as an injectable magnetic responsive hydrogel.

Bioinspired Stimuli-Responsive Materials for Soft Actuators

Biological species can walk, swim, fly, jump, and climb with fast response speeds and motion complexity. These remarkable functions are accomplished by means of soft actuation organisms, which are commonly composed of muscle tissue systems. To achieve the creation of their biomimetic artificial counterparts, various biomimetic stimuli-responsive materials have been synthesized and developed in recent decades. They can respond to various external stimuli in the form of structural or morphological transformations by actively or passively converting input energy into mechanical energy. They are the core element of soft actuators for typical smart devices like soft robots, artificial muscles, intelligent sensors and nanogenerators. Significant progress has been made in the development of bioinspired stimuli-responsive materials. However, these materials have not been comprehensively summarized with specific actuation mechanisms in the literature. In this review, we will discuss recent advances in biomimetic stimuli-responsive materials that are instrumental for soft actuators. Firstly, different stimuli-responsive principles for soft actuators are discussed, including fluidic, electrical, thermal, magnetic, light, and chemical stimuli. We further summarize the state-of-the-art stimuli-responsive materials for soft actuators and explore the advantages and disadvantages of using electroactive polymers, magnetic soft composites, photo-thermal responsive polymers, shape memory alloys and other responsive soft materials. Finally, we provide a critical outlook on the field of stimuli-responsive soft actuators and emphasize the challenges in the process of their implementation to various industries.

Nanomagnetic Elastomers for Realizing Highly Responsive Micro- and Nanosystems

Evolution has produced natural systems that generate motion and sense external stimuli at the micro- and nanoscales. At extremely small scales, the intricate motions and large deformations shown by these biosystems are due to a tipping balance between their structural compliance and the actuating force generated in them. Artificially mimicking such ingenious systems for scientific and engineering applications has been approached through the development and use of different smart materials mostly limited to microscale dimensions. To push the application range down to the nanoscale, we developed a material preparation process that yields a library of nanomagnetic elastomers with high magnetic particle concentrations. Through this process, we have realized a material with the highest magnetic-to-elastic force ratio, as is shown by an extensive mechanical and magnetic characterization of the materials. Furthermore, we have fabricated and actuated micro- and nanostructures mimicking cilia, demonstrating the extreme compliance and responsiveness of the developed materials.

Synthesis of Manganese Zinc Ferrite Nanoparticles in Medical-Grade Silicone for MRI Applications

The aim of this project is to fabricate hydrogen-rich silicone doped with magnetic nanoparticles for use as a temperature change indicator in magnetic resonance imaging-guided (MRIg) thermal ablations. To avoid clustering, the particles of mixed MnZn ferrite were synthesized directly in a medical-grade silicone polymer solution. The particles were characterized by transmission electron microscopy, powder X-ray diffraction, soft X-ray absorption spectroscopy, vibrating sample magnetometry, temperature-dependent nuclear magnetic resonance relaxometry (20 °C to 60 °C, at 3.0 T), and magnetic resonance imaging (at 3.0 T). Synthesized nanoparticles were the size of 4.4 nm ± 2.1 nm and exhibited superparamagnetic behavior. Bulk silicone material showed a good shape stability within the study’s temperature range. Embedded nanoparticles did not influence spin–lattice relaxation, but they shorten the longer component of spin–spin nuclear relaxation times of silicone’s protons. However, these protons exhibited an extremely high r2* relaxivity (above 1200 L s−1 mmol−1) due to the presence of particles, with a moderate decrease in the magnetization with temperature. With an increased temperature decrease of r2*, this ferro–silicone can be potentially used as a temperature indicator in high-temperature MRIg ablations (40 °C to 60 °C).

Nanoparticle-Based Magnetorheological Elastomers with Enhanced Mechanical Deflection for Haptic Displays

Haptics allows tactile interactions between humans and digital interfaces. Magnetorheological elastomers (MREs) constitute a promising candidate material for creating the tactile interface of the future─one able to recreate 3D shapes that can be sensed with touch. Furthermore, an MRE formed by using nanoparticles, as opposed to previously used microparticles, is necessary to generate a variety of shapes involving sharp curvatures over small, micrometer-scale horizontal distances to pave the way for haptic displays with microtexture resolution. Here we fabricated both isotropic and anisotropic MREs with different concentrations (2-8 vol % nanoparticles) of soft, low-remanence ferromagnetic nanoparticles. When placed in a magnetic field gradient, isotropic MREs, nonintuitively, show higher deflection than anisotropic MREs, with the former achieving displacement on the order of a millimeter at just 100 mT. This enhanced performance in the isotropic case is explained based on the soft magnetic nature of the nanoparticles. We show that performance improves with magnetic content up to a composition of 6 vol %, where it plateaus. This behavior is attributed to the stiffness of the composite material increasing at a faster rate than the magnetization as the rigid magnetic nanoparticles are added to the elastomeric matrix. Moreover, 6 vol % microparticle-based isotropic and anisotropic MREs were fabricated and compared with the nanoparticle-based MREs. Anisotropic nanoparticle-based films show higher deflection when compared with their microparticle-based counterparts. The latter is only able to match the nanoparticle film deflection at higher applied fields of almost 300 mT. This performance difference between nanoparticle and microparticle-based films is attributed to the increased anisotropic film stiffness resulting from the larger micrometer-size particles. Finally, the optimally designed nanoparticle-based isotropic film was utilized to create a programmable and real-time reconfigurable braille-inspired interface.

Magnetic Soft Materials and Robots

In conventional classification, soft robots feature mechanical compliance as the main distinguishing factor from traditional robots made of rigid materials. Recent advances in functional soft materials have facilitated the emergence of a new class of soft robots capable of tether-free actuation in response to external stimuli such as heat, light, solvent, or electric or magnetic field. Among the various types of stimuli-responsive materials, magnetic soft materials have shown remarkable progress in their design and fabrication, leading to the development of magnetic soft robots with unique advantages and potential for many important applications. However, the field of magnetic soft robots is still in its infancy and requires further advancements in terms of design principles, fabrication methods, control mechanisms, and sensing modalities. Successful future development of magnetic soft robots would require a comprehensive understanding of the fundamental principle of magnetic actuation, as well as the physical properties and behavior of magnetic soft materials. In this review, we discuss recent progress in the design and fabrication, modeling and simulation, and actuation and control of magnetic soft materials and robots. We then give a set of design guidelines for optimal actuation performance of magnetic soft materials. Lastly, we summarize potential biomedical applications of magnetic soft robots and provide our perspectives on next-generation magnetic soft robots.

Stable graphene oxide hydrophobic photonic liquids

Graphene oxide (GO) is an important nanomaterial for producing photonic liquids due to its ability to display full-color reflections in water. However, the poor stability of GO photonic liquids and unsatisfactory dispersibility of GO nanosheets in hydrophobic liquid media have been significant drawbacks to developing photonic materials based on GO. Here, stable GO hydrophobic photonic liquids are demonstrated for the first time. GO nanosheets are directed into different hydrophobic liquid media, including reactive liquid precursors like tetraethoxysilane and ethyl acrylate, in the presence of phase transfer additives. These liquids exhibit tunable reflection wavelength up to ∼1300 nm with improved stability relative to aqueous GO photonic suspensions at elevated temperatures or under ambient conditions. Supported by an entropy-driven depletion mechanism, hydrophobic additives can effectively mediate the self-assembly of GO to produce tunable photonic liquids without the need to adjust GO concentrations. Furthermore, simultaneous infrared and visible light reflection can be achieved, enabling infrared photonic GO liquids to display visible colors. The improved stability and tunable photonic properties of hydrophobic GO liquids will open a way for developing GO-based optical materials and devices.

Highly motile nanoscale magnetic artificial cilia

Cilia are hair-like microscopic structures present abundantly in our body and producing motions at the smallest scales. They perform a wide range of critical functions and are crucial for the normal functioning of our body. Abnormal functioning of cilia results in a number of diseases jointly known as ciliopathies. Artificially mimicking cilia is aimed at understanding their normal/abnormal functionality and at developing cilia-inspired micro-/nanoengineering devices. In this study, we present a magnetic polymer preparation process yielding a material with optimum properties and a cilia fabrication method producing the smallest highly motile artificial cilia with sizes equal to their biological counterparts. This opens avenues for biological studies and for creating submicrometer manipulation and control.

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.

Magnetically responsive polymer nanopillars with nickel cap

Embedding magnetic particles within polymer matrix is a common and facile method to fabricate magnetically responsive micro-/nanoscale pillars. However, the balance between mechanical compliance and magnetic susceptibility cannot be decoupled and the particles are limited by the pillar feature size, which can limit the actuation performance. Here we demonstrate a new type of magnetically responsive nanostructure consisting of a polydimethylsiloxane (PDMS) nanopillar array with deposited nickel caps, that has successfully achieved such decoupling with multiple cap-geometry designs for a better actuation control. The actuation result of nanopillars with 540 nm period and 1.3 μm height has been analyzed using image processing, leading to a maximum displacement of 180 nm with a ratio of 13.9% with respect to the pillar height. Magnetic and mechanical models based on magnetic force and torque have been developed and used to mitigate the weakening effect of the actuation by the residual magnetic layer. This structure demonstrates a feasible strategy for magnetic actuation at the sub-micrometer scale with freedom to design magnetic cap and polymeric pillar separately. This structure can also be utilized in multiple applications such as tunable optical elements, dynamic droplet manipulation, and responsive particle manipulation.

Recent Progress in Artificial Muscles for Interactive Soft Robotics

Artificial muscles are the core components of the smart and interactive soft robotic systems, providing the capabilities in shape morphing, manipulation, and mobility. Intense research efforts in the development of artificial muscles are based on the dielectric elastomer actuators, pneumatic actuators, electrochemical actuators, soft magnetic actuators, and stimulus responsive polymers. Recent progress has presented artificial muscles with impressive specific power output exceeding that of the natural muscles, dexterous shape morphing behavior that can be programmed and reconfigured, and exceptional high maneuverability to traverse surfaces with obstacles and different textures. Here, a succinct and critical summary is provided on the materials and strategies that have contributed to the important advancement of the artificial muscles in recent research. On that basis, the exciting opportunities are discussed in the integration of soft electronic devices with artificial muscles to enable smart and interactive soft robotic systems.

Modeling and theoretical description of magnetic hybrid materials—bridging from meso- to macro-scales

Magnetic gels and elastomers consist of magnetic or magnetizable colloidal particles embedded in an elastic polymeric matrix. Outstanding properties of these materials comprise reversible changes in their mechanical stiffness or magnetostrictive distortions under the influence of external magnetic fields. To understand such types of overall material behavior from a theoretical point of view, it is essential to characterize the substances starting from the discrete colloidal particle level. It turns out that the macroscopic material response depends sensitively on the mesoscopic particle arrangement. We have utilized and developed several theoretical approaches to this end, allowing us both to reproduce experimental observations and to make theoretical predictions. Our hope is that both these paths help to further stimulate the interest in these fascinating materials.

Active Periodic Magnetic Nanostructures with High Aspect Ratio and Ultrahigh Pillar Density

Magnetically actuated micro/nanoscale pillars have attracted significant research interest recently because of their dynamic properties. These structures can be used for various applications, such as dry adhesion, cell manipulation, and sensors or actuators in microfluidics. Magnetically actuated structures can be fabricated by mixing magnetic particles and polymers to yield a favorable combination of magnetic permeability and mechanical compliance. However, the pillar density of demonstrated structures is relatively low, which limits the potential applications in active surface manipulation of microscale objects. Here, we demonstrate active periodic nanostructures with a pillar density of 0.25 pillar/μm², which is the highest density for magnetically actuated pillars so far. Having a structure period of 2 μm, diameter of 600 nm, and high aspect ratio of up to 11, this structure can be magnetically actuated with a displacement of up to 200 nm. The behaviors of the pillars under various cyclic actuation modes have been characterized, demonstrating that the actuation can be well controlled. This work can find potential applications in particle manipulation and tunable photonic elements.

Magnetically Actuated Dynamic Iridescence Inspired by the Neon Tetra

Inspired by the tropical fish neon tetra, we report a mechanism to achieve dynamic iridescence that can be magnetically tuned. This approach is based on the tilting of periodic photonic nanostructures, as opposed to the more common strain-induced color tuning. In this method, a periodic array of magnetic nanopillars serves as a template to guide the assembly of iron oxide nanoparticles when magnetized in a liquid environment. The periodic local fields induced by the magnetic template anchor the assembled particle columns, allowing the structure to tilt about the base when the angle of the applied field is changed. This effect emulates a microscopic "Venetian blind" and results in dynamic optical properties through structural coloration that is tunable in real time. The fabricated prototype demonstrates tunable reflectance spectra with peak wavelength shift from 528 to 720 nm. The magnetic actuation mechanism is reversible and has a fast response time around 0.3 s. This structure can be implemented on an arbitrary surface as dynamic camouflage, iridescent display, and tunable photonic elements, as well as in other applications such as active fluidic devices and particle manipulation.

In Situ Generated Medical Devices

Medical devices play a major role in all areas of modern medicine, largely contributing to the success of clinical procedures and to the health of patients worldwide. They span from simple commodity products such as gauzes and catheters, to highly advanced implants, e.g., heart valves and vascular grafts. In situ generated devices are an important family of devices that are formed at their site of clinical function that have distinct advantages. Among them, since they are formed within the body, they only require minimally invasive procedures, avoiding the pain and risks associated with open surgery. These devices also display enhanced conformability to local tissues and can reach sites that otherwise are inaccessible. This review aims at shedding light on the unique features of in situ generated devices and to underscore leading trends in the field, as they are reflected by key developments recently in the field over the last several years. Since the uniqueness of these devices stems from their in situ generation, the way they are formed is crucial. It is because of this fact that in this review, the medical devices are classified depending on whether their in situ generation entails chemical or physical phenomena.

Reversible magnetomechanical collapse: virtual touching and detachment of rigid inclusions in a soft elastic matrix

Soft elastic composite materials containing particulate rigid inclusions in a soft elastic matrix are candidates for developing soft actuators or tunable damping devices. The possibility to reversibly drive the rigid inclusions within such a composite together to a close-to-touching state by an external stimulus would offer important benefits. Then, a significant tuning of the mechanical properties could be achieved due to the resulting mechanical hardening. For a long time, it has been argued whether a virtual touching of the embedded magnetic particles with subsequent detachment can actually be observed in real materials, and if so, whether the process is reversible. Here, we present experimental results that demonstrate this phenomenon in reality. Our system consists of two paramagnetic nickel particles embedded at finite initial distance in a soft elastic polymeric gel matrix. Magnetization in an external magnetic field tunes the magnetic attraction between the particles and drives the process. We quantify our experimental results by different theoretical tools, i.e., explicit analytical calculations in the framework of linear elasticity theory, a projection onto simplified dipole-spring models, as well as detailed finite-element simulations. From these different approaches, we conclude that in our case the cycle of virtual touching and detachment shows hysteretic behavior due to the mutual magnetization between the paramagnetic particles. Our results are important for the design and construction of reversibly tunable mechanical damping devices. Moreover, our projection on dipole-spring models allows the formal connection of our description to various related systems, e.g., magnetosome filaments in magnetotactic bacteria.

Effect of Sonication Time on Magnetorheological Effect for Monomodal Magnetic Elastomers

The effect of sonication time on the storage modulus and particle morphology for magnetic elastomers was investigated by dynamic viscoelastic measurements and morphological studies. An ultrasonic wave using a homogenizer was irradiated to magnetic liquids containing 70 wt % carbonyl iron, for up to 30 min before cure. SEM photographs revealed that magnetic particles were randomly dispersed in the polyurethane matrix for magnetic elastomers with sonication. A parameter showing nonlinear viscoelasticity for magnetic elastomers with sonication decreased from 0.75 to 0.4, indicating that the aggregations of magnetic particles had been destroyed by the sonication. The storage modulus at 500 mT at the linear viscoelastic regime significantly increased with the irradiation time, reaching saturation after 10 min; this suggests an increase in the number of chains of magnetic particles by sonication, due to the random dispersion of magnetic particles. At high strains, the storage modulus at 500 mT increased by 8.9 kPa by sonication, indicating the number of chains of magnetic particles which were not destroyed by increased sonication. It was also found that the storage modulus for polyurethane elastomers without magnetic particles was not varied by sonication, suggesting that the polyurethane network was not broken. The effect of sonication time on the viscoelastic properties, and on the magnetorheological response for magnetic elastomers, is discussed.

Tunable dynamic moduli of magnetic elastomers: from characterization by x-ray micro-computed tomography to mesoscopic modeling

Ferrogels and magnetorheological elastomers are composite materials obtained by embedding magnetic particles of mesoscopic size in a crosslinked polymeric matrix. They combine the reversible elastic deformability of polymeric materials with the high responsivity of ferrofluids to external magnetic fields. These materials stand out, for example, for significant magnetostriction as well as a pronounced increase of the elastic moduli in the presence of external magnetic fields. By means of x-ray micro-computed tomography, the position and size of each magnetic particle can be measured with a high degree of accuracy. We here use data extracted from real magnetoelastic samples as input for coarse-grained dipole-spring modeling and calculations to investigate internal restructuring, stiffening, and changes in the normal modes spectrum. More precisely, we assign to each particle a dipole moment proportional to its volume and set a randomized network of springs between them that mimics the behavior of the polymeric elastic matrix. Extending our previously developed methods, we compute the resulting structural changes in the systems as well as the frequency-dependent elastic moduli when magnetic interactions are turned on. Particularly, with increasing magnetization, we observe the formation of chain-like aggregates. Interestingly, the static elastic moduli can first show a slight decrease with growing amplitude of the magnetic interactions, before a pronounced increase appears upon the chain formation. The change of the dynamic moduli with increasing magnetization depends on the frequency and can even feature nonmonotonic behavior. Overall, we demonstrate how theory and experiments can complement each other to learn more about the dynamic behavior of this interesting class of materials.

Chained Iron Microparticles for Directionally Controlled Actuation of Soft Robots

Magnetic field-directed self-assembly of magnetic particles in chains is useful for developing directionally responsive materials for applications in soft robotics. Using materials with greater complexity allows advanced functions, while still using simple device architectures. Elastomer films containing chained magnetic microparticles were prepared through solvent casting and formed into magnetically actuated lifters, accordions, valves, and pumps. Chaining both enhances actuation and imparts a directional response. Cantilevers used as lifters were able to lift up to 50 times the mass of the polymer film. We introduce the "specific torque", the torque per field per mass of magnetic particles, as a figure of merit for assessing and comparing the performance of lifters and related devices. Devices in this work generated specific torques of 68 Nm/kgT, which is significantly higher than in previously reported actuators. Applying magnetic fields to folded accordion structures caused extension and compression, depending on the accordion's orientation. In peristaltic pumps comprised of composite tubes containing embedded chains, magnetic fields caused a section of the tube to pinch closed where the field was applied. These results will facilitate both the further development of soft robots based on chained magnetic particles and efforts to engineer materials with higher specific torque.

Magnetically Responsive Negative Acoustic Contrast Microparticles for Bioanalytical Applications

Smart colloidal particles are routinely used as carriers for biological molecules, fluorescent reporters, cells, and other analytes for the purposes of sample preparation and detection. However, such particles are typically engineered to respond to a single type of stimulus (e.g., commercial magnetic beads to magnetic fields). Here, we demonstrate a unique class of particles that display both positive magnetic contrast and negative acoustic contrast in water. This dual functionality allows for fine spatiotemporal control, enabling multiple separation modalities and increasing the utility of the particles in various chemical and biological assays.

Magnetic Mesoporous Photonic Cellulose Films

Novel hybrid materials of cellulose and magnetic nanoparticles (NPs) were synthesized and characterized. The materials combine the chiral nematic structural features of mesoporous photonic cellulose (MPC) with the magnetic properties of cobalt ferrite (CoFe2O4). The photonic, magnetic, and dielectric properties of the hybrid materials were investigated during the dynamic swelling and deswelling of the MPC films. It was observed that the dielectric properties of the generated MPC films increased tremendously following swelling in water, endorsing efficient swelling ability of the generated mesoporous films. The high magnetic permeability of the developed MPC films in conjunction with their superior dielectric properties, predominantly in the swollen state, makes them interesting for electromagnetic interference shielding applications.

Dipole correlation effects on the local field and the effective dielectric constant in composite dielectrics containing high-k inclusions

Mixing dielectric polymers with high permittivity (high-k) inclusions can affect their electrical properties. In actuation applications of dielectric elastomers, the polarized inclusions generate additional volume polarization-related electrostriction. In energy storage applications, it is possible to store more energy in dielectric composites because of additional polarization of the inclusions and interfaces. However, mixing an electroactive polymer with high-k inclusions also brings several disadvantages. The expulsion of the field from the interior of high-k fillers and the presence of two poles on the filler surface along the applied field direction result in higher local fields EL near the inclusion poles. The resulting field enhancement lowers the breakdown field (Eb) threshold for the material and therefore compromises the actuation and energy storage capabilities of dielectric composites. To mitigate this issue, the dependence of EL on the morphology of inclusion distribution, the field localization effect in chained configurations, and the role of the dipole-dipole correlation effects in the enhancement of the dipolar field of inclusions are analyzed. We show that the dipolar correlation effects are strong in large inclusion composites and their contribution to the inclusion dipole moment μ and to the local fields EL can reach 30-50%. A new method for deriving the composite permittivity from the field EL distribution, based on a caged probe technique, is also presented.

Magneto-electro-responsive material based on magnetite nanoparticles/polyurethane composites

Multi-functional materials in actuator applications have been developed toward reversibility and sensitivity under various actuating fields. In this work, magneto-electro-responsive materials consisting of a polyurethane (PU) matrix and its composites embedded with magnetite nanoparticles (MNP) as a dispersed phase were fabricated to tailor the electromechanical properties and bending performance under electric, magnetic, and electromagnetic fields. Due to the superior characteristics of MNP over other magnetic materials, the composites fabricated with electronic polarization were highly responsive under electric field. The highest storage modulus sensitivity belonged to the 1.0% v/v MNP/PU composite which possessed the value of 3.46 at the electric field 2 kV mm(-1). Moreover, all of the PU composites behaved as an electrostrictive material in which the stress depended quadratically on the electric field. It was demonstrated that the PU composites also possessed very good recoverability, fast response (<15s) and large bending angle relative to that of pristine PU under applied electric field. Interestingly, the steady state storage modulus response was attained within the first electrical actuation cycle and the PU composite was a fully reversible material. In addition, it was shown that superparamagnetism was a common characteristic of all fabricated composites under magnetic field. The 3.0%v/v MNP/PU composite provided the largest bending distance up to 23.60mm, and 14.10mm under the magnetic field of 5000 G, and the electromagnetic field of 320 G, respectively. In summary, the MNP/PU composite material is a potential candidate to be used as a smart material under the influences of electric and/or magnetic fields over other existing dielectric materials.

High-permeability functionalized silicone magnetic microspheres with low autofluorescence for biomedical applications

Functionalized magnetic microspheres are widely used for cell separations, isolation of proteins and other biomolecules, in vitro diagnostics, tissue engineering, and microscale force spectroscopy. We present here the synthesis and characterization of a silicone magnetic microsphere which can be produced in diameters ranging from 0.5 to 50 μm via emulsion polymerization of a silicone ferrofluid precursor. This bottom-up approach to synthesis ensures a uniform magnetic concentration across all sizes, leading to significant advances in magnetic force generation. We demonstrate that in a size range of 5-20 μm, these spheres supply a full order of magnitude greater magnetic force than leading commercial products. In addition, the unique silicone matrix exhibits autofluorescence two orders of magnitude lower than polystyrene microspheres. Finally, we demonstrate the ability to chemically functionalize our silicone microspheres using a standard EDC reaction, and show that our folate-functionalized silicone microspheres specifically bind to targeted HeLa and Jurkat cells. These spheres show tremendous potential for replacing magnetic polystyrene spheres in applications which require either large magnetic forces or minimal autofluorescence, since they represent order-of-magnitude improvements in each. In addition, the unique silicone matrix and proven biocompatibility suggest that they may be useful for encapsulation and targeted delivery of lipophilic pharmaceuticals.

Magnetic-Induced Luminescence from Flexible Composite Laminates by Coupling Magnetic Field to Piezophotonic Effect

Magnetic-induced luminescence (MIL) is realized via a strain-mediated coupling strategy. MIL composite laminates composed of magnetic actuator and phosphor phases are developed. The MIL performance is tested under low magnetic fields at room temperature. The results provide a novel type of promising luminescent and magnetic material for developing some new concept devices.

Self-Sensing, Ultralight, and Conductive 3D Graphene/Iron Oxide Aerogel Elastomer Deformable in a Magnetic Field

Three-dimensional (3D) graphene aerogels (GA) show promise for applications in supercapacitors, electrode materials, gas sensors, and oil absorption due to their high porosity, mechanical strength, and electrical conductivity. However, the control, actuation, and response properties of graphene aerogels have not been well studied. In this paper, we synthesized 3D graphene aerogels decorated with Fe3O4 nanoparticles (Fe3O4/GA) by self-assembly of graphene with simultaneous decoration by Fe3O4 nanoparticles using a modified hydrothermal reduction process. The aerogels exhibit up to 52% reversible magnetic field-induced strain and strain-dependent electrical resistance that can be used to monitor the degree of compression/stretching of the material. The density of Fe3O4/GA is only about 5.8 mg cm(-3), making it an ultralight magnetic elastomer with potential applications in self-sensing soft actuators, microsensors, microswitches, and environmental remediation.

Highly responsive core-shell microactuator arrays for use in viscous and viscoelastic fluids

We present a new fabrication method to produce arrays of highly responsive polymer-metal core-shell magnetic microactuators. The core-shell fabrication method decouples the elastic and magnetic structural components such that the actuator response can be optimized by adjusting the core-shell geometry. Our microstructures are 10 μm long, 550 nm in diameter, and electrochemically fabricated in particle track-etched membranes, comprising a poly(dimethylsiloxane) core with a 100 nm Ni shell surrounding the upper 3-8 μm. The structures can achieve deflections of nearly 90° with moderate magnetic fields and are capable of driving fluid flow in a fluid 550 times more viscous than water.

A simulation study of the electrostriction effects in dielectric elastomer composites containing polarizable inclusions with different spatial distributions

Controlled actuation of electroactive polymers with embedded high dielectric nanoparticles is theoretically analyzed.

Superior elastomeric nanocomposites with electrospun nanofibers and nanoparticles of CoFe2O4 for magnetorheological applications

Cobalt ferrite nanofiber filled polydimethylsiloxane nanocomposites show 100–400% improvement in magnetorheological properties compared to that with nanoparticles.