Magnetorheological (MR) materials are classified as smart materials due to their responsiveness to external magnetic stimuli. Intensive research on MR materials has led to broad applications in several potential fields. A solid carrier matrix state called MR elastomer with its exceptional magnetic responsive feature is obtained by merging magnetizable particles within an elastomeric polymer. This integration results in outstanding characteristics on the rheological performances. Special prominence is given to the understanding of the base materials and fabrication as well as the functional behavior through various characterization methods. Broad applications of MREs are also explored to provide a profound market picture and to motivate researchers to develop novel technology. The functional behavior of MREs is briefly explained. The art of the materials provides the current position and mapping of the matrix and filler particles. Types of matrix and particles are mentioned together with the level of the research interest on MREs. Description of the fabrication is provided in simple diagrams as summarized from previous works to enhance the MREs performance. The possible tests to reveal the characteristics of the MREs are delivered with the global experimental setup. The review also explains the applications of MREs as well as discussion on the MREs future promising applications.

Recent Progress on Magnetorheological Solids: Materials, Fabrication, Testing, and Applications†

Owing to their water-rich structures, which are similar to those of biological tissues, hydrogels have long been regarded as promising scaffolds for artificial tissues and organs. However, in terms of the structural anisotropy, most synthetic hydrogels are substantially different from biological systems. Synthetic hydrogels are usually composed of randomly oriented three-dimensional polymer networks whereas biological systems adopt anisotropic structures with hierarchically integrated building units. Such anisotropic structures often play essential roles in biological systems to exhibit particular functions. In this context, anisotropic hydrogels provide an entry point for exploring biomimetic applications of hydrogels. Reflecting these aspects, an increasing number of studies on anisotropic hydrogels have been reported recently. This Minireview highlights the use and perspectives of these anisotropic hydrogels, particularly focusing on their preparation, structures, and applications.

Synthesis of Anisotropic Hydrogels and Their Applications.

Magnetic separation techniques in sample preparation for biological analysis: a review.

Embedding magnetic nanoparticles into soft host media offers the opportunity to externally control material properties via a magnetic field. Choosing a hydrogel as the host medium allows modification of not only the elastic properties, but also the degree of swelling of the gel and the shape changes of the sample. Hydrogels where magnetic nanoparticles serve as the only crosslinking reagent of the network are a promising new class of such stimuli-responsive gels. The well-defined magneto-mechanical coupling present in these materials should allow for a better understanding and optimization of field-induced changes.

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Stimuli-responsive hydrogels cross-linked by magnetic nanoparticles

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.

Magnetic Soft Materials and Robots.

A new class of magnetoelastic gel that demonstrates drastic and reversible changes in storage modulus without using strong magnetic fields was obtained. The magnetic gel consists of carrageenan and carbonyl iron particles. The magnetic gel with a volume fraction of magnetic particles of 0.30 exhibited a reversible increase by a factor of 1400 of the storage modulus upon a magnetic field of 500 mT, which is the highest value in the past for magnetorheological soft materials. It is considered that the giant magnetoelastic behavior is caused by both high dispersibility and high mobility of magnetic particles in the carrageenan gel. The off-field storage modulus of the magnetic gel at volume fractions below 0.30 obeyed the Krieger–Dougherty equation, indicating random dispersion of magnetic particles. At 500 mT, the storage modulus was higher than 4.0 MPa, which is equal to that of magnetic fluids, indicating that the magnetic particles move and form a chain structure by magnetic fields. Morphological study r...

Magnetically Tunable Elasticity for Magnetic Hydrogels Consisting of Carrageenan and Carbonyl Iron Particles

Magnetism, dynamic viscoelasticity, mechanical property, and durability of magnetic elastomers consisting of polyurethane and carbonyl iron particles were investigated. Magnetic measurements revealed that the magnetic permeability of the magnetic elastomers can be explained by the linear combination of the magnetic permeability of magnetic particles and polyurethane. Dynamic viscoelastic measurements showed that the magnetic particles are randomly dispersed in the elastomer. On applying a magnetic field of 500 mT, the magnetic elastomers demonstrated a drastic change in the dynamic modulus; the storage modulus increased from 6.5 kPa to 1.6 MPa, and the loss modulus increased from 3.6 kPa to 0.16 MPa. This drastic change in the dynamic modulus was observed at all frequencies from 0.01 Hz to 3 Hz. The critical volume fraction showing the transition from a random dispersion to a chain structure decreased significantly with the magnetic field. Compression tests revealed that the magnetic elastomers exhibited high mechanical toughness with high breaking strain exceeding 0.8. Durability tests showed the magnetoviscoelastic behavior of the magnetic elastomers was maintained for 1.5 years after the synthesis without degradation.

Magnetism and viscoelasticity of magnetic elastomers with wide range modulation of dynamic modulus

The magnetoelastic behavior of bimodal magnetic elastomers consisting of magnetic particles, carbonyl iron, and nonmagnetic particles, zinc oxide, has been investigated by dynamic viscoelastic measurements. The storage modulus of bimodal magnetic elastomers increased under a magnetic field of 500 mT. The change in the storage modulus was enhanced by adding nonmagnetic particles at volume fractions above a certain volume fraction of 0.02, indicating the occurrence of stress transfer by a chain structure of magnetic particles via nonmagnetic particles. The critical volume fraction at 500 mT determined by percolation analysis was nearly independent of the diameter of nonmagnetic particles. However, at low magnetic fields below 160 mT, the critical volume fraction was found to decrease with the particle diameter. Substituting magnetic particles with nonmagnetic ones, the change in the storage modulus of bimodal magnetic elastomers monotonically decreased with the substitution ratio of magnetic particles. The mechanism of the enhanced magnetoelastic response for bimodal magnetic elastomers is discussed.

Enhancement of magnetoelastic behavior of bimodal magnetic elastomers by stress transfer via nonmagnetic particles

We have investigated the electric conductivity, dielectric relaxation behavior, and viscosity for the aqueous solution of cyanobacterial megamolecules, molecular weight $=1.6\ifmmode\times\else\texttimes\fi{}{10}^{7}$ g/mol, named sacran. Sacran is an anionic polyelectrolyte which has carboxylate and sulfate groups on the saccharide chain. The electric conductivity and the zero shear viscosity demonstrated three crossover concentrations at 0.004, 0.02, and 0.1 wt$%$. The viscosity was found to be scaled as \ensuremath{\sim}${c}^{1.5}$, \ensuremath{\sim}${c}^{0.5}$, \ensuremath{\sim}${c}^{1.5}$, and \ensuremath{\sim}${c}^{3.0}$ with increasing the sacran concentration. At 0.1 wt$%$, the sacran chain formed a weak gel which exhibits macroscopic liquid crystal domains including Schlieren texture. Therefore, these crossover concentrations are considered to be the overlap concentration, entanglement concentration, and gelation concentration (or critical polyelectrolyte concentration), respectively. Dielectric relaxation analysis exhibited the fact that sacran has two types of counterions with different counterion-polyion interaction, i.e., strongly bound and loosely bound counterions. The dielectric parameters such as relaxation time or relaxation strength are sensitive to both the entanglement concentration and the gelation concentration, but not the overlap concentration. The number density of bound counterions calculated from the relaxation strength revealed that the counterion is condensed on the sacran chain with raising the sacran concentrations. The decrease in the charge density of the sacran chain reduces the repulsive force between the chains and this would cause the helix transformation or gelation behavior. The chain conformation of sacran in pure water and the gelation mechanism are discussed in relation with the behavior of polyelectrolytes and liquid crystals.

Ionic state and chain conformation for aqueous solutions of supergiant cyanobacterial polysaccharide.

The effect of plasticizer content on the vibration absorbing properties for polyurethane elastomers was investigated. A natural frequency appeared on the frequency spectra at around 100 Hz. The natural frequency linearly decreased, and the transmissibility also decreased with the plasticizer content. Magnetic elastomers containing carbonyl iron particles with a dimeter of 7.0 mm also showed a natural frequency at ~260 Hz, and the natural frequency significantly decreased with the plasticizer content. The decrease in the transmissibility with the plasticizer content was larger than that for polyurethane elastomers. The natural frequency for magnetic elastomers increased by several ten Hz by a magnetic field of 60 mT although the transmissibility was independently of the plasticizer content. The effect of load on the natural frequency for these elastomers was also investigated, and it was found that the natural frequency is proportional to the storage modulus, G, with different two slopes depending on the mass of the system, m. The linear relation between the natural frequency and (G’/m)1/2 revealed that the observed vibration can be basically described by a simple harmonic oscillation.

Mika Kawai

Papers

Magnetically Tunable Elasticity for Magnetic Hydrogels Consisting of Carrageenan and Carbonyl Iron Particles

Magnetism and viscoelasticity of magnetic elastomers with wide range modulation of dynamic modulus

Enhancement of magnetoelastic behavior of bimodal magnetic elastomers by stress transfer via nonmagnetic particles

Ionic state and chain conformation for aqueous solutions of supergiant cyanobacterial polysaccharide.

Frequency spectra of vibration transmissibility for magnetic elastomers with various plasticizer contents