In recent years, smart/stimuli-responsive hydrogels have drawn tremendous attention for their varied applications, mainly in the biomedical field. These hydrogels are derived from different natural and synthetic polymers but are also composite with various organic and nano-organic fillers. The basic functions of smart hydrogels rely on their ability to change behavior; functions include mechanical, swelling, shaping, hydrophilicity, and bioactivity in response to external stimuli such as temperature, pH, magnetic field, electromagnetic radiation, and biological molecules. Depending on the final applications, smart hydrogels can be processed in different geometries and modalities to meet the complicated situations in biological media, namely, injectable hydrogels (following the sol-gel transition), colloidal nano and microgels, and three dimensional (3D) printed gel constructs. In recent decades smart hydrogels have opened a new horizon for scientists to fabricate biomimetic customized biomaterials for tissue engineering, cancer therapy, wound dressing, soft robotic actuators, and controlled release of bioactive substances/drugs. Remarkably, 4D bioprinting, a newly emerged technology/concept, aims to rationally design 3D patterned biological matrices from synthesized hydrogel-based inks with the ability to change structure under stimuli. This technology has enlarged the applicability of engineered smart hydrogels and hydrogel composites in biomedical fields. This paper aims to review stimuli-responsive hydrogels according to the kinds of external changes and t recent applications in biomedical and 4D bioprinting.

Smart and biomimetic 3D and 4D printed composite hydrogels: opportunities for different biomedical applications

This study investigates the influences of electric and magnetic fields applied both separately and superimposed on the dielectric and elastic properties of membranes (Ms) composed of cotton fabric and carbonyl iron microparticles (CI). To this end, three membranes were fabricated: one without CI microparticles and the remaining two with various proportions of CI microparticles. Additionally, scanning electron microscopy and energy emission X-ray spectroscopy were used to perform morphological and elemental analysis of the cotton fabric sample, CI microparticles, and the prepared Ms. Subsequently, flat capacitors (FCs) were manufactured using these Ms, encapsulated in a silicone rubber sheath. The FCs were introduced in static electric and magnetic fields superimposed over a sinusoidal electric field of frequency f = 1 kHz, and the electric capacity Cpand the tangent of dielectric loss angle Dpof the FCs were measured, depending on the E values of the static electric and magnetic fields superimposed over the medium-frequency electrical field. The values of the static electric field intensity and magnetic flux density were varied in steps of 4 kV/m within an interval 0 ≤ E (kV/m) ≤ 48 and in steps of 100 mT within an interval 0 ≤ B (mT) ≤ 300, respectively. Based on the obtained data, the components of the complex dielectric permittivity, those of the deformations, and the coefficients of elasticity of the Ms were determined to characterize the dependence of these values on the composition of Ms under the applied fields. Furthermore, the observed physical mechanisms were described based on the elements of physics of linear dielectrics and small deformation theory. Ultimately, the development of these materials can aid the design of various electrical devices for diverse applications such as sensors with electric field transducer, magnetic field, vibrations, and mechanical deformations. 

Effects of electric and magnetic fields on dielectric and elastic properties of membranes composed of cotton fabric and carbonyl iron microparticles

Magnetorheological (MR) fluids are smart materials that show enormous potential in vibration control, mechanical engineering, etc. However, the effects of the solid-liquid interface strength and the interaction strength between carrier liquid molecules on the mechanical properties and sedimentation stability of MR fluids have always been unresolved issues. This work presents a new type of MR fluid that has a novel carrier liquid, i.e., silicone oil (SO) mixed with a hydroxyl-functionalized ionic liquid (IL-OH). An all-atomic Fe/SO/IL-OH interface model for studying the relationship between mechanical properties and interface strength and intermolecular interactions is established. On the basis of simulation results and theoretical analyses, the mechanical properties and sedimentation stability of the SO/IL-OH-based MR fluids are thoroughly investigated by experiments. The results show that functional ionic liquids significantly improve the mechanical properties and sedimentation stability of MR fluids. These results are essentially attributed to the stronger solid-liquid interface strength, van der Waals forces, and hydrogen bonds between the silicone oil and the functional ionic liquid. The explicit results not only help elucidate the numerous phenomena involved in the research process for MR fluids at the atomic scale but also provide insightful information on the fabrication of high-performance MR fluids.

Molecular Dynamics Simulations and Experimental Studies of the Microstructure and Mechanical Properties of a Silicone Oil/Functionalized Ionic Liquid-Based Magnetorheological Fluid.

Magnetorheological Elastomer Films with Controlled Anisotropic Alignment of Polystyrene-Modified Fe3O4 Nanoplates

ABSTRACT A magnetorheological fluid (MRF) is a smart composite suspension composed of nonmagnetic liquid and soft magnetic particles. Carrier fluids can considerably influence the performance of MRFs; therefore, to investigate the effect of carrier fluids on MRFs, an SO/IL-MRF was prepared by mixing an ionic liquid (IL) with silicone oil (SO) in this study. Three types of MRF samples were prepared for experiments (pure SO, pure IL, and SO/IL). According to the experimental results, the SO/IL-MRF has better sedimentation stability than those based on pure SO and pure IL. Further, three methods were used to determine the shear yield stresses of the MRFs. The SO/IL-MRF achieved a higher shear yield stress than those of the other two because a network structure is formed between the ionic fragments and the molecular chains of the SO in the SO/IL-MRF. This increases the movement resistance of the particles in the carrier fluid, and it is unlike the mechanism of the IL-enhanced MRF. This work provides new ideas for improving the MRF performance. Graphical abstractSO/IL-MRF prepared by mixing ionic liquid with silicone oil exhibited better sedimentation stability and higher shear yield stress. This is because a network structure is formed between the ionic fragments and the molecular chains of the silicone oil, which increases the movement resistance of the particles in the carrier fluid

https://www.tandfonline.com/doi/pdf/10.1080/19475411.2022.2069876?needAccess=true

Properties and mechanism of ionic liquid/silicone oil based magnetorheological fluids

This study is focused on surface-modified Fe3O4@SiO2 particles with precisely controlled sizes and shapes applied in magnetorheological (MR) fluids. After the preparation of the monodisperse spindle-shaped and cubic Fe3O4@SiO2 particles, surface modification with dodecyltrimethoxysilane (DTM) was carried out via a silane coupling reaction to increase the dispersion stability of the particles. Afterward, MR fluids were prepared by mixing the DTM-modified Fe3O4@SiO2 particles with silicon oil. Transmission electron microscopy observations demonstrated that spindle-shaped Fe3O4@SiO2 particles could form a more stable chain-like structure than cubic Fe3O4@SiO2 particles upon application of an external magnetic field. The rheological measurements of MR fluids also indicated that the surface modification with DTM, the introduction of anisotropic shapes, and the increase in the particle size all played positive roles in the improvement in MR properties.

Shinya Yamanaka

Papers

Magnetorheological Fluids with Surface-Modified Iron Oxide Magnetic Particles with Controlled Size and Shape

Synthesis of Fine Particles by Arc Plasma Method and their Application to Functional Fluids