Distribution Of Magnetic Particles Research Articles

Symmetrically Controlled Design of Twin Alumina-Co Composite Thin Films

Self-assembled dual-working electrode electrolytic cells were designed to produce twin alumina-Co composite films with highly symmetrical microstructures using a deflected electric field-assisted alternating current electrodeposition method. The results show that the deposition current density, microstructure, and optical and magnetic properties of the twin composite films exhibit a high degree of symmetry. The distribution of magnetic Co particles in the alumina nanopores can be changed by adjusting the magnitude of the deflected electric field, resulting in a synchronous symmetrical change in the microstructure of the composite films, which enables the fine-tuning of the magneto-optical properties of the twin alumina-Co composite films at the microscopic scale. The current density distribution on the surface of the twin composite films along the direction of the deflected electric field was quantitatively analyzed by theoretical calculations and numerical simulations. The results show that the deposition current density gradually increases from 0.024 A/m2 in region C to 0.056 A/m2 in region A at 6 V deflection voltage. The saturation magnetization intensity gradually increases along the radial direction, which is 118, 130, and 150 kA/m, respectively

Investigation of the Morphological Structure of Needle-Free Electrospun Magnetic Nanofiber Mats

Electrospun magnetic nanofibers are promising for a variety of applications in biomedicine, energy storage, filtration or spintronics. The surface morphology of nanofiber mats plays an important role for defined application areas. In addition, the distribution of magnetic particles in nanofibers exerts an influence on the final properties of nanofiber mats. A simple method for the production of magnetic nanofiber mats by the addition of magnetic nanoparticles in an electrospinning polymer solution was used in this study. In this work, magnetic nanofibers (MNFs) were prepared by needle-free electrospinning technique from poly(acrylonitrile) (PAN) in the low-toxic solvent dimethy lsulfoxide (DMSO) and 20 wt% Fe3O4 at different parameter conditions such as PAN concentration, voltage and ultrasonic bath. The distribution of nanoparticles in the fiber matrix was investigated as well as the chemical and morphological properties of the resulting magnetic nanofibers. In addition, the surface morphology of magnetic nanofiber mats was studied by confocal laser scanning microscope (CLSM), scanning electron microscope (SEM), Fourier transform infrared microscope (FTIR) and ImageJ software, and distribution of Fe3O4 particles in the matrix was investigated by energy dispersive X-ray spectroscopy (EDX).

Magnetic Particle Reinforced Elastomer Composites for Additive Manufacturing

Development of magnetorheological elastomer (MRE) materials suitable for fused filament fabrication (FFF)-type additive manufacturing processes is performed. Filament materials are thermoplastic polyurethane (TPU) pellets and Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> magnetic particles. A trickling method is employed to introduce the materials into the filament extruder at a constant rate, after which they are mixed in a hopper and extruded at 160 °C. The trickling rate of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> was controlled to produce different volume concentrations [0%–13% (v/v)]. Micrograph analysis on the cross section of the filaments indicates that magnetic particles are uniformly dispersed in the TPU matrix. As the volume concentration of particles in the matrix is increased over 10% (v/v), the size distribution of magnetic particles is slightly increased, indicating formation of aggregation of magnetic particles. Micropores are observed in the filaments, and the number of pores seem to increase with increasing volume concentration of magnetic particles. A commercial off the shelf (COTS) FFF 3-D printer is successfully used to produce cylindrical MRE samples with different magnetic particle volume concentrations. Magnetorheological (MR) properties of the printed MRE samples are characterized by uniaxial compression testing under different applied magnetic field. Material elastic modulus was found to increase with the intensity of magnetic field for a given magnetic particle volume concentration. The relative MR effect varies from −1.3% to 60% within the studied volume concentrations and applied fields. The samples with 5% (v/v) show the highest MR effect when compared with samples with higher magnetic particle volume concentrations. The results of these experiments confirm that the equivalent energy dissipation of the material rises with the increase in the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> concentration of MRE.

Study on the trajectory of magnetic particles in the process of magnetorheological elastomer pre-structure

Magnetorheological elastomers (MREs) is made by embedding micro/nano-sized magnetic particles in a polymer matrix. Among them, the distribution and arrangement of internal magnetic particles have an important influence on the performance of magnetorheological elastomers. Therefore, the current research mainly focuses on the influence of the distribution and arrangement of magnetic particles inside MREs on its performance. However, there are few studies on the trajectory of magnetic particles in the prefabrication process of magnetorheological elastomers. In order to solve the above problems, this paper firstly calculated the magnetic interaction force and fluid force of the magnetic particles through numerical simulation. Then, the equation of motion of the magnetic particles is obtained through the coupling of the magnetic interaction force and the fluid force. Finally, the numerical simulation is verified through experiments. In addition, it is found through numerical simulation that the trajectory of magnetic particles is determined by the direction of the magnetic interaction force, instead of its magnitude. After that, the relationship between the direction of the magnetic interaction force and the motion trajectory is analyzed and summarized. The findings of this study have a directive function in explaining the mechanism of magnetic particle arrangement.

Shape Programmable Magnetic Pixel Soft Robot

Magnetic response soft robot realizes programmable shape regulation with the help of magnetic field and produces various actions. The shape control of magnetic soft robot is based on the magnetic anisotropy caused by the orderly distribution of magnetic particles in the elastic matrix. In the previous technologies, magnetic programming is coupled with the manufacturing process, and the orientation of magnetic particles cannot be modified, which brings restrictions to the design and use of magnetic soft robot. This paper presents a magnetic pixel robot with shape programmable function. By encapsulating NdFeB/gallium composites into silicone shell, a thermo-magnetic response functional film with lattice structure are fabricated. Basing on thermal-assisted magnetization technique, we realized the discrete magnetization region distribution on the film. Therefore, we proposed a magnetic coding technique to realize the mathematical response action design of software robot. Using these methods, we prepared several magnetic soft robots based on origami structure. The experiments show that the behavior mode of robot can be flexibly and repeatedly regulated by magnetic encoding technique. This work provides a basis for the programmed shape regulation and motion design of soft robot.

Traffic-related magnetic pollution in urban dust from the Xiamen Island, China

Pollution by magnetic particles is a rising environmental issue with possible adverse health effects such as the Alzheimer's disease. Magnetic particles in urban dust originate from traffic-related activities, industrial products and detrital minerals, yet distribution of magnetic particles in cities is poorly known. Here, we measured magnetic properties of surface roadside dust and topsoils by low-frequency magnetic susceptibility (χLF), saturation isothermal remanent magnetization (SIRM), anhysteretic remanent magnetization (χARM) and advanced rock-magnetic methods. Results show that χLF, SIRM and χARM of roadside dust are much higher than topsoil values. Roadside dusts had much lower percent frequency-dependent susceptibility of 1.83 ± 0.12% than topsoil of 2.96 ± 0.14%. Findings also demonstrate a minor contribution of fine pedogenic grains to the elevated χLF. The high values of dust χLF, SIRM and χARM are explained by the contribution of local, exogenous magnetite-like materials.

Development of Magnetic Particle Distribution Imaging Using Magnetic Field Reconstruction for Biopsy of the Sentinel Lymph Node

The sentinel lymph node is the first lymph-node-draining cancer metastasis. The identification of the sentinel lymph node using magnetic particles and a magnetic sensor has attracted attention in recent years, as this method is less invasive than the conventional method of radiotracer injection. However, the development of a two-dimensional measurement method for sentinel lymph nodes using magnetic nanoparticles remains an issue. In the present study, a method and apparatus for the two-dimensional imaging of magnetic particle distribution were developed to detect a lymph node with magnetic particles concentrated within lymphoid tissues. The method comprises the reconstruction of the magnetic field measured with a high-sensitivity magnetic sensor and with a magnetic detection ability of 2 nT/√Hz at 100 Hz (5 nT/√Hz at 1 Hz). The proposed system measures the two-dimensional magnetic field distribution in an area of up to 25 × 25 mm2 using a coil generating a 0.77 mT external magnetic field applied to the measurement target. The improved spatial resolution of the images makes it possible to use two-dimensional imaging for diagnostics of breast cancer metastases.

Affordable Magnetic Hydrogels Prepared from Biocompatible and Biodegradable Sources.

Magnetic hydrogels composed of poly(vinyl alcohol) (PVA)/water-soluble tricarboxy cellulose (CO)/magnetic fluids (MFs) have been prepared by a freeze–thaw cycle technique. The system designed here combines the renewability and biocompatibility aspects of PVA and CO, as well as the magnetic properties of MFs, thereby offering special properties to the final product with potential applications in medicine. In the first step, the water-soluble CO is synthesized using a one-shot oxidation procedure and then the aqueous solutions of CO are mixed with PVA solutions and magnetic fluids in the absence of any additional cross-linking agent. The magnetic hydrogels were thoroughly investigated by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), magnetometry (VSM), and thermogravimetric analysis. The morphological results show an excellent distribution of magnetic particles and CO inside the PVA matrix. The VSM results show that the magnetic hydrogels possess superparamagnetic properties.

Evolutionary design of magnetic soft continuum robots

Worldwide cardiovascular diseases such as stroke and heart disease are the leading cause of mortality. While guidewire/catheter-based minimally invasive surgery is used to treat a variety of cardiovascular disorders, existing passive guidewires and catheters suffer from several limitations such as low steerability and vessel access through complex geometry of vasculatures and imaging-related accumulation of radiation to both patients and operating surgeons. To address these limitations, magnetic soft continuum robots (MSCRs) in the form of magnetic field-controllable elastomeric fibers have recently demonstrated enhanced steerability under remotely applied magnetic fields. While the steerability of an MSCR largely relies on its workspace-the set of attainable points by its end effector-existing MSCRs based on embedding permanent magnets or uniformly dispersing magnetic particles in polymer matrices still cannot give optimal workspaces. The design and optimization of MSCRs have been challenging because of the lack of efficient tools. Here, we report a systematic set of model-based evolutionary design, fabrication, and experimental validation of an MSCR with a counterintuitive nonuniform distribution of magnetic particles to achieve an unprecedented workspace. The proposed MSCR design is enabled by integrating a theoretical model and the genetic algorithm. The current work not only achieves the optimal workspace for MSCRs but also provides a powerful tool for the efficient design and optimization of future magnetic soft robots and actuators.

Mirror-mode Wave Identification Methods and Their Application to Martian Magnetosheath

Mirror-mode waves are structures usually seen in plasma with temperature anisotropy, identifiable through features in magnetic field and particle distribution and fluctuation. Two identification methods are analyzed and compared in this paper. Method A uses magnetic field data only, while Method B combines magnetic field and particle data. Method A is based mainly on features of magnetic field variation such as large amplitude fluctuation along background field direction, using magnitude of magnetic field fluctuation ∆<italic>B</italic>/|<italic>B</italic>| and angles between background field and maximum/minimum variation direction θ_max, θ_min as criteria. Method B is based on features such as wave compression, total pressure balance and zero velocity in plasma frame. Identification using data from MAVEN probe in the Martian magnetosheath shows that Method A can cause misidentification under certain circumstances, <italic>e.g.</italic> magneto-sonic waves. Results of identification using Method A with varying criteria ∆<italic>B</italic>/|<italic>B</italic>| and θ_min/θ_max are studied in the Martian magnetosheath, suggesting threshold values: θ_min> 40º, θ_max< 40º and ∆<italic>B</italic>/|<italic>B</italic>| > 80% can yield satisfying results.

Cu 전극 사이에 존재하는 항체 접합용 자성입자의 크기와 분포에 따른 자기저항 특성 연구

항체치료제 개발용으로 자성입자의 코어에 Fe₃O₄를 두고 쉘에는 실라놀기와 카르복시기가 각각 붙어있는 초상자성체 자성비드와 자성나노입자들의 크기와 분포에 따른 자기저항 특성을 조사하였다. 부피 10 μl 만큼 들어가는 플라스틱 수조 중앙에 1mm 공간 내 존재하는 크기가 다른 입자들이 분포가 되도록 자기장을 인가하였다. 자성비드 용액을 통과하는 외부자기장 세기에 따라 측정한 전압곡선으로부터 얻은 최저 자기저항값, 자기저항비, 그리고 용액보자력은 각각 3.567 MΩ, 1.1%, 그리고 400Oe 이었고, 자성나노입자 용액 경우는 각각 0.245 MΩ, 50%, 그리고 184Oe 이었다. 0.35 μm 크기의 자성나노입자 용액이 1 μm 크기의 자성비드 용액보다 자기저항 특성이 훨씬 민감하게 변하였다. 이러한 특성은 크기가 다른 자성입자들이 외부자기장의 세기에 따라 이동으로 생긴 예상분리거리의 크기와 용액의 예상경로정도의 양에 따라 생기게 되는 것으로 분석하였다. 이 연구결과는 나노자성입자의 표면에 접합될 항체를 포함한 여러 가지 생체분자들의 기능을 조절하는 데 활용될 수 있음을 논의하였다.

Thermal phase transition controlling electromagnetic wave absorption behavior of PAN fiber derived porous magnetic absorber

The increasing excess electromagnetic wave (EMW) energies causes potential electromagnetic radiation pollution. It gives rise to more and more attention on designing environmentally friendly, efficient and suitable for large-scale industrial production of daily EMW absorbers. Carbon matrix composites modified with magnetic particles occupy the majority of these candidate absorbers. However, the preparation is still in its infancy. Here, we creatively mixed Fe(acac)3 with polyacrylonitrile (PAN) spinning solution, and then prepared magnetic carbon-based composites by in-situ pyrolysis of the obtained electrospun fibers (Fe(acac)3/PAN) at different temperatures. The SEM images display a fractured fibrous material with porous structure and uniform distribution of magnetic particles on the outer surface and inside. The XRD and XPS results indicate the composites consist of Fe3O4, Fe3C and carbon matrix. Besides, the thermal phase transition occurs with the pyrolysis temperature increasing from 300 to 1000 °C, and the formed Fe3O4 reacts with the carbon matrix to form Fe3C. As a result, the corresponding magnetic properties and electromagnetic properties improve. MCF-1000 exhibits enhanced EMW absorption performance with a minimum RL ( $${RL}_{\mathrm{min}}$$ ) value of − 29.27 dB at a matching frequency ( $${f}_{m}$$ ) of 11.68 GHz, a matching thickness ( $${t}_{m}$$ ) as low as 1.50 mm, and an effective frequency bandwidth as broad as 4.50 GHz from 13.50 to 18.00 GHz at a fixed thickness as thin as 1.13 mm. Further investigations indicate that such good EMW absorption behavior is attributed to the optimal impedance matching and the weakened eddy current effect which are both benefited by the thermal phase transition effect. The results provide theoretical and practical experience for the industrial large-scale preparation of a kind of EMW absorbers.

Viscoelastic characteristic iron powder core for reducing vibration and noise radiated by switched reluctance machine

This paper presents a new direction on the mitigation of acoustic noise and vibration in a switched reluctance machine using viscoelastic property of iron powder material also known as SMC – Soft Magnetic Composite. SMC material is made of iron powder particles whose surface is insulated by an oxide layer which is continuous. With a lubricant and a binder at high pressure these particles are compacted to result a bulk material. Soft magnetic composite material is characterized by the ferromagnetic property which is isotropic and three dimensional. The reduction in eddy current loss can be attributed to the insulation coating which is tantamount to the increase in resistance in laminated core. The reduced ripple torque due to soft magnetic electrical core is because of isotropic distribution of magnetic particles. The finite element method (FEM) based simulation studies on torque ripple, vibration and noise has been described. The test results of prototype SMC-SRM, vibration and performance results have been analyzed to validate the simulation results. The study, analysis and experimentation concludes that use of SMC as core in a SRM results in reduced vibration and noise.

Relationship between the response of microscopic and magnetic properties with highly uniform dispersion of carbonyl iron particles in magnetorheological polyurethane foam

Magnetorheological (MR) foam, which is comprised of magnetic particles inside the matrix phase, is capable of changing its physical and rheological properties corresponding to the applied magnetic field. However, the performance of this material seems unsatisfactory due to the nonuniform distribution of magnetic particles throughout the composite area resulting in uneven magnetic properties. Since uniform magnetic properties are significant regarding MR foam, in situ fabrication using a polyvinyl chloride mould can be used to achieve the targetted improvement. MR foam was prepared with various amounts of carbonyl iron particles (CIPs), 0, 35, 45 and 55 wt%, and investigated at three different positions (top, middle and bottom parts) with a vibration sample magnetometer. The results showed that the hysteresis loop at all CIP concentrations of 35, 45 and 55 wt% exhibited superparamagnetic properties with magnetic saturation. The M s of the top, middle and bottom positions were almost the same with a small standard deviation below 2 Am2 kg–1. Moreover, it is observed that the effect of porosity results in the enhancement of the magnetic saturation and remanence values. These results reveal the promising uniform magnetic properties of MR foam, which can benefit the industry by helping produce a consistent MR foam.

Study on surface quality improvement of the plane magnetic abrasive finishing process

Magnetic abrasive finishing (MAF) is an effective surface finishing method. At present, most of the research on plane MAF focuses on finishing characteristics. However, due to the “edge effect” of the magnetic field and the rotational movement of the magnetic brush, the uniformity and flatness of the finished surface are poor. In order to further improve the accuracy of the finished surface, the surface uniformity and flatness are improved by changing the shape of the magnetic pole and the trajectory of the magnetic brush. At the same time, the surface flatness is evaluated not only by the maximum height difference of the cross section but also by the standard deviation to evaluate the surface uniformity and flatness. Through magnetic field simulation and experiments, it is proved that the bottom groove of the magnetic pole helps to make the magnetic particle distribution more uniform in the processing area, which can effectively improve the surface quality. In addition, through experiments, changing the trajectory of the magnetic brush can also effectively improve the surface flatness of the finished surface.

A multi-proxy magnetic approach for monitoring large-scale airborne pollution impact

The interpretive utility of environmental magnetic proxies for investigating airborne particulate matter (PM) pollution impact is restricted by differences in soil composition, land cover and land use. For soil magnetic applications, land use strongly influences magnetic particle distribution down the soil profile, even in homogeneous soil environments. Here, an adaptive approach is engineered to provide accurate magnetic proxy information for pollution monitoring across different land use types. In an 81-km2 area between two industrial harbours, the irregular distribution of forests, arable lands, pasture and residential areas prevented robustly relating topsoil magnetic susceptibility data to known pollution impacts. Although normalized topsoil susceptibility values showed improved potential for deriving airborne pollution impacts, optimal results were obtained by depth-integrating magnetic susceptibility logs, revealing long-term impacts of both active and decommissioned industrial facilities. Complementing soil magnetic observations, active and passive (bio)magnetic monitoring allowed discriminating short-term pollution patterns and evaluating changes in PM impact across the study area. Hereby, active PM receptors (strawberry leaves and plastic coated cardboards (PCCs)) provided promising results, yet passive receptors allowed estimating pollution impacts more efficiently. For the latter, species-independent grass leaf sampling reflected airborne PM depositional patterns most accurately, whereas wiped anthropogenic surfaces proved too sensitive to wash-off.

The influence of crosslinkers and magnetic particle distribution along the filament backbone on the magnetic properties of supracolloidal linear polymer-like chains

Diverse polymer crosslinking techniques allow the synthesis of linear polymer-like structures whose monomers are colloidal particles. In the case where all or part of these colloidal particles are magnetic, one can control the behaviour of these supracolloidal polymers, known as magnetic filaments (MFs), by applied magnetic fields. However, the response of MFs strongly depends on the crosslinking procedure. In the present study, we employ Langevin dynamics simulations to investigate the influence of the type of crosslinking and the distribution of magnetic particles within MFs on their response to an external magnetic field. We found that if the rotation of the dipole moment of particles is not coupled to the backbone of the filament, the impact of the magnetic content is strongly decreased.

Inversion of displacement fields to quantify the magnetic particle distribution in homogeneous elastic media from magnetomotive ultrasound

Magnetomotive ultrasound (MMUS) contrasts superparamagnetic iron-oxide nanoparticles (SPIOs) that undergo submicrometer-scale displacements in response to a magnetic gradient force applied to an imaging sample. Typically, MMUS signals are defined in a way that is proportional to the medium displacement, rendering an indirect measure of the density distribution of SPIOs embedded within. Displacement-based MMUS, however, suffers from ‘halo effects’ that extend into regions without SPIOs due to their inherent mechanical coupling with the medium. To reduce such effects and to provide a more accurate representation of the SPIO density distribution, we propose a model-based inversion of MMUS displacement fields by reconstructing the body force distribution. Displacement fields are modelled using the static Navier-Cauchy equation for linear, homogeneous, and isotropic media, and the body force fields are, in turn, reconstructed by minimizing a regularized least-squares error functional between the modelled and the measured displacement fields. This reconstruction, when performed on displacement fields of two tissue-mimicking phantoms with cuboidal SPIO-laden inclusions, improved the range of errors in measured heights and widths of the inclusions from 54%–282% pre-inversion to-15%–20%. Likewise, the post-inversion contrast to noise ratios (CNRs) of the images were significantly larger than displacement-derived CNRs alone (p = 0.0078, Wilcoxon signed rank test). Qualitatively, it was found that inversion ameliorates halo effects and increases overall detectability of the inclusion. These findings highlight the utility of model-based inversion as a tool for both signal processing and accurate characterization of the number density distribution of SPIOs in magnetomotive imaging.

Evaluating the potential of topsoil magnetic pollution mapping across different land use classes

Soil magnetic measurements are used increasingly to estimate the impact of airborne, combustion-related particulate matter (PM) pollution in dense measurement grids. Although many studies have proven the potential of topsoil magnetic measurements in environmental monitoring, their application is not straightforward when factors such as parent material or land use have to be accounted for. Often, the influence of land use on the soil magnetic signal is circumvented by targeting forest soils, where deposited magnetic particles are best preserved in the topsoil. However, when large forests are absent, e.g. in densely populated areas or environments with more heterogeneous land use, this approach often impedes reliable and comprehensive spatial sampling. We evaluated if topsoil magnetic pollution mapping across different land use classes, against a homogeneous geological environment of sandy soils, could help increase the spatial reliability of results in regional scale surveys. Although detailed magnetic property analysis and evaluation of trace metal concentrations in soils on arable land, forest and pasture showed the impact of atmospheric pollution, topsoil susceptibility measurements did not allow delineating the magnetic footprint of PM pollution. Land use strongly influenced the distribution of magnetic particles through soil, and the evaluation of anomalous magnetic topsoil enhancement required the integration of downhole susceptibility soundings. We conclude that topsoil susceptibility mapping remains a useful tool to evaluate PM pollution impact, yet its application potential across land use classes is limited.

Investigation of a Magnetic Field-Assisted Digital-Light-Processing Stereolithography for Functionally Graded Materials

Abstract Stereolithography (SLA) is one of the most efficient additive manufacturing (AM) methods in terms of dimensional resolution and accuracy. The use of multiple or functionally graded materials in this process can broaden the design space for applications such as sensors and biomedical implants. However, it is still challenging to fabricate such materials using SLA. In this study, we propose a method for fabrication of functionally graded materials with embedded magnetic particles using digital-light-processing SLA in a single step. Magnetite (Fe3O4) particles with an approximate diameter of 300 nm were mixed with a diluted photocurable resin, and used in fabrication of the samples. A digital-light-processing SLA printer was modified to allow integration of a magnet holder above the print bed. Magnet holders with three different configurations of magnets were built and used to fabricate parts with controlled distribution of magnetic particles. Several parts have been printed and characterized by evaluating the concentration of ferromagnetic particles in different regions based on image analysis. The results of this study show that field assisted digital-light-processing SLA can be used to fabricate functionally graded materials with embedded magnetite particles. The resulting concentration of the particles depends on the strength and surface area of the magnets, and the distance between the magnets and the print bed. Experimental results are also used to verify a Finite Element model, which is developed to understand the effect of magnet location on particle concentration. The feasibility of the proposed process has been verified and this process can be used in additive manufacturing of magnetic field-responsive smart polymeric structures.