High-frequency electromagnetic fields (EMFs) are increasingly recognized either as environmental risk factors or as tools for electromagnetic attacks, which are difficult to detect in situ. Existing high-frequency EMF sensors face significant limitations related to structural simplicity, integration with mobile technology, and low energy consumption. To address these challenges, we propose a novel sensor concept based on a magnetically hybridized liquid crystal (LC) microdevice. The hybrid LC chip is designed to exhibit an optical response to external radio-frequency fields without the need for electronic components or an external power supply, relying solely on ambient light. Both sides of the chip are covered with polymer-based crossed polarizer films. The chip is filled with flexible matrices containing thermotropic LCs, such as the rod-like 4-cyano-4'-pentylbiphenyl, into which a network of thin ferromagnetic wires is embedded. The resulting field-responsive LC microdisplay operates via a simple magnetothermal mechanism, and its optical response is sufficiently strong to be visible to the naked eye.

This study compares the propulsion of scaled-up helical microrobot models, based on hard- and soft-magnetic elements under rotating magnetic fields. The experiments were performed at the millimeter scale and interpreted using hydrodynamic scaling laws to predict microscale behavior. Results show that hard-magnetic microrobots achieved step-out frequencies and maximum propulsion speeds 4.5 times higher than soft-magnetic microrobots. Below saturation magnetization, soft-magnetic microrobots demonstrated similar performance irrespective of magnetic susceptibility, highlighting that torque generation in these materials is purely geometry-dependent. Employing a tapered ribbon design increased propulsion speed by a factor of 3.5 compared to regular helical designs. These results show that the impact of using soft rather than hard magnets is manageable, allowing for biodegradable magnets such as pure iron. The theory and experiments in this paper provide a quantitative basis for selecting materials and designs.

This study compares the propulsion of helical microrobots based on hard- and soft-magnetic elements under rotating magnetic fields. Results show that hard-magnetic microrobots achieved step-out frequencies and maximum propulsion speeds 4.5 times higher than soft-magnetic microrobots. Below saturation magnetization, soft-magnetic micro-robots demonstrated similar performance irrespective of magnetic susceptibility, high-lighting that torque generation in these materials is purely geometry-dependent. Employing a tapered ribbon design increased propulsion speed by a factor of 3.5 compared to regular helical designs. These results provide a quantitative basis for selecting materials and designs, enabling designers to weigh the propulsion benefits of hard magnets against the biocompatibility of soft-magnetic microrobots.

We optimized designs of permanent magnet systems to study the effect of magnetic nanoparticles on cell cultures in microslide channels. This produced two designs, one of which is based on a large cylindrical magnet that applies a uniform force density of 6 MN/m$^3$ on soft magnetic iron-oxide spherical nanoparticles at a field strength of over 300 mT. We achieved a force uniformity of better than 14% over the channel area leading to a concentration variation that was below our measurement resolution. The second design was aimed at maximizing the force by using a Halbach array. We indeed increased the force by more than one order of magnitude at force density values over 400 MN/m$^3$, but at the cost of uniformity. However, the latter system can be used to trap magnetic nanoparticles efficiently and to create concentration gradients. We demonstrated both designs by analyzing the effect of magnetic forces on the cell viability of human hepatoma HepG2 cells in the presence of bare Fe$_2$O$_3$ and cross-linked dextran iron-oxide cluster-type particles (MicroMod). Python scripts for magnetic force calculations and particle trajectory modeling as well as source files for 3D prints have been made available so these designs can be easily adapted and optimized for other geometries.

Halloysite nanotubes (HNTs) have been extensively investigated for potential utilization due to their unique structure and properties as a type of natural, eco-friendly clay. The synthesis and modification of magnetic halloysite nanotubes was studied using several experimental techniques including SEM, TEM, FT-IR, Raman spectroscopy, UV-Vis spectroscopy, and BET. Dye absorption experiments were conducted to understand bonding using EDS, XPS, XRD, and Raman spectroscopy. In this study, we evaluated Sunset Yellow FCF (SY) dye removal as a model to understand bonding structures among magnetic HNTs, magnetic particles, and dye molecules. We focus on the interactions of SY-magnetic HNTs and characteristics of magnetization by VSM after SY dye adsorption, which highlight the notable features of magnetic halloysite nanotubes. We used different pH environments to study the behavior of magnetic HNTs after dye absorption. The application of these modified HNTs is promising for future organic dye removal and wastewater treatment.

Magnetically guided untethered devices are used in a variety of medical applications. These devices are typically powered by onboard battery units. Hydrogen fuel cells (FC) are a promising alternative power source for such small-scale devices since they rely on a sustainable fuels which produce electric power from the redox reaction of hydrogen and oxygen across a proton exchange membrane (PEM). Understanding the impact of decreasing the active electrode area in FCs is crucial for deploying FCs in untethered devices and gaining insights into the challenges of downscaling the devices. This paper investigates the performance of PEM FCs (PEMFCs) when their active area is reduced, and when the FC is supplied with reactants at different flow rates from a PEM electrolyzer. PEMFCs with three active electrode areas, 3.5x3.5 cm(2), 2.7x2.7 cm(2), and 1.6x1.6 cm(2) were designed, fabricated, and characterised. Maximum fuel cell output powers of 0.3 W, 0.09 W, and 0.03 W (maximum power densities of 0.025 W/cm(2), 0.012 W/cm(2), and 0.013 W/cm(2)) corresponding to the three aforementioned areas were achieved. Mathematical modeling of the PEMFC simulated the FC response, providing insights into the activation kinetics of the fuel cell. The smallest PEMFC with an active area of 1.6x1.6 cm(2) was used to power an inductor coil (rated 130 mA, 150 mH, 8 Omega). This study can guide the development of FCs to power untethered devices.

Cell membrane potential affects the electrostatic self-assembly of magnetizable nanoparticles around the flagellum of sperm cells, leading to the formation of biohybrid microrobots (i.e., IRONSperm) with various bending stiffness. Here we explain the influence of bull sperm cell membrane potential on the formation of two types of IRONSperm samples that are produced by electrostatic self-assembly. The first type is a proximal-coated soft body with nanoparticles concentrated on the head to maintain high flexibility of the flagellum and create a passively propagating transverse bending wave under the influence of an external rotating magnetic field. The second type is a rigid-body with nanoparticles approximately uniformly distributed along the length to provide arbitrary geometry that maintains a constant chiral shape and propel by rotation about its long axis. We present a magneto-elastohydrodynamic model to predict the swimming speed at low Reynolds number for rigid IRONSperm with arbitrary shapes, and show that decreasing the bending stiffness allows the model to capture the behavior of its soft counterpart. While the response of a rigid chiral IRONSperm is distinguished by a greater swimming speed with a smooth decay with frequency, the benefit of a soft flagellum in certain scenarios would present a much smaller range of frequencies for wireless actuation.

Magnetic continuum manipulators (MCMs) are a class of continuum robots that can be actuated without direct contact by an external magnetic field. MCMs operating in confined workspaces, such as those targeting medical applications, require flexible magnetic structures that contain combinations of magnetic components and polymers to navigate long and tortuous paths. In cylindrical MCM designs, a significant trade-off exists between magnetic moment and bending flexibility as the ratio between length and diameter decreases. In this study, we propose a new MCM design framework that enables increasing diameter without compromising on flexibility and magnetic moment. Magnetic soft composite helices constitute bending regions of the MCM and are separated by permanent ring magnets. Local dipole interactions between the permanent magnets can reduce bending stiffness, depending on their size and spacing. For the particular segment geometry presented herein, the local dipole interactions result in a 31% increase in angular deflection of composite helices inside an external magnetic field, compared to helices without local interactions. In addition, we demonstrate fabrication, maneuverability, and example applications of a multisegment MCM in a phantom of the abdominal aorta, such as passing contrast dye and guidewires.

Magnetic nanoparticles can be electrostatically assembled around sperm cells to form biohybrid micro robots. These biohybrid microrobots possess sufficient magnetic material to potentially allow for pulse-echo localization and wireless actuation. Alternatively, magnetic excitation of these nanoparticles can be used for localization based on Faraday's law of induction using a detection coil. Here, we investigate the influence of the electrostatic attraction between positively charged nanoparticles and negatively charged sperm cells on the activation of the nanoparticles during nonlinear differential magnetometry and wireless magnetic actuation. Activation of clusters of free nanoparticles and nanoparticles bound to the body of sperm cells is achieved by a combination of a high- frequency alternating field and a pulsating static field. The nonlinear response in both cases indicates that constraining the nanoparticles is likely to yield significant decreases in the magnetometry sensitivity. While the attachment of particles to the cells enables wireless actuation (rolling locomotion), the rate of change of the magnetization of the nanoparticles decreases one order of magnitude compared to free nanoparticles.

Well-defined and technically relevant domain configurations are sought in patterned magnetic thin films. We used Magnetic Force Microscopy to investigate these in square-patterned Permalloy films. The films were prepared using dc sputter deposition by varying the Argon pressure from 1.5×10−3 to 30.0×10−3 mbar. The Landau domain configuration was found in films prepared at 1.5×10−3 mbar pressure. With an increase in pressure, tulip and irregular domains were consecutively formed. Based on magnetic and structural characterizations, an increase in coercivity and a decrease in Permalloy film density were observed at the same time.

We present a computational framework for two-scale asymptotic homogenization to determine the intrinsic magnetic permeability of composites. To this end, considering linear magnetostatics, both vector and scalar potential formulations are used. Our homogenization algorithm for solving the cell problem is based on the displacement method presented in Lukkassen et al. 1995, Composites Engineering, 5(5), 519-531. We propose the use of the meridional eccentricity of the permeability tensor ellipsoid as an anisotropy index quantifying the degree of directionality in the linear magnetic response. As application problems, 2D regular and random microstructures with overlapping and nonoverlapping monodisperse disks, all of which are periodic, are considered. We show that, for the vanishing corrector function, the derived effective magnetic permeability tensor gives the (lower) Reuss and (upper) Voigt bounds with the vector and scalar potential formulations, respectively. Our results with periodic boundary conditions show an excellent agreement with analytical solutions for regular composites, whereas, for random heterogeneous materials, their convergence with volume element size is fast. Predictions for material systems with monodisperse overlapping disks for a given inclusion volume fraction provide the highest magnetic permeability with the most increased inclusion interaction. In contrast, the disk arrangements in regular square lattices result in the lowest magnetic permeability and inadequate inclusion interaction. Such differences are beyond the reach of the isotropic effective medium theories, which use only the phase volume fraction and shape as mere statistical microstructural descriptors.

We optimized designs of permanent magnet systems to study the effect of magnetic nanoparticles on cell cultures in microslide channels. This produced two designs, one of which is based on a large cylindrical magnet that applies a uniform force over the channel area. We achieved a force uniformity of better than 14 %, leading to a concentration variation that was below our measurement resolution. The second design was aimed at achieving as high a force as possible by using a Halbach array. We indeed increased the force by more than one order of magnitude, but at the cost of uniformity. However, the latter system can be used to trap magnetic nanoparticles efficiently and to create concentration gradients. We demonstrated both designs by analyzing the effect of magnetic forces on the cell viability of human hepatoma HepG2 cells in the presence of bare Fe2O3 and cross-linked dextran iron-oxide cluster-type particles (MicroMod). Python scripts for magnetic force calculations and particle trajectory modeling as well as source files for 3D prints have been made available so these designs can be easily adapted and optimized for other geometries.

We investigate the predictive performance of specific analytical and numerical methods to determine the effective magnetic properties of two-phase steels at the macroscale. We utilize various mixture rules reported in the literature for the former, some of which correspond to rigorous bounds, e.g., Voigt (arithmetic) and Reuss (harmonic) averages. For the latter, we employ asymptotic homogenization together with the finite element method (FEM) and periodic boundary conditions (PBC). The voxel-based discretization of the representative volume element is conducted with digital image processing on the existing micrographs of DP600-grade steel. We show that unlike the considered isotropic mixture rules, which use only the phase volume fraction as the statistical microstructural descriptor, finite element method-based first-order asymptotic homogenization allows prediction of both phase content and directional dependence in the magnetic permeability by permitting an accurate consideration of the underlying phase geometry.

There is an increasing need for wireless autonomous micro electromechanical systems (MEMS) and microrobots that can perform various functions such as sensing, diagnosis, locomotion, actuation, implantation, material removal, manipulation, and localized drug delivery. A major problem with these systems is the production, storage, and transduction of power at the micro scale. In addition, these miniature devices cannot use existing battery packs that are commonly used to power electronic devices. These MEMS and microrobots need on-board power sources that are miniaturized to their size. Together with the energy of an external source, some basic functions of microrobots can be powered simultaneously. This study seeks to develop a theoretical framework based on a chemo-electromagnetic model for use in the design of microrobots with full energetic autonomy. We first conceive a microrobot design and derive its mathematical model; the design consists of an on-board fuel generator, electrochemical device, electromagnetic device, and a locomotion mechanism. Then we present numerical simulations to show the relationship between the consumption rate of the H 2 source, power density, and angular and translational velocities at low Reynolds number. We find that power density decreases approximately linearly with the diameter, while the relative velocity with respect to the body-length is approximately inversely proportional to the size, making downscaling favourable for this class of untethered devices.

Absolute position detection in sensorless electric stepper motors potentially allows for higher space efficiency, improved shock resistance, simplified installation, reduced number of parts and lowered cost. A prototype is demonstrated measuring 42 × 42 × 34 mm 3 with seven coils arranged in a star configuration. The rotor is ϕ 25.8 × 12.5 mm 2 and has 51 teeth which are irregularly spaced. At the driver side, the coil currents are measured during motion in order to reconstruct the absolute position of the motor. Calibration and smoothing techniques are used to reduce systematic and stochastic measurement errors, respectively. The motor is able to detect and correct its position after externally-induced stalls at the tested motor speeds from 40 rpm to 108 rpm. The holding torque is 0.23 N m at an armature current of 1 A; on average the torque is 7% lower than that of a reference bipolar stepper motor with the same dimensions. The results show that dynamic position sensing and correction are possible for a range of velocities, but not at standstill. The driver requires seven current sensors and sufficient computational power, and proper calibration of motor intrinsics is required beforehand. The presented technology could make existing 3-D printers and other machines with open-loop stepper motors more robust and increase the range of operating speeds and accelerations, without the adverse side-effects of increased complexity and cost associated with dedicated position sensors.

A novel principle of passive magnetic field shimming for permanent magnet assemblies is introduced, based on defined amounts of a ferrofluid suspension contained in a microliter volume plastic container that is accurately positioned within the magnet bore. The ferrofluid volume acts as a micromagnet that compensates in-homogeneities in the magnetic field profile. A proof of principle is shown for a permanent magnet assembly, derived from a Halbach design, with two additional movable rings of magnets. These rings result in an average magnetic field strength of 1.06 T inside the bore, 19 % higher than the Halbach alone. Two options for field shimming with ferrofluids are shown: changing the material volume or choosing a ferrofluid with different saturation magnetization value. With the tested simple single-cube ferrofluid implementation the field homogeneity is improved from 86 ppm to 8 ppm over a sampling length of 5.5 mm. Better homogeneity is expected with more sophisticated ferrofluid arrangements. The complete assembly has a size of 5 × 5 × 4.2 cm 3 and a weight of 332 g. The demonstrated concept is particularly attractive for mobile magnetic resonance systems, as it does not require electrical power during operation.

Cell membrane potential affects the electrostatic self-assembly of magnetizable nanoparticles around the flagellum of sperm cells, leading to the formation of biohybrid microrobots (i.e. IRONSperm) with various bending stiffness. Here we explain the influence of bull sperm cell membrane potential on the formation of two types of IRONSperm samples that are produced by electrostatic self-assembly. The first is a proximal-coated soft body with nanoparticles concentrated on the head to maintain high flexibility of the flagellum and create a passively propagating transverse bending wave under the influence of an external rotating magnetic field. The second is a rigid-body with nanoparticles approximately uniformly distributed along the length to provide arbitrary geometry that maintains a constant chiral shape and propel by rotation about its long axis. We present a magneto-elastohydrodynamic model to predict the swimming speed at low Reynolds number for rigid IRONSperm with arbitrary shapes, and show that decreasing the bending stiffness allows the model to capture the behavior of its soft counterpart. While the response of a rigid chiral IRONSperm is distinguished by a greater swimming speed with a smooth decay with frequency, the benefit of a soft flagellum in certain scenarios would present a much smaller range of frequencies for wireless actuation.

We present a spectrophotometer (optical density meter) combined with electromagnets dedicated to the analysis of suspensions of magnetotactic bacteria. The instrument can also be applied to suspensions of other magnetic cells and magnetic particles. We have ensured that our system, called MagOD, can be easily reproduced by providing the source of the 3D prints for the housing, electronic designs, circuit board layouts, and microcontroller software. We compare the performance of our system to existing adapted commercial spectrophotometers. In addition, we demonstrate its use by analyzing the absorbance of magnetotactic bacteria as a function of their orientation with respect to the light path and their speed of reorientation after the field has been rotated by 90°. We continuously monitored the development of a culture of magnetotactic bacteria over a period of 5 days and measured the development of their velocity distribution over a period of one hour. Even though this dedicated spectrophotometer is relatively simple to construct and cost-effective, a range of magnetic field-dependent parameters can be extracted from suspensions of magnetotactic bacteria. Therefore, this instrument will help the magnetotactic research community to understand and apply this intriguing micro-organism.

Individual biohybrid microrobots have the potential to perform biomedical in vivo tasks such as remote-controlled drug and cell delivery and minimally invasive surgery. This work demonstrates the formation of biohybrid sperm-templated clusters under the influence of an external magnetic field and essential functionalities for wireless actuation and drug delivery. Ferromagnetic nanoparticles are electrostatically assembled around dead sperm cells, and the resulting nanoparticle-coated cells are magnetically assembled into three-dimensional biohybrid clusters. The aim of this clustering is threefold: First, to enable rolling locomotion on a nearby solid boundary using a rotating magnetic field; second, to allow for noninvasive localization; third, to load the cells inside the cluster with drugs for targeted therapy. A magneto-hydrodynamic model captures the rotational response of the clusters in a viscous fluid, and predicts an upper bound for their step-out frequency, which is independent of their volume or aspect ratio. Below the step-out frequency, the rolling velocity of the clusters increases nonlinearly with their perimeter and actuation frequency. During rolling locomotion, the clusters are localized using ultrasound images at a relatively large distance, which makes these biohybrid clusters promising for deep-tissue applications. Finally, we show that the estimated drug load scales with the number of cells in the cluster and can be retained for more than 10 h. The aggregation of microrobots enables them to collectively roll in a predictable way in response to an external rotating magnetic field, and enhances ultrasound detectability and drug loading capacity compared to the individual microrobots. The favorable features of biohybrid microrobot clusters place emphasis on the importance of the investigation and development of collective microrobots and their potential for in vivo applications.

In this work an optical and inductive calibration procedures for calibrating multi-material fused filament fabrication 3D printers in the x and y directions is presented. The inductive calibration is based on the principle that the inductance of a detection coil placed on the print bed changes when the (metallic) extrusion nozzle passes it. This calibration method shows a repeatability of up to 2µm. To determine the accuracy of this calibration method, another calibration method is proposed that directly measures the position of the deposited material. During this alternative process, a calibration structure is printed on an A4-format sheet of paper using every tool. The paper is subsequently scanned using a digital scanner, and the resulting image is analyzed using an image-processing script. Using this method as a reference, it was determined that the inductive method has an accuracy of approximately 45µm. For applications where this accuracy is sufficient, the inductive method provides a fast solution that requires little to no user interaction. For more demanding applications, the optical calibration might be the better choice, since it is more time-consuming but yields a more accurate solution. It is expected that the accurate calibration of tool offsets will reduce both the chance of poor adhesion between materials and the mixing of filaments due to local over- and under-extrusion at material interfaces.