Magnetomyography (MMG) holds great potential as a contactless alternative to electromyography (EMG). However, the spatiotemporal dynamics of the myomagnetic field in vivo remain largely unknown, limiting signal interpretability and hence the use of MMG. Here, the myomagnetic field of the abductor digiti minimi muscle is measured in vivo at 69-by-2 effective positions by scanning a grid of six triaxial optically pumped magnetometers (OPMs) along the muscle, while reproducible muscle contractions are evoked electrically. In this way, the compound muscle action field (CMAF) is measured with high density (1 mm sampling steps) and its evolution is referenced to the underlying anatomy based on 3D scans and magnetic resonance images. A model is used to simulate the expected signals. It suggests that the clinically relevant innervation zone (IZ) manifests as a zero-crossing of the azimuthal CMAF component. Based on this, contactless IZ localization is demonstrated in three subjects by projecting the zero-crossing onto the skin. The estimated IZ localizations deviate by 1.7 mm to 4.1 mm from estimates with simultaneously recorded 64-channel surface EMG. With the first millimeter-scale spatiotemporal mapping of magnetic muscle activity in vivo, this study paves the way for the use of MMG in biomedical applications.

ZusammenfassungOptische Magnetometer sind eine neue Klasse hochempfindlicher Quantensensoren. Sie werden bereits in der Größe einer Streichholzschachtel hergestellt und können Magnetfeldänderungen messen, die milliardenfach geringer als das Erdmagnetfeld sind. Sie werden eingesetzt, um die bioelektrischen Signale im menschlichen Körper magnetisch – und somit nichtinvasiv und prinzipiell berührungslos – zu messen. Am weitesten fortgeschritten ist ihr Einsatz in flexiblen Multisensorsystemen für die Magnetoenzephalographie. Darüber hinaus wird ihr Einsatz auch für Muskelsignalmessungen (Magnetomyographie) oder zur Defektanalyse von Batterien erforscht.

Objective. Mapping the myomagnetic field of a straight and easily accessible muscle after electrical stimulation using triaxial optically pumped magnetometers (OPMs) to assess potential benefits for magnetomyography (MMG). Approach. Six triaxial OPMs were arranged in two rows with three sensors each along the abductor digiti minimi (ADM) muscle. The upper row of sensors was inclined by 45 degrees with respect to the lower row and all sensors were aligned closely to the skin surface without direct contact. Then, the electromagnetic muscle activity was electrically evoked utilizing stepwise increasing currents at the cubital tunnel at the ulnar nerve. Evoked myomagnetic activity was recorded with 18 channels, three per sensor. As the measurements were performed in PTB's magnetically shielded room (BMSR-2) no averaging and only moderate filtering was applied. Main results. The myomagnetic vector field could be successfully mapped. The obtained spatial structure with a radial symmetry corresponds to the expectations from the ADM's parallel muscle architecture. The temporal evolution exhibits an up to four-phasic shape. Implications for future experiments are derived and needs for sensor performance improvements are identified. Significance. The use of an OPM array with small (similar to 3 mm edge length) sensing voxels enabled the mapping of the magnetic vector field of the ADM. This allowed visualization of the spatiotemporal evolution of the muscle's evoked magnetic field and gives implications for future experiments. In the future, high density OPM grids may enable high-accuracy determination of muscle parameters such as innervation zone position, pennation angle, and propagation velocities.

Biomagnetic measurements of muscles have increasingly gained interest in recent years. While before hardly any sensors other than superconducting quantum interference devices (SQUIDs) could be considered for non-invasive biomagnetic measurements, since a few years ago, zero-field optically pumped magnetometers (ZF-OPMs) are available as small, flexibly placeable and highly sensitive magnetic field sensors, opening up new opportunities for magnetomyography (MMG). But the benefits come with challenges, as the comparatively fast muscular signals demand a higher bandwidth than today's commercial ZF-OPMs can offer, while still requiring high sensitivity, a small detecting volume, and vectorial measurement capability. In this study, we consider MMG measurements with a commercial zero-field OPM. As exemplary MMG signal, we selected the abductor digiti minimi (ADM) muscle of the hand, which was electrically stimulated at the ulnar nerve. Although we see only slight distortions of the MMG signal with the commercial ZF-OPM in this case, when considering faster MMG signals, stronger distortion and higher directional uncertainty limit the performance of MMG with current commercial ZF-OPMs, as our analysis reveals. To this end, we present measurements of fast artificial MMG signals measured with an in-house developed OPM with an extended bandwidth of 1 kHz but reduced sensitivity.

While magnetomyography (MMG) using optically pumped magnetometers (OPMs) is a promising method for non-invasive investigation of the neuromuscular system, it has almost exclusively been performed in magnetically shielded rooms (MSRs) to date. MSRs provide extraordinary conditions for biomagnetic measurements but limit the widespread adoption of measurement methods due to high costs and extensive infrastructure. In this work, we address this issue by exploring the feasibility of mobile OPM-MMG in a setup of commercially available components. From field mapping and simulations, we find that the employed zero-field OPM can operate within a large region of the mobile shield, beyond which residual magnetic fields and perturbations become increasingly intolerable. Moreover, with digital filtering and moderate averaging a signal quality comparable to that in a heavily shielded MSR is attained. These findings facilitate practical and cost-effective implementations of OPM-MMG systems in clinical practice and research.

Results are reported from the first full-scale search for transient signals from exotic fields of astrophysical origin using data from a newly constructed Earth-scale detector: the Global Network of Optical Magnetometers for Exotic physics searches (GNOME). Data collected by the GNOME consist of correlated measurements from optical atomic magnetometers located in laboratories all over the world. GNOME data are searched for patterns of signals propagating through the network consistent with exotic fields composed of ultralight bosons such as axion-like particles (ALPs). Analysis of data from a continuous month-long operation of the GNOME finds no statistically significant signals consistent with those expected due to encounters with topological defects (axion domain walls), placing new experimental constraints on such dark matter scenarios.

Magnetic nanoparticles (MNP) offer a large variety of promising applications in medicine thanks to their exciting physical properties, e.g., magnetic hyperthermia and magnetic drug targeting. For these applications, it is crucial to quantify the amount of MNP in their specific binding state. This information can be obtained by means of magnetorelaxometry (MRX), where the relaxation of previously aligned magnetic moments of MNP is measured. Current MRX with optically pumped magnetometers (OPM) is limited by OPM recovery time after the shut-off of the external magnetic field for MNP alignment, therewith preventing the detection of fast relaxing MNP. We present a setup for OPM-MRX measurements using a commercially available pulsed free-precession OPM, where the use of a high power pulsed pump laser in the sensor enables a system recovery time in the microsecond range. Besides, magnetometer raw data processing techniques for Larmor frequency analysis are proposed and compared in this paper. Due to the high bandwidth (≥100 kHz) and high dynamic range of our OPM, a software gradiometer in a compact enclosure allows for unshielded MRX measurements in a laboratory environment. When operated in the MRX mode with non-optimal pumping performance, the OPM shows an unshielded gradiometric noise floor of about 600 fT/cm/Hz for a 2.3 cm baseline. The noise floor is flat up to 1 kHz and increases then linearly with the frequency. We demonstrate that quantitative unshielded MRX measurements of fast relaxing, water suspended MNP is possible with the novel OPM-MRX concept, confirmed by the accurately derived iron amount ratios of MNP samples. The detection limit of the current setup is about 1.37 μg of iron for a liquid BNF-MNP-sample (Bionized NanoFerrite) with a volume of 100 μL.

Ultralight bosons such as axion-like particles are viable candidates for dark matter. They can form stable, macroscopic field configurations in the form of topological defects that could concentrate the dark matter density into many distinct, compact spatial regions that are small compared with the Galaxy but much larger than the Earth. Here we report the results of the search for transient signals from the domain walls of axion-like particles by using the global network of optical magnetometers for exotic (GNOME) physics searches. We search the data, consisting of correlated measurements from optical atomic magnetometers located in laboratories all over the world, for patterns of signals propagating through the network consistent with domain walls. The analysis of these data from a continuous month-long operation of GNOME finds no statistically significant signals, thus placing experimental constraints on such dark matter scenarios.

We present a fast and robust optically pumped magnetometer that is based on a feedback-controlled spin ensemble of cesium atoms in spin-polarized vapor. The table-top system is intended for operation in unshielded environment, and its design allows conversion into a handheld sensor head. Under strongly disturbed environmental conditions in the laboratory, the sensor exhibits a speed of more than 56 kHz, while having a slew rate of 39 mT/s and a full dynamic range of 10 - 120 mu T. Under these conditions a sensitivity of 33 pT/root Hz is reached. By reducing the speed to 3.6 kHz the sensitivity can be improved to 6 pT/root Hz, which is close to the shot noise limit. We describe the sensor design and its optimization and demonstrate the performance of the sensor under conditions appropriate for magnetic susceptometry measurements.

The Global Network of Optical Magnetometers for Exotic physics searches (GNOME) is a network of time-synchronized, geographically separated, optically pumped atomic magnetometers that is being used to search for correlated transient signals heralding exotic physics. GNOME is sensitive to exotic couplings of atomic spins to certain classes of dark matter candidates, such as axions. This work presents a data analysis procedure to search for axion dark matter in the form of topological defects: specifically, walls separating domains of discrete degenerate vacua in the axion field. An axion domain wall crossing the Earth creates a distinctive signal pattern in the network that can be distinguished from random noise. The reliability of the analysis procedure and the sensitivity of the GNOME to domain-wall crossings are studied using simulated data.

We introduce a Magnetic Particle Imaging Susceptometer (MPIS) that uses a high-sensitivity atomic magnetometer (AM) for recording the spatial distribution of fluid-suspended magnetic nanoparticles. We have evaluated the MPIS performance by one-dimensional scans of structured nanoparticle phantoms, demonstrating, in particular, resolutions of ≈2.5 mm prior to deconvolution and << 1 mm after deconvolution. Our instrument conceptually follows the general principle of Magnetic Particle Imaging (MPI) for encoding spatial distributions into magnetic flux density variations. Conversely to previously demonstrated MPI methods, MPIS works in time-space by recording time series of the sample's magnetic response including all Fourier components. The device deploys a specifically designed system of coils, a low-frequency excitation scheme, and a simple source localization algorithm. The difference of the AM's frequency response with respect to the conventional receive coil detection allows us to work at much lower driving frequencies. We demonstrate operation at frequencies on the order of 100 Hz, enabling the beneficial use of larger nanoparticles. The spatial distribution encoded into the particles' susceptibility needs a much lower excitation field amplitude compared to conventional MPI scanners. These two features make MPIS least harmful for biological samples and subjects compared to conventional MPI scanners. We also address performance characteristics and other possible applications of MPIS.

With the emergence of environmental, biomedical, and medical monitoring technologies, development of flexible and lightweight sensors is ongoing. This work presents a flexible lightweight bilayer (BL) film (polycarbonate/polycrystalline layer of crystalline (BEDT-TTF)(x)Br(H2O) y salts [BEDT-TTF = bis (ethylendithio)tetrathiafulvalene]) as a promising material for humidity sensing which may be applied in a number of monitoring scenarios. X-ray structural characterization revealed the presence of three different crystal types in the crystalline layer of the BL film, i.e., [phase I, [(BEDT-TTF)(5) (Br-4(H5O2))]; phase II, [(BEDT-TTF)(2)Br(H2O)(3)]; and crystals of neutral BEDT-TTF compound], showing that phase I is highly sensitive to humidity. The humidity testing of the BL film showed that it is capable of monitoring relative humidity (RH) levels from 15 up to 90% with a well-defined and reproducible electrical signal. Electrical resistance measurements revealed that the crystalline conducting layer can absorb moisture reaching equilibrium at constant RH as reflected in a stable relative resistance response. The structural response of the BL film to variations of RH clearly demonstrated that crystallite interlayer spacing (d) of phase I is strongly affected, exhibiting a reversible metal-nonmetal transition, while phase 11 was insensitive to humidity. An overview of mechanical and humidity sensing properties of the developed BL film corroborates that it can be used as flexible hygrometer as well as moisture sensing units on the board of low-cost electronic sensing devices.

The Global Network of Optical Magnetometers to search for Exotic physics (GNOME) is a network of geographically separated, time-synchronized, optically pumped atomic magnetometers that is being used to search for correlated transient signals heralding exotic physics. The GNOME is sensitive to nuclear- and electron-spin couplings to exotic fields from astrophysical sources such as compact dark-matter objects (for example, axion stars and domain walls). Properties of the GNOME sensors such as sensitivity, bandwidth, and noise characteristics are studied in the present work, and features of the network's operation (e.g., data acquisition, format, storage, and diagnostics) are described. Characterization of the GNOME is a key prerequisite to searches for and identification of exotic physics signatures.

We have performed quantitative measurements of the variation of the on-resonance absorption coefficients k(0) of the four hyperfine components of the Cs D-1 transition as a function of laser power P, for pumping with linearly and with circularly polarized light. Sublevel populations derived from rate equations assuming isotropic population relaxation (at a rate gamma(1)) yield algebraic k(0)(P) dependences that do not reproduce the experimental findings from Cs vapor in a paraffin- coated cell. However, numerical results that consider spin-exchange relaxation (at a rate gamma(se)) and isotropic relaxation fit the experimental data perfectly well. The fit parameters, viz., the absolute value of k(0), the optical pumping saturation power Psat, and the ratio gamma(se)/gamma(1), are well described by the experimental conditions and yield absolute values for gamma(1) and gamma se. The latter is consistent with the previously published Cs-Cs spin-exchange relaxation cross section.

This study is stimulated by the discovery of high sensitivity of nanostructured layers of organic semiconductor alpha'-BEDT-TTF)2I(x)Br(3-x) [BEDT-TTF = bis(ethylendithio)-tetrathiafulvalene] to heat radiation. We present the development and assessment of the flexible lightweight highly sensitive film-based thermistor as (i) a separate sensor, (ii) a sensor integrated in e-textile and (iii) a sensor embedded in a wireless sensor node. Wireless Sensor Networks (WSN) and Internet of Things (loT), being two promising technologies, have already been applied in a number of monitoring scenarios. In spite of great progress achieved in sensing technologies and wireless embedded systems there is a gap in multidisciplinary research aimed at investigating the aggregate potential of these technologies. Experimental results demonstrate that the developed bi-layer organic thermistor has high potential for environmental and biomedical monitoring. They can be used as a part of wearable units or as sensing units on board of wireless sensing devices. (C) 2016 Elsevier B.V. All rights reserved.

We report on our progress in the development of an atomic magnetometer (AM) based low-frequency magnetic particle imaging (MPI) scanner, expected to be free from Specific Absorption Rate (SAR) and Peripheral Nerve Stimulation (PNS) constraints. We address major challenges in coil and sensor design due to specific AM properties. Compared to our previous work we have changed the AM's mode of operation towards its implementation for detecting weak magnetic nanoparticles (MNP) response fields in the presence of nearby-located strong drive/selection fields. We demonstrate that a pump-probe AM scheme in a buffer gas filled alkali vapour cell can tolerate mT/m gradients while maintaining a sensitivity in the one-digit pT/Hz^(1/2) range over a bandwidth from DC to several kHz. We give a detailed description of the drive/selection coils' geometry and their hardware implementations that provides a field-free-line (FFL) operation, compatible with a best performance AM operation. We estimate the achievable field of view and spatial resolution of the scanner as well as its sensitivity, assuming mechanical scanning of a Resovist sample through the field-free point/line.

We have investigated the dependence of lock-in-demodulated $$ {M}_x $$ -magnetometer signals on the orientation of the static magnetic field $$ {\mathbf{B}}_0 $$ of interest. Magnetic resonance spectra for 2400 discrete orientations of $$ {\mathbf{B}}_0 $$ covering a $$ 4\pi $$ solid angle have been recorded by a PC-controlled steering and data acquisition system. Off-line fits by previously derived lineshape functions allow us to extract the relevant resonance parameters (shape, amplitude, width, and phase) and to represent their dependence on the orientation of $$ {\mathbf{B}}_0 $$ with respect to the laser beam propagation direction. We have performed this study for two distinct $$ {M}_x $$ -magnetometer configurations, in which the rf-field is either parallel or perpendicular to the light propagation direction. The results confirm well the algebraic theoretical model functions. We suggest that small discrepancies are related to hitherto uninvestigated atomic alignment contributions.

We review our ongoing work on deploying optical (atomic) magnetometry for measuring the magnetic response of magnetic nanoparticle (MNP) samples, yielding MNP size distributions, and other sample parameters like Néel relaxation time τ, saturation magnetisation Ms, anisotropy constant K and magnetic susceptibility χ. We address magnetorelaxation (MRX) signals, in which the decaying magnetisation M(t) following a magnetising pulse is recorded by a single atomic magnetometer or by a novel magnetic source imaging camera (MSIC) allowing spatially resolved MRX studies of distributed MNP samples. We further show that optical magnetometers can be used for a direct measurement of the M(H) and dM/dH(H) dependencies of MNP samples, the latter forming the basis for an optical magnetometer implementation of the MPI (Magnetic Particle Imaging) method. All experiments are in view of developing biomedical imaging modalities.