Determination of carbon nanotubes concentration by tabletop dynamic susceptometer.

Carbon nanotubes (CNTs) can be incorporated in various materials to enhance their mechanical or electrical properties. Information on their precise concentration and local distribution is difficult to access non-invasively. For example, electron microscopy studies require cutting of samples. Another way to measure the concentration of CNTs is by the magnetic susceptibility of the ferrocene present in the CNTs by the synthesis process, which can be performed on sample coupons on a vibrating sample magnetometer (VSM); VSM is a bulky laboratory instrument, and the size of the samples studied is constrained. In order to provide a technique that is fast, easy, cheap, and adaptable to the size of the samples, we have developed a benchtop device that measures the CNT concentration through an original inductive dynamic measurement of the ferrocene magnetic susceptibility. We present the method for extracting CNT concentrations and show the results obtained on cement matrices with CNT concentrations of the order of a few percent.

In vivo magnetic recording of single-neuron action potentials.

Measuring fast neuronal signals is the domain of electrophysiology and magnetophysiology. While electrophysiology is much easier to perform, magnetophysiology avoids tissue-based distortions and measures a signal with directional information. At the macroscale, magnetoencephalography (MEG) is established, and at the mesoscale, visually evoked magnetic fields have been reported. At the microscale however, while benefits of recording magnetic counterparts of electric spikes would be numerous, they are also highly challenging in vivo. Here, we combine magnetic and electric recordings of neuronal action potentials in anesthetized rats using miniaturized giant magneto-resistance (GMR) sensors. We reveal the magnetic signature of action potentials of well isolated single units. The recorded magnetic signals showed a distinct waveform and considerable signal strength. This demonstration of in vivo magnetic action potentials opens a wide field of possibilities to profit from the combined power of magnetic and electric recordings and thus to significantly advance the understanding of neuronal circuits.

Proof of concept of a two-stage GMR sensor-based lab-on-a-chip for early diagnostic tests.

The development of rapid, sensitive, portable and inexpensive early diagnostic techniques is a real challenge in the fields of health, defense and in the environment. The current global pandemic has also shown the need for such tests. The World Health Organization has defined ASSURED criteria (affordable, sensitive, specific, user-friendly, rapid and robust, equipment-free and deliverable to end-users) that field diagnostic tests must fulfill, which proves the real need in terms of public health. Giant magnetoresistance (GMR) sensors, which have flourished in a wide variety of spintronic applications (automobile industry, Information Technology, etc.), also have real potential in the field of health, particularly for the development of early diagnostic point-of-care devices. This work presents a new type of innovative biochip, consisting of GMR sensors arranged on both sides of a microfluidic channel which allow on the one hand to count magnetic objects one by one but also to better distinguish false positives (aggregates of beads, etc.) from labelled biological targets of interest by determining their magnetic moment. We present the operating principle of this new tool and its great potential as a versatile diagnostic test.

Magnetoresistive Sensor in Two-Dimension on a 25 µm Thick Silicon Substrate for In Vivo Neuronal Measurements.

Neuronal electrical activity is widely studied in vivo, and the ability to measure its magnetic equivalent to obtain an undisturbed signal with both amplitude and direction information leading to neuronal signal mapping would be a promising tool for neuroscience. To provide such a tool, a probe with spin-electronics-based magnetic sensors with orthogonal axes of sensitivity for two directions of measurement is realized, thanks to a local magnetization re-orientation technique induced by Joule heating. This probe is tested under in vivo measurement conditions in the brain of an anesthetized rat. To be as close as possible to neurons and to create minimal damage during the probe's insertion, the tip thickness has been drastically decreased using a silicon-on-insulator substrate. Our probes provide the ability to perform in vivo magnetic measurements on two orthogonal axes on a 25 µm thick silicon tip with a sensitivity of 1.7%/mT along one axis and 0.9%/mT along the perpendicular axis in the sensor plane, for a limit of detection at 1 kHz of 1.0 and 1.3 nT, respectively. These probes have been tested through a phantom study and during an in vivo experiment. The robustness and stability over one year are demonstrated.

Evaluation of In-Flow Magnetoresistive Chip Cell-Counter as a Diagnostic Tool.

Inexpensive simple medical devices allowing fast and reliable counting of whole cells are of interest for diagnosis and treatment monitoring. Magnetic-based labs on a chip are one of the possibilities currently studied to address this issue. Giant magnetoresistance (GMR) sensors offer both great sensitivity and device integrability with microfluidics and electronics. When used on a dynamic system, GMR-based biochips are able to detect magnetically labeled individual cells. In this article, a rigorous evaluation of the main characteristics of this magnetic medical device (specificity, sensitivity, time of use and variability) are presented and compared to those of both an ELISA test and a conventional flow cytometer, using an eukaryotic malignant cell line model in physiological conditions (NS1 murine cells in phosphate buffer saline). We describe a proof of specificity of a GMR sensor detection of magnetically labeled cells. The limit of detection of the actual system was shown to be similar to the ELISA one and 10 times higher than the cytometer one.

In Vivo Magnetic Recording of Neuronal Activity.

Neuronal activity generates ionic flows and thereby both magnetic fields and electric potential differences, i.e., voltages. Voltage measurements are widely used but suffer from isolating and smearing properties of tissue between source and sensor, are blind to ionic flow direction, and reflect the difference between two electrodes, complicating interpretation. Magnetic field measurements could overcome these limitations but have been essentially limited to magnetoencephalography (MEG), using centimeter-sized, helium-cooled extracranial sensors. Here, we report on in vivo magnetic recordings of neuronal activity from visual cortex of cats with magnetrodes, specially developed needle-shaped probes carrying micron-sized, non-cooled magnetic sensors based on spin electronics. Event-related magnetic fields inside the neuropil were on the order of several nanoteslas, informing MEG source models and efforts for magnetic field measurements through MRI. Though the signal-to-noise ratio is still inferior to electrophysiology, this proof of concept demonstrates the potential to exploit the fundamental advantages of magnetophysiology.

Local recording of biological magnetic fields using Giant Magneto Resistance-based micro-probes.

The electrical activity of brain, heart and skeletal muscles generates magnetic fields but these are recordable only macroscopically, such as in magnetoencephalography, which is used to map neuronal activity at the brain scale. At the local scale, magnetic fields recordings are still pending because of the lack of tools that can come in contact with living tissues. Here we present bio-compatible sensors based on Giant Magneto-Resistance (GMR) spin electronics. We show on a mouse muscle in vitro, using electrophysiology and computational modeling, that this technology permits simultaneous local recordings of the magnetic fields from action potentials. The sensitivity of this type of sensor is almost size independent, allowing the miniaturization and shaping required for in vivo/vitro magnetophysiology. GMR-based technology can constitute the magnetic counterpart of microelectrodes in electrophysiology, and might represent a new fundamental tool to investigate the local sources of neuronal magnetic activity.

Femtotesla magnetic field measurement with magnetoresistive sensors.

The measurement of magnetic fields in the femtotesla (fT, 10(-15) tesla) range is important for applications such as magnetometry, quantum computing, solid-state nuclear magnetic resonance, and magnetoencephalography. The only sensors capable of detecting these very small fields have been based on low-temperature superconducting quantum interference devices operating at 4.2 kelvin. We present a magnetic field sensor that combines a superconducting flux-to-field transformer with a low-noise giant magnetoresistive sensor. The sensor can be operated up to 77 kelvin. Our small-size prototype provides the capability of measuring 32 fT.

Micromagnetic phase transitions and spin wave excitations in a ferromagnetic stripe.

Magnetic excitations of micrometer-wide ferromagnetic stripes subjected to a transverse applied field have been measured between 1 and 20 GHz. The complexity of the observed response is attributed to the spatially nonuniform equilibrium spin distribution. This one is modeled analytically and numerically, which allows one to distinguish two micromagnetic phases governing the ground state. The nucleation-related phase transitions are evidenced by soft modes, while the different observed resonances are attributed to spin wave modes localized in the two phases and at their interface.