Science News

In the future, highly-sensitive sensors could be able to detect magnetic signals from the body in order to draw conclusions on heart or brain functions. In contrast with established electrical measurement techniques, they would achieve contactless measurement, i.e. without direct skin contact. At present, such measurements are still associated with considerable expense and effort. This is because the sensors must be cooled dramatically, or shielded against other magnetic fields. Now, researchers at Kiel University built an important basis for biomagnetic diagnostics. In the Collaborative Research Center (CRC) 1261 "Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics", they are researching the development of magnetic field sensors, which in the long-term - with better spatial resolution - could be easily put to use in medical practice. The interdisciplinary research team developed a magnetic field sensor system that not only includes the detection of a magnetic signal, but also its processing. The researchers presented their results in the journal Scientific Reports.

Full press release can be found at: http://www.uni-kiel.de/pressemeldungen

Our new phase noise analyzer FSWP from Rohde & Schwarz has just arrived. With this measurement device the phase noise (and also the amplitude noise) of both one-port and two-port devices under test (DUT) can be measured. One-port measurements are particularly necessary for the noise characterization of the oscillators used for the excitation of our sensors. However, the main feature of this device is the additional radio frequency (RF) source for so-called additive phase noise measurements, e.g. two-port measurements. Thus, we are now able to comprehensively analyze our new surface acoustic wave (SAW) magnetic field sensors [Kit 2018] regarding their noise behaviour. Last but not least the FSWP greatly supports the development of low-noise sensor electronics.

 

[Kit 2018] A. Kittmann, P. Durdaut, S. Zabel, J. Reermann, J. Schmalz, B. Spetzler, D. Meyners, N. X. Sun, J. McCord, M. Gerken, G. Schmidt, M. Höft, R. Knöchel, F. Faupel, and E. Quandt: Wide Band Low Noise Love Wave Magnetic Field Sensor System; Scientific Reports, vol. 8, no. 278, January 2018; http://dx.doi.org/10.1038/s41598-017-18441-4

A highlight, especially for the team of the projects B2 and B6 of the CRC 1261, was the magnetic measurement of nerve signals with a 304 SQUID vector magnetometer at the PTB in Berlin. For further development and also for optimization of our uncooled magnetoelectric (ME) sensors, a better understanding of spectral power distribution and signal strength of nerve signals is of particular interest. Since the magnetic field of human nerve pulses is quite low, only signal amplitudes in the fT range from the deep nerve are measurable. The project B6 intensively prepared these measurements, since an earlier attempt at measuring the signals had completely failed. Finally, Christin Bald and Eric Elzenheimer succeeded in measuring nerve signals magnetically, which also fits to the current electrical gold standard (electroneurography). Signal amplitudes were subject dependent and ranged from 17 fT to 60 fT in a frequency range from 100 Hz to 1 kHz. The required averaging time was in the range of minutes, while for current ME sensors significantly longer averaging times are expected to be necessary.

 

Ron-Marco Friedrich recently reported on great progress the project project B7. Here, the aim is the detection of magnetically labeled cells for biomaterial scaffold characterization and first measurements of the magnetic field and localization of magnetic nanoparticles were successful. For the measurement, an ME sensor samples space over a surface with magnetic nanoparticles (Fig. 1). The magnetic field is measured and the inverse problem is solved to localize the sources of that magnetic field (Fig. 2).

Fig. 1: Sampling of the surface with magnetic nanoparticles using an ME sensor. The magnetic moment is aligned with the AC magnetic field.

 

   
Fig. 2: a) Reference sample with magnetic nanoparticles (red), b) Measured magnetic field (red – positive, blue - negative), c) Reconstructed sources of the magnetic field.

Good news from our central project Z2. Alexander Teplyuk (project Z2) has finished a new scheme for mounting and characterization of our cantilever-based ME sensors. Instead of of permanenty fixing the cantilevers on a mouning plate (as we did it so far), we can contact the cantilevers now in a non-permanent manner. This allows to adjust the resonance frequency of our sensors in an easy way. Consequently, sensor characterisation with a predetermined center frequency is possible now.

The video above shows how easy it is to install and contact the ME cantilevers with the new measurement setup. By using a simple torque wrench, it is possible to ajust the claming force which dircetly influcenes the resonance frequency. This allows to characterize a large set of sensors with exactly the same resonance behaviour.

Alexander Teplyuk (project Z2) had improved the head scanner with respect to scanning speed, robustness, and saftety means. It is ready now to be used for patient measurements. First evaluations have been performed in close cooperation with our medical project B5.

The scanner has reached the next level ...

 

Furthermore, a head phantom for emulation of a network of connected sources in the brain has been designed in close cooperation with the rearchers from project B3. Small coils can be arbitrarily placed at designated postions. This allows us to generate a variety of source configurations and we are able now to create the corresponding magentic fields on the surface of our artifical head.

 
  Schematic of an magnetoelectric cantilever sensor the magnified region represents the layer stacking of piezolectric (AlN), and the self biased magnetostrictive stack (Ta, Cu, MnIr, FeCoSiB). The arrow represents the bias field orientation.

For his work about magnetoelectric sensors in medical engineering Dr. Enno Lage, post doc at Kiel University, received an award for young researchers from the German Society for Materials Science (DGM, Deutsche Gesellschaft für Materialkunde). The prize was awarded at the annual conference of the DGM in Darmstadt on September 26th 2016. It honors outstanding achievements of young researchers in material science.

During his PhD, Enno Lage conducted his research on highly sensitive magnetoelectric sensors within the collaborative research center “Magnetoelectric Composites” at the Faculty of Engineering.

Those sensors show their highest sensitivity in presence of well-defined magnetic bias fields. For vector sensors and in densely arranged sensor-arrays each component needs an individually oriented bias field. Thus, the sources of bias fields, either generated with electromagnetic coils or with permanent magnets, are detrimental in terms of miniaturization.

In order to overcome these limitations, the researchers utilized the exchange bias effect which is well established in magneto-resistive sensing. The challenge in their approach was the precise adjustment of the bias field. The orientation of the internal bias field is crucial for the sensitivity. If one would simply substitute the external bias field by an internal field with same orientation and strength, the sensors would show a vanishingly small response and hence an alternative orientation had to be considered. Additionally, if the field is too large the sensitivity decreases, if it is too small the sensor behaves partially unbiased.

The followed approach led to the successful realization of self-biased magnetoelectric sensors and was suitable to combine sensor elements for vector sensing.

Corresponding publication:

Lage, E., Kirchhof, C., Hrkac, V., Kienle, L., Jahns, R., Knöchel, R., Quandt, E. and Meyners, D., 2012. Exchange biasing of magnetoelectric composites. Nature materials, 11(6), pp.523-529.

Lage, E., Woltering, F., Quandt, E. and Meyners, D., 2013. Exchange biased magnetoelectric composites for vector field magnetometers. Journal of Applied Physics, 113(17), p.17C725.

Lage, E., Urs, N.O., Röbisch, V., Teliban, I., Knöchel, R., Meyners, D., McCord, J. and Quandt, E., 2014. Magnetic domain control and voltage response of exchange biased magnetoelectric composites. Applied Physics Letters, 104(13), p.132405.

Today (September 15th, 2016) the inventors Robert Jahns, Holger Runkowske, and Reinhard Knöchel were informed from the European Patent Office that their idea on the "tuning fork" sensor principle is protected now by the European patent EP 2 811 314 B1. Congratulations to the inventors!

A "tuning fork" sensor basically consists of two ME sensors, which are arranged on top and at the bottom of a mounting block. The inverse orientation of the individual sensors with respect to the suspension point (FR-4 substrate in the picture below) leads to distinguishable output signals for magnetic and vibrational excitations. If a magnetic field is applied to the tuning fork, the cantilevers are bent in opposite directions (e.g. both away from the centre, black arrows). The outputs of the upper and lower cantilevers are opposite in phase with respect to a common ground and thus produce a differential-mode signal. In the case of vibrational coupling, the cantilevers are predominantly bent in the same direction (green arrows) and produce co-phase signals of the two sensor outputs (common mode). The comparison of a tuning fork sensor with a single cantilever sensor (see figure below) reveals that the tuning fork shows a limit of detection of approximately 500 fT/Hz1/2 – a very good LOD for magnetoelectric thin film sensors - whereas the individual magnetoelectric cantilevers, similar to those of which the tuning fork is composed, have sensitivities of approximately 5 pT/Hz1/2. With superimposed white noise the effect of the tuning fork is even more distinct. Whereas the tuning fork experiences an increase in noise level of about a factor of roughly 4, the single magnetoelectric sensor shows a rise of approximately two decades.

   
Magnetoelectric tuning fork sensor (a). LOD plots of tuning fork setup (c) in comparison to a single sensor (b) with and without additional superimposed wideband noise. The dashed auxiliary lines indicate the noise level and the LOD. The resonance frequency was 958 Hz.

 

Contact

sfb1261@tf.uni-kiel.de

Chairman:

Prof. Dr. Eckhard Quandt

Kiel University
Institute for Materials Science

 

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