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”It can be used for many different applications“

Miniature Sensors: ”It can be used for many different applications“


Photo: Franz Faupel

Professor Franz Faupel; ©

Tracking magnetic particles through the human body – this is one potential application for a new sensor that was developed by Professor Franz Faupel and his team at the Christian Albrecht University of Kiel (CAU) in Germany. In a conversation with the project manager, found out what makes it different from other sensors and what benefits it can have for medical technology. How does this sensor work?

Franz Faupel: The sensor has a very small oscillating beam of 120 micrometers, similar to a spring – it is about as thick as a strand of hair and therefore incredibly tiny. A special material is vapor-deposited onto this beam. This material is magnetostrictive, an amorphous iron-cobalt-silicon-boron alloy. This alloy has the characteristic of changing its form if you apply a magnetic field. The coating either expands or contracts, depending on the alloy and direction of the magnetic field. This dimensional change is called magnetostriction. It results in the material becoming softer or harder in the magnetic field. You can picture this by thinking of a tuning fork where the frequency changes. Meaning, the speed in which the oscillator vibrates is getting slower or faster. The idea is that the hardness changes in this particular alloy through an applied magnetic field. In a strong magnetic field, the coating becomes softer and vibrates slower, and in the case of a weak magnetic field, it is exactly the opposite. You can measure the change in oscillation frequency very precisely, by which you are also able to measure very small magnetic fields.

Photo: Blue match head in addition to sensor

A prototype of the new magnetic field sensor. The actual sensor tip on the left end of the silicon chip can barely be seen with the naked eye; © Björn Gojdka For what purpose was this mini-sensor developed? Just for medical science?

Faupel: No, we are materials scientists and physicists and we developed this sensor without exclusively having medical applications in mind. So it can definitely be utilized for many different applications that require a magnetic field sensor. Yet the sensor is also meant to assist in specifically releasing drugs in the body. How is this possible?

Faupel: In medical science there are several applications where drugs are used that are marked with magnetic particles. They are specifically transported in the body to a specific location and released in a magnetic field. The sensor could serve to detect such marked magnetic substances in the body and trace their way through the body. The sensor itself is outside of the body of course.

Photo: A man holds the tiny sensors with tweezers

Björn Gojdka got the idea for the new sensor concept from his previous work with the atomic force microscope. Here the tiny spring hangers are used to scan surfaces; © Björn Gojdka Where was the difficulty in developing such a miniaturized device?

Faupel: We wanted to develop something that was fully integrable. The difficulty was that it was supposed to be a measurement principle that makes due without an external magnetic field. So far, for most sensors you need a second magnetic field to take the sensor to a favorable working point area, so it can exhibit the necessary sensitivity. Our sensor does without such an external magnetic field and is therefore integrable in the first place. Another point is that unlike other sensors that only have a small bandwidth - that is to say the frequencies of alternating magnetic fields they are able to measure are in a very narrow range – it can measure magnetic fields in a wide frequency range and also detect static magnetic fields, to the point of magnetic fields that change with very high frequencies. Many sensors are not able to do this.

Photo: The bar of the magnetic field sensor

The coated beam of the magnetic field sensor measures only 125µm. The principle of the sensor is suited for miniaturization and has a large application potential. (Atomic force microscope image); © American Institute of Physics In which equipment for instance could it be integrated?

Faupel: We do not make that decision. We are primarily scientists that developed this sensor, published our knowledge and make it available to the general public. Although we have built a demonstrator to show the measurement principle and are currently still working on improving the sensor’s sensitivity and the measurement results, we do not want to commercialize it ourselves.

The interview was conducted by Simone Ernst and translated by Elena O'Meara.


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