The aim of the project is to develop magnetic sensors for biomedical applications with increased sensitivity by elaboration of stable technology of deposition of thin film detectors based on a giant magnetoimpedance effect (GMI) being adapted to the present day semiconductor electronics. The creation of highly sensitive film sensors for biomedical applications is an engineering problem strongly demanded by the society. Its solution is only possible in frame of a multidisciplinary approach joining together engineering sciences, physics, chemistry, materials science, nanotechnology, biology and medicine, provided that the fundamentals are developed.
One of the fundamental characteristics of a magnetic biosensor is sensitivity to the external magnetic field. The GMI provides a very high sensitivity to the magnetic field. There are GMI detectors of weak fields (up to 1 nT with a resolution of up to 1 pT) based on amorphous wires operating under pulsed excitation. However, in comparison with sensors excited by a harmonic current, such detectors have a number of disadvantages: the inability to register signals at a signal-to-noise ratio of less than 0 dB; the design is not well matched with semiconductor integration technology; the possibility of using digital signal processing is limited. It is known that the maximum sensitivity to an external magnetic field is achieved in the symmetric GMI structures. The applicants have own experience in the field of technology for the preparation of film nanostructures of the [FeNi/Cu]x/Cu/[FeNi/Cu]y and [FeNi/Ti]x/ Cu/[FeNi/Cu]y. The aim of this project is: to develop technologies for the production of thin film GMI structures of the appropriate architecture on rigid or flexible substrates, providing a the highest of existing sensitivity for the detection of the biomedical signals.
Film geometry is most adapted to the semiconductor electronics. Sensors excited by a high-frequency harmonic signal provide faster measurements: when operating at a frequency of about 80 MHz, it is possible to record changes in the magnetic field occurring in hundreds of nanoseconds. The flat form of film structures allows to vary the active surface area of the sensor, but the main technological advantage is the existence of stable technologies for obtaining multilayer structures. The project proposes to develop a sensor based on a quadrature demodulator. The latter is characterized by the presence of an additional synchronous detector, to which the signal is shifted in phase by π/2 as the reference. Both in-phase and quadrature components of the signal reflect slow changes in the external magnetic field, and can be converted to digital form using analog-to-digital converters. The use of a quadrature demodulator will allow an extreme increase in the signal-to-noise ratio (SNR) in the measurement of weak magnetic fields due to the transfer of the spectrum of the measured signal to the region of relatively high frequencies, where the effect of noise is much lower. It is supposed to create a magnetic field detector based on a film-based GMI sensor and a quadrature demodulator operating at room temperature and providing a sensitivity of 0.5 nT in the magnetic field induction measurement range B = ± 10 μT. To establish a more accurate position of the sensor operating point, increase stability of operation and reduce dependence on fluctuations in ambient temperature, a bridge or differential switching circuit is planned.
To study the parameters of the sensors, the systems based on the Agilent E4991 impedance analyzer and the vector network analyzer ZVA-67 Rohde & Schwarz (VNA) will be used to in the power range from microwatts to tens of milliwatts. It is possible to measure the parameters of films using a high-frequency signal with amplitude modulation, as well as using a regime of connecting a constant component of the current of different polarity. VAC ZVA-67 is equipped with four independent channels, which allow to measure the parameters of differential and bridge high-frequency magnetic field sensors. It is planned to develop a film detector with a sensitivity comparable to the sensitivity of SQUID, but operating at room temperature and with dimensions of a cell phone that has a combination of technological characteristics that do not have world analogues. Extreme sensitivity of the order of 0.5 nT will be achieved due to the excitation of a high-frequency harmonic signal and the use of circuitry solutions that compensate the external noise.
The use of quadrature detection of a stable high-frequency harmonic signal determines important advantages over pulsed excitation: the possibility of operating under conditions of considerable external electromagnetic interference exceeding the amplitude of the signal; the ability to apply digital signal processing; extremely low level of own noise; the possibility of increasing the speed of the detector, etc.
It is assumed that the GMI sensors of the new class will be tested in the mode of detecting biomedical signals. To do this, biomimetics of biological tissues based on synthetic polymers (gels) filled with magnetic particles will be created and conditions for the synthesis of stable suspensions of nano/microparticles of iron oxide and hydrogels based on them will be worked out. To increase the biocompatibility of the tested materials, gels will be synthesized with chemical and physical grids, the latter of which will be represented by biological polysaccharides and/or proteins. It is supposed to obtain the dependence of the intensity of the GMI sensor response on the concentration of magnetic particles in the gel, its physicochemical structure and functional properties (viscoelastic and electrical characteristics). In addition, in vitro experiments, an evaluation of the proliferation potential of cells in ferrogels will be performed, by describing the relationships between the features of cell culture on gels and the parameters of the GMI signal. At the final stage, experiments are planned on animals in vivo in the processes of regeneration of damaged sections of the tissues of the locomotor system of animals.
It is expected that the developed film detector will become the basis of a line of sensor magnetic detection devices in the field of biomedical applications