Wireless Stress Sensor Based on Magnetoelastic Microwires for Biomedical Applications: detection of collagen concentration

In recent years, much interest and effort have been devoted to develop soft magnetic materials due to their technological potential [1]. Amorphous microwires are one of the most widely studied soft materials. They are fabricated by means of extracting melt-spinning Taylor technique. Those microwires are composed by a metallic core and a Pyrex cover both in the micrometer range. The ratio between the total diameter and the magnetic core, often called aspect ratio, is one of the key parameters of such microwires, since magnetic properties depend dramatically on it. Many properties of these materials have been deeply studied both from the point of view of the basic physics and the applications. This is the case of the giant magnetoimpedance effect [2], bistability, ferromagnetic resonance [3], and magnetoelastic resonance [4]. It is easy, also, to find much literature regarding microwave-related applications of microwires or microwire-based materials [5]. In the frequency range of GHz, some experimental and theoretical studies of the effect of the magnetization on the scattering properties of a single microwire have been developed [6]. This kind of work gives experimental evidence showing that the microwave scattering by a single microwire depends on the magnetic permeability with sufficient strength to be experimentally detected as an effect of the GMI. This dependence was used to show the potential of such microwire as a wireless field and/or stress sensor. Experimental results are followed by a theoretical approach where the influence of the microwire magnetic state in its microwave reflection features is taken into account. Besides these investigations on magnetic microwires, it should be stated that technological development has spurred the growing interest in the investigation of new biosensors aimed at simplifying present day diagnostic methods and thereby improving medical care, so that it improves the quality of life of the patients and allows for outpatient treatment for a number of pathologies, avoiding unnecessary hospital admissions. Magnetic sensors are at the helm of technological development seen in this field over the last decades, offering numerous advantages attributed to their elevated sensitivity, reduced size, systems without the need for an external source of energy, and wireless connections. The use of wireless sensor network (WSN) technologies offers the possibility of developing implantable biomedical sensors allowing for the monitorization and follow-up of certain physiological parameters with precise and, up until now, unthinkable measurements. The aim of the present work is to show the physical fundamentals and the particular biomedical applications of magnetic microwires as wireless stress sensors. Two main applications will be described. On one side we proposed a flexible magnetic element able to be integrated both in artery and prosthesis suitable for wireless localized blood pressure monitoring. The sensor made of a ring of glass-covered magnetic microwire is simple and inexpensive and could be detected by means of a simple exciting and detecting set up able of emitting and detecting microwaves simultaneously applying a low-frequency magnetic field. The reflectivity of the microwire is determined by mechanical changes. In the experimental study, a piece of a cardiovascular prosthesis and a piece of an artery both sensed with a ring of magnetic microwire have been situated in a hydraulic setup simulating cardiovascular human circuit. Reflectivity changes of the sensor show its capability of measuring pressure variations in the circuit. The same kind of material and based also in the same kind of experiment has been used to wireless determine collagen density and stress distribution to determine stress distribution on bioreactors.