**Magnetic Sensors for Biomedical Applications**

Guillermo Rivero, Marta Multigner and Jorge Spottorno *Instituto de Magnetismo Aplicado (Universidad Complutense de Madrid) Spain* 

### **1. Introduction**

124 Magnetic Sensors – Principles and Applications

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5th ed., Wiley: New York, NY, USA.

The aim of this chapter is to give an overview of the applications of magnetic and magnetoelastic sensors and actuators in Biomedicine.

Over the last century, life expectancy on our planet has experienced a remarkable increase. That is if we exclude those regions ravaged by war and those others in which the population lives, or more probably survives, under conditions that we would not tolerate. In Europe this increase has been spectacular. Average life expectancy has reached 80 years old among the male population and even more for the female population and the tendency is upward. However, this has caused important health problems, as our bodies are not usually prepared to function for such a long time without repairs and/or replacements.

As a result, so-called substitution surgery is now taking up an increasing percentage of the operations performed in hospitals; such as bone prostheses for hips, knees and teeth, implants for eyes and ears and artificial sphincters and penises. Besides this, it is necessary to make periodic analyses in order to test that the levels of glucose, and cholesterol, etc. are correct. So, the application of magnetism in Biomedicine covers a wide field of devices, from sensors to determine the concentration of several elements in the blood to prostheses, both active and passive, for replacing organs or articulations.

Apart from all the above, some experimental treatments also use magnetic sensors or actuators. For example, magnetic nanoparticles are used for hyperthermia treatments against tumour cells or for drug delivery. In addition, much more sophisticated and expensive treatments, like magneto encephalography techniques, use magnetic elements.

In this chapter we are going to describe a small number of devices, some of which are now being researched. They are divided in two categories: magnetic sensors and magnetic actuators, according to the following scheme:

	- In situ measurement of the mass evolution of cell culture
	- Test of blood coagulation
	- Sensor system for early detection of heart valve bio prostheses failure
	- Magnetic endoluminal artificial urinary sphincter

Magnetic Sensors for Biomedical Applications 127

magnetizing field is used not only in the magnetization process but also in elastically deforming the material. The opposite phenomenon also takes place. When the material is deformed due to an external stress, not all the mechanical work is used in deforming the material, a part of the stress is used in magnetizing the material. In the free energy of the solid, a third term has to be added to the terms of elastic and magnetic energy that is called the magnetoelastic energy term. It takes into account the energy transfer between the elastic

) *Fmg* (*m*) *Fmgel*(*m*,

As a consequence, in these materials the magnetic susceptibility changes with the deformation (and consequently with stress) of the material and, inversely, the elastic constant of the material changes with its magnetization (and thus the applied magnetic field). The magnetic coupling term, being the one that takes into account the energy exchange between both systems, shows a resonance phenomenon: it has a maximum for a particular frequency that depends on the magnetic and elastic properties of the material.

The materials that are most widely used are the amorphous magnetostrictive alloys, in which the absence of magnetocristalline anisotropy makes it easy to control the resonance frequency, acting on the magnetic anisotropy of the material by means of appropriate thermal treatments. The amorphous magnetic materials with positive magnetostriction constant, made by the Taylor technique (Marín & Hernando, 2004), are well known due to their availability for use in wireless sensors based on magnetoelastic resonance (Vázquez & Hernando, 1996). Some recent works show the possibility of using these materials as magnetoelastic biosensors(Shem et al., 2009; Xie et al., 2009). Other works show a detailed study of the magnetoelastic coupling phenomenon in ribbons (Hernando 1983). Recently, there has been published a development, for monitoring in situ the mass of a cell culture, based on the magnetoelastic resonance of ribbon shaped metallic glasses (Rivero et al., 2008). On the other hand, magnetic amorphous microwires coated with Pyrex have attracted much interest due to their particular properties and their simple manufacturing process based on the Taylor technique. The use of these materials can reduce the dimensions of the set-up and

the Pyrex cover makes them appropriate for many applications in the medical field.

Cell cultures constitute one of the most frequently used assays in biology and also one of the diagnostic methods most used in Medicine. The growing of several microorganism strains, like cells, bacteria and virus etc., under conditions in which atmosphere, temperature,

On the other hand, the research on the treatment of tumours by hyperthermia, in the last decade, has been focused on the different behaviour of normal and tumour cells over temperature. Generally, normal cells show better temperature resistance than the tumour ones. Consequently, it is very important to determine the optimum temperature for

The monitoring of the progress of a cell culture is conventionally done by direct observation of the evolution that takes place on a culture plate, (the support on which the cells are

**2.1** *In situ* **measurement of the mass evolution of cell culture** 

humidity and nutrients are controlled have been studied.

hyperthermia treatments.

) (1)

*F Fel*(

system and the magnetic one:

 Hyperthermia HeLa cell treatment with silica-coated manganese oxide nanoparticles

In the development of these magnetic devices, as well as others not described here, there are some aspects that are common to the magnetic devices used in other very different fields as, for example, railway sensors, automotive sensors or aerospace vehicles. These may apply the basic magnetic properties or the signal acquisition and control process. Nevertheless, there are some aspects, as for instance, biocompatibility, corrosion resistance, size limitation and patient comfort, in the case of a human implant, that are specific to this application. We will try to describe them through our work in this area.
