**2.3 Sensor system for early detection of heart valve bioprostheses failure**

Heart valves are widely used in cardiac diseases. For example in 2002 in Spain 9269 heart valves were implanted as against 310 heart transplants. There are two kinds of heart valves, mechanical and biological ones. Biological heart valves, currently made with calf pericardium, have a similar shape to that of the original ones and better hemodynamic conditions than the mechanical ones. Moreover they have the advantage that the patient does not need lifelong treatment with anticoagulants, as happens with the mechanical valves. On the other hand, biological cardiac valves, or bioprostheses, have the inconvenience of their limited durability (about 10 years on average). What is more troublesome is the fact that they have an important dispersion that may be estimated in plus/minus 3 years (Kouchoukos et al, 2003).

These bioprostheses fail for several reasons, similar to those of the original ones and there are many factors that may contribute to their deterioration. The most important are biochemical degradation and mechanical damage of the tissue. Mechanical fatigue is the result of the great number of opening and closing cycles (approximately 30 million per year), which the valve is submitted to. Their effects are cumulative, and are expressed by lineal ruptures and/or perforations.

This means that it is difficult to determine the best moment for replacing the valve. The problem of the user getting it done too late must be balanced against the economic costs to the Social Security system, if is carried out unnecessarily early. So, a sensor is needed that

Magnetic Sensors for Biomedical Applications 137

The sensor works as follows. An excitation field is generated by an external coil. This field magnetizes the soft materials attached to the cusps and this magnetization triggers to a signal, the carrier signal, having the same frequency as that of the excitation one (2 kHz). The working of the valve causes a movement of the magnetic elements together with the cusps, producing a change in their position with respect to the exciting and detection coils (fig 7). This change in the relative position causes a modulation in the carrier signal (2 kHz) that clearly has the

The dysfunctions produced in the movement of the cusps during the opening and closing of the valve, due to degradation, hardening or rupture processes, provoke a change in the modulated signal, which is monitored against the signal obtained in the standard working conditions. The comparison between both signals may allow the medical services to establish precisely the moment when the valve must be replaced (Rivero at al., 2007). All this may be done by wireless, without any need to implant, in the patient, any electrical or electronic device for detection or transmission, or to establish any type of material joint with

This sensor has been tested in a hydrodynamic setup. Figure 8 shows the signal that is received in the pickup coils. Three different series of images corresponding to the opening process of the valve with different damage levels can be seen. The electrical signal obtained by the sensor system is superimposed on the corresponding image. The first row (a) corresponds to the opening process of the undamaged valve. The second row (b) corresponds to the valve with a slightly glued commissure, and the third row (c) corresponds to the same valve but with the three commissures slightly glued. In the last case it can be seen that there is a dramatic change in the electrical signal, as well as in the

Fig. 8. Opening process of the valve with different damage levels.

The working of most conventional electromagnetic actuators is based on the force exerted by a magnetic field on a moving element, made of a ferromagnetic element, that eventually can

frequency of the opening and closing of the valve (cardiac rhythm, 60–80 cycles/min).

the valve to the patient.

movement of the cusps.

**3. Magnetic actuators** 

can continuously monitor the working of the valve and give physicians objective data in order for them to make the substitution of the valve at the right time. This sensor has to be non-invasive and biocompatible. If small samples of soft magnetic material are inserted in the valve their movement can be detected when the valve is submitted to an alternating field of a frequency ranging from a few tens of Hz to tens of MHz, depending on the magnetic material used and its morphology (fig 7).

The material inside the body cannot interfere with the normal working of the valve. Each element's mass should not be higher than a few micrograms and its section must be of 10-8 m2 or lower. Different soft magnetic materials meet these requirements as for example amorphous ribbons and wires made by melt spinning or amorphous microwires made by rapid quenching.

Fig. 7. Detection of motion of bioprostheses cusps with microwires inserted.

The best results are obtained by using an amorphous Co-based microwire as a magnetic element. This microwire has a diameter of 20 m and is coated in the manufacturing process itself by a glass layer. The total diameter of the coated microwire is 45 m. It presents remarkable advantages over other materials (Marin P. & Hernando A. 2004):


can continuously monitor the working of the valve and give physicians objective data in order for them to make the substitution of the valve at the right time. This sensor has to be non-invasive and biocompatible. If small samples of soft magnetic material are inserted in the valve their movement can be detected when the valve is submitted to an alternating field of a frequency ranging from a few tens of Hz to tens of MHz, depending on the magnetic

The material inside the body cannot interfere with the normal working of the valve. Each element's mass should not be higher than a few micrograms and its section must be of 10-8 m2 or lower. Different soft magnetic materials meet these requirements as for example amorphous ribbons and wires made by melt spinning or amorphous microwires made by

Fig. 7. Detection of motion of bioprostheses cusps with microwires inserted.

remarkable advantages over other materials (Marin P. & Hernando A. 2004):

It can be integrated in the cusps without the usage of biological glues.

their mass in an appreciable way.

its integration.

with the other models.

The best results are obtained by using an amorphous Co-based microwire as a magnetic element. This microwire has a diameter of 20 m and is coated in the manufacturing process itself by a glass layer. The total diameter of the coated microwire is 45 m. It presents

Due to its minimum size it neither diminishes the cusps flexibility, nor does it change

It is coated, during the process of manufacture, by a biocompatible glass, so facilitating

Despite its reduced size the magnetic signal obtained is comparable to that obtained

 Its high-shape anisotropy (10mm×45m) together with its high-magnetic susceptibility makes it possibly to greatly reduce the energy expense needed for the sensor working.

material used and its morphology (fig 7).

rapid quenching.

The sensor works as follows. An excitation field is generated by an external coil. This field magnetizes the soft materials attached to the cusps and this magnetization triggers to a signal, the carrier signal, having the same frequency as that of the excitation one (2 kHz). The working of the valve causes a movement of the magnetic elements together with the cusps, producing a change in their position with respect to the exciting and detection coils (fig 7). This change in the relative position causes a modulation in the carrier signal (2 kHz) that clearly has the frequency of the opening and closing of the valve (cardiac rhythm, 60–80 cycles/min).

The dysfunctions produced in the movement of the cusps during the opening and closing of the valve, due to degradation, hardening or rupture processes, provoke a change in the modulated signal, which is monitored against the signal obtained in the standard working conditions. The comparison between both signals may allow the medical services to establish precisely the moment when the valve must be replaced (Rivero at al., 2007). All this may be done by wireless, without any need to implant, in the patient, any electrical or electronic device for detection or transmission, or to establish any type of material joint with the valve to the patient.

This sensor has been tested in a hydrodynamic setup. Figure 8 shows the signal that is received in the pickup coils. Three different series of images corresponding to the opening process of the valve with different damage levels can be seen. The electrical signal obtained by the sensor system is superimposed on the corresponding image. The first row (a) corresponds to the opening process of the undamaged valve. The second row (b) corresponds to the valve with a slightly glued commissure, and the third row (c) corresponds to the same valve but with the three commissures slightly glued. In the last case it can be seen that there is a dramatic change in the electrical signal, as well as in the movement of the cusps.

Fig. 8. Opening process of the valve with different damage levels.
