**3. Magnetic actuators**

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

Magnetic Sensors for Biomedical Applications 139

reaching the limit where gold nanoparticles of diameter 1.5 nm demonstrate a spontaneous

Urinary incontinence is not considered an illness but a symptom and is only treated when it becomes a social problem. Urinary incontinence is defined by the complaint of any involuntary leakage of urine (Abrams et al., 2002) or unintentional loss of urine that occurs with such frequency and in such quantities as to cause physical and/or emotional distress in the person experiencing it. Moreover, the number of people suffering from this is estimated to be around a million people among the adult population in western countries

Depending on the origin and the severity of the affection, it is treated with absorbent pads, pharmacological or surgical methods. For more than 30 years physicians have implanted inflatable artificial sphincters that simulate the sphincter function and permit the voluntary control of the micturition (American Medical System). However this device requires highly invasive surgery since it is necessary to implant a cuff around the urethra, in addition to a balloon, that regulates the cuff pressure and a bulb that controls the inflation and deflation

The objective of developing a magnetic artificial urinary sphincter is to look for a minimally invasive device that permits voluntary micturition control, making use of the potential of

The device consists of a magnetic valve placed in the urethra. The urine is evacuated by bringing a permanent magnet near the body of the patient. The magnetic valve consists on (Figure 9) a hollow cylindrical body valve with a toroidal magnet fixed at one of their ends and a soft magnetic material piston that is attracted by the magnet, closing the evacuation

If a more powerful external magnet is brought near the valve, the magnetization of the piston (made of a soft magnetic material) is reversed and a repulsion force between the internal magnet and the piston appears, causing the evacuation hole to open. When the external magnet is moved away, the interaction between the internal magnet and the piston turns attractive again and the valve closes automatically. Furthermore, the system is provided with a safety system preventing overpressure in the bladder. By adjusting the distance between the piston and the internal magnet, a pressure level can be fixed in such a way that if the pressure inside the bladder reaches that value, then the valve

magnetic moment (Crespo et al., 2004).

(Irwin et al., 2006).

of the cuff.

**3.1 Magnetic endoluminal artificial urinary sphincter** 

the magnetic field to exert a force without physical contact.

Voluntary opening by the patient

Automatic closing

Immunity to RF fields

opens automatically.

The preliminary specifications for the device are the following:

Automatic opening when the safety pressure is reached

hole. As a sealing gasket a medical grade silicone O-ring is used.

External magnetic drive, without other devices

Urethral implantation without surgery

become a permanent magnet. The magnetic field that acts on the material can be created by a coil, with the appropriate feeding (with alternate or direct current) or by a permanent magnet. This force is used to change the moving element.

Other kind of magnetic actuators are based on the change in dimensions or the deformation that takes place in a magnetostrictive magnetic material (see section 2) when there exists a variation of the magnetic field that is acting on it. As in the previous case this field can be originated by a coil or a permanent magnet when it changes its position or its orientation. This kind of actuators, based on the effect commentated in section 2 of this chapter, present great advantages for their application on biomedicine since their magnetic field is not shielded by human body tissues due to their low conductivity. In other words the actuator field can be established by devices placed outside the human body although the actuator element is an internal implant. The limits imposed by the size and biocompatibility requirements are the same as the ones described in section 2 for the magnetic sensors.

There are other kinds of recent applications of magnetic actuators related to temperature. A general property of magnetic materials, except the ferrites, is an electrical conductivity in the range of metal and metal alloys, ranging between 104 and 107 S/m. This means that if the actuator field is an alternating one with a frequency , eddy currents will be induced in the element that will dissipate a Joule power proportional to the conductivity value, the square of the magnetic field amplitude and the square of the frequency.

$$W\_{\text{Joule}} \propto \sigma \text{ oo}^2 \text{ B}\_0^{-2} \tag{9}$$

Moreover, any ferromagnetic material will dissipate, through heat, the power needed for the magnetization process, which will be proportional to the frequency.

$$\mathcal{W}\_{\text{Hyst}} = \alpha \prod\_{\text{hystl@p}} \mathcal{H}d\mathcal{B} \tag{10}$$

If the frequency is high, (between 10 KHz and 103 KHz), the power dissipated through both causes may mean a significant local increase in the medium temperature. This behavior has been used by numerous researchers in the past decade in order to try to find a treatment against tumors (hyperthermia) which would be not too aggressive for the patient. In order to avoid the danger of creating an embolism in narrow capillaries, nanoparticles with diameters ranging from five to a few hundreds of nanometers are used. To avoid the particles aggregating because of the attraction of their magnetic moment, superparamagnetic materials or ferromagnetic materials functionalized with polar radicals that cause electrostatic repulsion are used. Another application of these particles, (in their hollow mode), is to use them as a vehicle for transporting medicines to the place in the organism where they have to act. Once they are there they are heated with a high frequency magnetic field that provokes their fragmentation and frees the medicine inside the nanoparticle.

These magnetic actuators have given rise to a new kind of materials, using traditional materials as a starting point but reducing them to smaller and smaller sizes. One only has to consider that in a gold cube with a 1 micron side just one part in a million of their atoms are arranged on the surface while in a cube of 2 nm 60 % of them are on the surface. So it is not strange that their physical properties become very different from that of bulk gold when

become a permanent magnet. The magnetic field that acts on the material can be created by a coil, with the appropriate feeding (with alternate or direct current) or by a permanent

Other kind of magnetic actuators are based on the change in dimensions or the deformation that takes place in a magnetostrictive magnetic material (see section 2) when there exists a variation of the magnetic field that is acting on it. As in the previous case this field can be originated by a coil or a permanent magnet when it changes its position or its orientation. This kind of actuators, based on the effect commentated in section 2 of this chapter, present great advantages for their application on biomedicine since their magnetic field is not shielded by human body tissues due to their low conductivity. In other words the actuator field can be established by devices placed outside the human body although the actuator element is an internal implant. The limits imposed by the size and biocompatibility requirements are the same as the ones described in section 2 for the magnetic sensors.

There are other kinds of recent applications of magnetic actuators related to temperature. A general property of magnetic materials, except the ferrites, is an electrical conductivity in the range of metal and metal alloys, ranging between 104 and 107 S/m. This means that if the actuator field is an alternating one with a frequency , eddy currents will be induced in the element that will dissipate a Joule power proportional to the conductivity value, the square

> 2 2 *W B Joule*

Moreover, any ferromagnetic material will dissipate, through heat, the power needed for the

If the frequency is high, (between 10 KHz and 103 KHz), the power dissipated through both causes may mean a significant local increase in the medium temperature. This behavior has been used by numerous researchers in the past decade in order to try to find a treatment against tumors (hyperthermia) which would be not too aggressive for the patient. In order to avoid the danger of creating an embolism in narrow capillaries, nanoparticles with diameters ranging from five to a few hundreds of nanometers are used. To avoid the particles aggregating because of the attraction of their magnetic moment, superparamagnetic materials or ferromagnetic materials functionalized with polar radicals that cause electrostatic repulsion are used. Another application of these particles, (in their hollow mode), is to use them as a vehicle for transporting medicines to the place in the organism where they have to act. Once they are there they are heated with a high frequency magnetic field that provokes their

These magnetic actuators have given rise to a new kind of materials, using traditional materials as a starting point but reducing them to smaller and smaller sizes. One only has to consider that in a gold cube with a 1 micron side just one part in a million of their atoms are arranged on the surface while in a cube of 2 nm 60 % of them are on the surface. So it is not strange that their physical properties become very different from that of bulk gold when

*hystloop*

*Hyst*

*W*

<sup>0</sup> (9)

*HdB* (10)

magnet. This force is used to change the moving element.

of the magnetic field amplitude and the square of the frequency.

magnetization process, which will be proportional to the frequency.

fragmentation and frees the medicine inside the nanoparticle.

reaching the limit where gold nanoparticles of diameter 1.5 nm demonstrate a spontaneous magnetic moment (Crespo et al., 2004).
