ABSTRACT
Figure 9.1 Different varieties of quartz (smoked, citrine and amethyst) as a piezoelectric material. Therefore, this chapter is somewhat similar to Chapter 7, which took a detailed look at piezoelectric polymers like “PVDF” and its copolymers and at its most innovative applications in the medical industry. Notwithstanding, this chapter will focus more closely on piezoelectric activation rather than on the subject of electroactive polymers as sensors, since piezoelectric ceramics have some advantages as actuators over their polymer counterparts (which, in turn, make better sensors, as we have already explained). Electroactive ceramics were discovered before other families of active materials and have been showing promising results since the 1880s (with the first research done by the brothers Pierre and Jacques Curie) and particularly outstanding results since the 1960s. This has allowed decades of in-depth examination of the problems concerning the synthesis, processing, characterisation, modelling and simulation of these “intelligent” ceramics. This means that the appearance of commercial devices has been a gradual ongoing process, mainly actuators or sensors, in the form of film (and more recently in the form of multilayer film) supplied by over twenty multinational companies. The following section starts with an analysis of the most recent major proposals for medical applications concerning active device development using ceramic materials as electromechanical transducers. 9.2 Applications in Medical Devices
Besides the benefits linked to the ceramic coatings of numerous prostheses, either to increase resistance to wear or to enhance body
compatibility, also worthy of mention is the use of active ceramics with piezoelectric properties, mainly as component parts of medical devices capable of detection or activation (Cheremisinoff, 1990; Haertling, 1999; Davis, 2003; Schwartz, 2006). Some of the most implemented active ceramics used in such devices are calcium titanates, barium or lead, lead zirconate-titanate (“PZT”) or tungsten and bronze alloys. On the other hand, being able to deposit them in the form of thin layers in combination with polymer or metal substrates also endows these materials with numerous capabilities (Albella, 2006; Janas and Safari, 2005). Some major applications are listed below, firstly looking at their applications as sensors and then as actuators. Pressure sensors In order to evaluate the pressure or contact forces between different parts of the body, suitably prepared piezoelectric ceramics can be used to measure footstep pressure for use in rehabilitation or high performance physiotherapy (Pons et al., 2007). They are particularly suited to monitoring the evolution of lesions and for analysing progressive improvements until the optimal state is reached in top athletes. Active orthopaedic devicesTheir use as transducers in inertial sensors has been tested for various orthopaedic devices, such as motorised leg orthosis that require a real-time electronic control by means of information from inertial sensors (Moreno et al., 2006). Monitoring patients Piezoelectrics processed in thin film form are light and convenient for being used to monitor patients. Some of the physiological magnitudes that can be monitored are any body movements that are capable of deforming the material (and so generate a voltage), as is the case with cardiac and respiratory movements and other involuntary movements. These kinds of sensors can be used directly on patients or embedded in mattresses, carpets or other structures. The signals generated can be managed remotely, which means that a larger number of patients can be supervised in special circumstances (Dent and Smith, 2009). Electronic stethoscopesStethoscopes are acoustic instruments used in medicine, kinesiology, phonoaudiology, veterinary science and nursing for listening to the
sounds inside a human or animal body. It is generally used to listen to heart sounds or respiratory sounds, although it is sometimes used to listen to intestinal noise or unusual murmurs in arteries and veins. The use of piezoelectric film has also led to the development of electronic stethoscopes. These consist of a piezoelectric microphone, a signal amplifier and a connection to other devices such as a PC so that the signals recorded can then be analysed (Dent and Smith, 2009). These devices enable more objective approaches to be taken that can supplement the traditional procedure which simply amplifies the acoustic signal so that the doctor can make an assessment based on their experience (listening). Moreover, these electronic devices are especially suitable for making assessments in noisy surroundings (ambulances, emergency services and others). Active cochlear implantsThese are high-precision, high-tech active implantable health products that help bring back hearing to persons whose cilia cells of the cochlea are damaged, by stimulating the ganglionar cells (of the auditory nerve) whose mission is to send coded information to the brain. The use of electromechanical transducers that can generate electric stimuli in response to mechanical impulses (sound pressure waves, in this case) have played a major role in the development of these implants. Piezoelectric ceramics and piezoelectric polymers have both been used to find different solutions (Mukherjee et al., 2000). They have evolved remarkably since the first implant in 1957, culminating in the present-day devices from firms like Advanced Bionics, Cochlear and Med-el o MXM. Sensors in heart pacemakersPiezoelectric sensors can also be used to monitor patient activity from the inside of heart pacemakers. A cantilever microstructure is usually integrated into the interior of the pacemaker on which a piezoelectric material is deposited or adhered. This microstructure vibrates according to the activity of the patient, which causes the piezoelectric material to deform and generate a signal that helps manage the device and adjust the stimulation frequency (Dent and Smith, 2009). As the signal is generated by the piezoelectric material itself, it is often unnecessary to use a battery to provide more power, so this makes the material ideal as it helps minimise the end size of the implantable devices.
