ABSTRACT
The advances in microfabrication technologies of the last four decades has led to new approaches being used for synthesising, shaping and processing materials that make it possible to operate on the their microstructure with greater precision in order to obtain mechanical, electrical, chemical and thermal properties that are ever more outstanding and controlled. Microstructured materials are usually made up of the superposition of layers of different ceramic, metallic and polymer materials (multilayered materials), that succeed in combining the properties of mechanical, thermal and chemical strength in a particularly effective manner and can even be used to design materials intended for a specific application with “a la carte” properties. For example, combining a hard core with very hard micrometric surface coatings has many applications in tribology for minimising the wear resulting from contact phenomena. On other occasions, when there is contact between parts, surface coverings are sought that have small friction coefficients or certain self-lubricating properties as a way of minimising wear and optimising the useful life of machine parts. Another common and increasingly widely used example of microstructured materials is composite materials with a polymer matrix and reinforcement fibre, which have become essential for the aeronautic and automobile industry, among others. They will not be dealt with in this chapter due to the many excellent texts available that deal exclusively with composite materials and also because their main application is to obtain passive structural elements, whereas this handbook is focused on active devices intended for sensors or actuators. One specific example of microstructured materials is functionally gradient materials or FGMs, where the variation in properties is not as abrupt as with multilayers comprising different materials, since their composition and structure change gradually throughout their volume. This is a result of their being manufactured using complex processes such as laser, physical, chemical or electrolytic deposition, powder metallurgy, plasma spray among others (Schwartz, 2006). When microstructured materials are designed to act as transducers in a role as sensors or actuators, we can refer to them by the more specific term of “microstructured active materials”. The fact that they are made up by bonding materials with different properties makes them able to respond, usually mechanically (by deforming), to different external thermal, electrical, magnetic or
mechanical stimuli, which means they can be used for detection or activation requirements. These microstructured synthetic materials have numerous applications in the field of health sciences. This will be clearly seen in the chapter, as on many occasions the marked mechanical properties of the biological tissue (which is replaced or complemented) is based on a very specifically ordered microstructure and on the combination of different materials that mutually strengthen one another. Bone tissue, for example, combines compact zones that contribute stiffness and mechanical strength, with other porous or spongy zones that help absorb impact and vibrations. Teeth have an enamel surface coating with an extremely high surface hardness that protects against wear on a dentine core that is remarkably strong. The phenomenon also occurs in soft tissue, as with heart valves and the surrounding tissue which reveals additional deposits of collagen in the areas that are subjected to higher circumferential stress. These kinds of examples can be appreciated in all types of living organisms, and by careful observation new innovative biomimetic microstructured materials are being developed. Using these materials to fabricate implants will bring better results as they are able to behave in the same way as the materials comprising the original tissue. It should be pointed out that the considerable advantages of these innovative materials are not only confined to developing passive devices to replace body structures and tissue. Their ability to combine materials that respond in different ways to electrical, thermal, mechanical or magnetic stimuli in a multilayer structure or a structure that gradually varies its properties means that some responses can be given a wider range in order to produce more effective sensors or actuators than those previously analysed. Their major applications are to be found in the medical industry and in the development of micro-electromechanical systems (MEMS), that include all kinds of micromachines, microactuators, and microsensors whose precision and speed benefits from acting on a microscale (Gad-el-Hak, 2003). This chapter deals with all these aspects and describes certain limitations that can be resolved by structuring the materials on an even tinier scale, as we will see in the Chapter 14, that can also be used for developing nano-electromechanical systems (NEMS). As an additional introduction to macro-, micro-and nanoscale dimensions, Fig. 13.1 shows a diagram with some examples and standard measurements.
