Shape memory alloys and polymers

Piezoelectric and magnetostrictive actuators in particular, as well as actuators with electrically controllable fluids, are coimted among the unconventional actuators. Actuators with shape memory alloys and polymers as well as other, less common actuators sometimes require very simple and sometimes fairly complex amplifiers, which have to be timed to the actuator and the signals that are to be processed. However, we will not go into the details of such special cases. 

Materials or surfaces are said to be responsive if they display a pronounced response to an environmental stimulus, particularly a response that may be suitable for application. Some responses in the form of physical or phase changes can be switchable or reversible. With the development of materials science, especially with the development of synthetic polymers and surface chemistry, these materials and surfaces have been designed for broad applications. Smart or intelligent has also been used to describe these materials since the 1980s. Shape-memory alloys and polymers, piezoelectric materials, and switchable glass are all good examples. 

Kirkby, E.L., Michaud, V.J., MSnson, J.A.E., Sottos, N.R., and White, S.R. (2009) Performance of self-healing epoxy with microencapsulated healing agent and shape memory alloy wires. Polymer, 50, 5533-5538. 

By far the most common actuator for electrically powered prostheses is the permanent magnet dc electric motor with some form of transmission (Fig. 32.10). While there is much research into other electrically powered actuator technologies, such as shape memory alloys and electroactive polymers, none is to the point where it can compete against the dc electric motor. A review of the available and developing actuator technologies with their associated advantages and disadvantages as well as their power and force densities can be found in Hannaford and Winters (1990) and Hollerbach et al. 

The most popular smart materials are piezoelectric materials, magnetostric-tive materials, shape-memory alloys, electrorheological fluids, electrostrictive materials, and optical fibers. Magnetostrictives, electrostrictives, shape-memory alloys, and electrorheological fluids are used as actuators optical fibers are used primarily as sensors (see Shape Memory Polymers)... 

Among shape memory materials, shape memory alloys and bimetals are well known. Compared to these metallic compounds, SMP have a lower density, high shape recoverability, easy processability, and lower cost. Similar to conventional thermoplastics, SMP can be easily molded by the common methods, such as injection, extrusion, compression, and casting. In addition, their shape recovery temperature can be set at any value in the range room temperature 50 C, which allows a wide variety of applications. Also, SMP can be colored if desired because they are transparent. However, since the retractive force of the polymer is based on the small entropy elasticity, the SMP applications differ from those of metallic alloys. The basic differences between shape memory polymers and alloys are listed in Table 1. 

A review of micro-electromechanical systems (MEMS)-based delivery systems provides more detailed information of present and future possibilities (52). This covers both micropumps [electrostatic, piezoelectric, thermopneumatic, shape memory alloy bimetallic, and ionic conductive polymer films (ICPF)] and nonmechanical micropumps [magnetohydrodynamic (MHD), electrohydrodynamic (EHD), electroosmotic (EO), chemical, osmotic-type, capillary-type, and bubble-type systems]. The biocompatibility of materials for MEMS fabrication is also covered. The range of technologies available is very large and bodes well for the future. 

Shape memory polymers are here to stay, not only because of their unique ability to display double existence under the influence of a triggering mechanism, but also because, unlike shape memory alloys, their elastic deformation and recoverable strains are huge, and their transition dependence can be tailored to fit specific requirements as well as having excellent biocompatibility, nontoxicity, ease of manufacture, and, perhaps most importantly, low cost of manufacture. 

In fact, not only alloys have shape memory properties, some polymers, ceramics, and even biological systems also possess such properties. For example, bacteriophages can use the shape... 

TABLE 16.2 Comparison between Two Mechanical Properties of Different Actuating Materials Skeletal Muscles, Thermomechanical (Thermal Liquid Crystals and Thermal Shape Memory Alloys), Electrochemomechanical (Conducting Polymers and Carbon Nanotubes) and Electromechanical (Ionic Polymer Metal Composites, Field Driven Liquid Crystal Elastomers, Dielectric Elastomers)... 

The first materials known to exhibit shape memory were shape memory metal alloys. Shape memory pol)nners are being developed to replace the use of Shape memory alloys, in part because the polymers are light, high in shape recovery ability, easy to manipulate, and economical in comparison to shape memory alloys... 

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