# A State-of-the-Art Review on Robots and Medical Devices Using Smart Fluids and Shape Memory Alloys
## 3 Robots Using SMA  
### Material Characteristics  
Shape memory alloys (SMA) are a class of metallic alloys that can return to their original shape because of the phase-transformation phenomenon, known as the shape memory effect (SME), when subjected to changes in temperature or magnetic field. The primary application of these materials is therefore in actuation systems, where the SMAs can be readily contracted or recovered to their original form. The actuating motions could be achieved by controlling temperature beyond a certain threshold temperature, by internal Joule heating. SMAs possess two different phases with three different crystal structures: twinned martensite, detwinned martensite, and austenite. Typically, the austenite phase is stable at high temperatures, and the martensite phase is stable at lower temperatures. Upon heating an SMA, the initial martensite phase begins to transform into the austenite phase, as shown in Figure 7. The onset temperature is \(A_{s}\), where the austenite transformation starts, and \(A_{f}\) is the terminal temperature, where this transformation is complete. Once an SMA is heated beyond \(A_{s}\), it begins to transform into the austenite structure, resulting in recovering (i.e., contracting) to its original shape. During cooling, the transformation starts to reverse to the martensite at \(M_{s}\), and is completed when it reaches \(M_{f}\). Above the highest temperature (i.e., \(A_{f}\)), SMA is permanently deformed, similar to ordinary metals and alloys. In addition, SMAs exhibit other shape-memory characteristics, such as pseudoelasticity or superelasticity. The SMAs can return to their original shapes after applying mechanical loading at temperatures between \(A_{f}\) and \(M_{d}\), without any thermal  
Figure 6: Two different micro-robots featuring MRE body structures [34]. activation. This property is currently used for passive vibration-damping applications. Hysteresis is a measure of the difference in transition temperatures between heating and cooling, and is generally defined between the temperatures at which 50% of the material is transformed to austenite upon heating, and 50% is transformed to martensite upon cooling. This hysteresis has significant design considerations for SMA applications. For example, a small hysteresis is typically required for fast actuation applications, such as in robotics, while larger hysteresis may be required to retain the predefined shape, such as in deployable structures within a large temperature range. The hysteresis loop associated with different transition temperatures is known to be affected by the composition of the SMA material, and the thermomechanical processing. In addition, some of the SMA's properties also vary between these two phases, including the Young's modulus, electrical resistivity, thermal conductivity, and thermal expansion coefficient. Detailed information on the mechanical properties of SMAs is well summarized in relevant references [35; 36; 37]. SMAs are particularly attractive for actuator applications because their strain (length change or stroke) during the contraction phase is relatively larger than other smart materials, such as piezoelectric transducers, and it is typically 4-5% of their initial length. If the contraction is constrained, large block forces can be generated, and stresses may reach the order of magnitude of 1000 MPa. However, for actuator applications with a large number of load cycles, up to \(10^{5}\), the stress level should be alleviated, up to 200 MPa, to prevent fatigue damage. Compared to other actuation systems, high work output along with lower weight (i.e., high power density) can be achieved with SMAs, which is important for robot applications. While the temperature during the contraction process is controlled electrically through Joule heating, cooling is simply controlled by convection in surrounding air, which results in a much slower recovery response.