# A State-of-the-Art Review on Robots and Medical Devices Using Smart Fluids and Shape Memory Alloys
## 3 Robots Using SMA
### SMA-Based Robots  
SMA actuators for robotic applications can be categorized according to various actuation configurations in terms of loading types. Most of the actuator designs are based on an SMA spring-wire type, as a large stroke change can be generated with a relatively small microscopic strain, and because of its simplicity of fabrication, by annealing an SMA wire wound on a rod [38]. Most key parameters--such as the wire diameter, rod diameter, pitch angle, and number of active coils--can be successfully designed, based on reliable SMA models [39]. However, the stress distribution over the cross-section of an SMA spring is no longer constant, and therefore it requires greater material volume to generate the same force, which will have an effect on the efficiency and the bandwidth of the actuator. Therefore, a straight, linear wire type for tension loading is more advantageous because of its significantly higher efficiency (i.e., more work generated from a minimal amount of SMA material). Various versions of linear SMA wire actuators can be found in recent literature. For example, [PERSON] et al. [40] suggested a wet SMA actuator embedded within a compliant fluid-filled tube to produce a linear contraction and extension of the SMA wire. [PERSON] et al. [41] proposed a hybrid linear actuator by using an SMA wire and DC motor to resolve the low driving frequency issue. A new flexible SMA actuator, composed mainly of an SMA wire and a multilayer tubular structure, was proposed to make the conventional linear SMA wire actuator flexible, with high axial stiffness [42]. As robots have become a focus of interest, they are now required to complete various diverse tasks in a variety of environments, using factors such as a torsional motion. In order to produce a torsional (or twist) deformation, a smart soft-composite torsional actuator using SMA wires has been proposed by [PERSON] et al. [43]. The proposed twisting actuator is composed of a torsional-restrained SMA wire embedded in the center of a polydimethylsiloxane matrix. [PERSON] et al. [44] proposed a torsion actuator composed of a pair of antagonistic SMA torsion springs, capable of bi-directional actuation. [PERSON] at al. [45] manufactured a hollow-tube-shaped actuator, with multiple curved SMA wires that follow the curvature of the tube, and which is capable of pure-twisting deformations, while sustaining a cantilever load. This wrist actuator showed twisting deformation up to 25\({}^{\circ}\) while holding objects weighing 100 g, and could sustain loads above 2 N without buckling. Recently, different loading types of SMA wire actuation have been explored for robotic origami applications, which require multiple sequential folding of tiled sheets for recovering the original shape. [PERSON] et al. [46; 47] presented a preliminary research on a low-profile bidirectional folding actuator based on annealed thin NiTi sheets (thickness of 0.1 mm), for meso- and micro-scale robotic applications. A torsional actuator composed of an SMA wire, with a diameter of 0.25 mm in the array form, and two supporting elements, was developed to provide large rotational motion by connecting single actuator modules in series [48]. A new soft SMA actuator was developed, capable of fast bending actuation with large deformations. Multiple thin SMA wires were used to increase heat dissipation for faster cooling, and the bending driving frequency of an SMA wire was increased up to 35 Hz [49]. SMAs have been widely used in a diverse range of humanoid-robotic applications since the 1980s, especially in artificial muscles, because SMA wire actuators can mimic human muscle. For example, many researchers created a dexterous robot manipulator that can mimic the human hand using SMAs. A recent study by [PERSON] and [PERSON] [50] divided the robot hand into several categories based on locomotion styles and multi-DOF. For the design of a facial-expressive baby robot, SMAs were embedded in the skull of a humanoid head and connected to the elastomeric skin at control points.