Shape memory alloys (SMAs)

Shape memory alloys (SMAs) are metallic alloys that can memorise their original shape and, after being plastically deformed, return to their original shape on heating above its transition temperature. Their advantages of high energy density, significant recovery stress (e.g. 800 MPa), high strain (typically ~8%) and no noise make them promising as embedded actuators for soft robots. However, SMAs show non-linear characteristics, including pseudo-elasticity that is associated with stress-induced phase transitions, shape memory effect and high damping capacity due to their high mechanical hysteresis and internal friction, also associated with its stress-induced phase transitions. The need for a high-temperature control mechanism and long cooling time can restrict their applications due to the low frequency of operation.
Stirling et al. fabricated an active SMA-based soft orthotic, as illustrated in Figure 6a [57]. The active, soft orthotic (ASO) is comprised of four sets of SMA (NiTi) actuators on the dorsal surface of the knee to assist flexion and an SMA set on the frontal surface to assist extension. Each actuator set consisted of four lines of the SMA coils. This design enabled a large deflection angle of 35° for knee motion and demonstrated the concept of embedding SMA actuators into soft materials for creating soft rehabilitation devices. The ASO was able to provide individualised control since the actuators were able to mimic muscle functionality and assist muscle rehabilitation effectively. However, some key challenges remain; for example, the response time of the SMA actuators and cooling time are 25 s and 40-50 s, respectively, which are significantly slower than fluid-powered driven soft actuators.
Lai et al. designed a novel SMA-based splint for finger joints rehabilitation, as shown in Figure 6b [58]. Four wires of a shape memory alloy (SMA – Ti50Ni45Cu5 alloy 1.0 mm in diameter and 110 mm in length) were arranged in parallel within silica-gel tubes to form a splint, which is a flexible joint that actuates during rehabilitation when the SMA is controlled by heating and cooling, see Figure 6c. A fixture splint that has an adjustable mechanism to fit a variety of fingers with a PC-based control system was developed for a medical clinic milieu use, while a portable splint controlled by a microcontroller (PIC18F452) was developed for home rehabilitation. The maximum output force of a finger splint was 22 N, which is sufficient for a finger rehabilitation process. However, the need for temperature control during the actuation of the SMAs leads to a long response time of the splint, typically 16-19s.
To effectively assist in hand rehabilitation, Hadi et al. presented a novel lightweight hand exoskeleton robot actuated by SMA-based tendons [59]. A glove was used as the interface between the SMA actuators and human hands, where SMA wires for both proximal and distal tendons of each finger were assembled on the glove. For each finger, two DoFs were actuated via the SMA wires connected to the proximal phalanx and the end of the distal phalanx, as shown in Figure 6d. To avoid mutual interference of SMA wires during actuation, several guides were mounted on the glove for improved connection and transfer of the associated force to the desired phalanxes. When the SMA actuators were activated, their tensile force and length variation could be transformed to the joint angles, as shown in Figure 6e. This glove was able to generate a 90° angular movement and produce a grasping force of over 40N, and the speeds of assisting to hand flexion and extension were 3 and 4 seconds, respectively. Kazeminasab et al. improved the performance of this glove by exploiting variable structure controllers, and the optimised glove has the potential for physiotherapy and assistance [60]. During operation in a physiotherapy mode, hand motion and the required joint trajectories are controlled, while in the object manipulation mode, the grasping force is controlled. The maximum output force generated by the glove is approximately 45 N, and the speed of hand flexion and extension are 2.5 and 4 seconds, respectively. Compared to the initial design [57], the size of the device is still quite large and more compact and portable designs are needed for remote use; for example by exploiting coiled or finer scale SMA materials.
The wrist joint is critical for force transmission between the forearm and the hand, and its stability affects the capability and motion of the hands and fingers [61]. Therefore, wrist rehabilitation aims to restore the normal range of movement of joints and facilitate hand and finger function. Serrano et al. developed an SMA-based soft glove for wrist joint rehabilitation [62]. The device was integrated with a rigid skeleton to provide a range of motion between -10° to 30° and a relatively low response frequency of 0.04 Hz. A PID controller was designed to effectively control motion, however the control of non-linearly requires more investigation. The device weighed 960 g and was low noise, which is suitable for rehabilitation.  Villoslada et al. proposed a soft wearable prototype for wrist joint rehabilitation based on flexible NiTi SMA actuators [63]. The SMA wire was placed into the interior of a Bowden cable sheath made of nylon. When the SMA wire was actuated, an extension of the wrist was achieved. The SMA actuator was designed to be bent at angles up to 180°, providing free movement and improved integration with wearable robotic devices. In addition, PID control algorithms were used for velocity and position control of the SMA, which enhanced its actuation bandwidth and increased its service life by preventing overheating during operation.