# Design of Shape Memory Alloy-Based Soft Wearable Robot for Assisting Wrist Motion
## 2 Design of Muscle-Like Actuator Based on Shape Memory Alloy (SMA) Coil Spring  
Typical type of SMA for actuator applications is in the form of wires [47; 48; 49; 50]. The advantage of the wire-shaped SMA actuator is that it can produce high forces compared to the other soft actuator technologies such as Ionic Polymer Metal Composites [51], dielectric elastomers [52], shape-morphing polymers [53] and hydrogels [54]. However, the strain that the actuator can produce is relatively low (\(<\)8% of its initial length) [46], which is insufficient for the use in making a wearable robot. In order to enhance actuating performance, SMA can be shaped in the shape of coiled springs that result in substantially larger displacement when it is utilized as an actuator [46; 55; 56; 57; 58; 59]. The coil spring shaped SMA actuators have large extension strain over 200% [46; 55], which is enough for the desired application as the strain of a SMA coil spring is larger than that exhibited by human muscles [60]. Another shortcoming faced when working with SMA actuators is the slow response due to the difficulty of both heating up and cooling down the SMA rapidly in the same system. Generally, many SMA-based actuators are operated in ambient air, which offers a fast heating rate, but the limitation is a slow cooling rate as well [46; 47; 48; 59]. To enhance the response speed of SMA based actuators, a few works have applied water as an alternative coolant during the actuation of the SMA [50; 55; 56; 57; 58]. [PERSON] et al. developed SMA coil spring-based actuator that was composed of the SMA coil spring bundles and a stretchable polymer tube that can stretch over 300% of its initial length [55; 56; 57]. The heat transfer from water activated the SMA coil spring, and the temperature of the water that flows through the tube was controlled by the faucet-like valve system, which adjusts the amount of hot and cool water from the reservoir. The above-mentioned actuator showed up to 50% contraction strain when 5 kg load was applied with 1 Hz actuation speed. However, this actuation method is not suitable for wearable applications since large reservoirs for hot and cool water are needed. The highly stretchable muscle-like actuator proposed in this work is designed based on SMA coil springs and coolant circuit for a thin, high force, displacement, and fast response. Figure 1 shows the schematic design of the proposed actuator. In order to make the actuator thin, one SMA coil spring is integrated with a coolant circulation system. An electrical wire is connected to both the ends of the SMA coil spring while attaching it tightly at the end of a Polycarbonate connector containing the coolant flow inlet and outlet. The temperature of the SMA increases when the electrical current is applied to the SMA as the SMA itself is resistance. A stretchable polymer tube that is designed to surround the SMA coil spring is used for the flow of coolant. The polymer tube is made with Ecoflex 00-30 (Smooth on Co). It stretches and contracts in conjunction with the SMA coil spring thanks to its high stretchability of over 300% of its initial length. The tubes attach to the pump and the radiator was connected to the other end of the connectors. The coolant flows in closed circulation. The initial diameter of the stretchable polymer tube before stretching is selected as 7 mm. The width and height of the connectors are less than 15 mm. Mineral oil is chosen as a coolant that flows inside the tube. In order to design the SMA coil spring for a wearable robot, the actuator parameters which determine the maximum force as well as displacement should be analyzed.