# Design of Shape Memory Alloy-Based Soft Wearable Robot for Assisting Wrist Motion
## 4 Conclusions and Future Work  
A need for wearable robots that are flexible, easy to wear, lightweight and provide enough force has increased for patients who have difficulties in manipulating their bodies. This paper shows the first step in the development of a new type of soft wearable robot. We demonstrated an SMA-based soft wearable robot made for the assistance of wrist motion. To design a wearable robot that possesses such advantages for supporting the wrist, several design processes are carried out. To design a wearable robot that possesses such advantages for assisting the wrist, several design processes are carried out in this paper. Firstly, a highly stretchable muscle-like actuator is designed to achieve the target performances of force and contraction ratio required for wearable applications. The optimal design of the SMA coil spring is selected by comparing the force-displacement relationships of five coil springs samples. In order to enhance the response speed, an active cooling system using mineral oil was integrated with the SMA coil spring. The fabricated muscle-like actuator shows a maximum force of 10 N and a maximum contraction ratio of 40% respectively. Furthermore, by attaching the proposed muscle-like actuators to multiple positions on a wearable anchor, we propose a new soft wearable robot for the wrist named SWA, which is able to assist the 2-DOF motion of the wrist. The fabricated SWA is totally flexible and lightweight. The mass of the wearable parts of the robot was only 151 g. Additionally, the highest torque measured was 1.32 N for wrist extension, while the robot provided a torque of higher than 0.5 Nm for the other wrist motions. In order to observe the wearability and the ROM of the robot, we conducted user tests on five healthy male subjects with no impairment in the upper extremity. The average ROM is measured as 33.8, 30.4, 21.4, and 15.4 degrees for flexion, extension, radial deviation, and ulnar deviation. In addition, the wearability of the SWA was observed by measuring the time cost for wearing the device. The self-wearing time is measured as 87 s, and the assistive wearing time is measured as 75 s. The experimental results imply that the SWA can be used routinely to assist patients facing motor impairments at the wrist since it can be worn easily, even when under single hand use. However, there are some issues that have to be resolved in the design process. Firstly, although we designed the anchor system that prevents the dislocation of the wearable parts, it could not be perfectly prevented. Additional anchoring at the forearm to increase the fixation force can be applied. Secondly, since the size and the shape of the hands and forearms vary from person to person, the force and motion imbalance resulting from the misalignment of the actuators can occur while using the robot. In order to optimize the robot performances for all users, the attachment positions of the actuators should be easily modified, while having strong fixing force when it is attached. Adoptinglocking mechanisms that allow fixing the actuators on the wearable anchor regardless of the attachment position can be considered. As a future work, we will integrate highly stretchable skin patch-like sensors fabricated using a stretchable polymer and a waved optical fiber with the proposed robot to measure its position. Furthermore, the position of the wrist can be possibly controlled by feedback provided from the integrated sensors. Recognizing the intention of the users by adopting intention sensing systems like EMG, force, or ultra-sensitive strain measurement systems will be covered in upcoming studies. We expect that the development of the SWA could be helpful for patients who have difficulties in manipulating their lower arm by suggesting a new portable assistive wearable robot that supports the wrist motion in everyday ADL. Conceptualization, [PERSON] and K.-U.K.; methodology, J.J., I.B.Y., C.H.P., J.H., S.-K.B. and K.-U.K.; writing-original draft preparation, [PERSON], I.B.Y., J.H.