# Design of Shape Memory Alloy-Based Soft Wearable Robot for Assisting Wrist Motion
## 1 Introduction  
Loss of mobility and power in the upper extremity (UE) is a common symptom resulting from damage to the peripheral nervous system, muscle weakness, tendon disorders, muscle contracture and change in muscle tone after an accident or a medical disorder such as cerebral palsy, stroke, traumatic brain injury or damage to the spinal cord [1; 2; 3; 4; 5; 6]. The disabilities of the upper extremity can manifest themselves individually more often, but they also occur in combination. Also, several of these neurological diseases are chronic as well as progressive [3; 4]. Consequently, the ability to perform voluntary movement and fine motor and manipulation skills are lost [7]. This results in loss of independence during the performance of tasks required in activities of daily living (ADL) such as feeding, self-hygiene, dressing and transferring [8]. Especially for rehabilitation treatment, there is less improvement of patients in the wrist mobility compared to the shoulder or elbow. The radiocarbon joint is one of the major and complex parts of the upper extremity, comprising of bones, ligaments, nerves, tendons and connective tissues. Its anatomy allows a wide spectrum of intricate movements necessary in performing day-to-day tasks, which in turn, makes it susceptible to injuries [9]. Compression or damage to the radial, ulnar and median nerve of the wrist such as wrist drop, ulnar claw or carpal tunnel syndrome also lead to chronicwrist impairment. Owing to the motor, neurological and perceptual defects of the wrist joint, the quality of life and independence of the patient reduces along with the adverse effect on the ability to perform ADL. Moreover, the reduced mobility at the wrist results in compensatory movement of the shoulder, and the elbow [10]. In many cases, physical therapy leads to recovery by stimulating the affected muscles together with leveraging neuroplasticity [11; 12]. Similarly, rehabilitation in the form of repetitive motions and exercises have resulted in restoring some degree of motor loss [13] and cortical reorganization while regaining some portion of the lost sensorimotor skills [14]. Findings of 6 months of longitudinal studies reveal that about 30 to 66% of hemiplegic patients, had little or no function in the paralyzed arm, whereas only 5 to 20% showed complete functional recovery [15]. Nevertheless, recovery is not limited to the months following stroke [16] with rehabilitation of the UE being reported many years after stroke [17]. Also, manual therapeutic rehabilitation where the therapist has to guide the paralyzed limb directly requires a daily treatment, which makes the treatment difficult and expensive [18]. In the past decades, researchers have developed many robotic rehabilitation systems [19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35]. A few investigations show that the improvement of recovery for acute and chronic patients facing stroke, cerebral palsy and injury to the spinal cord was observed from rehabilitation treatment by applying robotic systems [19; 20; 21; 22; 23]. Conventionally used rehabilitation robots have rigid structures and the force to the patients is delivered primarily by a motor located at the respective joint. However, these robots are not suitable in wearable applications since the rigid links and joints are heavy and bulky. Also, the limitation of the degrees of freedom (DOF) causes inconvenience for the user. Applying soft robotic technologies is a great approach for a compact, lightweight, user-friendly and portable exoskeleton for the upper limb, since the soft actuators and sensors are lightweight, compact and have high DOFs [36; 37; 38; 39; 40; 41; 42; 43; 44; 45]. Pneumatic actuators are widely used for wearable applications since they can produce high force compared to that of other soft actuators along with large deformations. Several soft wearable robots for the wrist using multiple pneumatic actuators that produce bending or linear contraction motion were suggested [36; 37; 38; 39; 40].