# Mechanical Design of a Bioinspired Compliant Robotic Wrist Rehabilitation Equipment
## 1 Introduction
The WaveFlex Hand system developed by Remington Medical is a light construction deployable both in hospital and in patients' homes. It supports the performing of rehabilitation exercises within the limits of hand joint biomechanics [8]. Recovery of hand joints is also conducted by means of a system with 3 degree-of-freedom that can be attached on a MIT-MANUS robot. This piece of equipment limits motion to 60\({}^{\circ}\)/60\({}^{\circ}\) in flexion/extension, 30\({}^{\circ}\)/45\({}^{\circ}\) in abduction/adduction, and 70\({}^{\circ}\)/70\({}^{\circ}\) in pronation/supination [9]. Besides CPM-based rehabilitation literature also discusses game therapy for poststroke recovery of upper limbs. Thus, motor-training software on tablets or smartphones offer a low-cost, widely-available solution to supplement arm physiotherapy after stroke. Studies involving 127 therapists revealed that the most commonly used device was Nintendo Wii. Gaming was reported to be enjoyable but therapists described barriers, which relate to time, space and cost [10; 11; 12]. This category of game therapy includes also a smart mobile device for the assessment and training of hand functions called GripAble Device [13], which is connected to a tablet by means of a dedicated software application. Its deployment is not based on CPM, as the equipment is driven by the patients themselves, patients who have not suffered total mobility loss. Given the benefits of soft robotics several research teams have developed variants of wearable orthotic devices for the rehabilitation of the hand. Known are for example the Exo-Glove PM--a customizable modularized pneumatic assistive glove [14] or the Harvard Soft Robotic Glove for Neuromuscular Rehabilitation developed by researchers of the Wyss Institute at Harvard University [15]. In both cases the soft actuators are mobilized by compressed air. During rehabilitation exercising motion can often exceed the limits of patient supportability with the consequence of onsetting pain. The system's response time since the moment of pain onset needs to be as small as possible and thus is an essential characteristic of any rehabilitation equipment. While the electrically actuated systems described above do have such a self-adaptive behavior, this is made possible only be excessive sensorization and complicated control diagrams, all of this leading to cost-intensive rehabilitation equipment. Rethinking their construction makes it possible to reduce the cost of rehabilitation equipment. By eliminating many of the sensors, simplifying control diagrams and most importantly by using adjustable compliance actuators (ACAs) performant equipment can be developed at significantly lesser costs. An example of adjustable compliance actuator is the pneumatic muscle whose inherently compliant behavior is due to air compressibility. Characteristics of pneumatic muscles are safe interaction with human operators and their ability of storing and releasing energy into passive elastic elements [16]. Conception of pneumatic muscle actuated rehabilitation equipment for hand joints is still in its incipient stage as to date merely few published patents are known and just a small number of prototypes or functional equipment. One such piece of equipment is the Hand Mentor Pro (manufactured by Motus Nova) actuated by a pneumatic muscle. The Hand Mentor is a stroke rehabilitation device that provides active assistance. It moves the patient like a skilled physical therapist, and is designed for recovering the gripping ability of the hand [17]. Another example of rehabilitation equipment is the EXOWRIST, using four pneumatic muscle actuators to undertake the 2-degrees-of-freedom movements performed by the human wrist. It is characterized by adjustable performance to meet the needs of individualized configuration, assisted movement capabilities, high reliability in different treatment environments and conditions for safe human-robot interactions, low development and construction costs, and high portability for autonomous and independent use [18].