Soft pneumatic fluid-powered actuators have also been developed and applied for upper extremity rehabilitation and assistance. They are used for assistance and rehabilitation of fingers [43], hands [39-42], wrists [47], and elbows [38], and support of upper limbs [45,46] and the back [44]. They are lightweight and cost-efficient, but more complex to control for a variety of motions. For example, controlling the devices for different finger movements synchronously and accurately is challenging. Real-time feedforward and feedback control systems (e.g. PID control [39], adaptive assistive control [38], integrated electromyography (EMG) -radio-frequency identification (RFID) control [40]) have been developed and used for the devices to achieve desired performance. To increase the comfortability and wearability, fabric-based pneumatic actuators are popular, and they can provide a reasonable force range for hand movement (e.g. 14.3 N at 70 kPa [42]) and perform a wide range of motion. In addition, McKibben pneumatic muscle-based devices are also developed for higher force (e.g. 450 N at 60 kPa [44]) for support purposes, such as lower back and upper limb support. Advanced manufacturing technologies, such as 3D-printed technologies, has also been used in the field. The soft robotic wrist sleeve (SWS) is a promising example which provides flexible foldable actuators, which has significant potential for the development of cost-efficient and easy-to-manufacturing assistive devices. It will enhance our capability for ‘making it ourselves’ assistive devices at home for daily activities.
Hydraulic fluid-powered actuation
Hydraulic fluid-powered systems provide a high-power density by pressurising fluids of high viscosity and low compressibility in their actuators; the power density is particularly high when compared to pneumatic fluid-power systems which often use air as the fluid medium. Hydraulic fluid-powered actuation therefore combines the advantages of high-power density (e.g. 180 kW/m3), fast dynamic response (within a few millisecond), and good controllability. This actuation mechanism has been used for soft rehabilitation and assistive devices. Polygerinos et al. [48] developed a soft robotic glove using fluid-driven fibre-reinforced elastomer actuators to augment hand rehabilitation for individuals with functional grasp pathologies, as shown in Figure 4. The soft actuators were mechanically programmed using a specific arrangement of fibres and limit layers to match the range of motion (ROM) of human hands. To operate the soft robotic glove, a closed-loop control system was designed to regulate the actuation pressure and a sliding mode control was used to control the soft actuator precisely. The actuator was able to respond to a step response within 0.2 s (65%) and reach a steady-state after 2.2 s (95%). A qualitative test showed that the glove could fulfil the range of activities need for daily life where the glove was used by a healthy individual. The results showed that the glove could generate sufficient force to perform grasping a 500 g tin can without biological muscle effort. The glove was also evaluated by a participant with muscular dystrophy and preliminary testing showed that the participant could effectively grasp, hold, and release a wooden block using a glove with sEMG control. The actuator was also customised based on the user’s biomechanics and evaluated by a motion capture system, showing an almost equivalent ROM to the user’s biomechanics [49]. Preliminary evaluation of the glove on a patient with reduced hand function was also conducted, and the result showed that the patient can perform tasks faster with a greater degree of functional grasping in a standardized Box-and-Block test with the assistance of the sEMG controlled glove.