Shape memory alloys (SMAs) wires and springs are gradually used in the field due to their high energy density, large recovery stress (e.g. 800 MPa), high strain (typically ~8%), lightweight nature and zero noise. Ni-Ti based alloys is widely used as the material. Current SMA-based rehabilitation and assistance devices include soft knee, ankle and foot orthoses [57, 67, 69-71], hand, finger and elbow assistive devices [58-60, 70], soft gloves for wrist joint rehabilitation [62, 63], soft devices for climbing assistance [71] and muscle strength assistance [72]. They provided force in the range of 10 N – 100 N. However, since they require changes in temperature to produce phase changes of the alloy they present a long response time (10s – 30s) and significant nonlinearity, resulting in the challenge in the design of control systems. The state-of-the-art devices use PID controllers [62-63, 68], adaptive control systems [31, 33] and Bluetooth® tracing technology [70] in order to achieve desired motions. The control of SMAs needs to be further investigated to enhance the capabilities of the soft devices for better assistance. SMA-based soft rehabilitation and assistive devices are still in their infancy. Research in the development of SMA materials, fundamental understanding in material properties, control and sensing methodologies need to be explored for the potential of SMA-based assistive devices for patients’ rehabilitation and assistance of people’s daily activities.
Dielectric elastomers actuators (DEAs)
Dielectric elastomers actuators (DEAs) have been used for soft actuators and robots because of their advantages of large strain and high compliance. Currently, commercially available acrylate-based (e.g., VHBTM) polymers are often used for DEAs because they exhibit large area strains of up to 380% and possess relatively high dielectric constants (r ~ 4.7) and breakdown field strengths [73]. Since Young’s modulus of dielectric elastomers is in the range of biological tissues, rehabilitation and assistive devices that aim to exploit DEAs can provide safe human-device interaction. DEAs also have good muscle-like behaviour, which can mimic the behaviour of skeletal muscles. Lidka et al. studied the feasibility of using a 40 μm thick silicone-based DEA as an actuator within a wearable assistive device for wrist rehabilitation [74]. The DEA was sprayed with a graphite powder to act as a stretchable electrode, which was tested for its capabilities for wrist rehabilitation in terms of force and range of motion. The results showed that this single layer DEA is insufficient to actuate an assistive wearable wrist rehabilitation device, as the maximum output force is only ~ 0.93 N. Therefore, a DEA device based on multiple layers was recommended to enhance the device output performance. Behboodi et al. investigated the electromechanical properties of DEAs by assessing their capabilities and limitations for the use in robotic rehabilitation devices [75]. A commercial stacked DEA manufactured by CTSystem [76] was used for investigating the concept. The strain of this stacked DEA device was 3.3%, which is lower than human skeletal muscles of 20%. While the maximum cycle life of mammalian skeletal muscles is 1000 million cycles, the cycle life of stacked DEA is 50 million cycles, which is considered sufficient for rehabilitations.
Increasing interest of DEA-based soft rehabilitation and assistive devices has focused on the improvement of the output strain and force, along with the reduction of the size. Duduta et al. developed a stacked DEA to actuate elbow rehabilitation robotic devices, as shown in Figure 8a [77]. This soft composite DEA combines a UV-curable strain-stiffening elastomers with ultra-thin carbon nanotube (CNT)-based percolative electrodes. Compared with conventional carbon grease electrodes, the CNT electrodes can help increase the dielectric breakdown field, leading to operation at high electric fields and therefore higher strain or force. Since the output force of a single-film DEA is usually small, they used a multi-layer structure to achieve the desired forces (>10 N) and displacements (>1 cm). The stacked DEA demonstrated a peak energy density of 19.8 J/kg, which is close to the upper limit for natural muscle (0.4–40 J/kg). Carpi et al. [78] proposed the concept of using a folded DEA to optimise the dynamic hand splints for finger rehabilitation, see Figure 8b. The elastic band in conventional dynamic hand splints is substituted with a folded DEA to achieve active control. The folded DEA was fabricated using a silicone substrate and a silicone/carbon black stretchable electrode. As shown in Figure 8c, the folded DEA contracts when actuated by an applied potential and pulls up the finger; when the actuation signal is removed, the folded DEA returns to its original state. The electrical activation of the DEA is utilised to control the compliance of the system, enabling the regulation of the force against finger movements. Behboodi et al. developed an elbow joint rehabilitation robotic device actuated by a 3×5 DEA array, see Figure 8d [79]. The 3×5 DEA muscle was directly attached to the forearm of a phantom model. The resulting DEA array was able to significantly improve the driving force of artificial muscles up to 30.47 N and provide 19.5° of elbow flexion and 16.2°/s angular velocity for the elbow, under a 1 N tensile load.
To reduce the size of actuators, Saharan et al. [80] utilised Twisted and Coiled Polymer (TCP) muscles to assemble 3D printed hand orthosis for facilitating the motion of hand joints, see Figure 8e. The TCP muscles were fabricated using silver coated Nylon 6,6 threads. The soft orthosis consisted of an arm attachment, stitched fabric for the wrist and the palm with guideways, and three rings per finger with one for each of the three finger joints. The TCP muscles were actuated by an electrothermal field and could achieve temperatures up to 250°C during actuation. A thermal protection layer below the TCP muscles was designed for the safety of the user. The orthosis was capable of restoring basic human hand movements, such as flexion and extension. While the device weighed only 90 grams, it was able to grasp everyday use items. Amin et al. [81] proposed a concept of a soft hand rehabilitation device, as shown in Figure 8f, which was actuated by a spring-roll DEA that provided the advantages of small size, simple structure and ease of control, as shown in Figure 8g. The DEA was made of VHB 4910 elastomer sheets and carbon conductive grease electrodes. The application of a pre-strain to the dielectric elastomer was able to improve the actuator performance and the breakdown strength [82]. A metallic screwed core was used to compress the spring before wrapping the pre-stretched dielectric elastomers. The spring roll DEA was small and easy to control and the output axial forces under activated and deactivated status were 12.66 N and 14.71 N, respectively.