An ankle-foot orthosis (AFO) is an assistive device for improving the function of the foot and ankle joint [66]. Passive AFOs apply springs and dampers to provide motion control during the human gait, but they generally create excessive resistance to plantarflexion in the stance phase, thereby restricting ankle motion and disturbing stability of the leg. Since SMA-based active AFOs have the potential to improve AFO performance, Mataee et al. designed two SMA-based AFOs by taking advantage of their super-elasticity [67]; this is a phase induced phase transformation that provides large elastic deformations. In the first design, an SMA rod was used to provide variable torsional stiffness by exerting an axial preload, while in the second design, the bending stiffness of the SMA element was controlled. As shown in Figure 7a, the super-elastic element was designed as a hinge of the articulated orthosis. The linear actuator was able to control the vertical position of the slider, which determines the active length of the SMA hinge, resulting in the variation of the bending stiffness, which benefits the AFO. Sadeghian et al. also designed a knee-ankle-foot orthosis with stance and swing for patients with quadriceps muscle weakness that operate SMAs, see Figure 7b [68]. The orthosis is needed to maintain the stability of the knee joint during the stance phase and assist in the extension of the leg using SMAs during the swing phase. The swing phase mechanism was based on an SMA, and a cantilever SMA beam was subjected to a bending load during flexion, and the extension motion of the knee joint was achieved by releasing the stored energy in the beam, exploiting the super-elastic properties of SMA. For correcting the walking gait for patients, a longitudinal slider was designed at the end of the SMA beam to adjust the stiffness of the SMA element by varying its effective length. The return torque of the SMA bending element was controlled by the walking speed.
To enhance the performance of SMAs and their controllability, Hau et al. developed an ankle rehabilitation device using SMA wires with a periodic cooling system to provide foot plantarflexion and dorsiflexion during rehabilitation exercise, see Figure 7c [69]. The device consisted of eight aluminium metal plates, a plastic shaft, an aluminium cylinder, a rubber ring, and an aluminium connector. The SMA wire was coiled around the aluminium metal plates, which also act as a cooling system to rapidly cool the SMA wires to improve the speed of response. Four conventional on-off fans were also used to enhance the cooling rate of the SMA wires, producing an airflow of 61 m3/h at 3500 rpm from each fan. The periodic cooling system for SMA wires provided an excellent heating/cooling cycle time of only 5.7 seconds. Most of the SMA actuators presented above use SMA elements with large length-diameter aspect ratios, such as wires, beams, and rods. The deformation of these shapes is mainly in the form of elongation, bending or twisting, which could result in stress concentrations and mechanical failures on the SMA materials, including fatigue. As an alternative approach, Pittaccio et al. created a novel hinge-shape SMA that can be readily applied to the rehabilitation of the elbow and ankle joint, see Figure 7d [70]. Inside the hinge-shaped SMA actuator, an SMA spring was inserted that was shaped like a capital letter omega (Ω). When loaded, the actuator was compressed, and the output force could reach up to 30 N. The hinge-shaped SMA can be trained by using thermo-mechanical treatments in the shape setting stage to suit patients’ needs and provide more personalised characteristics.
Since soft assistive devices have the potential for daily use in the home and to assist the elderly for staircase climbing, Joudzadeh et al. [71] proposed a conceptual design of an SMA-based lightweight wearable device, see Figure 7e. A tendon driven system was used for motion and as illustrated in Figure 7f, the length of the SMA tendon varied when a rotation of the knee angle was produced. Motion during staircase climbing can be divided into two stages: in the first stage, a constant torque was applied at the knee joint to make the concept device move up the stair; in the second stage, a tendon system was substituted in the concept device, and a constant force was applied to the knee-tendon to create the same torque of the first stage. The simulated results show the feasibility of using SMA wires as actuators for this form of assistive device. However, the time for cooling SMA wires is approximately 90% of the climbing cycle, therefore an efficient cooling system is needed to achieve a shorter cycle time. To assist the upper limb muscular strength, Park et al. proposed a SMA-based fabric muscle (SFM) which is placed in a wearable arm suit, see Figure 7g [72]. When a wearer lifts an object and moves, the SFMs contract and assist the muscular strength of the arms and the contraction length of the SFM can be regulated to operate at a range of conditions. A position controller for the length of the SFM was developed, in which a K-type thermocouple was used to monitor the SFM’s temperature and control the input current for Joule heating. While the wearable device weighs less than 1 kg, it can lift over 4 kg in weight. In addition, the response time of moving to the target position was only 3 seconds, but the required relaxation time during cooling is approximately 30 s. Therefore, an additional cooling system can further enhance the performance of the soft arm suit.