# Mechanical Structural Design and Actuation Technologies of Powered Knee Exoskeletons: A Review
## 3 Actuation Design
### Pneumatic Muscle Actuators  
Pneumatic muscle actuators inflate a sheathed bladder by employing a compressed gas to make it contract lengthwise when expanded radially and have been successfully applied in robotics and robotic therapy. In comparison with conventional driving actuators in human-robot interaction, the pneumatic muscle actuator has unique advantages including light weight, simple structure and intrinsic compliance [67]. Thus, the application of pneumatic artificial muscle in the knee exoskeleton robots have been explored and a large number of related prototypes have been developed.  
[PERSON] et al. [68] developed a unilateral exoskeleton with a knee joint to assist the rehabilitation of gait, as shown in Figure 5a. The powered exoskeleton was actuated by two antagonistically pleated pneumatic artificial muscles, and the force was transmitted to the lower leg link by means of two four-bar linkages. In order to realize a wearable robot for enhancing human walking, [PERSON] et al. [69] developed a robotic knee exoskeleton with soft McKibben style pneumatic artificial muscles (PAMs). As shown in Figure 5b, the actuator could improve the compliance of the exoskeleton by controlling the internal air pressures of agonist-antagonist PAMs and respectively adjusting its balance point and stiffness linearly. In addition, [PERSON] et al. [70] developed a powered suit actuated by straight-fiber-type artificial muscles for knee auxiliary, as shown in Figure 5c. The assistive suit was composed of wire, pneumatic artificial muscles, and some fixed parts, and it could generate torque in the knee joint as the rotation center in case the wire was tensioned by the contraction of the artificial muscle. As shown in Figure 5d, [PERSON] et al. [71] developed a flexible wearable exoskeleton with a novel pleated pneumatic interference actuator to assist knee movement. As a soft structure, the actuator could stably and comfortably allow the knee to generate sufficient forces for extension and flexion. [PERSON] et al. [72] developed a compliant exoskeleton system which utilized artificial pneumatic muscles to provide torque assistance at the knee, as shown in Figure 5e, and the experiment showed that the actuator performed more positive mechanical work than negative mechanical work compared to normal walking conditions without the device. Besides, many other exoskeletons have been designed to provide torque assistance to the knee joint through pneumatic muscle actuators, such as a lower-limb exoskeleton with hybrid actuation for the strength augmentation [73], a power exoskeleton to assist weakened individuals during locomotor training [74], and an assistive exoskeleton system [75] for normal walking.  
Figure 5: Knee exoskeletons with pneumatic muscle actuators: (**a**) the powered exoskeleton actuated by two antagonistic pleated PAMs; (**b**) the robotic knee exoskeleton with soft McKibben PAMs; (**c**) the power assist suit actuated by straight-fiber-type artificial muscles; (**d**) the flexible wearable exoskeleton with a novel pleated pneumatic interference actuator; (**e**) the compliant exoskeleton system actuated by artificial pneumatic muscles.  
As mentioned above, pneumatic muscle actuators are suitable for the exoskeleton type of robotic devices due to their intrinsic compliance and large force-to-weight ratio. Nevertheless, pneumatic muscles-based system may not be further developed because the pneumatic power supply device could bring a burden and generate noise. In addition, considering that the dynamics of PMAs is highly nonlinear and subjected to hysteresis, these developed knee exoskeletons may produce uncomfortable wearing experiences due to the difficulty in precise tracking control.