# Mechanical Structural Design and Actuation Technologies of Powered Knee Exoskeletons: A Review
## 2 Mechanical Structural Design of Knee Exoskeletons
### Complex Design Based on Biological Geometries  
In fact, the human knee joint, as a complex multi-center form, contributes to its highly flexible and adaptable human lower limb locomotion. However, the single mechanical rotation model for the knee joint is prone to cause joint dislocation when the rotation centers of human body and the machine do not coincide with each other [38]. In the last decade, a number of gait rehabilitation devices were developed to eliminate negative effects of the closed leg-exoskeleton kinematic chain on a human knee. The multi-bar mechanism can provide a polycentric motion similar to the human knee joint to achieve self-alignment, so it can reduce the potential discomfort caused by the movement inconsistency between the wearer and assistive devices. [PERSON] et al. [11] proposed a modular knee exoskeleton system to support the knee joint of hemiplegic patients, as shown in Figure 3a. This device aims to realize the multi-center movement of the real human knee joint through a four-bar mechanism. For the sake of providing redundant DOFs to compensate for the knee joint, [PERSON] et al. [39] explored a power-aided exoskeleton for the knee joint utilizing two generalized conforming quadrilateral structures. To well adjust the translational motion of the knee joint and allow for a natural gait, [PERSON] et al. designed a novel mechanism that combined a four-bar linkage and a prismatic joint into a two-degree-of-freedom hinge for knee osteoarthritis [40]. However, increasing the degree of freedom by increasing connecting rods inevitably led to the complexity and inertia of the structural design. Figure 2: Knee exoskeletons with a simplified structure design: (**a**) the ERF based knee device; (**b**) the RoboKnee exoskeleton; (**c**) the EICOSI orthosis; (**d**) the KAFO; (**e**) the knee exoskeleton using IMU sensors. Meanwhile, [PERSON] et al. [41] presented a Gait-Enhancing Mechatronic System (GEMS) equipped with a pulley and several rolling cams to adapt to the joint motion of a wearer, as shown in Figure 3b, the articulated joint could effectively deliver the required torque and prevent discomfort in the process of physical gait assistance. Based on the knowledge of a knee-joint kinematics, [PERSON] et al. [9] introduced an adaptive knee-joint exoskeleton that incorporated a pin slider/cam, and the investigation indicated that it could effectively minimize internal joint forces and torque via the human-machine interaction. Besides, to achieve the alignment with the instantaneous knee center of rotation, [PERSON] [42] and [PERSON] et al. [43] developed a wearable walking assistance device employing cam and slider mechanisms. These optimized mechanisms were designed to replicate the reference path of the human knee, however, the misalignment is the main problem to be considered that other design objectives have not been considered. Pulley, belt, and other mechanical parts are combined together to enhance the flexibility and self-aligning ability of the knee joint. To provide effective torque assistance for the knee joint without disrupting the natural gait of the wear, [PERSON] et al. [44] developed a personal joint mechanism in conjunction with two-stage timing belt transmission system, as shown in Figure 3c. [PERSON] et al. [45] introduced a novel knee exoskeleton with three co-centered rings driven through a belt drive transmission for robot-assisted rehabilitation and indicated that the robot was capable of accommodating transitional movements of the knee joint along with its rotation. Obviously, the intelligent design utilizing the belt and machine parts can also effectively minimize or eliminate the misalignment between robot and human knee joint. However, these knee assistance devices are not ideal due to the complexity and large size of the belt transmission system. Furthermore, many other mechanisms have been designed to adapt to the mismatching between the instantaneous center of the rotation of human lower limb locomotion and the knee exoskeletons.