# Mechanical Structural Design and Actuation Technologies of Powered Knee Exoskeletons: A Review
## 3 Actuation Design
### Other Actuators  
In recent studies, smart and composite materials have demonstrated enormous potential in knee exoskeletons to realize compliant human-robot interaction, including shape memory alloy (SMA) actuators [83], vacuum-actuated rotary actuators [84], and plasticized polyvinyl chloride (PVC) gel soft actuators [85]. These innovations have their own unique means to efficiently imitate the nonlinear characteristics of the human knee joints. Nevertheless, to the best of our knowledge, some essential problems such as nonlinear hysteresis effect, high technical difficulty, and complex control remain to be solved before their further applications.  
Based on the above studies, Table 2 shows the comparison results of the actuation designs of existing powered knee exoskeletons, including the characteristics, advantages, and disadvantages of each category. Regardless of the application domain or body area, the actuator should be selected according to the intention or purpose of the designer because each device has its own advantages and disadvantages. In practice, human-computer interaction security, motion flexibility, and adaptability to complex environments are required in wearable knee exoskeletons. Besides, in the design of knee exoskeletons, the tradeoff between weight and the output torque should be fully considered so as to develop a more compact and lightweight mechanism with improved wearing comfort.  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Type of Actuator** & **Driving Devices** & **Advantages** & **Disadvantages** \\ \hline \multirow{4}{*}{\begin{tabular}{} \end{tabular} } & (a) linear hydraulic actuators; & (a) fast response speed; & (a) with large stiffness; \\  & (b) linear pneumatic cylinder; & (b) easy to control; & (b) poor human-machine interaction; \\  & (c) motor in combination with & (c) large drive and auxiliary & (c) defects in the weight and drive flexibility; \\  & (d) other transmission mechanisms; & torque; & (a) slow response time; \\ \hline \multirow{3}{*}{\begin{tabular}{} \end{tabular} } & (a) antagonistic pleated PAMs; & (a) simple structure; & (a) slow response time; \\  & (b) McKibben style PAMs; & (b) compliant driving; & (b) fixed movement; \\  & (c) straight-fiber-type PAMs; & (c) small motion inertia; & (c) low control precision; \\ \hline \multirow{4}{*}{\
\begin{tabular}{} \end{tabular} } & (a) adopted the combination of series and parallel springs; & (a) behaves similar to biological muscles; & (a) multiple components and complex structure; \\  & (b) clutchable elastic device based & (b) adaptable to multiple & (b) the drive system is relatively bulky; \\  & on Hill muscle model; & application; & (c) relatively bulky; \\  & (c) multi-configuration elastic driving model; & (c) friendly to user; & (c) lack of robustness; \\ \hline \multirow{2}{*}{
\begin{tabular}{} \end{tabular} } & (a) shape memory alloy; & (a) soft, light and flexible; & (a) geometries must be exact; \\  & (b) plasticized polyvinyl chloride; & (b) biomimicry; & (b) difficult to control; \\  & (c) other novel function material; & (c) high force/weight ratio; & (c) nonlinear hysteresis effect \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison results of the actuation designs of existing powered knee exoskeletons.  
Figure 6: Knee exoskeletons with variable stiffness actuators: (**a**) the wearer’s knee joint equipped with two springs; (**b**) the knee exoskeleton employing two springs working both in series and in parallel; (**c**) the wearable exoskeleton actuated by tendon-sheath artificial muscles; (**d**) the robotic knee exoskeleton with a variable stiffness actuator.