# Review of the Research Progress in Soft Robots
## 2 Driven Methods and Materials for Soft Robots
### Twisted and Coiled Polymer Actuator (TCPA)  
TCPA is a novel soft actuator. It is fabricated by twisting nylon thread or fishing line. It can be thermally activated and has remarkable properties such as a high power/mass ratio and large deformation. By applying conductive nylon fibers to the actuator, it can  
Figure 7: (**a**) Computer-aided design model of the entire robot [74]. (**b**) Fully soft, autonomous robot, Octobot [76].  
be electrically driven by Joule heating. IPMC can achieve a large deformation with low voltage (1-3 V) and high speed motion, but the manufacturing cost is relatively high. DEA can achieve a large strain (<400%), but DEA requires a high driven voltage [78]. Conductive polymer actuators can achieve large deformations at low voltages (1-4 V), but the response is slow [79]. In contrast, TCPA has the following remarkable properties.  
* Its fabrication cost is extremely low since it is easily fabricated by commercially available nylon fibers.
* It realizes large deformation over 20%.
* It is flexible and lightweight.
* Its motion is silent.
* The actuation environment is unbounded, such as in water, in wet conditions, or in a magnetic field.
* Compared to the shape memory alloy actuators, a hysteresis property of TCPA is small.  
TCPA has been developed for various applications in robotic systems, such as robotic hands [80], power assist systems [81], mobile robots [82], deformation mechanisms for flying robots [83], and bionic robots [84]. Therefore, TCPA has good prospects for soft robots.  
Due to the wide range of smart materials and the obvious differences in performance between the different smart materials, as shown in Table 2, only the performance of the fluid driven and chemical reaction driven were comprehensively compared.  
Gas media are widely used in soft robots because of their light weight, a wide range of sources, lack of pollution, and good safety. The pneumatic-driven requires an external compressed air pump to store the gas, which is bulky and consumes a lot of gas, greatly limiting the application of soft robots in non-structural environments. In addition, air pressure and flow rate vary nonlinearly with time, which makes it difficult to control the robot accurately in real time, and the control has hysteresis. Liquids have good incompressibility, high response frequency, can withstand high loads, and can be used with specific pumps to build a recirculation loop, which in turn can supply energy to the soft robot for a long time. However, the hydraulic-driven requires an external hydraulic pump to provide power, and the hydraulic system is generally larger and more energy-consuming.  
Due to the different types of chemical reactions, the performance of the chemical reaction driven varies greatly. The chemical reaction speed inside the actuator is generally difficult to accurately control; thus, the controllability of the chemical actuator is poor [73]. Although the chemical reaction driven has fast response, it has low energy utilization, poor stability, and short life.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & & **Advantages** & **Disadvantages** \\ \hline \multirow{4}{*}{Fluid driven} & \multirow{4}{*}{Pneumatic driven} & Good safety, low production cost, large deformation, and light weight & Low output force, hysteresis, and external air supply required \\  & & & \\ \cline{2-3}  & & Fast response speed, large output force, circular loop, and lower working noise & Heavy weight, external hydraulic pump, and high energy consumption \\ \hline \multirow{4}{*}{Chemical reaction driven} & \multirow{4}{*}{Pneumatic driven} & \multirow{4}{*}{Fast response and large output force} & Low energy utilization, poor stability, poor controllability, and short service life \\  & & & \\ \cline{1-1} \cline{3-3}  & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Performance comparison of fluid driven and chemical reaction driven.