# Review of the Research Progress in Soft Robots
## 2 Driven Methods and Materials for Soft Robots
### Smart Materials and Their Driven Methods
#### 2.2.1 Shape Memory Material (SMM)  
SMM is a kind of smart material with good development prospects. It can sense and respond to external stimuli [29], while it can adjust its mechanical properties to return to the initial state. Typical SMM is mainly Shape Memory Alloy (SMA) and Shape Memory Polymer (SMP). SMA is an intelligent material with a shape memory effect, super elasticity, and high damping; it can experience reversible martensitic transformation due to the applied thermo mechanical load, and can recover permanent strain when heated to a certain transformation temperature [30]. It can sense temperature changes and convert thermal energy into mechanical energy, as well as it can output force and displacement, or store and release energy [31]. At present, the advantages of SMA are simple fabrication, low cost, and light weight, high power density, high driving force, and large driving displacement; the disadvantages of SMA are easy aging of material, difficult to accurately control the accuracy of the driven temperature, and low driven frequency. Shown in Figure 3a, inspired by the inchworm, [PERSON] invented a soft crawling robot whose motion is based on the interaction between SMA driven and continuous elastomers [32]. [PERSON] et al. created a new class of fast, high-curvature, low-voltage, reconfigurable, micron-scale shape memory actuators. Shown in Figure 3b, they made the world's smallest self-folding paper bird [33]. Shown in Figure 3c, [PERSON] and [PERSON] used the SMA spring to simulate the structure of the trunk corresponding to the muscle of the inchworm. The new type of microrobot's body structure, which could realize the bending deformation by simulating the body structure of the inchworm, was designed [34]. Shown in Figure 3d, IIT Indore developed a soft robot jellyfish based on SMA polymer. It could expand and contract the tentacles by heating and cooling, thereby generating propulsion to move through the water [35]. [PERSON] et al. invented a variable stiffness lower limb exoskeleton robot based on SMA. The variable stiffness exoskeleton knee joint could effectively assist the wearer to walk. It had a good actuation effect, light weight, and high strength, so it could protect the wearer well [36]. Professor [PERSON] fabricated soft composite actuators by embedding SMA wires into soft polymer matrices. The actuator could deform in large bends, retain its deformed shape, and regain its original shape by changing temperature only [37]. SMP is a kind of material that can fix in a temporary shape. It can also revert from the temporary shape to the initial shape under external stimulation such as light, electric field, magnetic field, heat, acid, and base [38]. SMP has the advantages of large deformation, light weight, simple structure, etc. Compared with SMA, SMP has more forms of stimulation, including heat, light, electric field, magnetic field, humidity, chemical stimulation, etc. The rich forms of stimulation make the design of the SMP driven more diversified and the application range is wider. However, the driving force of the SMP driven is smaller, less stable with less precise control, and more demanding on the working environment. In order to solve the problem of variable stiffness in soft robots, [PERSON] et al. processed the SMP into a thin sample strip embedded in the bottom layer of the flexible sensor [39]. Inspired by a multi-bit screwdriver, [PERSON] et al. created a self-healing, light-driven SMP called \"five-petal flower\", which enabled soft robots to adapt quickly to changes in their environments [40]. Professor [PERSON]'s team proposed a universal gripping strategy that used SMP smart plastic as the material. The gripper could be scaled according to different sizes, arbitrary shapes, and different quantities of objects [41]. [PERSON] et al. combined programmableSMP with 3D printing technology and designed a shape memory occlusion that was biodegradable, remotely controlled, and personalized.