2.9.1. Electric Motor

LCE has outstanding advantages such as large deformation, softness, multi‐function, multiple response modes, durability, etc. However, the limitation of LCE as a wearable actuator material lies in its inherent properties. Phase changing from isotropic to smectic can only be induced by heating, which restricts its application. Although a variety of actuation modes have been achieved through the modification of LCE, they are realized through the form of energy conversion. In addition, the excessively high driving temperature of LCE (>100 °C) hinders its application on the human body. Some research has shown that the LCE molecular structure can be modified to reduce its driving temperature [158,159]. However, it will weaken the performance. Aside from these, slow response speed is a major problem. However, its excellent deformability, structure stability and processability endow its significant advantages as actuator. The application of LCE as a wearable actuator still needs further research. Besides pneumatic and hydraulic actuators, electric-driven actuators are also widely applied in exoskeletons. Compared to pneumatic or hydraulic actuators, electric actuator systems inevitably come with many problems, such as: friction, stiffness, electrical disturbance and complex systems [13]. Nevertheless, electric actuators are still the most common actuators in the industry due to their irreplaceable processability and designability. Gassert et al. present a wearable glove system for grasping assistance [160]. Due to a compliant finger mechanism and versatile thumb mechanism, this system can provide assistance for the most commonly used grabbing exercises. An exoskeleton that can assist walking and running separately was first reported by Kim et al. [161]. Through the electrical motors connected to the cable, this device applies tension between the waist belt and the thigh wrap, creating an external extension moment around the hip joint. Based on the estimation of potential energy fluctuation of the wearer's center of mass, this device can switch the actuation modes between running or walking.

#### *2.9. Other Actuators* 2.9.2. Carbon Nanotubes

2.9.1. Electric Motor Besides pneumatic and hydraulic actuators, electric‐driven actuators are also widely applied in exoskeletons. Compared to pneumatic or hydraulic actuators, electric actuator systems inevitably come with many problems, such as: friction, stiffness, electrical disturbance and complex systems [13]. Nevertheless, electric actuators are still the most Carbon nanotubes (CNTs) have attracted great interest in recent research due to their distinguished mechanical, electrical and chemical properties [162]. With the advancement of process methods, carbon nanotube-based sensors, electrodes, actuators and energy storage materials have been widely used in smart systems. In particular, CNTs-based yarns with twist, or introducing a yarn structure in CNTs, can further enhance its flexibility, allowing it to be used in actuators such as artificial muscles [163]. By far, researchers

common actuators in the industry due to their irreplaceable processability and designability. Gassert et al. present a wearable glove system for grasping assistance [160].

find that CNTs can be actuated by electrical, chemical, thermal, or photonic power [164]. Among them, electrochemical actuation of CNTs received special attention [165]. Compared to electrothermally driven CNT yarns, electrochemical driven CNT yarns provide higher efficiencies and larger torsional or tensile actuation with lower voltage requirements [166]. Similar to CPs, the mechanism of electrochemical actuation is the strain and volume change on the graphene layers induced by electrochemical double-layer charge injection (Figure 25a) [162]. Foroughi et al. have first demonstrated a torsional and rotational CNT yarn which can provide a reversible 15,000◦ rotation and 590 revolutions per minute. [167]. This kind of CNT yarn, which was twist-spun from forests of multiwalled carbon nanotubes (MWCNTs), can undergo partial untwisting in the electrolyte applied with an electric field to achieve actuation. Kim et al. have designed a hierarchically twisted electrochemical driven CNT yarn with a maximum tensile stroke of 15.1% and work capacity of 3.78 kJ/kg. [168]. This twisted and coil structure achieves the transformation of electrochemical driven CNT yarn from rotation actuation to stretching actuation (Figure 25b). researchers find that CNTs can be actuated by electrical, chemical, thermal, or photonic power [164]. Among them, electrochemical actuation of CNTs received special attention [165]. Compared to electrothermally driven CNT yarns, electrochemical driven CNT yarns provide higher efficiencies and larger torsional or tensile actuation with lower voltage requirements [166]. Similar to CPs, the mechanism of electrochemical actuation is the strain and volume change on the graphene layers induced by electrochemical double‐ layer charge injection (Figure 25a) [162]. Foroughi et al. have first demonstrated a torsional and rotational CNT yarn which can provide a reversible 15,000° rotation and 590 revolutions per minute. [167]. This kind of CNT yarn, which was twist‐spun from forests of multiwalled carbon nanotubes (MWCNTs), can undergo partial untwisting in the electrolyte applied with an electric field to achieve actuation. Kim et al. have designed a hierarchically twisted electrochemical driven CNT yarn with a maximum tensile stroke of 15.1% and work capacity of 3.78 kJ/kg. [168]. This twisted and coil structure achieves the transformation of electrochemical driven CNT yarn from rotation actuation to stretching actuation (Figure 25b).

can assist walking and running separately was first reported by Kim et al. [161]. Through the electrical motors connected to the cable, this device applies tension between the waist belt and the thigh wrap, creating an external extension moment around the hip joint. Based on the estimation of potential energy fluctuation of the wearer's center of mass, this

Carbon nanotubes (CNTs) have attracted great interest in recent research due to their distinguished mechanical, electrical and chemical properties [162]. With the advancement of process methods, carbon nanotube‐based sensors, electrodes, actuators and energy storage materials have been widely used in smart systems. In particular, CNTs‐based yarns with twist, or introducing a yarn structure in CNTs, can further enhance its flexibility, allowing it to be used in actuators such as artificial muscles [163]. By far,

*Textiles* **2021**, *1*, 31

2.9.2. Carbon Nanotubes

device can switch the actuation modes between running or walking.

CNTs yarn can be considered as CPs and applied into textile structures. For example, they can be fabricated with other yarns or applied in smart textile systems. However, due to their poor stretchability, high cost and low energy conversion efficiency, CNTs based wearable actuators need further research. For more detail about CNTs actuators, we refer to a review by Jang et al. [163]. CNTs yarn can be considered as CPs and applied into textile structures. For example, they can be fabricated with other yarns or applied in smart textile systems. However, due to their poor stretchability, high cost and low energy conversion efficiency, CNTs based wearable actuators need further research. For more detail about CNTs actuators, we refer to a review by Jang et al. [163].

**Figure 25.** (**a**) Schematic of the mechanism for electrochemical actuation of CNT [162]. Copyright 2015, Elsevier. (**b**) MWNT yarn structures for torsional and tensile actuation [166]. Copyright 2014, American Chemical Society. **Figure 25.** (**a**) Schematic of the mechanism for electrochemical actuation of CNT [162]. Copyright 2015, Elsevier. (**b**) MWNT yarn structures for torsional and tensile actuation [166]. Copyright 2014, American Chemical Society.

#### 2.9.3. Hydrogels 2.9.3. Hydrogels

Stimuli‐responsive gels refer to a category of hydrogels that can respond to external stimuli. These stimuli include temperature, electric field, light and PH change. The mechanism of actuation of hydrogels is similar to the material mentioned in Section 2. These materials are widely used in biomedical applications and wearable actuators. The Stimuli-responsive gels refer to a category of hydrogels that can respond to external stimuli. These stimuli include temperature, electric field, light and PH change. The mechanism of actuation of hydrogels is similar to the material mentioned in Section 2. These materials are widely used in biomedical applications and wearable actuators. The exoskeleton made by PVC gel in Section 2 is one example of hydrogels as wearable devices. The chemical stability and durability are the limitation for them to be applied in wearable actuators. For more information we refer to a review by Mirvakili et al. [8].

#### 2.9.4. Organic Molecule-Driven Polymeric Actuators

Organic molecule-driven polymeric actuators can be defined as actuators that can have mechanical motions induced by organic solvents. This driving principle is usually the volume change or molecular structure change caused by the absorption and release of the organic solution. For example, LCEs can generate inhomogeneous swelling when absorbing polar organic solvents. Such materials can also be twisted into artificial muscles. However, the actuation mechanism and complex synthesis process are the major problems. For detailed information, we refer to a review of Lin et al. [169].
