2.3.4. Merits and Limitations

There are some advantages and limitations for the thermal or hygroscopic driven actuators. Commercially available fibers such as nylon, PE, and bamboo are inexpensive and they can be easily converted into torsional and tensile actuators. In terms of the structure, although several film-based actuators have been demonstrated, they are difficult to be developed into a textile structure for wearables. Fiber-based actuators are more promising and have demonstrated their capabilities to be used for personal thermal regulation. Moreover, thermal responsive actuators usually need a high temperature for desired stress and strain, causing possible discomfort and injuries for humans, and thus hinders their applications in smart textiles. In addition, the energy efficiency of heat driven actuators is very low (<1%), and the cycling rate is low due to the poor heat diffusion and dissipation, especially during the cooling process, so they take a longer time for heating

and cooling [2]. For moisture responsive actuators, their durability and performance need to be further explored. *Textiles* **2021**, *1*, 10

**Figure 6.** Moisture and heat responsive smart textiles. (**a**) Nafion sheet schematics with openable flaps mimicking thermo‐adaptive functionality of human skin [70]. Copyright 2017, Springer Nature. (**b**) The schematic of the thickness reversible structure using nafion as a thermally adaptive interlayer [70]. Copyright 2017, Springer Nature. (**c**) Schematic diagram of moisture sensitive clothing changing sleeve and pant length before and after exercise [57] Copyright 2017, Wiley‐VCH. (**d**) Running suit prototype with ventilation flaps based on the moisture sensitive biohybrid two‐ layer film [72]. Copyright 2017, the authors, published by AAAS. **Figure 6.** Moisture and heat responsive smart textiles. (**a**) Nafion sheet schematics with openable flaps mimicking thermo-adaptive functionality of human skin [70]. Copyright 2017, Springer Nature. (**b**) The schematic of the thickness reversible structure using nafion as a thermally adaptive interlayer [70]. Copyright 2017, Springer Nature. (**c**) Schematic diagram of moisture sensitive clothing changing sleeve and pant length before and after exercise [57] Copyright 2017, Wiley-VCH. (**d**) Running suit prototype with ventilation flaps based on the moisture sensitive biohybrid two-layer film [72]. Copyright 2017, the authors, published by AAAS.

#### 2.3.4. Merits and Limitations *2.4. Dielectric Elastomer Actuators*

#### There are some advantages and limitations for the thermal or hygroscopic driven 2.4.1. Mechanism

2.4.1. Mechanism

actuators. Commercially available fibers such as nylon, PE, and bamboo are inexpensive and they can be easily converted into torsional and tensile actuators. In terms of the structure, although several film‐based actuators have been demonstrated, they are difficult to be developed into a textile structure for wearables. Fiber‐based actuators are more promising and have demonstrated their capabilities to be used for personal thermal regulation. Moreover, thermal responsive actuators usually need a high temperature for desired stress and strain, causing possible discomfort and injuries for humans, and thus hinders their applications in smart textiles. In addition, the energy efficiency of heat driven actuators is very low (<1%), and the cycling rate is low due to the poor heat diffusion and dissipation, especially during the cooling process, so they take a longer time for heating and cooling [2]. For moisture responsive actuators, their durability and performance need to be further explored. *2.4. Dielectric Elastomer Actuators* Dielectric elastomer actuators (DEAs) are electronic electroactive polymers that enable electromechanical transduction at effective electrical fields, transferring electrical energy into mechanical work [7,73]. The working principle of a DEA is based on the basic configuration that two compliant electrodes are coated on each side of a thin polymer film to form a deformable capacitor (Figure 7). According to the elastic electrostatic model first established by Pelrine et al. [74,75], due to the inherent volume incompressibility, when an electrical field is applied perpendicularly to the plane of the two electrodes, two mechanisms are generally induced: the attractive electrostatic forces among opposite charges on each electrode cause the polymer film to compress in thickness and the repulsive electrostatic forces among like charges over the same electrode render further expansion in area, reducing the electrical energy [7,74]. For an ideal elastic model, electrostatic energy is equally converted to mechanical energy through lateral and transverse displacement [74]. The capacitance (*C*) of the dielectric film is

$$\mathbf{C} = \varepsilon\_0 \mathbf{e} \mathbf{A} / \mathbf{z} \tag{2}$$

Dielectric elastomer actuators (DEAs) are electronic electroactive polymers that enable electromechanical transduction at effective electrical fields, transferring electrical and thus the stored electrostatic energy (*U*) is given by:

$$\mathcal{U} = 0.5 \frac{\mathcal{Q}^2}{\mathcal{C}} = 0.5 \frac{\mathcal{Q}^2 z}{\varepsilon\_0 \varepsilon \mathcal{A}} \tag{3}$$

established by Pelrine et al. [74,75], due to the inherent volume incompressibility, when an electrical field is applied perpendicularly to the plane of the two electrodes, two mechanisms are generally induced: the attractive electrostatic forces among opposite where *Q* is the fixed charge on the electrodes, *A* is the area of the electrode, *z* is the thickness of the DE film, *ε*<sup>0</sup> is the free-space permittivity, and *ε* is the relative permittivity (dielectric constant) of the DE material.

charges on each electrode cause the polymer film to compress in thickness and the repulsive electrostatic forces among like charges over the same electrode render further expansion in area, reducing the electrical energy [74,7]. For an ideal elastic model,

Acrylics

Silicones

High, resulting in long‐ term relaxation and slower response

Low, due to flexible backbone (‐Si‐O‐)

**Figure 7.** Schematic diagram of the operational principle of a DEA. **Figure 7.** Schematic diagram of the operational principle of a DEA.

As described in Equations (5) and (6), the dielectric constant (), elastic modulus (), electric field ()/applied voltage () and film thickness () proportional to the Maxwell stress and thickness strain are key parameters to consider in assessing the actuation performance of DEAs. Normally high voltage (>1 kV) or electric fields (~100 MV/m) are The electrostriction effect of a DEA upon the application of a proper electric field is believed to be caused by the formation of the effective compressive pressure *P*, also known as the Maxwell stress, which is defined as the electrostatic energy change per unit thickness displacement per unit area [74]:

$$p = \varepsilon\_0 \varepsilon E^2 = \varepsilon\_0 \varepsilon \left(\frac{V}{z}\right)^2\tag{4}$$

[73,76]. Hence, key pathways of countering the common limitation of high driving voltage and improving the electrical stimulated actuation performance of a DEA membrane Given that *Az* = *constant*, the effective stress (*p*) can be obtained:

$$p = \varepsilon\_0 \varepsilon E^2 = \varepsilon\_0 \varepsilon \left(\frac{V}{z}\right)^2\tag{5}$$

modifying the elastomer to tune its modulus/stiffness and electromechanical properties where *E* is the applied electric field, and *V* is the applied voltage.

ability of self‐clearing such as carbon particle‐based electrodes [77].

High

through composites, polymer blends and copolymers, and employ highly compliant electrodes that have high conductivity, little/no stiffening effect upon large strains and the For low strains (<10%), the actuated strain in the thickness direction (*sz*) can be defined as below according to the Hooke's Law in compression [3,5]:

$$s\_z = -\frac{p}{Y} = -\frac{\varepsilon\_0 \varepsilon \left(\frac{V}{z}\right)^2}{Y} \tag{6}$$

to other commonly reported materials such as polyurethane, polyisoprene and fluoroelastomers [75,76]. General comparison between the properties of the two main where *Y* is the elastic modulus of the dielectric elastomer.

elastomeric matrices: acrylics (mixtures of aliphatic acrylate) and silicones (polysiloxanes, e.g., most commonly used polydimethylsiloxane (PDMS)), is summarized in Table 3. **Table 3.** Comparison between acrylic and silicone elastomers [77–79]. **Material Viscoelasticity Dielectric Constant (at 1 Hz) Actuated Strain (Prestrained) Adhesion Property Thermal Stability Moisture Property** The above equations are built on the assumption of linearly elastic behavior of the DEA under relatively small strains. However, polymers are generally nonlinear materials and more complex constitutive relationship need to be taken into account. For example, the elastic modulus essentially depends on the strain (i.e., *Y* = *Y*(*s*)) for nonlinear materials. In addition, as large in-plane extension is exhibited, the actual thickness under actuation can be described in relation to the original value (*z*0) as:

$$z = z\_0(1 + s\_z) \tag{7}$$

(−10~80 °C)

Sensitive to humidity

moisture

(4.5–4.8) (~380%, area) Good Low and the constancy of volume can be expressed as

High

Low

2.4.2. Structure

$$(1+s\_x)(1+s\_y)(1+s\_z) = 1\tag{8}$$

(2.5–3) (~120%, linear) energy) (−65~240 °C) absorption If the in-plane deformation is symmetric, i.e., *s<sup>x</sup>* = *s<sup>y</sup>* = *sxy*, more accurate thickness strain can be expressed by [76]:

(low surface

$$s\_z = \left(1 + s\_k\right)^{-2} - 1\tag{9}$$

the basic operational mechanism to transduce electrical power into mechanical work to achieve amplified performance of in‐/out‐of‐plane deformation such as contraction, expansion and bending. Common structures derived from the fundamental membrane As described in Equations (5) and (6), the dielectric constant (*ε*), elastic modulus (*Y*), electric field (*E*)/applied voltage (*V*) and film thickness (*z*) proportional to the Maxwell stress and thickness strain are key parameters to consider in assessing the actuation performance of DEAs. Normally high voltage (>1 kV) or electric fields (~100 MV/m) are needed for better actuation output and allow for higher energy efficiency as they lead to smaller current, yet they are often considered disadvantageous for practical applications [73,76]. Hence, key pathways of countering the common limitation of high driving voltage and improving the electrical stimulated actuation performance of a DEA membrane consist in: (1) decreasing film thickness, (2) increasing DE dielectric constant, and (3) reducing DE intrinsic stiffness and viscoelastic loss. Common approaches of enhancing electromechanical responses include physically pre-straining (pre-stretching), chemically modifying the elastomer to tune its modulus/stiffness and electromechanical properties through composites, polymer blends and copolymers, and employ highly compliant electrodes that have high conductivity, little/no stiffening effect upon large strains and the ability of self-clearing such as carbon particle-based electrodes [77].

Since the early investigation of DEAs, acrylic and silicone elastomers have stood out and been widely studied due to their superior overall actuation performance compared to other commonly reported materials such as polyurethane, polyisoprene and fluoroelastomers [75,76]. General comparison between the properties of the two main elastomeric matrices: acrylics (mixtures of aliphatic acrylate) and silicones (polysiloxanes, e.g., most commonly used polydimethylsiloxane (PDMS)), is summarized in Table 3.


**Table 3.** Comparison between acrylic and silicone elastomers [77–79].

#### 2.4.2. Structure

Configurational designs of DEAs differ with applications and generally manipulate the basic operational mechanism to transduce electrical power into mechanical work to achieve amplified performance of in-/out-of-plane deformation such as contraction, expansion and bending. Common structures derived from the fundamental membrane prototype include multi-stack [80], interdigitated [81], and twisted [82] contractile devices; rolled [83], cone-shaped [84], and diaphragm [4]. DEAs exhibit linear or areal expansion. Multi-stack architecture is an effective approach to augmenting the effect of the Maxwell stress and generating amplified contractile actuation and load-bearing capacity (Figure 8a). Twisted actuators are developed from spiral cylindrical stacked DEAs to enhance their operational stability and ensure inherent flexibility, the actuated strain of which depends on the helix angle (Figure 8b) [82]. In contrast, cone-shaped (Figure 9a) and rolled (Figure 9b) DEAs utilize areal expansion to produce axial extension and amplified forces. Tunable two-cone actuators with proper voltage excitation modes enable the control of multi-directional and rotational actuation [85]. Zhao et al. [83] developed rolled multilayer DEAs that are able to produce a force of 1 N and strain of 10% at small driving voltages lower than 1 kV. The out of plane protrusion of diaphragm DEAs is triggered by two working modes: one is based on a physical support or boundary constraint and the other requires an external pressure for amplified actuation [86], which has found its applications in optical lenses [87], haptic interfaces [4] and braille displays [88], etc. Additionally, bendable DEAs with functional elements such as laminated passive layers in the form of unimorphs or bimorphs are also commonly investigated (Figure 10a) [89,90]. Bending motion has also been realized by multi-degree-of-freedom spring roll configuration with patterned electrodes that are circumferentially aligned (Figure 10b) [91]. The actuation performance of several DEAs with common structures is shown in Table 4.

DE material

Strain/displacement

Blocking/output force

Acrylic (VHB 4910, IPN post‐processed)

mixture

Electrode material Silicone/ carbon‐black

*Textiles* **2021**, *1*, 13

*Textiles* **2021**, *1*, 13

prototype include multi‐stack [80], interdigitated [81], and twisted [82] contractile devices; rolled [83], cone‐shaped [84], and diaphragm [4]. DEAs exhibit linear or areal expansion. Multi‐stack architecture is an effective approach to augmenting the effect of the Maxwell stress and generating amplified contractile actuation and load‐bearing capacity (Figure 8a). Twisted actuators are developed from spiral cylindrical stacked DEAs to enhance their operational stability and ensure inherent flexibility, the actuated strain of which depends on the helix angle (Figure 8b) [82]. In contrast, cone‐shaped (Figure 9a) and rolled (Figure 9b) DEAs utilize areal expansion to produce axial extension and amplified forces. Tunable two‐cone actuators with proper voltage excitation modes enable the control of multi‐directional and rotational actuation [85]. Zhao et al. [83] developed rolled multilayer DEAs that are able to produce a force of 1 N and strain of 10% at small driving voltages lower than 1 kV. The out of plane protrusion of diaphragm DEAs is triggered by two working modes: one is based on a physical support or boundary constraint and the other requires an external pressure for amplified actuation [86], which has found its applications in optical lenses [87], haptic interfaces [4] and braille displays [88], etc. Additionally, bendable DEAs with functional elements such as laminated passive layers in the form of unimorphs or bimorphs are also commonly investigated (Figure 10a) [89,90]. Bending motion has also been realized by multi‐degree‐of‐freedom spring roll configuration with patterned electrodes that are circumferentially aligned (Figure 10b) [91]. The actuation

prototype include multi‐stack [80], interdigitated [81], and twisted [82] contractile devices; rolled [83], cone‐shaped [84], and diaphragm [4]. DEAs exhibit linear or areal expansion. Multi‐stack architecture is an effective approach to augmenting the effect of the Maxwell stress and generating amplified contractile actuation and load‐bearing capacity (Figure 8a). Twisted actuators are developed from spiral cylindrical stacked DEAs to enhance their operational stability and ensure inherent flexibility, the actuated strain of which depends on the helix angle (Figure 8b) [82]. In contrast, cone‐shaped (Figure 9a) and rolled (Figure 9b) DEAs utilize areal expansion to produce axial extension and amplified forces. Tunable two‐cone actuators with proper voltage excitation modes enable the control of multi‐directional and rotational actuation [85]. Zhao et al. [83] developed rolled multilayer DEAs that are able to produce a force of 1 N and strain of 10% at small driving voltages lower than 1 kV. The out of plane protrusion of diaphragm DEAs is triggered by two working modes: one is based on a physical supportor boundary constraint and the other requires an external pressure for amplified actuation [86], which has found its applications in optical lenses [87], haptic interfaces [4] and braille displays [88], etc. Additionally, bendable DEAs with functional elements such as laminated passive layers in the form of unimorphs or bimorphs are also commonly investigated (Figure 10a) [89,90]. Bending motion has also been realized by multi‐degree‐of‐freedom spring roll configuration with patterned electrodes that are circumferentially aligned (Figure 10b) [91]. The actuation

**Figure 8.** Schematics of contractile DEAs: (**a**) multi‐stack structure; (**b**) twisted structure. **Figure 8.** Schematics of contractile DEAs: (**a**) multi-stack structure; (**b**) twisted structure. **Figure 8.**Schematics of contractile DEAs: (**a**)multi‐stack structure; (**b**) twisted structure.

performance of several DEAs with common structures is shown in Table 4.

**Figure 9.** Schematics of expansion DEAs. (**a**) Cone‐shape structure [78]. (**b**) Rolled structure [78] Reproduced under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Copyright 2020, The authors. Licensee MDPI, Basel. **Figure 9.** Schematics of expansion DEAs. (**a**) Cone‐shape structure [78]. (**b**) Rolled structure [78] Reproduced under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Copyright 2020, The authors. Licensee MDPI, Basel. **Figure 9.** Schematics of expansion DEAs. (**a**) Cone-shape structure [78]. (**b**) Rolled structure [78] Reproduced under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Copyright 2020, The authors. Licensee MDPI, Basel. *Textiles* **2021**, *1*, 14

**Figure 10.** Schematics of bending DEAs: (**a**) multilayer bistable structure with bending features [90]. Copyright 2018, Wiley‐VCH; (**b**) 2‐degree‐of‐freedom spring roll structure and a prototype [91]. Copyright 2004, IOP Publishing. **Figure 10.** Schematics of bending DEAs: (**a**) multilayer bistable structure with bending features [90]. Copyright 2018, Wiley-VCH; (**b**) 2-degree-of-freedom spring roll structure and a prototype [91]. Copyright 2004, IOP Publishing.

Silicone (Wacker Elas Tosil)

PDMS‐based DEAs with a pyramidal microstructure allowed for the tunability of modulus and dielectric properties and achieved enhanced actuation performance (pressure up to 25 kPa and vibrotactile cycles up to 250,000) [93]. A novel design of a dielectric liquid actuator utilized hydraulic‐coupled electrostatic effect to amplify electrical simulated actuation and enable peak performance surpassing that of natural muscle (specific power of 614 W/kg and load capacity of 700 g for a 2‐unit planar hydraulically amplified self‐healing electrostatic (HASEL) actuator) [94]. Inspired by the HASEL actuators, a recent effort incorporated the mechanism into origami and created programmable shape‐altering interfaces with three dimensional folding features [94].

DEAs with bistable structures have also been developed for the purpose of shape transformation between two equilibrium states. Shao et al. [90] developed disk and tape‐ spring bistable actuators with a support layer sandwiched by two DE layers based on the

**Diaphragm**

PDMS

(AgNWs) SWCNTs

Carbon black Silver nanowires

**Bending Unimorph**

Formulated acrylic (oligomer: CN9014)

Prestrain 200% areal None None 10% areal None None Voltage (kV) 4.2 6.5 1 4.5 4 4 Specific power (W/kg) ‐ 13.7 55 ‐ ‐ 19.5 ± 1.01 Energy density (J/kg) 12.9 ‐ 0.275 ‐ ‐ 1.95 ± 0.10

(without load) 30% 5.2% 10% 2.3% <sup>650</sup> μ<sup>m</sup> <sup>16</sup> mm

(mN) ‐ ‐ <sup>1000</sup> <sup>185</sup> <sup>255</sup> 12.5 <sup>±</sup> 0.9 Reference [80] [92] [83] [84] [4] [89]

184 and Ecoflex 0030)

Single‐walled carbon nanotubes (SWCNTs)

**Table 4.** Performance of dielectric elastomer actuators with common structures.

**Structure Multi‐Stack Twisted Rolled Cone Buckling**

Silicone (Wacker Elas tosil P7670)

Carbon black, graphite and ethanol


**Table 4.** Performance of dielectric elastomer actuators with common structures.

PDMS-based DEAs with a pyramidal microstructure allowed for the tunability of modulus and dielectric properties and achieved enhanced actuation performance (pressure up to 25 kPa and vibrotactile cycles up to 250,000) [93]. A novel design of a dielectric liquid actuator utilized hydraulic-coupled electrostatic effect to amplify electrical simulated actuation and enable peak performance surpassing that of natural muscle (specific power of 614 W/kg and load capacity of 700 g for a 2-unit planar hydraulically amplified selfhealing electrostatic (HASEL) actuator) [94]. Inspired by the HASEL actuators, a recent effort incorporated the mechanism into origami and created programmable shape-altering interfaces with three dimensional folding features [94].

DEAs with bistable structures have also been developed for the purpose of shape transformation between two equilibrium states. Shao et al. [90] developed disk and tapespring bistable actuators with a support layer sandwiched by two DE layers based on the bioinspired snap-through mechanism. A pair of electrodes on each DE layer and the control of a dual power supply enable the disk DEAs to shift the curvature bidirectionally, while with two electrodes on one DE layer, the tape-spring DEAs, snap upon electrical stimulation mimicking a chameleon's tongue [90]. Another established bistable mechanism lies within the bistable electroactive polymers (BSEPs). The actuated shape can be maintained by decreasing the temperature below the glass transition temperature of the DE after electrical excitation, saving the need for a constant power supply and thus energy consumption and increasing stiffness for adaptive applications, the rubbery-to-rigid transition of which is recoverable and repeatable [95].

### 2.4.3. Applications

The properties of DEAs fit into a wide realm of applications owing to their versatility and outstanding actuation performance. For wearable assistance in particular, research has mainly explored communication and rehabilitation. As haptic feedback is an important part of electronic textiles and wearable technologies, integrating DE tactile actuators into wearable systems through haptic sensation has been a rising field of interest attributed to material softness, small size, ease of fabrication, and adaptability compared to traditional rigid motors with fixed frequencies [96]. A crucial factor for effective sensing of haptic cues is skin perceptibility, which means the output force and resonance frequency should reach perceivable levels and depends on both human and device factors, such as placement on bodies and material stiffness [96,97]. For example, typically fingertips are ideal positions on tactile devices due to their higher sensitivity than other parts such as arms, and are usually susceptible to a force threshold around 30 mN depending on the fingers [98]. Hence, one of the main goals in the current development of DEA haptic wearable interfaces is to generate large force and displacement outputs at such a small (millimeter) scale [96,99]. Meanwhile, as human contact is involved, the design of wearable haptic interfaces using DEAs needs to account for various aspects such as compactness, weight, comfort, safety, etc.

Most current designs of wearable haptics have leveraged the out-of-plane deformation of diaphragm/membrane [96,100] and linear displacement of rolled multilayer [101] configurations, as well as the hydraulic amplification with liquid coupling effect [99]. Multilayered PDMS membranes coated by AgNWs electrodes with a perforated polymeric frame support were able to generate an output force up to 255 mN and a protrusive displacement of 650 µm [14]. More recently, hydraulically amplified taxel (HAXEL) actuators designed with four quadrants consisting of P(VDF-TrFE-CTFE) liquid dielectric and segmented aluminum electrodes were able to simulate directional motions and produce forces over 300 mN and a vertical displacement of 500 µm (Figure 11) [99]. Prototype demonstrations have integrated the haptic functions into wearables such as armbands [4,99,101], rubber gloves [4] and fingertip devices (Figure 12) [96,100] for potential applications of human-machine interaction and virtual/augmented reality. Untethered wearable tactile actuators were realized by on-board battery integration into compact feel-through lowvoltage (<500 V) DEAs (FT-DEAs) with a thickness of only 18 µm and a wide vibrotactile frequency range of 1–500 Hz, and the fingertip prototype with incorporated photodetector was demonstrated to be applicable in assisting text-reading for people with vision impairment (Figure 12) [96]. *Textiles* **2021**, *1*, 16

**Figure 11.** Schematics and prototypes of wearable HAXEL actuators. (**a**) Schematic diagrams of a HAXEL actuator and an armband prototype of 5x5 array HAXELs [99]. Copyright 2020, Wiley‐VCH. (**b**) Haptic testing modes with various directional motions based on a four‐quadrant HAXEL [99]. Copyright 2020, Wiley‐VCH. **Figure 11.** Schematics and prototypes of wearable HAXEL actuators. (**a**) Schematic diagrams of a HAXEL actuator and an armband prototype of 5x5 array HAXELs [99]. Copyright 2020, Wiley-VCH. (**b**) Haptic testing modes with various directional motions based on a four-quadrant HAXEL [99]. Copyright 2020, Wiley-VCH.

**Figure 12.** Schematics and prototypes of wearable fingertip tactile DEAs. (**a**) The wireless driver circuit. (**b**) Untethered "feel‐through" haptic device on a fingertip. (**c**) Operating principle of Untethered FT‐DEA. (**d**) The blindfolded user correctly identifies randomly rotated and placed

letters E, P, F, and L [96]. Copyright 2020, Wiley‐VCH.

**Figure 11.** Schematics and prototypes of wearable HAXEL actuators. (**a**) Schematic diagrams of a HAXEL actuator and an armband prototype of 5x5 array HAXELs [99]. Copyright 2020, Wiley‐VCH. (**b**) Haptic testing modes with various directional motions based on a four‐quadrant HAXEL [99].

*Textiles* **2021**, *1*, 16

**Figure 12.** Schematics and prototypes of wearable fingertip tactile DEAs. (**a**) The wireless driver circuit. (**b**) Untethered "feel‐through" haptic device on a fingertip. (**c**) Operating principle of Untethered FT‐DEA. (**d**) The blindfolded user correctly identifies randomly rotated and placed letters E, P, F, and L [96]. Copyright 2020, Wiley‐VCH. **Figure 12.** Schematics and prototypes of wearable fingertip tactile DEAs. (**a**) The wireless driver circuit. (**b**) Untethered "feel-through" haptic device on a fingertip. (**c**) Operating principle of Untethered FT-DEA. (**d**) The blindfolded user correctly identifies randomly rotated and placed letters E, P, F, and L [96]. Copyright 2020, Wiley-VCH.

Apart from the aforementioned properties, the muscle-like and variable-stiffness behavior of DEAs without bulkiness and noise makes it favorable for wearable rehabilitation devices. Examples include active hand splints for hand/finger rehabilitation (Figure 13a) [102], ankle-foot orthosis (AFO) remedying foot drop (ankle dorsiflexion inability) (Figure 13b) [103] and compression bandages targeting the disorder of venous systems (Figure 13c) [104]. Compared to conventional passive ones with elastic bands, dynamic hand splints with folded contractile silicone DEAs enable flexible modulation of mechanical compliance and finger exercise by varying the driving voltage [102]. AFO designed with DEAs allows for lighter weight and less energy consumption with a charge recovery power system, and at the same time does not hinder plantarflexion function [104].

#### 2.4.4. Merits and Limitations

Copyright 2020, Wiley‐VCH.

As an outstanding smart material, DEAs hold promising potentials in wearable actuator technologies attributed to their intrinsic softness, light-weight and compactness, straightforward mechanism and fabrication, and high electromechanical performances. Compared to other existing actuator technologies such as shape memory alloys and electromagnetic and piezoelectric polymers, the distinctive actuation mechanism of DEAs has been reported to be capable of producing prominent resulting strain (>100%) and stress (~7 MPa), fast response speed (µs), high energy density (~3.4 MJ/m<sup>3</sup> ) and high efficiency [75,78]. In addition, the self-sensing characteristic of DEAs, i.e., the phenomenon that electrical properties such as capacitance alter under deformation, makes it possible to monitor the actuation response and develop closed-loop systems for smart human assist. Meanwhile, various challenges to some extent prevent their widespread applications in real-life scenarios. First and foremost, high driving voltage often requires an external power supply, constraining the integration of DEAs into electronic textile systems and bringing about safety concerns. Furthermore, biocompatibility, the need for additional rigid frames supporting pre-stretched films, and unstable electrical and mechanical properties are also

*Textiles* **2021**, *1*, 17

crucial obstacles to overcome towards improving wearability, efficiency and sustainability of DEAs. designed with DEAs allows for lighter weight and less energy consumption with a charge recovery power system, and at the same time does not hinder plantarflexion function [104].

Apart from the aforementioned properties, the muscle‐like and variable‐stiffness behavior of DEAs without bulkiness and noise makes it favorable for wearable rehabilitation devices. Examples include active hand splints for hand/finger rehabilitation (Figure 13a) [102], ankle‐foot orthosis (AFO) remedying foot drop (ankle dorsiflexion inability) (Figure 13b) [103] and compression bandages targeting the disorder of venous systems (Figure 13c) [104]. Compared to conventional passive ones with elastic bands, dynamic hand splints with folded contractile silicone DEAs enable flexible modulation of mechanical compliance and finger exercise by varying the driving voltage [102]. AFO

**Figure 13.** Schematics and prototypes of wearable DEAs for rehabilitation. (**a**) A schematic diagram and prototype of a DEA‐based hand splint protected by a plastic guard [102]. Copyright 2008, SPIE. (**b**) A DEA AFO prototype consisting of a DEA strap and a knee brace [103]. Copyright 2014, IOP Publishing. (**c**) An active compression bandage prototype for the ankle, mid‐calf and knee region [104]. Copyright 2021, Elsevier. **Figure 13.** Schematics and prototypes of wearable DEAs for rehabilitation. (**a**) A schematic diagram and prototype of a DEA-based hand splint protected by a plastic guard [102]. Copyright 2008, SPIE. (**b**) A DEA AFO prototype consisting of a DEA strap and a knee brace [103]. Copyright 2014, IOP Publishing. (**c**) An active compression bandage prototype for the ankle, mid-calf and knee region [104]. Copyright 2021, Elsevier.

#### 2.4.4. Merits and Limitations *2.5. Ionic-Polymer/Metal Composites and Conducing Polymers*

As an outstanding smart material, DEAs hold promising potentials in wearable actuator technologies attributed to their intrinsic softness, light‐weight and compactness, straightforward mechanism and fabrication, and high electromechanical performances. Compared to other existing actuator technologies such as shape memory alloys and electromagnetic and piezoelectric polymers, the distinctive actuation mechanism of DEAs has been reported to be capable of producing prominent resulting strain (>100%) and stress (~7 MPa), fast response speed (μs), high energy density (~3.4 MJ/m3) and high efficiency [75,78]. In addition, the self‐sensing characteristic of DEAs, i.e., the phenomenon that electrical properties such as capacitance alter under deformation, makes it possible to monitor the actuation response and develop closed‐loop systems for smart human assist. Meanwhile, various challenges to some extent prevent their widespread applications in real‐life scenarios. First and foremost, high driving voltage often requires an external power supply, constraining the integration of DEAs into electronic textile Ionic-Polymer/Metal Composites (IPMCs), as a category of Electroactive polymers (EAPs), have been proposed as the active material in a wide-range of applications, including biomimetic sensors and mechanical actuators [105]. Kuhn [106] and Katchalsky [107] are generally considered to be the first reporters of deformable polyelectrolytes ion solution such as polyacrylic acid (PAA) and polyvinyl chloride (PVA) systems. After that, numerous researchers have demonstrated that IPMCs can show a large deformation when placed in a time-varying electric field due to their inherent properties [108]. IPMCs can be seen as an electrolyte sandwiched by two layers of ionically conducting membranes with two electrodes. Typically, the ionically conducting membranes are made of Nafion and Flemion due to their fast response time and good durability [92]. Gold and platinum were generally used as electrodes for IPMCs. Recently, CNTs appeared as an attractive alternative material for metal electrodes due to their extraordinary electrical and mechanical properties (Figure 14a). However, compared to metal electrodes, CNTs have some limitations such as poor dispersibility and special requirements for electrolytes [109].

#### systems and bringing about safety concerns. Furthermore, biocompatibility, the need for 2.5.1. Mechanism

additional rigid frames supporting pre‐stretched films, and unstable electrical and mechanical properties are also crucial obstacles to overcome towards improving wearability, efficiency and sustainability of DEAs. *2.5. Ionic‐Polymer/Metal Composites and Conducing Polymers* Ionic‐Polymer/Metal Composites (IPMCs), as a category of Electroactive polymers (EAPs), have been proposed as the active material in a wide‐range of applications, including biomimetic sensors and mechanical actuators [105]. Kuhn [106] and Katchalsky [107] are The mechanism for IPMCs varies slightly depending on the different electrodes, electrolyte and conductive membranes. Briefly, under the applied electric field, cations (or anions) in IPMCs electrolyte move to the cathode (anode) [8]. This transportation of ions generates the volumetric difference, which leads to a pressure gradient in the ionic polymer followed by a swelling of one side and shrinkage of another side of the membrane [110]. In traditional IPMCs, water was commonly used as solvent. However, water has several major problems: poor electrochemical stability in high-voltage and gradual leakage and a high vapor pressure inside a membrane due to evaporation [111]. Therefore, some researchers use other solvents such as organic solvents instead of water. Ionic liquids are considered to be a good alternative for water due to their negligible vapor pressure and high conductivity. In water driven IPMCs, the cations gather under the applied electric field squeezing out the water molecules, which create strain pressure on the ionic polymer (Figure 14b) [108]. In ionic liquid driven IPMCs, the deformation is caused by the different transfer rates of

cations and anions. The anions moves slower to the anode due to electrostatic repulsion from the polymer matrix (Figure 14c) [110]. the ionic polymer (Figure 14b) [108]. In ionic liquid driven IPMCs, the deformation is caused by the different transfer rates of cations and anions. The anions moves slower to the anode due to electrostatic repulsion from the polymer matrix (Figure 14c) [110].

The mechanism for IPMCs varies slightly depending on the different electrodes, electrolyte and conductive membranes. Briefly, under the applied electric field, cations (or anions) in IPMCs electrolyte move to the cathode (anode) [8]**.** This transportation of ions generates the volumetric difference, which leads to a pressure gradient in the ionic polymer followed by a swelling of one side and shrinkage of another side of the membrane [110]. In traditional IPMCs, water was commonly used as solvent. However, water has several major problems: poor electrochemical stability in high‐voltage and gradual leakage and a high vapor pressure inside a membrane due to evaporation [111]. Therefore, some researchers use other solvents such as organic solvents instead of water. Ionic liquids are considered to be a good alternative for water due to their negligible vapor pressure and high conductivity. In water driven IPMCs, the cations gather under the applied electric field squeezing out the water molecules, which create strain pressure on

generally considered to be the first reporters of deformable polyelectrolytes ion solution such as polyacrylic acid (PAA) and polyvinyl chloride (PVA) systems. After that, numerous researchers have demonstrated that IPMCs can show a large deformation when placed in a time‐varying electric field due to their inherent properties [108]. IPMCs can be seen as an electrolyte sandwiched by two layers of ionically conducting membranes with two electrodes. Typically, the ionically conducting membranes are made of Nafion and Flemion due to their fast response time and good durability [92]. Gold and platinum were generally used as electrodes for IPMCs. Recently, CNTs appeared as an attractive alternative material for metal electrodes due to their extraordinary electrical and mechanical properties (Figure 14a). However, compared to metal electrodes, CNTs have some limitations such as poor

*Textiles* **2021**, *1*, 18

dispersibility and special requirements for electrolytes [109].

2.5.1. Mechanism

**Figure 14.** Schematic diagram of some IPMCs. (**a**) Structure of bucky‐gel actuator (CNT as electrodes) [109]. Copyright 2013, Wiley‐VCH. (**b**) A typical IPMCs and its actuation mechanism [108]. Copyright 2001, IOP Publishing. (**c**) Schematic illustrations of the actuation mechanism and performance differences of IPMCs driven by an IL or by water [110]. Copyright 2021, Elsevier. **Figure 14.** Schematic diagram of some IPMCs. (**a**) Structure of bucky-gel actuator (CNT as electrodes) [109]. Copyright 2013, Wiley-VCH. (**b**) A typical IPMCs and its actuation mechanism [108]. Copyright 2001, IOP Publishing. (**c**) Schematic illustrations of the actuation mechanism and performance differences of IPMCs driven by an IL or by water [110]. Copyright 2021, Elsevier.

Conducting polymers (CPs) are another category of EAPs which have similar mechanisms to IPMCs. CPs are typically composed of the constituent monomers of pyrrole, aniline, and thiophene and its derivatives (Figure 15) [8]. These polymers can be electrochemically oxidized and reduced repeatedly. During the redox reaction, ions can move in or out from the CPs depending on the ion (called a dopant) and the reaction [112]. Typically, there are two different reactions. When CPs contain mobile anions (*e* −), the anions will leave the polymer during reaction and the anions (*A* −) in electrolyte will enter CP [8].

$$(M)\_n - me^- + mA^- \rightarrow (M)\_n^{m^+} \left(A^-\right)\_m \tag{10}$$

(*M*) is the monomer, *m* is the number of electrons transferred, and *A* − is the anion responsible for maintaining the electroneutrality.

On the contrary, if CPs contain immobile anions (*A* −), they will require cations for the charge neutrality inside the polymer matrix. The cations *M<sup>+</sup>* will move in and out of the CP during redox reaction [8].

$$(M)\_n + me^- + mA^- \rightarrow (M)\_n^{m^+} \left(A^+\right)\_m \tag{11}$$

*Textiles* **2021**, *1*, 19

responsible for maintaining the electroneutrality.

CP during redox reaction [8]**.**

Conducting polymers (CPs) are another category of EAPs which have similar

(*M*) is the monomer, *m* is the number of electrons transferred, and *A<sup>−</sup>* is the anion

On the contrary, if CPs contain immobile anions (*A−*), they will require cations for the

charge neutrality inside the polymer matrix. The cations *M+* will move in and out of the

ሺሻ ି ି → ሺሻ

<sup>శ</sup>

<sup>శ</sup>

ሺିሻ (10)

ሺାሻ (11)

mechanisms to IPMCs. CPs are typically composed of the constituent monomers of pyrrole, aniline, and thiophene and its derivatives (Figure 15) [8]. These polymers can be electrochemically oxidized and reduced repeatedly. During the redox reaction, ions can move in or out from the CPs depending on the ion (called a dopant) and the reaction [112]. Typically, there are two different reactions. When CPs contain mobile anions (*e*−), the anions will leave the polymer during reaction and the anions (*A−*) in electrolyte will enter CP [8].

ሺሻ െ ି ି → ሺሻ

**Figure 15.** Chemical structure of some common CPs [8]. Copyright 2013, Wiley‐VCH. **Figure 15.** Chemical structure of some common CPs [8]. Copyright 2013, Wiley-VCH.

#### 2.5.2. Applications

2.5.2. Applications IPMCs and CPs, have been widely applied in textiles, sensors, and soft exoskeletons (Figure 16a). The electrochemical conductive polymer is combined with textile weaving technology to provide an electrochemical artificial muscle. Jager et al.reported a wearable, soft, and strain adjustable rayon fabric, which was achieved by continuously coating cellulose fibers with a PEDOT layer as a conductive electrode and a PPY layer for actuation [11]. By weaving and knitting, this actuator has an increasing output force with the increase of the number of actuator yarns. Similarly, a PEDOT:PSS coated SWCNT wired ZIF‐8 structure has been reported (Figure 16b) [113]. Hydrophilization treatments enable the formation of electrode ink on the surface of the fabric and highly increase the ionic conductivity. The textile‐structure actuator demonstrated large strain (0.28%) and a high blocking force (0.62 mN at 0.1 Hz). Li group demonstrated a plasticized polyvinyl chloride (PVC) gel‐based exoskeleton for hip joint support. Electroresponsive hydrogels have demonstrated that they can be deformed under an electric field, similar to the mechanism of IPMCs [114]. This device can provide up to 94 N and 10% biological torque IPMCs and CPs, have been widely applied in textiles, sensors, and soft exoskeletons (Figure 16a). The electrochemical conductive polymer is combined with textile weaving technology to provide an electrochemical artificial muscle. Jager et al. reported a wearable, soft, and strain adjustable rayon fabric, which was achieved by continuously coating cellulose fibers with a PEDOT layer as a conductive electrode and a PPY layer for actuation [11]. By weaving and knitting, this actuator has an increasing output force with the increase of the number of actuator yarns. Similarly, a PEDOT:PSS coated SWCNT wired ZIF-8 structure has been reported (Figure 16b) [113]. Hydrophilization treatments enable the formation of electrode ink on the surface of the fabric and highly increase the ionic conductivity. The textile-structure actuator demonstrated large strain (0.28%) and a high blocking force (0.62 mN at 0.1 Hz). Li group demonstrated a plasticized polyvinyl chloride (PVC) gelbased exoskeleton for hip joint support. Electroresponsive hydrogels have demonstrated that they can be deformed under an electric field, similar to the mechanism of IPMCs [114]. This device can provide up to 94 N and 10% biological torque at hip joints during walking (Figure 16c). In addition to being able to undergo shape changing under an electric field, some IPMCs can generate voltage under deformation, which makes them a potential sensor (Figure 16d). An IPMCs based heart rate monitoring sensor was first explored by Chattaraj et al. [115]. The mechanical impact induced by pulsating blood flow generates a potential difference on the metal plate electrode. Data retrieved from this device exhibited error percentages of 4–15% when evaluated against standard plethysmographic measures.

*Textiles* **2021**, *1*, 20

standard plethysmographic measures.

at hip joints during walking (Figure 16c). In addition to being able to undergo shape changing under an electric field, some IPMCs can generate voltage under deformation, which makes them a potential sensor (Figure 16d). An IPMCs based heart rate monitoring sensor was first explored by Chattaraj et al. [115]. The mechanical impact induced by pulsating blood flow generates a potential difference on the metal plate electrode. Data retrieved from this device exhibited error percentages of 4–15% when evaluated against

**Figure 16.** (**a**) Processing and integration of electroactive textiles [11]. Copyright 2017, the authors, published by AAAS, (**b**) textile actuator based on PEDOT:PSS/MOF‐derivative electrode ink [113]. Copyright 2020, Wu, Yang, Li, Li and Chen. (**c**) PVC gel soft actuator‐based assist wear [114]. Copyright 2017, IOP Publishing. (**d**) IPMCs based heart rate monitoring sensor [115]. Copyright 2018, Elsevier. **Figure 16.** (**a**) Processing and integration of electroactive textiles [11]. Copyright 2017, the authors, published by AAAS, (**b**) textile actuator based on PEDOT:PSS/MOF-derivative electrode ink [113]. Copyright 2020, Wu, Yang, Li, Li and Chen. (**c**) PVC gel soft actuator-based assist wear [114]. Copyright 2017, IOP Publishing. (**d**) IPMCs based heart rate monitoring sensor [115]. Copyright 2018, Elsevier.

#### 2.5.3. Merits and Limitations 2.5.3. Merits and Limitations

IPMCs are considered to be promising actuators for practical application and have been investigated since 1965 [116]. They can deform by themselves through charge transfer. IPMCs based actuators have fast response times and good repeatability. At the same time, this type of material can be designed at a millimeter‐sized scale and can work in synergy with other structures such as textile structures. Large deformation under low voltage, extremely short reaction time and high stability allow them to have unique advantages. However, the main problem of IPMCs is the poor durability due to the loss of traditional electrodes in air or liquid. There are many methods for preparing IPMCs using non‐metal electrodes. The effect of these new type of electrodes needs further research. In addition, when water is used as the electrolyte solvent, problems such as ionization and leakage may occur. Although IPMCs with ionic liquid as electrolyte have good stability, their response time and actuation force are decreased. As the research progresses, the performance of IPMCS is still improving. Complicated preparation processes and expensive costs are the main limitations of IPMCs. The electrolyte and IPMCs are considered to be promising actuators for practical application and have been investigated since 1965 [116]. They can deform by themselves through charge transfer. IPMCs based actuators have fast response times and good repeatability. At the same time, this type of material can be designed at a millimeter-sized scale and can work in synergy with other structures such as textile structures. Large deformation under low voltage, extremely short reaction time and high stability allow them to have unique advantages. However, the main problem of IPMCs is the poor durability due to the loss of traditional electrodes in air or liquid. There are many methods for preparing IPMCs using non-metal electrodes. The effect of these new type of electrodes needs further research. In addition, when water is used as the electrolyte solvent, problems such as ionization and leakage may occur. Although IPMCs with ionic liquid as electrolyte have good stability, their response time and actuation force are decreased. As the research progresses, the performance of IPMCS is still improving. Complicated preparation processes and expensive costs are the main limitations of IPMCs. The electrolyte and polymer matrix may have certain limitations as well.

polymer matrix may have certain limitations as well. In wearable applications, CPs are more advantageous than traditional IPMCs because they eliminate the limitations of electrodes. CPs can be considered a storage element of a capacitor that can change shape during charging and discharging [8]. Therefore, reducing their thickness can improve the response time, but it will reduce the driving force [117]. Due to the existence of the degradation potential, the ion concentration In wearable applications, CPs are more advantageous than traditional IPMCs because they eliminate the limitations of electrodes. CPs can be considered a storage element of a capacitor that can change shape during charging and discharging [8]. Therefore, reducing their thickness can improve the response time, but it will reduce the driving force [117]. Due to the existence of the degradation potential, the ion concentration of CPs is limited [118]. The single actuation mode and low accuracy are also limiting factors in the application of such materials in wearable drives.

#### *2.6. Piezoelectric Actuators* 2.6.1. Mechanism

*2.6. Piezoelectric Actuators*

#### 2.6.1. Mechanism Piezoelectric effect is a phenomenon in which mechanical energy and electric energy

Piezoelectric effect is a phenomenon in which mechanical energy and electric energy are exchanged in dielectric materials. There are two kinds of piezoelectric effect, namely positive piezoelectric effect and inverse piezoelectric effect. Positive piezoelectricity is the electric charge that accumulates in certain solid materials in response to applied mechanical stress (Figure 17a). Inverse piezoelectric effect refers to when an electric field is applied in the polarization direction of piezoelectric sensing elements; this will produce mechanical deformation or mechanical stress in a certain direction. When the applied electric field is removed, this deformation or stress will disappear (Figure 17b). The stress deformation can be divided into five basic forms: thickness deformation, length deformation, volume deformation, thickness shear deformation, and plane shear deformation. are exchanged in dielectric materials. There are two kinds of piezoelectric effect, namely positive piezoelectric effect and inverse piezoelectric effect. Positive piezoelectricity is the electric charge that accumulates in certain solid materials in response to applied mechanical stress (Figure 17a). Inverse piezoelectric effect refers to when an electric field is applied in the polarization direction of piezoelectric sensing elements; this will produce mechanical deformation or mechanical stress in a certain direction. When the applied electric field is removed, this deformation or stress will disappear (Figure 17b). The stress deformation can be divided into five basic forms: thickness deformation, length deformation, volume deformation, thickness shear deformation, and plane shear deformation.

of CPs is limited [118]. The single actuation mode and low accuracy are also limiting

*Textiles* **2021**, *1*, 21

factors in the application of such materials in wearable drives.

(b) Inverse piezoelectric——the applied electric field causes the crystal to deform

**Figure 17.** Schematic diagram of piezoelectric effect. (**a**) Direct piezoelectric effect. (**b**) Inverse **Figure 17.** Schematic diagram of piezoelectric effect. (**a**) Direct piezoelectric effect. (**b**) Inverse piezoelectric.

piezoelectric. For piezoelectric actuators, the inverse piezoelectric effect is the basic working For piezoelectric actuators, the inverse piezoelectric effect is the basic working principle, and the governing equation can be expressed as follows:

$$S\_{\bar{j}} = d\_{i\bar{j}} E\_i \tag{12}$$

 ሻଶଷଵଷ

 ൌ (12) where *S* is the strain, *E* is the electric field intensity, *i* and *j* are the electric field and strain direction respectively, is its piezoelectric strain constant. The basic deformation mode where *S* is the strain, *E* is the electric field intensity, *i* and *j* are the electric field and strain direction respectively, *dij* is its piezoelectric strain constant. The basic deformation mode and its main parameters are shown in Table 5 [119].


Bending ଷଵ ൌ 3ሺ \* L: Length of piezo-wafer, T: Width of piezo-wafer.

\* L: Length of piezo‐wafer, T: Width of piezo‐wafer.

Piezoelectric materials include inorganic piezoelectric materials and organic piezoelectric materials [120]. Inorganic piezoelectric materials are divided into piezoelectric crystals and piezoelectric ceramics. Organic piezoelectric materials, also known as piezoelectric polymers, are being increasingly pursued for wearable applications because of their soft nature. Current piezoelectric actuators are based on organic piezoelectric materials, such as polyvinylidene fluoride (PVDF); it is necessary to mention that the PVDF piezoelectric film is softer compared to piezoelectric ceramics, light weight, has a large piezoelectric constant, high application sensitivity, good matching state, a compliance coefficient that is 30 times of PZT, and can be used in tactile measurement, mechanical measurement, energy collection, medical monitoring and so on. It is worth noting that the PVDF piezoelectric film requires polarization and stretching to improve the piezoelectric properties. Different tensile rate and other factors will affect its piezoelectric properties, but change in the preparation process can improve piezoelectric properties without additional polarization, such as electrostatic electricity. In the electrospinning process, the viscous solution is stretched and solidified into nanofibers in a strong electric field so that the molecular dipole of the polymer is oriented in the nanofiber length direction, and the transformation of α is converted into β phase completion. However, PVDF-TRFE does not require additional polarization and stretching. The parameters of some piezoelectric materials are shown in Table 6.



#### 2.6.2. Structure

The unimorph, single crystal structure is the basic structure for piezoelectric actuators. The deformation of the piezoelectric actuator depends on the basic principle of the piezoelectric inverse effect. The piezoelectric film layer is bent or elongated in alternating current; meanwhile, the passive layer is not shortened, and the stress between the two layers causes bending deformation of the piezoelectric actuator due to subsequent bending or volume mismatch. For example, Akther et al. [122] used piezoelectric single crystal actuators to amplify the haptic feedback of the vibration signal, so that humans can clearly perceive the vibration (Figure 18a,b). Wu et al. [123] combined PVDF with PET and adopted a piezoelectric single crystal structure to create a soft robot. The soft robot was bent and a leg structure was added to increase the walking speed. The soft robot has reached the rapid movement of 20 body length/s at the resonant frequency, and it can still move even if crushed one million times, demonstrating strong robustness. Maccabi et al. [124] designed three piezoelectric spiral arms using a curved piezoelectric single crystal structure which generated torsional deformation through alternating current to achieve parallel plane piston movement (Figure 18c).

**Figure 18.** (**a**) Schematic side view of a piezoelectric actuator, which includes PZT at the bottom, glycerin/water solution in the middle, thermoplastic polyurethane (TPU) at the top and polymethyl methacrylate (PMMA) at both sides [124]. (**b**) Power on [124]. Copyright 2019, The Royal Society of Chemistry. (**c**) In the picture of the actuator, the angles between the interdigitated electrodes (IDEs) and the spiral arm are 90° and 45° respectively, and the spiral arm is made of PZT [126]. Copyright 2016, Elsevier. **Figure 18.** (**a**) Schematic side view of a piezoelectric actuator, which includes PZT at the bottom, glycerin/water solution in the middle, thermoplastic polyurethane (TPU) at the top and polymethyl methacrylate (PMMA) at both sides [122]. (**b**) Power on [122]. Copyright 2019, The Royal Society of Chemistry. (**c**) In the picture of the actuator, the angles between the interdigitated electrodes (IDEs) and the spiral arm are 90◦ and 45◦ respectively, and the spiral arm is made of PZT [124]. Copyright 2016, Elsevier.

**Figure 19.** (**a**) The experimenter touched the piezoelectric actuator and felt the vibration [134]. Copyright 2017, Elsevier. (**b**) Wearable gloves vibrate to irritate finger skin [135]. Copyright 2020, the authors, published by AAAS. The piezoelectric bimorph three-layer structure, has piezoelectric materials on the top and bottom layers. This structure has different mechanisms compared to the unimorph structure. The two piezoelectric film layers are bent in alternating current. One of the piezoelectric sheets is elongated, and the other is shortened, and their bending strain is enlarged. The advantage of this structure is that it doubles the displacement of the soft robot and achieves more excellent moving characteristics. Piezoelectric ceramics are widely used in this structure. For example, Xu et al. [125] developed a piezoelectric actuator that can rotate rapidly. When using a piezoelectric bimorph structure, driven by piezoelectric vibration, the maximum rotation speed is as high as 118.3 r/min. Kwon et al. [126] developed a mobile phone vibration driver by using bimorph piezoelectric ceramics. This structure can also be applied in other materials; for example, Park et al. [127] used the upper and lower layers of PVDF piezoelectric film and magnetic tape as the main body and the PVDF piezoelectric film and magnetic tape as the legs to make the soft robot. Under the 160 Hz resonance frequency, the robot applied ±65 V AC voltage and achieved a good mobility performance of 35.3 mm/s.

In addition, according to the direction of vibration, the piezoelectric actuator can be divided into a longitudinal structure [128], a longitudinal bending structure [129], a longitudinal torsion [130], and a curved bending structure [128]. The length affects the modal frequency of the longitudinal structure, and length and cross-sectional dimensions structure affect the modal frequency of the bending and torsional structure [131].

#### *Textiles* **2021**, *1*, Firstpage–Lastpage. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/textiles 2.6.3. Applications

Piezoelectric sensors have many applications in the medical field to detect human health, and much wearable research has also demonstrated great potential for them in biomechanical energy collection. Moreover, piezoelectric actuators can be applied in various fields, such as miniature pumps and piezoelectric motors, etc. This section mainly introduces the application of piezoelectric drivers in wearable applications. Wearable

devices based on piezoelectric actuators are light in weight, especially organic piezoelectric materials, which can be made very thin and move unconstrained. Theoretically, there are many applications for piezoelectric actuators in wearable applications. First, piezoelectric actuators can be directly applied to the skin to recognize fonts or objects and their colors through vibrational tactile feedback. For example, Sauvet et al. [132] study a wearable actuator that can make the skin feel. Through experiments they verified that people can correctly perceive the location and strong level of vibration when piezoelectric actuators achieve vibration response with the index finger and palm soft contact. This further improves the user interaction with the product, while providing inspiration for identifying fonts, color and so on (Figure 19a). Additionally, Zhu et al. [133] developed a tactile feedback smart glove. Its accuracy of the target recognition can reach 96%, in which the piezoelectric unit is applied to PZT, the main function is to use the reverse piezoelectric effect to vibrate and stimulate the touch. They can achieve a different interaction result by adjusting power at a resonant frequency (Figure 19b). Piezoelectric actuators also have applications in medicine. For example, in the study of Dagdeviren et al. [134], PZT piezoelectric plates, where the actuator is the larger part and the sensor is the smaller part, were installed on the human skin surface. The combination of the driver and the sensor was used to accurately measure the elastic modulus of the skin and predict the pathophysiological conditions. **Figure 18.** (**a**) Schematic side view of a piezoelectric actuator, which includes PZT at the bottom, glycerin/water solution in the middle, thermoplastic polyurethane (TPU) at the top and polymethyl methacrylate (PMMA) at both sides [124]. (**b**) Power on [124]. Copyright 2019, The Royal Society of Chemistry. (**c**) In the picture of the actuator, the angles between the interdigitated electrodes (IDEs) and the spiral arm are 90° and 45° respectively, and the spiral arm is made of PZT [126]. Copyright 2016, Elsevier.

**Figure 19.** (**a**) The experimenter touched the piezoelectric actuator and felt the vibration [134]. Copyright 2017, Elsevier. (**b**) Wearable gloves vibrate to irritate finger skin [135]. Copyright 2020, the authors, published by AAAS. **Figure 19.** (**a**) The experimenter touched the piezoelectric actuator and felt the vibration [132]. Copyright 2017, Elsevier. (**b**) Wearable gloves vibrate to irritate finger skin [133]. Copyright 2020, the authors, published by AAAS.

#### 2.6.4. Merits and Limitations

*Textiles* **2021**, *1*, Firstpage–Lastpage. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/textiles Advantages of the piezoelectric actuator include a simple manufacturing method, and for organic piezoelectric polymer with the flexible combination of organic ground substance, advantages include good flexibility, light quality, good sensitivity, small size, and low power consumption. Additionally, compared with the traditional rigid or general shape memory alloy's heat, humidity, pH value, and chemical stimulation, the piezoelectric actuator has the advantage for quick response. Theoretically, this kind of soft robot has infinite degrees of freedom. It can be in smooth contact with the skin, integrate sensors and actuators, and can make small self-driven devices by using piezoelectric power generation. Its limitations are mainly manifested in the small amplitude of motion in the wearable, the required voltage is large, and the combination of textile problems.

#### *2.7. Electromagnetic Actuators*

#### 2.7.1. Mechanism

Recently, electromagnetic actuators have been widely used in robots and wearable tactile feedback devices. Electromagnetic actuators are mainly composed of energized coils and magnetic materials, and the interaction force generated between them is used as the power source. The basic mechanism of action is mainly divided into two situations. The first situation is that it is affected by magnetic force or magnetic moment, causing deformation/displacement of the magnetic object, which is expressed as follows [135]:

$$f\_m = \int\_{Vm} (M \cdot \Delta)dV\_m \tag{13}$$

$$
\pi\_{\mathfrak{M}} = \int\_{Vm} (M \times B)dV\_{\mathfrak{m}} \tag{14}
$$

where *f <sup>m</sup>* is the magnetic force experienced by a magnetic field, *τ<sup>m</sup>* is the magnetic moment experienced by an external magnetic field, *M* is the magnetization of the external magnetic field, ∆ is the gradient of the gradient magnetic field, *V* is the volume of the magnetic shield, and B is the magnetic flux density of the external magnetic field.

The second is that the electrified conductor is deformed by Lorentz force in the external magnetic field, which is expressed as follows [136]:

$$F = I \int dl \times B \tag{15}$$

where *F* is the Lorentz force, *I* is the current through the wire, *l* is the length of wire, and *B* is the magnetic flux density of the external magnetic field.

At present, the driving magnetic field of many electromagnetic actuators is generated by the energized coil, and the magnetic flux density generated by them conforms to the Biot–Saffar law, which is expressed as follows [137]:

$$
\stackrel{\rightarrow}{B} = \int\_{L} \frac{\mu\_0 I}{4\pi} \frac{dl \times \stackrel{\rightarrow}{e\_r}}{r^2} \tag{16}
$$

where *I* is the current through the wire and does not change with time, *dl* is an infinitesimal segment of the wire, *e<sup>r</sup>* is the unit vector of the current element pointing to the field point to be solved, *µ*<sup>0</sup> is the permeability of vacuum.

### 2.7.2. Structure

Electromagnetic actuators used in wearable tactile devices are usually composed of coils, magnets, vibration generating parts, and flexible materials. The changing magnetic field generated by the coils drives the magnets to move. The movement of the magnets and the vibration generating parts will cause the surface of the human skin to be generated. The tactile and vibration generating part is usually a rigid shell, and the flexible material is usually wrapped around the outside of the device to fit the skin. For instance, the wearable electromagnetic actuator researched by Pece et al. uses [138] this type of structure. The common magnetic materials that have been reported in the literatures and include nickel-plated neodymium and NdFeB magnet and the Flexible materials include PMMA, parylene, polyimide and PDMS elastomer [139].

The electromagnetic actuator can not only move in a straight line, but also complete the bending effect. It can be used as a soft gripper. For instance, Do et al. [140] demonstrated a flexible electromagnetic actuator. The magnet is mounted on the flexible beam and driven by the magnetic field generated by soft 3D coil to achieve bending deformation (Figure 20a).

*Textiles* **2021**, *1*, 26

a maximum displacement amplitude of 21.5 mm [141].

**Figure 20.** Structure of electromagnetic actuators. (**a**) Electromagnetic actuator with bending motion [140]. Copyright 2018, Wiley‐VCH. (**b**) Electromagnetic actuator composed of liquid metal coil (69%gallium, 22%Indium, 9%tin) and silicon elastomer [136]. Copyright 2020, the authors, published by AAAS. **Figure 20.** Structure of electromagnetic actuators. (**a**) Electromagnetic actuator with bending motion [140]. Copyright 2018, Wiley-VCH. (**b**) Electromagnetic actuator composed of liquid metal coil (69%gallium, 22%Indium, 9%tin) and silicon elastomer [136]. Copyright 2020, the authors, published by AAAS.

2.7.3. Applications Among wearable devices, electromagnetic actuators are an area that has not yet been completely developed. Due to their advantages of low driving voltage and large deformation, it is one of the important directions for the future development of wearable tactile devices. Ozioko et al. demonstrated [142] a dual‐function wearable tactile device that can collect tactile information and provide tactile vibration feedback. The tactile feedback component was realized by an electromagnetic actuator composed of flexible coils and permanent magnets assembled in PDMS. The tactile collection component was realized by the tablet capacitor; these components were integrated. The information was transmitted between deaf‐blind and sighted and hearing individuals via a mobile app through different touch and vibration modes, similar to Morse code (Figure 21a). In their later work, they made gloves for communication between the blind [143]. In each pair of Recently, many scholars have studied a new structure of electromagnetic actuator that uses liquid metal instead of traditional copper as the coil for the electromagnetic actuator and is integrated into flexible materials. When the flexible coil is energized, it is driven by Lorentz force. This kind of structure has more degrees of freedom than traditional structures. For instance, Mao et al. [136] embedded liquid metals (69% gallium, 22% indium, 9% tin) into silicon elastomers to replace traditional metal coils as an actuator. The soft actuator is placed on the flat magnet, and the Lorentz force generated by different types of voltage signals makes the actuator move in different ways. They use actuators as fins, agitators, etc. to demonstrate their greater deformability (Figure 20b). Likewise, liquid metal coils made of Ga-In alloys are also used in soft electromagnetic actuators. The actuator that was composed of a liquid metal coil, and PDMS could reach a maximum displacement amplitude of 21.5 mm [141].

actuators as fins, agitators, etc. to demonstrate their greater deformability (Figure 20b). Likewise, liquid metal coils made of Ga‐In alloys are also used in soft electromagnetic actuators. The actuator that was composed of a liquid metal coil, and PDMS could reach

#### gloves, six tactile devices were placed on the index finger, middle finger and ring finger 2.7.3. Applications

of each glove, representing six points in Braille. The tactile feedback information generated by tactile device and the collected touch information were processed by the microcontroller integrated on the glove and transmitted wirelessly through the Bluetooth module (Figure 21b). Likewise, Rogers et al. integrated 32 electromagnetic actuators with a wireless power supply as wearable devices for tactile feedback. The primary coil obtained power from the outside to supply power to the coil, so as to drive the magnet to vibrate. The force acting on the skin of the actuator with the input power of 1.75 mW was 135 mN, and the NFC antenna received control information from the outside. Each actuator can be controlled individually. This device can be applied in games, helping the disabled, and in other fields [133,144]. Among wearable devices, electromagnetic actuators are an area that has not yet been completely developed. Due to their advantages of low driving voltage and large deformation, it is one of the important directions for the future development of wearabletactile devices. Ozioko et al. demonstrated [142] a dual-function wearable tactile device that can collect tactile information and provide tactile vibration feedback. The tactile feedback component was realized by an electromagnetic actuator composed of flexible coils and permanent magnets assembled in PDMS. The tactile collection component was realized by the tablet capacitor; these components were integrated. The information was transmitted between deaf-blind and sighted and hearing individuals via a mobile app through different touch and vibration modes, similar to Morse code (Figure 21a). In their later work, they made gloves for communication between the blind [143]. In each pair of gloves, six tactile devices were placed on the index finger, middle finger and ring finger of each glove, representing six points in Braille. The tactile feedback information generated by tactile device and the collected touch information were processed by the microcontroller integrated on the glove and transmitted wirelessly through the Bluetooth module (Figure 21b).

**Figure 21.** Application of electromagnetic actuators in wearable devices. (**a**) Working principle of dual‐function wearable tactile device [142] copyright 2020 by the authors. Licensee MDPI, Basel, Switzerland. (**b**) Structure and application of tactile feedback glove [143]. Reproduced under the terms of the CC‐BY Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), Copyright 2020, published by IEEE. **Figure 21.** Application of electromagnetic actuators in wearable devices. (**a**) Working principle of dual-function wearable tactile device [142] copyright 2020 by the authors. Licensee MDPI, Basel, Switzerland. (**b**) Structure and application of tactile feedback glove [143]. Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International License (https://creativecommons. org/licenses/by/4.0/), Copyright 2020, published by IEEE.

Generally speaking, the electromagnetic actuator has the advantages of low driving voltage (typically from 0 to 30 V) [140], fast response speed (millisecond) [144] and large displacement, but it is difficult to be completely flexible. Furthermore, most electromagnetic actuators must be driven by wires or in an external driving magnetic field, which cannot achieve remote wireless control. *2.8. Liquid Crystal Elastomer* Likewise, Rogers et al. integrated 32 electromagnetic actuators with a wireless power supply as wearable devices for tactile feedback. The primary coil obtained power from the outside to supply power to the coil, so as to drive the magnet to vibrate. The force acting on the skin of the actuator with the input power of 1.75 mW was 135 mN, and the NFC antenna received control information from the outside. Each actuator can be controlled individually. This device can be applied in games, helping the disabled, and in other fields [133,144].

#### 2.8.1. Mechanism 2.7.4. Merits and Limitations

2.7.4. Merits and Limitations

Liquid crystal polymers are materials that exhibit liquid crystallinity and can be divided into liquid crystal elastomers (LCEs) and liquid crystal polymer networks (LCNs) [145]. Compared to LCNs, LCEs consist of loosely crosslinked liquid‐crystal side‐chain and/or main‐chain mesogenic units with a low crosslink density, which cause greater deformability and flexibility [146]. LCEs can undergo reversible transitions between polydomain, monodomain, and isotropic phases (Figure 22). These different phases Generally speaking, the electromagnetic actuator has the advantages of low driving voltage (typically from 0 to 30 V) [140], fast response speed (millisecond) [144] and large displacement, but it is difficult to be completely flexible. Furthermore, most electromagnetic actuators must be driven by wires or in an external driving magnetic field, which cannot achieve remote wireless control.

#### depend on the orientation of mesogens, which normally refers to the aromatic groups [147]. Phase transition from polydomain to monodomain can be achieved through *2.8. Liquid Crystal Elastomer*

#### mechanical deformation while the phase transition from monodomain or polydomain to 2.8.1. Mechanism

isotropic can be achieved through heating up the temperature above isotropic clearing temperature (Ti) [148]. They have large (~40%) and reversible actuation, high processability, and programmability, making LCEs a desired material for soft actuators. Liquid crystal polymers are materials that exhibit liquid crystallinity and can be divided into liquid crystal elastomers (LCEs) and liquid crystal polymer networks (LCNs) [145]. Compared to LCNs, LCEs consist of loosely crosslinked liquid-crystal sidechain and/or main-chain mesogenic units with a low crosslink density, which cause greater deformability and flexibility [146]. LCEs can undergo reversible transitions between polydomain, monodomain, and isotropic phases (Figure 22). These different phases depend on the orientation of mesogens, which normally refers to the aromatic groups [147]. Phase transition from polydomain to monodomain can be achieved through mechanical deformation while the phase transition from monodomain or polydomain to isotropic can be achieved through heating up the temperature above isotropic clearing temperature (T<sup>i</sup> ) [148]. They have large (~40%) and reversible actuation, high processability, and programmability, making LCEs a desired material for soft actuators.

**Figure 22.** Schematic of different phases of LCE. **Figure 22.** Schematic of different phases of LCE.

#### 2.8.2. Structure 2.8.2. Structure

LCEs based on a two‐stage thiol–acrylate Michael addition and photopolymerization (TAMAP) reaction were reported by Yakacki et al. [149]. Two step reactions allow for pre‐ stretch before full crosslinking of polymer networks, which enable up to 100% strain for actuation. Cai et al. reported an LCE artificial muscle film using the two step reaction. By embedding heating wires, this artificial film can lift a load of 3.92 N (the stress was 0.312 MPa) by 38% of its initial length under electrical control [148]. An electrically actuated soft artificial muscle made by flexible electrothermal film and liquid crystal elastomer was reported by Liu et al. [150]. The LCE was assembled with MWCNT/AgNW composite to achieve fast responsive uniform temperature deformation and constant resistance. At 6.5 V, a saturation temperature of 189 °C can be reached with a heating rate of 21 °C/s, leading LCEs based on a two-stage thiol–acrylate Michael addition and photopolymerization (TAMAP) reaction were reported by Yakacki et al. [149]. Two step reactions allow for pre-stretch before full crosslinking of polymer networks, which enable up to 100% strain for actuation. Cai et al. reported an LCE artificial muscle film using the two step reaction. By embedding heating wires, this artificial film can lift a load of 3.92 N (the stress was 0.312 MPa) by 38% of its initial length under electrical control [148]. An electrically actuated soft artificial muscle made by flexible electrothermal film and liquid crystal elastomer was reported by Liu et al. [150]. The LCE was assembled with MWCNT/AgNW composite to achieve fast responsive uniform temperature deformation and constant resistance. At 6.5 V, a saturation temperature of 189 ◦C can be reached with a heating rate of 21 ◦C/s, leading to a work density of 9.97 kJ/m3 and an actuating stress of 0.46 MPa.

to a work density of 9.97 kJ/m3 and an actuating stress of 0.46 MPa. Beside direct heating and electrical heating, photothermal actuation is another common method to trigger the deformation of LCE. To achieve photothermal control, conventional energy converters such as gold nanoparticles, carbon nanotubes and liquid metals have been reported to apply in the LCE matrix. An AuNR/LCE film reported by Yang et al. obtained a 100° bending angle under laser (800 nm, ≈1.0 W/cm2) (Figure 23a) [151]. Yang et al. demonstrated a heat/UV/near‐infrared (NIR) triple‐stimuli‐responsive LCE material using a two‐step cross‐linking process coupled with a uniaxial stretching technique [152]. Carbon nanotubes (CNTs) enable the conversion of near‐infrared light to thermal energy while the azobenzene group A44 V6 can trigger the deformation under UV light (Figure 23b). Ware et al. exhibited a 4D‐printed LM‐LCEs which can absorb NIR (730 nm) (Figure 23c) [153]. By the dispersion of Eutectic gallium−indium (EGaIn) in LCE, the composite is 4D‐printable to different patterns, achieving up to 150° bending angle Beside direct heating and electrical heating, photothermal actuation is another common method to trigger the deformation of LCE. To achieve photothermal control, conventional energy converters such as gold nanoparticles, carbon nanotubes and liquid metals have been reported to apply in the LCE matrix. An AuNR/LCE film reported by Yang et al. obtained a 100◦ bending angle under laser (800 nm, <sup>≈</sup>1.0 W/cm<sup>2</sup> ) (Figure 23a) [151]. Yang et al. demonstrated a heat/UV/near-infrared (NIR) triple-stimuli-responsive LCE material using a two-step cross-linking process coupled with a uniaxial stretching technique [152]. Carbon nanotubes (CNTs) enable the conversion of near-infrared light to thermal energy while the azobenzene group A44 V6 can trigger the deformation under UV light (Figure 23b). Ware et al. exhibited a 4D-printed LM-LCEs which can absorb NIR (730 nm) (Figure 23c) [153]. By the dispersion of Eutectic gallium−indium (EGaIn) in LCE, the composite is 4D-printable to different patterns, achieving up to 150◦ bending angle under 800 mw/cm<sup>2</sup> NIR light within 40 s.

under 800 mw/cm2 NIR light within 40 s. Photoisomerization is another mechanism for photoexcited actuators. Polymers with azobenzene, diarylethene and spiropyrans functional group can undergo cis–trans photoisomerization transitions triggered by radiation with high energy photons [154]. This microscopic chemical structure change can cause macroscopic deformation to a certain extent. This method normally accompanies other polymer networks to induce photoexcited actuation and improve deformability. Priimagi et al. designed a self‐ regulating iris based on light‐actuated liquid crystal elastomer (Figure 23d) [155]. It can automatically adjust the shape by reacting to the power density of the incident light. When Photoisomerization is another mechanism for photoexcited actuators. Polymers with azobenzene, diarylethene and spiropyrans functional group can undergo cis–trans photoisomerization transitions triggered by radiation with high energy photons [154]. This microscopic chemical structure change can cause macroscopic deformation to a certain extent. This method normally accompanies other polymer networks to induce photoexcited actuation and improve deformability. Priimagi et al. designed a self-regulating iris based on light-actuated liquid crystal elastomer (Figure 23d) [155]. It can automatically adjust the shape by reacting to the power density of the incident light. When the light intensity increases, the device will close, and when the minimum pupil size is reached, the light transmittance is reduced to one seventh.

the light intensity increases, the device will close, and when the minimum pupil size is

reached, the light transmittance is reduced to one seventh.

**Figure 23.** Modification of LCEs for multi‐actuation modes. (**a**) Illustration of the fabrication process of AuNR/LCE films [151]. Copyright 2018, Wiley‐VCH. (**b**) NIR/UV actuated CNT/Azobenzene/LCEs [152]. Copyright 2016, American Chemical Society. (**c**) 4D‐Printable Liquid Metal–Liquid Crystal Elastomer Composites [153]. Copyright 2016, American Chemical Society. (**d**) Light‐actuated LCE [155]. Copyright 2017, Wiley‐VCH. **Figure 23.** Modification of LCEs for multi-actuation modes. (**a**) Illustration of the fabrication process of AuNR/LCE films [151]. Copyright 2018, Wiley-VCH. (**b**) NIR/UV actuated CNT/Azobenzene/LCEs [152]. Copyright 2016, American Chemical Society. (**c**) 4D-Printable Liquid Metal–Liquid Crystal Elastomer Composites [153]. Copyright 2016, American Chemical Society. (**d**) Light-actuated LCE [155]. Copyright 2017, Wiley-VCH.

#### 2.8.3. Applications 2.8.3. Applications

The main applications of LCE in wearable actuators are artificial muscles, smart textiles and exoskeletons. Qi et al. demonstrate a loom woven smart textile using LCE fibers [156]. LCE undergoes shrinkage during heating process, which creates pores in the textile. This shrinkage starts at 40 °C and reaches the maximum at 80 °C. After cooling, the LCE fiber expands and the textile returns to its original shape (Figure 24a). These results indicate that LCE can use loom weaving to create a stimulus responsive, two‐way shape memory textile. Inspired by vascular artificial muscle, Cai et al. designed a vascular LCE‐based artificial muscle (VLAM) which showed a potential application for LCE to be used as exoskeletons (Figure 24b) [157]. With the injection of hot and cold water in its internal fluidic channel, VLAM achieved fast thermal actuation and recovery, and can be applied in a wide range of external temperatures. In Figure 24b, VLAM demonstrated The main applications of LCE in wearable actuators are artificial muscles, smart textiles and exoskeletons. Qi et al. demonstrate a loom woven smart textile using LCE fibers [156]. LCE undergoes shrinkage during heating process, which creates pores in the textile. This shrinkage starts at 40 ◦C and reaches the maximum at 80 ◦C. After cooling, the LCE fiber expands and the textile returns to its original shape (Figure 24a). These results indicate that LCE can use loom weaving to create a stimulus responsive, two-way shape memory textile. Inspired by vascular artificial muscle, Cai et al. designed a vascular LCE-based artificial muscle (VLAM) which showed a potential application for LCE to be used as exoskeletons (Figure 24b) [157]. With the injection of hot and cold water in its internal fluidic channel, VLAM achieved fast thermal actuation and recovery, and can be applied in a wide range of external temperatures. In Figure 24b, VLAM demonstrated different actuation modes of motion of a skeleton model.

29

#### different actuation modes of motion of a skeleton model. 2.8.4. Merits and Limitations

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.

**Figure 24.** Wearable LCE actuators. (**a**) LCE based thermal management textile [156]. Copyright 2019, American Chemical Society. (**b**) A vascular LCE‐based artificial muscle (VLAM) [157]. Copyright 2018, Wiley‐VCH. **Figure 24.** Wearable LCE actuators. (**a**) LCE based thermal management textile [156]. Copyright 2019, American Chemical Society. (**b**) A vascular LCE-based artificial muscle (VLAM) [157]. Copyright 2018, Wiley-VCH.

2.8.4. Merits and Limitations *2.9. Other Actuators*
