*2.1. Pneumatic and Hydraulic Actuator*

Artificial muscles are born out of human needs for biological muscles, with a wide range of applications in soft robots, wearable devices, and medicine [9,10]. They are typically demonstrated to be alternatives to rigid electrostatic and electromagnetic actuators, since they possess unique advantages such as being silent, soft, and compliant. Among different forms of artificial muscles, fluid-driven actuators are commonly used due to their simplicity, large driving stress and deformation, good energy efficiency, and processability [11]. By far, pneumatic and hydraulic actuators are one of the most applied actuators in the industry. According to previous research, fluid-driven actuators can be simply divided into three major types of devices: elastic fluidic actuator, piston–cylinder fluidic actuators, and drag-based fluidic actuators [12]. Note that in this review we mainly focus on elastic fluidic actuators, specifically McKibben actuators, since they are by far the most common in wearable actuators [12]. Pneumatic and hydraulic actuators will be introduced together since they share similar structures and mechanisms. Compared to pneumatic, hydraulic actuator systems are more complicated and normally require the installation of a pump, valves, hoses, and an electric motor [13]. Nevertheless, hydraulic motors typically possess the better power to weight ratio [14].

#### 2.1.1. Structure

McKibben actuators are typical pneumatic artificial muscles (PAMs). These actuators generally consist of an expandable chamber, normally a balloon, with other structures. By the pressurization of fluid in the chamber, these actuators can transform the expansion to a contraction force. Pneumatic and hydraulic actuators can generate linear, torsional, and bending actuation through the control of internal structures, which are normally inspired by bio-architectures. For example, inspired by muscular hydrostat, Schaffner et al. demonstrated complex motion modes on soft actuators by printing stiff silicone stripes on top of a soft silicone cylinder [15] (Figure 1a). With the arrangement of stiff fiber, this actuator can achieve bending, elongation, and other movements. Similarly, Kim et al. reported a lamina composed of not stretchable fiber, super-elastic matrix, and an adhesive backing [16]. This lamina is also named Stretchable Adhesive Uni-Directional prepreg (STAUD-prepreg) (Figure 1b). By adhering multiple prepreg on a stack, this soft actuator demonstrates complex motion. It is worth noting that with the rearrangement of prepregs, this actuator is reprogrammable, which makes it different than other predefined fluid-driven actuators. Based on vacuum-actuated muscle-inspired pneumatic structures (VAMPs), Li et al. proposed fluid-driven origami-inspired artificial muscles (FOAMs) composed of a folding skeleton, flexible fluid-tight skin, and fluid medium (Figure 1c) [17]. Through programing the geometry of the skeleton, various motions and contractions can be achieved. Experiments reveal that these muscles can contract over 90% of their initial lengths, generate stresses of ~600 kPa, and produce peak power densities over 2 kW/kg. composed of a folding skeleton, flexible fluid‐tight skin, and fluid medium (Figure 1c) [17]. Through programing the geometry of the skeleton, various motions and contractions can be achieved. Experiments reveal that these muscles can contract over 90% of their initial lengths, generate stresses of ~600 kPa, and produce peak power densities over 2 kW/kg.

**Figure 1.** Schematic diagram of structures of pneumatic and hydraulic actuators. (**a**) Silicone‐based 3D‐printing pneumatic actuators [15] copyright 2018, the authors, published by Springer Nature. (**b**) Stretchable Adhesive Uni‐Directional prepreg (STAUD‐prepreg) [16] copyright 2019, the authors, published by Springer Nature. (**c**) Fluid‐driven origami‐ inspired artificial muscles (FOAMs) [17] copyright 2017, the author(s), published by PNAS. **Figure 1.** Schematic diagram of structures of pneumatic and hydraulic actuators. (**a**) Silicone-based 3D-printing pneumatic actuators [15] copyright 2018, the authors, published by Springer Nature. (**b**) Stretchable Adhesive Uni-Directional prepreg (STAUD-prepreg) [16] copyright 2019, the authors, published by Springer Nature. (**c**) Fluid-driven origami-inspired artificial muscles (FOAMs) [17] copyright 2017, the author(s), published by PNAS.

#### 2.1.2. Applications A typical application of pneumatic and hydraulic actuators is Robotic orthoses, also 2.1.2. Applications

called exoskeletons. These devices are usually used to assist human movement, or for rehabilitation of upper or lower limbs [18]. The electric, hydraulic, and pneumatic actuators are the most popular in current orthoses. Here, we briefly introduce some recent research of pneumatic/hydraulic actuators on exoskeleton devices. These devices use simple but well‐designed materials with novel actuation mechanisms to achieve lightness and high efficiency. An inflatable wrinkle pneumatic actuator with fast inflation and deflation responses A typical application of pneumatic and hydraulic actuators is Robotic orthoses, also called exoskeletons. These devices are usually used to assist human movement, or for rehabilitation of upper or lower limbs [18]. The electric, hydraulic, and pneumatic actuators are the most popular in current orthoses. Here, we briefly introduce some recent research of pneumatic/hydraulic actuators on exoskeleton devices. These devices use simple but well-designed materials with novel actuation mechanisms to achieve lightness and high efficiency.

was proposed [19]. A theoretical model was built to improve the design of the torque required to fit the joints that need to be assisted. A pneumatic circuit was designed to instantly generate negative pressure at the exit of the actuator, thereby shortening the exhaust time. This wrinkle actuator was applied on a wearable knee suit to exhibit rapid inflation and deflation times (0.40 s and 0.16 s) (Figure 2a). A textile based pneumatic actuator was designed to assist the movement of the upper arm through shoulder abduction and horizontal flexion/extension [20]. By coordinated movement of Abduction Actuators (ABA) with the Horizontal Extension/Flexion Actuator (HEFA), the device can provide comprehensive support for movement in different directions of the shoulder. With only 0.48 kg for the whole actuator systems, this device can generate 8 Nm under 70 kpa. A soft robotic glove based on a fluid‐driven actuator demonstrated assistance in the grasping movement of the hand [21]. Different actuation modes for the thumb and the An inflatable wrinkle pneumatic actuator with fast inflation and deflation responses was proposed [19]. A theoretical model was built to improve the design of the torque required to fit the joints that need to be assisted. A pneumatic circuit was designed to instantly generate negative pressure at the exit of the actuator, thereby shortening the exhaust time. This wrinkle actuator was applied on a wearable knee suit to exhibit rapid inflation and deflation times (0.40 s and 0.16 s) (Figure 2a). A textile based pneumatic actuator was designed to assist the movement of the upper arm through shoulder abduction and horizontal flexion/extension [20]. By coordinated movement of Abduction Actuators (ABA) with the Horizontal Extension/Flexion Actuator (HEFA), the device can provide comprehensive support for movement in different directions of the shoulder. With only 0.48 kg for the whole actuator systems, this device can generate 8 Nm under 70 kpa.

rest of the fingers were applied to achieve a typical grasping movement (Figure 2b). Inspired by sheet‐like biological muscles, the Zhu group presented a new family of soft actuators, named Fluidic Fabric Muscle Sheets (FFMS) [22]. The elastic tubes were stitched

100% engineering strain (Figure 2c).

*Textiles* **2021**, *1*, 4

**Figure 2.** Applications of pneumatic/hydraulic actuators as wearable devices. (**a**) Pneumatic based wearable knee suit [19]. (**b**) A hydraulic soft glove for combined assistance and at‐home rehabilitation [21]. Copyright 2015, Elsevier. (**c**) Fluidic Fabric Muscle Sheets (FFMS) [22]. **Figure 2.** Applications of pneumatic/hydraulic actuators as wearable devices. (**a**) Pneumatic based wearable knee suit [19]. (**b**) A hydraulic soft glove for combined assistance and at-home rehabilitation [21]. Copyright 2015, Elsevier. (**c**) Fluidic Fabric Muscle Sheets (FFMS) [22].

2.1.3. Merit and Limitations A major problem with pneumatic/hydraulic actuators is poor portability [23]. These actuators usually require a large fluid tank and assorted control units. In addition, the seal of the actuator is also a challenge [24]. Although they are made of soft materials, these actuators still contain some hard parts, which limit their applications. At the same time, their advantages are obvious. The actuation mechanism determines that these actuators are not limited by material. Through the integration of the control system and structure, pneumatic/hydraulic actuators can achieve very complex movements [25]. Fluid pressure can generate a greater driving force than other soft materials, which promise their applications for exoskeletons. Due to the maturity of the process technique, these actuators have been made into commercial products for daily use [26]. A soft robotic glove based on a fluid-driven actuator demonstrated assistance in the grasping movement of the hand [21]. Different actuation modes for the thumb and the rest of the fingers were applied to achieve a typical grasping movement (Figure 2b). Inspired by sheet-like biological muscles, the Zhu group presented a new family of soft actuators, named Fluidic Fabric Muscle Sheets (FFMS) [22]. The elastic tubes were stitched into fabric to achieve actuation by the movement of fluid in and out. By the design of the fluid route, these actuators exhibit multiple deformation. Through the application of textile technology, this type of actuator can be made into a micro execution unit, or can be developed as a large, meter-level actuator. Data shows that this type of actuator can withstand a force of more than 150 n, which is more than 115 times its weight, and up to 100% engineering strain (Figure 2c).

into fabric to achieve actuation by the movement of fluid in and out. By the design of the fluid route, these actuators exhibit multiple deformation. Through the application of textile technology, this type of actuator can be made into a micro execution unit, or can be developed as a large, meter‐level actuator. Data shows that this type of actuator can withstand a force of more than 150 n, which is more than 115 times its weight, and up to

#### *2.2. Shape Memory Effect* 2.1.3. Merit and Limitations

Shape memory effect (SME) is a phenomenon in which a material recovers to its original size and shape when heated above a certain characteristic transformation temperature [27]. The two most prevalent shape‐memory materials are shape memory alloys (SMAs) and shape memory polymers (SMPs). 2.2.1. SMAs SMAs are characterized by solid state displacive transformations between austenite and martensite phases in response to a stimulus such as heat. This provides the materials with the capability for sustaining and recovering from strains up to 10% which imbues them with unique actuator and potential sensor capabilities in smart material systems [28]. As shown in Figure 3, upon heating, phase transformation from martensite to austenite A major problem with pneumatic/hydraulic actuators is poor portability [23]. These actuators usually require a large fluid tank and assorted control units. In addition, the seal of the actuator is also a challenge [24]. Although they are made of soft materials, these actuators still contain some hard parts, which limit their applications. At the same time, their advantages are obvious. The actuation mechanism determines that these actuators are not limited by material. Through the integration of the control system and structure, pneumatic/hydraulic actuators can achieve very complex movements [25]. Fluid pressure can generate a greater driving force than other soft materials, which promise their applications for exoskeletons. Due to the maturity of the process technique, these actuators have been made into commercial products for daily use [26].

#### starts at temperature As and stops at temperature Af, whilst the reverse phase transformation starts at temperature Ms and stops at temperature Mf during cooling *2.2. Shape Memory Effect*

(Figure 3a). There are three major shape memory characteristics for SMAs, namely one‐ way memory effect, two‐way memory effect, and pseudoelasticity [29]. In one‐way SMAs, the material is deformed at a low temperature, and the shape can be restored after heating, in which the SME only exists in the heating process. In two‐way SMAs, the material has Shape memory effect (SME) is a phenomenon in which a material recovers to its original size and shape when heated above a certain characteristic transformation temperature [27]. The two most prevalent shape-memory materials are shape memory alloys (SMAs) and shape memory polymers (SMPs).

SME during both heating and cooling by training. In pseudoelasticity, the phase transition

#### of the material comes from an external mechanical stress instead of thermal excitation. 2.2.1. SMAs

The SME was first observed in gold‐cadmium alloy by Arne Ölander in 1932 [30]. So far, more than 50 alloy metals with shape memory effect have been found. Among them, Nickel‐titanium (NiTi) has become the most popular and studied SMA due to its SMAs are characterized by solid state displacive transformations between austenite and martensite phases in response to a stimulus such as heat. This provides the materials with the capability for sustaining and recovering from strains up to 10% which imbues them with unique actuator and potential sensor capabilities in smart material systems [28]. As shown in Figure 3, upon heating, phase transformation from martensite to austenite starts at temperature A<sup>s</sup> and stops at temperature A<sup>f</sup> , whilst the reverse phase transformation starts at temperature M<sup>s</sup> and stops at temperature M<sup>f</sup> during cooling (Figure 3a). There are three major shape memory characteristics for SMAs, namely one-way memory effect, two-way memory effect, and pseudoelasticity [29]. In one-way SMAs, the material is deformed at a low temperature, and the shape can be restored after heating, in which the SME only exists in the heating process. In two-way SMAs, the material has SME during both heating and cooling by training. In pseudoelasticity, the phase transition of the material comes from an external mechanical stress instead of thermal excitation. The SME

was first observed in gold-cadmium alloy by Arne Ölander in 1932 [30]. So far, more than 50 alloy metals with shape memory effect have been found. Among them, Nickel-titanium (NiTi) has become the most popular and studied SMA due to its outstanding mechanical and thermomechanical properties, such as biocompatibility, high corrosion, and high work capacity [31,32]. SMA wires can achieve a high stress of about 700 Mpa and strain of 10% in length. Higher strains can be obtained with special geometries such as helix or zigzag but with lower stresses. Due to their excellent properties, SMAs are widely used in aerospace, mechatronics, biomedicine, bridge construction, automobiles, and daily life [33].

**Figure 3.** Shape memory principles of SMAs (**a**) and SMPs (**b**).

#### 2.2.2. SMPs

SMPs are polymeric smart materials that have the ability to return from a deformed state (temporary shape) to their original (permanent) shape in response to an external trigger, such as temperature, an electric or magnetic field, light, or solution [34–36]. As demonstrated for a heat triggered SMP in Figure 1b, the original shape of an SMP is determined after manufacturing by conventional methods such as extrusion, spinning, pressing, etc. The SMP is changed into a temporary shape by processing through heating, deformation, and cooling. The material maintains this temporary shape until it is activated by a predetermined external stimulus. This cycle of programming and recovering can be repeated multiple times. The mechanism behind this phenomenon depends on their molecular network structure, which contains at least two separate phases, namely a fixing phase and a reversible phase [37]. The fixing phase, showing the highest thermal transition, is the temperature that must be exceeded to establish the physical crosslinks responsible for the permanent shape. The fixing phase can be the cross-linked structure, the partial crystalline structure or the glassy state of the polymer, while the reversible phase can be a partial crystalline phase with reversible change of crystallization and melting, or a phase structure with reversible transition between a glass state and rubber state. SMPs can be a single component polymer or a copolymer, a mixture of two components with different softening temperatures but good compatibility. SMPs also cover a wide property-range from stable to biodegradable, from soft to hard, and from elastic to rigid, depending on the structural units that constitute the SMP [38].

#### 2.2.3. Applications

SMAs, especially SMA wires, have been widely employed in various soft exoskeletons to replace conventional rigid motors or pneumatic/hydraulic actuators for rehabilitation and assisting patients' daily life including hand [39], elbow [40], wrist [41], ankle [42], etc. For instance, a lower limb-worn, soft wearable robot using an SMA wire has been designed to assist ankle plantar flexion, which can generate a stroke of 3 cm and an ankle moment of 100 N cm in each ankle during walking (Figure 4a) [42]. A soft muscle glove containing SMA wires has been developed to replicate the functionalities of a human hand. The glove can achieve a functional range of motion of the human hand and can perform a wide range

of grasp types [43]. A suit-type wearable robot (STWR) containing SMA fabric muscle has been developed to assist the muscular strength of wearers [44]. The STWR can lift barbells weighing 4 kg to a certain target position and demonstrated a fast response time of less than 1 s (Figure 4b). A medical rehabilitation exoskeleton using SMA wires as the actuator for the elbow has been proposed for proper patient elbow joint articulation. The proposed exoskeleton is lightweight and has low noise, which improves the medical rehabilitation process and their ability to perform daily activities (Figure 4c) [45]. containing SMA fabric muscle has been developed to assist the muscular strength of wearers [44]. The STWR can lift barbells weighing 4 kg to a certain target position and demonstrated a fast response time of less than 1 s (Figure 4b). A medical rehabilitation exoskeleton using SMA wires as the actuator for the elbow has been proposed for proper patient elbow joint articulation. The proposed exoskeleton is lightweight and has low noise, which improves the medicalrehabilitation process and their ability to perform daily activities (Figure 4c) [45].

*Textiles* **2021**, *1*, 6

**Figure 4.** Soft wearable robots using SMA wires. (**a**) A soft wearable robot using a SMA wire to assist ankle plantar flexion [42]. Copyright 2020, IOP Publishing Ltd. (**b**) An STWR to assist the muscular strength of wearers [44]. Copyright, 2019, Springer Nature. (**c**) A medical rehabilitation exoskeleton for proper patient elbow joint articulation [45] Copyright 2017, Dorin Copaci et al. **Figure 4.** Soft wearable robots using SMA wires. (**a**) A soft wearable robot using a SMA wire to assist ankle plantar flexion [42]. Copyright 2020, IOP Publishing Ltd. (**b**) An STWR to assist the muscular strength of wearers [44]. Copyright, 2019, Springer Nature. (**c**) A medical rehabilitation exoskeleton for proper patient elbow joint articulation [45] Copyright 2017, Dorin Copaci et al.

SMPs have plentiful applications in textiles, such as wrinkle‐free fabrics, self‐cleaning fabrics, breathable garments, and self‐adaptable textiles [6]. Lamination, coating, knitting, and weaving are methods that integrate SMPs into textiles [5]. Thermal responsive SMPs can be applied onto textile fabrics via finishing process. Wrinkle‐free, crease retention, or anti‐shrinkage textiles can be fabricated by treating SMPs on fabrics. The cotton fabric treated with SMPU showed a good wrinkle‐free effect and can return to its original flat shape quickly upon blowing steam over it [46]. Taking the advantages of the change of water vapor permeability of SMPs with temperature makes them capable of regulating the human body temperature [47]. As the body temperature is above the glass transition temperature of SMPs, the molecular free volume of the SMPs significantly increases, which aids the transfer of vapor and heat through perspiration, and vice versa. SMPs have plentiful applications in textiles, such as wrinkle-free fabrics, self-cleaning fabrics, breathable garments, and self-adaptable textiles [6]. Lamination, coating, knitting, and weaving are methods that integrate SMPs into textiles [5]. Thermal responsive SMPs can be applied onto textile fabrics via finishing process. Wrinkle-free, crease retention, or anti-shrinkage textiles can be fabricated by treating SMPs on fabrics. The cotton fabric treated with SMPU showed a good wrinkle-free effect and can return to its original flat shape quickly upon blowing steam over it [46]. Taking the advantages of the change of water vapor permeability of SMPs with temperature makes them capable of regulating the human body temperature [47]. As the body temperature is above the glass transition temperature of SMPs, the molecular free volume of the SMPs significantly increases, which aids the transfer of vapor and heat through perspiration, and vice versa.

#### 2.2.4. Merits and Demerits 2.2.4. Merits and Demerits

SMAs have outstanding mechanical and thermomechanical properties, such as biocompatibility, high corrosion, and high work capacity. These allow them to have several commercialized applications in medical tools. However, SMAs exhibit large thermal hysteresis, which makes it hard to control the actuation process. Moreover, the low durability and high material cost (\$200–300/kg) hinder their applications in wearable robots. In contrast to SMAs, SMPs possess many advantages such as light weight, flexibility, high elastic deformation (up to 800%), high shape recovery (>90%), and low recovery temperature, etc. SMPs can be integrated into textiles by various textile processes. Although many textile prototypes and commercial trials have been demonstrated, many issues remain to be resolved. Several difficulties lie in meeting the stringent textile requirements, such as color, dimensional stability, comfort and tactile properties, washability, strength, flexibility, stretchability, as well as compatibility with many other chemical, mechanical, and thermal processing standards, and low‐cost production [37]. SMAs have outstanding mechanical and thermomechanical properties, such as biocompatibility, high corrosion, and high work capacity. These allow them to have several commercialized applications in medical tools. However, SMAs exhibit large thermal hysteresis, which makes it hard to control the actuation process. Moreover, the low durability and high material cost (\$200–300/kg) hinder their applications in wearable robots. In contrast to SMAs, SMPs possess many advantages such as light weight, flexibility, high elastic deformation (up to 800%), high shape recovery (>90%), and low recovery temperature, etc. SMPs can be integrated into textiles by various textile processes. Although many textile prototypes and commercial trials have been demonstrated, many issues remain to be resolved. Several difficulties lie in meeting the stringent textile requirements, such as color, dimensional stability, comfort and tactile properties, washability, strength, flexibility, stretchability, as well as compatibility with many other chemical, mechanical, and thermal processing standards, and low-cost production [37].
