*2.3. Thermal and Hygroscopic Expansion*

*Textiles* **2021**, *1*, 7

#### 2.3.1. Mechanism *2.3. Thermal and Hygroscopic Expansion*

Thermal or hygroscopic expansion is the tendency of matter to change its volume in response to a change in temperature or moisture content. The macroscopic effect of hygroscopic expansion is similar to that of thermal expansion, but the microscopic causes are very different. Thermal expansion is a result of molecules' vibration and movement when heated, while hygroscopic expansion is caused by hygroscopy, the phenomenon of attracting and holding water molecules via either absorption or adsorption from the ambient environment. The coefficient of thermal expansion (αT) is defined to quantify the magnitude of the volume change in relation to temperature change, which is given by 2.3.1. Mechanism Thermal or hygroscopic expansion is the tendency of matter to change its volume in response to a change in temperature or moisture content. The macroscopic effect of hygroscopic expansion is similar to that of thermal expansion, but the microscopic causes are very different. Thermal expansion is a result of molecules' vibration and movement when heated, while hygroscopic expansion is caused by hygroscopy, the phenomenon of attracting and holding water molecules via either absorption or adsorption from the ambient environment. The coefficient of thermal expansion (αT) is defined to quantify the magnitude of the volume change in relation to temperature change, which is given by

$$a\_T = \frac{1}{V} \left(\frac{\partial V}{\partial T}\right)\_p \tag{1}$$

where *V* is the volumetric expansion, *T* is the temperature, and *p* is the pressure held constant during the expansion. In the case of hygroscopic expansion, the temperature (*T*) in Equation (1) is replaced by the relative humidity (ϕ). where *V* is the volumetric expansion, *T* is the temperature, and *p* is the pressure held constant during the expansion. In the case of hygroscopic expansion, the temperature (*T*) in Equation (1) is replaced by the relative humidity (φ).

High thermal expansion materials include low-density polyethylene (500 ppm/K) [48], polydimethylsiloxane (~300 ppm/K) [49], biaxial oriented polypropylene (~120 ppm/K) [50], etc., while textile cellulose and protein fibers have a high expansion in diameter when wet, such as cotton (7–20%), jute (20–21%), wool (14–17%), and silk (16–19%) [51]. These materials' expansion behavior has been utilized to fabricate various tensile, bending, and torsional actuators for wearable applications, as shown in Figure 5. High thermal expansion materials include low‐density polyethylene (500 ppm/K) [48], polydimethylsiloxane (~300 ppm/K) [49], biaxial oriented polypropylene (~120 ppm/K) [50], etc., while textile cellulose and protein fibers have a high expansion in diameter when wet, such as cotton (7–20%), jute (20–21%), wool (14–17%), and silk (16– 19%) [51]. These materials' expansion behavior has been utilized to fabricate various tensile, bending, and torsional actuators for wearable applications, as shown in Figure 5.

**Figure 5.** A schematic diagram of heat or moisture induced actuators. **Figure 5.** A schematic diagram of heat or moisture induced actuators.

#### 2.3.2. Structure 2.3.2. Structure

Bimorph structure is a widely adopted strategy to generate bending motion via asymmetric deformation of the two layers, an active layer that contracts or expands by an external stimulation and a passive layer that remains intact. The interfacial stress generated between the two layers due to the volume mismatch leads to bending deformations. There are many examples of bending actuators in the film form, such as LDPE/PVC film [48], poly(vinyl alcohol‐co‐ethylene) (EVOH)/cellulose film [52]. Alternatively, asymmetric exposure of water vapors or heat to the thin film of moisture or heat responsive materials can also achieve the bending motions, but the bending performance is thickness sensitive. Bimorph structure is a widely adopted strategy to generate bending motion via asymmetric deformation of the two layers, an active layer that contracts or expands by an external stimulation and a passive layer that remains intact. The interfacial stress generated between the two layers due to the volume mismatch leads to bending deformations. There are many examples of bending actuators in the film form, such as LDPE/PVC film [48], poly(vinyl alcohol-co-ethylene) (EVOH)/cellulose film [52]. Alternatively, asymmetric exposure of water vapors or heat to the thin film of moisture or heat responsive materials can also achieve the bending motions, but the bending performance is thickness sensitive. Examples includes microfibrillated cellulose film [53] and nylon beam coated with thermally conductive graphene flakes [54]. Some bimorph fibers have also been developed. Cyclic olefin copolymer elastomer/HDPE fiber was developed by using a fiber-drawing technique, and can lift more than 650 times its own weight at a low temperature [55]. A moisture trigged acetate-based conjugate fiber (VentcoolTM) was created by the Kaiteki company to automatically adjust ventilation of clothing [56].

Torsional fiber actuators are realized by twisting moisture and heat responsive fibers to form a yarn like structure with free-standing torque balanced status. The yarn torque or torsional movement is generated by anisotropic swelling of fibers in the radial direction when exposed to a stimulus. When the stimulus is off, the deswelling of fibers causes the actuator to rotate in the opposite direction and finally returns to the balanced status. Accompanying the torsional movement, the twisted yarn or fiber actuators also show a tensile expansion and contraction behavior due to the twist contraction effect. For example, degummed silk fibers were twisted and folded into torsional silk muscles that provided a reversible torsional stroke of 547◦ mm−<sup>1</sup> , a maximum rotary speed of 975 rpm, and a peak torque of 0.063 Nm/kg [57]. Twisted graphene oxide (GO) fiber demonstrated remarkable performance as a reversible rotary motor with a torsional stroke 588◦ mm−<sup>1</sup> , a rotary speed of up to 5190 rpm, a tensile expansion of 4.7%, and a peak power output of 71.9 W/kg. The moisture-triggered electric generator based on GO fibers produced an open-circuit voltage of up to 1 mV, and a short-circuit current of up to 40 µA [58]. Hydrophobic carbon nanotube (CNT) twisted yarns offered a maximal torsional moment of 0.4 Nm/kg, close to the commercial electric motor (the Aerotech model 1410-01motor) in response to water and moisture after oxygen plasma treatment [59]. Other fiber materials include bamboo [60], cotton [61], lotus [62], chitosan [63], etc. The performance of several fiber-based torsional actuators is summarized in Table 1.


Fiber-based tensile actuators are achieved by twisting and coiling the fibers to form a spring-like structure. The actuating principle lies in anisotropic swelling of the fibers in the radial direction, causing the yarn to untwist and in turn the coil to change its writhe, which pulls the adjacent coils close to each other, shortening the coil. When used as a tensile actuator, the cylindrical yarn coil should be torsionally tethered such that the two ends can slide but not be allowed to rotate, which prevents the yarn from untwisting. This concept was first proposed by Harins et al. [65] in 2014, in which low-cost high-strength nylon fibers, used as fishing line or sewing thread, have demonstrated a high stroke of 34% for a temperature variation of ~220 ◦C. Likely, composite yarns made of polyimide and PDMS have achieved a tensile actuation of 20.7% and a competitive specific work of 158.9 J/kg, four times that of natural muscle [64]. Surfactant-treated wool yarn coils generated a contraction stroke up to 38% and a maximum work capacity of 194 J/kg [66]. Viscose fiber artificial muscles demonstrated a 35% contraction and a maximum work capacity of 90.4 J/kg [67]. Degummed silk fibers were twisted and wrapped around a mandrel to form tensile actuators that provided a maximum 70% contraction and peak work capacity up to 73 J/kg [57]. Other fiber materials include carbon fiber (CF)/PDMS [68], lotus [62], bamboo [60], etc. The performance of several fiber-based tensile actuators is summarized in Table 2.


**Table 2.** Performance of fiber-based tensile actuators.

### 2.3.3. Applications

There are many applications of moisture or heat responsive materials for smart textiles, which dynamically change the structure or pore size of clothing for enhanced personal thermal management [69]. For instance, two kinds of moisture responsive bendable smart clothing were designed based on the successful application of the Nafion film from DuPont that can reversibly adapt their thermal insulation functionality [70]. The first design is pre-cut flaps, which open to produce pores in Nafion sheets when humidity increases, allowing air flow and reducing both the humidity level and the apparent temperature (Figure 6a). The second design is thickness adjustable clothes by inserting the bent Nafion films between two fabrics. When the humidity increases, the films become thinner, thus reducing the gap between the two fabrics to reduce the thermal insulation (Figure 6b). Knitted fabrics made of CNTs coated triacetate-cellulose bimorph fibers effectively shifted the infrared radiation (IR) by more than 35% as the relative humidity of the underlying skin changed [71]. When hot and wet, the multiple metafibers move close to each other, leading to resonant electromagnetic coupling that modulates the IR emissivity to spectrally overlap with that of the human body and enhance radiative cooling effect. A woven textile from silk fiber muscles demonstrated excellent comfort and drapability. The sleeves, made by weaving coiled silk muscle fibers in the warp direction and untwisted fibers in the weft direction, shrink in the warp direction when humidity increases, and then expand when humidity decreases [57]. This moisture-responsive textile, which can change macroshape or microstructure, is promising to be very effective for moisture and thermal management to increase comfort between skin and fabric (Figure 6c). In addition to hygroscopic polymer materials, living cells have been engineered to design biohybrid wearables [72]. A bilayerstructured biohybrid film was proposed by depositing genetically tractable microbes on a humidity-inert material to form a heterogeneous multilayered structure, which can reversibly change shape within a few seconds in response to environmental humidity gradients (Figure 6d).
