Extremely Large‐Stroke Hair Artificial Muscles with Fast Recovery Prepared by a Facile and Green Method

Twisted artificial muscles based on natural fibers are of special interest in diverse fields. However, current research is limited either by the material itself, or by the preparation methods requiring complex processing or specialized equipment. In this study, a facile and green method is introduced to prepare hair artificial muscles with extremely large tensile stroke and fast recovery. The maximum tensile stroke for the hair artificial muscles upon water actuation is as large as 10 000% and the large‐stroke muscles can recover fast in ethanol. In addition, these hair artificial muscles maintain their excellent performance after either 100 water–ethanol stimulation cycles or staying in open air for 5 months. Moreover, the hair artificial muscle is able to contract by 59% when lifting 10 times its own weight, pull a wheel model or climb a long distance under water, and work as a smart water‐sensitive switch. This work provides new insights into the development of simpler and more environmentally friendly methods for preparing advanced natural fiber‐based artificial muscles that have broad applications in soft robotics and biomedical engineering. An interactive preprint version of the article can be found here: https://doi.org/10.22541/au.167385630.01748766/v1.

DOI: 10.1002/aisy.202300014 Twisted artificial muscles based on natural fibers are of special interest in diverse fields. However, current research is limited either by the material itself, or by the preparation methods requiring complex processing or specialized equipment. In this study, a facile and green method is introduced to prepare hair artificial muscles with extremely large tensile stroke and fast recovery. The maximum tensile stroke for the hair artificial muscles upon water actuation is as large as 10 000% and the large-stroke muscles can recover fast in ethanol. In addition, these hair artificial muscles maintain their excellent performance after either 100 water-ethanol stimulation cycles or staying in open air for 5 months. Moreover, the hair artificial muscle is able to contract by 59% when lifting 10 times its own weight, pull a wheel model or climb a long distance under water, and work as a smart water-sensitive switch. This work provides new insights into the development of simpler and more environmentally friendly methods for preparing advanced natural fiber-based artificial muscles that have broad applications in soft robotics and biomedical engineering. An interactive preprint version of the article can be found here: https://doi.org/10.22541/au.167385630.01748766/v1. also environmentally friendly. By simply adjusting the twist density and the diameter of the coiled hair muscles, a tensile stroke of as large as 10 000%, more than 3 times of the largest stroke reported so far, was achieved upon stimulation. Meanwhile, ethanol could significantly shorten the recovery time of the artificial muscle to merely 10 s. Moreover, the hair artificial muscles were also demonstrated for weight lifting, climbing, and sensitive control of the switch of a circuit.

Fabrication and Characterization of the Twisted Hair Fibers
A facile two-step method combining twisting and steaming was used to fabricate the hair artificial muscles. The fabrication process is schematically illustrated in Figure 1.
First, a certain length of human hair was hanged vertically with a load torsionally tethered, and twisted counterclockwise with a motor for some time (Figure 2A). By adjusting the twisting time, hair fibers with different twist densities (1000, 1500, 2000, 2500, 2650 turns m À1 ) were obtained. The morphology of these hair fibers was examined with a micro lens and shown in Figure 2B. It can be seen that the longitudinal elements changed from straight to helical after twisting and their angular displacement increased with the increase of the twist density. The helical angles can be approximated with the following equation (1) where r is the radial distance from the center of the hair fiber (%100 μm), and T is the amount of inserted twist divided by the length of the hair fiber. In addition to the theoretical value, ImageJ was also used to measure the actual helical angles of these fibers. It can be seen from the graph in Figure 2C that the measured values of these angles agree well with the calculated values. After twisting, the torsional stress generated during the twist insertion tends to cause twist release of the hair fiber when the  www.advancedsciencenews.com www.advintellsyst.com torsional tethering is removed. To balance the strong untwisting torque, the hair fiber was folded at its middle point and plied together to achieve a self-balanced structure. When the twist density was less than 1000 turns m À1 or more than 2650 turns m À1 , either the self-plied fiber was too loose at the end or the hair fiber would break during twisting. Therefore, twist densities of 1000, 1500, 2000, 2500, and 2650 turns m À1 were used for the following experiments.

Preparation of the Hair Artificial Muscles
The torque-balanced two-plied hair fibers ( Figure 3A) were then wrapped around cylindrical mandrels clockwise or counterclockwise and steamed for 30 min. When the twisting direction of the fiber matches the winding direction of the coil, the obtained coiled muscles are referred to as homochiral artificial muscles. In contrast, when the fiber is twisted in the opposite direction to winding of the coil, the obtained coiled muscles are referred to as heterochiral artificial muscles. After hydrothermal setting, coiled hair artificial muscles were untied from the mandrels and relaxed in the ambient air. The relaxed heterochiral hair artificial muscles and the homochiral muscles have distinct morphology. The coils of the heterochiral hair artificial muscles remained in contact with each other regardless of their diameters ( Figure 3B), while the coils of the homochiral muscles gradually loosened up ( Figure 3C-i) and extended to a long thin squiggle shape. For convenience, the diameter of the homochiral hair muscle before relaxation is considered as the homochiral muscle's diameter, and the length before relaxation is considered as the homochiral muscle's initial length, which can be achieved after water actuation ( Figure 3C-ii). It can be seen from Figure 3D that no significant correlation was found between the extension of the homochiral hair muscles and their twist densities. As for the influence of the diameter of the mandrel on the elongation of the homochiral hair muscles, however, it was found that the larger the initial diameter of the muscle, the longer the muscle extended ( Figure 3E,F). The distance between the adjacent muscle coils is referred to as coil pitch and denoted as δ, which can be used to quantitatively analyze the extension of the homochiral artificial muscles ( Figure 3G). ImageJ was used to measure the coil pitch of the homochiral hair muscles. Results in Figure 3H show that the coil pitch increases with the diameter of the homochiral muscle. When the diameter of the hair muscle is increased to 8 mm, the largest coil pitch for the fully relaxed homochiral muscles is %9 mm, about 3 times larger than that of the disulfide cross-linked hair muscle. [8] The large coil pitch www.advancedsciencenews.com www.advintellsyst.com provides a structural basis for the large-stroke contraction of the artificial muscle, providing enough distance for adjacent coils to come closer.

Actuation Performance of the Hair Artificial Muscles
The actuation performance of both the homochiral and heterochiral hair artificial muscles in response to water and ethanol was studied. To investigate the impact of both the twist densities and the diameter of the hair muscles on their actuation performance, homochiral and heterochiral hair muscles with the same twist density but different diameters or with the same diameter but different twist densities were prepared. It can be seen that homochiral hair artificial muscles contract in water and elongate in ethanol ( Figure 4A, Videos S1 and S2, Supporting Information), while the heterochiral muscles elongate in water and contract in ethanol ( Figure 4B). Figure 4C presents the photos of a homochiral hair muscle with a twist density of 2500 turns m À1 and a diameter of 7.0 mm at different time points of the actuation process. The muscle was about 235 mm long in the beginning. When activated by water, the hair coils would rotate radially and contract along the muscle axis, bringing the coils closer until they contacted each other, forming a tight spring. Then the hair muscle was put into ethanol to extend. It rotated radially and extended axially to about 100 times the length of the tight spring in 50 s.
Similarly, the length change of a heterochiral hair muscle with a twist density of 2650 turns m À1 and a diameter of 8.0 mm in response to water and ethanol is shown in Figure 4D. The length of the hair muscle increased by 100 times in 32 s in water and returned to its initial length in only 10 s in ethanol. Results in Figure 4E,F (Videos S3 and S4, Supporting Information) show that the actuation speed was high for the first 10-20 s and then decreased. The maximum actuation speed could reach 500% s À1 . Tensile stroke of both homochiral and heterochiral hair artificial muscles increases with not only the diameter of the helical muscles ( Figure 4E,F), but also the twist density of the hair fibers ( Figure 4G,H). Surprisingly, when the twist density reached 2500 turns m À1 and the diameter reached 7.0 mm, the tensile stroke was as large as 10 000% for either the heterochiral or homochiral hair muscles. Since the homochiral hair muscle extended after untying from the mandrel, the length before extension is considered as its initial length. In addition to the heterochiral hair muscle with the highest twist density and largest diameter, nearly all heterochiral hair muscles could return to its initial length within 20 s ( Figure 4F,H).

Mechanisms of the Hydrothermal Setting and the Hygroscopic Actuation
Whether a twisted fiber artificial muscle can be actuated reversibly or irreversibly is determined by whether the twist in the fiber can be retained. Previous works have shown that water can fix the shape of the animal hair with much higher strain than heat and the degree of shape recovery is also greater. [11] Human hair is also one of the natural keratinous fibers based on α-keratin. [12]  The stability of the keratin is maintained by intramolecular hydrogen bonds, making it susceptible to the ambient water content. [13] Herein, attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy was used to analyze the hydrogen bonds in the human hair. Results are shown in Figure 5A. As can be seen from the colored zones in the figure, the intensity of the characteristic peaks related to hydrogen bonds in the hydrothermal-treated hair sample increased significantly. Compared to normal heating process, more aqueous molecules would penetrate into the hair and forming new intermolecular hydrogen bonds (HBs) with the residues during hydrothermal setting, thus better locking the coiled shape of the hair artificial muscle. The reason could be that hydrogen bonds in keratinous materials can act as switching units for the shape memory effect, and the destruction of HBs and forming new ones release the inner stress from the coiling and twist, thus enhancing the shape fixation rate of the hair after hydrothermal setting. This is in agreement with Hu's work that the α-keratinbased hair with the most amount of hydrogen bonds displays the highest shape fixation rate. [11a] Previous works have shown that twisted artificial muscles from keratinous fibers, including hair and wool, can be water sensitive by controlling the disulfide cross-linking. [8,14] The opening and closing of the disulfide bonds in keratin require chemical treatment such as reduction and oxidation, or ultraviolet-light irradiation. Herein, instead of the covalent disulfide bond, abundant but weaker hydrogen bonds were manipulated to set the coiled shape of the artificial muscles. Compared with the treatment methods for disulfide bonds, the hydrothermal method introduced in this work to reconstruct hydrogen bonds is much simpler, more environmentally friendly, less time-consuming, and has lower requirement on equipment.
Human hair has a fine filament-matrix structure. The internal alignment of the hair comes from the filaments, also known as the crystal, and gives the hair fiber a key property called fiber anisotropy. When soaked in water, the hair fiber's radial expansion differs from its length-direction expansion. Results of the water absorption and desorption in Figure 5B show that the pristine hair fiber expands %10% in diameter but barely changes in length while the radial expansion of the steamed hair increases to %15%. This effect might derive from the matrix protein of the hair, wherein water may act as a cross-link between the protein chains and increase the interchain space as a swelling agent. [15] Inserting twist to the anisotropically expanding hair fiber leads to the phenomenon of hygroscopic torsion effect. Because the oriented hair fiber expands in radius more than expanding in length, it will reversibly untwist when soaked in water and recover the twist when it is dried, thus forming a torsional actuator. By transforming the twisted fibers into coiled muscles, the untwisting torque-derived torsional actuation upon external stimulus can be translated into tensile actuation.
In addition, the structural change of the hair after treatment was characterized by DSC analysis. As shown in Figure 5C, in spite of the peak at around 230°C that generally depicts the Figure 5. A) Fourier-transform infrared (FTIR) characterization of the pristine hair, the heated hair, and the steamed hair. Zone I and Zone II display the absorption band related to water and the hydrogen bond. Insert shows the hydrogen bond-related macromolecular structural formula of the hair. B) Differences of the diameter change of the hair fiber after water absorption and desorption, where blue represents the pristine hair and red represents the steamed hair. Insert (top) displays the photographs of the dry, wet, and re-dried hair fiber. C) Differential scanning curves (DSCs) of the pristine hair, hair treated with only heat and with steam. D) Comparison of the tensile strokes between different artificial muscles. [2b,3b,4a,4d,7c,8À10] E) Schematic of the homochiral hair artificial muscle where the chirality of the twist and coil is the same. When stimulated by water, the hair fiber untwists, bringing the adjacent coils closer, thus reducing the coil bias angle (α c ) and leading to tensile contraction.
www.advancedsciencenews.com www.advintellsyst.com melting of the α-form crystallites, another peak at around 239°C appeared on the DSC curve of the steamed hair. This endothermic event indicated the irreversible denaturation of α-keratin caused by steaming. Interestingly, no obvious peak showed at around 239°C on the DSC curve of the heat hair, suggesting the important role of water vapor. It might be inferred that the intramolecular hydrogen bonds of the keratin were interfered with the ambient water vapor during the steaming process.
To the best of our knowledge, the tensile stroke achieved in this study is far larger than that of the previously reported artificial muscles ( Figure 5D). Furthermore, it is more than 3 times higher than the largest stroke reported so far, which results from hair artificial muscles prepared through disulfide cross-linking (3000%). [8] The actuation mechanism of the torsion-originated coiled muscle can be explained by the rearranged Love's equation as follows where ΔL is the change of the coiled muscle, l is the length of the precursor fiber, ΔT is the change of the inserted twist, and N is the number of coils. [16] In the case of the homochiral hair artificial muscle, the water-induced volume change of the hair fiber reduces T (untwist) in the equation, resulting in the decrease of L, so that the coiled hair muscle can be seen shortened in water. Another equivalent explanation is the knot theory. In this theory, the linking number which is defined as the sum of twist and writhe should be reserved. As twist represents the net rotation of the ends of a straight fiber, writhe represents the in-plane loop of the fiber. To fix the linking number, for a fiber with both ends tethered, the change of twist must bring about the change of writhe. In the case of the coiled hair artificial muscle, one coil equals to 1-sin(α c ) writhe, where α c is the angle between the coil axis and the plane orthogonal to it. It can be written as where C represents the linking number, T is the twist of the hair fiber, and N is the number of coils. It can be inferred that when the hair fiber untwists in water, the value of T decreases. To keep the linking number C fixed, α c must also be reduced to increase www.advancedsciencenews.com www.advintellsyst.com the writhe, which will be presented as adjacent coils getting closer and shortening the coiled muscle ( Figure 5E). Studies have shown that the anisotropically hygroscopic expansion of the cellulose fibers together with the coil-spring mechanics can be used to explain the working mechanism of the natural fiber-based artificial muscles, which is similar to the working principle of the hair artificial muscles in this work. [17] As for the cellulose fibers, water diffuses into the amorphous domains of the fiber and the oriented microfibrils also absorb water and swell, leading to the untwist of the fiber and contraction of the fiber-based muscle. Similarly, hair fiber can also absorb water and experience a predominant radial expansion, resulting in untwist of the hair fiber in water and contraction of the hair artificial muscles.

Reversibility and the Long-Term Stability of the Hair Artificial Muscles
To investigate the reversibility of the hair artificial muscles, 100 fully reversible water-ethanol stimulation cycles were demonstrated. The homochiral hair muscles contracted in water and re-elongated in ethanol, while the heterochiral hair muscles elongated in water and re-contracted in ethanol. This water and ethanol stimulation cycle is called a reversibility cycle and can be repeated many times. As shown in Figure 6A, after 100 water-ethanol stimulation cycles, the homochiral hair muscle could still contract to a tight spring in 21 s in response to water and re-elongate to 100 times its initial length in ethanol, while the heterochiral hair muscle could elongate to 100 times its initial length in 40 s in water and return to its initial length in only 25 s in ethanol. The change of the tensile stroke for both the homochiral and heterochiral hair muscles during the 100 reversible cycles is shown in Figure 6B,C. It can be seen that there is no significant performance decrease, indicating the reversibility for both the homochiral and heterochiral hair artificial muscles.
To investigate long-term stability, hair muscles were first placed in ambient environment for 5 months before testing. Figure 6D shows the tensile stroke as a function of time for a homochiral hair muscle with a diameter of 8 mm and twist density of 2650 turns m À1 . It responded to both the water and ethanol stimulation and achieved a tensile stroke as large as 10 000% within 30 s. The response rate shown in Figure 6F indicates no significant degradation in actuation performance after 5 months. Figure 6E,G shows similar results for the heterochiral hair muscle, suggesting the good long-term stability of the hair artificial muscles.

Applications of the Hair Artificial Muscles
The extremely large tensile stroke upon water stimulation and its fast recovery in ethanol makes the hair artificial muscle suitable for various applications. Figure 7 displays several different application scenarios. First of all, a robotic "sea cucumber" that could climb long distances was made from a heterochiral hair muscle with a diameter of 3.0 mm and twist density of 2500 turns m À1 . The sea cucumber could crawl on a barbed plastic cord as long as 200 mm under the stimulation of water and ethanol ( Figure 7A). The barbs on the surface of the cord all tilted in one direction, guiding the sea cucumber to move in the fixed direction. In addition, the energy generated from water actuation of a homochiral hair muscle was used to pull a wheel model (%2.8 g). The wheel model could move 40 mm within 143 s in water ( Figure 7B), suggesting that the homochiral hair muscle could work as an engine to actuate the movement of a wheel model which was approximately 500 times heavier than itself. Moreover, a single homochiral hair artificial muscle was shown to lift a weight 10 times heavier than itself by 57 mm (59% strain) in 60 s in water ( Figure 7C, Video S5, Supporting Information). The contractile work generated by this hair muscle during weight lifting was normalized to the total muscle weight, thus the work capacity of this hair muscle was 5.38 J kg À1 .
The high sensitivity of the hair muscle to water can be used for smart switches due to its extremely large tensile stroke upon water actuation. As illustrated in Figure 7D, two homochiral hair muscles were used to control the switch of a circuit. When the environment was dry, the circuit was connected and the light emitting diode (LED) light was on. But if there is water in the ambient environment and it comes into contact with the hair muscles, they would contract and disconnect the circuit, thus turning off the LED light. Photos in Figure 7E (Video S6, Supporting Information) show that the smart switch turned off the LED light when the hair muscle came into contact with water.

Conclusion
In this study, tether-free hair artificial muscles with extremely large tensile stroke and fast recovery were successfully made through a facile and green method, without any chemical processing. Coiling the twisted hair fiber with large diameters increases the muscles' tensile stroke, and ethanol stimulation enables their fast recovery. The tensile stroke upon water actuation could be as large as 10 000%, three times larger than the largest tensile stroke reported so far. [8] The fastest actuated muscle could fully recover in 10-20 s. Various applications such as long-distance climbing robots, weight pulling or lifting, and a smart water-sensitive switch were also demonstrated. This study provides new insight into developing simple, cost-effective, and environmentally friendly methods for preparing advanced natural fiber-based artificial muscles for applications in soft robotics and biomedical fields.

Experimental Section
Materials: Ethanol was purchased from Sinopharm Chemical Reagent Co., Ltd. Pure water was provided by the water purification systems (Direct-Q8UV-R, Merk Millipore, Germany). Human hair was provided by the author Qian-Ru Xiao, who had no objection to the hair being used in this study. Appropriate ethics committee approval and informed written consent of all participants were obtained.
Fabrication and Characterization of the Twisted Hair Fibers: Long black hair with a diameter of 90-100 μm was cleaned and used in this study. The length of the hair fibers ranged from 40 to 70 cm. For preparation of the twisted hair fiber, a raw hair fiber was placed vertically with a weight hanging on its bottom end. Twist was inserted at the top end of the hair fiber by a motor in the counterclockwise direction. Meanwhile, the weight at the bottom was tethered to avoid twist release during the twist insertion. After twisting, the fiber was folded at its middle point and formed a torquebalanced two-plied structure. Then, the single twisted hair filament was attached to a folded paper with an observation window and observed with a macro lens. ImageJ was used to measure the helical angles.
Preparation of the Tensile Hair Artificial Muscles: For preparation of the tensile hair artificial muscles, the obtained twist-stable double-plied hair fibers were wrapped around cylindrical mandrels of different diameters (1.6, 3.0, 5.0, 7.0, 8.0 mm) in clockwise and counterclockwise directions, respectively, with both ends tethered. After steaming for 30 min, the coiled hair muscles were untied from the mandrel and relaxed in the open air. The fully relaxed hair muscles were observed with a macro lens and imageJ was used to measure the coil pitch. Differential scanning calorimetry (DSC) spectra for pristine hair fibers, hair fibers heated for 30 min, and hair fibers steamed for 30 min were obtained on a thermal analysis (TA) DSC (USA, Q2000). FTIR spectroscopy (Thermo Scientific Nicolet iN10, USA) in the scan range of wave numbers between 4000 and 650 cm À1 using ATR method was used to characterize the pristine hair, heated hair, and the steamed hair.
Water Absorption and Desorption of the Hair: The water absorption and desorption properties of the hair was studied by measuring the changes in diameter and the length of the hair. Briefly, a short segment of the pristine hair and the steamed hair was fixed and exposed to a water droplet, respectively. An optical microscope was used to take the images of the hair segment before water exposure, 5 min after water exposure, and after drying, respectively. The diameter and the length of the hair segment were measured with ImageJ from the pictures.
Muscle Actuation Test: The actuation performance of both homochiral and heterochiral hair artificial muscles with different diameters and different twist densities were characterized under water. Then, the actuated hair muscles were put into ethanol for recovery. Digital camera was used to record the video of the real-time actuation process. The tensile stroke of the hair artificial muscles was calculated. In addition, both the homochiral and heterochiral muscles were put into water and ethanol repeatedly for 100 times to test their reversibility. And the actuation performance of both the homochiral and heterochiral hair muscles was investigated after 5 months to test their stability.
Applications of the Hair Artificial Muscles: Homochiral hair artificial muscles with a diameter of 1.6 mm and twist density of 2500 turns m À1 were used for weight lifting, pulling a model of car wheels under water, and sensitively switched off the LEDs when exposed to water. The heterochiral hair muscles with a diameter of 3.0 mm and twist density of 2500 turns m À1 were used as a "sea cucumber" for climbing along a barbed cord under water.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.