Moisture-Responsive Natural Fiber Coil-Structured Artificial Muscles

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Moisture-responsive natural fiber coil-structured artificial muscles Xiaohui Yang, Weihong Wang, and Menghe Miao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12144 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on September 2, 2018

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ACS Applied Materials & Interfaces

Moisture-responsive natural fiber coil-structured artificial muscles Xiaohui Yang1,2,3, Weihong Wang3, and Menghe Miao2* 1

College of Materials Science and Engineering, Guizhou Minzu University, Guiyang 550025, China

2

CSIRO Manufacturing, 75 Pigdons Road, Waurn Ponds, Victoria 3216, Australia

3

Key Lab of Bio-based Material Science & Technology of Ministry of Education, Northeast Forestry University, Harbin 150040, China.

ABSTRACT: Miniature linear actuators, also known as artificial muscles, mimick the contractile action of skeletal muscles and have potentials in applications, such as soft robotics, prosthetics, exoskeletons and smart textiles. Natural fibers commonly used in textiles, such as wool, cotton and flax, are highly anisotropic materials in response to moisture stimulus. Here we report that this anisotropic property of the natural fibers can be utilized to provide muscle-like contractile motions when they are constructed into springlike cylindrical coils by twist insertion. The treatment and conversion of these natural fibers into high-performance muscle-like actuators are described. The muscle-like actuators made from natural fibers can provide output strain, stress and work capacity that are orders of magnitude higher than animal skeletal muscles and are higher than many artificial muscles made from synthetic materials. The natural fiber artificial muscles are demonstrated for potential applications in smart textiles to alleviate body discomfort caused by sweating during sports and other physical activities.

KEYWORDS: linear actuators; artificial muscles; natural fibers; anisotropicity; hydroexpansion; smart textiles --------------------------------* Corresponding author. M Miao (Email address: [email protected]). ORCiD 0000-0003-1799-1704.

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1. Introduction Smart textiles refer to textile devices that can detect changes in the surrounding environment and take appropriate actions. The simplest smart textile devices are electronicfree and can be constructed from materials driven by stimulus such as sweating or body temperature change. Haines, et al, reported that spring-like yarn coils formed by twisting a nylon fishing line (a monofilament yarn) can be used as a heat-responsive actuator 1. These yarn coils, referred to as artificial muscles, utilize the large difference between radial and axial expansions of the constituent fibers (anistropicity) in response to heat stimulation to generate a contractile actuation far exceeding the work and power capabilities of human skeletal muscle 1, 2. Since then, a number of artificial muscles based on the twisted yarn coil structure have been developed and have demonstrated potentials for a wide range of applications. As summarized in Table S1 in Supporting information, most of these yarn coil-structured artificial muscles require some kind of enabling systems to supply high temperature heat

1

or volatile solvents 3. Water-driven artificial muscles reported recently

were made from carbon nanotube yarns infiltrated with swelling polymers

4, 5

. The carbon

nanotubes, which are essentially isotropic material, form the skeleton of the yarn and lateral expansion of the yarn is caused by actuating the swelling polymers infiltrated in the yarn. Twist insertion is an essential process for the production of most textile yarns. Under the twist level commonly encountered in textile processes, the yarn axis maintains in a straight line. When the twist in the yarn is increased and thus yarn torque becomes excessively high in relation to the tension applied to the yarn, local buckling (instability) 2


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occurs thereby the straight yarn axis jumps to assume a curved shape, a phenomenon also known as localized writhing 6. This distorted yarn structure is known as a snarl in the textile industry. Depending on the twist introduced to the yarn and the tension applied to the yarn, two types of snarls can be formed. The more common type of snarl, which is formed at relatively low twist and tension, is the side snarl in the form of a two-ply yarn loop standing perpendicular to the original yarn axis (Figure 1a). The other type of yarn snarl takes the shape of a spring-like cylindrical coil aligned with the direction of the original yarn axis, as shown in Figure 1b. The cylindrical snarl is formed at much higher twist and tension than the side snarl. Snarls are normally considered to be faults in textile processing as they block the smooth passage of the yarn in processing machines, although purposely produced snarls can be used as a visual feature for fashion garments 7 and have also been utilized to provide stretch in false-twist texturized thermoplastic filament yarns 8.

Figure 1. Snarls generated in excessively twisted yarns. (a) side-snarl; (b) cylindrical snarl, taking the shape of a spring-like coil; and (c) formation of a spring-like cylindrical coil by inserting extremely high twist into a tensioned yarn.

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The conditions for the onset of side-snarling of twisted elstatic rods, fibres and yarns, known as torsional instability in mechanics of materials or writhing in Knot Theory, have been studied by many researchers6, 9-16. The mechanical conditions to form the cylindrically coiled snarls, which are particularly relevant to the investigation reported here, were studied by Hearle and Yegin

17

and Ghatak and Mahadevan

18

. These studies focused on

predicting the onset of snarling, i.e., conditions triggering a straight yarn to jump to form a snarl due to the presence of excessive twist. After the onset, an indefinitely long cylindrical yarn coil can be formed by continuous insertion of twist to the yarn. In Knot Theory terminology, each turn of the yarn coil represents (1-sinα) unit of a “writhe 2, where α is the rising angle of the coil helix, as shown in Figure 1c. The sum of the twist in the yarn and the writhe in the coil is known as the linking number of the twisted yarn coil. When used as an artificial muscle, the cylindrical yarn coil is torsionally tethered such that the two ends are allowed to slide but not allowed to rotate. This prevents the yarn from losing its twist – or untwisting, but the twist turns in the yarn can be transformed into writhe in its coil, and vice versa. In other words, the linking number of a tethered twisted coil conserves when the coil geometry changes. Haines, et al, used theory of elasticity to explain the length contraction of the twisted yarn coil 1. In summary, when a stimulus is applied to the coil, the yarn diameter expands much more than its length, causing the yarn to untwist and in turn the coil to change its writhe, which can be seen as pulling the adjacent coils closer together, shortening the coil. Many naturally occurring materials have shape memory functions that can be utilized as 4


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actuators. For example, horse hairs can generate a pull due to length change in response to moisture and have been used in hygrometers since 1700s. However, the length contraction generated by such natural fibers is quite small and the response time is too long for use as muscle-like actuators. Here, we report water-responsive linear actuators, or artificial muscles, that can be constructed from treated natural protein and cellulose fibers, such as cotton, wool and flax, which have been used traditionally as safe and comfortable everyday clothing materials. The natural fiber artificial muscles provide output strain, stress and work capacity that are similar to or higher than many artificial muscles made from synthetic materials and have the additional advantage over synthetic artificial muscles for applications in smart textiles to provide next-to-skin comfort and safety.

2. Experimental section Materials Commercial cotton and wool yarns were used in this investigation. The flax yarn was spun from flax sliver supplied by a textile mill in China, using a Caipo SRL ring spinning machine

19

. The tensile strength of the three yarns were tested on a Uster automatic yarn

tensile testing machine. The characteristics of the three yarns are summarized in Table S2.

Preparation of coils (artificial muscle yarns) The spun yarns were folded to form a ply yarn to achieve the desired thickness (linear density). One end of the yarn is attached to a motor-driven twister and the other end tied to 5



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a sliding weight. Twist was inserted to the yarn by operating the twister at a rotating speed of ~600 rpm (Video S1 - Supporting information). During the twisting operation, the coiling yarn was maintained in the horizontal direction. The vertically sliding weight applied a constant tension to the yarn while allowing length contraction as the coil structure was being formed. By adjusting the number of plies for each yarn, the resulting coils from the three types of fibres were made to a similar diameter of about 1.2 mm. The parameters of the twisting operation and the resulting coils are summarized in Table S3.

Treatments Textile auxiliaries Croscolor SDCF, Verolan NBO, Albatex FFC and Triton X-100 were supplied by Crosfield, Rudolf, Ciba-Geigy and Bio-Rad Laboratories, respectively. In surfactant treatment, oven-dry cotton, wool and flax coils were immersed in 1 wt% non-ionic surfactant (Triton X-100, t-Oct-C6H4-(OCH2CH2)xOH, x= 9-10) aqueous solution for 15 min and then dried at 105℃ for 2 hours. In scouring treatment, cotton coil was treated in an Ahiba Lab dyeing machine (Datacolor, USA) run at a rotational speed of 10 rpm. The scouring bath was set at room temperature with Albaflow FFW (0.25 g/l), Croscolor SDCF (4.0 g/l), Irgasol CO new (1.0 g/l) and Caustic Soda (2.5 g/l), heated to 100°C at 3°C/min and held for 30 minutes, cooled to 80°C and then drained. The cotton coil was rinsed with 50°C water for 10 minutes and tap water for 10 minutes, followed by a 50°C rinse with 0.5 g/L acetic acid solution for 15 minutes to neutralize residual alkali. The resulting coil was finally rinsed with tap water for 6


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10 minutes and dried for 2 hours at 105°C.

Wicking test The coil specimens were conditioned (20oC and 65% RH) for 48 h before testing. A 10cm-long coil was held by its two ends on a stainless steel ruler. The wicking experiments were conducted at 20◦C and 65% RH. To enhance visibility of the liquid front advancement, drops of a colorant (Flexobrite blue C5, Degussa) were added to the distilled water. To start the test, one end of the steel ruler was placed vertically in the water bath. The wicking process was recorded using a digital camera.

Tensile testing A coil specimen, slightly tensioned to keep the specimen straight, was fixed on a paper frame with a 2 cm window. The specimen was tested on an Instron 5500R Universal Testing Instrument. The tensile tests were conducted at 20oC and 65% RH. The crosshead speed was set to 60 mm/min. Both dry and wet coils were tested up to 0.05 strain and their respective strain-stress curves were recorded.

Tensile stress and modulus at constant length To measure the contractile force at constant length, a coil specimen on a paper frame was held stationary on the Instron 5500R Universal Testing Instrument (i.e., cross-head speed set to zero). Water was dropped over the coil until it was saturated while the waterinduced contractile force was recorded. To accelerate drying, a hot air gun set to 100oC and positioned 3 cm from the specimen was used to dry the specimen. The measured force was 7



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converted to stress based on the outer diameter of the coil in dry state.

Contraction at constant force A yarn coil specimen was tethered at the two ends by clamps to prevent rotation during actuation. The tensile actuation (percent contraction) was tested under a constant force by hanging a weight at the lower end. In all tests the initial isobaric load was applied to the specimen in wet state. The specimen was then dried for 2 minutes using the hot air gun. This dry-state position was marked as the starting point of the actuation experiment to follow. Such a procedure was adopted to minimize the effect of any previous working history of the specimen. The shortening of the specimen was measured at a specified time after applying water to the specimen (e.g., 30 s or 75 s). The experiment was repeated to required number of dry-wet cycles. A digital camera was used to record the experiment, and the muscle yarn contraction was measured from the recorded images.

3. Results and discussions The three yarns were twisted into cylindrical coils (shown in Figure 2a) using a motordriven twisting device under constant tension applied by hanging a weight restricted from rototation. The as-prepared cylindrical coils were tethered to a sliding weight for actuation testing. When water was applied, the as-prepared coils contracted in length at a very slow speed. The process was captured in video and provided as Video S2 in Supporting

Information . The reason for the slow actuation was attributed to the hydrophobic surfaces of the untreated natural fibers. Wax and pectin on cotton fiber act as a hydrophobic barrier 8


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that restricts the penetration of water into the fiber 20. Similarly, wool fiber is covered by a fatty acid layer that creates a hydrophobic surface, preventing water absorption into the fiber 21. These hydrophobic barriers on the fiber surface slow down the uptake of water and the contraction of the as-prepared natural fiber coils. As exhibited in Figure 2b, the contact angles of water droplets on the surfaces of the these as-prepared natural fiber coils were in a range between 110 o and 125o.

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Figure 2. Water absorption by natural fiber coils. (a) optical microscope images of coils (artificial muscles) prepared from cotton, wool and flax yarns, (b) water drops sitting on natural fiber coils due to hydrophobicity, (c ) wicking test on surfactant-treated natural fiber coils – images and plots of testing results from the treated coils, (d) wicking test on untreated, surfactant-treated and scoured cotton yarn coils – images and plots of testing results from the treated coils.

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3.1 Treatments to improve wicking rate The wettability of these natural fiber coils was dramatically improved by applying a non-ionic surfactant Triton X-100 aqueous treatment. Wicking tests were carried out on pairs of untreated and surfactant-treated coils, as shown in Figure 2c. The wicking test was recorded on video, from which a relationship between wicking height and time was plotted. The water front in the untreated coils (left of each pair in the image in Figure 2c) moved less than 5 mm in 10 min whilst the water front in the surfactant-treated coils moved up by up to 60 mm in the same duration. The relationships between wicking time and distance for the surfactant-treated cotton, wool and flax yarn coils are presented in Figure 2c. The wicking speed in the treated wool yarn coil was the highest, followed by the treated cotton yarn coil. The treated flax yarn coil showed a considerably slower wicking speed than the other two treated yarn coils. Wettability of the natural fiber yarn coils can also be improved by alkaline scouring treatment. The as-prepared cotton yarn coil was scoured in a lab dyeing machine as described in the Method Section. The wicking of the untreated, surfactant-treated and scouring-treated cotton yarn coils was recorded on video (see Video S3a and Video S3b -

Supporting information). As shown in Figure 2d, the untreated cotton yarn coil did not show any visiable wicking, whereas the surfactant-treated and the scouring-treated coils showed 20 mm and 30 mm wicking in 2 min. The initial absorption rate of the scouringtreated cotton yarn coil (the initial slope of the plots) was higher than the surfactant-treated cotton coil. This is probably because the surfactant treatment introduces hydrophilic groups 11



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to the cotton fiber surface but the hydrophobic substances (e.g., wax) remain in the cotton fiber after the treatment, which hinders water infiltration further into the fiber. The scouring treatment, on the other hand, removes the hydrophobic substances from the cotton fiber so that water can come into direct contact with the cellulose molecules. After the treatments, the contractile actuation of natural fiber coils became dramatically faster, as exhibited in Video S4 - Supporting information.

3.2 Actuation of treated natural fiber coils To determine the contractile force of the coils, the cylindrical coil samples, free of tension, were mounted on a tensile testing machine with the two jaws set at fixed positions and the contractile force developed was measured over time while the coils were wetted and dried (Figure 3a). The contractile force was then converted into contractile stress based on the cross-sectional area of the cylindical envelope formed by the external surface of the coil. As shown in Figure 3b, once water contacted the coil, a contractile stress started to develop and reached its maximum and remained there for as long as the coils were kept wet. The scoured cotton yarn coil contracted more quickly and developed a higher maximum stress than the surfactant-treated cotton yarn coil. The rise of the contractile stress was completed in about 1 s for the scoured coil and in about 5 s for the surfactanttreated coil. The corresponding rate of contractile stress increase was 0.10 MPa/s for the scoured cotton coil and 0.04 MPa/s for the surfactant-treated cotton coil. Obviously, the rate

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of contractile stress increase reflected the speed of water movemont in the coils, which can be explained by the previous wicking test results in Figure 2d.

Figure 3. Contractile stress at fixed length without pre-straining for natural fiber yarn coils. (a) actuation on a tensile testing machine by holding a tension-free coil at fixed length, (b) contractile stress and stress rate plots of surfactant-treated and scoured cotton coils, (c) contractile stress curves for surfactant-treated cotton, wool and flax yarn coils throughout wetting, maintaining wet and drying stages. The inset in each plot is a magnified view of the circled part of the main curve. (d) Contractile stress in cyclic wetting-drying experiment.

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Figure 3c shows one actuation cycle (wetting, maintaning wet, and drying) of the surfactant-treated cotton, wool and flax yarn coils. After initial wetting, the coils were kept wet for 30 minutes by adding water drops. When allowed to dry in room temperature air, the coils were able to recover to their original lengths, but the return stroke took a much longer time than the initial contraction because the time taken to dry the yarn coils was much longer than the time taken to wet the coils. In the following experiments, the return stroke was accelerated by drying the wet coils with a hot air gun. The three types of coils were wetted and dried in this way for cyclic actuation testing (the first 8 cycles are shown in Figure 3d) and the maximum contractile stress achieved at consecutive cycles maintained very well given that the manual operation could introduce variations to wetting and drying times and conditions. 3.3 How the natural fiber actuators work The working principle of the natural fiber muscle-like actuators can be explained by anisotropic hydroexpansion of natural fibers, yarn structural mechanics and coil-spring mechanics. Cellulose fibers (cotton, flax) and protein fiber (wool) contain hydroxyl groups (-OH) and amide groups (-NH-) respectively which can form hydrogen bonds with water molecules

22

. For wool, the hierarchical structure of the fiber cortex with the macro- and

micro- fibrils and helical coils provide the mechanism for moisture absorption

23

. Plant

fibers such as cotton and flax are composed of oriented micro-fibrils that swell anisotropically when absorbing water, as illustrated in Figure 4a. When water is applied to natural fibers, the water molecules penetrate into the inter- and intra-fibril spaces of the 14


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fiber, leading to lateral swelling of the fiber. So the hygroscopic expansion of natural fibers happens predominantly as transverse swelling with little expansion or even shrinkage in length

22-24

. We will disregard any length change of the fiber caused by wetting for

simplicity of the following discussion.

Figure 4. Mechanics of natural fiber twisted yarn coil actuators. (a) Hygroexpansion of fibrils within individual natural fibers; (b) fiber helix paths (red and blue curves) in a straight twisted yarn; and (c) force and torque acting on a spring-like yarn coil. The most commonly used geometrical model of twisted yarns was proposed by 15



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Gégauff in 1907 25 and it has a coaxial helix structure as shown in Figure 4b. According to this model, all the fibers follow perfect helices with a common axis which coincides with the yarn axis. The helix angle (θ) of fibers on the yarn surface can be related to the twist (T, turns per unit length) and the yarn radius (r) by tanθ = 2πrT. This means that the fiber helix angle increases as the yarn radius increases if the yarn twist maintains a constant. Obviously all the fiber helix paths are all longer than the corresponding length of the yarn axis. In average 22, the ratio between fiber length and yarn length is 1+ tan2(θ/2). This ratio represents the shortening of the yarn when a twist T is inserted to an initially zero twist yarn, and this ratio is known as twist retraction of the yarn. The twist retraction of a yarn is related only to the twist angle (referring to the angle of surface fibers on a yarn). If we tether the two ends of the yarn in such a way that one end of the yarn can slide without losing its twist (i.e., T is constant) and we swell the yarn (i.e., yarn radius r increases), the twist angle θ will increase and the yarn will further retract in length. This yarn retraction will reduce the length of the coil assuming that the coil diameter D and rising angle α (Figure 4c) are kept constant, resulting in a contraction of the coil. In fact, the yarn also becomes more rigid when it swells. This leads to a rebalancing among the tension, torque and bending moment of the yarn in the coil, resulting in an increase of the coil diameter D and in turn an increase of the length of yarn required for each turn of the coil. Consequently, the number of turns in the coil (i.e., the writhe) will decrease, which reduces the total length of the coil, contributing to the coil contraction. Figure 4c is a free body diagram showing that the lifting force F of the spring-like 16


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yarn coil is balanced by the torque Q developed in the twisted yarn. Clearly,

=

(1)

The torque developed in a twisted yarn depends on the mechanical state of the constituent fibers in tension, in torsion and in bending. Postle, et al 26, showed that the part of yarn torque derived from fiber tensile stresses (Qt) is an extremely large component of the total yarn torque and can amount to more than 90% of the total torque

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. The yarn

torque derived from fiber tension can be expressed as

=

(2)

where Ef is the tensile modulus of the fiber, εy is the tensile strain of the yarn, θ is the surface helix angle of the yarn (twist angle) and r the yarn radius. From equations 1 and 2, we can see that the lifting force of the coil (F) increases with the cubic power of the yarn radius (r) and with the increases of yarn strain (related to tension) and twist angle. As the natural fiber yarn coil is actuated by wetting, both the yarn radius r and the twist angle θ will increase, causing a decrease of the coil length and an increase of the yarn torque (Qt in equation 2), which means that the coil becomes shorter as well as stiffer (from equation 1, F increases with Q) and thus lifts the weight further up, i.e., which manifests a contractile actuation. The modulus Ef of natural fibers can be affected to some degree by moisture absorption and surrounding temperature

22, 28, 29

,

which mitigates part of the overwhelming effect of anisotropic hydroexpansion on yarn torque.

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3.4 Spring stiffness of the twsited yarn coils at dry and wet conditions The change of coil stiffness (tensile modulus) due to water absorption was verified by testing the tensile behaviours of the natural fiber coils at both dry and wet states. The dry samples of natural fiber yarn coils were kept under standard condition (21⁰C and 65%RH) for 24 h and the wet samples were immersed in water for 24 h before testing. The specimens were elongated up to 0.05 strain (5%) and then allowed to return to their orginal length at constant rate of extension. As shown in Figure 5, the stress for each coil followed the higher curve when strain was increased and followed the lower curve when strain was decreased, forming a hysteresis loop in both dry test and wet test. The moduli (spring stiffness of coil) for increasing strain were 110 MPa for the wet flax coil and 10 MPa for dry flax coil; 25 MPa for the wet cotton coil and 0.2 MPa for dry cotton coil; and 11 MPa for the wet wool coils and 0.4 MPa for dry wool coil. So the spring stiffness for the wet coils was 11 – 27 times higher than that of their dry coils. At 5% strain, the wet wool and flax coils showed tensile stress 15 times higher than their corresponding dry coils, whereas about 5 times for the cotton coils. This demonstrates the actuation capacity of the natural fiber hydroexpansion, which obeys a cubic power relationship with the yarn radius according to equation 2.

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Figure 5. Tensile hysteresis of cotton, wool and flax yarn coils in dry (room conditions 65% RH, 21°C) and wet conditions. In Figure 5, all the three types of coils exhibited stress-strain hysteresis loops at both dry and wet states, which is typical for all viscoelastic materials within their elastic recovery limits. The three wet coils showed much higher hystersis than their corresponding dry coils, as shown by the larger areas of their hysteresis loops. The hysteresis loop represents the dissipation of internal energy (heat generated by internal friction) during the tensile cycle and can be associated with the viscoelastic behavior of the constituent fibers

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and the

friction between fibers in the yarn structure 31, both of which are altered by the absorption of water. It is also understandable that due to water evaporation, the disspipation of heat from a wet coil is much quicker than that from a dry coil, resulting in the larger loop area for the wet coil. 3.5 Actuation of pre-strained coils According to equation 2, if the yarn strain (εy ) is increased, for example, by hanging a heavier weight or by applying a pre-strain on a testing machine, the yarn torque Qt will increase, which in turn increases the lifting force of the coil F according to equation 1. 19



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This means that the contractile stress achieved in these coils depends on the pre-strain applied to the coil at the start of the experiment. A series of experiments were conducted on the surfactant-treated natural fiber yarn coils under different pre-strains using the tensile testing machine with the jaws fixed in position. Figure 6a shows the relationship between the pre-strain and the contractile stress measured at 30 s wetting time for the three types of yarn coils. The 30 s reading time was adopted because we found that it took much longer time to fully actuate a coil at high prestrain than at low pre-stain. The time-series plots for the three types of coils at relatively high pre-strain levels in Figure 6b show that the contractile stress climbed further up until they were truncated at 75 s wetting time and then declined when drying started. The plots in Figure 6a show that the contractile stress at 30 s increased dramatically with increasing pre-strain until it plateaued at 200% pre-strain for cotton, 90% pre-strain for wool and 60% pre-strain for flax. Back to Figure 6a, the cotton coil reached the maximum contractile stress of 9 MPa at 200% pre-strain, which was more than 60 times higher than that reached at zero pre-strain (0.14 MPa). The wool yarn coil reached a maximum stress of 2.6 MPa at 90% pre-strain, 14 times higher than that achieved at zero pre-strain. The flax yarn coil reached a maximum stress of 8.5 MPa at 60% pre-strain, eight times of that at zero prestrain.

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Figure 6. Efect of pre-strain on contraction stress of surfactant-treated natural fiber yarn coils. (a) Maximum contractile stress developed as a function of the pre-strain applied to the coils, (b) contractile stress curves of cotton, wool and flax yarn coils at pre-strain corresponding to the highest stress in (a): cotton 200%, wool 90%, and flax 60%). These contractile stress outputs achieved by the cotton, wool and flax yarn coils were remarkable. To put them in context, the stress output generated by typical skeletal muscles is about 0.1 MPa

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, and the stress output generated by the thermally responsive nylon-6

twisted coil artifacial muscles actuated at 190°C was 8.4 MPa 33. So the output contractile stress developed in the cotton yarn coil was higher than the values of the nylon-6 coils, 90 times the value of the skeletal muscles, as well as more than 20 times higher than that reported for CNT yarn filled with paraffin wax (~0.4 MPa) thermally-actuated by a 0.5 V pulsed power source

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and six time higher than that reported for hierarchically arranged

helical CNT fibers (∼1.5 MPa) 35. Another widely used method for assessing the performance of coiled artificial muscles 21



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is measuring the stroke (contraction distance) of the coil under a given tension by hanging a weight at the lower end of the coil, as shown in Figure 7a and Video S5 in Supporting Information. By varying the mass of the hung weight and measuring the corresponding maximum contraction, a relationship between the hung weight (i.e., tension exerted on the coil) and the contraction stroke (travel distance of the weight as a percentage of coil length) was established. Figure 7b shows such relationships for the surfactant-treated cotton, wool and flax yarn coils. The cotton yarn coil was capable of lifting a 200 g weight (~200 cN tension), which was more than 14,000 times the weight of the coil. Interestingly, the contraction stroke was relatively low at a low tension (7.9% at 2.8 cN) but it was more than doubled (16.6%) as the mass of the hung weight was increased to 20 g (~20 cN). This was because under a low tension, there was small or even no space between neighboring turns of the coil, and thus little or no space for the coil to contract before neighboring turns contacting each other (“lock-up” position of the coil). Hanging a heavier weight could open a larger gap between neighboring turns so that the coil could contract to a greater extent. When even heavier weight was used, the contraction stroke reached a peak. At this point, coil lock-up was no longer a limit because the energy released by wetting the coil was not sufficient to lift the weight up to the lock-up position. Further increasing the weight caused a decrease of contraction stroke although the work capacity (output energy) continued to increase. When the weight reached 200 g (tension 200 cN), the stroke decreased to about 7%, similar to the value at 2.8 g weight. The output energy or work capacity generated by the contraction was calculated and normalized to the mass of the coil, also shown in Figure 22


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7b. As the mass of the hung weight on the cotton coil was increased from 2.8 g to 200 g (tension from about 2.8 cN to 200 cN), the work capacity increased from 3 J/kg to 200 J/kg. This is 25 times the work capacity of typical skeletal muscles (~8 J/Kg) 32 and 7.5 times of that of the hierarchically arranged CNT (HHF) actuator (26.7 J/Kg) 35.

Figure 7. Efect of tension on percent contraction of surfactant-treated natural fiber yarn coils. (a) photo showing contraction of wool coil before and after wetting, (b) contraction (%) and work capacity (J/kg) versus tension exerted by hung weight. In comparison, the surfactant-treated wool yarn coils generated much larger contraction stroke (up to 38%) than its plant fiber counterparts (cotton and flax), and this was almost twice as much as contraction stroke of typical skeletal muscles (20%) about three times that of the HHF actuator (10-15%)

32

and

35

. The maximum work capacity of

194 J/kg achieved at 60 g hung weight was similar to that of the cotton yarn coil achieved 23



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at 200 g hung weight. On the other hand, the surfactant-treated flax yarn coil showed a decrease of contraction stroke from 16.6% to 11.4% as the mass of the hung weight increased from 15 to 60 g, while the work capacity of the coil increased from 16 to 44 J/kg, which is about 75% lower than the value for the cotton or wool coil. 3.6 Applications Several applications for moisture-actuating coiled artificial muscles made from synthetic materials have been suggested, such as, humidity switches 36 and smart windows that respond to weather conditions

4, 5

. These are also viable applications for the natural

fiber coil-structured artificial muscles. In addition, the artificial muscles made from wool, cotton and flax fibers can be safely used in smart textiles because the constituent fibers have been traditionally used as safe and comfortable textiles worn next to the skin. These natural fiber yarn coils could be conveniently integrated into textile structures to form smart garments that respond to body sweating (perspiration) and weather conditions. As an example, Figure 8 illustrates a design to release body heat generated during physical activities. The matrix of flaps strategically constructed in the garment are operated by the natural fiber yarn coils stitched on the flaps . These flaps are normally closed. When the coils and the fabric become wet due to perspiration, the coils contract and pull the flaps to curl, opening gaps around the edges of the flap to let out hot moist air from the body. A video demonstration of this working principle is presented as Video S6 and Video 7 in Supporting Information.

24


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Figure 8. An examples of integrating natural fiber coils yarns into smart textiles: ventilation flaps that close when dry and open when wet.

4. Conclusions Natural fibers commonly used in textiles, such as wool, cotton and flax, expand laterally much more than longitudinally in response to moisture stimulus. This anisotropic property can be utilized to produce muscle-like contractile motion using a twisted coil structure. In this paper, we reported the treatment and conversion of these natural fibers into high-performance muscle-like actuators. The natural fiber artificial muscles can provide output strain, stress and work capacity that are similar to artificial muscles made from synthetic materials and are orders of magnitude higher than animal skeletal muscles. We also demonstrated potential application of the natural fibre artificial muscles in smart textiles to alleviate discomfort caused by sweating during sports and other physical activities.

25



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Supporting information Supporting Information associated with this article (file list below) can be found in the online version or from the authors: Video filename ID VS 1 VS 2 VS 3a VS 3b VS 4 VS 5 VS 6 VS 7

File Title coil preparation actuation of untreated cotton coil wicking test of treated and untreated cotton coils wicking of treated cotton coil yarn under optical microscope actuation of surfactant-treated cotton coil yarn actuation of surfactant-treated wool coil with a 2.8 g hung weight fabric wrinkling – cotton coil ventilation flap opening – flax coil

Acknowledgment We

acknowledge

the

financial

support

from

China

Scholarship

Council

(201606600008) that enabled X Yang to carry out this work at CSIRO in Australia.

Author information Corresponding author, M Miao, Email address: [email protected]. ORCiD 00000003-1799-1704.

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Abstract Graphic

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Moisture-Responsive Natural Fiber Coil-Structured Artificial Muscles

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