Twist and shout

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Editor's Introduction

Artificial muscles from fishing line and sewing thread

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Artificial muscles have many potential applications, such as muscle transplantation and use in humanoid robots, but are currently expensive and inefficient. Haines et al. found a way to decrease the cost of artificial muscle synthesis by designing them out of common, inexpensive materials, such as sewing thread and fishing line. The authors subjected these fibers to different tests that examine their strength, durability, and work capability. They found that coiling fibers through twist insertion allows for strong, durable fibers that can produce large amounts of work due to torque generated from untwisting. Because these fibers have the ability to both contract and expand when external energy was applied to them, they have the potential to function as normal muscles found in the human body.

Paper Details

Original title
Artificial muscles from fishing line and sewing thread
Original publication date
Reference
Vol. 343 no. 6173 pp. 868-872
Issue name
Science
DOI
10.1126/science.1246906

Abstract

The high cost of powerful, large-stroke, high-stress artificial muscles has combined with performance limitations such as low cycle life, hysteresis, and low efficiency to restrict applications. We demonstrated that inexpensive high-strength polymer fibers used for fishing line and sewing thread can be easily transformed by twist insertion to provide fast, scalable, nonhysteretic, long-life tensile and torsional muscles. Extreme twisting produces coiled muscles that can contract by 49%, lift loads over 100 times heavier than can human muscle of the same length and weight, and generate 5.3 kilowatts of mechanical work per kilogram of muscle weight, similar to that produced by a jet engine. Woven textiles that change porosity in response to temperature and actuating window shutters that could help conserve energy were also demonstrated. Large-stroke tensile actuation was theoretically and experimentally shown to result from torsional actuation.

Report

Artificial muscle fibers are needed for diverse applications, ranging from humanoid robots, prosthetic limbs, and exoskeletons to comfort-adjusting clothing and miniature actuators for microfluidic “laboratories on a chip.” However, performance, scalability, and cost problems have restricted their deployment. Electrothermally driven shape-memory metal wires can contract fast and deliver large strokes under heavy loads, but are expensive and hysteretic, which makes them difficult to control (12). Thermally powered shape-memory polymers have low work capacity unless they are fiber-reinforced (34), and giant-work-capacity polymer/carbon nanotube (CNT) composite fibers must be redrawn between cycles (5). High-performance hybrid CNT muscles (6), in which a guest (such as paraffin wax) is infiltrated into a twist-spun carbon nanotube yarn, are expensive because of the cost of CNT yarn. Electrochemically driven fibers of organic conducting polymers can provide large strokes but have limited cyclability and cycle rate and require an electrolyte, counter-electrode, and containment system, which adds to system weight and cost (79). Polymeric electric field–driven electrostrictive rubbers and relaxor ferroelectrics (1012) are attractive because of their large strokes and high efficiencies but would be difficult to deploy as muscle-like fibers because of the high required electric fields.

The present goal is to convert inexpensive (~$5/kg) high-strength polymer fibers into artificial muscles that match or exceed the performance of mammalian skeletal muscle to deliver millions of reversible contractions and over 20% tensile stroke, while rapidly lifting heavy loads. These muscles should provide hysteresis-free actuation to enable convenient control, be scalable in force-lifting capability without decreasing stroke or gravimetric work capabilities, and be weavable into textiles that actuate to accomplish amplified mechanical work or change textile porosity.

We started from low-cost high-strength fibers, most often those used as fishing line or sewing thread (table S1). Commercially produced polyethylene and nylon fibers are important muscle precursors, because they combine reversible–fiber-direction thermal contraction, large volumetric thermal expansion, and large anisotropy in thermally induced dimension changes to provide enhanced muscle stroke.

These precursor fibers are composed of flexible polymer chains that are highly oriented in the fiber direction. Although crystalline regions of highly drawn polymers, such as polyethylene and nylon, can have small negative thermal expansion coefficients (13), fiber-direction–aligned polymer chains in neighboring noncrystalline regions are less conformationally constrained, so they can provide large reversible contractions as they access conformational entropy when heated (1415) (fig. S1). The resulting thermal contraction of nylon 6,6 fibers (Fig. 1A) can be as large as 4% (Fig. 2A and fig. S2A), which is similar to that of commercial NiTi shape-memory wires.

140221-muscle-f1.jpg

Fig. 1. Muscle and precursor structures using nylon 6,6 monofilament sewing thread. Optical images of (A) a nontwisted 300-μm-diameter fiber; (B) the fiber of (A) after coiling by twist insertion; (C) a two-ply muscle formed from the coil in (B); (D) a braid formed from 32 two-ply, coiled, 102-μm-diameter fibers produced as in (C); (E) a 1.55-mm-diameter coil formed by inserting twist in the fiber of (A), coiling it around a mandrel, and then thermally annealing the structure; and (F) helically wrapping the fiber of (A) with a forest-drawn CNT sheet and scanning electron microscope images of a CNT-wrapped, 76-μm-diameter nylon 6,6 monofilament (G) before and (H) after coiling by twist insertion.

Question

How are common original materials, such as sewing thread, transformed into artificial muscle?

Panel A and B

Sewing thread can be twisted and coiled in many different ways in order to form artificial muscle. Panel  A and B show sewing thread before (A) and after (B) any twisting is performed.

To see this process in action, go here.

Panel C-E

After initial twist insertion is applied, the coiled fibers can be further manipulated to enhance their structure, durability, and strength.

Two twist insertion fibers (like the ones in Panel B)  can be wrapped around each other (C) and additionally braided (D). You can also wrap twist insertion fibers around a flexible metal bar, known as a mandrel, which provides an even larger amount of contraction (E).

Panel F-H

Artificial muscles can also be synthesized so that they can be externally powered by different sources of energy. For example, sewing thread can be wrapped in CNT sheets, which act as conductors (F). Panels G and H show a magnified scanning electron microscope (SEM) image of the CNT wrapped fibers before and after twisting.

Next Step

The researchers then subjected all of these different coiled fibers to various tests to measure their ability to move and withstand stress.

140221-muscle-f2.jpg

Fig. 2. Thermomechanical analysis of various muscles. (A) Comparison of the negative thermal expansion of braided polyethylene, nylon 6 monofilament, nylon 6,6 monofilament, and silver-coated nylon 6,6 multifilament fibers before twisting (inset) and after coiling by twist insertion. (B) Tensile stroke and nominal modulus versus temperature for a coiled, 300-μm-diameter nylon 6,6 monofilament muscle under 7.5 MPa static and 0.5 MPa dynamic load. During contraction, neighboring coils come into complete contact at ~130°C, which dramatically increases nominal elastic modulus and causes the thermal expansion coefficient to become positive. Optical micrographs (top) are shown of the coils before and after contact. (C) Tensile stroke versus load, as a percent of the loaded muscle length, for a 127-μm-diameter nylon 6,6 monofilament fiber that was coiled by twist insertion under loads of 10, 16, and 35 MPa, which resulted in spring indices of 1.7, 1.4, and 1.1, respectively. Optical images of the coils are inset. (D) The results of (C) when normalized to the initial nonloaded muscle length, indicating that the absolute displacement during actuation remains nearly constant at loads above those at which coils contact.

Question

Which fiber makes the best artificial muscle?

Panel A

First, the researchers needed to test if twist insertion made a difference in the functionality of the fibers. To do this, they tested fibers before and after twist insertion.

First, the researchers measured how much each fiber could contract with temperature. The best artificial muscle is the material that exhibits the greatest amount of contraction strain before melting. The smaller, inset graph shows how much braided fibers can contract at different temperatures BEFORE twist insertion. Each colored line represents a different fiber material (black: polyethylene; green: nylon 6; red: nylon 6,6; blue: Nylon 6,6 with silver coating).

They found that single nylon 6,6 monofilaments and nylon 6,6 yarns with a silver coating can contract the most as the temperature increases. Conversely, polyethylene behaved the worst, and melts at only around 135°C to 140°C. Nylon 6,6 with and without silver coating exhibited the greatest actuation capability at high temperatures.

The researchers then repeated the experiment, except they inserted enough twist into each of the fibers to cause them to fully coil. The larger graph shows how much braided fibers can contract AFTER twist insertion. Again, the muscle that will exhibit the greatest amount of movement will be one that has the high percentage on the y-axis. By tracking these values on the graph, we can see that nylon 6,6 exhibited the greatest amount of movement. We can also see that each of the fibers was able to contract by a large amount after twisting. This is visible by comparing the amount of strain on the y-axis of the smaller graph to the amount of strain on the y-axis of the larger graph.

Panel B

Why are fibers that have been coiled by twist insertion able to move better than nontwist inserted fibers?

This figure demonstrates what happens when a coiled fiber is heated and cooled. The picture at the top illustrates that at cool temperatures, the fiber is relaxed, and when the fiber is heated, it contracts and gets very tight.

The graph tells us that when the temperature is low, there is no movement. However, as the temperature increases, we can see the tensile actuation increase as the line slopes down. It continues to move until it reaches about 130°C, where it stays constant around –18 or –19% tensile actuation. This is because once the fiber reaches 130°C, the coils are completely touching and they can’t contract anymore. However, right before the fiber reaches 130°C, the nominal modulus increases drastically. This tells us that once the coils are touching, they increase in stiffness and become harder to pull apart.

Panel C

Because the researchers found that twist insertion allows for strong, high-stroke artificial muscles, the next question was how much twist should be inserted into the fibers.

To test this, the researchers twisted and coiled the fibers using various loads. The load corresponds to how tight the fibers are coiled. In order to coil a fiber very tightly, a bigger load is needed. To measure how tight the coils are, the researchers used the spring index, which is a measure of the average diameter of the coil divided by the diameter of the fiber it is made from. So as the coil gets tighter, the spring index decreases.

The researchers tested fibers with spring indices of 1.7 (blue line), 1.4 (red line), and 1.1 (black line). The blue line, or the fiber with the highest spring index, exhibited the greatest actuation. The picture illustrates that this is because the coils will have more room to contract. The fiber on the right on the other hand is coiled so tightly that there is little room left to move and contract.

The researchers showed that coiled fibers, which cannot contract as much, do have some advantage. For example, the most tightly coiled fiber (C = 1.1) was able to handle the greatest amount of stress. The fiber that exhibited the greatest amount of movement handled the least amount of stress, breaking most easily out of the three. This is important information because it allows us to take advantage of certain properties when designing artificial muscles. For example, some artificial muscles might need to exhibit a wide range of movement, but will not be placed under high stress, whereas others might be placed under high stress but will not need to exhibit a wide range of movement. To make the perfect artificial muscle to fit your needs, you can simply vary the load during coiling!

Panel D

To test how much twist should be inserted into the fibers in Panel C, the researchers placed a weight on each of the fibers and measured their movement. In order to ensure that the added weight did not have an effect on the length of the coils, the researchers normalized their results shown in Panel C to the initial coil length before any weight was added. They found that the absolute expansion of the fibers was not dependent on the amount of weight loaded onto the fiber. You can see this by examining and comparing the values in Panel C with those in Panel D.

As with CNT yarn muscles (616), twist is inserted into these polymer fibers to make them chiral, which enables them to function as torsional muscles. Most importantly, we greatly amplified tensile stroke by inserting such a large amount of twist that some twist converted to fiber coiling (movie S1), called writhe (1718). By completely coiling the fibers (Fig. 1B), tensile contractions (Fig. 2A) exceeding the maximum in vivo stroke of human skeletal muscles (~20%) (19) were obtained. This coiling is more compact than that used to amplify the stroke of shape-memory metal wires, thereby providing contraction against higher applied stress (19 MPa for nylon) than reported for NiTi coils (~1.6 MPa) (1), in which stress is obtained by normalization to the nonactuated coil’s cross-sectional area. The spring index (C), the ratio of mean coil diameter to the fiber diameter, for such polymer muscles will typically be less than 1.7, whereas for NiTi coils this ratio exceeds 3.0.

The weight applied during coiling is important and is adjustable over a narrow range for a given fiber: Too little weight and the fiber snarls during twist insertion; too much weight and the fiber breaks. For example, the load during coiling can be varied between 10 and 35 MPa for a 127-μm-diameter nylon 6,6 sewing thread, yielding coils with spring indices between 1.7 and 1.1, respectively. Immediately after coiling, adjacent coils are in contact, limiting contraction during actuation, and must be separated by increasing load or reducing twist.

Coils formed by twist insertion maintain some twist liveliness, meaning that they can untwist, especially when under load. This problem can be avoided by preventing end rotation during actuation, by thermal annealing to set the structure, or by forming torque-balanced structures. Figure 1C depicts a coiled polymer muscle that has been torque-balanced by plying (in the Z direction) two S-twisted fibers. Thereby stabilized, the plied, highly coiled muscles can be woven into textiles or braids (Fig. 1D).

Coiled muscles can also be made by wrapping highly twisted fibers around a mandrel and then stabilizing the coils by thermal annealing (Fig. 1E). This process enables the formation of larger-diameter coils than by direct, unconfined (i.e., mandrel-free) twist insertion. Although such structures have reduced load capacity, they can contract more before adjacent coils contact, thereby achieving larger stroke. The relative directions and amounts of twist in the fibers and the coils can be varied using this method. When the chirality of fiber twist matches the coil’s chirality, the muscle contracts during heating. However, when these chiralities are opposite, the coiled polymer muscle expands during heating (movie S2). We hereafter refer to such coils as homochiral and heterochiral, respectively.

Thermomechanical analysis (TMA) results for fiber thermal expansion before and after coiling are shown in Fig. 2A for four polymer fibers (Table 1 and table S1). Unless otherwise indicated, tensile stress and modulus are calculated as nominal values by normalizing applied force to the diameter of the initial nontwisted fiber, because coil and fiber diameter are difficult to measure accurately during isotonic (constant applied force) measurements, where coil diameter varies during large-stroke actuation. Gravimetric capabilities were used to provide performance comparisons between natural and artificial muscles.

table_1_2.png

Table. 1. Summary of the polymer fibers found to be most useful for actuation as coiled tensile muscles, and the parameters used to fully coil them. The tensile actuation results are given in Fig. 2A.

The reversible thermal contraction of nylon 6,6 monofilament between 20° and 240°C increased from 4 to 34% as a result of coiling (Fig. 2A and fig. S2). Polyethylene’s lower melting temperature limited contraction to less than 0.3% for the noncoiled fiber and 16% for the coiled muscle between 20° and 130°C. However, the higher modulus and strength of coiled polyethylene fibers (a nearly 10 times higher nominal modulus than for coiled nylon, fig. S8) are especially useful for muscles that lift heavy loads and provide increased energy efficiency.

When adjacent coils contact, due to insufficient applied load or excessive twist, the muscle-direction thermal expansion becomes positive, as in Fig. 2B. Under low tensile load (7.5 MPa), upon coil contact at ~130°C, the nylon 6,6 muscle expands at a rate comparable to the fiber’s radial thermal expansion. After inter-coil contact, the coiled structure stiffens with increasing temperature, producing a 24-fold increase in nominal tensile modulus (Fig. 2B). Such large temperature-controlled changes in compliance may be useful for humanoid robots, in which actions such as catching a ball require both tensile actuation and tunable stiffness.

The tensile stroke and load-carrying capabilities of coiled muscles can be varied by adjusting the coil spring index, which is inversely related to spring stiffness (1). Figure 2C plots the load dependence of tensile stroke for coils having spring indices of 1.1, 1.4, and 1.7, produced by coiling under stresses of 10, 16, and 35 MPa, respectively. For each muscle, maximum stroke was realized for the lowest applied load (called the optimal load) that prevented inter-coil contact over the temperature range used (20° to 120°C). For the largest diameter coil (C = 1.7), this maximum stroke (21%) occurred for an optimal load of 22 MPa. When the polymer muscle was tightly coiled (C = 1.1), the maximum stroke decreased to 9.3%, but the optimal load increased to 50 MPa. Large-diameter mandrel-formed coils can yield even larger strokes, such as the 49% contraction provided at 1 MPa for a nylon 6 fiber having = 5.5 (movie S2).

When coils are noncontacting, absolute stroke and stroke normalized to the nonloaded muscle length do not substantially depend on applied load (Fig. 2D), even though stroke normalized to loaded initial muscle length decreases with increasing load (Fig. 2C) because of muscle lengthening. Hence, the work done during contraction increases up to loads where the muscle breaks. The maximum specific work during contraction was 2.48 kJ/kg for the C = 1.1 nylon 6,6 muscle of Fig. 2C (1), which is 64 times that for natural muscle (19). The average mechanical output power during contraction (27.1 kW/kg) was 84 times the peak output of mammalian skeletal muscles (0.323 kW/kg) (120). However, although natural muscles have a typical energy conversion efficiency of 20%, the maximum energy conversion efficiency during contraction was 1.08 and 1.32% for the coiled nylon and polyethylene fibers, respectively (1). These polymer muscle efficiencies are similar to those of commercial shape-memory metals, which can reach 1 or 2% (21).

Shape-memory NiTi muscles suffer from over 20°C hysteresis in stroke, complicating actuator control (222). Scanning at a slow 2°C/min rate, to reduce artificial hysteresis due to temperature measurement errors, reveals that coiled nylon 6,6 actuators exhibit little or no inherent hysteresis (less than 1.2°C, Fig. 3A). This substantial absence of hysteresis, combined with the far more linear temperature dependence than for the commercially important NiTi shape-memory wires, makes these coiled polymer fiber muscles well suited for robotics and prosthetics, where a continuous range of control is desired. Although very recent work has provided shape-memory metal wires exhibiting down to 2°C hysteresis, these muscles comprise ~56.5 weight % gold (23).

140221-muscle-f3.jpg

Fig. 3. Performance of torsional and tensile artificial muscles. (A) Comparison of hysteresis for a 152-μm-diameter NiTi wire muscle and a coiled, 127-μm-diameter nylon 6,6 monofilament muscle, measured using a 2°C/min scan rate. Less than 1.2°C of hysteresis is observed for the nylon muscle versus the 27°C hysteresis for the NiTi muscle. (B) Tensile actuation versus cycle for a coiled, 76-μm-diameter, nylon 6,6 monofilament wrapped in a CNT sheet and driven electrothermally at 1 Hz under a 22-MPa load (each point averages 1000 cycles). The inset provides creep as a function of cycle. (C) The optically measured fiber bias angle induced by an applied torque and the torsional stroke and work during thermal actuation (between 20° and 160°C) as a function of this applied torque for a noncoiled torsional muscle made from 860-μm-diameter nylon 6 fishing line (1). The inset photograph was used to optically determine the fiber bias angle by measuring the displacement of a black line from its initial orientation parallel to the fiber axis.

Question

How do the new artificial muscles compare with previously synthesized, more expensive artificial muscles? 

Panel A

Some current, expensive artificial muscles exhibit hysteresis, a term that describes how actuation depends on the history of applied temperature. These muscles are not optimal, because it is hard to predict their length. Therefore, the researchers want to determine whether the inexpensive nylon 6,6 coiled fibers would exhibit hysteresis.

They compared the nylon 6,6 fibers to a current fiber used for artificial muscle that exhibits a high amount of hysteresis (a shape-memory nickel-titanium alloy, aka NiTi). The researchers then applied weight to both of these wires, while increasing the temperature. As the temperature increased, both of the materials contracted, as shown by the negatively sloping line in the graph. If a material does not exhibit hysteresis, you will see a line that overlaps during heating and cooling, which is not the case here. The hysteresis exhibited by NiTi wires is great. There is a wide gap in the graph, showing that NiTi wires do not behave the same at a given temperature. Instead, their movement is dependent on previously experienced temperatures. Although there is also a gap for the nylon 6,6, the gap is much smaller, which tells us that the nylon 6,6 shows very little hysteresis. This means that the strain of the nylon 6,6 is more predictable at a given temperature. Furthermore, it tells us that the length of coiled nylon 6,6 muscles will not be as sensitive to how temperature is applied as NiTi wires.   

Panel B 

Because nylon 6,6 exhibited very little hysteresis, the researchers wanted to see how it would behave when subjected to multiple actuation cycles.

First, the researchers needed to consistently power the nylon 6,6 by wrapping a sheet of carbon nanotubes (CNTs) around the fiber. This enabled a power source to drive the movement of the fibers. The larger graph shows a somewhat straight line at about –10 or –11% of tensile actuation and stays relatively straight after 1.2 million cycles. This means the fiber was able to contract without degrading for 1.2 million cycles. This is a large amount of cycles, showing that this fiber is stable when powered electrothermally. These are important data for muscle development. Artifical muscles have to last a person their whole life, so you need muscles that will be able to withstand millions and millions of contractions and expansions without failing. But the smaller, inset graph shows that there was also some amount of creep, which measures irreversible lengthening of the muscle. As the cycle numbers increased, the percentage of creep also increased. The creep increase, however, was relatively low (less than 2%).  

Panel C 

After determining that their fibers were less hysteric and could handle multiple cycles of movement, the researchers asked how the fibers were able to actuate so much when coiled.

The researchers looked at the torsional actuation of the muscles. They examined the fiber bias angle of the fiber in relation to twisting and the specific energy a muscle could generate by twisting to lift a weight. The bias angle is a measure of how twisted a fiber is. (Greater angles mean a greater amount of twist.) A weight was applied to force the fiber to twist, as shown on the x-axis. At greater torques, the fiber twisted to a greater bias angle. When this twisted fiber was heated, it generated an untwisting torque which lifted the weight. Importantly, at greater torques the amount of rotation did not change significantly when heated, but each time it lifted a greater weight. Therefore, the greatest work from these torsional muscles occurred at the highest applied torque.

Although such muscles can be driven chemically, photonically, hydrothermally, or by ambient temperature changes (1), electrothermally driven muscles must contain an electrical heating element. This can be provided by helically wrapping the coiled muscle or precursor fiber with a CNT sheet (24) (Fig. 1F-1H), using commercial metal-coated sewing thread, or placing a conductor on the inside or outside of the coiled muscle fiber (such as wires woven into an actuating textile or placed interior to actuating fiber braids, movies S5 and S6).

A coiled nylon 6,6 muscle delivered over 1 million cycles during periodic actuation at 1 Hz (Fig. 3B), raising and lowering a 10-g weight producing 22 MPa of nominal stress. This actuation was powered by applying a 30 V/cm square-wave potential (normalized to coil length) at a 20% duty cycle. Although the coiled fiber did experience creep (inset of Fig. 3B), this creep was below 2% over the 1.2 million investigated cycles, stroke amplitude was negligibly affected, and the creep rate decreased with cycling.

Similar to other thermally or electrochemically driven artificial muscles, muscle cycle rate decreases with increasing fiber diameter. This response time is unimportant when harvesting energy from slowly varying ambient temperature changes or for clothing textiles that change porosity to provide wearer comfort, but it is critically important when maximizing average output power. Passive cooling offers an economical solution to increase cycle rate. For instance, when immersed in water, a two-ply, coiled, silver-plated, 180-μm-diameter nylon fiber can be electrothermally actuated at 5 Hz to produce ~10% stroke while lifting a 22-MPa load (movie S4). Similarly, in helium, a coiled, 26-μm-diameter CNT-wrapped, nylon 6,6 monofilament was capable of actuation at over 7.5 Hz (1).

Fast, high-force actuation can be driven hydrothermally. A coiled polymer muscle made from 860-μm-diameter nylon 6 fishing line (Fig. 4, A and B) was driven at 1 Hz by switching between cold (~25°C) and hot (95°C) water (movie S3), achieving 12% reversible actuation under a 0.5-kg load (8.4 MPa). Even though nylon muscles absorb water (25), 1500 reversible actuation cycles were observed.

140221-muscle-f4.jpg

Fig. 4. Mechanism and applications for coiled polymer muscles. Hydrothermal actuation of a coiled 860-μm-diameter nylon 6 fishing line lifting a 500-g load by 12% when switched at 0.2 Hz between (A) ~25°C water (dyed blue) and (B) 95°C water (dyed red). (C) Calculated temperature dependence of tensile actuation (dashed lines) compared to experimental results (using an applied stress of 2.2 MPa, solid symbols, and 3.1 MPa, open symbols, respectively) for twisted 450-μm-diameter, nylon 6 monofilament fibers that are mandrel-wrapped to the indicated initial coil bias angles (1). Contracting and expanding coils were homochiral and heterochiral, respectively. From Eq. 3, fiber untwist during heating was calculated for the coiled fiber with a 17° bias angle to provide the data in the inset, which was then used to predict tensile actuation for the other coiled fibers. (D and E) Schematic illustration of the mechanism by which torsional fiber actuation drives large-stroke tensile actuation for (D) heterochiral and (E) homochiral coiled fibers. (F) Measured tensile actuation versus fiber bias angle for coiled, 860-μm-diameter nylon 6 muscles actuated between 25° and 95°C. These results show, for highly twisted fibers, that the homochiral muscles thermally contract when coils are noncontacting, and the heterochiral muscles expand. (G) An actuating textile woven from conventional polyester, cotton, and silver-plated nylon (to drive electrothermal actuation) yarn in the weft direction and coiled nylon monofilament muscle fibers in the warp direction.

Question

What are the practical implications of these artificial muscles?

Panels A and B 

Both panels show how nylon 6 fishing line can be powered to generate movement. This is an exciting finding because it shows that these fibers could have many possible applications beyond electrically powered actuators. The experimental setup in both pictures shows coiled nylon 6 fishing line attached to a weight in cold and hot water. The fibers contracted when they were subjected to hot water. This decreased the length of the fibers by 12%, causing the weight to lift.

To see this process in action, go here.

Panels C 

Because temperature was shown to drive the movement of the fibers, the researchers tested how temperature affects the movement of the coils.

They heated and cooled four different coils with different bias angles and recorded their movement in relation to temperature. As the temperature increased, the movement of the coils also increased. However, the bias angle of the coils dictated their movement at higher temperatures. To understand why, a second experiment was performed to see how temperature affected fiber untwist (inset panel). Fiber untwist increased with temperature. This demonstrates that the untwisting of the fiber drives its movement. To validate this, the researchers calculated the theoretical movement (dashed lines) and plotted it against the experimental values and found that they matched. This confirmed that movement of the fibers was due to fiber untwist during heating. It also showed that movement of the coils is impacted by how tightly they are twisted. 

Panels D and E 

The diagrams help explain how the fibers move when twisted in a certain chirality. If they are twisted in the opposite chiral direction (heterochiral), they will expand when energy is applied. This is due to the fibers’ bias angle, which causes untwisting of the fibers, generating a large torque that produces the movement. Conversely, if nylon fibers are twisted in the same chiral direction (homochiral), they will contract and come closer together. This is also because the fiber generates an untwisting torque that pulls the coils together, which causes it to contract and tighten when heat is applied. 

Panel F 

This panel demonstrates the properties of the fibers’ chirality explained in Panel D and E. The graph shows that homochiral fibers exhibit zero tensile actuation under compressive loads. This is because the fiber is pushed together, which causes the coils of the fiber to touch one another so they cannot move. However, they can exhibit tensile actuation under tensile load because they are able to expand due to the pulling motion from the load. On the other hand, heterochiral fibers exhibit tensile actuation under both tensile and compressive loads.

To see this process in action, go here

Panel G 

This panel shows how the artificial nylon muscles can be incorporated into other items, such as temperature sensitive clothing that will provide comfort to the wearer in extreme temperatures. The yellow and white yarnlike fibers are typical polyester and cotton fibers that are normally used in clothing. We can also see silver plated nylon fibers woven into the cotton and polyester fibers. Because silver is a conductive material, it can make the textile conductive. This is important because we know that heating a heterochiral coiled fiber will cause it to expand. In times of extreme heat, the fibers have the ability to expand or contract, making the material more porous or breathable. This will cool the wearer off by creating holes or pores into the textile, which allows the heat to dissipate. Therefore, if the wearer gets too hot, heat has a way to escape, cooling the wearer down. Conversely, if the weather is too cold, the fibers contract, pulling the textiles closer together. This will provide insulation for the wearer because heat will be retained within the textiles, which in turn warms the wearer up.

To see this process in action, go here.

Polyethylene fiber muscles, which do not absorb water, can similarly be driven hydrothermally if a surfactant (dishwashing soap) is added to facilitate polymer wetting. A coiled polyethylene muscle was prepared by twisting an 800-μm-diameter bundle of four polyethylene fishing lines. Containing the muscle in a flexible silicone tube to allow fast water flow provided 4.5% contraction at 2 Hz while lifting a 7.2 kg (140 MPa) load (movie S3). The mechanical work output during contraction (2.63 kJ/kg), normalized to the total cycle time, was 5.26 kW/kg (7.1 horsepower/kg), which is over 100 times that of a human biceps muscle (26) and roughly the same as for a modern jet engine (27).

Polymer fibers that are twisted, but not coiled, are capable of producing surprisingly large amounts of mechanical work as torsional actuators. Constant-torque torsional actuation measurements were performed (using the apparatus of fig. S5) on an 860-μm-diameter, 5.5-cm-long, twisted nylon 6 fiber. When heated from 20° to 160°C, this muscle lifted a 1-kg load by 7 mm by rotating a 2.8-mm-diameter axle by 286°, thereby providing 2.1 kJ/kg of mechanical work (Fig. 3D). This work during rotation is similar to the 2.48 kJ/kg provided during tensile actuation of a coiled nylon 6,6 muscle.

Why do two-end–tethered, fully coiled homochiral polymer fibers thermally contract during heating, independent of whether the nontwisted fibers have a positive or negative axial thermal expansion coefficient (fig. S2), and why is such thermal contraction so large? The answers are found in the ability of twisted fibers to generate giant torque by reversibly untwisting when heated, as shown in Fig. 3C.

Until just before coiling, initially parallel polymer chains are twisted into helices that have a bias angle relative to the fiber direction of approximately αf=tan1(2πrT) where r is the radial distance from the fiber center and T is the amount of twist inserted per initial fiber length.

After twisting, both length contraction of these helically configured polymer chains and fiber diameter expansion will cause fiber untwist (1). To enable physical understanding, consider the case where these effects are artificially decoupled, so that polymer chain length contracts while the fiber diameter is kept constant, or fiber diameter increases while the polymer chain length is kept constant. Both cases result in fiber untwist. Although thermal untwist can occur in any fiber whose radial expansion exceeds its axial expansion, polymers such as nylon are ideal because negative axial expansion and positive radial expansion additively contribute to untwist.

This thermally induced fiber untwist (ΔΤ, measured in turns per initial fiber length) generates the torsional actuation observed for twisted fibers and drives the length change for coiled fibers by requiring coil bias angle to change from αc to αc′, as described by the spring mechanics equation (28)ΔT=sin(αc')cos(αc')πD'sin(αc)cos(αc)πD where D and D′ are the coil diameters, taken through the fiber centerline before and after heating, and the coil bias angle αc is the angle between the fiber and the coil’s cross-section. For a coil of N turns and length L made from a fiber of length l: sin(αc) = L/l and cos(αc) = πΝD/l. Using these relationships and assuming negligible change in fiber length l (fig. S4), stretching a coil clamped to prevent end rotation creates a change in fiber twist of>ΔT=NΔLl2

From this equation, we can predict the giant contractions and expansions in coil length resulting from fiber untwist during heating [Fig. 4C and table S2 (1)]. This twist-driven coil actuation mechanism is best demonstrated using mandrel-formed coils. Upon heating of a homochiral muscle, the fiber generates an untwisting torque that pulls coils together, providing work by contracting in length (Fig. 4E). Conversely, when oppositely twisted and coiled to form a heterochiral muscle, fiber untwist during heating increases coil length (Fig. 4D). This relationship is depicted in Fig. 4F, which shows, for highly twisted fibers, that the strokes for homochiral and heterochiral coils are maximized when using tensile and compressive loads, respectively. For compressive loads, the homochiral muscle stroke is near zero because adjacent coils are in contact. By preventing inter-coil contact, thermally driven fiber untwist can cause giant muscle contraction (>45%, Fig. 4F and movie S2).

Retention of muscle stroke and specific work capacity as fiber diameter changes by orders of magnitude is important for the diverse family of targeted applications, ranging from microscopic actuators for microfluidic circuits to those for giant-force-capacity exoskeletons and morphing air vehicles. Present experimental results for twist-insertion–coiled nylon 6 (150- to 2.45-mm-diameter fibers coiled under 16 MPa of nominal stress) provided nearly constant percent stroke and specific energy during contraction against 32 MPa of nominal stress (fig. S9, 1), despite spanning a 267-fold range in cross-sectional area and a 325-fold range in load rating for precursor fishing lines (0.91 to 295 kg).

This near-invariance of actuator performance with fiber diameter implies near-perfect scaling of structure and therefore of properties. Indeed, we find to good approximation that the twist insertion per muscle length needed to initiate and complete coiling is inversely proportional to precursor fiber diameter (fig. S10B). This means that the fiber bias angle is nearly scale-invariant, and the number of coils per fiber length inversely depends on fiber diameter. Hence, images of twist-insertion–coiled fibers having vastly different diameters look much the same when scaled by adjusting magnification to have the same diameter (fig. S10A).

The performance of coiled muscle fibers suggests many possible applications, such as for window shutters that noiselessly open and close to conserve energy (movie S8). Additionally, spools of both conductive and nonconductive nylon are cheaply obtainable, used in clothing, and easily processed into high-stroke artificial muscles. These advantages encourage incorporating coiled fiber muscles in actuating textiles and braids.

Figure 1D shows a braid produced from 32 two-ply coils of 102-μm-diameter nylon fiber. This braid was used as a sleeve over a glass tube containing a nichrome heating element. Upon heating to 120°C, the braid delivered 16.4% stroke while lifting a 630-g load (movie S6). During contraction, the structure of the braid changed (fig. S10), decreasing the braid helix angle from 66.2° to 62.1°, corresponding to a 20.6% drop in pore areaFigure 4G shows a textile woven from coiled nylon muscles, conductive silver-plated fibers for electrothermal heating, and polyester and cotton fibers. Twelve muscle fibers were deployed in parallel to lift 3 kg (movie S5), while providing increased cycle rate capabilities by dissipating heat over a much larger area than for a single large-diameter muscle of similar strength.

Textiles and braids that change porosity in response to temperature can potentially be used for clothing that increases wearer comfort or protects emergency responders from intense heat. For instance, movie S7 demonstrates a braid with a coiled nylon muscle inserted in the center. When heated electrically, the muscle contracts, increasing the diameter of the braid, and thereby opening its pores. The braid bias angle and muscle chirality can be selected so that pores either open or close during heating. Using novel textile weaves, comfort-adjusting clothing might be created by combining polymer muscles having large thermal contractions (up to 1.2%/°C, Fig. 4C) with those that thermally expand, to thereby amplify textile porosity changes.

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Acknowledgments: We thank C. Mozayan, D. B. Hagenasr, Y. Zhang, D. A. Tolly, D. E. Wait, and P. E. Javidnia for assistance with sample preparation and measurements. Support is largely from Air Force Office of Scientific Research grant FA9550-12-1-0211, with additional support from Air Force grants AOARD-10-4067 and AOARD-13-4119, Office of Naval Research MURI grant N00014-08-1-0654, Robert A. Welch Foundation grant AT-0029, the Creative Research Initiative Center for Bio-Artificial Muscle, the Korea–U.S. Air Force Cooperation Program grant 2012-00074 (Korea), Centre of Excellence funding from the Australian Research Council and the Australian National Fabrication Facility, China National 973 Project (nos. 2007CB936203 and S2009061009), NSF China (no. 51003036), and a Natural Sciences and Engineering Research Council of Canada Discovery grant. Correspondence and requests for materials should be addressed to ray.baughman@utdallas.edu. A provisional patent application (61784247) and an international patent application (PCT/US2013/053227) have been filed by N. Li et al. on “Coiled and non-coiled twisted nanofiber yarn and polymer fiber torsional and tensile muscles.”