Long Liquid Crystal Elastomer Fibers with Large Reversible Actuation

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Applications of Polymer, Composite, and Coating Materials

Long Liquid Crystal Elastomer Fibers with Large Reversible Actuation Strains for Smart Textiles and Artificial Muscles Devin J Roach, Chao Yuan, Xiao Kuang, Vincent Chi-Fung Li, Peter Blake, Marta Lechuga Romero, Irene Hammel, Kai Yu, and H. Jerry Qi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04401 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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ACS Applied Materials & Interfaces

Long Liquid Crystal Elastomer Fibers with Large Reversible Actuation Strains for Smart Textiles and Artificial Muscles Devin J. Roach1, Chao Yuan1,2, Xiao Kuang1, Vincent Chi-Fung Li1, Peter Blake1, Marta Lechuga Romero1, Irene Hammel1, Kai Yu1,3, H. Jerry Qi1* 1 G.W.W.

School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA

30332 2 State

Key Laboratory for Strength and Vibration of Mechanical Structures, School of

Aerospace Engineering, Xi’an Jiaotong University, Xi’an 710049, China 3Department

of Mechanical Engineering, University of Colorado, Denver, CO

*Corresponding author: HJQ: [email protected]

KEYWORDS: fibers; liquid crystal elastomer; soft robotics; smart textiles; wearables.

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ABSTRACT A method for fabricating long, soft, and reversibly actuatable liquid crystal elastomer (LCE) fibers by using direct ink write (DIW) printing was developed. Here, the LCE was produced based on a two-stage Michael-addition reaction between a difunctional acrylate monomer and thiol. The LCE ink, mixed with nanoclay to increase the viscosity, was extruded through a nozzle onto a rotating mandrel to obtain a long fiber. After printing, the fiber was first thermally cured on the mandrel then mechanically stretched and photocured to achieve liquid crystal chain alignment for stress-free reversible activation.

Upon optimizing the ink

viscosity and DIW printing parameters, long fibers (up to 1.5m long from the laboratory) were obtained. The resulting fiber had a modulus of 2MPa, 51% actuation strain, and a failure strain of well over 100%.

The potential of these fibers for applications was

demonstrated. The LCE fibers were knit, sewn, and woven to form a variety of smart textiles. The fiber was also used to mimic bicep muscles with both large activation force and activation strain. By incorporating further intelligent characteristics, such as conductivity and bio-sensing into a single fiber, the LCE fibers could be potentially used for smart clothing, soft robotics, and biomedical devices.


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INTRODUCTION Fibers are abundant in nature and play essential roles in both plants and animals. They provide a myriad of essential properties, such as strength in plants1, durability in spider webs 2,

driving force for dramatic shape change in muscles 3, and even reproduction in lizard tails4.

They are also found in a wide variety of man-made objects, ranging from composites to fiber optics, providing a vast array of technical advantages5-7. Moreover, and most notably, fibers are known for their uses in textiles. Recently, a new generation of functional textiles are under development for use in wearable electronics. These textiles can exhibit capabilities ranging from bio-sensing 8-9 to data collection 10-11 and information processing10, 12. However, to date, many wearable electronics simply use off-the-shelf electronic components that come as patches to be woven within a shirt or worn directly on the skin

13-14.

Therefore, it is very

desirable to create fabrics with similar characteristics to those traditionally worn, yet with fibers containing conductive, sensing, or energy harvesting functions 15. According to Tao, et. al,

16,

fibers that are able to withstand a minimum of 100% strain and are under 750µm in

diameter can be easily integrated into textiles using existing manufacturing strategies, such as loom knitting, weaving, and sewing. This indicates opportunities for the incorporation of stretchable, mechanically robust, smart fibers into textiles. Materials such as hydrogels and shape memory polymers (SMP) have been used in “smart” structures that can respond intelligently to external stimuli (e.g. light or heat) 17-27. By combining the advances in these smart materials and fiber fabrication techniques, some groups have produced smart fibers able to achieve self-healing, piezo actuation, and one-time shape memory

11, 28-29.

Still, there are some drawbacks of using SMP and hydrogels. For

SMPs, they either only exhibit one-time response without human intervention or are too stiff for most smart textile applications

30.

For example, Ahir et. al. utilized a twin-extruder to

extrude and draw shape memory fibers

31.

There, they heated the pre-fabricated triblock

copolymer to ~70°C to create a melt ink for drawing into long fibers. Immediately afterwards, 3 ACS Paragon Plus Environment


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the fibers were quenched to 20°C, a temperature below the TNI, to achieve crosslinking, making the fiber glassy and brittle at room temperature. After heating above the TNI the fibers could achieve large, reversible shape changes for many cycles. The formation of brittle fibers at room temperature, however, presents a major drawback of this approach as smart textile requires large strain at room temperature. Soft hydrogels, on the other hand, can provide reversible shape change but their response time is relatively slow

21, 32-33.

Furthermore, the

requirement of an aqueous environment limits their potential application space. Better materials, therefore, are still needed for producing fibers that can be used for smart textile applications. Liquid crystal elastomers (LCE), due to their ability to achieve rapid, reversible shape change in response to stimulus, could be an excellent candidate for smart textile applications 34-41.

The reversible shape change observed in LCE is due to the transition of mesogens,

usually consisting of 2-3 linked benzene rings, between nematic and isotropic states in response to various stimuli such as light

38, 42-44,

heat

45-46,

electrical

47

or magnetic fields 48.

Yakacki and coworkers recently developed a simple two-step crosslinking approach using a two-stage thiol-acrylate Michael addition and photopolymerization (TAMAP) reaction45. There, a first-stage thiol-acrylate polymerization is used to fabricate a flexible LCE network; this network is then stretched and a second-stage photo-crosslinking reaction is used to fix the LCE mesogens in an aligned, or nematic state. By using this two-step mechanism, the fabricated LCE can be actuated immediately after fabrication. Upon heating above the isotropic transition temperature, the mesogens lose their alignment, driving a large, macroscopic shrinkage due to the LCE’s transition into the isotropic state. Previously, to fabricate elastomeric fibers, researchers relied on electrospinning which extracts polymer droplets and extrudes them using electrostatic charges

49.

However,

collecting individual fibers from electrospinning method is challenging. In addition, the requirement of large pre-stretching in LCE fabrication makes it difficult to prepare LCE fibers 4 ACS Paragon Plus Environment

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using electrospinning. Direct ink write (DIW) printing is an emerging 3D printing technology where a high viscous liquid resin, or slightly crosslinked elastomer is extruded through a nozzle50-52 to form a 3D structure. It has the advantage of being capable to accommodate a large group of different polymers and elastomer. In the past, DIW was used to 3D printing hydrogels, cell scaffolds, batteries, and micro-fiber mats exhibiting high strength, ductility, and conductivity 17, 50, 53-57. Because a typical DIW method uses a nozzle to write thin lines, it has the potential to be used for fiber production. In this paper, we explored the design and fabrication of long LCE fibers using the DIW method. Our method was based on the simple two-step chemistry using the TAMAP reaction. We added nanoclay as a viscosity modifying agent to achieve room-temperature DIW printing of long LCE fibers onto a rotating mandrel that could maintain their distinct fiber shape after printing and the first-stage curing. Then a roll-to-roll method was used to stretch the firststage

printed

LCE

fiber

with

a

simultaneous

second-stage

crosslinking

via

photopolymerization to fix the LCE monodomain. Using this approach, fibers of up to 1.5 meters in length were fabricated in the laboratory setting. The resulting fibers could achieve up to 51% reversible actuation after printing. To demonstrate the applications of these LCE fibers, they were knit, sewn, and woven to form a variety of smart textiles, including a temperature-responsive smart shirt that can help regulate increased body temperatures. In addition, the smart fibers were twined together to mimic bicep muscle fibers that could lift up to 250 times their own weight. RESULTS & DISCUSSION Fabrication Process of Printed LCE Fibers Long LCE fibers were obtained after a 3-step fabrication process utilizing a custombuilt set up shown schematically in Figure 1a. An image of the fabrication setup can be seen in Figure S1 in the Supporting Information (SI). The LCE ink was first extruded through a DIW printing nozzle via pressure applied by a regulator onto a rotating mandrel (Step 1). 5 ACS Paragon Plus Environment


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(Video S1). The first-stage curing of the printed fibers was performed by using convective heating (step 2). Then the precured fibers were stretched between two mandrels rotating at different speeds to stretch the fiber and align the LCE mesogens followed by a UV curing to fix the LCE mesogen alignment (step 3). The detailed procedures and printing parameters can be found in the experimental section. A close-up photograph of the DIW 3D printing process with the extrusion nozzle and the resulting LCE fiber is shown in Figure 1b. The two-stage curing process involved in the above process was analyzed by FTIR and can be seen in Figure S2 in the SI. After the first stage thermal cure, the Michael addition reaction of thiol-acrylate proceeded and most of the acrylates was consumed to form a polymer network. The residual acylates in the aligned polymer network was further reacted by the second stage radialinduced photopolymerization. By controlling the nozzle or mandrel movement speeds, the extrusion pressure, and the nozzle size, it was possible to print LCE fibers with desirable diameters and actuation strains. To print an LCE fiber that can be easily integrated into current textile fabrication techniques, fibers must fit within the range of 500 to 750µm

16.

To determine the optimal

printing parameters needed to obtain an LCE fiber within this range, a parametric study was performed. For this study, nozzles of five different diameters were used (230, 300, 460, 630, and 990 µm), while the extrusion pressure was adjusted from 20psi to 80psi, in an increment of 10psi. The resulting LCE fiber diameters as a function of nozzle diameter and extrusion pressure can be seen in Figure 1c. For the images of fibers printed under different conditions, see Figure S3 in the SI. In Figure 1c, red lines are drawn in the figure to show fibers that fit within the desired range. As seen, fibers obtained using the 300µm diameter nozzle and a pressure of 50psi most consistently fit within the target range and were thus used for the remainder of this study. Figure 1d shows photographs of a meter-long fiber produced using these settings with the inset showing a close-up of the fiber. Fibers of up to 1.5m were 6 ACS Paragon Plus Environment

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fabricated in the laboratory. Figure 1e shows the LCE fiber axially contracting upon heating above the nematic to isotropic transition temperature, TNI, and returning to its initial length after cooling. The TNI was measured to be approximately 65°C using Differential Scanning Calorimetry (DSC) and the results are shown in Figure S4 in the SI.

Figure 1. (a) Schematic of the 3-step fabrication process of an LCE fiber. (b) Close-up photograph of the DIW extrusion nozzle and the resulting LCE fiber. (c) Demonstration of the printed LCE fiber diameter as a function of the extrusion pressure and nozzle diameter. (d) A photograph of the meter long LCE fiber produced using the 3-step fabrication method. (e) Demonstration of the two-way, stress-free actuation the LCE fiber upon heating and cooling above its TNI. Mechanical and Actuation Properties LCE Fibers LCE fibers were cut into 50mm long strips and used to perform characterization tests. The samples were loaded into the DMA tester to understand the thermomechanical properties of the LCE fibers. Figure Figure 2a shows the mechanical actuation strain of an LCE fiber 7 ACS Paragon Plus Environment


between -20°C and 120°C. 51% bi-directional actuation was observed, similar to that observed by Yuan, et al 41 for conventionally mold-made bulk LCE. Importantly, the amount of pre-strain on the LCE due to stretching is directly related to the resulting actuation characteristics. As seen in Figure 2b, a relatively linear relationship was observed between the pre-strain percentage and the mechanical actuation of the LCE fibers. This can be attributed to the increased LCE mesogen alignment achieved through pre-straining and fixing during second-stage photocuring.

Therefore, controllable actuation properties can be

imparted into the LCE fibers based on the desired applications. The formulation used for prestrain can be found in Section S5 in the SI. Figure 2b also demonstrates the LCE fiber’s excellent mechanical robustness, indicated by the fiber failure strain percentage, which increases as the pre-strain percentage is decreased. It was noted that all fibers had the ultimate strains well above 100%. This is important because, as previously noted, fibers must withstand axial strains of 100% at room temperature during traditional weaving, knitting, and sewing processes

16.

Here, fibers were pre-strained up to 120%, as the first-stage fibers

stretched above this value would fail prior to the second-stage UV curing. The stress-strain behavior of the LCE fiber at various temperatures is a critical indicator of its mechanical robustness during activation. Figure 2c shows the stress-strain behavior for the 120% prestrained fiber at different temperatures. As the temperature increased, the ultimate strain decreased, however, still remained to be relatively large. For example, at 80°C, the fiber failed at 40% axial strain. In addition to achieving large reversible actuation strains, it is desirable that LCE fibers can produce a large force. Therefore, the axial stress generated by an LCE fiber in the activation temperature range was evaluated and is shown in Figure 2d. Here, a maximum actuation stress of 0.038 MPa was observed. In practical, fiber-based textile applications, it is typical to utilize twines, or a thread of many twisted fibers to increase the strength and 8 ACS Paragon Plus Environment

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processability. Therefore, activation force measurements were recorded for multiple fibers woven together using a standard twining procedure. A diagram of the twining process used and a photograph of the resulting LCE fiber twine can be seen in Figure S5 in the SI. Figure 2e shows the maximum activation force measurement obtained for 1, 2, 4, 6, and 8 fibers. A maximum axial force of approximately 0.049 N was observed for 8 fibers twisted together. Figure 2f shows the resulting stress vs. strain curves of 2, 4, and 8 fibers demonstrating an increase in strength and toughness as more fibers are added to the twine.

Figure 2. (a) Two-way actuation strain of the LCE fiber during cooling. (b) The activation strain and the total strain at failure of the LCE fiber as a function of pre-strain percentage. (c) Stress strain curves of the LCE fiber at various temperatures. (d)Activation force generated by the LCE fiber during heating. (e) Increasing activation force generated by twining multiple LCE fibers together. (f) Stress vs. strain properties of 2, 4, and 8 fibers after being twined together. Stimulus Responsive LCE Fiber-based Smart Textiles Early efforts in smart textiles revealed interesting shape change and even produced complex folding patterns such as miura-origami 58. These studies, however, relied on human 9 ACS Paragon Plus Environment


intervention to apply mechanical deformation to the textile.

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Here, to demonstrate the

functionality of LCE active fibers, smart temperature-responsive textiles were fabricated using some common textile fabrication processes including knitting, loom weaving, and sewing, together with our LCE fibers. The LCE fiber’s reversible actuation property was first utilized to activate knitted structures, creating smart and temperature responsive textiles. To highlight the capabilities of the LCE fibers, seamed textiles were knitted such that folding along the seams would occur upon heating. The following textiles were worked in garter stitch; a sturdy stitch which lies flat, reducing the likelihood that any results could be influenced by the textile’s inherent relaxation tendencies. In addition, the seam folding direction could be influenced by using either mountain or valley seams. Seams were created by taking a piece of knit work, which was already completed, and attaching a second piece. If the pieces were attached at the edges of the work, the last stitch of every row would be knit through the completed piece. Based on the desired fold direction, the last stitch was either worked from the front-to-back or back-tofront of the completed textile to produce the desired mountain or valley seam. After creating the structures, the LCE fiber was woven into the center of the closing side of the folds to ensure that the LCE fiber contraction produced the desired fold. Upon heating and LCE activation, the seams folded to alter the structure from its resting state into an activated state determined by the placement of the folds throughout the structure. Using this technique, a variety of smart textiles were knitted and activated as seen in Figures 3a-d. First, a black cylindrical textile was fabricated by knitting in the round and can be seen in Figure 3a. This textile did not have any seams around its edges and did not have any folds, so its activation behavior was based solely on the placement of the LCE fibers. For this structure, LCE fibers were sewn into the edge of the cylinder in a helix like fashion. This orientation of the fibers caused the cylinder to constrict upon heating to a temperature of approximately 120°C and expand upon cooling back to room temperature (Video S2). An 10 ACS Paragon Plus Environment

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effective diameter change of 40% was observed during heating. After cooling, the textile returned to a diameter of 22mm because, at a specific point during cooling, the LCE expansion force is not sufficient to overcome the friction force of the table. From this new configuration, however, completely reversible actuations could be performed. Figure 3b shows a parallel knit textile which shrinks upon heating to a temperature of approximately 120°C and expands back to its original configuration after cooling to room temperature. This textile was made by knitting together multiple pieces to create alternating mountain and valley folds. Two LCE fibers were then preferentially added into the textile to ensure proper mountain and valley folding upon heating. In contrast to the cylindrical textile, the parallel knit structure could reversibly shrink 31% from 120 mm to 83.5 mm. In previous demonstrations, the LCE fiber activation was achieved using an external heat source such as an oven or a heat gun. However, it is possible that the LCE fiber can be activated by Joule hearting when it is used with conductive fibers. To demonstrate, LCE fibers were twined together with AgSIS conductive fibers (supplied by Syscom Advanced Materials, Columbus, Ohio, USA) and then sewn into the textiles. A photograph of an LCE fiber twined together with an AgSIS conductive fiber is shown in Figure S5 in the SI. Figure 3c and d show the textile which was knit one section at a time. Alternating mountain and valley folds were created by attaching individually knit petals. The LCE and AgSIS fibers were woven through the middle of each of the petals and tied to complete a loop through the petals of the textile. A power supply was then connected to either end of the conductive fiber and a current of 2A was applied. A study was performed to determine the temperature and resulting strain of the LCE fiber as a function of current and the results can be seen in Figure S6 in the SI. Figure 3c shows the textile activated to create a deployable dome-like structure (Video S3). Figure 3d shows the same textile activated to create a blooming flower structure (SI video 4). The ability to activate the LCE fibers by applying current makes it possible to provide sequential activation to activate specific folds in a textile. In addition, weaving LCE 11 ACS Paragon Plus Environment


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fibers with conductive fibers indicates the simple integration of LCE fibers with other functional fibers to achieve multifunctionality such as sensing and autonomous actuation.

Figure 3. (a) Cylindrical knit textile achieving 40% effective diameter shrinkage upon heating. (b) Parallel knit shrinking textile obtaining ~31% reversible contraction. (c) Expanding dome textile utilizing an LCE/AgSIS conductive fiber twine which can be activated using Joule heating when 2 Amps current was applied. (d) The same structure can also be activated to demonstrate a flower-like blooming textile. The LCE fibers were also used to create smart textiles using a traditional loom weaving procedure and woven together with standard cotton fibers.

Loom weaving utilizes a

combination of weft, or vertical, and weave, or horizontal fibers woven and pressed together to form a textile 59. The weft fibers consisted of cotton fibers while a combination of cotton and LCE fibers were used in the weave direction. To visually highlight the difference between the LCE fibers and the cotton fibers in the weave direction, green cotton fibers were used in contrast to the white LCE fibers. Figure 4a shows the smart textile resembling a traditional woven textile with 2 disconnected sections. A video showing the looming process used to weave the smart textile is in the SI (Video S5). To activate the smart textile, it was heated to 80°C. During heating, the LCE fibers shrank, creating pores in the textile as seen in the lower image in Figure 4a. Upon cooling, the LCE fibers expanded and the textile returned to its initial configuration. These results demonstrate that a smart textile can be fabricated using loom weaving to create a stimulus responsive, two-way shape memory textile 12 ACS Paragon Plus Environment

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(Video S6). Another common methodology for fabricating textiles is sewing. For this method, the fiber was thread through a needle and the needle was passed through an existing textile creating a path for the fiber to be woven into the existing textile. Here, the LCE fibers were sewn into a typical cotton shirt to create a smart shirt. During exercise, clothing represents the largest barrier to efficient heat transfer and evaporation of sweat from the skin’s surface 60. Therefore, LCE fibers were sewn into a typical cotton shirt such that as the wearer’s body temperature or the environmental temperature increases, the LCE fibers will shrink causing pores to open within the shirt, effectively increasing convective cooling and sweat evaporation. After a reduction in the wearer’s body or environmental temperature the LCE fibers will expand, closing the pores, and returning the shirt to its original configuration. Figure 4b shows two pores in the smart shirt with the top image showing the closed, cool configuration, and the bottom image showing the pores generated upon heating. The pore size was measured as a function of the environmental temperature and the results can be seen in Figure S7 in the SI. To further demonstrate the capabilities of the smart shirt, further pores were created in a smart textile as shown in Figure 4c. Here, the wearer stood in a warm environment for 10 minutes. Photographs of the smart shirt before and immediately after being in the warm environment were taken and are shown in Figure 4c. Here, the sewn LCE shrank to create open pores during exercise providing a breathable shirt. After approximately 5 minutes in room temperature, the LCE fibers expanded to close the pores and the smart shirt returned to its initial configuration. Alternatively, conductive fibers could be woven together with the LCE and sewn into a standard cotton shirt to create on-demand pore to help reduce wearer’s body temperature during exercise.



Figure 4. (a) Loom woven smart textile using LCE fibers to develop pores upon heating. (b) LCE fibers sewn into a cotton shirt creating pores upon heating. (c) Demonstration of the LCE smart shirt demonstrating pores for improved heat transfer and sweat evaporation during exercise. LCE Fiber-based Bicep Muscle One of the most important part of animals are their muscles which produce force and motion for locomotion, heart contractions, and digestion. Vertebrate animal muscles are comprised of a series of fibers capable of obtaining rapid, reversible, and highly controllable actuations. The characteristics of LCE fibers’ rapid and reversible actuation were therefore utilized to mimic muscles, specifically, a human’s fiber-based bicep muscle. Figure 5a shows a schematic demonstrating a relaxed and contracted bicep muscle which creates movement.

The human bicep muscle begins at the bottom of the shoulder and inserts

approximately 3cm from the elbow joint 61. An arm was 3D printed and the LCE fibers were placed between the two holes designed at the insertion points of the bicep muscle. Figure 5b shows the use of a single fiber to demonstrate the activation characteristics of the LCE fiber when used as bicep muscle fiber mimicking muscle contraction and relaxation. Upon heating, the LCE fiber contracted causing the arm to achieve an activation angle of 70 degrees. To demonstrate the activation force generated, multiple fibers were twined together and pennies, 14 ACS Paragon Plus Environment

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weighing 2.5 grams each, were placed in the arm’s hand. As shown previously, by increasing the number of woven fibers, the activation force increases and therefore the number of lifted pennies can also increase (Video S7 in SI). Here, the LCE fiber could lift 250 times its weight. As shown previously, twisted bundles of LCE fibers can also be used to create tougher, more damage tolerant muscles for soft robotics.

Figure 5. (a) Schematic of the anatomy of a relaxed and contracted bicep muscle fibers to achieve a lifting motion. (b) Activation of a single relaxed and contracted bicep muscle fiber using an LCE fiber achieving a 70° rotation angle. (c) Demonstration of the increased activation force generated by combining multiple fibers without sacrificing activation strain to lift increasing number of pennies. CONCLUSION We have demonstrated the design and fabrication of soft and long LCE fibers by using the direct ink write method. The LCE fiber achieved a significant reversible actuation strain of up to 51%. The LCE fiber was then used in three primary textile fabrication methods, namely, sewing, loom weaving, and knitting. LCE fibers were knit into complex patterns creating smart textiles able to drastically change their shape upon heating. Loom weaving and sewing were used to create a smart textile which was able to respond to the wearer’s increased body 15 ACS Paragon Plus Environment


and environmental temperature by creating breathable pores within the shirt. Lastly, LCE fibers were used to mimic a fiber-based bicep muscle which could find use in biomimetic soft robotics. We fabricated fibers up to 1.5m long in the laboratory, but the developed approach can be easily scaled to make even longer fibers in more advanced settings. DIW of smart polymers opens the door for the simple fabrication of smart fibers able to respond to a variety of different external stimuli. EXPERIMENTAL SECTION LCE ink preparation: For the LCE ink, we began with the two-stage Michael-addition chemistry developed by Yakacki, et al

40.

First, a diacrylate mesogen 1,4-bis-[4-

(3acryloyloxypropyloxy) benzoyloxy]-2-methylbenzene (RM 257) (Wilshire Technologies, Princeton, NJ, USA) was diluted in toluene and combined with a dithiol flexible spacer 2,2’(ethylenedioxy)diethanethiol (EDDET) and a tetra-functional thiol crosslinker pentaerythritol tetrakis (3-mercaptopropionate) (PETMP) (Sigma Aldrich, St. Louis, MO, USA). The photoinitiator 2-hydroxy-2-methylpropiophenone (HHMP) was also added to allow the second-stage UV-crosslinking reaction.

Next, 2 w.t.% nanoclay was added (Nanoclay,

surface modified from Sigma Aldrich) to modify the viscosity of the LCE resin for DIW printing 62. Lastly, 3 w.t.% dipropryl amine (DPA), in a 1:20 ratio with toluene, was added to trigger the first-stage crosslinking reaction. The LCE ink was ready for use. LCE Fiber Fabrication: Long LCE fibers were obtained after a 3-step fabrication process utilizing a custom-built set up shown schematically in Figure 1a. In Step 1, the LCE ink was extruded through a DIW printing nozzle via pressure applied by a regulator (Ultimus V, Nordson EFD, East Providence, RI, USA) onto a rotating mandrel driven by a stepper motor. SI video1 shows this printing process where the ink was extruded using a pressure of 50psi, the nozzle moved in the x-direction at 5 mm/s, and the mandrel rotated at 1mm/s. The DIW printing nozzle was placed approximately 2mm to generate an equi-dimensional line 16 ACS Paragon Plus Environment

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according to Zhao, et. al 63. In Step 2, the first-stage curing of the fibers was performed by using convective heating from a portable compact heater (Lasko Inc., West Chester, PA) set to 80°C for 10 minutes while the mandrel rotated. Lastly, in Step 3, the fibers were stretched between two mandrels rotating at different speeds to align the LCE mesogens. Just prior to being rolled onto the second mandrel, the LCE fiber was photocured to fix the LCE mesogen alignment by using a 365 nm UV light source at an intensity of ~10mW/cm2, measured at the location of the LCE fiber, 150mm from the UV light source. The total UV curing time is 2 minutes and 30 seconds. FTIR results of the LCE in each of the three stages is shown in Figure S2 in the SI. A close-up photograph of the DIW 3D printing process with the extrusion nozzle and the resulting LCE fiber is shown in Figure 1b. By controlling the nozzle or mandrel movement speeds, the extrusion pressure, and the nozzle size, it was possible to print LCE fibers with desirable diameters and actuation strains. LCE Fiber Characterization Methods: The diameters of the LCE fibers were measured using a digital caliper (Hornady Inc., Grand Island, NE, USA). The amount of pre-strain imparted on the LCE fibers during stretching was calculated as follows (see SI for derivation): pre  strain(%) 

where

2  1 100 1

(1)

are the rotation speeds of the first and second mandrel, respectively. To

characterize the thermomechanical properties of the LCE fibers, they were loaded into a Dynamic Mechanical Analysis (DMA) tester (Model Q800, TA Instruments, New Castle, DE, USA). The stress-strain behavior was obtained by pulling the fiber at a rate of 0.5mm/s at room temperature until failure. For measuring the activation strain, a preload of 1mN was applied and the temperature was decreased from 120°C to -20°C at 5°C/min. To measure the activation stress, the fibers were held at a constant strain of 0.5% and the temperature was ramped from 20°C to 80°C at 5°C/min.



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Supporting Information. Photographs of the experiment set-up, FTIR data, photographs of the LCE fibers with varing nozzle diameters and extrusion pressures, pre-strain formulation, DSC data to show the TNI, the twining methodology, Joule heating using an LCE/AgSIS fiber twine, thermal imaging of the smart shirt.

The Supporting Information is available free of charge on the ACS Publications website at DOI:xxx AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (H.J.Q.). ORCID

H. Jerry Qi: 0000-0002-3212-5284 Xiao Kuang: 0000-0001-6596-1417 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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TOC use only


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Long Liquid Crystal Elastomer Fibers with Large Reversible Actuation

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