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# Highly twisted supercoils for superelastic multi-functional fibres

## Abstract

Highly deformable and electrically conductive fibres with multiple functionalities may be useful for diverse applications. Here we report on a supercoil structure (i.e. coiling of a coil) of fibres fabricated by inserting a giant twist into spandex-core fibres wrapped in a carbon nanotube sheath. The resulting supercoiled fibres show a highly ordered and compact structure along the fibre direction, which can sustain up to 1,500% elastic deformation. The supercoiled fibre exhibits an increase in resistance of 4.2% for stretching of 1,000% when overcoated by a passivation layer. Moreover, by incorporating pseudocapacitive-active materials, we demonstrate the existence of superelastic supercapacitors with high linear and areal capacitance values of 21.7 mF cm-1 and 92.1 mF cm-2, respectively, that can be reversibly stretched by 1,000% without significant capacitance loss. The supercoiled fibre can also function as an electrothermal artificial muscle, contracting 4.2% (percentage of loaded fibre length) when 0.45 V mm-1 is applied.

## Introduction

Yarn or fibre-based one-dimensional (1-D) conductors can serve as effective and universal substrates for various wearable applications, such as textile electronics, implantable devices, and microscale devices. In addition to providing various functionalities, these fibres need to have elastic properties so that the devices can be highly deformable without performance loss during application of tensile strain, which is important for more advanced and human-friendly applications1,2,3.

Recently, important advances have been made toward realising highly stretchable one-dimensional (1-D) conductors. To achieve both high conductivity and stretchability, fibre conductors have been developed by either incorporating conducting additive materials into elastic substrates or designing microscopically or macroscopically stretchable structures. From a material aspect, composite fibres consisting of conductive metal fillers and elastomeric substrates have been introduced widely. For example, Ag nanoparticles or nanowires were embedded within elastomer fibres using various spinning technologies4,5,6. These metal/polymer composite fibres exhibit excellent length-normalised conductivity and high stretchability; however, they still suffer from significantly increased resistance upon a threshold strain. Alternatively, a liquid metal such as a GaIn was injected in hollow elastomer fibres7,8. Although the fibres injected with a liquid metal were reported to have excellent stretchy-invariant electrical conductivity because of high viscosity of the liquid metal, this method is based on expensive elements and needs to be contained in hollow structured containers to prevent leakage, which limits wider application. On the other hand, from a structural aspect, microscopically buckled structures1,9,10,11, macroscopically coiled structures2,3,12,13,14,15,16, or a combination of both17 were employed for carbon nanotube (CNT) yarns or twist-spun CNT sheet-wrapped fibres to provide structural stretchability. These buckled or coiled fibres exhibited relatively high electrical and elastic properties and were demonstrated to be effective electrodes for elastic sensors1,10,11 and energy storage2,3,9,10,15.

Ultimately, however, realising highly elastically deformable fibrous conductors with microscale diameters and assigning them with multiple functionalities remain challenging. Here, we report on achieving both high stretchability and highly improved quality factor (percent strain divided by percent resistance change). The supercoil structure of DNA is mimicked in a twisting and bending of a double helical axis that not only affects structural transitions and interactions18, but also enables tight compaction to store substantial genetic information within the cell. Although the coiling strategy has been previously reported, the presented supercoil structure has not been reported, to the best of our knowledge. The presented supercoiled fibres are fabricated by highly over-twisted insertion. Due to the high degree of structural compaction, by storing large linear strain in the fibre tensile direction, the supercoiled fibres exhibit superelasticity (~ 1500%) without significant electrical fracture. This high degree of structural organisation can store the ultra-high linear strain, and thus allow the supercoil structure to be used for extendable conductive transmission lines, pseudocapacitive supercapacitors, and artificial muscles.

## Results

### Fabrication and morphology of supercoil fibre electrodes

The schematic illustrations of the supercoiled fibres and buckled surface are presented in Fig. 1a and its inset, respectively. We used spandex fibres wrapped a in CNT-spun sheet (spandex@CNT fibre) as starting materials. Such spandex fibres (~ 200 μm diameter) are stretchable, low-cost, commercially available in effectively unlimited length, and require no treatment before use. More specifically, the spandex fibre core is fully stretched in the tensile direction (fabrication strain, εfab = 400%) and conducting CNT sheath layers mechanically drawn from multiwalled forests19 were wrapped on the stretched fibre using a custom-made twisting machine (Supplementary Fig. 1). The CNT orientation was parallel to the core fibre length direction during wrapping. Ethanol was subsequently dropped on the fibre for surface tension-based densification of the CNT sheets during ethanol evaporation. The length of the relaxed spandex@CNT fibre was longer than the pristine spandex because of the constraint by the mechanically robust CNT sheath wrapping1. Relaxation of pre-stretched fibre after CNT wrapping resulted in a buckled microstructure on the fibre surface, as shown in scanning electron microscopy (SEM) image presented in Fig. 1b, consisting of uniaxially aligned CNT bundles in the fibre length direction (Fig. 1c).

The micro-buckled fibres relaxed from fabrication strain (Fig. 1d) became highly deformable elastically by inserting a sufficiently high twist for supercoiling. One end of the relaxed spandex@CNT fibre was attached to the electrical motor for the insertion of the twist while the other was fixed. The supercoiling process can be categorised into three stages. The first stage is a CNT sheath bias angle increase. In the initial twisting stage, only the CNT sheet bias angle (θ, the angle between the CNT bundle alignment direction and the fibre axis) increases up to 48.3° just before the first coil formation (Supplementary Fig. 2), showing no observable macroscopic changes in fibre morphology. In the second stage, after sufficient application of twisting, the microscopically buckled spandex@CNT fibre is fully transformed into a macroscopically coiled fibre, as shown in the SEM images in Fig. 1e. This first coil formation starts at a twisting of about 1350 turns m-1 (relative to the initial fibre length just before twisting insertion), and the coiling process propagates along the fibre length direction (inset of Fig. 1e). The last stage is a supercoil transformation in which the coiled fibres undergo coiling once again, resulting in a highly packed structure, when more than 3000 turns m-1 twisting is inserted (Fig. 1f). The fibre transformation from the first coil into a supercoil image is shown in the inset of Fig. 1f. The surface tension-based densification-induced binding of the CNT sheets onto the spandex fibre was so strong that no noticeable delamination was observed during the giant twist insertion until supercoiling.

Structural differences of the supercoil with other coils are presented in the schematic illustrations (Supplementary Fig. 3). For examples, first coil fibres (Supplementary Fig. 3b) are widely reported for various stretchable fibrous applications such as supercapcaitors2,3,15,17, actuators20,21, and high toughness22, and can be fabricated by moderate twisting insertions. Due to the simple structure, however, first coil fibres have normally reported less than 400% stretchability. Helical-coil fibres are different from other coils in that they are fabricated by not just twisting but by winding active fibres onto the surface of non-active core substrate fibres (Supplementary Fig. 3c)23,24,25. Such helical coil fibres are reported to be stretchable up to 800% when the core substrates have appropriate elasticity25. However, they can suffer from low specific performances when normalised by system dimension including thick and bulk core substrate. This is because the as-used core substrate does not contribute to the fibre functionalities such as mechanical actuation or energy storage, but just provides a mechanical stretchability, which might especially lead to low volumetric or surface areal capacitance, energy, or power densities.

The electrical and morphological properties of the fibre before fully supercoiling were dynamically changed during the twisting as shown in Fig. 1g. The initial electrical resistance normalised by the maximum (fully stretched) fibre length of the noncoiled spandex@CNT fibre significantly decreased by 82% when fibres were fully supercoiled; this occurred because the electrical pathway along the fibre length was enlarged by the intercoil contact area increase after the supercoil formation. The relaxed fibre length from the fabrication strain continuously decreased during twist insertion; about 67% and 83% of the fibre length contracted after the first and supercoil formations, respectively. By contrast, the total diameter for the first coiled and supercoiled fibres discretely increased by 33.6% and 113% over the noncoiled fibre, respectively, roughly showing the volume conservation of the total fibre. Due to the ultra-high twist insertion, the supercoiled fibre is untwisted when both ends of the fibre are not tethered. Torque-balance locking can be an effective way to prevent this unwanted untwisting. Photograph of the double-helix structured supercoiled fibre made by self-inter locking is shown in Fig. 1h. This type of internally torque-balanced structure eliminates the need for external torsional tethering. Plying a single, fully supercoiled fibre in the direction opposite to the internal twist of the fibre creates a structure in which the chiralities of fibre twist and plying are opposite by double-helix structure (Fig. 1i)26. This was accomplished by folding the supercoiled fibre itself, while prohibiting relative rotation of fibre ends. Another possible strategy to prohibit untwisting is to upscale the supercoil fibres into a neat textile structure because the supercoiled fibres could be mechanically tethered by adjacent fibres. The supercoiled fibres were mechanically strong enough so they can be woven into a commercial mock rib-structured textile (Fig. 1j) or be assembled into the textile by themselves (Fig. 1k). In addition, the over-twisting process is stable for scalable supercoil fabrication. Hence, much longer supercoil fibre (60 cm in length), fabricated using a 3-m long spandex fibre, with uniform morphological and electrical properties was successfully demonstrated, showing resistance-linearity property with 100 Ω cm-1 slope value (Supplementary Figs. 4 and 5).

### The strain dependence of electrical and mechanical properties for supercoiled fibres

As there is normal trade-off relation between the CNT sheet loading level and fibre stretchability, as previously reported1,17, we used five-layer CNT sheets for conductive wrapping, enabling the realisation of highly stretchable and electrically conductive supercoiled fibres. A comparison of the resistance normalised by the maximum fibre length (R(ε)/Lmax) versus tensile strain for supercoiled, coiled, and noncoiled spandex@CNT fibres is shown in Fig. 3a. The initial resistance of the noncoiled spandex@CNT fibre increased by about 15% with 200% strain application. For a coiled fibre, at a strain of 620%, the resistance increased up to 90% over the initial resistance. The decrease in initial resistance after coiling is caused by the increase in contact area between coils, providing a larger electrical pathway in the longitudinal direction of the fibre. The trend is more evident for the supercoiled fibre that the initial resistance drastically increased at the low strain range (below 450%), which mainly corresponds to the progressive opening of the first coils, and the supercoils during strain application. The separation between coils during supercoil fibre stretching is shown in the optical images of Fig. 3b. In the higher strain region (450%–1300%), micro-buckles unfolding rather than coil opening mainly contributed to fibre stretchability, showing very little change in resistance.

Although the maximum stretchability before electrical failure for supercoiled spandex@CNT fibres was 1500%, we used the stable and mechanically or electrically reversible strain region (1000%) to investigate the performance of the supercoiled fibre. The resistance change ratio ((R(ε)R0)/R0), where R0 is the resistance at the initial fibre length versus strain for the supercoiled fibre, was measured during 1000% stretching and releasing (Fig. 3c). The resistance recovered to the initial value after repeated tensile deformations at 1000%. The reversible resistance change against the cyclically applied maximum reversible strain (εmax = 1000%) is presented in the inset of Fig. 3c. The supercoil fibre exhibited stable electrical properties, with 4.06 average resistance change ratio (R(ε)-R0)/R0) with 0.12 standard deviation during repeated 1000% strain applications. Mechanical properties of the supercoil fibre is characterised by stress–strain curves (Fig. 3d). The ultimate tensile strain before mechanical fracture of the presented supercoiled fibre was about 1500%. Moreover, the mechanical properties were largely retained after repeated stretching that no significant deformation was detected after 100 times of 1000% strain loading-unloading cycles (inset of Fig. 3d). The photographs in Fig. 3e show a supercoiled fibre mounted on Vernier calipers (initial fibre length, L0 = 1.5 cm) progressively stretched to εmax = 1000% (Lmax = 16.5 cm) and recovered to the initial state (ε = 0%), respectively, without showing any observable residual deformation (the left SEM image in the inset shows a pristine supercoil and the right SEM image shows a relaxed supercoil after 1000% strain application).

### Passivation for high quality factor and transmission line applications

Although the large and reversible resistance change of the supercoiled fibre by varying the intercoil distance is advantageous for strain sensor applications, strain-insensitive properties are also required for use in other applications27. To avoid abrupt resistance increase during fibre stretching, particularly during coil opening, the supercoiled fibres were overcoated by styrene-ethylene-butylene-styrene (SEBS) layers (~ 3.85 μm thickness) using the spray coating method. This SEBS passivation layer largely prevents supercoils from directly contacting during the fibre deformation without degrading the mechanical elasticity of the supercoiled fibre; therefore, the resistance change could be dramatically reduced. The resistance increase of SEBS/supercoiled fibre during 1000% strain application was 4.2% (Fig. 4a), which is about two orders of magnitude lower than the resistance change of nonpassivated supercoiled fibres. The scheme presented in the inset in Fig. 4a schematically depicts that the SEBS passivation (green layer) is uniformly overcoated on the buckled surface of the supercoiled fibres. SEM images of the uniform SEBS coating layers on the supercoiled fibres are presented in Supplementary Fig. 6. To investigate the usefulness of a supercoil fibre with a high quality factor, the SEBS/supercoiled fibre was used as an electrical signal transmission line. A square wave signal with 20 Hz frequency generated by a function generator was transmitted to an oscilloscope through bare and SEBS-overcoated supercoiled fibres, and the resulting transmitted signal amplitudes were compared (Fig. 4b). The quality factor of the SEBS/supercoiled fibre was compared with previously reported fibre-based conductors with diameters similar to the present work (Fig. 4c). Specifically, the SEBS/supercoiled fibre showed an impressively higher quality factor (Q = 238.8) than (a) the buckled CNT@sandwich structured rubber fibre (Q = 54)28, (b) buckled CNT@SEBS fibre (Q = 65.1)10, (c) CNT@coiled nylon fibre (Q = 0.1)3, (d) buckled CNT@coiled rubber fibre (Q = 1.8)17, and (e) CNT/elastomer polymer fibre (Q = 1.6)29. A detailed comparison of quality factors of the present supercoiled fibre and prior-art yarn or fibre electrodes is presented in Supplementary Table 1. Elastomeric conductors with very-low-quality factors are useful as strain sensors, but other applications, such as signal transmission lines or conventional conductors, would benefit from very-high-quality factors1. The amplitude transmitted through the supercoiled fibre without passivation was drastically reduced when the fibre was fully stretched. However, the amplitude transmitted through the SEBS-passivated supercoiled fibre was largely retained even at εmax (1000%) application. This voltage amplitude retention performance was plotted versus strain for supercoiled fibres (inset of Fig. 4c). The signal from the SEBS/supercoiled fibre showed 92% amplitude retention, whereas the signal from the bare supercoiled fibre showed only 28% amplitude retention when both fibres were fully stretched.

A few groups have reported stretchable signal transmission lines with strain-insensitive resistance, which remains nearly constant even when the lines are subjected to large strains. Such properties are desired for transmitting analogue voltage or current signals, for example, which may contain vital information from sensors attached to or implanted in a human body. Combined with high signal-to-noise ratios (SNRs), these stretchable conductors are likely to be promising in developing health care and wound-monitoring systems, or other medical applications30,31. An effective demonstration of such resistance-invariant, highly expandable electrical signal transmission lines is electrocardiogram (ECG) measurement (Fig. 4d). ECG signals measured by the “three leads ECG” method were transmitted by a supercoiled fibre; a commercial cable was also used for comparison. Moreover, the supercoils were also applicable as highly extendable cables to transmit the audio and video signals. Figure 4e, f show photograph images for audio and video signals measurement setups, respectively. The ECG patterns, peak magnitude, and normal sinus rhythm transmitted by the SEBS/supercoiled fibre were clear, independent of strain, and highly comparable to the peak from reference measurements using a commercially available cable (Fig. 4g), implying high versatility of the present passivated supercoiled fibre as an expandable cable for biomedical signal transmissions. Audio and video signals were split into the supercoil fibre and reference line simultaneously for comparison. In general, relevant frequencies range from 20 Hz to 20 kHz for acoustic electronics devices and are on the order of a few MHz for composite video signals; these frequencies are much higher than those considered in ECG measurements27. Real-time signal transmissions using passivated supercoil fibres under dynamic strain and comparisons with reference signals were shown for audio (Fig. 4h) and video (Fig. 4i) signals. The transmission of video signals requires much higher accuracy in terms of the signal period (or frequency) and amplitude because the information on brightness, colour, and synchronisation of a signal is delivered at the same time27. Specifically, various significant signals such as horizontal synchronisation, colour reference burst, active pixel region, and front and back porch should be transmitted without delay and distortion. The transmitted audio and video signals by supercoil fibres were almost identical with that of reference cables without distorting the original information in terms of its resistance and frequency. Moreover, the signals did not distort even at 1000% strain applications as shown in Fig. 4h, i, respectively. Successful demonstrations of audio and video signal transmission during dynamic supercoil fibre stretching/releasing (up to 1000%) were demonstrated in Supplementary Movie 1 and 2, respectively.

### Performance of supercoiled fibre-based artificial muscle

The supercoiled spandex@CNT fibre can also function as electrothermally driven artificial muscles to provide large contractile stroke under heavy loads. The actuation performance for a supercoiled fibre artificial muscle was measured during the application of a voltage for Joule heating (Fig. 6a). The CNT sheath acts as a heating element to induce volume change of core spandex fibre during voltage application. The tensile stroke of a polymer muscle can be amplified by inserting a large amount of twisting21. The supercoil fibre-based muscles delivered maximum contractile tensile actuation of 4.2% based on a loaded fibre length (or 5.5% actuation based on nonloaded fibre length) when 0.45 V mm-1 was applied under a load of 175 kPa (Fig. 6b). The stress is obtained by normalising to the nonactuated, nontwisted fibre cross-sectional area. The specific work capacity of the supercoil muscle during contraction was 0.17 kJ kg-1. The supercoiled muscle also showed stable actuation performance during repeated voltage application (inset of Fig. 6b).

## Discussion

In summary, we developed a highly twisted supercoil structure of superelastic conducting fibres with various potential applications. Table 1 compares the fabrication method, stretchability, and functionality of the supercoil to prior-art various coil structured fibres for clarifying the novelty of this work. We showed that the fibres can be used as a cable or transmission line that are extendable up to 11 times their initial length without significant resistance change for applications such as ECG or other audio and video signal transmissions. Additionally, the introduction of electrochemically active materials enables the assigning of pseudocapacitive energy storage functionality into the supercoiled fibres, resulting in superelastic supercapacitor fibres. When electrical energy was applied, the supercoiled fibres showed electrothermally operated tensile actuation originating from the core fibre volume expansion, which was amplified by the insertion of twist. Other possible applications for such highly elastic fibres include the morphing of structures in space, robotic arms or exoskeletons capable of extreme reach, and interconnects for highly elastic electronic circuits1.

## Methods

### Supercoiled fibre fabrication

Multiwalled carbon nanotube (MWNT) sheets were mechanically drawn from a carbon nanotube (CNT) forest with 750-μm height (NTAD 10, PDSI Corporation, Korea) and were wrapped on a 200 μm-diameter commercially available spandex fibre (Hyosung, Korea). The spandex fibre was stretched (400%) during the CNT wrapping. After wrapping the CNT on the spandex fibre, the spandex@CNT fibre was highly twisted for full supercoiling. For supercoiled fibre passivation, the SEBS particles were dissolved in cyclohexane (Sigma-Aldrich, USA) and the solution was spray-coated on the supercoiled fibre while it was fully stretched. For supercapacitor electrodes fabrication, MnO2 nanoparticles (Sigma-Aldrich, USA) were dispersed in alcohol (20 mg mL-1) using sonication, and the solution was drop-cast onto the surface of the spandex@CNT fibre before supercoiling. PVA/LiCl gel was used as electrolytes for electrochemical performance characterisation. More specifically, the gel electrolyte was prepared by mixing 3 g PVA (Sigma-Aldrich, USA) and 6 g LiCl (Sigma-Aldrich, USA) in 30 mL deionized water and heating it at 90 °C until it became transparent.

### Characterisation

MWNT sheet wrapping was performed using homemade fibre twisting machines. Images of fibre morphology were obtained with an SEM (S-4600, Hitachi, Japan) and an optical camera (D750, Nikon, Japan). For electrical resistance measurements for fibres during the statically applied tensile strain, the fibres were mounted on digital Vernier calipers (500 series, Mitutoyo, Japan), with both fibre ends mechanically fixed by a bolt–nut pair, and electrically connected to multimeter probes (15 + , Fluke) for resistance measurements. A function generator (AFG1062, Tektronix) and an ECG measurement system (MP36, Biopac) were used to investigate the electrical signal transmission performance. An electrochemical analyser (Vertex EIS, Ivium) was used for electrochemical performance characterisation.

### Calculation of the electrochemical and actuation performances

The capacitance of the two-electrode system was calculated from CV curves. From C = I/(dV/dt), where I is average discharge current from the CV curve and the dV/dt is the scan rate, the single-electrode specific areal capacitance was calculated from the following Eq. 1:

$${\mathrm{Areal}}\,{\mathrm{capacitance}}\,\left( {{\mathrm{F}}\,{\mathrm{cm}}^{{\mathrm{ - 2}}}} \right){\mathrm{ = 2}}C/A_{{\mathrm{surface}}}$$
(1)

where Asurface is the total external surface area of a single spandex@MnO2/CNT fibre electrode. The total fibre length was used for linear capacitance calculation. For actuation performance characterisations, the fibre length changes were measured during voltage application using a voltage source (E3633A, Agilent). The tensile stroke of the supercoiled fibre was calculated using Eq. 2:

$${\mathrm{Contraction}}\,{\mathrm{tensile}}\,{\mathrm{stroke}}\left( {\mathrm{\% }} \right) = \left( {L_{{\mathrm{final}}} - L_{{\mathrm{initial}}}} \right)/L_{{\mathrm{initial}}} \times 100$$
(2)

where Linitial is the initial supercoiled fibre length under isobaric load and Lfinal is the final fibre length under isobaric load after voltage application. The specific work capacity per mass was calculated using Eq. 3:

$${\mathrm{Work}}\,{\mathrm{capacity}}\left( {{\mathrm{kJ}}\,{\mathrm{kg}}^{{\mathrm{ - 1}}}} \right) = m_{{\mathrm{load}}}gh/m_{{\mathrm{fibre}}}$$
(3)

where mload is isobaric load, g is gravitational acceleration, h is the change in fibre height, and mfibre is the mass of the supercoiled fibre.

## Data availability

The data that support the findings of this study are available from the corresponding author on reasonable request.

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## Acknowledgements

This work was supported by the Basic Science Research Programme (NRF-2017R1A6A3A04004987) and Global Research and Development Center Program (NRF-2018K1A4A3A01064272) through the National Research Foundation of Korea funded by the Ministry of Education and Ministry of Science and ICT. This work was also supported by the DGIST R&D Programme (18-NT-02) of the Ministry of Science, ICT, and Future Planning.

## Author information

Authors

### Contributions

W.S., S.C., J.M.L. and C.C. conceived the idea and designed the experiments; D.W.L. and Y.K. contributed mechanical/electrochemical characterisation; S.M.J. fabricated material for experiments; W.S., S.C., S.K.L. and C.C. wrote the paper; Y.L., J.P., H.J., Y.J.K. and D.S. performed the signal transmission experiments; All authors discussed the results and commented on the manuscript.

### Corresponding author

Correspondence to Changsoon Choi.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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Reprints and Permissions

Son, W., Chun, S., Lee, J.M. et al. Highly twisted supercoils for superelastic multi-functional fibres. Nat Commun 10, 426 (2019). https://doi.org/10.1038/s41467-018-08016-w

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• Stephan Handschuh-Wang
• Xuechang Zhou

Chemical Research in Chinese Universities (2021)