Highly twisted supercoils for superelastic multi-functional fibres

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.


Response to Reviewer Comments
We appreciate the comments of the reviewers and the suggestions of the editor. Our responses to these comments are listed below, and the manuscript and supplemental materials have been accordingly revised. To help address the comments of the reviewers, we performed additional experiments and added pertinent new results. This includes (1) demonstration of large production possibility, more advanced applications, and free-standing supercoil fibres, (2) structural analysis, and characterization of structural effects on electrical/mechanical properties, (3) MnO2 weight percent measurement, electrochemical performance optimization, and the capacitance retention of PVA-LiCl gel electrolyte coated 1,000% stretchable supercapacitor.
Main revision contents are summarized in table as shown below. Moreover, we revised the key word 'fiber' into 'fibre' for NPG publication, and recalculate the length-normalized twisting number based on not the final length but the initial length of the fibre before twisting insertion in order to fairly compare the twisting numbers with other references.

Reviewer No. Revisions or new results based on reviewer suggestions
Revised or newly added figure   Table 1 - Fig. 1 (h), (i)

Reviewer #1 (Remarks to the Author)
The author reported a kind of superelastic fiber based on a supercoil structure. The fiber shows outstanding strain to 1300% due to the coil structure. However, the coil-strategy is not new (Adv. Mater. 2015, 27, 4982-4988, 10.1016/j.joule.2018.06.003). Moreover, the title is not appropriate. It looks more like an "over twisted yarn" other than "DNA", based on the SEM images provided in Figure 1f. Besides, the demonstrated applications seems ordinary without showing the advantage of 1300% superelstic. I suggest to make the novelty of this work clearer by better design. Following are some specific comments.
Response: Thank you for your kind comment about interest in our work. Although the coilstrategy is not new as a Reviewer #1 commented, presented supercoil is apparently a new structure that has not been reported yet. To help the readers clearly understand the structural difference of supercoil with others, we newly added a schematic illustrations ( Figure S1) showing structural differences of various coil structured fibres.  have appropriate elasticity. However, they can suffer from low specific performances when normalized by whole system dimension including thick and bulk core substrate. This is because the as-used core substrate does not contribute to the fibre functionalities (energy storage or actuation) but just provides a mechanical stretchability, which might especially lead to low specific capacitances, energy, or power densities. Presented supercoils are unique structure that is fabricated by only highly over-twist insertion and due to the high degree of structural compaction, they exhibited superelasticity (~ 1,500%) without significant loss in electrical property. We newly added Figure S1, Table 1 and added this information in page 3, line 8 and page 15, line 17 in the revised manuscript. A Reviewer #1 also pointed out that the supercoil looks more like an "over twisted yarn" other than "DNA", based on the SEM images provided in Figure 1f. According to the comments, we revised our title from "DNA-Inspired Supercoils for Superelastic Fibres" into "Highly Twisted Supercoils for Superelastic Fibres". In addition, we removed related key words (DNA, inspiration), and scheme ( figure 1a showing DNA structure in previous version manuscript).
Finally, we added new demonstrations of supercoil fibre applications, which will be discussed later, in order to showing advantage of present work better.
Major issue: (1) The resulting supercoiled fiber is fabricated by inserting a giant twist into spandex core fibers wrapped in a carbon nanotube sheath, which the authors call "CNT@spandex fiber".
This definition of the supercoiled fiber should be "spandex@CNT fiber" because carbon nanotubes are the sheath of the fiber.

Response:
We revised all "CNT@spandex fiber" into "spandex@CNT fibre" in revised manuscript, and have made appropriate changes elsewhere.
(2) The author could show more in-depth study of the special structure.
Response: According to the valuable comments, we performed additional experiments to study the structural effects, and characterization is included in Figure 2 which is newly added in revised version manuscript. Our unique structures which contribute to high fibre stretchability can be categorized into two parts: one is microscopic scale buckles, and the other is macroscopic scale supercoils. The morphological width of CNT micro-buckles is investigated against pre-strain (Figure 2a) applied to a bare, noncoiled spandex fibre before CNT wrapping, and CNT loading layers (Figure 2b). Upon observations, the width of the buckles is roughly, inversely proportional to the degree of pre-strain application (CNT loading level is fixed as five layers in this case). The average width of the CNT buckles formed from relaxation of 100% pre-strained fibre was about 38 μm and it significantly decreased to 16.3 μm by 400% prestrain relaxation. The effect of CNT loading level on buckles formation, where pre-strain is fixed at 400%, showed slight increase of buckle width from 14.6 to 20 μm as the number of CNT wrapping layer increased from 1 to 9. The fibre contraction force to recover initial length after pre-strain relaxation is inversely proportional to the fibre length ratio of before to after relaxation (ΔL/L0) as shown in fig. 2b. The larger contraction force by either larger pre-strain application or lower CNT loading level results in narrower buckle width.  (3) How to realize the large-production of the supercoil fibers? What is the length of the fiber achieved in this article? As far as I know, the over twisted process is not stable. The author should convince the readers of the possibility of large-production.

Response:
The final length of the most supercoil fibres presented in this work was mostly less than 2 cm for lab-scale fabrication. The fabrication processes were started from stretching 7 cm-long initial spandex fibre up to 400% in tensile direction (35 cm in length), and wrapping CNTs onto the surface of the stretched spandex fibres. Over-twisting process is a widely used and reliable strategy to fabricate the coiled yarns or fibres as previously reported [3,5]. To demonstrate the possibility of the large-production of supercoil fibres, we newly fabricated and demonstrated the 60 cm-long supercoil fibre as shown image below (two ends of the fibre were tethered using bolts and nuts). In fabrication detail, both ends of bare spandex fibre with 3 mlong were fixed to the twisting motor tips and 400% pre-strain was applied (15 m in length).
After appropriate CNT wrapping the spandex@CNT fibre was relaxed from pre-strain and the relaxed fiber length was 3.5 m. Total 7,000 turns/m of twisting was inserted for supercoiling and 60 cm-long supercoil fibre was successfully fabricated. Formation of supercoil was stable that the supercoil morphology observed by optical microscope was largely uniform as shown images below ( Figure S4). Electrical property of the 60 cm-long supercoil fibre was also investigated by measuring fibre resistances versus fibre length as shown below ( Figure S5). The fibre exhibited resistancelinearity property with 100 Ω/cm slope value. The inset photograph shows 60 cm-long supercoil fibre wounded in 1 cm-diamter glass tube. Therefore, with high length of the fibre, we confirmed that the morphological and electrical properties are uniform and the over-twisting method is revisable. We added this data in Supplementary figure S4, S5 and page 6, line 12 in the revised manuscript.  Figure 1j, and k. The supercoiled fibres were mechanically strong enough to be woven into a commercial textile that six-supercoil fibres were successfully sewn into mock rib-structured textile (Figure 1j). Moreover, the supercoil fibres can be also assembled into the textile themselves which comprising twenty-seven spandex@CNT fibres (Figure 1k).    Specifically, various significant signals such as horizontal synchronization, color reference burst, active pixel region, and front and back porch should be respectively transmitted without delay and distortion [14]. The transmitted 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 distorted even at 1000% strain applications as shown in Figure 4h, and i. We added this audio and video signal transmission data and description in       capacitance is plotted as shown in Figure 5a and 5c and d, respectively. It is observed that the CV curves from higher MnO2 loaded supercoil supercapacitor gets dented at higher scan rate. This degradation of rate-capability at high MnO2 loading is originated from low electrical conductivity (∼10 -5 to 10 -6 S/cm), which significantly limits charge transport [19].    Figure S8 shows PVA/LiCl gel coated, two parallel supercoil spandex@MnO2/CNT fibres at 1000% strain application. It is shown that the transparent gel electrolyte is well surrounded between the fibre electrodes without degrading the electrodes' stretchability.  Reviewer #2 (Remarks to the Author): This paper introduces a method to fabricate stretchable fibers with a coiled configuration and tested their mechanical and electrical performance. There are strong interests to develop fabricbased stretchable structures. The concept in this paper is interesting but this reviewer has the following concerns: (1) The title "DNA-inspired" is somewhat misleading. The stretchable fiber is just coil-shaped, or more specifically a fiber with coil shapes in two length scales. They are not really double helix structures.

Response:
We appreciate your very kind comment about the importance of our work.
According to the comments, we removed related key words ("DNA" and "inspiration"), and scheme showing DNA structure in the revised manuscript to prevent the authors to misunderstand our work. Moreover, we revised our title from "DNA-Inspired Supercoils for Superelastic Fibres" into "Highly Twisted Supercoils for Superelastic Fibres".
(2) The formation of the first buckled structure, the main innovation of this paper, was not clearly discussed. To form a buckled structure, the CNT sheath layers must have certain mechanical properties, such as mechanical modulus, and thickness. These factors all affect the buckling geometry and thus affects the stretchability. They are needed to be thoroughly discussed.
Response: According to the valuable comments, we performed additional experiments to study the structural effects, and characterization is included in Figure 2 which is newly added in revised version manuscript. The morphological width of CNT micro-buckles is investigated against pre-strain (Figure 2a) applied to a bare, noncoiled spandex fibre before CNT wrapping, and CNT loading layers (Figure 2b). Upon observations, the width of the buckles is roughly, inversely proportional to the degree of pre-strain application (CNT loading level is fixed as five layers in this case). The average width of the CNT buckles formed from relaxation of 100% pre-strained fibre was about 38 μm and it significantly decreased to 16.3 μm by 400% prestrain relaxation. The effect of CNT loading level on buckles formation, where pre-strain is fixed at 400%, showed slight increase of buckle width from 14.6 to 20 μm as the number of CNT wrapping layer increased from 1 to 9. The fibre contraction force to recover initial length after pre-strain relaxation is inversely proportional to the fibre length ratio of before to after relaxation (ΔL/L0) as shown in fig. 2b. The larger contraction force by either larger pre-strain application or lower CNT loading level results in narrower buckle width.    (Figure S1) showing structural differences of various coil structured fibres.  suffer from low specific performances when normalized by whole system dimension including thick and bulk core substrate. This is because the as-used core substrate does not contribute to the fibre functionalities (energy storage or actuation) but just provides a mechanical stretchability, which might especially lead to low specific capacitances, energy, or power densities. Presented supercoils are unique structure that is fabricated by only highly over-twist insertion and due to the high degree of structural compaction, they exhibited superelasticity (~ 1,500%) without significant loss in electrical property. We newly added Figure S1, Table 1 and added this information in page 3, line 8 and page 15, line 17 in the revised manuscript. In addition one significant issue is how to prevent fabricated fibers from uncoiling. Since coiled sample has to be under tension and there is explanation how to keep sample without release of twist and permanently set. Thermally annealing process was used for the coiled nylon or other thermoplastic polymer however Spandex (polyether-polyurea copolymer) has a very low Tg (below room temperature) and impossible to annealing method. I believe that the manuscript should be considered for other journal and it is not suitable for Nature Comm. Due to insufficient novelty and innovations Response: Torque-balance locking can be an effective way to make the supercoil fiber freestanding state, which is previously reported for CNT coil yarns [22]. Photograph for doublehelix structured supercoil fibers made by self-inter locking does not need tethering as shown image below (Figure 1h, and i). This type of internally torque-balanced structure eliminates the need for external torsional tethering. Plying a single, fully supercoiled fibre in the opposite direction to the fibre's internal twist creates a structure in which the chiralities of fibre twist and plying are opposite. This plying was accomplished by folding the supercoiled fibre itself, prohibiting relative rotation of fibre ends. Another possible strategy to prohibit the untwisting is to upscale the supercoil fibres into neat textile form. The supercoil fibres were mechanically strong enough to be woven into commercial mock-rib structured textile (Figure 1j). In this case, the woven supercoil fibres were mechanically fixed by adjacent fibres of neat structured textile, therefore, no any observable untwisting was generated while they were woven. The supercoil fibres could be also fully assembled into textile structure by themselves (Figure 1k).
We added this demonstration of free-standing supercoiled fibre, and textile assembly in Figure   1h, i, j, and k, and page 6, line 1, respectively, in the revised manuscript.