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Preparation of biomimetic hierarchically helical fiber actuators from carbon nanotubes

Abstract

Mechanically responsive materials that are able to sense and respond to external stimuli have important applications in soft robotics and the formation of artificial muscles, such as intelligent electronics, prosthetic limbs, comfort-adjusting textiles and miniature actuators for microfluidics. However, previous artificial muscles based on polymer materials are insufficient in generating large actuations, fast responses, diverse deformation modes and high cycle performances. To this end, carbon nanotubes (CNTs) are proposed as promising candidates to be assembled into artificial muscles, as they are lightweight, robust and have high surface-to-volume ratios. This protocol describes a reproducible biomimetic method for preparing a family of hierarchically arranged helical fiber (HHF) actuators that are responsive to solvents and vapors. These HHFs are produced through helical assembly of CNTs into primary fibers and further twisting of the multi-ply primary fibers into a helical structure. A large number of nanoscale gaps between the CNTs and micron-scale gaps between the primary fibers ensure large volume changes and fast responses upon the infiltration of solvents and vapors (e.g., water, ethanol, acetone and dichloromethane) by capillarity. The modes of shape transformations can be modulated precisely by controlling how the CNTs are assembled into primary fibers, multi-ply primary fibers, HHFs and hierarchical springs. This protocol provides a prototype for preparing actuators with different fiber components. The overall time required for the preparation of HHF actuators is 17 h.

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Figure 1: A hierarchically helical structure of Towel Gourd tendrils.
Figure 2: Formation of the HHF with multiscale gaps.
Figure 3: Actuation performance of helical fibers.
Figure 4: Hierarchically arranged gaps formed in the helical fibers.
Figure 5: The comparison of actuation performance of pristine HHFs and modified hydrophilic HHFs.
Figure 6: Actuation performance of helical fibers.

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Acknowledgements

This work was supported by the Ministry of Science and Technology of the People's Republic of China (2016YFA203302 to H.P.), the National Natural Science Foundation of China (21634003, 51573027 to H.P. ; 51403038, 51673043 to X.S.; 21604012 to B.W.) and the Science and Technology Commission of Shanghai Municipality (16JC1400702, 15XD1500400, and 15JC1490200 to H.P.). This work was supported in part by the Samsung Advanced Institute of Technology (SAIT) Global Research Outreach (GRO) Program (IO140919-02248-01).

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Authors and Affiliations

Authors

Contributions

H.P. and X.S. conceived and designed the research project. J.D., Y.X., S.H., P.C., L.B. and Y.H. performed the experiments. J.D., Y.X., S.H., B.W., X.S. and H.P. analyzed the data. J.D., Y.X., S.H., X.S. and H.P. wrote the paper.

Corresponding authors

Correspondence to Xuemei Sun or Huisheng Peng.

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The authors declare no competing financial interests.

Integrated supplementary information

Supplementary Figure 1 Growth of CNT arrays through CVD process by a tube furnace.

(a) Placing the catalyst-coated silicon wafer in the middle of the tube reactor. (b) Schematic illustration to the growth of spinnable CNT array.

Supplementary Figure 2 Preparation of the primary CNT fibre.

(a) Placing a CNT array on the rotor of a serve motor (left) with the end of the spun CNT sheet on a collecting drum of a stepping motor (right). (b) Twisting the primary CNT fibre by anticlockwise rotation of the servo motor and clockwise rotation of the stepping motor. Scale bar in a and b, 2 cm.

Supplementary Figure 3 Arrangement of the multi-ply primary CNT fibres.

(a) Twenty parallel primary fibres with the same length of 25 cm on a sheet of paper. (b) Assembling the twenty parallel primary fibres together. Scale bar in a and b, 1 cm.

Supplementary Figure 4 Preparation of an HHF.

(a) Clamping one end of the 20-ply fibre onto a rotating motor shaft. The other end is attached to a moveable paper, and the length of the fibre is 25 cm. The fibre lays on the top of the paper. (b) Cutting the left part of the paper. (c) Anticlockwise twisting of the CNT fibre with the moveable end being moved towards the shaft while keeping the fibre horizontal. (d) The completion of a coiled fibre. Scale bar in a-d, 2 cm.

Supplementary Figure 5 Post-treatment of an HHF.

(a) Infiltrating the whole fibre with ethanol. (b) Slightly relaxing the fibre to observe the excessive twisting by moving the moveable substrate towards the motor. (c) An entanglement confirming the excessive twisting. (d) Clockwise rotation of the motor to release the excessive twisting. Scale bar in a, b and d, 2 cm. Scale bar in c, 2 mm.

Supplementary Figure 6 The preparation of a spring by a thermo-hydro setting of an HHF.

(a) Spirally wrapping an HHF on a glass rod. Scale bar, 5 mm. (b) The glass rod being transferred into a quartz boat at the middle of a furnace tube. Scale bar, 3 cm. (c) The HHF being heated at 300 oC for 1 h in the furnace at argon atmosphere (it is heated to 300 oC in 15 min). Scale bar, 10 cm.

Supplementary Figure 7 The preparation of hydrophilic fibres.

(a) Collecting primary fibres on a drum with arrangement at regular intervals. Scale bar, 1 cm. (b) Modifying the fibre under oxygen microwave plasma. Scale bar, 5 cm.

Supplementary Figure 8 The measurement of contact angle.

(a) Experimental setup for the contact angle measurement. Scale bar, 3 cm. (b-e) The detailed process of a water droplet comes into contact with the sample.

Supplementary Figure 9 Schematic illustration to the actual and projected lengths of the copper paddle for calculating the rotation angle.

Supplementary Figure 10 Contractive measurements.

(a) Experimental setup for the contractive measurement. Scale bar, 10 cm. (b) Clamping the two ends of the fibre on the table-top universal testing instrument. Scale bar, 1 cm. (c) Cutting the two shoulders of the paper (the area marked in dashed line in b), followed by touching with the solvent. Scale bar, 1 cm.

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Deng, J., Xu, Y., He, S. et al. Preparation of biomimetic hierarchically helical fiber actuators from carbon nanotubes. Nat Protoc 12, 1349–1358 (2017). https://doi.org/10.1038/nprot.2017.038

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