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Preparation of biomimetic photoresponsive polymer springs

Nature Protocols volume 11, pages 1788–1797 (2016)Cite this article

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Abstract

Polymer springs that twist under irradiation with light, in a manner that mimics how plant tendrils twist and turn under the effect of differential expansion in different sections of the plant, show potential for soft robotics and the development of artificial muscles. The soft springs prepared using this protocol are typically 1 mm wide, 50 μm thick and up to 10 cm long. They are made from liquid crystal polymer networks in which an azobenzene derivative is introduced covalently as a molecular photo-switch. The polymer network is prepared by irradiation of a twist cell filled with a mixture of shape-persistent liquid crystals, liquid crystals having reactive end groups, molecular photo-switches, some chiral dopant and a small amount of photoinitiator. After postcuring, the soft polymer film is removed and cut into springs, the geometry of which is determined by the angle of cut. The material composing the springs is characterized by optical microscopy, scanning electron microscopy and tensile strength measurements. The springs operate at ambient temperature, by mimicking the orthogonal contraction mechanism that is at the origin of plant coiling. They shape-shift under irradiation with UV light and can be pre-programmed to either wind or unwind, as encoded in their geometry. Once illumination is stopped, the springs return to their initial shape. Irradiation with visible light accelerates the shape reversion.

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Figure 1: Helix-based motion and mirror-image helices in biological systems.
Figure 2
Figure 3: A variety of springs can be prepared by cutting thin stripes in a homogeneous thin film of liquid crystal polymer network (top panel).
Figure 4: A specific liquid crystal cell is required to prepare liquid crystal polymer springs.
Figure 5: Photo-actuated twisting and/or untwisting motion is encoded via orthogonal deformation modes: in all cases, the outside of the ribbon deforms perpendicularly to the inside of the ribbon.
Figure 6: Nonlinear mechanical character of a biomimetic photoresponsive polymer spring (φ = 45°).
Figure 7: Filling of the cell with the liquid crystal pre-polymer mixture.
Figure 8: Photopolymerization.
Figure 9: Opening of the cell.
Figure 10: The procedure allows production of a variety of photoresponsive polymer springs.

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Acknowledgements

This work was supported financially by the European Research Council (Starting Grant 307784 to N.K.), the Netherlands Organization for Scientific Research (Vidi Grant to N.K.), the EPSRC (Standard Grant EP/M002144/1 to S.P.F.) and the Royal Society through an International Exchange Grant to S.P.F. and N.K. The authors gratefully acknowledge R. Carloni, A. Cremonese (Robotics and Mechatronics, University of Twente), T. Kudernac and A. Leoncini (Molecular Nanofabrication, University of Twente) for discussions on the mechanical properties of the springs.

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Authors and Affiliations

  1. Bio-inspired and Smart Materials, MESA+ Institute for Nanotechnology, University of Twente, Enschede, the Netherlands

    Supitchaya Iamsaard, Federico Lancia, Sarah-Jane Aβhoff & Nathalie Katsonis

  2. Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Oxford, UK

    Elise Villemin & Stephen P Fletcher

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  1. Supitchaya Iamsaard
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  2. Elise Villemin
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Contributions

N.K. and S.P.F. initiated the project and designed the research. S.I., E.V., F.L. and S.-J.A. conducted the experiments and analyzed the data. F.L. conducted the tensile strength measurements. N.K., S.P.F., E.V. and F.L. wrote the manuscript and all authors contributed to discussing the results and the manuscript at all stages.

Corresponding authors

Correspondence to Stephen P Fletcher or Nathalie Katsonis.

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The authors declare no competing financial interests.

Integrated supplementary information

Supplementary Figure 1 Evaluating the thickness of the polymer springs

Cross-section of a liquid crystal polymer film prepared in a commercially available twist cell of nominal thickness 50 μm, observed by scanning electron microscopy. The measured thickness of the film is 43 μm. Scale bar 10 μm.

Supplementary Figure 2 Characterization of the liquid crystal mixture used for the preparation of the biomimetic polymer springs

Differential scanning calorimetry of the liquid crystal mixture without photoinitiator (Irgacure 819). The isotropic to nematic and nematic to crystalline transition are visible respectively at 64.8oC and 12.8 °C.

Supplementary Figure 3 Evaluating the mechanical properties of the springs

Photography of the set-up dedicated to stiffness measurements

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Iamsaard, S., Villemin, E., Lancia, F. et al. Preparation of biomimetic photoresponsive polymer springs. Nat Protoc 11, 1788–1797 (2016). https://doi.org/10.1038/nprot.2016.087

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