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Thermoresponsive actuation enabled by permittivity switching in an electrostatically anisotropic hydrogel

Abstract

Electrostatic repulsion, long used for attenuating surface friction1,2, is not typically employed for the design of bulk structural materials. We recently developed a hydrogel3 with a layered structure consisting of cofacially oriented electrolyte nanosheets4. Because this unusual geometry imparts a large anisotropic electrostatic repulsion5 to the hydrogel interior, the hydrogel resisted compression orthogonal to the sheets but readily deformed along parallel shear. Building on this concept, here we show a hydrogel actuator6,7,8,9,10,11 that operates by modulating its anisotropic electrostatics12 in response to changes of electrostatic permittivity associated with a lower critical solution temperature transition13,14. In the absence of substantial water uptake and release, the distance between the nanosheets rapidly expands and contracts on heating and cooling, respectively, so that the hydrogel lengthens and shortens significantly, even in air. An L-shaped hydrogel with an oblique nanosheet configuration can thus act as a unidirectionally proceeding actuator that operates without the need for external physical biases15,16,17,18.

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Figure 1: Thermoresponsive deformation of a hydrogel adopting a layered structure with cofacially oriented unilamellar titanate(IV) nanosheets (TiNSs).
Figure 2: Rapid thermoresponsive deformation of a PNIPA/TiNS hydrogel rod in a glass capillary.
Figure 3: Thermoresponsive deformation profiles of hydrogel films in open air.
Figure 4: Synchronous thermoresponsive changes in the macroscopic shape and nanostructure of a PNIPA/TiNS hydrogel rod.
Figure 5: Unidirectional procession of an L-shaped symmetric PNIPA/TiNS hydrogel actuator.

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Acknowledgements

This work was financially supported by a Grant-in-Aid for Specially Promoted Research (25000005) on ‘Physically Perturbed Assembly for Tailoring High-Performance Soft Materials with Controlled Macroscopic Structural Anisotropy’. We also acknowledge the ImPACT Program of the Council for Science, Technology and Innovation (Cabinet Office, Government of Japan). Y.S.K. thanks JSPS for a Young Scientist Fellowship. The small-angle X-ray scattering measurements were performed at BL45XU in SPring-8 with the approval of the RIKEN SPring-8 Center (proposal 20140073).

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Contributions

Y.S.K. designed and performed all experiments. M.L., Y.I. and T.A. co-designed the experiments. Y.E. and T.S. prepared colloidally dispersed TiNSs. M.O. conducted the permittivity measurements. Y.S.K., Y.I. and T.A. analysed the data and wrote the manuscript. T.H. and M.T. supported the small-angle X-ray scattering measurements at SPring-8.

Corresponding authors

Correspondence to Yasuhiro Ishida or Takuzo Aida.

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Kim, Y., Liu, M., Ishida, Y. et al. Thermoresponsive actuation enabled by permittivity switching in an electrostatically anisotropic hydrogel. Nature Mater 14, 1002–1007 (2015). https://doi.org/10.1038/nmat4363

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