Machine technology frequently puts magnetic or electrostatic repulsive forces to practical use, as in maglev trains, vehicle suspensions or non-contact bearings1,2. In contrast, materials design overwhelmingly focuses on attractive interactions, such as in the many advanced polymer-based composites, where inorganic fillers interact with a polymer matrix to improve mechanical properties. However, articular cartilage strikingly illustrates how electrostatic repulsion can be harnessed to achieve unparalleled functional efficiency: it permits virtually frictionless mechanical motion within joints, even under high compression3,4. Here we describe a composite hydrogel with anisotropic mechanical properties dominated by electrostatic repulsion between negatively charged unilamellar titanate nanosheets5 embedded within it. Crucial to the behaviour of this hydrogel is the serendipitous discovery of cofacial nanosheet alignment in aqueous colloidal dispersions subjected to a strong magnetic field, which maximizes electrostatic repulsion6 and thereby induces a quasi-crystalline structural ordering7,8 over macroscopic length scales and with uniformly large face-to-face nanosheet separation. We fix this transiently induced structural order by transforming the dispersion into a hydrogel9,10 using light-triggered in situ vinyl polymerization11. The resultant hydrogel, containing charged inorganic structures that align cofacially in a magnetic flux12,13,14,15,16,17,18,19, deforms easily under shear forces applied parallel to the embedded nanosheets yet resists compressive forces applied orthogonally. We anticipate that the concept of embedding anisotropic repulsive electrostatics within a composite material, inspired by articular cartilage, will open up new possibilities for developing soft materials with unusual functions.
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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 Council for Science, Technology and Innovation (Cabinet Office, Government of Japan). The synchrotron X-ray diffraction experiments were performed at BL45XU in SPring-8 with the approval of the RIKEN SPring-8 Center (proposal 20140073).
The authors declare no competing financial interests.
This file contains Supplementary Text and Data, Supplementary Figures 1–18, Supplementary Tables 1-2 and Supplementary References. (PDF 9710 kb)
This video shows the vibration isolation of hydrogel pillars containing horizontally oriented TiNSs. (MOV 5745 kb)
This video shows the vibration isolation of hydrogel pillars containing vertically oriented TiNSs. (MOV 2480 kb)
This video shows the vibration isolation of hydrogel pillars containing randomly oriented TiNSs. (MOV 4334 kb)
This video shows the oscillation-induced deformations of single hydrogel cylinders without any weight at their free edge. (MOV 31453 kb)
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Liu, M., Ishida, Y., Ebina, Y. et al. An anisotropic hydrogel with electrostatic repulsion between cofacially aligned nanosheets. Nature 517, 68–72 (2015). https://doi.org/10.1038/nature14060
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