Composite Hydrogel With Ultra-High Mechanical Anisotropy and Biotissue-Like Softness by Confining Electrostatic and Entropic Repulsion Between Cofacially Aligned Nanosheets.

Owing to their biotissue-like nature, mechanically anisotropic hydrogels have attracted extensive attention. Recent studies have achieved high mechanical anisotropy by densely assembling oriented polymers and/or nanofillers into stiff networks. However, this strategy involves soft-direction hardening to afford intrinsically hard hydrogels, limiting their applications, particularly in biomedical fields. Here, we developed the first hydrogel that simultaneously achieves excellent mechanical anisotropy and a low elastic modulus in the soft direction, using magnetically oriented, negatively charged nanosheets. The hydrogel is directionally reinforced by the electrostatic and entropic repulsions between nanosheets. Although increasing the nanosheet concentration enhances the anisotropy, it cannot exceed the threshold where the gel-precursor becomes viscous and unalignable. We discovered that a hydrogel, prepared with nanosheets at an alignable concentration and a less-crosslinked polymer network, spontaneously shrinks vertically toward the nanosheets with expelling ∼30% of water, driven by nanosheet-polymer adsorption and polymer reconfiguration. This densification concentrates the nanosheets while preserving their orientation, selectively amplifying the repulsion between nanosheets. Consequently, the hydrogel achieves an extraordinary anisotropy factor (hard modulus/soft modulus: E