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Stiff yet supple hydrogel mimics cartilage

9 February 2015
Magnetically aligned nanoparticles give the material its rare combination of mechanical properties.

In the world of bioengineering, hydrogels—absorbent networks of interlinked polymers—are something of an all-purpose material. Soft, transparent, and biocompatible, they can be used as scaffolds for tissue regeneration, capsules for controlled-release drug delivery, and even sensing elements in breathalyzers. Now researchers led by Takuzo Aida, Yasuhiro Ishida (both at Japan’s RIKEN research institute), and Takayoshi Sasaki (Japan’s National Institute for Materials Science) have created a hydrogel that mimics joint-lubricating cartilage: It can support a heavy load along one direction but still stretch and shear with ease in the others. Aida and his colleagues made the cartilage by adding titanium oxide nanosheets to a solution of hydrogel precursor molecules, as illustrated at left in the figure. When they placed the mixture in the field of a superconducting magnet, the nanosheets spontaneously organized into the crystal-like, layered arrangement shown at right. The researchers then polymerized the precursor molecules to form a hydrogel, which fixes the arrangement into place. The resulting material’s cartilage-like behavior results from interactions between the negatively charged nanosheets: Their electrostatic repulsion stiffens the hydrogel in the vertical direction, as seen in the figure, but because the layers can slide past one another, the material can easily deform along the horizontal plane. That mechanical anisotropy also causes the hydrogel to concentrate vibrational energy into horizontal modes, which could make the material useful as a vibration isolator: When a plate is supported by pillars of the hydrogel, it remains level even as the table underneath it shakes. (M. Liu et al., Nature 517, 68, 2015.)

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