Cartilage is the solid but pliable tissue that allows joints to move with very low friction, but it has been difficult to replicate artificially. “The functions of biological materials have been acquired through evolution over the course of millions of years,” explains Yasuhiro Ishida from the research team. “We hope to develop soft materials with functions similar or even superior to their natural counterparts.”
Figure 1: The composite material has properties that vary with direction. Here, a RIKEN logo can be viewed clearly through the material from two directions, whereas the material is opaque when viewed orthogonally to the aligned titanate nanosheets.
Ishida and his co-workers developed a hydrogel — a crosslinked polymer that entraps a large amount of water yet retains a relatively firm structure — containing titanate nanosheets. These nanosheets have a very large aspect ratio, being less than a nanometer thick but about 10,000 times wider, and the nanosheet surfaces are highly negatively charged. This charge gives rise to electrostatic repulsion between the sheets, causing them to disperse readily in the hydrogel.
When placed in a magnetic field, the titanate nanosheets align face-to-face, parallel to the magnetic field — an alignment that differs from that of other metal oxides. The ordering can then be fixed in place by the formation of a crosslinked hydrogel around the nanosheets. Although the alignment direction of the nanosheets can be confirmed using techniques such as transmittance spectroscopy and x-ray diffraction, the alignment is actually apparent to the naked eye — viewed along the direction of the applied magnetic field, the material is opaque, but from other directions it is highly transparent (Fig. 1).
Materials design has often exploited attractive forces between oppositely charged components to improve material strength. The use of repulsive forces as in this titanate nanosheet hydrogel is rare but in this system affords some potentially useful applications, such as artificial cartilage.
“As people age,” explains Ishida, “their cartilage becomes weak, and once someone begins to have difficulty walking, they quickly lose other abilities. The mechanical properties of this new material mimic those of natural cartilage, tolerating heavy loads vertically, but deforming easily horizontally. This anisotropic behavior is also maintained for a long time in physiological saline.” To achieve the goal of developing a fully compatible artificial cartilage material, the research team is now working to improve the material’s mechanical toughness, anisotropy and durability for long-term use.