The properties of articular cartilage, which lines bones in joints, depend partly on repulsion between components of the material. A new synthetic gel that mimics this feature has rare, direction-dependent properties. See Letter p.68
Materials design focuses overwhelmingly on attractive interactions, as in advanced polymer-based composites that exploit interactions between inorganic fillers and a polymer matrix. Electrostatic repulsion can be also harnessed to good effect, as demonstrated by the articular cartilage that covers the ends of bones where they form joints, providing near-frictionless mechanical motion. Taking this cartilage as the model, Mingjie Liu and colleagues have developed a hydrogel with mechanical properties dominated by repulsion between negatively charged titanate nanosheets embedded within it. The material deforms easily when sheared parallel to the sheets but resists compressive forces applied orthogonally. Composites of this type should open new possibilities for developing soft materials with unusual functions.