Taking a cue from nature, scientists have developed materials that can heal themselves thanks to a carefully constructed microvasculature network (Nat. Mater., DOI: 10.1038/nmat1934). These materials could be used, for example, in aerospace composites, where tiny fractures can cause big problems. Nancy R. Sottos, Kathleen S. Toohey, and colleagues at the University of Illinois, Urbana-Champaign, developed the system, which mimics the healing process of skin by using a network of channels in the substrate to deliver damage-repairing fluid into surface cracks and fractures. Chemically speaking, the material heals itself via ring-opening metathesis polymerization of dicyclopentadiene. The microvasculature delivers the monomer wherever a crack appears (shown). Grubbs' catalyst embedded in the material polymerizes the dicyclopentadiene, repairing any damage at the site. Besides repairing multiple fractures in the same material, the continuous supply of monomer also can repair the same crack over and over again—a feat that earlier versions of self-healing materials were unable to achieve.