Surface instabilities on curved film–substrate structures in nature and engineering applications. (a) Instabilities on flat structure (from left to right: crease, ridge, double, and wrinkle); (b) multimodal instabilities on concave structures (from left to right: cross section of colon, ductus deferens, muscular artery, and bronchus); and (c) multimodal instabilities on convex structures (from left to right: cross section of bitter melon, cactus, tree stump, and cereus).