The eight most common biological structural design elements (from upper left to lower right): Fibrous: high tensile strength in one direction, minimal compressive strength; Helical: toughness in multiple directions, in-plane isotropy; Gradient: gradual property transition to reduce interfacial stress, enhancing toughness; Layered: complex composites that improve toughness, especially in brittle materials; Cellular: lightweight porous structures for stress distribution and energy absorption, often in sandwich forms; Tubular: organized porosity for energy absorption and crack deflection; Overlapping: layered plates or scutes for flexible, often armored surfaces. In the center, a 3D printer is shown schematically; Suture: interdigitating interfaces for controlled strength and flexibility (figure inspired by: Naleway et al. 2015). Figure by Konrad Eyferth.