Two natural toughening strategies may inspire sustainable structures.

Contemporary designs of engineering structures strive to minimize the use of material in order to reduce cost and weight. However, the approach taken by focusing on materials selection and on the design of the exterior shape of structures has reached its limits. By contrast, nature implements bottom-up designs based on a multiple-level hierarchy, spanning from nanoscale to macroscale, which evolved over millions of years in an environmentally sustainable manner given limited resources. Natural structures often appear as laminates in wood, bone, plants, exoskeletons, etc., and employ elaborate micro-structural mechanisms to generate simultaneous strength and toughness. One such mechanism, observed in the scorpion cuticle and in the sponge spicule, is the grading (gradual change) of properties like layers thickness, stiffness, strength and toughness. We show that grading is a biological design tradeoff, which optimizes the use of material to enhance survival traits such as endurance against impending detrimental cracks. We found that such design, when applied in a more vulnerable direction of the laminate, has the potential to restrain propagation of hazardous cracks by deflecting or bifurcating them. This is achieved by shifting material from non-critical regions to more critical regions, making the design sustainable in the sense of efficient use of building resources. We investigate how such a mechanism functions in nature and how it can be implemented in synthetic structures, by means of a generic analytical model for crack deflection in a general laminate. Such a mechanical model may help optimize the design of bioinspired structures for specific applications and, eventually, reduce material waste.