Figure 1. Crack path profile and fractography within individual aragonite platelets.

(a), Schematic drawing showing sample geometry and experimental scheme under three-point bending loaded along [002]. (b), Fracture path observation on nacre cross section, at a glance, indeed demonstrates crack deflection along its biopolymer interlayer in a zigzag manner. (c), A close-up view reveals at least one of two aragonite platelets (blue arrowed) has been invaded by an advancing crack considering geometrically staggered arrangement. Subsequently, however, the advancing crack is not followed by successive cleavage, but defected to biopolymer interlayer (black arrow). (d), Zigzag crack path shows the inherent extension resistance arising from the interior of individual aragonite platelets (red arrowed). (e), AFM observation confirms the zigzag crack profile within an individual aragonite platelet. (f), A closer examination (from white boxed area in Fig. 1e) reveals that crack propagation path is along the boundaries of aragonite nanoparticles.