As already indicated by the lower tendency to cracking during the indentation tests and the plastic behavior observed during bending, ultimately leading to a remarkable increase in the strain at failure of about 3 % (from typically 0.5 % for pure supercrystalline materials ), the nature-inspired design shows an improved resistivity to crack propagation, which is a key feature attributed to such nacre-like, hierarchical structures. The crack growth resistance of the xSP-NH 2 composites was therefore further analyzed using prenotched bending bars as described in the SI, Ch. 4.2. Stable crack growth was obtained due to an increase in the fracture toughness, K IC , denoted K R upon crack propagation, also referred as R-curve.