Graded cross-links for stronger nanomaterials

Cross-links are nowadays recognized to play a key role in the overall mechanical strength of buckypapers, nanotube or graphene based materials; material scientists or chemists are thus developing new nanomaterials with denser and stronger cross-links in order to maximize their mechanical strength. However, in spite of some fascinating achievements of material science and chemistry today, we are evidently far from an optimal result; the reported mechanical strength of a buckypaper, for example, is comparable to that of a classical sheet of paper. In this concept article we try to solve the paradox showing that the cross-link stiffness, a parameter still ignored in the literature, governs (more than its strength) the overall mechanical strength. New strategies for the experimentalists, e.g. the use of graded cross-links, are consequently suggested.