Nanostructures, such as carbon nanotubes, are often added to polymers and composites to enhance their strength. The extreme mechanical properties of carbon nanotubes suggest an obvious rationale behind this approach. However, as Markus Buehler and Isabelle Su at Massachusetts Institute of Technology in the US highlight in their recent topical review, the behaviour that renders nanomaterials soft or strong can be far from trivial, often involving interactions on a range of scales from macrostructures to nanostructures and – in the case of biostructures – the amino acids and proteins they are built from.
Bone is a classic example of excellent natural material engineering. It primarily consists of tropocollagen fibrils – which would be too soft to support the weight of the skeleton under its daily loads – and hydroxyapatite, a stiff but fragile material prone to fracture. However, the alliance of these two imperfect candidates is an extremely tough, lightweight and robust material.
Based on a simple molecular model of mineralized collagen fibrils, Buehler showed that, as might be expected, the stiffness of mineralized fibrils lies somewhere between the two extremes of the component materials, with as more recent studies reveal, the mineral components bearing up to four times the stress of the collagen fibrils. However, in addition his 2007 study pointed out that the mineralization increases the energy dissipation during deformation. As he explains in his report, “The fibrillar toughening mechanism increases the resistance to fracture by forming large local yield regions around crack-like defects, a mechanism that protects the integrity of the entire structure by allowing for localized failure.”
Nanomechanics – the whole is more than the sum of its parts, Anna Demming