"We discovered that the rigidity of viruses is very high," team leader Alexei Sokolov of the University of Akron, Ohio, told nanotechweb.org. "We expected that viruses should be soft, like all other biological entities, and at most be as hard as proteins."

Instead, Sokolov and colleagues found that the Wiseana iridovirus (WIV) that they studied was at least twice as hard as proteins and stiffer than hard plastics like PMMA and PS. They also discovered that viruses mechanically couple together, in contrast to polymeric colloids, which was a surprise. Strong coupling between neighbour virions might be essential for nanostructure formation, explains Sokolov.

The observations, which were made using Brillouin light scattering, are important because although the structures of many viruses are known, their biophysical properties have been largely unexplored. Understanding these physical characteristics will be crucial if they are to be used in nanotechnology and other applications.

It is possible that nature specifically designed viruses to be rigid so that they could survive Alexei Sokolov, University of Akron, Ohio

The researchers measured the characteristic vibration frequencies of the virus using inelastic light scattering spectroscopy and used these frequencies to estimate the rigidity of the viruses. The advantage of this technique is that it is direct, non-contact and non-destructive and has already been successfully employed to measure the rigidity of photonic crystals and polymeric nanostructures.

Strength as high as 7 GPa
The team was expecting Young's moduli for the viruses of around 1–3 GPa (which is about the value for solid, soft plastics), but were surprised to find a value as high as 7 GPa. "It is possible that nature specifically designed viruses to be rigid so that they could survive," said Sokolov.

According to the scientists, the rigidity of viruses comes thanks to their DNA core. Indeed, specific simulations confirmed that DNA is the main source of such a high rigidity.

Viruses are tiny particles, just tens of nanometres across, and each virus type has the same size, chemistry and physical properties. This makes them the perfect building blocks in various nanotechnology applications. Indeed many groups around the world are investigating viruses for building electronic and photonic devices.

Sokolov and co-workers reported their work in Physical Review E.