Through millions of years of evolution, nature has produced biological systems with exceptional properties and functionalities. Many organisms have adapted to their particular environment by creating extraordinarily efficient materials and structures. These materials are optimised in terms of their mechanical, thermal, and optical properties in a way that sometimes even technology is still unable to reproduce.
These properties are often achieved by means of “hierarchical” structures, with characteristic lengths ranging simultaneously from the macro- to the nanoscale, hierarchical structures that are easily observed in materials such as wood, bones, spider webs or sea sponges. So far, the focus has mainly been on structures that nature has optimised from the point of view of the “quasi-static” mechanical properties such as, for example, fracture strength, toughness or adhesion, while there are far fewer studies on the dynamic properties such as vibration damping, noise absorption or sound transmission.
In particular, limited knowledge currently exists on how hierarchical structures play a role in the optimisation of natural structures. In the recent article “Optimized structures for vibration attenuation and sound control in nature: A review”, published in the journal Matter, researchers from the Politecnico di Torino Federico Bosia, Antonio Gliozzi and Mauro Tortello, together with colleagues from the Universities of Turin, Trento and the CNRS in Lille, collected and systematised some striking examples, existing in nature, of structural optimisation for wave and vibration control, highlighting some common traits and strategies in different biological systems.
The study will make it possible to “mimic” some of these structures, i.e. to adopt a bio-inspired approach, and to apply it to the design of acoustic metamaterials, i.e., innovative materials that have recently emerged to control sound waves.
Biological structures of interest from this point of view can be classified into three main categories: structures that are extremely resistant to impacts – such as the skull of the woodpecker, the “hammer-like club” of the mantis shrimp, or the structure of some sea shells; structures for perception and predation – spiders, scorpions, moths (one type of moth has evolved to form wings consisting of a natural metamaterial that makes them invisible to the sonar of bats), even elephants, each of which has developed an innovative strategy to generate and exploit vibrations of various frequencies; finally, structures for controlling, focusing and amplifying sound – for example the echo-localisation system of dolphins and the complex and exceptional structure found in mammals: the cochlea. Ultimately, it is often possible to find common traits in the various cases considered, such as heterogeneity of components, variable porosity, hierarchical organisation and efficient resonance mechanisms.
“This review work – comment Federico Bosia, Antonio Gliozzi and Mauro Tortello – helps to better understand the many systems that nature has optimised through millions of years of evolution. A better understanding of their functioning and common traits can help to develop materials that employ what nature has already optimised. This can be useful for a variety of applications involving the manipulation of acoustic or elastic waves, ranging, for example, from systems for protection against seismic waves to others that allow elastic wave energy to be ‘harvested’ at the microscale (energy harvesting)”.
This work was completed as part of the project “BOHEME: Bioinspired hierarchical Metamaterials”, funded by the European Commission (grant no. 863179).