Delicate balance in biological fibre networks causes unusual elastic behaviour
Theoretical physicists from the FOM Foundation and the VU University Amsterdam have, in collaboration with colleagues from the University of Pennsylvania (US), discovered an unusual elastic phenomenon in fibre networks. Three-dimensional fibre networks appear to have hardly any mechanical stability. However, this lack of stability does give rise to unusual elastic properties. Living cells might use this elastic behaviour to regulate their mechanical properties. Furthermore, this mechanism offers new design possibilities for soft materials with unusual elastic characteristics. The results were published online on 30 October 2011 in the leading journal Nature Physics.
Unusual elasticity
The cause of this unusual elasticity is a delicate mechanical balance in these chaotic networks. A fibre network contains too few connections to form a sufficiently rigid structure. The collective behaviour of the interconnected fibres determines the properties of the network in a comparable manner to critical phase transitions, for example between liquid water and water vapour. Consequently the mechanics of the network is not simply the sum of the parts but a far richer combination of both fibre and network properties. That results in an unusual elasticity.
Biological fibre networks
Living cells largely gain their mechanical properties from a chaotic fibre network: the cytoskeleton. By making use of different types of protein, the cell determines the architecture of the network. Experiments have demonstrated that this type of biological fibre network has unusual properties, which possibly play a major role in the biological function. For example, the mechanics of the network is highly sensitive for the number of connections in the network and the mechanical tension in the fibres. Exactly how these elastic properties of the network arise, however, is still a big mystery. Even in a small space such as a cell, the network consists of an entwined network of countless fibres and connections. Up until now it was very difficult to precisely calculate the mechanics of such a complex three-dimensional structure, even with the help of powerful computers.
Simple trick inspired by crystals
To obtain new insights into the unusual elastic properties of chaotic fibre networks, the researchers had to make the existing computer model easier to use without losing the essence of the problem. Inspired by models of crystals, they introduced a new model in which the fibre network was initially presented as a regular crystal structure. Then by randomly removing connections from this crystalline network, the researchers realised a chaotic system. This model proved to be a great success. Besides being able to simply and quickly model the elastic properties of large three-dimensional networks with a computer, the researchers also formulated a mathematical description of the network’s elasticity. The results of this model eventually led to a better understanding of the underlying mechanism that determines the mechanics of the network. This insight can serve as a source of inspiration for the development of new synthetic soft materials with unusual elastic properties.
This research emerged from the FOM programme 'Material properties of biological assemblies'. The first author Dr Chase Broedersz presented this research last January during Physics@FOM Veldhoven 2011. Broedersz gained his doctorate this year cum laude from the VU University Amsterdam. Subsequently he was a postdoc at FOM and he has recently received a prestigious research grant from Princeton University (US) where he will do research as a Lewis-Sigler Fellow over the next five years. This research is jointly financed by the FOM Foundation, NWO and the National Science Foundation in the United States.
Reference
Criticality and isostaticity in fiber networks, Chase P. Broedersz, Xiaoming Mao, Tom C. Lubensky and Frederick C. MacKintosh, Nature Physics, 30 October 2011, DOI 10.1038/NPHYS2127.
The article is further discussed in the 'News and views' section of Nature: http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2146.html
Contact
Chase Broedersz, +1 609 258 7194
Fred MacKintosh, +31 (0)20 598 7857