Legend has it that Isaac Newton’s insight into the natural world was inspired by an apple; Arindam Phani has taken his own inspiration from the insects that might have sought out that fruit.
“If you open a bottle of wine or a banana, fruit flies know you are eating something and they come and will follow you around when you move,” says Phani, a PhD student in the University of Alberta’s Department of Chemical and Materials Engineering.
What Phani finds remarkable about this behaviour is that it reflects sophisticated molecular sensing by creatures that do not seem to have the vast array of chemical receptors necessary to collect this information. Nor is it thought that they have the brain processing capability to make sense of it. Upon closer inspection, Phani concluded that the flies were probably employing an altogether different mechanism, one based on tiny hairs that cover the surface of their bodies. Detection is dependent upon how these delicate structures move through air containing molecules from wine or bananas. “This detection system is still poorly understood but we are beginning to appreciate that these hairs have a significant role while in motion in a fluid medium,” he says. Phani points to another fluid medium, the ocean, where corals that are similarly outfitted with tiny whiskers can sense the presence of a potentially threatening fish and move out of the way.
Phani’s interest in this subject began with a desire to create a nanoscale sensor technology for taking environmental readings. However, in order to achieve chemical sensitivity at that level, nanoscale devices typically need to operate under specific controlled conditions, such as a vacuum. Insects, on the other hand, appear to have no problem, doing much the same thing at standard atmospheric pressures and temperatures.
Taking inspiration from the sensing capabilities of insects at normal conditions, and based on the idea of behaviour of insect hairs, Phani designed experiments with a resonator having a forest of nanoscale hairs similar to that in insects. Along with his academic adviser Thomas Thundat, who holds the Canada Excellence Research Chair (CERC) in Oil Sands & Molecular Engineering, as well as Vakhtang Putkaradze, centennial professor of U of A’s Department of Mathematics and Statistical Sciences, Phani theoretically modelled the response, based on his experimental results, which showed a stark deviation from the age-old Stokes’ law.
The fundamental result of their study, recently published in Scientific Reports, shows an exponential change in the resonator’s damping as a function of the viscosity of the gas medium, in sharp contrast to a linear drag as predicted by Stokes law. According to the researchers, this deviation is forced by the presence of nanoscale features on the surface of the resonator. “Damping in nanomechanical resonators has traditionally been regarded as an impediment to sensitivity but we use it to our advantage,” the article reads. “We show that for oscillating surfaces modified with nanoscale hairs, dissipation offers a wealth of information on the nature of mechanical interactions of molecules with surfaces. We believe that in the future, the analysis of dynamics of the multitude of coupled nanostructures in complex gas mixtures may play an important role in purely mechanical, adsorption-free detection of chemicals.”
In other words, these resonators could operate as highly simplified spectrometers, taking measurements that can, in principle, reach to the parts per billion range. Phani, who had little background in biology before tackling this challenge, acknowledges that there is still much to be learned about how organisms successfully achieve and employ such sensitivity to find food or avoid danger. He suggests that the position of hairs, and different lengths of hairs, could be critical factors that would in turn drive the design of any synthetic resonator.
Phani also notes that the examples provided by these organisms should provide a significant head start in reaching such a practical design. “If nature has evolved to exploit the mechanical effects of nanohairs for millions of years,” he says, “so can we, in revolutionizing sensing by mimicking nature’s intricate designs.”