Understanding springs at the nanoscale: a step towards nanorobots

(Nanowerk Spotlight) Inspired by nature's ingenious biological designs, researchers have persistently attempted to mimic these biofunctionalities to bring technological breakthroughs. One of these morphologies – the unique shape of a helical coil – is not only interesting from a scientific standpoint but also pivotal, offering DNA its distinctive properties and propelling flagella in viscous fluids, to name a few.

Helically coiled springs are an integral element in many current mechanical systems. Nanotechnology researchers have proposed nanoscale helically coiled morphology for enabling elastic memory devices, flexible electronics, impact protection, nanoinductors, and efficient electromagnetic shielding (see for instance our previous Nanowerk Spotlight: "Nanosprings in action").

With the advent of personalized medicine on the horizon, researchers are now trying to use tiny springs made of carbon nanotubes, i.e. nanocoils, to propel nanorobots to perform microsurgeries.

A major challenge, however, is the lack of a detailed understanding of nanocoil mechanical properties. Indeed, the dynamic mechanical response of helically coiled structures is not fully understood due to the difficulties involved in exciting purely longitudinal/transverse resonances. This mathematical complexity has puzzled researchers for many years.

In a recent study, Prof. Apparao M. Raos group at Clemson Nanomaterials Center revisited this age-old problem and developed a three-pronged methodology which entails experimental, analytical and computational techniques.

Various resonances observed in a singly clamped helical carbon nanowire. Besides classical transverse modes (left), intriguing non-planar circular mode (center) and planar asymmetrical modes (right) were also detected. (Image: Rao Group, Clemson University)

The team reported their new findings in a recent online edition of Scientific Reports ("Mechanical Resonances of Helically Coiled Carbon Nanowires").

This study offers a better understanding of the shear and tensile contribution to the response when a helical coil is subjected to a transverse force.

The researchers, who had previously synthesized helically coiled carbon nanowires and nanotubes ("Rational Synthesis of Helically Coiled Carbon Nanowires and Nanotubes through the Use of Tin and Indium Catalysts"), isolated and clamped a single helically coiled nanowire akin to a diving board or cantilever. This nanosized cantilever was then electrically resonated and detected via a method invented at Clemson called the Harmonic Detection of Resonance (HDR) under a scanning electron microscope.

"Nanocoils are very special since they have multiple application ranging from simple shock absorbers to diverse nanorobotic tools," says Rao. As co-authors Herbert Behlow and Prof. Malcolm Skove note, "Our protocol can also be used as a non-destructive probe for determining the material properties of not only nanocoils but any helically coiled material, in general."

The team was successful in deriving a much needed closed form solution or a formula to predict transverse resonance frequencies of not just the first but also the second mode of any singly clamped helically coiled cantilever.

"The analytical solution was built upon the classical model for mechanical springs, and importantly, our protocol is applicable to any size of the coil," says Deepika Saini, the lead author of the paper. "Hence, it will prove beneficial across many engineering and research fields allowing more accurate designs and early prediction of mechanical failures."

In addition, the team also observed fascinating mechanical resonance modes – non planar as well as asymmetrical.