Tuesday, August 3, 2010

Spiderman suit based on nanotechnology

Imagine owning your own Spiderman suit, complete with gloves and boots to allow you to stick to walls, and even a sticky silk spinner to swing between buildings. That might be a step closer to reality, thanks to Nicola Pugno at the Polytechnic University of Turin in Italy, who has come up with a scheme for an adhesive material and “spider silk” based on carbon nanotubes.

Efforts to develop surface gripping materials have focused on mimicking geckos, which can hang upside down from just one toe because their feet are covered with millions of tiny elastic hairs called setae. Each seta is attracted to the wall, largely by an intermolecular force called the van der Waals force, allowing the gecko’s feet to adhere.

Researchers have previously created nanotechnology structures – carbon nanotubes – that mimic setae, but though these have around 200 times the adhesive force of gecko feet, they have yet to be scaled up to a size fit for superheroes. That’s because if you simply make the nanotubes longer, they become floppy and stick to each other rather than to the wall. On the other hand, if you make them fatter and stiffer, they become too inflexible to ensure a large enough contact area with the wall.

Now, in a paper to be published in Journal of Physics: Condensed Matter, Pugno suggests that the secret to developing an effective sticky material lies in creating a “hierarchical structure” – branching bristles of ever finer nanotubes, just as the setae of a gecko’s feet are divided at their ends into smaller branches called spatulae. Pugno calculates that this approach could result in stiff, non-tangling structures with tips still flexible enough to produce good adhesion.

Researchers at Rensselaer Polytechnic Institute in Troy, New York, have previously built similar branched nanotube structures by growing them inside aluminium oxide templates.

That just leaves the problem of swinging between buildings on Spiderman-style silk. Researchers can already spin metre-long carbon nanotube fibres (Strong, Transparent, Multifunctional, Carbon Nanotube Sheets), and much longer ones should be possible, so Pugno proposes making a cable consisting of around 4 million nanotube fibres.

Each nanotube is invisible, as they are much thinner than the wavelength of light. It is suggested that suggests that by anchoring the nanotubes through holes in a 1-centimetre-square spacer plate to keep the fibres 5 micrometres apart, the whole cable would remain invisible. The end of each fibre passing through the plate could be branched to create the seta structure, allowing it to stick to the target surface. Then all you would need to do is fire the cable from some kind of launcher device.

Stefano Mezzasalma at the University of Trieste in Italy says the approach could work. “The first prototype of a Spiderman suit might be ready in a decade or so.”

Microparticles Can Be Captured

To trap and hold tiny microparticles, engineers at Harvard have "put a ring on it," using a silicon-based circular resonator to confine particles stably for up to several minutes.

“We demonstrated the power of what we call resonant cavity trapping, where a particle is guided along a small waveguide and then pulled onto a micro-ring resonator," explains Kenneth Crozier, an Associate Professor of Electrical Engineering at the Harvard School of Engineering and Applied Sciences (SEAS) who directed the research. "Once on the ring, optical forces prevent it from escaping, and cause it to revolve around it."

The process looks similar to what you see in liquid motion toys, where tiny beads of colored drops run along plastic tracks -- but on much smaller scale and with different physical mechanisms. The rings have radii of a mere 5 to 10 micrometers and are built using electron beam lithography and reactive ion etching.

Specifically, laser light is focused into a waveguide. Optical forces cause a particle to be drawn down toward the waveguide, and pushed along it. When the particle approaches a ring fabricated close to the waveguide, it is pulled from the waveguide to the ring by optical forces. The particle then circulates around the ring, propelled by optical forces at velocities of several hundred micrometers-per-second.

While using planar ring resonators to trap particles is not new, Crozier and his colleagues offered a new and more thorough analysis of the technique. In particular, they showed that using the silicon ring results in optical force enhancement (5 to 8 times versus the straight waveguide).

"Excitingly, particle-tracking measurements with a high speed camera reveal that the large transverse forces stably localize the particle so that the standard deviation in its trajectory, compared to a circle, is as small as 50 nm," says Crozier. "This represents a very tight localization over a comparatively large distance."

The ultimate aim is to develop and demonstrate fully all-optical on chip manipulation that offers a way to guide, store, and deliver both biological and artificial particles.