e-whisker: how about a hairy robot?

No matter how quietly you approach a cat from behind, it is sure to detect your presence almost always. It is not its acute sense of hearing or smell that helps the cat, but its whiskers. They can sense the tiniest air turbulence caused by your movements. In fact, such tactile feedback gathered by most animals and insects with their whiskers and antennae makes it very efficient to coordinate their movements at striking speeds.

Such high-speed reflexes are possible because the feedback from the sensors is directly coupled to the insects’ locomotive actions and does not have to pass through much processing. Actually, there is absolutely no central processing or environmental data analytics to impede the information from multiple data sources.

Several insects and certain mammals use their antennae and whiskers – the hair-like tactile sensors – to monitor wind turbulences and for navigating around obstacles in tight spaces. Researchers at Berkeley Lab found this a new source of inspiration and they came up with e-whiskers or electronic whiskers. These e-whiskers are based on flexible polymer fibers with high aspect ratio, coated with a mixture of silver nano-particles and carbon nanotubes.

According to the lead researcher Ali Javed of the Materials Sciences Division of Berkley Lab, tests of these whiskers show they are ten times more sensitive compared to all previously reported resistive or capacitive pressure sensors. In addition, by changing the composition of the whiskers, researchers could manipulate their characteristics.

For example, a change the ratio of the nanoparticles and the nanotubes resulted in a change in resistance from a minimal 10% to around 260% with the application of a 2.4% strain on the whiskers. Scientists monitored the resistivity change by hooking up the e-whiskers arrays to a computer. The carbon nanotubes give the e-whiskers their excellent bendability with their conductive network matrix. On the other hand, the silver nanoparticles contribute to the conductivity of the coated fibers giving them the high mechanical strain sensitivity. That makes the e-whiskers so sensitive to pressures as low as 1Pa, representing 8%.

When scientists increased the weight content of the silver nanoparticles, the strain sensitivity of the e-whiskers was enhanced. This can be explained as the change in the distance between the silver nanoparticles in the film directly affecting the probability of electrons tunneling through neighboring conductive nanoparticles. As compressive and tensile stresses cause the gaps between the nanoparticles to become smaller and larger compared with the relaxed state, the e-whisker is able to detect the direction of bending.

Scientists at Berkley Lab built the e-whisker by patterning it with a micro-etched silicon mold with trenches which were 15mm long, 250µm wide, and 250µm deep. They then coated the fiber with the carbon nanotube and silver nanoparticle composite and cured it. The researchers claim that the whiskers can be made smaller still – they would have to use the MEMS processes for that.

With this e-whisker array of seven vertically placed fibers, scientists demonstrated mapping a weak wind flow (1m/s) in three dimensions as a proof-of-concept. More applications are planned for the future.