E-whiskers could turn robots hairy
Even the faintest optical and tactile feedback sources gathered by the most humble insects will efficiently coordinate their moves at striking speed, without relying on a lot of processing, at least much less than what engineers tend to pack into your typical autonomous robot. That is because their locomotive action is often directly coupled to sensory feedback, with no such thing as central processing or environment data analytics from multiple data sources.
A lot of insects and certain mammals have whiskers, in effect hair-like tactile sensors that help them monitor wind and navigate around obstacles in tight spaces. This was a new source of inspiration for researchers at Berkeley Lab who have recently demonstrated electronic whiskers (e-whiskers) based on a mixture of carbon nanotubes (CNT) and silver nanoparticles coated on flexible and high aspect-ratio polymer fibres.
Close-up of the fully fabricated e-whisker array.
"In our tests, these whiskers were 10 times more sensitive to pressure than all previously reported capacitive or resistive pressure sensors" stated lead researcher Ali Javey of Berkeley Lab’s Materials Sciences Division in a press release.
The results were published the Proceedings of the National Academy of Sciences, in a paper titled “Highly sensitive electronic whiskers based on patterned carbon nanotube and silver nanoparticle composite films”.
The feat doesn’t stop here, by changing the composition ratio of the nanoparticles and the nanotubes, the researchers could observe a minimal resistance change from around 10 % up to a maximum resistance change of around 260% when a 2.4 % strained was applied.
“To monitor the resistivity change, the e-whisker arrays were connected to a computer as a proof-of-concept. The tuneable range we checked was resistivity, in the order of 100x, and the sensitivity in the order of 26x” clarified Kuniharu Takei, Assistant professor in the Department of Physics and Electronics of the Osaka Prefecture University and lead author of the paper.
Whilst the nanotubes form a conductive network matrix with excellent bendability, the silver nanoparticles enhance the conductivity of the coated fibres and give them high mechanical strain sensitivity, responding to pressures as low as 1Pa with high sensitivity (around 8%).
Top: An e-whisker with 5 and 30 weight% of AgNP composite lines patterned on the top and bottom surfaces of the PDMS fibre.
Bottom: Circuit diagrams of the e-whiskers for resistivity sensing under strain.
The strain sensitivity is enhanced by increasing the weight content of the silver nanoparticles (AgNPs), explains the paper, as the distance change between the AgNPs in the CNT–AgNP composite film directly affects the electron tunnelling probability through neighbouring conductive nanoparticles. What’s more, compressive and tensile stresses induce smaller and larger gaps between AgNPs, respectively, compared with the relaxed state, which makes the e-whisker able to detect the direction of bending.
The e-whiskers are built by first imprinting a polymer substrate (polydimethylsiloxane) into a high aspect ratio fibre (this could be done by printing or painting, but in the lab it was patterned using a micro-etched silicon mould with trenches 15mm long, 250xμm wide and 250μm deep), then coating the fibre with the CNT–AgNP composite and curing it.
Although the researchers used a polymer substrate for the whisker structure, they could shrink them down using MEMS processes if necessary.
When asked if this research would lead to a form of hairy skin implementation for robotic applications, Takei told us “We are planning to demonstrate more application in near future. In this paper, we demonstrated a weak wind flow three-dimensional mapping as the first proof-of-concept”.
An array of seven vertically placed e-whiskers for wind mapping in 3D
“The results exhibited that the e-whisker array can detect a very weak wind flow (around 1m/s) that is like the wind you can generate by shaking your hand. We are not sure the human skin sensitivity, but we believe that the e-whisker has similar sensitivity to the human skin (hair)”.
Such e-whiskers could be used to offer immediate tactile sensing for the spatial mapping of nearby objects, they could also lead to wearable sensors for measuring heartbeat and pulse rate.
This research was supported by the Defense Advanced Research Projects Agency.
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