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Human touch extends down to the nano-level

Feeling groovy The human finger is so sensitive it can detect bumps as small as a few nanometres in height, according to a new study.

The finding, publish today in the Nature journal Scientific Reports, could have applications in fields as diverse as personal electronics and robotics, through hair products and tissues.

Lead researcher Professor Mark Rutland, a surface chemist at the KTH Royal Institute of Technology in Stockholm, Sweden, says until recently touch was a relatively unknown sense.

"What you're capable of sensing and how you use your finger to sense a surface, up until recently has been a little bit of black art," says Rutland, who grew up in Adelaide and completed his PhD at the Australian National University.

Previous studies have shown that there are three main 'dimensions' used to describe touch - rough-smooth, hard-soft and sticky-slippery.

"There are other stimuli like heat, cold, wetness, but we've excluded them just to be able to focus on the topographical stuff," says Rutland.

Rutland's team, which included material scientists and psychologists, developed a test to measure how sensitive the human finger is at differentiating between rough and smooth surfaces.

They constructed a set of 16 polymer surfaces that had a series of parallel ridges across its surface. The distance between each peak - known as wavelength - ranged from 300 nanometres up to 90 micrometres, and the height of the peaks were from 7 nanometres up to 4.5 micrometres. A blank surface with no ridges was also used.

Twenty volunteers were then blindfolded and presented with two surfaces at a time and asked to run their index finger over each one. They then rated how similar, or dissimilar, each pair was.

To Rutland's surprise, the results showed that the group was able to sense wrinkles at the nano-scale.

"The participants could distinguish a surface which had a 13-nanometre average amplitude from a smooth surface," he says. "I was surprised and very very excited."

Consumer applications

According to Rutland, understanding touch, and the response it elicits, could be used by manufacturers to attract consumers to their products.

"If you pick up a product in a shop, you [the manufacturer] want to be able to give something in addition to it looking nice or smelling nice. You want it to be able to feel a certain way and control that reaction on a multi-sensory level," he says.

Rutland says that while the consumer electronics industry is most interested in being able to understand and control tactile surfaces, there are other that could benefit.

"The shampoo industry would also be interested in this, because the way your hair feels is one of the things that consumer product testers find is important," he adds.

"In fact, anything that a consumer would touch before buying, like a paper surface, would all be candidates for this tactile control."