Flexible sensor key to 'electronic skin'

A team of scientists at the Technion-Israel Institute of Technology has found out how to make a certain kind of flexible sensor that could be integrated into electronic skin or e-skin. If scientists learn how to attach e-skin to prosthetic limbs, people with amputations might once again be able to feel changes in their environments. According to them, the invention can simultaneously sense touch, humidity and temperature, as real skin can do.

Researchers have long been interested in flexible sensors, but have had trouble adapting them for real-world use. To make its way into mainstream society, a flexible sensor would have to run on low voltage (so it would be compatible with the batteries in today's portable devices), measure a wide range of pressures, and make more than one measurement at a time, including humidity, temperature, pressure and the presence of chemicals. In addition, these sensors would also have to be able to be made quickly, easily, and cheaply.

The Technion team's sensor has all of these qualities. The secret is the use of monolayer-capped nanoparticles that are only 5-8nm in diameter. They are made of gold and surrounded by connector molecules called ligands. In fact, "monolayer-capped nanoparticles can be thought of as flowers, where the centre of the flower is the gold or metal nanoparticle and the petals are the monolayer of organic ligands that generally protect it," said research team leader Hossam Haick. Additionally, the new system "is at least 10 times more sensitive in touch than the currently existing touch-based e-skin systems."

The team discovered that when these nanoparticles are laid on top of a substrate, in this case, made of flexible polyethylene terephthalate (PET), the same plastic found in soda bottles, the resulting compound conducted electricity differently depending on how the substrate was bent. (The bending motion brings some particles closer to others, increasing how quickly electrons can pass between them.) This electrical property means that the sensor can detect a large range of pressures, from tens of milligrams to tens of grams. "The sensor is very stable and can be attached to any surface shape while keeping the function stable," stated Nir Peled, head of the thoracic cancer research and detection centre at Israel's Sheba Medical Centre, who was not involved in the research.

And by varying how thick the substrate is, as well as what it is made of, scientists can modify how sensitive the sensor is. Because these sensors can be customised, they could in the future perform a variety of other tasks, including monitoring strain on bridges and detecting cracks in engines.

"Indeed," noted Peled, "the development of the artificial skin as biosensor by Haick and his team is another breakthrough that puts nanotechnology at the front of the diagnostic era."