Electronic skin: Physicist develops multisensory hybrid material

Anna Maria Coclite's "smart skin" is very similar to human skin. It produces electronic signals by simultaneously sensing pressure, humidity, and temperature. As a result, more sensitive robots or intelligent prosthetics are possible.
The skin is the human being's greatest sense organ as well as his or her protective coat. It "feels" numerous sensory inputs at once and sends information to the brain regarding humidity, temperature, and pressure. 

For Anna Maria Coclite, a material with such multisensory properties is "a kind of 'holy grail' in the technology of intelligent artificial materials. In particular, robotics and smart prosthetics would benefit from a better integrated, more precise sensing system similar to human skin."

Using a revolutionary technique, an ERC grant winner and researcher from TU Graz's Institute of Solid State Physics has developed a three-in-one hybrid material "smart skin" for the next generation of artificial, electronic skin. The findings of this groundbreaking study were just published in the journal Advanced Materials Technologies.

As delicate as a fingertip

As part of Coclite's ERC project Smart Core, the team spent over six years developing smart skin. The composite material is more sensitive than a human fingertip, with 2,000 unique sensors per square millimetre. Each of these sensors is made up of a unique combination of components, including an inner smart polymer in the form of a hydrogel and a piezoelectric zinc oxide shell.

As explained by Coclite: "The hydrogel can absorb water and thus expands upon changes in humidity and temperature. In doing so, it exerts pressure on the piezoelectric zinc oxide, which responds to this and all other mechanical stresses with an electrical signal."

The result is a wafer-thin material that reacts to force, wetness, and temperature simultaneously with extraordinarily high spatial resolution and emits electronic signals in response. "The initial artificial skin samples are six micrometres (0.006 millimetres) in thickness. It may, however, be even thinner "Anna Maria Coclite agrees. The human epidermis, by comparison, is 0.03 to 2 mm thick. Things smaller than one square millimetre are perceived by the human skin. The smart skin has a resolution a thousand times smaller than human skin and can detect items that are too small for human skin to register (such as microorganisms).

Material processing at the nanoscale

The individual sensor layers are extremely thin while also include sensor elements that cover the entire surface. For the first time, the researchers integrated three recognized physical chemistry methods: chemical vapour deposition for the hydrogel material, atomic layer deposition for the zinc oxide, and nanoprint lithography for the polymer template. The research group "Hybrid electronics and structuring," led by Barbara Stadlober, was in charge of the lithographic preparation of the polymer template. The group is part of the Materials Institute of Joanneum Research in Weiz.

The skin-like hybrid material is now being used in a variety of fields. In healthcare, for example, the sensor material might identify germs independently and report them. Prosthetics that provide information regarding temperature or humidity, as well as robots that can detect their environment more sensitively, are also possibilities. Smart skin has a significant advantage on the way to application: the sensing nanorods — the material's "smart core" — are manufactured utilizing a vapor-based manufacturing technique. This procedure is already well established in integrated circuit manufacturing operations, for example. As a result, smart skin production may be readily scaled and integrated into current production processes.

Smart skin's properties are presently being improved much more. Anna Maria Coclite and her team, led by PhD student Taher Abu Ali, hope to expand the temperature range over which the material reacts and improve the artificial skin's flexibility.
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