E-skin with multiple sensors bring us closer to human-friendly soft robots

(Nanowerk Spotlight) Human skin is a sensitive detector of both pressure and temperature sets the performance benchmark for the development of artificial tactile systems. Researchers are working on the development of flexible and stretchable electronics and sensors that allow the fabrication of electronic skin (e-skin) that mimics human skin (read our primer on electronic skin).
E-skin technology will give prosthetics and soft robotics a finer sense of touch, of what's hard and soft, hot and cold, smooth and rough. For instance, when you stretch your arm, a network of nerves and sensors within the skin provides feedback to help you orient the arm in space and interact with its surroundings. Embedding a network of sensors into artificial e-skin would give soft robots similar sensory feedback. Among other applications, it could allow surgical robots to interact with their surroundings through touch.
Reporting their findings in Advanced Intelligent Systems ("Human-Like Electronic Skin-Integrated Soft Robotic Hand"), researchers in Japan have demonstrated a pneumatic balloon-based soft robotic hand integrated with four tactile force sensors and one temperature sensor to give it similar sensitivity to a human hand.
E-skin-integrated pneumatic soft robotic hand
E-skin-integrated pneumatic soft robotic hand. a) Schematic of the soft robotic hand structure with three layers: a pneumatic balloon layer, tactile force sensor layer, and temperature sensor layer. b) Photo of e-skin integrated with four tactile force sensors and a temperature sensor embedded in the soft robotic hand. Photos of c) opening the hand, d) closing the hand, e) holding a ball, and f) shaking hands with a human. g) Actuation bending angle as a function of applied air pressure. Inset shows the definition of the angle. h) Cycle test of the bending angle at an applied pressure of 50 kPa. i) Generated tactile force at different air pressures. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
The researchers fabricated a resistive temperature sensor comprised of conductive SnO2 nanoparticles and single-walled carbon nanotubes (SWCNTs). The tactile force sensors are based on a conductive SWCNT film layer overlaid with silver threads.
"For a human-friendly soft robot with functionalities similar to human skin, our study proposes a soft robotic hand integrated with an e-skin to monitor tactile pressure and temperature, without sacrificing the soft functionality of the robot," Professor Kuniharu Takei at Osaka Prefecture University, tells Nanowerk. "By integrating an e-skin, the tactile pressure to grab an object and the friction movement of an object from the hand can be monitored."
"There are two key challenges to build this platform," he adds. "The first is the integration of sensors in a high-strain region where bending occurs in a soft robot without sacrificing the actuation force. The other is to optimize the sensing sensitivity to match the actuation force, detectable threshold pressure, and temperature by touching an object."
In soft robotics, researchers usually use stretchable materials. However, bending and actuation operations affect the sensor output because the sensors' output changes as a function of stretching and bending. This makes it difficult to monitor tactile force and temperature precisely.
Takei's team addressed this challenge by arranging the materials and device structure in such a way that they are insensitive to the bending of the soft robot.
Thanks to the arrangement of multiple tactile sensors, this e-skin-integrated soft robotic hand also can monitor sliding of an object by detecting the time delay of the tactile force. This provides real-time feedback so that the robotic hand can adjust the actuation force to prevent dropping an object.
However, such a feedback system was not part of this demonstration. "We only demonstrated sensor integration embedded into a soft robotic hand," says Takei. "For practical applications, signal processing including artificial intelligence, power supply, and other functions to operate the sensors and robot together are required. Those are our next challenges for moving forward to realizing practical applications."
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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