Designing sensors that mimic those in human skin could lead to more realistic prosthetic limbs that can feel a light touch or the warm sun. But no flexible electronic skin so far has included the small feelers that make mammals unique: hair. Now, by placing microscopic polymer hairs on top of graphene, researchers have made a skinlike sensing device that can feel wind and detect its direction and angle (ACS Appl. Mater. Interfaces 2019, DOI: 10.1021/acsami.9b01427). The technology, they say, could lead to wearable sensors that are more sensitive and responsive and to new kinds of soft or winged robots.
Electronic skin has typically been made of an array of sensors embedded in or printed on a rubbery polymer. Recent innovations include e-skin that can monitor vital signs or can heal itself. Yet most research so far has focused on mimicking skin’s essential ability to sense touch and temperature, which would allow a robotic arm to pick up a cup or know if it’s hot.
Hair enhances the skin’s ability to sense pressure, especially light pressure such as a landing bug or a gentle breeze. Scientists have recently found that bats use hair on their wings to detect airflow and change flight direction in a split second. Artificial skin with hair would more closely mimic the real thing, says Changhyun Pang, a chemical engineer at Sungkyunkwan University. Others have attempted hairy sensors before, but they were complicated to make and mounted on rigid substrates. Such strategies won’t work for soft e-skins or flexible wearable electronics, he says.
Pang and his colleagues made a 4-by-4 array of sensors by spraying a suspension of graphene nanoflakes through a stencil onto a piece of flexible polyethylene. Each sensor is a 4 by 4 mm patch of graphene nanoflakes only 15 µm thick. Applying pressure on the sensors pushes the nanoflakes together, changing the electrical resistance. Using a different stencil, the researchers then top the sensor area with thicker 170 µm graphene nanoflake films that are highly conductive and form a pair of electrodes.
Next comes the hair. The researchers molded a thin poly(dimethylsiloxane) film covered with a forest of microscopic pillars. They placed this on top of one half of the sensor array so that one half is smooth while the other is hairy.
To test the device, the researchers blew air on it and measured the electrical current from each sensor in the array. The breeze bent the microhairs and put pressure on the graphene sensors, changing their output current. Stronger drafts generated higher current. Researchers calculated the flow’s direction by looking at a map of the sensors’ output; pillars directly facing the wind bent more. To measure the angle of airflow, the researchers measured the difference in the current from the smooth side, which monitors downward force, and the hairy side, which measures force across the array. The hairy sensors can detect an airflow pressure of 0.2 kilopascal—a gentle pressure that our skin can feel. Although previous e-skins have been able to sense pressures below a kilopascal, those devices cannot trace the flow direction and angle.
Finally, as a practical demonstration, the team attached the hairy sensor to a wheeled robot powered by a sail. The sensor detected airflow and rotated the sail, adjusting it to the wind direction so the robot could move forward.
Researchers have made whiskers that can detect surface texture before, says Darren J. Lipomi, a nanoengineer at the University of California, San Diego. But this work is novel because it uses rubbery micropillars to relay the mechanical forces from wind to the graphene sensors in the device, he says, thus mimicking the hair follicle receptors in human skin. Given the simple printing and molding techniques, it should be straightforward to produce practical sensors on a large area, he adds.