An electronic skin (e-skin) that simultaneously senses pressure and strain has been fabricated using a PVC-free pencil eraser that has been sliced into a thin layer and deposited with multi-walled carbon nanotubes on either side. The carbon nanotubes are the main sensing elements of the device. The low-cost, easily scalable sensor fabricated by a two-member team led by Prof. Sushmee Badhulika at the Indian Institute of Technology (IIT) Hyderabad may find applications in flexible electronics and medical diagnostics.
“The device can be used for human motion monitoring in the case of elderly and infants. The e-skin is sensitive… it can sense even a gentle touch,” says Prof. Badhulika from the Department of Electrical Engineering at IIT Hyderabad. The results were published in the journal Nanotechnology.
“The signal from the device can be interfaced with a microcontroller and the signal can be taken from the sensor to a smartphone using wireless technology,” she says.
The researchers tested the sensor for potential application as an e-skin by integrating it on the forefinger joint, wrist, neck and elbow and subjected it to stretching and compression. The sensor showed good sensitivity to both stretching and compression, and showed repeatability. The sensor was tested for both soft and hard touch where pressure was applied by a human hand. “The sensor showed repeatability for distinct pressure touches of the human hand,” she says.
Making of the sensor
The PVC-free pencil eraser is sliced and cut to optimal dimensions and optimised amount of multi-walled carbon nanotubes are pressed on the layer using a rolling pin and a pre-compaction mechanical press. The rolling pin is used to get a uniform thin film with carbon nanotubes deposited on the film to get a desired initial resistance. The film is further pressed using a mechanical press.
“Depositing the multi-walled carbon nanotubes using a solvent-free rolling pin method is done manually on the hydrophobic eraser substrate. So there could be variation from user to user.
The mechanical press is used for standardising the pressure applied for depositing the carbon nanotubes and achieve repeatability,” says Parikshit Sahatiya from the Department of Electrical Engineering at IIT Hyderabad and first author of the paper.
Both the sides of the eraser are patterned with carbon nanotubes so that the film together with the carbon nanotubes acts as a capacitor. Silver paste is applied to make a metal contact to draw the signal from the device.
When pressure is applied the thickness of the film decreases and the distance between the two carbon nanotube layers that behave like metal plates gets reduced. This increases the capacitance.
When the e-skin is stretched the distance between carbon nanotubes increases and therefore the resistance increases. But when the e-skin is compressed the distance between carbon nanotubes decreases and so the resistance reduces. “Basically the e-skin acts as a resistance sensor when subjected to strain and capacitive sensor when subjected to pressure,” says Prof. Badhulika.