MacGyver would be proud. By drawing a rectangle on a piece of paper with an ordinary pencil, researchers in China have made a strong, highly sensitive sensor that can measure how much it bends.
The idea could be used to customise wearable electronics, monitor the movement of robots and other machines, or trigger alarms when objects like doors or books are opened. And its creators say the process is cheap and simple enough to be useful in do-it-yourself projects and in research, or when resources are limited. It could also be scaled up to cut the cost of sensors for wearable technology.
The graphite rectangle drawn on the paper conducts electricity, but has quite a high resistance. As it bends, the resistance either increases or decreases, depending on which way it is distorted. By measuring that resistance, the precise angle of bend can be detected.
By gluing the paper to a finger, the researchers showed that it could monitor the movement of limbs. They also attached it to a ruler and showed that it could measure how bent the item was – a proxy for determining the pressure exerted on the ruler. And inside the fold of a book, it could measure the angle to which the book was opened.
They showed that the accuracy of the sensor is comparable to that of commercial sensors. It can also be used thousands of times without losing sensitivity and costs almost nothing, they say.
Yue Zhang from the University of Science and Technology Beijing says that the sensors are intended for use in wearable technology, which relies on other components that haven’t yet been developed. But in the shorter term, he thinks there are other uses. “We can foresee that alarm systems can be cheaper than existing ones,” he says. “We are making active efforts to find [a] related company to cooperate.”
Zhang’s work builds on earlier work by Jiaxing Huang and colleagues from Northwestern University in Evanston, Illinois, on similar sensors. “Pencil-drawn devices are cheap, versatile and fast to make. They could be useful in situations where resources are limited,” Huang says. “Pencil-drawing is a very convenient way to deposit materials on paper. Since it does not use any solvent, it bypasses many issues, such as toxicity of solvent, stability of ink, evaporation and spreading of ink, that one would need to consider in ‘wet’ printing techniques.”
Both Huang and Zhang say that the technique could be scaled up to produce these types of sensor in factories. But in the meantime they could be useful for DIY projects or education.
“People can use the [pencil-on-paper] technique to make devices such as wearable sensors, alarm devices or wearable rheostats which belong to them,” says Zhang.