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Versatile sensors made from low-cost, paper-based substrate

21 February 2016

Saudi Arabian researchers have discovered a way to make useful wearable sensors out of kitchen consumables that can detect pressure, temperature and humidity.

Paper Skin is a simple paper-based platform that detects changes in electrical conductivity according to external stimuli (photo: KAUST)

Everyday materials found in the kitchen, such as aluminium foil, sticky note paper, sponges and tape, have been used by a team of electrical engineers at King Abdullah University of Science and Technology (KAUST) to develop a low-cost sensor that can detect external stimuli, including touch, pressure, temperature, acidity and humidity. 

The sensor, which is called 'Paper Skin', performs as well as other artificial skin applications currently being developed while integrating multiple functions using cost-effective materials.

Wearable and flexible electronics show promise for a variety of applications, such as wireless monitoring of patient health and touch-free computer interfaces, but current research in this direction employs expensive and sophisticated materials and processes.

“Our work has the potential to revolutionise the electronics industry and opens the door to commercializing affordable high-performance sensing devices,” says Muhammad Mustafa Hussain, KAUST associate professor of electrical engineering at the University’s Integrated Nanotechnology Lab, where the research was conducted.

"Previous efforts in this direction used sophisticated materials or processes,” says Hussain. “Chemically functionalised inkjet printed or vacuum technology-processed papers — albeit cheap — have shown limited functionalities. Here we show a scalable ‘garage’ fabrication approach using off-the-shelf and inexpensive household elements.”

The team used sticky note paper to detect humidity, sponges and wipes to detect pressure and aluminium foil to detect motion. Colouring a sticky note with an HB pencil allowed the paper to detect acidity levels, and aluminium foil and conductive silver ink were used to detect temperature differences.

The materials were put together into a simple paper-based platform that was then connected to a device that detected changes in electrical conductivity according to external stimuli.

Increasing levels of humidity, for example, increased the platform’s ability to store an electrical charge, or its capacitance. Exposing the sensor to an acidic solution increased its resistance, while exposing it to an alkaline solution decreased it. Voltage changes were detected with temperature changes. Bringing a finger closer to the platform disturbed its electromagnetic field, decreasing its capacitance.

The team took advantage of the various properties of the materials they used, including their porosity, adsorption, elasticity and dimensions to develop the low-cost sensory platform. They also demonstrated that a single integrated platform could simultaneously detect multiple stimuli in real time.

Hussain says several challenges must be overcome before a fully autonomous, flexible and multifunctional sensory platform becomes commercially achievable. Wireless interaction with the paper skin needs to be developed, and reliability tests also need to be conducted to assess how long the sensor can last and how good its performance is under severe bending conditions.

“The next stage will be to optimise the sensor’s integration on this platform for applications in medical monitoring systems. The flexible and conformal sensory platform will enable simultaneous real-time monitoring of body vital signs, such as heart rate, blood pressure, breathing patterns and movement,” Hussain says.

“We may also transfer the achieved functionalities of the technology to biologically grown skin and develop mechanisms to connect it to neuronal networks in the human body to assist burn victims, for example. Other applications include robotics, vehicular technology and environmental surveys,” he adds.

An article describing this work was published on February 19, 2016 in the inaugural issue of Advanced Materials Technologies (Wiley-VCH Germany).

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