A Low-Cost Multisensor

A flexible temperature array made by drawing a resistor structure with a silver conductive ink pen on Post-it paper. (Image courtesy of KAUST.)

What can you do with aluminum foil, sticky notes, sponges and tape?

A team of electrical engineers used these and other low-cost materials to create a sensor called “paper skin” that is capable of detecting external stimuli including touch, pressure, temperature, acidity and humidity.

MacGyver-Style Engineering

Imagine this: using sticky note paper to detect humidity, sponges and wipes to detect pressure and aluminum foil to detect motion. All it takes is a little engineering ingenuity.

By coloring a sticky note with an HB pencil, the team created a simple way to detect acidity levels. Combining aluminum foil and conductive silver ink created a way to detect temperature differences.

The researchers combined these materials in a simple paper-based platform which was then connected to a device that detects of changes in electrical conductivity according to external stimuli.

a) Schematic of temperature sensors using silver ink pen and aluminum foil. b) Capacitive design of humidity sensor using post-it paper as sensing material. c) Representation of a capacitive-based disposable pH sensor. d) “Design 1” of pressure sensors using a parallel-plate structure and two different sensing materials: microfiber wipe and a sponge. e) Schematic of “Design 2” of pressure sensor based on air-gap structure. (Image courtesy of Advanced Materials Technologies.)

Increasing humidity levels, for example, increased the platform's capacitance. Exposing the sensor to an acidic solution increased its resistance, while exposing it to an alkaline solution decreased it.

Other tests include changing temperatures and observing the resulting change in voltage, as well as disturbances to the electromagnetic field when a finger approached the platform.

Matching current artificial skin applications in performance, the sensor is an attractive alternative to those which require more expensive materials to produce.

"Previous efforts in this direction used sophisticated materials or processes," said Muhammad Mustafa Hussain, associate professor of electrical engineering at King Abdullah University of Science and Technology (KAUST).

“Chemical 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,” Hussain added.

Is the paper skin sensor commercially viable?

The team certainly used the properties of their low cost materials to full effect, including their porosity, adsorption, elasticity and dimensions. If nothing else, the success of their initial tests shows that a single integrated platform can sense several stimuli simultaneously in real time.

However, they will need to develop wireless interaction with the paper skin sensor as well as conduct reliability testing under extreme bending to determine whether the sensor is commercially viable.

The most prominent applications for these particular sensors are in medicine for real-time assessment of patients. Other applications include robotics, transportation, and environmental monitoring in processing plants.

"The next stage will be to optimize 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 said.

For more information, check out the article “Paper Skin Multisensory Platform for Simultaneous Environmental Monitoring” in Advanced Materials Technologies.