Using household items to make a multi-sensory 'Paper Skin'
(Nanowerk Spotlight) Efforts in artificial skin development have been inspired by human skin which can simultaneously feel touch, pressure, temperature, humidity, strain and flow. The objective is to mimic the sensations and multi-functionality of human skin for medical technologies aiming to help burn and acid victims, but also advancing applications in robotics and vehicular technology (read our Nanowerk primer on electronic skin).
"To date, all demonstrations on artificial skin use sophisticated nanomaterials or processes, such as for instance carbon nanotubes, nanowires, and silver nanoparticles," Muhammad Mustafa Hussain, an Associate Professor of Electrical Engineering at King Abdullah University of Science and Technology (KAUST), tells Nanowerk. "Although advancements in the field of paper electronics are rapidly growing due to the low-cost and recyclability of paper, they still rely on chemical functionalization, use expensive vacuum technology, and show limited functionalities. Thus, performance and/or functionalities per cost has been limited."
They demonstrate a scalable fabrication approach using off-the-shelf household items such as aluminum foil, scotch tapes, sticky-notes, napkins and sponges to build 'Paper Skin'.
Paper Skin promises to be an affordable all-in-one flexibel sensing platform, applicable in health monitoring, 3D touchscreens, and human-machine interfaces, where sensing diversity, surface adaptability, and large-area mapping all are essential.
(a) Digital photograph of flexible 6 × 6 'Paper Skin' wrapped around an arm. (b) Schematic of 3D stacked paper skin structure composed of pressure, temperature and humidity arrays. (c) Digital photograph of flexible temperature sensors array. (d) High resolution photograph of the cross section of the pressure sensor, showing the microfiber wipe sandwiched in aluminum foil with an airgap cavity. (e) Zoom in picture of the air-gap assembly. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
"Our Paper Skin is a 3D stacked flexible platform capable of simultaneous and real-time monitoring of external stimuli such as pressure, touch, temperature, humidity, proximity, pH and flow," explains Joanna M Nassar, a PhD student in Hussain's group and first author of the paper. "Paper Skin is the first ever recyclable, eco-friendly and a fully functioning distributed sensor network platform for real-time environmental monitoring. Competitive study shows its effectiveness from functionality/cost perspective. Another important social aspect is it opens up new opportunity for using household elements to build useful electronics by commoners to give rise of citizen science."
According to the team, this device is based on out-of-box thinking and applications using simple and accessible materials. The design and fabrication process leverages the basic principles of porosity, adsorption, elasticity, and dimensions of off-the-shelf inexpensive materials, promoting high sensitivities and fast response for improved sensors applications.
"This a huge step towards the democratization of electronics," says Hussain. "Also, our work is the first ever artificial skin platform that integrates the most sensory functionalities available in human skin, demonstrating real-time and simultaneous sensing of various environmental stimuli – pressure, touch, proximity, temperature, humidity, flow, and PH – using one singular platform."
In their work, the researchers investigated the electrical properties and mechanical structures of cellulose paper, synthetic sponge (made from foamed polyester PES), adhesive tape, HB pencil, polypropylene wipe (PP), and aluminum foil, to demonstrate their effectiveness and advantageous properties for improved sensors performance.
Their findings were supported by experimental data using electrical characterization tools for determining capacitance, resistance, conductivity, and dielectric of the corresponding materials. They also used high quality scanning electron microcopy imaging (SEM) to inspect the density and porosity of the materials to assess their topographical qualities.
"We found that the intrinsic porosity of cellulose paper (they used Sticky Notes™) allowed a 10 times improved response and recovery times for humidity sensing, with respect to recent progresses in the field," notes Nassar. "This was supported with theoretical findings showing that adsorption and evaporation rates of water droplets are much faster in porous surfaces than in the commonly used flat polymeric films."
The scientists also found that the nanofibril structure of the polypropylene wipe promotes ultra-high sensitivity in the low-pressure regime, with detection levels as low as 9 Pa, allowing them to easily detect the heart pulse waveform from the tip of a finger upon a light touch on the surface of the sensor.
Finally, the researchers discovered that aluminum foil actually possesses paramagnetic properties, which further enhances the induced electromagnetic field around a capacitor, thereby improving the detection range in proximity sensing devices.
"Based on this, our pressure sensor structure revealed multi-functionality, with the ability to sense large forces/pressures, light touch, as well as conductive objects in proximity as far as 13 cm," Hussain summarizes these results. "This is a significant finding that will revolutionize touchless motion-based systems. The distinct responses received for pressure, touch and proximity allow for improved differentiation between multiple mechanical stimuli, enhancing user recognition for touchless control panel applications."
The next stage in the team's work 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 monitoring of body vitals in real-time, such as heart rate, blood pressure, breathing patterns, and movement. Also, reliability tests will be conducted to assess the platform’s lifetime and the sensor’s performance under severe bending conditions.
Hussain cautions that several challenges need to be overcome before a fully autonomous flexible and multifunctional sensory platform can be realized. "This will require the investigation of a compatible flexible integrated platform housing the driving and interfacing circuitry that will allow a wireless interaction with the Paper Skin platform."