Paper has many well-known properties. However, very few people know that it is a perfect sensor material. Scientists from UC Davis developed a new mechanism for inexpensive paper with pressure sensors.
 

Paper as Sensor Material

Paper is a common and widely used material. In the printing world, the properties of different papers and finishing techniques are familiar and used to create diverse products.

Now paper has also been recognized as a sensor material. It‘s the obvious choice because it‘s readily available, cost-effective, insulating, flexible and portable. Thus, low-cost paper-based sensors can be developed as measuring devices that are also energy-saving and disposable. The measuring devices could be used in the fields of medicine and for the detection of explosives and toxic substances.
 

Additives are possible due to the fibrous structure of paper. These can be carbon materials (e.g. carbon nanotubes and graphene), conductive polymers or metallic nanocomposites. This results in completely new functionalities and sensor modalities. Examples are paper sensors for the detection of toxic substances in water, biosensors on bioactive paper but also gas sensors based on nanotechnology paper.
 

But not only paper-based measuring instruments are possible. It is also possible to produce pressure-sensitive papers. These pressure sensors used to be based on one of the three mechanisms:
 

• Resistive: The materials react to pressure.
• Capacitive: The materials require conductive input devices, such as a finger.
• Triboelectric: An electric charge is created between two materials by contact with each other and subsequent separation.
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A New Development

But Tingrui Pan, professor at UC Davis, introduced a new mechanism for paper-based pressure sensors: This is the new, flexible iontronic measurement (FITS). It uses pressure-induced capacitive changes between electrodes and ionic surfaces. In contrast to the other mechanisms of a pressure sensor, this has the advantage of extremely high sensitivity and resolution. Disturbances of any kind are largely negligible due to the extremely high signal-to-noise ratio.
 

Pan and his team present their innovation in their paper in Advanced Functional Materials (?All-in-One Iontronic Sensing Paper?). The printable paper developed by Pan fulfills all the characteristics of a normal paper: it can be printed on, cut individually and also the foldability does not differ. Another important point is that the costs for a paper with pressure sensors are similar to those of normal paper. This development enables pressure measurement platforms that can be easily printed, cut, glued and folded. A 3D pressure measurement origami for recording the spatial pressure is therefore possible without any problems.
 

Future Outlook for Sensory Structures

The future prospects lead to a large application field of this technology. Every surface can be transformed into a sensory structure. Some examples are: toys, packaging, disposable electronics, wallpaper and furniture. But also many other areas can be reached with this technology. If the concept is further developed, it could be a flexible human-machine interface. This would enable tactile communication with objects.
 

However, at the moment there are still connection problems between the paper electronics and the signal processing system. The possible solution to work with low temperature soldering and special paper FPC (flexible printed circuit) slot is still being investigated, but is most probably an efficient approach. In addition, research into sensor devices in the future will not only have to focus on sensor elements. Equally important are power supply, signal processing and electronic circuits.