Be Careful What you Say...Your Walls Could be Listening!

Princeton researchers develop a way to
 embed ultra-thin radios  to plastic sheets. 

Eavesdropping just took on new heights with the creation of walls that can listen and even speak. 

Researchers at Princeton University have developed a way to embed ultra-thin radios to plastic sheets, which can be placed on a variety of structures.  These listening walls have the potential to be the foundation for invisible communication systems inside buildings or to function as sophisticated structural monitors for bridges and roads. 

Originally the intent of this technology was for the application of smart building energy management. Through the use of distributed radio arrays that are patterned on wallpaper, temperature sensors and occupancy sensors are able to communicate with a central management system. 

These thin plastic sheets have the potential for several applications. The sheets can be painted without diminishing their function and can be applied to irregular surfaces such as bridge decks or supporting columns because of their flexibility. 

Patterning Circuits on Plastic 

One of the major difficulties with this technology was patterning circuits on the plastic sheets, as the high temperatures needed to create circuitry will melt the plastic. New methods for patterning circuits on plastics have helped researchers surpass this problem. However, these new methods compromise the performance of electronic components such as transistors, which are vital in the operation of complex devices such as radio transmitters. 

Transistors, the building block of modern electronics, are devices that control or switch the flow of electrons in circuits. The silicon crystal that forms the base of transistors allows for electrons to move quickly.  

Because plastic is susceptible to melting at high temperatures, researchers turned to amorphous silicon transistors in place of crystalline silicone, as amorphous silicon does not require high temperatures like the crystal form.  However, amorphous silicon lacks the highly ordered inner structure of the crystal form. This inhibits the electrons ability to move efficiently, as it is like changing from a smooth superhighway to a gravel road. 

Researchers were poised with the challenge of speeding up the movement of electrons through the transistor (faster movement means higher frequency), despite the lower-performing amorphous silicon transistors. 

The Super-Regenerative Circuit 

To overcome this problem, the Princeton researchers found inspiration from the father of FM Radio, Edwin Armstrong. In 1922, Armstrong developed the super-regenerative circuit, which uses other components to increase the radio's frequency and bypass the poor performance of the amorphous silicon transistors. 

In 1922, Edwin Armstrong
developed the super-regenerative circuit.

By bouncing electrons between a capacitor and an inductor, the super-regenerative circuit, is able to store and discharge energy.  The energy change caused by the bouncing electrons depends on the super regenerated circuit's capacitor and inductor--not the transistor.  This allows the radio to operate at a high frequency despite the poor quality of the transistors. 

The key was to prevent the electrons from being lost as they bounced back and forth between the capacitor and the inductor, as lost electrons would than be compensated through the faulty transistors. This meant high quality capacitors and inductors.  

This was good news to researchers as large inductors are easier to build. And due to the fact that the radios were designed to fit on walls, there was plenty of space. In the end, despite the poor quality transistors, the circuit worked perfectly with the new system.  

The Future of Structural Monitoring Systems? 

Researchers are developing ways to use this technology to create flexible structural health monitoring systems for bridges, buildings, pipelines and other structures.  

Plastic sheets embedded with radios
could better detect structural problems. 

Currently, engineers are able to use single-point sensors or fiber optic strips to detect structural problems. Unfortunately these devices are limited and can only collect data from relatively small spaces. This makes detection of early problems difficult, as most problems occur on larger spaces. 

Plastic sheet technology could make monitoring these structures more efficient.  The linked sensors could potentially detect imminent structural problems over larger areas. 

While the research of this project still remains in early development the results are encouraging. A prototype is in the works, but it will take many years of research and development before this technology is utilized in such a manner. 

This article was originally published, "The Walls have Ears: Princeton researchers develop walls that can listen, and talk" from Phys.org on August 21, 2013.