Researchers have developed a multi-layered “sensing skin” to detect corrosive or otherwise harmful substances in structures. The skin can also detect cracks and other structural flaws that are invisible to the naked eye.

“We’ve created a skin that can be applied to the surface of almost any structure and used to monitor the structure’s integrity remotely and in real time, identifying potential problems long before they become catastrophic,” says Mohammad Pour-Ghaz, assistant professor of civil, construction and environmental engineering at North Carolina State University.

The skin consists of three layers, which can be painted onto the surface of a structure or pre-assembled and attached like wallpaper.The sensing skin consists of three layers, which can be painted onto the surface of a structure or pre-assembled and attached like wallpaper.

The first layer is electrically conductive and is used solely to detect cracks. The second layer serves as a buffer between the first and third layers. The third layer incorporates metal nanoparticles whose conductivity changes in the presence of specific ions. By changing the composition of the metal nanoparticles, this layer can be engineered to detect and respond to any particular chemical.

To monitor for chemicals emerging from inside a structure, the third layer would face inward. To monitor external chemicals, the third layer would face outward.

The sensing skin is set up with electrodes affixed around the perimeter of a structure. The skin is then applied to the structure over the electrodes. A computer program runs a small current between two of the electrodes at a time, cycling through a number of possible electrode combinations.

When the current runs between two electrodes, a computer monitors and records the electrical potential at all of the electrodes on the structure for both the first and third layers of the sensing skin. This data is then used to calculate the sensing skin’s spatially distributed electrical conductivity on both layers. The researchers have developed a suite of algorithms that can use changes in conductivity measured by the first and third layers of the skin to detect and locate both damage and the presence of target chemicals.

In a proof-of-concept study, the researchers applied the sensing skin to reinforced concrete, exposing the concrete to corrosive elements and subjecting it to strain to simulate the failure of real-world structures. For this study, the third layer of the sensing skin was engineered to detect chlorides, which can cause corrosion in reinforced concrete.

“The skin performed really well,” Pour-Ghaz says. “We were able to detect cracks as small as a few hundred micrometers. We could very accurately detect any instances in which chlorides came into contact with the skin."

While this proof of concept tested the skin on concrete, the technology—if properly applied—could also be used on structural materials ranging from metals to polymers, he adds.