In the wake of an earthquake, you need information about the structural integrity of buildings in the quake zone in order to determine if it’s safe to enter them. For this reason, buildings in earthquake-prone regions often have sensors attached to measure strain on the structure. There’s one problem though: what if the quake damages the sensor itself? The building might have severe structural damage, or it might not–with the sensor down, you just can’t be sure.
To help counter this known unknown, researchers at North Carolina State University have developed a structural sensor that’s able to heal itself when damaged. The sensor is made up of a pair of glass optical fibers embedded in a reservoir of liquid resin. This resin, though, has special properties–it is “ultraviolet-curable” resin, which means that when it comes into contact with UV light, it hardens. If the filament connecting the two glass fibers should snap during an earthquake, a beam of UV light will carve a new connection out of the reservoir of liquid resin. It is, in essence, “a sensor that automatically repairs itself,” in the words of Kara Peters, one of the researchers. “As far as I’m aware,” she tells Technology Review, “this is the first time” a self-healing sensor has been made.
The NCSU team, which received funding form the National Science Foundation for their research, tested the sensor’s abilities before and after failure, and found that it still worked well. Sometimes, though, as they note in their paper published recently in Smart Materials and Structures, the repaired filament didn’t always go exactly where it was supposed to, failing to bridge the gap between the two optical fibers. “In order to produce reliable self-repairing sensors,” they write, “the alignment and bending problems will need to be addressed.”
Are self-healing “smart materials” really necessary here? Aren’t there other ways to get around the problem of damaged sensors? There are, in fact, and the NCSU team’s paper lists them. In some cases, you can simply repair or replace a sensor (though none too easily, in cases where a sensor is embedded deep within a structure). “Sensor redundancy,” or putting a lot of sensors up in a building (or in other structures–airplanes also use similar sensors, to monitor forces on wings), is another solution. Some researchers have even used neural networking to get an intelligent composite picture from the remaining sensors in a damaged building.
These strategies are helpful, write the study authors. But since a damaged sensor is, almost by definition, likely to be located in the most critical and vulnerable part of a structure, there’s simply no replacement for actual, hard data coming from that sensor. Even if less high-tech solutions might seem possible, the bottom line is that self-healing sensors like the one developed at NCSU have the potential to save lives.