Whether it's monitoring the integrity of buildings during earthquakes or airplane wings during flight, so-called strain sensors are getting more and more important for our safety. But what if a sensor itself is broken? A pair of engineers claims to have developed an answer to this problem: a new type of strain sensor that heals itself as soon as it is damaged.
Strain is the amount that an object is stretched, bent, or deformed. Too much strain and a structure fractures, which is bad news if that structure happens to be keeping a building upright or people flying through the air. Some critical structures have sensors to continually record strain placed at various points, all linked up to a central hub, so that technicians can monitor it and flag areas for repair before any serious damage is done.
But once a sensor itself is damaged, repair might not be easy. Sensors can be placed in a part of a structure that doesn't have easy access, like the inside of an airplane wing, or even embedded in concrete. One solution is redundancy, packing in more sensors than necessary, so that if one goes, another can be switched on to take its place. Alternatively, engineers have tried using more complex "neural networks" of sensors, which estimate the strain at a broken sensor based on readings from other sensors throughout the structure. But either solution is a compromise: there is still no information being collected at the failed sensor, which, having clearly suffered the most, is probably the most important sensor of all.
The new self-healing sensor, developed by Kara Peters and Young Song of North Carolina State University in Raleigh, is a no-compromise solution. The sensor consists of an optical fiber with a small, half-millimeter gap in the middle, surrounded by resin. Because of this gap, the sensor cannot detect anything. To activate it, the researchers shine an ultraviolet beam down the fiber, which meets the resin and begins to cure it into a transparent polymer thread traveling in the same direction as the beam. With the gap bridged by the thread, the sensor becomes functional.
In practice, such a sensor might be one of many throughout a structure, all fed with light using fiber optics emanating from a central hub. The sensing works with a separate infrared beam, which travels alongside the ultraviolet beam in the fiber and through the polymer thread. Any strain in the local structure would bend the thread and allow some of the infrared light to escape. By recording the amount of infrared light transmitted through the thread, the sensor measures the amount of local strain.
And if the thread bends so much that it breaks? Resin flows into the fracture, and the ultraviolet beam encounters it once more and patches the thread.
Although they do not know the maximum number of times their sensor can self-heal, the researchers have demonstrated that it can do so at least five times and still work. But that's plenty, Peters says. "In most applications that we're envisioning, the sensor would only need to self-heal a few times, because the structure would be designed to withstand only a few [events causing damage] before inspection or failure." The results are published in this month's issue of Smart Materials and Structures.
Richard Wool, a materials engineer at the University of Delaware in Newark calls the self-healing sensor an "interesting concept." But he notes that the same idea could sometimes be used to make the structural material around the sensor self-healing, leaving the sensor redundant. Structural self-healing plastics are already being developed, and earlier this year scientists reported ones that healed using ultraviolet light. "In the grand scheme of things, the self-repair of the sensor may be overkill," Wool says.
Even if self-healing sensors do have a market, they will need refinement. Two potential problems are debris from a breakage might clog up the resin, or the filament only partially fractures, degrading the infrared signal without repairing itself. The researchers haven't seen either of these in practice, however. One drawback Peters and Song have seen is that the two fibers sometimes become misaligned after a break, so the new filament doesn't bridge the gap properly. The duo is still working on ways to address this. "This is ongoing research," Peters says.