A new fiber optic strain sensor can heal itself, which will help engineers collect data in the wake of earthquakes, explosions or other unexpected events and make informed decisions about structural safety. Strain sensors measure the force exerted on materials that make up civil infrastructure, airplanes and buildings. These sensors can provide data on, for example, how an airplane wing performs during flight and give advance notice to maintenance authorities when the wing may be near failure. This strain data provides a chance to address an issue before it becomes a problem. Historically, these sensors have broken easily under stress and thus no longer were able to provide information; moreover, as in the airplane example, their inaccessibility means they are difficult or impossible to replace. To address this problem, engineers at North Carolina State University have developed a sensor that automatically repairs itself. The research, which was funded by the National Science Foundation, appeared in the May 13 issue of Smart Materials and Structures (doi: 10.1088/0964-1726/20/6/065005). The top image shows a polymer filament connecting the glass fibers in a sensor. The middle image shows where the filament has snapped off. The bottom image shows where the resin has rushed into the gap, been exposed to UV light and reconnected the filament – effectively repairing itself. Courtesy of Kara Peters, North Carolina State University. The new sensor can stretch and compress along with the material it monitors. Infrared light passing through it detects the resulting changes in length, offering details about how much strain the material is undergoing. The sensor contains two glass optical fibers that run through a reservoir filled with ultraviolet-curable resin. The ends of the glass fibers are aligned with each other but separated by a small gap. To demonstrate how the sensor works, the investigators focused beams of IR and UV light through one fiber. When the resin is hit by the tightly focused UV beam, it hardens, creating a thin polymer filament that connects the glass fibers and forms a closed circuit for the IR light. The rest of the resin in the reservoir remains in liquid form, surrounding the filament. The liquid resin is important if the polymer filament breaks under stress. In this instance, the remaining resin would rush into the gap, where it would come into contact with the UV beam and harden, repairing the sensor automatically. The scientists hope that their design will help collect data about the structure being monitored.