Microscopic cracks in a material can spread and grow into larger fissures—ones that can split apart the plastics and composites used in airplanes, spacecraft, electronics, and pipes. A simple new technique can reveal tiny, invisible cracks in a wide variety of plastics by making the cracks glow (ACS Cent. Sci. 2016, DOI: 10.1021/acscentsci.6b00198). Such an early-warning strategy could allow engineers to replace or repair critical components and prevent catastrophes. Nancy R. Sottos, Jeffrey S. Moore, and colleagues at the University of Illinois, Urbana-Champaign, embedded various polymeric materials with polyurethane microcapsules filled with a dilute solution of 1,1,2,2-tetraphenylethylene (TPE), which fluoresces brightly when TPE molecules aggregate. Formation of tiny cracks in the plastic ruptures the capsules, leading to solvent evaporation and growth of TPE crystals on the capsule shell. Ultraviolet light causes the crystals to shine bright blue. Scratches made with a blade in plastics loaded with microcapsules at a concentration of 10% by weight blended in under visible light but glowed when illuminated with a UV lamp. The researchers could detect cracks smaller than 2 µm in size up to 40 days after the damage occurred.