Researchers at the Fraunhofer Institute for Mechanics of Materials in Germany have developed a bicycle helmet that emits a foul stench after an impact. This would inform users that the shock absorbing material inside is no longer effective and it's time to get a new helmet:
The polymer materials or plastics produced by the process start to smell if they develop small cracks. Large cracks really cause a stink. The smell comes from odoriferous oils enclosed in microcapsules. "Cyclists often replace their helmets unnecessarily after dropping them on the ground, because they cannot tell whether they are damaged or not. The capsules eliminate this problem. If cracks form, smelly substances are released," explains Dr.-Ing. Christof Koplin, research scientist at the IWM.
Link via DVICE | Image: Fraunhofer IWM