An environmental sensing device includes an interferometric modulator which permanently actuates, in a visually-detectable manner, in response to being exposed to a predetermined environmental threshold or condition. The device can include a reactive layer, coating, or proof mass disposed on a movable...http://www.google.com.au/patents/US7787171?utm_source=gb-gplus-sharePatent US7787171 - Human-readable, bi-state environmental sensors based on micro-mechanical membranes
