US Patent 8629081 Time/temperature indicators

By printing a bar code (comprising dark and light regions) or parts of such a bar code with an ink containing a dye, which changes color in the presence of an acid or a base to remove the contrast between the regions of the bar code, and a neutral compound which releases an acid or a base on exposure to energising radiation, a bar code can be produced that becomes unreadable after a pre-determined interval. This can be used as a time/temperature indicator to ensure that products, which might be perishable, associated with such a bar code are not used by the public.