| CPC G01N 31/229 (2013.01) [G01D 3/10 (2013.01); G01K 3/04 (2013.01); G09F 3/0291 (2013.01)] | 43 Claims |

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1. An activatable print medium comprising:
a first reactive component;
a second reactive component, the first and second reactive components configured to react to initiate a state change of the activatable print medium when exposed to each other; and
a barrier initially in an unactivated form and configured to transition to an activated form when activated by an application of an activation heat, the barrier comprising a polymer having a glass transition temperature, and the activation heat heats the polymer above the glass transition temperature, causing the barrier to transition from a nonpermeable state to a permeable state, wherein
in the unactivated form, the barrier is in the nonpermeable state and is configured to separate the first reactive component and the second reactive component, preventing their interaction,
in the activated form, the barrier is in a permeable state and configured to allow the first reactive component to interact with the second reactive component,
wherein, after the barrier is activated, but not before, the activatable print medium is configured so that it has a property selected from the list consisting of:
(a) the first and second reactive components are configured to interact in response to a predetermined environmental stimulus which occurs after the barrier was activated, thereby providing an indication of an historical exposure of the activatable print medium to the environmental stimulus that occurs after the barrier was activated, and
(b) the first and second reactive components are configured to interact over time at a rate dependent on the environmental stimulus which occurs after the barrier was activated, thereby providing an indication of a cumulative historical exposure of the activatable print medium to the environmental stimulus that occurs after the barrier was activated.
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