A method of activating a micro-cell in which the micro-cell includes a first compartment, a second compartment, a fluid in the first compartment, an element in the second compartment and a porous barrier separating the first compartment from the second compartment. The porous barrier, in a first state, is operable to prevent the fluid from entering the second compartment whereas the porous barrier, in a second state, is operable, in response to an event, to allow the fluid to enter the second compartment and interact with the element in the second compartment so as to generate an activation signal.