A device comprising: an isolated cavity that is initially inaccessible to an exterior environment; an electrically passive timing mechanism; and a mechanical structure separating the isolated cavity from the exterior environment, such that at the end of a timing interval the timing mechanism acts on the mechanical structure to rupture and/or collapse the mechanical structure, thus bringing the isolated cavity in contact with the exterior environment, the device being characterized in that the passive timing mechanism includes: a timing diaphragm having a protrusion or other shaped structure facing the mechanical structure; a timing cavity; and a microfluidic channel of predefined geometry having two ends and filled with a timing fluid having known timing fluid properties, the passive timing mechanism being arranged so that upon applying pressure to the timing fluid, said timing fluid advances within the microfluidic channel at a speed dictated by the predefined channel geometry, known timing fluid properties and a pressure difference between the two ends of the microfluidic channel, and upon reaching the timing cavity after a timing interval, the timing fluid applies pressure to the timing diaphragm so that the protrusion or other shaped structure ruptures the mechanical structure thus allowing external fluid to enter the isolated cavity. an isolated cavity that is initially inaccessible to an exterior environment; an electrically passive timing mechanism; and a mechanical structure separating the isolated cavity from the exterior environment, such that at the end of a timing interval the timing mechanism acts on the mechanical structure to rupture and/or collapse the mechanical structure, thus bringing the isolated cavity in contact with the exterior environment, the device being characterized in that the passive timing mechanism includes: a timing diaphragm having a protrusion or other shaped structure facing the mechanical structure; a timing cavity; and a microfluidic channel of predefined geometry having two ends and filled with a timing fluid having known timing fluid properties, the passive timing mechanism being arranged so that upon applying pressure to the timing fluid, said timing fluid advances within the microfluidic channel at a speed dictated by the predefined channel geometry, known timing fluid properties and a pressure difference between the two ends of the microfluidic channel, and upon reaching the timing cavity after a timing interval, the timing fluid applies pressure to the timing diaphragm so that the protrusion or other shaped structure ruptures the mechanical structure thus allowing external fluid to enter the isolated cavity. a timing diaphragm having a protrusion or other shaped structure facing the mechanical structure; a timing cavity; and a microfluidic channel of predefined geometry having two ends and filled with a timing fluid having known timing fluid properties, the passive timing mechanism being arranged so that upon applying pressure to the timing fluid, said timing fluid advances within the microfluidic channel at a speed dictated by the predefined channel geometry, known timing fluid properties and a pressure difference between the two ends of the microfluidic channel, and upon reaching the timing cavity after a timing interval, the timing fluid applies pressure to the timing diaphragm so that the protrusion or other shaped structure ruptures the mechanical structure thus allowing external fluid to enter the isolated cavity.