Device for controlling exposure of reservoir contents to surroundings

Microchip devices are provided that include a substrate; a plurality of reservoirs in the substrate, each reservoir defining an internal volume; and a reservoir cap positioned on or in each of the reservoirs bounding the internal volume from an external environment, the reservoir cap being formed of a material that undergoes a phase change upon heating, wherein the internal volume can communicate with the external environment upon heating said reservoir cap to cause said reservoir cap to undergo a phase change and rupture.