Miniature pumps for dispensing a volume a flowable substance are described. One pump embodiment includes an environmentally-responsive plug layer that can contract in size, e.g., when exposed to an environmental change such as temperature, to allow an activation solution to flow to a layer of expandable material, such as a superporous hydrogel. Expansion of the expandable material urges a diaphragm into a chamber that holds the dispensable fluid, gas, or gel, forcing the fluid, gas or gel out of the pump.