The fundamental principles necessary to create and model a pump for microfluidic systems using fluid-responsive polymer particles are described. The pump is "activated" by the addition of water to the particles, which induces a significant particle volume expansion and pushes a stored fluid from an adjacent reservoir at a predicted flow rate. Two particle systems were investigated to examine how polymer properties affect the rate and amount of fluid delivered. Poly (acrylic acid) (PAA) particles obtained from Pampers(R) diapers yield the best micropump swelling characteristics for delivering fluid at pressures above 1100 Pa, whereas the softer potassium-neutralized PAA particles from Aldrich are best only at lower pressures. The maximum flow rates produced by the Pampers(R) and Aldrich particles with minimal backpressure are 0.5 and 3.4 muL/s/mg of PAA, respectively. The experimental results demonstrate good agreement with an analytical model describing equilibrium and dynamic polymer swelling coupled with pressure-driven flow through cylindrical channels under conditions in which gel-blocking was not important. Fluid-responsive polymer micropumps could provide an inexpensive and lightweight method for driving fluid flow in microfluidic and other applications. (C) 2004 Elsevier Ltd. All rights reserved.