Micropumps are devices that can control and manipulate small fluid volumes. Although any kind of small pump is often referred to as micropump, a more accurate definition restricts this term to pumps with functional dimensions in the micrometer range. Such pumps are of special interest in microfluidic research, and have become available for industrial product integration in recent years. Their miniaturized overall size, potential cost and improved dosing accuracy compared to existing miniature pumps fuel the growing interest for this innovative kind of pump.
Note that the below text is very incomplete in terms of providing a good overview of the different micropump types and applications, and therefore please refer to good review articles on the topic.
Introduction and HistoryEdit
First true micropumps were reported in the mid-1970s, but attracted interest only in the 1980s, when Jan Smits and Harald Van Lintel developed MEMS micropumps. Most of the fundamental MEMS micropump work was done in the 1990s. More recently, efforts have been made to design non-mechanical micropumps that are functional in remote locations due to their non-dependence on external power.
Types and TechnologyEdit
Within the microfluidic world, physical laws change their appearance. As an example, volumetric forces, such as weight or inertia, often become negligible, whereas surface forces can dominate fluidical behaviour, especially when gas inclusion in liquids is present. With only a few exceptions, micropumps rely on micro-actuation principles, which can reasonably be scaled up only to a certain size.
Micropumps can be grouped into mechanical and non-mechanical devices. Mechanical systems contain moving parts, which are usually actuation and microvalve membranes or flaps. The driving force can be generated by utilizing piezoelectric, electrostatic, thermo-pneumatic, pneumatic or magnetic effects. Non-mechanical pumps function with electro-hydrodynamic, electro-osmotic, electrochemical  or ultrasonic flow generation, just to name a few of the actuation mechanisms that are currently studied.
A diaphragm micropump uses the repeated actuation of a diaphragm to drive a fluid. The membrane is positioned above a main pump valve, which is centered between inlet and outlet microvalves. When the membrane is deflected upwards through some driving force, fluid is pulled into the inlet valve into the main pump valve. The membrane is then lowered, expelling the fluid through the outlet valve. This process is repeated to pump fluid continuously.
A peristaltic micropump is a micropump composed of at least three microvalves in series. These three valves are opened and closed sequentially in order to pull fluid from the inlet to the outlet in a process known as peristalsis.
Static valves are defined as valves which have fixed geometry without any moving parts. These valves provide flow rectification through addition of energy (active) or inducing desired flow behavior by fluid inertia (passive). Two most common types of static geometry passive valves are Diffuser-Nozzle Elements  and Tesla valves. Micropumps having nozzle-diffuser elements as flow rectification device are commonly known as Valveless Micropumps.
In microfluidics, capillary pumping plays an important role because the pumping action does not require external actuation power. Glass capillaries and porous media, including nitrocellulose paper and synthetic paper, can be integrated into microfluidic chips. Capillary pumping is widely used in lateral flow testing. Recently, novel capillary pumps, with a constant pumping flow rate independent of the liquid viscosity and surface energy, were developed, which have a significant advantage over the traditional capillary pump (of which the flow behaviour is Washburn behaviour, namely the flow rate is not constant) because their performance does not depend on the sample viscosity.
Chemically Powered PumpsEdit
Chemically powered non-mechanical pumps have been fabricated by affixing nanomotors to surfaces, driving fluid flow through chemical reactions. A wide variety of pumping systems exist including biological enzyme based pumps, organic photocatalyst pumps, and metal catalyst pumps. These pumps generate flow through a number of different mechanisms including self-diffusiophoresis, electrophoresis, bubble propulsion and the generation of density gradients. Moreover, these chemically powered micropumps can be used as sensors for the detection of toxic agents.
Micropumps have potential industrial applications, such as delivery of small amounts of glue during manufacturing processes, and biomedical applications, including portable or implanted drug delivery devices. Bio-inspired applications include a flexible electromagnetic micropump using magnetorheological elastomer to replace lymphatic vessels. Chemically powered micropumps also demonstrate potential for applications in chemical sensing in terms of detecting chemical warfare agents and environmental hazards, such as mercury and cyanide.
|Wikimedia Commons has media related to Micropumps.|
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