Within the microfluidic world, physical laws are different. 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.
Mechanical micropumps Diaphragm micropumps 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. This mechanical strain results in pressure variation in the chamber, which causes inflow and outflow of the fluid. The flow rate is controlled by the polarization limit of the material and the voltage applied on the piezo. In comparison with other actuation principles piezoelectric actuation enables high
stroke volume, high actuation force and fast mechanical response, though requiring comparatively high actuation voltage and complex mounting procedure of the piezo ceramic. the world-renowned research organization with focus on
MEMS and Microsystem technologies. The micropump consists of three silicon layers, one of which as a pump diaphragm confines the pump chamber from above, while two others represent middle valve chip and bottom valve chip. Openings of the passive flap valves at the inlet and outlet are oriented according to the flow direction. The pump diaphragm expands with application of a negative voltage to the piezo thus creating negative pressure to suck the fluid into the pump chamber. While positive voltage vice versa drives the diaphragm down, which results in overpressure opening outlet valve and forcing the fluid out of the chamber. Currently mechanical micropump technology extensively uses Silicon and Glass based
micromachining processes for fabrication. Among the common microfabrication processes, the following techniques can be named: photolithography, anisotropic
etching, surface micromachining and bulk micromachining of silicon. Three
monocrystalline silicon wafers (100 oriented) are structured by doublesided lithography and etched by silicon wet etching (using
potassium hydroxide solution KOH). The connection between the structured wafer layers is realized by silicon fusion bond. This bonding technology needs very smooth surfaces (roughness lower than 0.3 nm) and very high temperatures (up to 1100 °C) to perform a direct silicon–silicon bond between the wafer layers. Absence of the bonding layer allows definition of the vertical pump design parameters. Additionally, the bonding layer might be affected by the pumped medium. The compression ratio of the micropump as one of the critical performance indicator is defined as the ratio between the stroke volume, i.e. fluid volume displaced by the pump membrane over the course of the pump cycle, and the dead volume, i.e. the minimum fluid volume remaining in the pump chamber in pumping mode. The compression ratio of Fraunhofer EMFT micropumps reaches the value of 1, which implies self-priming capability and bubble tolerance even at challenging outlet pressure conditions. Large compression ratio is achieved thanks to special patented technique of piezo mounting, when electrical voltage is applied on the electrodes on the top and bottom of the piezoelectric ceramic during the curing process of the adhesive used for the piezo mounting. Considerable reduction of the dead volume resulted from predeflected actuators along with shallow fabricated pump chamber heights increases the compression ratio.
Peristaltic micropumps 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.
Non-mechanical micropumps Valveless micropumps 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.
Capillary pumps 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 pumps 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. Recently, a combination of enzyme-based pumps has been used to enhance, suppress, and change the directionality of fluid flow.
Light-powered pumps Another class of non-mechanical pumping is light-powered pumping. Certain nanoparticles are able to convert light from a UV source to heat which generates convective pumping. These kinds of pumps are possible with titanium dioxide nanoparticles and the speed of pumping can be controlled by both the intensity of the light source and the concentration of particles. == Applications ==