Reciprocating Pump
Rotary Pump
Types and Technology
In this sense, first true micropumps were reported on in 1975.However, the micropumps developed by Jan Smits and Harald Van Lintel in the early 1980's are considered to be the first genuine MEMS micropumps, and sparked the interest in shrinking the size of a fully functional pump to new dimensions.
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 valve 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 flow generation, just to name a few of the actuation mechanisms that are currently studied.
Dispensing therapeutic agents into the body has long been a goal of micropump designers. Among the first micropumps, those developed by Jan Smits in the early 1980s were intended for use in controlled insulin delivery systems for maintaining diabetics’ blood sugar levelswithout frequent needle injections . Micropumps might also be used to dispense engineered macromolecules into tumors or the bloodstream. Highvolumetric flow rates are not likely to be required of implanted micropumps (the amount of insulin required by a diabetic per day, for example, is less than a milliliter) but precise metering is of great importance . The pressure generation requirements for implantable micropumps are not insignificant, as the back pressure encountered in vivo can be as high as 25 kPa. Reliability, power consumption, cost and biocompatibility are critical .To date, deficiencies in these areas have precluded widespread implantantion of micropumps.
A number of researchers have sought to develop micropumps for use in single- or two-phase cooling of microelectronic devices. Microelectronics cooling is highly demanding with respect to flow rate. For instance, Tuckerman and Pease’s seminal paper on liquid-phase chip cooling contemplated flow rates of several hundred milliliters per minute. Recent studies indicate that two-phase convective cooling of a 100Wmicrochip will require flowrates of order 10 ml min−1 or more . The fundamental scaling associated with pressure-driven flow dictates that high pressures (100 kPa or greater) will be required to force such high flow rates through microchannels and/or jet structures found in micro heat sinks. In the laminar regime, an order-of-magnitude decrease in the hydraulic diameter of a channel (the channel cross-sectional area multiplied by four and divided by its perimeter) increases by two orders of magnitude the pressure difference required to maintain a constant average flow velocity. Cost and power consumption are also important considerations, the latter especially for mobile units. Micropumps might also be built directly into integrated circuits to cool transient hot spots, and so fabricationmethods and temporal response characteristicsmay be particularly important. Insensitivity to gas bubbles is also important as bubbles are present in and detrimental to many microfluidic systems.
Electroosmotic Pump
Much attention has been focused recently on miniature systems for chemical and biological analysis . Miniaturization of chemical assays systems can reduce the quantities of sample and reagents required and often allows assays to be performed more quickly and with less manual intervention. Miniaturization also enables portability as in the case of a portable chemical analysis system under development at Sandia National Labs .Miniaturization sometimes offers the further advantage of enabling use of inexpensive disposable substrates. Although fluids (typically liquids) must typically be introduced into, and transported within, these micro total analysis systems (μTAS) during operation, micropumps are found in very fewcurrent-generation systems. Liquid transport is instead often accomplished through manual pipetting, with external pneumatic sources, or by inducing electroosmotic flow. The limited use of micropumps in μTAS may be partly due to the lack of available micropumps with the necessary combination of cost and performance. Compatibility with the range of fluid volumes of interest will be necessary if micropumps are to become more widely used in μTAS. Monitoring single cells may require manipulation of fluid volumes on the order of 1 pl—the volume contained in a cube 10 μm on a side. Microchipbased systems used in drug discovery amplify DNA, separate species through capillary electrophoresis, and/or interface with mass spectrometers with sample volumes ranging from hundreds of picoliters to hundreds of microliters. Patient pain considerations have prompted manufacturers of in vitro blood glucose monitors for diabetics to minimize sample size requirements; current systems need a sample volume of only one-third of a microliter.Detecting microbes in human body liquids often requires somewhat larger sample volumes; for example, a common immunoassaybased blood test for malaria uses a sample volume of 10 μl . Other parameters important for μTAS include working fluid properties such as pH, viscosity, viscoelesticity and temperature, as well as the presence of particles which may disrupt operation of pumps and valves. Secondary effects associated with reliability and corrosion include the impact of mechanically shearing the sample, chemical reactions, adsorption of analytes and wear of moving parts.
potential application of micropumps in space. For example, ion-based propulsion systems proposed for future 1–5 kg ‘microspacecraft’ may require delivery of compressed gases at 1 ml min−1 flow rates .Larger stroke volumes are generally required for pumping gases than for pumping liquids, making these space exploration applications particularly challenging. Inspired by this wide range of applications, over 200 archival journal papers reporting new micropumps or analyzing micropump operation have been published sinceSmits’ micropump was first developed in the 1980s. A robust, coherent system of categorization is helpful for making sense of the diverse set of devices that have been reported. In this review, we categorize micropumps according to the manner and means by which they produce fluid flow and pressure. Our system ofmicropump classification, illustrated in figure 1, is applicable to pumps generally and is essentially an extension of the system set forth by Krutzch and Cooper for traditional pumps .Pumps generally fall into one of two major categories: (1) displacement pumps, which exert pressure forces on the working fluid through one or more moving boundaries and (2) dynamic pumps, which continuously add energy to the working fluid in a manner that increases either its momentum (as in the case of centrifugal pumps) or its pressure directly (as in the case of electroosmotic and electrohydrodynamic pumps). Momentum added to the fluid in a displacement pump is subsequently converted into pressure by the action of an external fluidic resistance. Many displacement pumps operate in a periodic manner, incorporating some means of rectifying periodic fluid motion to produce net flow. Such periodic displacement pumps can be further broken down into pumps that are based on reciprocating motion, as of a piston or a diaphragm, and pumps that are based on rotary elements such as gears or vanes. Themajority of reportedmicropumps are reciprocating displacement pumps in which the moving surface is a diaphragm. These are sometimes called membrane pumps or diaphragm pumps. Another subcategory of displacement pumps are aperiodic displacement pumps, the operation of which does not inherently depend on periodicmovement of the pressure-exerting boundary. Aperiodic displacement pumps typically pump only a limited volume of working fluid; a syringe pump is a common macroscale example. Dynamic pumps include centrifugal pumps, which are typically ineffective at low Reynolds numbers and have only been miniaturized to a limited extent, as well as pumps in which an electromagnetic field interacts directly with the working fluid to produce pressure and flow (electrohydrodynamic pumps, electroosmotic pumps andmagnetohydrodynamic pumps) and acoustic-wave micropumps1. In figure 1, open boxes represent pump categories of which operational micropumps have been reported. In our use of the term micropump, we adhere to the convention for microelectromechanical systems, with the prefix micro considered to be appropriate for devices with prominent features having length scales of order 100 μm or smaller. Many pumps that meet this criterion are micromachined, meaning that they are fabricated using tools and techniques originally developed for the integrated circuit industry or resembling such tools and techniques (e.g., tools involving photolithography and etching). Techniques such as plastic injectionmolding and precision machining have also been used to produce micropumps. In keeping with the nomenclature associated with nanotechnology, we consider the term nanopump to be appropriate only for devices with prominent features having length scales of order 100 nm or smaller (so pumps that pump nanoliter volumes of liquid are not necessarily nanopumps). We suggest, that, in general, that the term nanopump should be used judiciously, with terms that more accurately describe the operation of a nanoscale device used when appropriate. Of course,subcontinuum effectsmay be important in nanopumps and somemicropumps,, particularly in the case of devices that pump gases .As an aside, we note that electric-motor-driven miniature reciprocating displacement pumps that are compact relative to most macroscopic pumps (but larger than the micropumps discussed here) are commercially available. The performance of several such pumps is reviewed by Wong et al .In this review, we consider the various categories of micropumps individually. We review important features, analyze operation, describe prominent examples and discuss applications. We then compare micropumps of all categories, recognizing that the enormous variation among micropumps makes such comparisons difficult. Throughout this review, we pay particular attention to the maximum measured volumetric flow rate reported for micropumps, Qmax, and the maximum measured micropump differential pressure, _pmax. Since many of the micropumps discussed here are explicitly targeted for applications where compactness is important, we also consider micropump overall package size, Sp. When Sp is not explicitly reported, we attempt to estimate size from images, by making inferences from known dimensions, etc. An interestingmetric is the ratio of maximum flow rate Qmax to package size Sp, which we refer to as the selfpumping frequency, fsp. We also discuss certain micropump operating parameters, particularly operating voltage, V, and operating frequency, f. These parameters partially determine the electronics and other components needed to operate the micropump—important considerations for sizeand/ or cost-sensitive applications. Power consumption P and thermodynamic efficiency η are also important operational parameters, but unfortunately these measures are rarely reported. We urge the community to collect and report power consumption and thermodynamic efficiency data on all micropumps of interest.
