Micropump displacement

Fig. 6.5. Principle of a valve-based micropump. Membrane deflection in a micropump displaces fluid through one of two integrated check valves.

As in high vacuum generation in macroscopic scale, the micropump is divided into two parts. As a backing pump for the pressure range from a few 102 Pa to atmospheric pressure a micro-sorption pump is under investigation, which relies on surface adsorption effects, as well as a scroll pump, which ranks with its displacement principle among the classic backing pumps. 

Unlike diffusion devices the displacement micropump pumps at increasing actuator volume (Fig. 13b) and fills the pump chamber at hydrogel shrinkage. The flow direction of the pump is defined by the valves placed inlet- and outletsided. 

Fig. 13 Displacement micropump, (a) Schematic set-up. (b) Operating principle, (c) Pump characteristics. Reproduced from (Richter et al. 2009a) by permission of The Royal Society of Chemistry...

Compared to other micropumps hydrogel-based devices can be classified as pumps with small dead volume suitable for low- (diffusion micropump) and medium-performance (displacement micropump) applications (Nguyen et al. 2002 Laser and Santiago 2004). 

Biochemical analysis on nanoliter scale is precisely carried out by micrototal analysis system (pTAS) which consists of microreactors, microfluidic systems, and detectors. Performance of the pTAS depends on micromachined and electrochemically actuated micropump capable of precise dosing of nanoliter amounts of liquids such as reagents, indicators, or calibration fluids [28]. The dosing system is based on the displacement of the liquid from a reservoir which is actuated by gas bubbles produced electrochemically. Electrochemical pump and dosing system consist of a channel structure micro-machined in silicon closed by Pyrex covered with novel metal electrodes. By applying pulsed current to the electrodes, gas bubbles are produced by electrolysis of water. The liquid stored in the meander is driven out into the microchannel structure due to expansion of gas bubbles in the reservoir as shown in Fig. 11.8. 

Membrane actuation for micropumps refers to the reciprocating periodic motion of a thin flexible layer - diaphragm or membrane made of silicon or other materials - bounding one side of a displacement micropump to create volume and pressure oscillations in a fluid (i.e., hquid or gas) stored in the chamber of the micropump that is rectified by other means to accomplish a net fluid flow through the micropump. The mechanical energy necessary for the membrane actuation in micropumps is generally derived from electrical, thermal, optical, or other forms of energy. 

Fig.1 Schematics of a generic reciprocating displacement micropump and its working principle, (a) Membrane in the initial flat configuration, (b) membrane bowing upwards during the suction stroke, (c) membrane bowing downwards during the discharge stroke...

Nonmechanical pumps or dynamic pumps, which continuously add energy to the working fluid in a manner that increases either its momentum or its pressure directly [1,2]. Displacement micropumps generally operate... 

One of the earliest types of rotary micropumps developed for microfluidics applications, drug delivery in particular, is the jet-type magnetically driven fluid micropump. It is based on a rotary micromotor which is attached to a toothed rotor (Fig. 1). Basically, it is a micro version of conventional positive displacement pump. Flow rates up to 24 pL/min at a pressure of 10 kPa have been obtained using this design [4]. 

Nonmechanical pumping. Micropumps in this class are usually continuous and include the use of effects such as electrochemical displacement (bubble generation), thermal expansion, electrohydrodynamics, capillarity, and evaporation forces. The most commonly used nonmechanical pumping method is based on electrokinetic flow. In comparison with mechanical micropumps, field-induced flow is advantageous as it acts as both a valve and a pump, enabling both the direction and the magnitude of the flow to be controlled. 

Peristaltic pumps are mechanical displacement pumps that induce flow in a fluid-fllled, flexible-walled conduit through peristalsis - transport due to traveling contraction waves. While macroscale peristaltic pumps appear in a variety of configurations, micropumps based on this principle almost exclusively use the sequenced contraction and expansion of a small number of discrete actuators - typically three - placed along the fluid channel. 

In many peristaltic micropump implementations, identical actuators provide both stroke displacement and flow valving. 

A typical present-day valve-less micropump may correspond to the example presented in Fig. 5. There is again the displacement by deformation of the compliant membrane, but the rectification is made by the fluidic suction and delivery valves, formed by cavities in the solid body of the pump. The valves shown in Fig. 5 are the Venturi diodes, currently very popular. Their rectification effect is based on the dissimilarity of the flow in the two opposite directions, as... 

Valve-Less Rectification Pumps, Fig. 14 Published data on micropumps with electrically drivtai moving mechanical component acting (as a pistrai) on the fluid in the displacement cavity. There is a distinct dependence betwetm relative operating fiequtaicy (related to resonance) and Stokes number, which is the similarity parameter of periodic fluid motion... 

Valve-Less Rectification Pumps, Fig. 31 An example of a planar micropump using the principle from Fig. 29. The cavities are made photochemically in a thin plate (above), the driving nozzles are in alternating positions on both sides of the channel. The alternating flow in them is generated by volume variations of an electrostrictive gel in displacement chambers... 

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