1)Microvalves [Group: Micro element]
There is no old definition.
[New]
Microvalves are one of the most important building blocks of microfluidic systems used for fluid flow control. They can be classified in two categories: active valves (with an actuator) and passive check valves (without an actuator).
Active Microvalves:
An active microvalve consists of a device body that contains the fluid under pressure. a valve seat to modify the fluid flow, and an actuator to control the position of the valve seat. The first reported microvalve was designed as an injection valve for use in inte-grated gas chromatography. It had a silicon valve scat and a nickel diaphragm actuated by an external solenoid. Following this first design, a large number of micro. valves have been designed and reported, and they can be classified on the basis of the actuation method employed. These methods include pneumatic, thermopneumatic. thermomechanic, piezoelectric, electrostatic, electromagnetic, electrochemical, and capillary force microvalves. Figure 10.7 shows some of these actuating devices.
Passive Microvalves (Check Valves)
This type of microvalve is typically designed for use in micropumps where a very small leakage under reverse applied pressure and a large reverse-to-forward flow resistance ratio is required. The dimensions of check valves are small in comparison with the valves with integrated or external actuators. A typical cantilever-type structure is shown in Fig. 10.8. Figure 10.9 shows functional, art, and SEM impressions of a 2 x 5 high-density in-plane check valve array.
(Handbook of Machine Olfaction: Electronic Nose Technology,Tim C. Pearce,Susan S. Schiffman,H. Troy Nagle,Julian W. Gardner,2003,pp. 238-141)
2)Micropumps [Group: Micro element]
There is no old definition.
[New]
In addition to microvalves, the inicropump is another essential and important component in the integrated microfluidic device. Based on different pump (actuation) mechanisms, conventional micropumps can be classified into two major groups: membrane-actuated (mechanical) and nonmembrane-actuated pumps . Membrane-actuated pumps can be further divided into different types: piezoelec-tric, electrostatic, and thermopneumatic, among others. Most of these conventional pressure-driven membrane-actuated micropumps suffer from complicated designs, complicated fabrication, or high cost. Nonmembrane pumping principles include electrohydrodynamic, electro-osmotic, traveling wave, diffuser, bubble, surface wetting, rotary, and so on. Although much progress has been made, micropumps with the appropriate combination of cost, performance, and operating requirements are still not available for many practical applications.
The microfluidic device reported here requires integrated micropumps for transport of a wide range of sample volumes (μL-mL). In our device, two simple pressure-driven micropumping methods were employed: a thermopneumatic air pump for pumping of ~μL volumes, and electrochemical pumps for ~mL volumes. The former made use of the air expansion in an air chamber, which was attached to a resistive heater in the PCB substrate, when heated up. The air expansion is a nearly linear function of temperature. The resulting air expansion pushed the solution from the storage chamber into the downstream channels and chambers. The latter relied on electrolysis of water between two platinum electrodes in a saline solution to generate gases when a DC current is applied. The gas generated a pressure that in turn moved liquid solutions in the device (Fig.3.6).
Both pumping mechanisms do not require a membrane and/or check valves in their designs. As a result, their fabrication and operation are much simpler than most conventional micropumps. Flow experiments demonstrated that the thermop-neumatic air pumps with air pockets of 50 μL internal volume could efficiently move tip to 60 μL, volume of fluids with a heater power consumption of less than 0.5W. For pumping of ~mL solution volumes, the electrochemical pump is more efficient and consumes less power. A steady flowrate of up to 0.8mL/min was achieved with a power consumption of <150mW.
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