Case study of a fluid mechanics MEMS

1、Case study of a fluid mechanics MEMSRecently,new MEMS applications, namely micro total analysis systems (TAS) or laboratory-on-a-chip (LOC) systems have been widely accepted in chemistry and biochemistry. advantages of TAS ：small sizethe need for only a small volume of samples and reagents, rapid re

2、sponse as a result of the chemical reaction and detection being completed in micrometer-scale channels. The size effect is the most remarkable feature of TAS. the feature of liquid flow in micrometer-scale channelsLiquid flow is incompressible, so, in micrometer-scale channels, the feature of liquid

3、 flow is different from micro : the flow has a small Reynolds number (Re), usually less than 1,the flow in simple microchannels is laminar, thus chaotic or turbulent flows are not observed.Under typical operating conditions, flows in these channels are laminar-the spontaneous fuctuations of velocity

4、 that tend to homogenize fluids in turbulent flows are absent, and molecular diffusion across the channels is slow. It is difficult to mix solutions in microchannels.Flow Control in MicrochannelsMany types of microfluidic device have been d e v e l o p e d o n t h e b a s i s o f t h i s f l o w beh

5、avior.Functional flow control methods based on laminar flow profiles have been proposed and applied in microflow devices and systems.Passive and active flow control methods and their applications are introduced as follows.Fig.1a.3D spiral flow along a microchannel is generated using slanted grooves,

6、 i.e.,obliquely oriented grooves, on the channel wall Fig. 1b.Much efficient spiral flow is achieved with short channel lengths by fabricating slanted grooves on the three walls.This flow behavior is useful for enhancing transverse components of flow in stretching and holding over a cross-section of

7、 the channel,which is necessary for effective mixing of materials in simple channels.The advantages of these devices:simple structures low pressure drops.spiral flowFrom:Sato H, Ito S, Tajima K, Orimoto N, Shoji S .PDMS microchannels with slanted grooves embedded in three walls to realize efficient

8、spiral flowJ. Sensors and Actuators A 119 (2005) 365371Sheath Flow and Flow FocusingHydrodynamic focusing is a useful function in the handling and sorting of reagents,particles, and biomolecules in microchannels. 3D flow focusing, i.e., 3D sheath flow, has been achieved in microchannels that can loc

9、alize the position of reagents or particles at the cross-sectional central region of the microchannel, and can minimize interaction with the channel walls. Using planar fabrication processes, 3D sheath flow devices have been fabricated with silicon, glass, and polymer substrates. Fig.2a:Ideal 3D flo

10、w focusing of a cylindrical sheath flow has been obtained with a pulled glass capillary and PDMS .However, these devices are not suitable for mass fabrication. Fig. 2 (a) Cylindrical sheath flowFrom:Jeong W, Kim J, Kim S, Lee S, Mensing G, Beebe DJ .Hydrodynamic microfabrication via “on the fly” pho

11、topolymerization of microscale fibers and tubes.J.Lab Chip (2004) 4:5765803D sheath flow devices3D sheath flow devicesFig.2b:3D sheath flow achieved by the two-step introduction of carrier flows to constrain the sample flow in a bonded silicon glass structure has been reported .Fig. 2(b) Two-step in

12、troduction of carrier flowFrom:Tashiro K, Sekiguchi T, Shoji S, Funatsu T, Masumoto W, Sato H (2000) Design and simulation of particles and biomolecules handling microflow cells with three-dimensional sheath flow. In: Proceedings 4th international conference on micro total analysis systems(TAS), Ens

13、chede, The Netherlands, 1418 May 2000. Kluwer, The Netherlands, pp 2092123D sheath flow devicesFig.2c:A 3D sheath flow device with four vertical and horizontal carrier inlets was obtained by a membrane sandwich method using PDMS Fig.2(c) Four carrier flows of vertical and horizontal carrier inletsFr

14、om:Sundararajan N, Pio MS, Lee LP, Berlin A .Three-dimensional hydrodynamic focusing in plolydimethylsiloxane (PDMS) microchannelsJ.IEEE JMEMS ,2004,13(4):559567Optically driven flow switches using sheath flow have been developed. Optical switch control gives simple flow device structures with no mo

15、vable mechanical structures and no electrical contacts.Fig. 3 Principle of the cell sorting method. (a) The window for detecting the fluorescence signal is located upstream of the junction. In the absence of a fluorescence signal, the collection channel continues to be plugged and flow directed to t

16、he waste channel. (b, c) Upon detection of a fluorescence signal, the entrance to the waste channel is plugged by switching the position of laser illumination, directing flow to the collection channel.Optically Controlled Flow DevicesFig.4 Schematic diagram of the cell sorting apparatus. A cell sort

17、er chip was placed on a fluorescence microscope equipped with an infrared laser system operating at 1480 nm. Biological specimens were introduced into the microchannel using a syringe pump. Fluorescence images were split by a half-mirror and projected onto both a CCD camera and a PMT, simultaneously

18、. Based on the detection of a fluorescence signal, the position of the infrared laser beam could be switched, enabling the position of laser illumination to be switched. Mirror 1 is a resonant scanner used to project laser light across the width of the microchannel, and mirror 2 is a scanner mirror

19、that changes the center of the laser illumination window.Fig.3 and 4 From:Shirasaki Y, Tanaka J, Makazu H, Tashiro K, Shoji S, Tsukita S, Funatsu T.On-chip cell sorting system using laser-induced heating of a thermoreversible gelation polymer to control flow.J.Anal Chem,2006, 78(3):695701Microvalves

20、 are one of the most important fluidic components for fluid control in microfluidic systems. Although various methods and structures using on-chip valves have been reported achieving on-chip fluid control, pneumatic actuation is the most widely employed because of its simple structure and easy fabri

21、cation. However, because the operation of pneumatic valves requires several world-to-chip connectors and off-chip solenoid valves, integration techniques for “pneumatic” logic circuits have been developed. An 8-bit microprocessor has been demonstrated for fluid control and mixing. A Baille display i

22、s used as the pressure driver instead of air actuation.Multiple Flow Control Systems: Arrayed Microvalves Fig.5(a) Schematic of the hydraulic valve and a top-down view with an open valve. Control channels are in pink and fluidic channels are in blue. The valve and piston can be centimeters apart. (b

23、) The same schematic with a vertical translation of a piezoelectrically driven Braille pin and a top-down view with a close valve and a pressurized control channel. (c) A top-down view of four intersections of pressurized control (red) and fluidic (blue) channels. All channels are 9m high and 100m w

24、ide except for the lower right control channel that is 40m wide. (d) A top-down view of the Braille pins aligned underneath pistons (left) and microfluidic valves (right). (e) A PDMS device with multiple hydraulic valves mounted onto a palm-sized Universal Serial Bus (USB) powered and controlled Bra

25、ille display module with 64 pin actuators.From:Gu W, Chen H, Tung YC, Meiners JC, Takayama S. Multiplexed hydraulic valve actuation using ionic liquid filled soft channels and Braille displaysJ.Appl Phys Lett,2007, 90(3):033505For chemical and biochemical experiments using microfluidic devices, prec

26、ise control of the fluid volume or fluid flow is required. The technique of sample metering and injection uses the fluid properties of droplet formation in heterophase or laminar flow. The use of rectangular channels with hydrophobic valves provides good performance and reproducibility as following

27、case.Fig.6 Schematic diagrams of the liquid behavior in a hydrophobic and rectangular channel. When the applied pressure was not high enough to enter into the narrow channel (a), liquid stops at the end of the broad channel (b). Liquid enters into the narrow channel only after applying a pressure hi

28、gher than the critical value (c).Fig.7 Layouts of microdevices with microdispenser array; (a) 3-reaction array,(b) 50-reaction array. Each device has one inlet (1) and one outlet (2) for the common liquid. There are 3 or 50 inlets (3) for the individual liquid in the 3-reaction array and the 50-reac

29、tion array, respectively.Fig.6 and7 From:Yamada M, Seki M.Nanoliter-sized liquid dispenser array for multiple biochemical analysis in microfluidic devicesJ.Anal Chem ,2004,76(4):895899Arrayed systems in microfluidic devices are promising for application in highfunctionality analysis and high-throughput screening. The method of fluidic access to microwell arrays is passive or