Micro-stirrers (mixers), Micro-Pumps, Valves for Flow Control (H. Bau)

Magnetohydrodynamic, chaotic stirrer-comparison of experimental observations with theoretical predictions at various times during the stirring process (Yi, 2002)

Magnetohydrodynamic, chaotic stirrer-comparison of experimental observations with theoretical predictions at various times during the stirring process (Yi, 2002)

Purpose: As the complexity of microfluidic experiments increases, it is desirable to integrate flow manipulation and control functions onto the chip.  Moving from off-chip control devices (e.g., syringe pumps) to on-chip systems allows for the complexity of devices to be increased and creates the opportunity for devices that can perform many measurements in parallel.  Here, we will develop modular components that can be integrated with other devices to add on chip functionality.  These modular components include stirrers/mixers, pumps, and valves for flow control.

Poincare cross-section of the flow pattern in an electroosmotic, chaotic stirrer (Qian, 2002)

Poincare cross-section of the flow pattern in an electroosmotic, chaotic stirrer (Qian, 2002)

Method of Fabrication/Use:  The basic principles, designs, and fabrication of strirrers, pumps, and valves that have been developed at used at Penn by faculty in our center are described below.  These devices have been primarily developed by Dr. Haim Bau and used in a variety of lab-on-a-chip devices. Based on these established technologies, we will develop designs for modular mixers, pumps, and valves that can be combined with  microfluidic components such that users can obtain functional devices with on-chip fluid control/actuation.

 

A passive stirrer fabricated with co-fired ceramic tapes. The stirrer consists of a sequence of bends (the geometry is depicted schematically in the top right inset).  Each bend induces two counter rotating vortices (right inset), which mix the solution transmitted in the meandering conduit (Yi, 2003).

A passive stirrer fabricated with co-fired ceramic tapes. The stirrer consists of a sequence of bends (the geometry is depicted schematically in the top right inset). Each bend induces two counter rotating vortices (right inset), which mix the solution transmitted in the meandering conduit (Yi, 2003).

Stirrers: In many circumstances involving microfluidic systems it is necessary to effectuate chemical reactions and biological interactions. Often, mass transfer is the limiting step in these processes, and it is desirable to stir the reactants.  Since in micro devices, the flow is typically at very low Reynolds numbers and well-organized, mixing is a challenge.  To facilitate effective stirring, we have designed and fabricated both active and passive mixers.  Active mixers are devices in which the fluid flow needed for mixing is purposely and internally induced. Passive mixers take advantage of externally driven fluid flow and of geometric alternations in conduit geometry to achieve secondary flows. To achieve effective mixing, we typically induce chaotic advection (Lagrangian Chaos) (Ottino, 1989). The chaotic convection is generated by either temporarily (active stirrers) or spatially (passive stirrers) alternating among two or more flow patterns.  We have constructed these stirrers in variety of materials such as plastics, PDMS, ceramic tapes, and silicon.  To induce agitation in the case of active stirrers we use electroosmois (Qian, 2002, Qian, 2005) induced charge electroosmosis (Zhao, 2007), magnetohydrodynamics (Bau, 2001, Qian, 2002, Yi, 2002)  and surface acoustic waves (Yi, 2002).  In the case of passive stirrers, we use alternations in conduit geometry (bends) to induce alternating secondary flow (Yi, 2003). The figure shows a few implementations of active and passive stirrers. The passive devices can be fabricated in PDMS through typical molding and laser machining approaches.  Active stirrers typically require the integration of electrodes to apply electric fields.  Such electrodes can be fabricated through physical vapor deposition (evaporation/sputtering) of metals onto glass or silicon wafers.

 

MHD-driven network. Blisters are used to pump reagents through a chip equipped with a bead array for concurrent detection of multiple analytes.

MHD-driven network. Blisters are used to pump reagents through a chip equipped with a bead array for concurrent detection of multiple analytes.

Pumps: We have also developed various pumping modalities that can be integrated with other microfluidic components to realize functional devices with on-chip flow actuation.  One attractive pumping method for microfluidics is magnetohydrodynamics (MHD). In MHD, electrodes are deposited along the opposing walls of a conduit (or a conduit segment), typically by physical vapor deposition or electroplating.  When the electrodes are subjected to a potential difference in the presence of a magnetic field (typically provided) by a permanent magnet, the current transmitted in the solution interacts with the magnetic field to produce a Lorentz force that drives fluid flow and/or generates pressure (Bau, 2001, Qian, 2005, Qian, 2006, Zhong, 2002). A more elegant application of MHD is to control flow in a network. Many of the conduits are equipped with electrodes.  By judiciously controlling the electrodes’ potentials, one can direct the fluid to follow any desired path without a need for valves or any other active members (Bau, 2003, Qin, 2011). The MHD devices can be fabricated on hard silicon/glass substrates or molded into plastic substrates.  Alternative approaches for pumping that we have demonstrated expertise in are devices based on capillary suction (Sinha, 2007) and blister-driven pumping (Chen, 2010, Qiu, 2009).

 

MHD-driven network. Each conduit is acts as pump. By judicious control of electrodes’ potentials, one can direct fluid flow along any desired path.

MHD-driven network. Each conduit is acts as pump. By judicious control of electrodes’ potentials, one can direct fluid flow along any desired path.

Valves: Flow control often requires the use of valves to allow or prevent fluid passage through a particular channel.  We have developed valves based on phase changes such as freezing (Wang, 2005) and effectuating phase transformation of temperature-sensitive hydrogels (Chen, 2005) and diaphragm valves (Chen, 2010).

Results: Our mixer, pump, and valve technologies have been demonstrated in a broad range of lab on a chip devices.  For example, mixing of various aqueous solutions via active methods has been shown in (Qian, 2005, Zhao, 2007, Qian, 2002).  MHD pumping of fluids on microfluidic chips of various materials have been demonstrated by our group, including fluid pumping and routing using devices with multiple sets of electrodes (Qin, 2011). Finally, we have fabricated and validated the designs of multiple types of valves, example are reported in (Wang, 2005) (Chen, 2005), and (Chen, 2010).

References

Ottino J. The Kinematics of Mixing: Stretching, Chaos and Transport. 1989.

Qian S, Bau HH. A chaotic electroosmotic stirrer. Anal Chem Aug 1 2002; 74 (15): 3616-3625.

Qian SZ, Bau HH. Theoretical investigation of electro-osmotic flows and chaotic stirring in rectangular cavities. Applied Mathematical Modelling Aug 2005; 29 (8): 726-753.

Zhao H, Bau HH. Microfluidic chaotic stirrer utilizing induced-charge electro-osmosis. Phys Rev E Stat Nonlin Soft Matter Phys Jun 2007; 75 (6 Pt 2): 066217.

Bau HH, Zhong JH, Yi MQ. A minute magneto hydro dynamic (MHD) mixer. Sensors and Actuators B-Chemical Oct 15 2001; 79 (2-3): 207-215.

Qian SZ, Zhu JZ, Bau HH. A stirrer for magnetohydrodynamically controlled minute fluidic networks. Physics of Fluids Oct 2002; 14 (10): 3584-3592.

Yi MQ, Qian SZ, Bau HH. A magnetohydrodynamic chaotic stirrer. Journal of Fluid Mechanics Oct 10 2002; 468 153-177.

Yi MQ, Bau HH. The kinematics of bend-induced mixing in micro-conduits. International Journal of Heat and Fluid Flow Oct 2003; 24 (5): 645-656.

Bau H. A case for Magneto-hydrodynamics (MHD). IMECE 2001, MEMS 23884 Symposium Proceedings. New York, NY; 2001.

Qian S, Bau HH. Magnetohydrodynamic flow of RedOx electrolyte. Physics of Fluids Jun 2005; 17 (6):

Qian SZ, Chen ZY, Wang J, Bau HH. Electrochemical reaction with RedOx electrolyte in toroidal conduits in the presence of natural convection. International Journal of Heat and Mass Transfer Oct 2006; 49 (21-22): 3968-3976.

Zhong JH, Yi MQ, Bau HH. Magneto hydrodynamic (MHD) pump fabricated with ceramic tapes. Sensors and Actuators a-Physical Jan 31 2002; 96 (1): 59-66.

Bau HH, Zhu JZ, Qian SZ, Xiang Y. A magneto-hydrodynamically controlled fluidic network. Sensors and Actuators B-Chemical Jan 15 2003; 88 (2): 205-216.

Qin MA, Bau HH. When MHD-based microfluidics is equivalent to pressure-driven flow. Microfluidics and Nanofluidics Feb 2011; 10 (2): 287-300.

Sinha S, Rossi MP, Mattia D, Gogotsi Y, Bau HH. Induction and measurement of minute flow rates through nanopipes. Physics of Fluids Jan 2007; 19 (1):

Chen D, Mauk M, Qiu X, Liu C, Kim J, Ramprasad S, Ongagna S, Abrams WR, Malamud D, Corstjens PL, Bau HH. An integrated, self-contained microfluidic cassette for isolation, amplification, and detection of nucleic acids. Biomed Microdevices Aug 2010; 12 (4): 705-719.

Qiu XB, Thompson JA, Chen ZY, Liu CC, Chen DF, Ramprasad S, Mauk MG, Ongagna S, Barber C, Abrams WR, Malamud D, Corstjens PLAM, Bau HH. Finger-actuated, self-contained immunoassay cassettes. Biomedical Microdevices Dec 2009; 11 (6): 1175-1186.

Wang J, Chen ZY, Mauk M, Hong KS, Li MY, Yang S, Bau HH. Self-actuated, thermo-responsive hydrogel valves for lab on a chip. Biomedical Microdevices Dec 2005; 7 (4): 313-322.

Chen Z, Wang J, Qian S, Bau HH. Thermally-actuated, phase change flow control for microfluidic systems. Lab Chip Nov 2005; 5 (11): 1277-1285.

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