Lecture 18: Microfluidic MEMS, Applications

Transcription

1 MECH 466 Microelectromechanical Systems University of Victoria Dept. of Mechanical Engineering Lecture 18: Microfluidic MEMS, Applications 1 Overview Microfluidic Electrokinetic Flow Basic Microfluidic Components Applications of Microfluidics 2

2 Electrokinetics The science of using electric fields to move fluids, or to use fluids to generate electric fields. There are two main electrokinetic phenomena that can be utilized: (a) Electro-osmotic effect (EO) (b) Electrophoresis and Dielectrophoresis These methods utilize electric fields to move fluids, and are primarily used in microfluidics. 3 Fluid Transport by Electro-osmotic Flow An electrochemical reaction will occur at liquid/solid interfaces, when an electrolyte solution is present, causing an electric polarization of the channel wall. For the glass surfaces used in microfluidics, the electrolytes will cause deprotonation of the wall surface, producing a negatively charged wall. Where deprotonation is the removal of a proton (H + ) from a molecule Here, water molecules absorbed by the glass wall will be subject to deprotonation, resulting in a negative charge distribution on the surface of the glass

3 Fluid Transport by Electro-osmotic Flow The charged wall will attract ions from the bulk liquid, and they will form an ion layer called a electric double layer on the wall surface. If an electric field is applied parallel to the wall, these ions adjacent to the wall will move in response to the E-field, and will drag the surrounding fluid. This fluid flow is called electro-kinetic flow Fluid Transport by Electrophoresis Here, the electric field is used to act upon the particles within the fluid, for the purposes of separation, transportation and characterization The force exerted on a particle due to the electric field can be defined as: Where: Q - Charge P - Polarization E - Electric Field 6

4 Fluid Transport by Electrophoresis Electrophoresis force is defined as: Dielectrophoresis force is defined: 7 Capillary Gel Electrophoresis Size-based separation of biological macromolecules such as DNA restriction fragments and proteins

5 Microfluid channels and chambers for transporting and storing fluid Microfluid pumps for moving fluid Microfluid valves for isolation of fluid Basic Microfluidic Components Mixers structures to prompt mixing at the micro scale Electrodes (metal) for provide potential or current, or to detect signals Sensors flow parameter sensors and chemical parameter sensors 9 Fabrication of Micro-Channels Micro-Channels are often fabricated in Glass or Pyrex Substrates using Isotropic Wet Etching Processes. (a) Glass (b) Chrome Mask Mask Glass (c) Isotropic Etch (d) Remove Mask (e) Glass Glass Fabrication Process Sequence for creating Micro-Channels in a Glass Substrate Bond Overlying Layer of Glass to Create Channel 10

6 Fabrication of Micro-Channels Micro-Channels fabricated using Silicon Substrate, and subsequent wafer to wafer bonding. (a) (b) (c) (d) [Images from Chang Liu] Step (a): Create pattern in silicon bulk using isotropic, anisotropic or DRIE etch process. Step (b): Conformal growth of layer (i.e. silicon nitride) to create channel wall. Step (c): Bond original wafer to main wafer using anodic bonding. Step (d): Selectively etch away original silicon material, without removing Silicon Nitride channels, to create structure shown. 11 Fabrication of Micro-Channels Micro-Channels fabricated into Silicon Substrate, and sealed with oxide growth. (a) Deep Reactive Ion Etching (b) Passivation of Sidewalls (c) Isotropic Etching (d) Channel Sealing [Images from Chang Liu] 12

7 Pumping of microfluids can be done in a number of ways, including: Pressure driven flow fluid flow caused by pressure differential Electrokinetic flow fluid flow caused by movement of charged particles or molecules Surface acoustic wave Capillary force driving Micro-Pumps 13 Review of Macro-Scale Pumps Recall two conventional technologies for pumping fluids: Centrifugal Water Pump [ Peristaltic Pump 14

8 Pressure Drive Flow in Microchannels To create flow in the micro-channel, a conventional macro-scaled pump pressurizes the fluid, and is connected to the chip via flexible tubes. [labs.pharmacology.ucla.edu/tsenglab] 15 Micro-Pump with One-Way-Valves Micro-Pump using a pair of one-way valves. Pump membrane is actuated using an external magnetic field. 16

9 Surface Acoustic Wave Driven Flow Piezoelectric-Actuator driven flow [Ogawa J., Kanno I., Kotera H., Wasa K., Suzuki T., Development of liquid pumping devices using vibrating microchannel walls, Sensors and Actuators A, 152, pp , (2009). ] 17 Micro-Mixers Increase interfacial area to reduce diffusion length - sinusoidal, square-wave, or zigzag channels - divide and conquer approach - lamination-splitting Lamination splitting mixer Lamination Splitting Mixer [J. Branebjerg, et al., IEEE MEMS, 1996, p. 442] LIGA micro mixer LIGA Micro-Mixer [W. Erhfeld, et al., Ind. Eng. Chem. Res., 1999, 38, p. 1077] 18

10 Micro-Magnetic Stir Bar Parylene channel Inputs Parylene Flow offset Stirrer bar formation a) perspective view Tip to channel clearance = 10 microns Output b) Mixer c) Pump Micro-Magnetic Stir Bar in a Channel [Chang Liu] Hub and chamber formation Photoresist Copper seed layer Permalloy (10 microns Parylene Fabrication Process to create the Micro-Magnetic Stir Bar in a Channel [Chang Liu] 19 Micro-Magnetic Stir Bar In Out T=0s Two Micro-Channels Mixing. Note: Micro- Magnetic Stir Bar is Off. [Chang Liu] Micro-Pumping Using a Micro-Magnetic Stir Bar Time = 0. [Chang Liu] T=2s Micro-Pumping Using a Micro-Magnetic Stir Bar Time = 2 sec. [Chang Liu] T=3s Micro-Pumping Using a Micro-Magnetic Stir Bar Time = 3 sec. [Chang Liu] 20

11 Micro-Fluidic Applications: Case Study 13.2: Electrophoresis in Microchannels Case Study 13.4: PDMS Microfluid Channels Case Study 13.6: PDMS Pneumatic Valves 21 Case Study 13.2: Electrophoresis in Microchannels (a) A buffer injection is done to fill in the entire channel (b) Analyte injection using electrokinetic flow (c) Sample introduction and analyte electrophoretic separation Optical detection is done near the waste port. (a) (b) (c) 22

12 Case Study 13.4: PDMS Microfluid Channels PDMS (polydimethysiloxane) is part of the silicone group of plastics. They produce a thermoset plastic, that is transparent and flexible. Additionally, PDMS is porous, allowing liquids and gasses to slowly diffuse through the material. Microchannels are made using micro-molds (a) (c) Fig PDMS Molding (b) (d) liquid 23 Case Study 13.6: PDMS Pneumatic Valves Soft membrane micro-valves actuated by air pressure photoresist mold photoresist mold (a) wafer (d) wafer (b) (e) (c) (f) fluid channel top view (g) A A liquid membrane control pressure control line Fig PDMS Microvalve Fabrication high pressure A-A cross-section 24

13 Case Study 13.6: PDMS Pneumatic Valves 1/12-3+/ )*+,-*!.-/00$-/ )*+,-*!.-/00$-/ )*+,-*!.-/00$-/ &'(!"#$"% Fig Fabrication of peristalic pump 25 Case Study 13.6: PDMS Pneumatic Valves Microscopic Images of PUMP Fluid VLSI 26