1Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, 2Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign
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Johnson-Chavarria, E. M., Tanyeri, M., Schroeder, C. M. A Microfluidic-based Hydrodynamic Trap for Single Particles. J. Vis. Exp. (47), e2517, doi:10.3791/2517 (2011).
The ability to confine and manipulate single particles in free solution is a key enabling technology for fundamental and applied science. Methods for particle trapping based on optical, magnetic, electrokinetic, and acoustic techniques have led to major advancements in physics and biology ranging from the molecular to cellular level. In this article, we introduce a new microfluidic-based technique for particle trapping and manipulation based solely on hydrodynamic fluid flow. Using this method, we demonstrate trapping of micro- and nano-scale particles in aqueous solutions for long time scales. The hydrodynamic trap consists of an integrated microfluidic device with a cross-slot channel geometry where two opposing laminar streams converge, thereby generating a planar extensional flow with a fluid stagnation point (zero-velocity point). In this device, particles are confined at the trap center by active control of the flow field to maintain particle position at the fluid stagnation point. In this manner, particles are effectively trapped in free solution using a feedback control algorithm implemented with a custom-built LabVIEW code. The control algorithm consists of image acquisition for a particle in the microfluidic device, followed by particle tracking, determination of particle centroid position, and active adjustment of fluid flow by regulating the pressure applied to an on-chip pneumatic valve using a pressure regulator. In this way, the on-chip dynamic metering valve functions to regulate the relative flow rates in the outlet channels, thereby enabling fine-scale control of stagnation point position and particle trapping. The microfluidic-based hydrodynamic trap exhibits several advantages as a method for particle trapping. Hydrodynamic trapping is possible for any arbitrary particle without specific requirements on the physical or chemical properties of the trapped object. In addition, hydrodynamic trapping enables confinement of a "single" target object in concentrated or crowded particle suspensions, which is difficult using alternative force field-based trapping methods. The hydrodynamic trap is user-friendly, straightforward to implement and may be added to existing microfluidic devices to facilitate trapping and long-time analysis of particles. Overall, the hydrodynamic trap is a new platform for confinement, micromanipulation, and observation of particles without surface immobilization and eliminates the need for potentially perturbative optical, magnetic, and electric fields in the free-solution trapping of small particles.
The hydrodynamic trap consists of a two-layer hybrid (polydimethylsiloxane (PDMS) /glass) microfluidic device for particle confinement. Steps 1-2 describe fabrication of microfluidic devices, and Steps 3-4 discuss device design and operation.
1. SU-8 Mold Fabrication (not shown in video)
2. Microfluidic Device Fabrication
Steps 3-4 describe implementing the hydrodynamic trap using the microfluidic device described above.
3. Hydrodynamic Trap Experimental Setup
4. Hydrodynamic Trapping Procedure
LabVIEW Code: Usage Note for Feedback Controller
Automated particle trapping is achieved using a linear feedback control algorithm implemented using a custom LabVIEW code. The LabVIEW code captures images from a CCD camera and transmits an electric potential (voltage) to a pressure regulator, which actively modulates the position (partially open/closed state) of an on-chip dynamic pneumatic valve. As the valve position changes, the hydrodynamic flow rate in one outlet line is adjusted, thereby re-positioning the stagnation point and enabling hydrodynamic trapping. The steps in the feedback loop are sequentially and iteratively executed at the rate of image capturing (10-60 Hz). The LabVIEW code executes the following steps during each feedback loop cycle:
The LabVIEW code records the following data for every image captured during particle trapping: 1) time elapsed, 2) centroid (x,y) position of the trapped particle, 3) position of the trap center, 4) distance of the particle from the trap center, 5) pressure applied to the on-chip valve. In addition, the code also records a movie of the trapped particle in AVI file format.
5. Representative Results
We trapped fluorescent polystyrene beads of various size (100, 540, 830 nm, and 2.2 μm diameter) using a hydrodynamic trap. Figure 1(a) shows an image of a particle trapped at the cross-slot junction in a microfluidic device. The trajectory of a trapped particle may be determined directly from the centroid position data recorded by the LabVIEW code during a trapping event or by tracking and localizing the trapped particle from the recorded movie file. Figure 1(b) shows the trajectory of a trapped particle (2.2 μm fluorescent polystyrene bead) along the outlet channel direction. The bead is initially trapped (squares) for 3 min and is then released from the trap and escapes along one of the outlet channels (circles). Particle trajectories along the compressional flow axis (inlet channel direction; data not shown) are similar to particle trajectories along the extensional flow axis (outflow direction) as shown in Figure 1(b). A histogram of particle displacement from the trap center for a trapped bead (2.2 μm diameter) along the outlet channel directions is shown in Figure 1(c). Using the feedback control algorithm described in this work, trapped particles are confined to within ±1 μm of the trap center along the inlet and outlet channel directions.
A schematic of the microfluidic device used for hydrodynamic trapping is shown in Figure 2. The integrated microfluidic device consists of a fluidic layer and a control layer and is fabricated using standard multilayer soft lithography as described in this article. The fluidic layer contains the buffer and sample channels, as well as the cross-slot channel geometry to facilitate hydrodynamic trapping. The control layer consists of a pneumatic valve positioned above one of the outlet channels in the fluidic layer, and the control and fluidic layers are separated by a thin elastomeric membrane. During device operation, the valve in the control layer is pressurized with nitrogen gas, which forces the thin membrane into the fluidic layer, thereby inducing a constriction in the outlet channel. The dynamic pneumatic valve constricts the outlet channel by variable amounts by changing the pressure applied to the control layer, which adjusts the relative flow rates in the outlet channels and enables fine-scale control of the stagnation point.
Figure 1: Particle Trapping. (a) Image of a single bead confined in the hydrodynamic trap. In addition to the bead at the trap center, several untrapped beads are shown in the trapping region. (b) Trajectory of a trapped particle along the outlet channels (squares). When the particle is released from the trap (arrow), it escapes along one of the outlet channels (circles). (c) Histogram of displacements of a trapped bead (2.2 μm diameter) from the trap center along the outlet channels.
Figure 2: Schematic of microfluidic device for hydrodynamic trapping. The hydrodynamic trap is constructed using a two-layer microfluidic device. The fluidic layer consists of a sample inlet, four buffer inlets, and two waste outlets. The control layer consists of a pneumatic membrane valve situated on top of one of the outlet channels in the fluidic layer. A constriction in the opposing outlet channel provides an offset pressure for the pneumatic valve. Typical channel dimensions range between 100-500 μm. In region (A), sample inlet is flow focused by two buffer inlets. In region (B), opposing inlet streams converge at the cross-slot junction where trapping occurs. The pneumatic valve (C) is positioned on top of one of the outlet channels. The stagnation point position is modulated by regulating pressure to this valve.
Current microfluidic methods for particle manipulation based on hydrodynamic flow can be characterized as contact-based or non-contact methods. Contact-based methods use fluid flow to physically confine and immobilize particles against microfabricated channel walls 9, whereas non-contact methods rely on circulating flow or microeddies 10. In this work, we present a method for free-solution particle trapping using the sole action of fluid flow. The hydrodynamic trap enables confinement and manipulation of small particles at a fluid stagnation point in a microfluidic cross-slot device. In this device, an automated feedback control mechanism is used to confine particles by fine-scale and active adjustment of the stagnation point position in a flowing fluid.
What is the tightness of confinement for particles in the hydrodynamic trap and how can this be optimized? The accuracy of confining a particle to the trap center depends on the precision of centroid determination when localizing particle position. To achieve robust particle trapping, the user should ensure maximum image contrast between the particle and the background for optimal tracking and localization. In addition, special care should be taken to avoid bubbles or debris in the microchannels, which may affect particle tracking. A stable flow source should be used to minimize perturbations in fluid flow, as the stability of the stagnation point position is sensitive to flow fluctuations. Using this approach, hydrodynamic trap stiffness was measured to be ~1E-4 pN/nm for ~2 μm particles1, which is comparable to alternative methods including electrokinetic traps or optical tweezers. Micron-scale particles are confined to within 1 μm of the trap center for extended periods of time, which allows for precise positioning and manipulation of particles in free-solution. With further technology development, trapped particles may be transiently exposed to variable microenvironments when coupling the hydrodynamic trap with chemical gradients generated using laminar flow in microchannels. Finally, hydrodynamic trapping occurs at a stagnation point, where fluid convection tends to zero. In an ideal trap, particles are confined at a location of zero fluid velocity where particle motion is largely dominated by Brownian motion. From this perspective, the hydrodynamic trap is a non-pertubative method of trapping based on continuous fluid flow.
Hydrodynamic trapping and manipulation is readily achieved for any arbitrary "target" particle, given that the particle can be imaged, tracked, and localized using optical microscopy. Therefore, fluorescent and non-fluorescent particles and non-isotropic objects can be trapped without regard to the chemical/physical/optical nature of the trapped particle. In addition, the hydrodynamic trap can be easily integrated into existing soft lithography-based microfluidic systems without the need for complicated fabrication, patterning of electrodes or extensive optical setups. The hydrodynamic trap is a low-cost and user-friendly tool for particle trapping with minimal laboratory equipment requirements, including a microfluidic device, a pressure regulator, and a computer-based feedback controller. Overall, the hydrodynamic trap has the potential to transform fundamental and applied science studies of micro- and nanoscale particles.
No conflicts of interest declared.
We thank the Kenis group at the University of Illinois at Urbana-Champaign for helpful discussions and generously providing use of cleanroom facilities.
This work was funded by an NIH Pathway to Independence PI Award, under Grant No. 4R00HG004183-03 (Charles M. Schroeder and Melikhan Tanyeri).
This work was supported by the National Science Foundation through a Graduate Research Fellowship to Eric M. Johnson-Chavarria.
|21 gauge blunt needle||Zephyrtronics||ZT-5-021-1-L||For punching port holes in PDMS|
|3 ml plastic syringe||BD Biosciences||309585||For filling valve with oil|
|Si wafers||University Wafer||3” P(100) single side polished 380 μm test grade|
|Cover glass||VWR international||48404-428||24 x 40 mm #1.5|
|DAQ card||National Instruments||PCI 6229|
|Fluorescent beads||Spherotech, Inc.||FP-2056-2||2.2 μm Nile red|
|Fluorinert||3M||FC 40||Fluorinated carrier oil|
|Inverted Microscope||Olympus Corporation||IX-71|
|LabVIEW||National Instruments||Version 9.0f3 (32bit)|
|Stereo Microscope||Leica Microsystems||MZ6||For aligning PDMS control layer to fluidic layer.|
|Mechanical Convection Oven||VWR international||1300U||For baking devices to create monolithic PDMS slabs with two layers.|
|Microfluidic tubing and connectors||Upchurch Scientific||1/16 x .020 PFA tubing and super flangeless fittings|
|PDMS||GE Healthcare||RTV 615 A&B|
|Plasma Chamber||Harrick Scientific Products, Inc.||PDC-001|
|Pressure Transducer||Proportion Air||DQPV1|
|Spin Coater||Specialty Coating Systems||G3P-8 Spin Coat|
|Photoresist||MicroChem Corp.||SU 8 2050|
|Syringe Pump||Harvard Apparatus||PHD 2000 Programmable|
|Terminal Block||National Instruments||BNC 2110||For analog output to pressure regulator and read out.|
|UV Collimated Light Source and Exposure System||OAI||Model 30 Enhanced Light Source|