Purpose: Sorting and positioning of objects in microfluidic channels such as cells, macromolecules, and micro/nano-particles is useful in a broad range of studies. For example, positioning and holding a specific cell sitting in suspension via a dielectrophoretic trap can facilitate detailed inspection via microscopy or other interrogation techniques. Furthermore, dielectrophoretic forces can be used to controllably displace and sort particles and other small objects in solution in channels.
Method of Fabrication: The Bau group at the University of Pennsylvania has extensive experience building dielectrophoretic devices by fabricating patterned metal electrodes on glass or silicon surfaces. Glass is often preferable because of its transparency. Briefly, the patterned electrodes are made by first patterning a photoresist layer, then depositing a metal layer (often Au) via e-beam evaporation, and then dissolving the resist in a solvent to lift off the metal in the regions where it is not needed. As these are thin film planar electrodes, a PDMS channel can subsequently be bonded on top of the patterned electrodes to place the devices in a channel. Electrical connections are made to sections of the electrodes that extend beyond the channel walls.
Use/Results: Using dielectrophoretic devices based on patterned electrodes, the group has positioned macromolecules such as actin filaments (Arsenault, 2007), microtubules, and carbon nanopipes at predetermined locations. Electrical polarization forces have also been applied to control the tension of actin filaments [Arsenault 2010]. For example, actin filaments of various tensions were stretched across trenches to study the unhindered motility of various processive myosin motors (Arsenault, 2009) (Yuan, 2013). In addition to localizing macromolecules, polarization forces have been applied to localize nematodes such as C. elegans to enable close inspection of the animals (Chuang, 2011). We have also modeled the polarization process, accounting for the concentration polarization in the electric double layer to predict particle’s trajectories (Liu, 2004, Zhao, 2008, Zhao, 2009, Zhao, 2010).
We use flow fractionization to sort particles such as bacteria by size. To this end, we utilized a combinations of hydrodynamic and electrokinetic (both dielectrophoresis and electrophoresis) forces. In the device featured a stream laden with heterogeneous mixture of fluorescently-labeled particles enters in the top conduit. Clear fluid entered in the bottom conduit and squeezes the top stream to form a thin layer next to edge of the wide conduit. The trajectories of the fluorescing particles are readily visible in the right column. Dielectrophoretic forces were applied with a set of slanted electrodes to deflect the particles causing them to leave the device through one of the five collection conduits on the right. Each exit conduit collects particles in a particular size range. The histograms compare the measured fractions of particles of various sizes that were collected in the exit conduits with theoretical predictions.
Arsenault ME, Zhao H, Purohit PK, Goldman YE, Bau HH. Confinement and manipulation of actin filaments by electric fields. Biophys J Oct 15 2007; 93 (8): L42-44.
Arsenault ME, Sun Y, Bau HH, Goldman YE. Using electrical and optical tweezers to facilitate studies of molecular motors. Phys Chem Chem Phys Jun 28 2009; 11 (24): 4834-4839.
Yuan J, Pillarisetti A, Goldman YE, Bau HH. Orienting actin filaments for directional motility of processive myosin motors. Nano Lett Jan 9 2013; 13 (1): 79-84.
Chuang HS, Raizen DM, Lamb A, Dabbish N, Bau HH. Dielectrophoresis of Caenorhabditis elegans. Lab Chip Feb 21 2011; 11 (4): 599-604.
Liu H, Bau HH. The dielectrophoresis of cylindrical and spherical particles submerged in shells and in semi-infinite media. Physics of Fluids May 2004; 16 (5): 1217-1228.
Zhao H, Bau HH. Effect of double-layer polarization on the forces that act on a nanosized cylindrical particle in an ac electrical field. Langmuir Jun 17 2008; 24 (12): 6050-6059.
Zhao H, Bau HH. The polarization of a nanoparticle surrounded by a thick electric double layer. J Colloid Interface Sci May 15 2009; 333 (2): 663-671.
Zhao H, Bau HH. Polarization of nanorods submerged in an electrolyte solution and subjected to an ac electrical field. Langmuir Apr 20 2010; 26 (8): 5412-5420.