Electric Tweezers

  • Edwards BE, Engheta N, Evoy S. "Theory of simultaneous control of orientation and translational motion of nanorods using positive dielectrophoretic forces.” Journal of Applied Physics (98), 124314, 2005. (pdf)
  • Edwards B, Mayer TS, Bhiladvala RB. "Synchronous electrorotation of nanowires in fluid" Nano Letters (6) 2006.
  • Edwards B, Engheta N, Evoy S. "Electric tweezers: Experimental study of positive dielectrophoresis-based positioning and orientation of a nanorod" Journal of Applied Physics (102) 2007. (pdf)
  • Edwards B, Engheta N. "Electric Tweezers: negative dielectrophoretic multiple particle positioning" New Journal of Physics (14) 063012, 2012. (pdf)
  • Edwards B. "Electric Tweezers: Arbitrary Dielectrophoresis Based Positioning and Orientation of Sub-Micron Particles" Dissertation, 2009. (pdf)

Introduction

Superposition of all of the images contained in the movie to the right

Superposition of all of the images contained in the movie to the right

The Electric Tweezers is a technology that can be used to apply arbitrary forces to cell sized objects. The goal is to perform much of the same functionality of Optical Tweezers in that it maneuvers particles without actually touching them. It is similar Magnetic Tweezers, ABEL and Fluidic Drag manipulation in that it is based on long range forces and a feedback mechanism. It employs electroorientation to apply an orienting torque and dielectrophoresis to apply a force. While a tremendous amount of work has been done in the past with dielectrophoresis, it has all been to produce motion in a single, non-reversible direction. This is what makes the Electric Tweezers unique. By surrounding one or more particles with an array of electrodes, the Electric Tweezers calculates the voltage that is needed on each one to create the force and orientation that the operator desires. Additionally the Electric Tweezers works with any microscope with video output.

Theory

fourforceH.png

The physics and mathematics behind calculating the electrode voltages in order to make a set of particles do what we want has been discussed in my papers and dissertation. However, without getting bogged down in too many details we'll summarize the basic idea:
1.  When there is an an electric field applied across a small particle, it will tend to orient to minimize its potential energy. For a positively polarized particle like a gold nanorod in water, this will align it with the electric field. Therefore, we can control the orientation of a particle by controlling the angle of the electric field at the particle's location.
2.  The particle will also attempt to minimize its potential energy in space, which for a positively polarized particle like a gold nanorod, means moving in the direction of greater electric field. For negatively polarized particle like a glass bead, this means moving in the direction of lesser field. Therefore we can control the force on a particle by controlling the way the electric field changes in space.
3.  Both of these conditions can be expressed in terms of the local derivatives of the voltage field at the location of the particle which in turn puts constraints on the voltages that can be applied to the electrodes. A computer then finds the minimum voltages that satisfy these constraints, making the particles do exactly what we want.

The surrounding plots and movies show some examples of what this might look like. The voltage fields are shown with the black equi-electricpotential lines. Remember that the electric field (E) is perpendicular to these lines. Additionally in the field quantity -abs(E)2) is plotted below in color. The gold rods will tend to want to "roll down-hill" on these plots while glass beads would "float up-hill" in these plots.

 

Experimental

The Experimental Setup

Even though this was an academic project, my goal was to create a commercial device from it. This had less to do with making money (not that there's anything wrong with that), and more about making an impact. In contrast to Optical Tweezers, this is a purely electronic/software based device and like most electronics, it is very difficult to build the first one, but once the design is complete, additional devices can be made for a fraction of the cost, providing it was built right and not duct-taped together at the outset. For this reason you'll see that it has a much more polished appearance than many dissertation works.

The Square Wave Generator Array in contact with the patterned microscope slide

The device consists of a board of synchronous bipolar square wave generators. The challenge was that they needed to be bipolar, covering an amplitude of 10V to -10V that could be changed at 60Hz with frequencies up to the low MHz. I could not find a signal generators will not do this quickly, at high frequencies, programmably, and synchronously. Therefore, I had build mine from scratch. I put the entire electronics assembly on a single PCB board that wrapped around the microscope slide but left room for an upright or inverted microscope to view and track the particles. Everything was controlled from a laptop running LabVIEW, which did all the real-time image analysis and calculated the voltages. Between particles, microscope camera, LabVIEW, and electronic hardware can be thought of as a feed back loop that can be used to hold particles in their locations

Results

When the electrodes are activated, positively polarized particles at locations where we are not controlling the electric field will move toward the electrode edges where the electric field is greatest. Negatively polarized particles will cluster around the center of the array where there tends to be an electric field null. However, regardless of polarizability, particles at locations where we are controlling the electric field will be herded into position against the randomizing effects of Brownian motion.

Engagement (positive polarization)Engagement (negative polarization)

From there we can manipulate the particle. In a relatively dramatic example, I made a gold nanorod spell out PENN.

Additionally, I demonstrated that we can manipulate two particles, and even push around a particle that is relatively unaffected by electric field.