|
  
Team Combines Modeling and Experimentation to Improve
Microfluidics
CAMBRIDGE, Mass., February 2, 2004 - Researchers at the ISN
are currently working to demonstrate experimentally a theoretical
phenomenon that may have powerful implications for the development
of microchip sensors for detecting chem/bio agents or monitoring
physiological conditions. The theoretical effect, “induced-charge
electro-osmosis,” (patent pending) was predicted in
2001 by Prof. Martin
Bazant of the Department of Mathematics with Dr. Todd
Squires, then a Ph.D. student at Harvard and now a postdoctoral
fellow at Caltech.
One of the most promising approaches to micro-sensing is
the development of tiny chips (less than one square centimeter)
containing nano-engineered surfaces that can detect specific
compounds, like proteins, viruses, and antibodies. However,
such micro-reactors require the manipulation of fluids through
micron-sized channels, and standard approaches require high
pressures for mechanical pumping and long times for molecular
diffusion. If a sensor is to be of use to the Soldier in the
field, it must operate quickly and not require bulky pumps
or batteries.
Non-mechanical pumping strategies, such as electro-osmosis,
are a good alternative since the entire device, including
the power source, can be integrated onto the chip. However,
conventional electro-osmosis using a DC field has several
drawbacks including the use of relatively large voltages,
limited device lifetime due to dissolution of the anode, and
sample contamination due to electrochemical reactions. Overcoming
these difficulties with AC fields is not possible with normal
electro-osmosis because the fluid flow will reverse direction
each time the field sign changes, resulting in zero net flow
over time.
In the induced-charge electro-osmosis (ICEO) approach proposed
by Bazant and Squires, a weak AC field is applied around polarizable
objects, such as metal posts or patterned surfaces, in the
micro-channels to pump fluids or force them into circulating
motions. The field interacts with diffuse surface charges
that it itself induces on the micro-posts and, because the
flow is independent of field direction, it persists in an
AC field. Only relatively small voltages are needed to generate
flow speeds that can reduce the sensing time for large molecules
from hours or days to a few minutes or seconds. The enhanced
mixing of the flow can also increase the sensitivity of the
device by improving the chances that a rare probe molecule
will meet a detector site on a surface.
Bazant and his graduate student, Jeremy Levitan (who is also
the ISN’s first Army Research Assistant in Nanotechnology,
or ARANT), are continuing to develop computer simulations
of the magnitude of ICEO flows, including those involving
more complex geometries. They are also working to demonstrate
the theory in a simple micro-channel device with one metal
post using particle tracking and flow visualization with fluorescent
dyes, in collaboration with Prof.
Todd Thorsen of Mechanical Engineering. If successful,
this experimental demonstration would be a major scientific
advance that would lead to the next phase of engineering applications
for these systems.
Combining simulations with real experiments speeds the pace
of discovery, according to Bazant. “You can save months
modeling these devices over having to build them, and you’re
saving costs too,” he says. “And if you understand
the theory of small-scale phenomena, you can actually predict
new phenomena and new devices that might be worth building.”
The next step in the research will be considering how to
integrate these devices with nanotechnologies for biological
and chemical analysis, such as DNA micro-array hybridization
assays.
Prof. Bazant and his collaborators have recently submitted
papers on this work to the Journal of Fluid Mechanics
and Physical Review Letters.
Contact:
Franklin Hadley
617-324-6413
fhadley@mit.edu
Back to Research News
|
