Wednesday, August 26, 2009
Johns Hopkins engineers are using a popular children's toy
to visualize the behavior of particles, cells and molecules in environments too
small to see with the naked eye. These researchers are arranging little LEGO
pieces shaped like pegs to re-create microscopic activity taking place inside
lab-on-a-chip devices at a scale they can more easily observe.
A tiny white ball is release into a Lego board with peg pieces, immersed in a tank filled with glycerol to help researchers visualize what happens at nanoscale in microfluidic arrays. Credit: Will Kirk/JHU
These lab-on-a-chip devices, also known as microfluidic
arrays, are commonly used to sort tiny samples by size, shape or composition,
but the minuscule forces at work at such a small magnitude are difficult to
measure. To solve this small problem, the Johns Hopkins engineers decided to
think big.
Led by Joelle Frechette and German Drazer, both assistant
professors of chemical and biomolecular engineering in the university's Whiting
School of Engineering, the team used beads just a few millimeters in diameter,
an aquarium filled with goopy glycerol and the LEGO pieces arranged on a LEGO
board to unlock mysteries occurring at the micro- or nanoscale level. Their
observations could offer clues on how to improve the design and fabrication of
lab-on-a-chip technology. Their study concerning this technique was published
in the Aug. 14 issue of Physical Review Letters.
The idea for this project comes from the concept of
"dimensional analysis," in which a process is studied at a different
size and time scale while keeping the governing principles the same.
"Microfluidic arrays are like miniature chemical
plants," Frechette says. "One of the key components of these devices
is the ability to separate one type of constituent from another. We
investigated a microfluidic separation method that we suspected would remain
the same when you scale it up from micrometers or nanometers to something as
large as the size of billiard balls."
With this goal in mind, Frechette and Drazer constructed an
array using cylindrical LEGO pegs stacked two high and arranged in rows and
columns on a LEGO board to create a lattice of obstacles. The board was
attached to a Plexiglas sheet to improve its stiffness and pressed up against
one wall of a Plexiglas tank filled with glycerol. Stainless steel balls of
three different sizes, as well as plastic balls, were manually released from
the top of the array; their paths to the bottom were tracked and timed with a
camera.
The entire setup, Drazer said, cost a few hundred dollars
and could easily be replicated as a science fair experiment.
Graduate students Manuel Balvin and Tara Iracki, and
undergraduate Eunkyung Sohn, all from the Department of Chemical and
Biomolecular Engineering, performed multiple trials using each type of bead.
They progressively rotated the board, increasing the relative angle between
gravity and the columns of the array (that is, altering the forcing angle). In
doing so, they saw that the large particles did not move through the array in a
diffuse or random manner as their small counterparts usually did in a
microfluidic array. Instead, their paths were deterministic, meaning that they
could be predicted with precision, Drazer said.
The researchers also noticed that the path followed by the
balls was periodic once the balls were in motion and coincided with the
direction of the lattice. As the forcing angle increased, some of the balls
tended to shift over one, two, three or as many as four pegs before continuing
their vertical fall.
"Our experiment shows that if you know one single
parameter—a measure of the asymmetry in the motion of a particle around a
single obstacle—you can predict the path that particles will follow in a
microfluidic array at any forcing angle, simply by doing geometry." Drazer
said.
The fact that the balls moved in the same direction inside
the array for different forcing angles is referred to as phase locking. If the
array were to be scaled down to micro- or nanosize, the researchers said they
would expect these phenomena to still be present and even increase depending on
the factors such as the unavoidable irregularities of particle size or surface
roughness.
"There are forces present between a particle and an
obstacle when they get really close to each other which are present whether the
system is at the micro- or nanoscale or as large as the LEGO board,"
Frechette said. "In this separation method, the periodic arrangement of
the obstacles allows the small effect of these forces to accumulate, and
amplify, which we suspect is the mechanism for particle separation."
This principle could be applied to the design of micro- or
nanofluidic arrays, she added, so that they could be fabricated to "sort
particles that had a different roughness, different charge or different size.
They should follow a different path in an array and could be collected
separately."
Phase locking is likely to become less important, Drazer
cautioned, as the number of particles in solution becomes more concentrated.
"Next," he said, "we have to look at how concentrated your
suspension can be before this principle is destroyed by particle-particle
interactions."
Both Drazer and Frechette are affiliated faculty members of
Johns Hopkins Institute for NanoBioTechnology. The research was funded by
grants from the National Science Foundation and the American Chemical Society
Petroleum Research Fund.
Citation: "Directional
locking and the role of irreversible interactions in deterministic
hydrodynamics separations in microfluidic devices"
German Drazer's
Lab website
Joelle Frechette Lab
website
Institute for
NanoBioTechnology
SOURCE: Johns Hopkins
University