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Nanoengineered
Surfaces:
Enabling New Technologies
By Marlys Amundson
A team of UCLA researchers
has reduced the amount of pressure needed to move liquids through
channels by 30 to 40 percent. In a field where even a five percent
reduction is significant, the breakthrough will enable entirely new
applications, including in the field of microfluidics.
Led by Chang-Jin (CJ) Kim, a mechanical and aerospace engineering
professor in the UCLA Henry Samueli School of Engineering and Applied
Science, the multidisciplinary group has designed and manufactured
a surface with significantly less drag for liquid flows.
"Despite the explosive growth in microfluidics - especially for high-profile
applications such as biochips and lab-on-a-chip - transport of liquids
through long, nano- and microscale channels incurs losses too great
to be practical," notes Kim. "It takes a disproportionately high pressure
to move liquid through microchannels, limiting the feasibility of
building a miniaturized lab-on-a-chip."
Microfluidics is concerned with the physics and performance of fluids
and fluidic systems at the microscale. Unlike macrofluidic systems,
these systems are significantly affected by surface tension, energy
dissipation, and electrokinetics. Microfluidics has applications in
lab-on-a-chip technologies and micro-propulsion and micro-thermal
devices.
Kim and mechanical and aerospace engineering professor Chih-Ming Ho
are collaborating with professors Fred Wudl and Robin L. Garrell in
the chemistry and biochemistry department to design and create a nanomachined
surface that allows liquids to move more easily though microchannels.
The team manufactured a structure that creates a gap between the liquid
and the channel surface, reducing the amount of pressure needed to
move the fluid through the tubes. Their solution was inspired in part
by the lotus leaf.
"A lotus leaf has microstructures on its surface, as well as a natural
coating of wax that prevents mud and water from sticking to the plant,"
explains Kim. "By recreating that in the lab we were able to manufacture
a surface with considerably less drag."
The nano-size gaps on the surface of the new material decrease the
interface area between the surface and the liquid, which reduces the
drag, allowing the liquid to move more easily. The tiny size of the
gaps, as well as the hydrophobic coating on the posts, prevents the
liquid from sinking down and filling the holes.
"Typically, a drop of water sits on a surface like Teflon with contact
angle as high as 120 degrees," says Kim. "But on our nano-engineered
surface a drop of water sits at an almost 180 degree angle, decreasing
the surface area that is touching the structure to almost none."
Indeed, the nano-engineered surface was so slippery that graduate
student Joonwon Kim was not able to keep the droplet stationary even
on the most leveled surface he could prepare. A droplet moves on this
surface with less than one percent of the drag a regular flat surface
would impose.
A 99 percent reduction in drag for a droplet rolling on a surface
is amazing, but the real impact is the 30 to 40 percent reduction
of required pressure to ensure a continuous flow in tubes, according
to Kim. "My logic had suggested we might get a reduction of this magnitude,
but I am still in awe we are actually getting it," says Kim.
Their work is funded in part by a $1,000,000 grant from the National
Science Foundation.
Another group in Kim's lab has developed programmable arrays that
support lab-on-a-chip technology for microfluidics. The new technology
brings researchers significantly closer to being able to recreate
on a micro scale the things a lab technician can do with a pipette.
"Our goal is to be able to mimic typical biological or chemical lab
practices on a chip," says Kim. "Our goal is a setup that allows for
complicated lab-on-a-chip functions with minimal wait and energy consumption."
Kim's digital microfluidics system uses surface tension and electrical
signals to move the tiny droplets, and can move them either in a north-south
or east-west direction. Signals from electronics embedded in the chip
determine the path of the fluids, which are not constrained to pre-determined
channels. The user can also create, cut, and mix drops of the chemical
solutions. Mixing tiny amounts of fluid in current systems is a fairly
complicated process. Kim's system mixes fluids evenly and quickly
by moving a merged droplet around in the chip.
By altering the surface tension with voltage to move the droplets,
Kim's system bypasses the fabrication, power, and pressure constraints
encountered by traditional lab-on-a-chip set-ups.
"We're able to develop it as a battery operated system, since power
consumption is so low," notes Kim. "Because there are no channels,
the user is able to move the chemical solutions along any paths, giving
the operator much greater flexibility. They determine which drops
mix with which and in what ratio."
The fluidic functions can be programmed on a PDA and wirelessly downloaded
to the chip, making the system entirely reconfigurable.
The group's first successful demonstration was a two-dimensional checkerboard
layout. They are closer now to a multiple droplet system that places
biological and chemical samples on one grid and reagents on another
grid. The prototype circuit board developed by graduate student Shih-Kang
Fan is slightly larger than a PDA, and houses the electronics, drivers,
a microprocessor, and a battery.
Eventually, Kim hopes to shrink the system enough to enable applications
such as a system on a Band-Aid that can sense an infection, synthesize
a solution, and apply it to the surface of the cut.
Kim's digital microfluidics were introduced on National Geographic
TV as a promising technology to combat bioterrorism. The team is currently
developing a digital microfluidics demonstration unit for the Boston
Museum of Science.
For more about Kim's lab, please see http://cjmems.seas.ucla.edu/.
Nanopost image appears courtesy of CJ Kim. |
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