Heated plastic holds proteins
Technology Research News
Imagine walking on a floor that your feet
sometimes stick to and sometimes do not. Now imagine if that floor could
sort people by causing those under a certain height to stop in their tracks
while everyone over that height walked freely by.
Researchers from Sandia National Laboratory have found a way to
switch the surface chemistry of a biochip that allows them to trap and
release protein molecules.
The method could eventually allow protein molecules to be sorted
by size, or even by types specific to different species, which would make
it able to sense pathogens, including those intentionally released in
acts of bio-terrorism, said Bruce Bunker, a principal number of the technical
staff at Sandia National Laboratories.
The researchers' prototype sorting device consists of a tiny heating
surface coated with an array of polymer chains. Polymers are long molecules
made from small, repeating units.
At room temperature, these polymer molecules interact with water
to swell to 10 times their normal size. At the same time a sheaf of water
surrounds the swollen polymer, keeping other molecules away. "This ordered
water not only promotes expansion of the polymer, but it forms a protective
barrier that prevents... proteins from coming into contact with the chains,"
Above a transition temperature of about 30 degrees Celsius, the
ordered water formation breaks down, allowing the chains to contract and
proteins to contact and stick to the polymer. "The net result is... proteins
do not stick to the polymer at room temperature, but readily [stick] above
the transition temperature," said Bunker.
Making proteins stick or not stick is simply a matter of changing
the temperature of the heating surface, said Bunker. "When we put the
coating on top of a micro heater device, we can thermally program the
surface to grab or release proteins on command," he said.
The setup traps and releases proteins in a matter of seconds,
and causes little damage to the fragile molecules, according to Bunker.
The tricky part of making the surface is making sure there are
no bare spots, said Bunker. "If the [polymer] chains are spaced too far
apart, bare patches open up between the chains when the film collapses
above the transition temperature, allowing proteins to penetrate the film
and stick to the underlying substrate," he said.
The heating device is a thin silicon nitride membrane striped
with gold heater lines, and covered with a thin layer of silica, said
Bunker. The silica "provides a platform for... the polymer film," he said.
The chip can be added to devices that already contain electrical and fluidic
connections, making the chip easy to replace, he said.
The current prototype traps relatively small numbers of protein
molecules, said Bunker. "Only a single monolayer is adsorbed on surfaces
in the existing device, so the total quantities of adsorbed protein are
low." The researchers are looking to increase the surface area by using
substrates that contain pores, he said.
The researchers' are also working to use the switchable films
to sort different types of proteins, like large proteins from small, said
Bunker. They found that in mixtures of large and small proteins, small
proteins stick first and that are replaced over about 10 minutes by the
large proteins. The researchers could use timing to sort proteins by size,
said Bunker. "An example... would be the removal of... small concentrations
of cell-signaling proteins -- cytokines -- from blood serum, which contains
high concentrations of the large protein albumin," he said.
Ultimately the researchers are aiming to make the protein traps
sort specific proteins or proteins from specific biological species, he
said. The researchers' current approach involves using the protein traps
to grab antibodies that in turn grab specific types of proteins," said
Bunker. "Antibody monolayers are known to have the ultimate selectivity
for [trapping] specific species from complex biological mixtures," he
Switchable films would be more economical than existing techniques,
which use tethered antibodies, said Bunker. This is because the researchers'
method is reusable, he said. "We can release the antibody film after the
bio-species are adsorbed and then regenerate a new antibody film for the
next separation or analysis procedure using the same or a different antibody,"
The researchers' technique is a combination of existing technologies
that may eventually prove useful for separating certain types of proteins,
said Ronald Siegel, a professor of pharmaceutics at the University of
Minnesota. Separation is one operation that could be included on labs-on-a-chip,
The method will have to be more extensively analyzed to see whether
it can achieve the throughput and efficiency needed for such operations,
however, he added.
The device could be used in practical microfluidic systems in
as little as two years, said Bunker.
Bunker's research colleagues were Dale L. Huber, Ronald P. Manginell,
Michael A. Samara, and Byung-Il Kim. The work appeared in the July 18,
2003 issue of Science. The research was funded by the Department of Energy
(DOE) and Sandia National Laboratories.
Timeline: 2 years
TRN Categories: Microfluidics and BioMEMS; Biotechnology
Story Type: News
Related Elements: Technical paper, "Programmed Adsorption
and Release of Proteins in a Microfluidic Device," Science, July 18, 2003.
September 24/October 1, 2003
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