Heated plastic holds proteins

By Kimberly Patch, 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," said Bunker.

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 said.

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," he said.

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, he added.

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
Funding:   Government
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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