Tiny Labs: Polymers on silicon chip catch, release proteins

Using polymers as tiny molecule-absorbing sponges, researchers have taken a step toward shrinking room-size chemical laboratories to the size of a crumb.

Microchips full of tiny channels and mixing chambers may eventually enable scientists to analyze minute amounts of any solution quickly and accurately (SN: 8/15/98, p. 104: https://www.sciencenews.org/sn_arc98/8_15_98/bob1.htm). Such so-called laboratories-on-a-chip could be useful for detecting the first molecular signs of disease in a blood sample or the presence of a bioterrorism agent in the environment.

For the technology to work, however, it has to manipulate molecules to be analyzed on the chip. Toward that end, researchers at Sandia National Laboratories in Albuquerque have developed a new method for gently grabbing and releasing proteins at particular places on a silicon wafer. This system could be used to concentrate biological molecules from dilute solutions for analysis, says Bruce C. Bunker.

He and his Sandia colleagues describe their research in the July 18 Science.

The investigators started with a silicon wafer on which they’d applied a silicon nitride coating, says team member Dale L. Huber. After etching a narrow channel in the silicon, the scientists deposited thin gold lines on top of the nitride layer so that the lines formed bridges over the silicon nitride–covered trench. Because the trench acts as an insulator, each gold bridge can be electrically heated. Each is essentially “a microtoaster,” says Bunker.

Finally, the researchers grew a dense thicket of polymer molecules into a thin film over the gold lines. At 35C, about body temperature, the polymer switches from a water-attracting state to a water-repelling one. When the researchers introduce a couple of microliters of protein-containing solution to the chip at room temperature, the polymer swells with water. When a gold bridge is then heated above 35C, the overlying polymer expels the water and quickly adsorbs the nearby protein molecules. Finally, by lowering the temperature, the researchers can induce the polymer to release the concentrated proteins over the gold bridge.

The system can’t yet adsorb particular proteins from a mixture such as blood, says Bunker. However, when a complex brew of proteins is applied to the chip, the polymer appears to adsorb smaller ones first and then replace them with larger ones, which take longer to enter the polymer layer but stick more strongly once inside. This property hints at one way that the researchers might build chemical selectivity into their chips.

The power of the Sandia approach is that it enables researchers to use electrical signals to control protein adsorption and release, comments Mark Burns of the University of Michigan in Ann Arbor.

With such “switchable surfaces,” adds Matthew Tirrell of the University of California, Santa Barbara, researchers potentially could release or mix components in a laboratory-on-a-chip just as they do in a room-size set-up.

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