A schematic of the bubble still.

A Real Microbrewery

The happy art of distillation has been around for a good 5,000 years or so. But in the future, a household distillery could be essential for your health—and not by making moonshine for medicinal purposes. Caltech researchers have crafted the world’s tiniest still to concentrate scant amounts of biomolecules, which could help detect the extremely low-abundance biomarkers that herald some diseases.

“Distillation is a well-established technology. You wouldn’t think there’d be many new avenues to develop,” comments David Boyd, a lecturer in mechanical engineering at Caltech who grew up in Alabama and is the lead author of a paper describing the work. “But in our approach, you don’t need to boil the fluid anymore.”

In a still, the vapor from a boiling liquid passes through a condenser, and the chilled condensate collects in a separate pot. If the liquid is a mixture of fluids with different boiling points, like ethyl alcohol in a watery corn mash, the lower-boiling one—the white lightning—will be concentrated in the condensate. Alternatively, boiling off the liquid leaves the heavier molecules behind, concentrating trace chemicals—the biomarkers—in the still pot. The catch, of course, is that the high heat will turn your precious proteins into so much gravy stock. Boyd and his colleagues have created a microstill that operates at room temperature, and thus could be used as a biomonitor.

The still is a microfluidic chip whose channels are studded with gold nanoparticles. Air bubbles, normally the bane of microfluidic designs, are actually the key to this one’s success. Each bubble acts as a tiny still pot. A laser no more powerful than a classroom pointer zaps the gold nanoparticles behind the bubble. The particles quickly transfer the heat to the surrounding fluid, and a little bit of liquid on the bubble wall vaporizes, passes through the bubble, and condenses again on the cooler wall of the bubble’s front. “Only the most volatile molecules cross over the bubble. Everything else is left behind,” Boyd says. “Typically, air bubbles are a real annoyance, because they pin the flow in the fluid, and are hard to get rid of. We’ve learned to love them.”

The bulk fluid stays at room temperature, preserving the fragile protein molecules intact, because of a unique heating property of gold in its nanoform. The particles absorb green light very strongly, “acting like antennas for visible light,” says coauthor David Goodwin, professor of mechanical engineering and applied physics.

A microphotograph of the real thing. The channel, 100 millionths of a meter wide, runs sideways; the bubble is the square in the middle. The spheres in the bubble are smaller bubbles—at 10 billionths of a meter in diameter, the gold nanoparticles are invisible at this scale.

In a demonstration experiment, the team dissolved a blue dye in ethanol, then mixed it with water. As the mixture coursed through the microchannels, the color behind each bubble intensified, while the liquid in front of each bubble turned clear.

Moving even beyond clinical diagnostic tools, the ultimate goal is to use this microstill as part of a personal sensor, perhaps even one worn as a patch, to track the level of some substance in one’s blood. “Say you are on some prescription whose optimum dose is 10 micrograms per liter of blood,” says Boyd. “The still could concentrate that med so another part of the chip could measure it accurately, and automatically control the release of more of it as needed to maintain the activity level.”

The paper, whose other authors are James Adleman (MS ’04, PhD ’08), a graduate student in electrical engineering, and Demitri Psaltis, Caltech’s Myers Professor of Electrical Engineering, appeared in the April 1 issue of Analytical Chemistry.—DS/EN