The towering coastal redwoods of northern California have long inspired awe in scientists and laymen alike. Sequoia sempervirens can grow to a height of more than 100 metres, ranking as Earth's tallest trees. When Abraham Stroock first visited a sequoia forest a decade ago as a graduate student in engineering at Harvard University, his thoughts turned to a question that had puzzled biologists for hundreds of years: how do these giants transport water all the way from their roots to distant leaves? Ten years later, Stroock, now an assistant professor of chemical engineering at Cornell University in Ithaca, New York, has the answer. He and graduate student Tobias Wheeler built a 'synthetic tree' — a microfluidic device that mimics the water-transportation capabilities of nature's tallest plants.

“I knew enough about biology to know that a cell is about 10 micrometres across, and enough about capillary phenomena to estimate what the capillary pull of a structure like that would be,” recalls Stroock of his first encounter with the redwoods. “And the result would have been a miserably small tree.” Biologists had long grappled with the same conclusion that capillary action alone couldn't account for water transportation in plants. Most accepted a passive 'wicking' model in which evaporation from the leaves pulls a long chain of water molecules up from the ground in a process known as transpiration. Stroock and Wheeler are the first to reproduce this phenomenon in the lab, with a microfluidic system in which chemically cross-linked hydrogel membranes represent natural root and leaf membranes, and are joined together by a fluid-filled microchannel that represents a plant's xylem capillaries (see page 208).

Their creation is no Sequoia — owing to materials and fabrication limitations, Stroock and Wheeler are working on a scale of tens of centimetres, not tens of metres. “But you can extrapolate from our system and see that you could get water transport against gravity over more than 100 metres,” Stroock says.

The biggest challenge for Stroock was “how to convince a chemical engineer to work on the problem for his PhD”. The solution? Wheeler hails from Sebastopol, California, and grew up with an appreciation for the giant native trees. “The transpiration process really is a collection of chemical engineering processes, mediated by membranes and changes in phase, heat and mass transfer,” says Stroock. “If you bring the process into a synthetic system, it becomes a chemical engineering problem.” With two engineers motivated by grand biology, Stroock says, “we were able to stick it out and make it work”.

Stroock has also collaborated with plant physiologists, and says that this new system will be used as a research platform to tease out further details of the transpiration process — such as how trees recover from inevitable failures of the system, when the long columns of water within the xylem break, leaving a vapour space. But as an engineer, Stroock has an eye on biomimetic applications as well. These include the possibility of designing artificial root systems that can extract and purify water from deep, sparse deposits. And because the synthetic tree can transport thermal energy as well as water, it could be used to design sophisticated heat-transfer devices similar to the heat pipes used to cool a laptop's motherboard, “but on an architectural scale” to cool buildings and other large structures.