In Abraham Stroock’s lab at Cornell University, the world’s first synthetic tree sits in a palm-sized piece of clear, flexible hydrogel — the type found in soft contact lenses. Stroock and graduate student Tobias Wheeler have created a “tree” that simulates transpiration, the process that allows trees to wick moisture upward to their highest branches. Their work, reported in the Sept. 11 issue of the journal Nature, supports the long-standing theory that transpiration in trees and plants is a purely physical process, requiring no biological energy. It may lead to new passive heat transfer technologies for cars or buildings, better methods for remediating soil, and more effective ways to draw water out of partially dry ground.

Stroock’s synthetic tree doesn’t look much like a tree at all. It consists of two circles side by side in the gel, with evenly spaced microfluidic channels to mimic a tree’s vasculature. In nature, trees use water in tubular tissues, called xylem, like ropes that pull water from the ground and delivering it to leaves. They manipulate the water in the xylem under negative pressure — what’s called a metastable liquid state — on the verge of becoming a vapor. Xylem-like capillaries are relatively easy to create by microfabrication, but the researchers’ innovation was in the material used to simulate membranes in the leaf and root. They used pHEMA hydrogel, or polyhydroxyethyl methacrylate, to form the plant membranes. The hydrogel is a solid embedded with water and has nanometer-scale pores. It acts as a wick by holding liquid in the pores, through which capillary action creates tension in the water. By mimicking a real tree’s xylem capillaries within the gels, the scientists were able to create negative pressures of the magnitude observed in nature and pump water against large resistances and out of partially dry sources. Heat-transfer technology commonly used for cooling laptops could be scaled up using the technology developed for the synthetic tree, Stroock said — for example, in a building’s heating system. He also envisions the synthetic tree helping to build better soil remediation systems. Instead of having to soak contaminated soil to pump contaminants out, transpiration could help pull the contaminated fluid out of the soil without the use of more liquid. Similarly, the technology could be used to draw water out of relatively dry soil without having to dig a well down to the water table. Go to: ScienceDaily

Posted September 17th, 2008 under Innovation of the Week, Tech Transfer

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