Fake Plastic Trees

A tree is growing at Cornell University. Unlike the English elms, Japanese maple, and swamp oaks lining the Ithaca campus, this specimen is about seven centimeters tall and made of plastic. It is a synthetic tree, the brainchild of two biomolecular engineers, and the first man-made system able to mimic the powerful pumping capability of plants. It could also be the seed of an engineering revolution.

The Telegraph (UK)

Most plants transport water through internal channels called xylem. This system operates upon the principle of negative pressure. Water evaporating from the surface of a leaf creates a tensile force akin to pulling on a rope. This force draws water from the ground, through the roots and up the entire plant. Tobias Wheeler, the leader of the project, explained that the mechanism of water transport in plants was deduced in the 1890s. Efforts to produce a replica of this model, however, have progressed little since then.

The first and greatest hurdle to be overcome was the selection of the material used to construct the tree. Wheeler and his mentor, Abraham Stroock, knew they needed something that would form tiny pores to hold water within the artificial xylem. They eventually hit upon the idea of using a hydrogel, the material used in soft contact lenses. In hydrogels, “the polymer network and the water are… mixed, such that the pores are effectively molecular in scale”, said Stroock.

The finished product is a relatively simple apparatus: two circular regions on the surface of the hydrogel, etched with 80 parallel channels, connected by a central “trunk” groove. Water moved through the synthetic tree at forces of up to 10 atmospheres. The previous record in any liquid pumping system was 0.7 atmospheres.

Wheeler said that scaling up their tiny tree to the size needed for civil engineering projects was “not a trivial matter”. Nevertheless, he and Stroock enthusiastically speculated about a slew of possible applications for their discovery. Biologists could use them to study the finer aspects of plant physiology. Viticulturalists could use them to closely monitor water levels in the vines of wine grapes. Synthetic trees could be used to construct deeper wells and larger heat pipes, cooling homes for a fraction of the cost of air conditioning. The heat transfer systems used today to cool electronics only exist on the centimeter scale.

“We could potentially build the sequoia of heat pipes,” said Stroock.

The original article: Wheeler, Tobias and Stroock, Abraham.  “The transpiration of water at negative pressures in a synthetic tree.” Nature 455:208-212 (11 September 2008)