Wood transistor could let us embed electronics in trees

An electrical switch made from conductive wood could become a building block for future electronic devices embedded within living trees and other plants.

“There is an emerging research field called electronic plants, where scientists look at different ways to send signals inside plants or to incorporate functionality such as sensors in plants, even in living plants,” says Isak Engquist at Linköping University in Sweden.

Engquist and his colleagues developed the wood equivalent of a transistor – an electronic component that can boost electric currents or act as a switch for electric signals. A single computer chip the size of a fingernail contains billions of tiny transistors made from semiconductor materials such as silicon. Each semiconductor transistor can switch on and off billions of times per second. Compared with silicon transistors, the wood transistors are significantly larger, each 3 centimetres long. They also have much slower switching speeds that only allow them to switch off in about 1 second, and to switch on in about 5 seconds.

But the wood transistors could prove more sustainable and biocompatible for certain electronic applications in agriculture or forestry, such as monitoring plants’ resistance to environmental stress and climate change.

To create the wood transistor, Engquist and his colleagues used a heating and chemical process to remove lignin – an organic binding substance within wood and plants – from pieces of balsa wood. That process freed up space within and between the natural network of tubes, called lumina, that transport water within wood.

They then immersed the wood within a liquid solution containing a conductive polymer, allowing the polymer to soak into the wood and coat the lumina. That created conductive wood capable of interacting with electrolytes – chemicals that conduct electricity when dissolved in water – as the foundation for constructing a wood transistor.

The researchers demonstrated and measured the wood transistor’s operations during multiple switching test runs. This represents an “exciting engineering possibility for utilising wood” as a scaffold “that can incorporate electrical materials into devices”, says Tian Li at Purdue University in Indiana, who wasn’t involved in the study.

The team initially tried several types of wood, including birch and ash. But balsa turned out to have the ideal characteristics for this approach, including retaining its structural integrity after the removal of the lignin, absorbing the conductive polymer without problems and having no significant seasonal difference between summer and winter wood.

Researchers may eventually grow conductive wood with the polymer already inside, says Engquist. That could involve using different conductive polymers to penetrate the wood without having to first remove the lignin.

“It seems most probable that each piece of wood, or each plant, would incorporate only a few wood electrochemical transistors, and that these would be on the millimetre-size scale,” says Engquist.

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