Wood is good for a lot of things. Building boxes, boats, and bookcases, for instance. Making tools, or campfires. Feeding termites. And beavers.
You’ll note powering functional electrical appliances isn’t among them.
Researchers at Linköping University and the KTH Royal Institute of Technology in Sweden clearly never paid much attention to lists of things wood is bad at, so they went ahead and made the world’s first wooden transistor.
To be upfront, the team behind the innovation added a special ingredient to get around the whole issue of wood being so lousy at conducting a current. And while it works, don’t expect to find one inside your next smartphone.
“Yes, the wood transistor is slow and bulky, but it does work and has huge development potential,” says electrical engineer Isak Engquist from Linköping University.
The humble transistor is a fundamental component in just about any piece of electronic technology. Functioning as a tiny ‘gate’ that controls the flow of one current through the application of another, it can amplify signals, store data as a string of switches, or work together to carry out logic operations.
It can do all of this thanks to a property of semiconductive materials that permits them to carry a current only when they themselves have enough of a charge.
Where the first transistors were lumpy objects you could maybe balance on a fingertip, they’ve since been constructed on a scale that crams tens of billions into the same space.
Wood isn’t semiconductive. In fact, the carbohydrate building blocks of plant fibers are downright stubborn when it comes to kicking electrons down the road.
One way to get around that problem is to reduce wood to a stick of charcoal, leaving a relatively pure carbon highway for an electrical charge to tumble along.
But charcoal isn’t really wood, so where’s the fun in that?
Using the hardwood balsa for its relatively high strength, low density, and homogenous structure, Engquist and his team stripped it free of its tough lignin and filled the remaining material with a mixed electron-ion conducting polymer called poly(3,4-ethylenedioxythiophene)–polystyrene sulfonate, or PEDOT:PSS.
If we’re being technical, the wood acts more as a housing for the conductive material, like a messy bundle of wires. The idea isn’t a new one either, with researchers having previously explored the idea of using plant material as a porous container for electrochemical and conductive substances.
Engquist built on this experience by attempting to optimize the removal of lignin to create a more efficient channel for the polymer to operate within, before analyzing the conductive properties of the balsa-polymer unit.
By stacking millimeter-thick units that operated as electrodes and channels, the team found they could create a rather crude transistor. With no voltage, the entire structure can be considered open and the switch set in the ‘on’ position. Pump in 6 volts, and the channel fills with electrons, slowly squeezing the channel shut and flicking the switch into the ‘off’ position.
Turning the wooden device off takes around 1 second while returning it to the on position takes roughly 5 seconds. Considering the transistors in your computer operate at speeds measured in gigahertz – flicking on and off hundreds of billions of times a second – you can forget cobbling together enough sticks to play Fortnite.
So what is the point, you ask?
“We didn’t create the wood transistor with any specific application in mind,” says Engquist.
“We did it because we could. This is basic research, showing that it’s possible, and we hope it will inspire further research that can lead to applications in the future.”
Rapid computations aside, biodegradable electronics made from easily harvested resources could be used in remote sensors that need to break down easily or inconspicuous devices powered by movements in the environment.
These are just a few more things wood could be rather good for.
This research was published in PNAS.