“Talk about flower power. Researchers have crafted flexible electronic circuits inside a rose. Eventually such circuitry may help farmers eavesdrop on their crops and even control when they ripen. The advance may even allow people to harness energy from trees and shrubs not by cutting them down and using them for fuel, but by plugging directly into their photosynthesis machinery.
Flexible electronics are made from pliable organic materials. That makes them potentially compatible with tissues and has spurred research efforts to use them to diagnose and treat diseases. “Organic electronics is booming in the area of medical applications,” says Magnus Berggren, a materials scientist and electrical engineer at Linköping University, Norrköping, in Sweden and a leader in devising such medical applications.
About 15 years ago one of Berggren’s plant biology colleagues asked whether it would be possible to place electronics inside trees in order to eavesdrop on the biochemical processes going on there. If so, perhaps they could control, for example, precisely when a tree flowers. “We thought it was a joke,” Berggren says. After all, he notes, biologists have made steady strides in genetic engineering techniques to control myriad biochemical functions in plants. However, genetically engineered plants have a much harder time being approved for release in Sweden than they do in the United States. “We felt those technologies were never going to make it into the forests and fields here,” Berggren says. So a couple of years ago he and his colleagues decided to give electronic plants a second look.
Their idea was to use the plants’ own architecture and biology to help them assemble devices on the inside. To do so, they aimed to assemble polymer-based “wires” on the inside of a plant’s xylem, the tubelike channel that transports water up a plant’s stem to the leaves. They thought that if they could dissolve conducting polymer building blocks in water, perhaps plants could pull them up the channels and link them together into wires.
Berggren and his colleagues tried more than a dozen different polymer electronic building blocks. They dissolved them in water, then placed roses—either with intact roots or cut at the stem—in the water to see whether the organics would be wicked upward. All of the building blocks either clogged the base of the stem or didn’t assemble into wires.
Finally they tried an organic electronic building block called PEDOT-S:H. Each of these building blocks consists of a short, repeating chain of a conductive organic molecule with short arms coming off each link of the chain. Each of the arms sports a sulfur-containing group linked to a hydrogen atom. Berggren’s group found that when they placed them in the water, the rose stems readily pulled the short polymer chains up the xylem channels. The intact plants pulled the organics up through the roots as well, though much more slowly, Berggren says. Once inside, the chemistry in those channels pulled the hydrogen atoms off the short arms, a change that prompted the sulfur groups on neighboring chains to bind together. The upshot was that the myriad short polymer chains quickly linked themselves together into continuous strings as long as 10 centimeters. The researchers then added electronic probes to opposite ends of these strings, and found that they were, in fact, wires, conducting electricity all down the line.
Once that worked, Berggren’s team added other electronic patches on the surface of their rose stems to create transistors that were able to switch the current in a wire on and off. As they report today in Science Advances, they went on to use a set of different techniques to show they could get leaves to take up organic electronics, essentially creating an array of pixels. By applying different voltages to the pixels, they could change their colors to create a living display.
“It sounds really cool,” says Zhenan Bao, an organic electronics expert at Stanford University in Palo Alto, California. Though after a quick read of the paper, Bao says she’s not clear what the application would be.
Berggren says he, too, is just beginning to try to sort that out. One possibility, he says, is to embed electronic sensors in a few plants in a field to detect when they begin to release hormones that initiate the process of flowering or other changes in the plant. This could allow growers to better time watering and fertilizer applications to aid the plants. In time, he adds, it may even be possible to use plant electronics to speed or delay the onset of flowering to protect them from coming harsh weather. Finally, he says, perhaps in the distant future it may be possible to harness plants’ photosynthesis abilities to generate electricity directly, enabling us to reap the sun’s power without destroying the plants.”