“Every rose has its thorn,” the song goes, but not every rose has electronic wires running through its body. The futuristic idea of plant cyborgs is making the leap from science fiction to real-world science.
What’s the big deal?
Swedish researchers have been working on ways to regulate plant growth, using electronic wires grown inside the plants own nutrient channels to host sensors and drug-delivery systems. The aim is to provide just the right amount of plant hormones at just the right time. Such efforts could provide even more precise human control over plant production and agriculture.
A separate but no less exciting project involves embedded biofuel cells that could literally turn plants into solar power plants. If all goes well, sensors and other devices could someday harvest electricity from the natural process of photosynthesis that enables plants to turn sunlight into chemical energy. It’s not often that such a sweet-smelling prospect begins with a humble garden rose. But that’s where the first successful steps toward electronic plants has begun. A team at Linköping University in Sweden has taken a huge step forward with the first experiments demonstrating electronic circuits within the living bodies of plant stems and leaves. Their research is detailed in the 20 November 2015 issue of the journal Science Advances.
They grew electronic wires as long as 10 centimeters within garden rose stems and turned leaves into patchy electronic displays capable of changing colors between light and dark on demand. They also built working transistors—the basic switches at the heart of modern electronics—based on the wires embedded within the plants.
“In a sense, we are then introducing a nervous system into the plants,” says Magnus Berggren, a professor of organic electronics at Linköping University in Sweden.
But the researchers didn’t perform Frankenstein-style surgery to implant the wires. Instead, they made use of the xylem, plants’ natural system of channels that typically carry water and nutrients from the roots to stems, leaves, and flowers.
The team’s early attempts to thread conductive polymer wires through the xylem led to the xylem being clogged or the plants exhibiting severe toxic reactions. But the researchers eventually discovered that a liquid solution containing a polymer called poly(3,4-ethylenedioxythiophene), or PEDOT, could readily be taken up by the xylem and distributed evenly throughout. What’s more, they found, it would eventually form a solid wire capable of conducting electricity. The presence of such “xylem wires” still allows the channels to carry the necessary water and nutrients for plant survival.
Berggren explained how the liquid solution containing dissolved chains of PEDOT-S:H—a chemical variation of PEDOT—was able to form solid wires with the help of both the xylem’s vascular channels and the plants’ delayed immune response:
After some time, the plant reacts against this unknown material. A common reaction against pathogens or toxic materials involves exchange of monovalent ions with divalent ones. The increase of divalent ions promote self-organization and formation of the actual conducting wires along the inner walls of the xylem channels. In a sense, the plant is helping us to separate the the event of distribution of the conducting and electronic materials from the event of film formation along the xylem walls.
Successful creation of the xylem wires also allowed the researchers to create “organic electrochemical transistors” within the plants; these transistors convert chemical signals into electronic outputs. Such transistors could form the basic hardware for more sophisticated plant cyborg devices. The team even used the plant circuitry to demonstrate digital logic gates—the building blocks for performing more complex electronic and computing operations.
Other experiments turned the leaves of roses into living electronic displays. The Swedish researchers accomplished this by encapsulating a leaf in a syringe filled with a different PEDOT solution. When the syringe’s plunger was pulled up, it created a vacuum that sucked gas out of the leaf through the “stomata” pores on the leaf surface. Once the syringe plunger was pushed down, the PEDOT solution rushed into the pores to fill the spaces between the leaf’s veins.
The result was a patchy network of conductive material within the leaf. Researchers sandwiched the leaves between PEDOT films to create electrical contacts with the PEDOT inside the leaves. That enabled the team to remotely manipulate the material within the leaves, changing their color between lighter and darker patterns. The switch between light and dark typically took about 20 seconds. The researchers observed that a pattern, whether light or dark, would remain visible for about 10 minutes.
The researchers mostly experimented with cut rose stems and leaves, but what works in garden roses could also help create other electronic plants, Berggren said. The basic structure of roses resembles those of larger plants such as trees, which means trees could also theoretically become living plant cyborgs or “e-plants.”