An ordinary houseplant sitting in Assistant Professor of Physics and Astronomy Ramesh Adhikari’s lab has become the star of his latest publication. It’s a golden pothos that he purchased at Home Depot for maybe $15.
When Adhikari and his students started plugging their equipment into the glossy, heart-shaped leaves of the houseplant, they discovered properties that might make it perfect for building leaf-based electronics.
This potted plant won’t microwave your food or show you Netflix. But someday, leaves like the pothos’ could be part of a device that harnesses the power of nature to do computing or monitoring in an Earth-friendly way.
In his lab, Adhikari is working to address the problem of electronic waste. Globally, people generate tens of millions of tons of electronic waste every year, and that number is growing. Some of this waste could be kept out of landfills, Adhikari says, if the electronics were biodegradable.
Of course, no one wants their desktop computer decomposing in their office. Short-lived electronics might make sense, though, for environmental sensors or wearable patches that gather health data. These devices could be made out of biodegradable materials such as paper — or living plants.
But what kinds of plants?
Biologists divide plants into two major types, called monocots and dicots. You can see the difference if you look closely at a leaf. Monocots, such as grasses, usually have veins that run parallel to each other along the length of a leaf. Dicots have veins that branch and spread out.
In earlier experiments, Adhikari had worked with monocot leaves. He and his students pumped polymers into the long, straight veins of a plant called the Louisiana iris. The result was a leaf with embedded wires that could conduct electricity and be incorporated into electronic devices.
Next, Adhikari wanted to see what would happen if he put conducting polymers inside dicot leaves. The golden pothos, a dicot, just happened to be sitting in his lab. He had bought the pothos for its flat, water-repellant leaves, hoping to use it for a device that generated power from rainwater falling on the plant. The pothos had turned out not to work especially well for this device, but Adhikari decided to give it another chance. “Since the plant was already in the lab, we decided, OK, let’s try to see what happens with this leaf,” he says.
Pumping polymers into the dicot’s branching web of veins would be difficult. So the students in Adhikari’s lab used a different method, sucking air out of the pothos’ leaves and then letting polymer flow through the leaves’ pores into the spaces where air used to be. When the team then observed the subsequent electrical behavior of the leaves, they found they “were getting some interesting data,” Adhikari says.
He discovered that the dicot leaves showed the behavior of a resistive switching device. This is when a material’s resistance — how strongly it inhibits the flow of electricity — can be changed by applying different amounts of current. “By applying different voltages, you can control the resistance of the material,” Adhikari says.
Furthermore, he didn’t even need to add conducting polymer, or anything else, to the leaves. The leaves displayed this property naturally. “All we do is we put our electrical probes into the leaf to apply voltage and measure the current,” Adhikari says. “And that’s it.” Naturally occurring ions in the plant cause the resistive switching behavior, Adhikari says.
Resistive switching devices are most often used in memory devices, Adhikari says. Computers store information in ones and zeros, or switching on and off, like the switching between two resistance states.
Adhikari hopes his work will inspire technology that imitates the leaves’ behavior. He’s especially interested in something called neuromorphic computing. This is where the memory of an electronic device works in a similar way to our own brain synapses: Repeating an experience often enough establishes a connection between brain cells. But without reinforcement, that connection is lost or forgotten. “Our next goal is to see if we can make devices that can have long-term memory,” Adhikari says.
As for using leaves themselves in biodegradable electronics, leaf tissues don’t last very long once they’re snipped from a parent plant. But Adhikari says leaves that are still attached to a living plant could be incorporated into electronics.
Another possible way to use the switching technology, Adhikari says, would be with crops in a field. “You could potentially use this technique to see if you have plants that have been dehydrated,” he says. Some of a farm’s plants could be incorporated into electronic devices. As leaves lose water, Adhikari’s experiments show, the difference between their two resistance states gets smaller. When a plant on this hypothetical farm becomes dehydrated enough, it could send a “Water me!” signal to a farmer.
Discovering that pothos leaves have these electrical properties naturally, with no added materials, was important, Adhikari says. “When you actually introduce external materials into the living system, then you probably would be poisoning that living system,” he says. But maybe the pothos — or a tomato plant in a field — can be part of an electronic device while also living and growing the way it should. “Now you have an integrated electronic component by just taking advantage of the leaf’s architecture and not messing with it,” Adhikari says.
Another benefit of leaves as electronic components: “They’re extremely cheap,” Adhikari says. That single potted pothos might provide research materials for one or two years.
Adhikari doesn’t have many houseplants at home, aside from a golden pothos that’s on top of the fridge so his young daughter doesn’t knock it over. But working in this field of research has made him see the plants around him with new eyes.
“As I’m walking around, I’m always paying attention to the different plants and different leaves,” he says: their veins, their shapes, the features that might make them useful for building the electronics of the future. “Before I started in this area,” he says, “plants were just plants.”