Graphene Plant Tattoos Turn Leaves Into Live Sensors

But the traditional techniques to monitor leaf hydration require cutting them from their plants, which is time-consuming and cannot give live measurements. That’s why many researchers are building sensors that measure a plant’s health in real time.

Now, researchers in Texas have developed a graphene “tattoo” that can be stuck directly onto a leaf to provide real-time moisture readings. The researchers also believe it could one day be the building block for a new kind of plant monitoring, by turning the patches into a neural network that computes on the plants themselves.

“Not only are we just sensing the moisture level, but we can have that sensor act as this artificial synapse, which then we can put into a neural network,” says Jean Anne Incorvia, an associate professor of electrical and computer engineering at the University of Texas at Austin. Incorvia and colleagues (including her graduate student Utkarsh Misra) published their work in Nano Letters in February.

A forest of the future, Incorvia and colleagues think, might hold a whole grove of sensors networked to gauge the risk of fire or drought in real time.

A Graphene Leaf “Tattoo” as a Moisture Sensor

The sensor is a graphene patch that can be pasted onto the leaf of a plant (the researchers used Monstera) like a stick-on tattoo. It’s functionally a sort of three-terminal transistor, with a graphene channel, gold strips as its electrodes, and the leaf itself as its dielectric insulator.

The sensor can gauge a leaf’s hydration level in real time by sending an electric pulse into the leaf, which moves around ions within the leaf and changes the graphene’s conductance. The magnitudes of these conductance changes depend on moisture inside the leaf, so the researchers can read out a leaf’s hydration without a need for external processing.

Graphene is a good material for a leaf tattoo. It’s nearly transparent, so it won’t block light and disrupt photosynthesis. It can stretch and squeeze as the leaf grows, shrinks, or twists.

This isn’t the first graphene sensor of its kind, but real-time hydration sensors aren’t common in the field. The researchers hope this new sensor can change that by fitting into a neural network, because it also acts like a synapse in a brain.

In particular, just as neural activity can strengthen or weaken a synapse, the researchers could use particular electric pulses to slightly adjust their sensor’s conductance up or down. Moreover, after a pulse ended, the sensor returned to its original conductance slowly, over about 90 seconds. In that time, their sensor could act as a sort of short-term memory.

The researchers imagine they could one day use such artificial synaptic qualities to tune and store a neural network’s weights.

Researchers study a Monstera plant in the lab, with…