Researchers Develop a Self-Powered Sensor Made From Plants

A tiny sensor built from the stems of a grass-like plant could be used to power the next generation of wearable tech, a new study suggests.

The new research, published across two papers in the journals Advanced Functional Materials and Cellulose, demonstrated the use of the open foam-like internal structure of the plant to create an alternative resource for plant-based foams. Such foams are a key component of triboelectric nanogenerators, which can harness mechanical energy distributed through an active layer of foam and transform it into electricity.

An unexpected source of inspiration

Around the grounds of the University of Groningen’s campus grows several patches of a common wetland weed, called soft rush (Juncus effusus L.). While most of the city might see the soft rush as an annoying weed, it has taken on new importance for one research team at the Zernike Institute for Advanced Materials.

“The structure of the soft rush stem consists of layers of interconnected stars, a bit like tiny snowflakes,” explained Qi Chen, PhD student at the University of Groningen and the lead author of both new papers. “The plant needs this open structure to breathe, because with their roots in a wet environment, they need to take oxygen from the air and transport it through the stem.”

These snowflake-like internal layers lead to a very open and airy internal structure, as Chen soon discovered.

“My samples were ultra-lightweight,” she said. “Once, I left the samples uncovered and as I opened the lab door, the samples were blown away. It looked like it had snowed in the hallway.”

Chen’s research had previously been focusing on generating electricity from bacteria, using foamy materials as an environment for the bacteria to live in. But after a chance encounter with the soft rush, she decided to take a closer look at whether the plants could be utilized for making plant-based foam materials.

New nanogenerator could power wearable tech

In the new research papers, Chen and colleagues demonstrate how the soft rush stem can be processed to create foam-like layers. They then demonstrate the use of this foam in fabricating a hollow stellate cellulose-based triboelectric nanogenerator (HSC-TENG).

TENGs can be based on many different materials, but at their core, they are self-powered devices that can convert mechanical energy into electricity.

The devices effectively act as a type of motion sensor, as co-author Dina Maniar, an assistant professor in the faculty of science and engineering at the University of Groningen, explained: “You can put it in your shoe and when you walk, jump or run, it releases a distinct signal that we can recognize.”

In addition to being a motion sensor, some TENGs can also function as an energy source. For this reason, these nanogenerators are increasingly being investigated as a more sustainable alternative for batteries in wearable technologies, or for very small devices that require a nano-scale power source.

TENGs function due to the triboelectric effect – the phenomenon where electric charge can transfer between two objects as they contact or slide against each other. This is also the reason for all the times you’ve received a minor static shock when reaching for something after scuffing your feet along the carpet. But in a TENG, the triboelectric effect creates a meaningful electric charge as two rough surfaces rub against each other and cause friction.

Once processed and dried, the researchers found that the interior of the soft rush plant created a very rough, foam-like surface with many pores along its layers – a perfect texture for exploiting the triboelectric effect as part of a TENG device.

Making new tech with sustainable building blocks

Cellulose has been proposed as a sustainable feedstock material for TENGs in the past, as well as for other applications that require foamy materials. But making these materials a reality has been a challenge.

“Usually, a lot of resources go into extracting cellulose, breaking down the structure as it was,” explained Katja Loos, a professor of applied chemistry at the University of Groningen and the senior author on both new papers. “Then, a lot of resources go into producing the desired structure for new materials.”

In their studies, the researchers highlight how they were able to obtain cellulose films that preserved the unique star-like structures in the soft rush plants by peeling their stems and dissolving the plants in a simple solvent mixture.

“So, we can really call it sustainable,” said Chen, adding that no oil-based fossil fuel materials and very little energy was needed to carry out this process.

Chen has big plans for the humble soft rush, saying that she is currently working on the feasibility of using the star-like soft rush structures as part of a battery, or as a way to clear up pollutants in water.

“In our lab, it’s not a weed; it’s a valuable resource,” Chen said.

References: Chen Q, Li W, Yan F, et al. Lightweight triboelectric nanogenerators based on hollow stellate cellulose films derived from Juncus effusus l. Aerenchyma. Adv Funct Materials. 2023. doi: 10.1002/adfm.202304801

Chen Q, Van Dijken J, Maniar D, Loos K. Aerenchyma tissue of Juncus effusus L.: A novel resource for sustainable natural cellulose foams. 2023. doi: 10.1007/s10570-023-05453-9

This article is a rework of a press release issued by the University of Groningen. Material has been edited for length and content.