Tree 2.0. From computer chips to wood-based touch screens
The technique for making clear wood is to remove the lignin (the glue that holds the bundles of cells together and give the wood its color) and fill the resulting air pockets with epoxy resin. The latter is necessary to achieve the same refractive index of light and make the material completely transparent.
Interestingly, transparency is not the only characteristic that the researchers managed to achieve. Surprisingly, wood treated in this way turned out to be many times stronger than plastic and glass.
Technology for producing transparent wood. Source.
Electronic plants
It would seem that humanity has gained access to a new durable material, albeit experimental. You can come up with something similar, say, in construction. But remember that we started with the idea of sustainability and the search for new materials for electronics? It was then that scientists from Sweden had an idea: why not create a transistor out of wood!
Even at the start of their work, the researchers were prepared for the fact that if they succeeded, their device might turn out to be larger and slower than traditional transistors made from semiconductor materials. But at the same time, it will definitely be the “greenest”. And thanks to its high biocompatibility, it is also well suited for agriculture or forestry, for example.
It's time to define the terms that will appear in abundance below:
- WECT – wooden electrochemical transistor,
- OECT – organic electrochemical transistor,
- PEDOT:PSS – a composite material with the unpronounceable name poly-3,4-ethylenedioxythiophene: polystyrene sulfonate (in Russian-language sources you can simply find the term PEDOT: PSS, described as a highly conductive polymer).
To create a wooden transistor (WECT), Prof.
from
in Sweden and his colleagues used heat and a chemical process to remove lignin from lumps
.
The team initially tried birch and ash. But balsa proved to be an ideal option due to its retention of its structural integrity after lignin removal, absorption of conductive polymer, and lack of significant seasonal differences between summer and winter wood. Some researchers believe it might even be possible to grow conductive wood with a polymer already inside. Although the question remains of how to add new polymer as the tree grows.
They then dipped the wood into a liquid solution containing a conductive polymer to allow it to soak into it. This created conductive wood capable of interacting with electrolytes as the basis for creating a wood transistor.
There were also attempts to make wood electrically conductive by carbonizing it, but they were unsuccessful.
The researchers successfully demonstrated and measured the wood transistor's performance during several tests. This result proves that it is possible to modulate the electrical conductivity of electroactive wood by applying an external voltage. WECT works on the same principle as dual-gate OECT, where the two gates and transistor channel are made of delignified wood.
A: removal of lignin and impregnation of wood with a highly conductive polymer. B: Making an electrochemical transistor. C: Front view of a wood transistor, front view of its configuration, and a cutaway view of the wood grain.Source.
Current modulation occurs through electrochemical oxidation/reduction of a layer of highly conductive polymer. Although the device performance is inferior to conventional PEDOT:PSS-based OECT, WECT clearly demonstrates the ability to transform wood into a functional transistor. The structure of the tree itself plays a significant role in this process: all these channels and fibers make it possible to control and regulate the electronic current.
Balsa, or balsa wood. Source.
Structure of balsa under a microscope.
Of course, the transistor can be improved. For example, by optimizing the conductivity of wood or changing the configuration of the device. Additional scope for creativity is provided by the increased strength of transparent wood. Finally, due to the great interest in bioelectronics research, such a device and its working principle could be a stepping stone to various new developments.
Paper boards
Let's return to the impact of electronics on the environment. In 2019 was produced in the world a record 53 million tons of electronic waste. By 2030, this figure is expected to increase to 82 million tons.
Additionally, in the same year of 2019, only about 20% of the world's e-waste was recycled. And a significant share of them are printed circuit boards. Because they are a layered matrix of materials—usually resins, plastics, and copper—they are difficult to recycle.
To develop environmentally friendly electronics such as sensors and printed circuit boards, Dr. Valerio Beni from the Swedish research RISE Institute also turned his attention to wood. He is leading a research project HyPELignum to study ways to manufacture consumer electronics using wood-based materials.
The HyPELignum team develops two types of wooden printed circuit boards. The first ones are made of thin layers of wood. The latter are made from cellulose fibers obtained from wood waste. The idea in both cases is simple: take high-carbon electronics components and replace them with low-carbon ones.
The circuits are printed, rather than etched, onto wooden boards using conductive metallic inks developed as part of the project. These inks also contain cellulose and bioplastics made from wood. At the end of their life, wooden boards are easier to recycle than regular boards. They might even be compostable.
A key challenge in electronics recycling is separating components from circuit boards. To solve this, HyPELignum researchers are developing thermally and chemically degradable layers that can be placed between wood and printed circuits. When they break down at the end of the product's life, the circuits and electrical components fall off the wood. The wooden board, metal circuitry and components can then be sent into various recycling streams.
Although here we can speculate about the potential problems of this approach. For example, about the reliability of boards that initially provide for separation of components after some time.
The decomposed layers themselves are also made from wood, more precisely from lignin. According to the researchers, such “green chemistry” emits much less carbon dioxide because it uses biogenic materials that can be renewed rather than fossil oil.
By the way, it works out conveniently: they took a tree, the trunk was used for transistors, and the lignin extracted from it was used for circuit boards. At least in theory and experiments.
Green electronics
Woodoo is one of the companies using biochemistry to create devices using wood instead of glass and plastic. Among the announced products planned to be made using wood, there is already a translucent LED display, sensors and various lighting devices.
Woodoo's creations are made by extracting air and lignin from wood cells, then filling the empty pores with bio-based polymers. Their raw materials are low-grade wood species: beech, birch, pine and aspen. At the same time, wood is supplied only from forestry farms located within 300 km from the manufacturer.
Woodoo claims its bio-based materials are 17 times less energy efficient than glass, 130 times less energy efficient than steel and 475 times less energy efficient than aluminum. And they are also lightweight. Wood-based digital components in smart cars have the potential to reduce emissions. But, frankly speaking, the thesis looks quite dubious. To reduce the amount of CO2 emitted by reducing the weight of the car, it is not enough to replace a couple of parts on the dashboard.
Another electronics segment where wood could become an important biomaterial is wearable devices. Currently they are made from non-biodegradable polymers, but in 2021, researchers from the University of Missouri created paper-based wearable material. It is breathable, flexible and repels liquids. Beauty, well! Moreover, compared to polymers, it is easily degraded by soil fungi, bacteria and yeasts in the soil.
And researchers from South China University of Technology created another wood-based innovation by developing a low-cost composite electronic substrate. Substrates are any structural elements that support the electronics in devices. The material, according to the researchers, provides mechanical properties like plastic and degradability like paper. It is made from lignin and nanocellulose.
The wood can also be used to make the base of computer chips, such as demonstrated in 2015 by researchers at the University of Wisconsin. They claim that their material has the same performance as the base of a silicon chip. However, the active components on such a wood chip still consist of gallium arsenide – a toxic material that is a typical representative of hazardous substances in electronic systems.
In New Zealand, researchers also created a flexible electronic circuit that is based on wood right down to the wiring. The circuit substrate is made by removing lignin and hemicellulose from organic matter. The wiring is made from bioinks that use carbon nanofibers derived from lignin. Interestingly, this circuit is recyclable because it dissolves easily in water.
Researchers are still working on how to make wood-based materials conductive so they can replace metals in digital circuits. Creating conductive biomaterials represents the biggest challenge for green electronics researchers, but it doesn't seem insurmountable.
