Knock on Wood: Researchers Turn to Wood as a Surprising Electronic Substrate

While wood and electronics seem like a contradictory pair, researchers are increasingly exploring the benefits of wood-based substrates for high-performance, multi-functional, and eco-friendly devices. In particular, by removing lignin and hemicellulose in wood, collapsing its cell walls, and printing on the resulting wood film, researchers are yielding high-strength and transparent substrates that have made their way into electronics, including flexible circuits and sensors.

This article examines a few interesting trends in wood-based electronics and their limitations compared to conventional silicon-based electronics.

 

How Do You Mesh Wood With Electronics?

The most common way researchers have adapted wood for electronic applications is by extracting and incorporating nano-cellulose and lignin at the component level. Researchers have also processed the nanostructure of bulk wood for various functional applications, including compressible sensors, smart-packaging, wearable devices, foldable electronics, and triboelectricity generation.

 

One example of how wood is used as a substrate for a printed circuit. Image used courtesy of ACS Publications

 

Wood-based electronics offer several benefits. Compared to conventional substrates, such as silicon or GaN, wood is readily available and less expensive. Due to their recyclability and biodegradability, these wood-based solutions are also environmentally friendly. Wood substrates are lightweight—promoting miniaturization—and highly flexible—enabling a wide range of bendable, rollable, and twistable electronics. The substrates’ high strength and electromechanical stability make them a candidate for reliable and high-performance applications as well.

Here are a few recent studies that have exploited the (surprising) promise of wood in the electronics sphere. 

 

Energy-harvesting Wood Floors

Imagine powering LED bulbs and small home appliances by walking across a wooden floor. In a study published in Matter, a group of Switzerland-based researchers recently created a wood-based triboelectricity nanogenerator by incorporating silicone coating and embedding crystals into the wooden panels.

 

When stepped on, this wood-based triboelectric generator was able to power a lightbulb. Image used courtesy of Matter

 

To create this generator, the researchers started with two pieces of wood sandwiched between electrodes, effectively charging the wood through the contact and separation of footsteps on the floor. While this study successfully generated electricity via the triboelectric effect, it wasn't without its challenges.

 

Wood-based triboelectric nanogenerator. Image used courtesy of Matter

 

Because wood is "triboneutral," it doesn't naturally have a tendency to acquire or lose electrons, which hinders its ability to create electricity. On one piece of wood, the researchers coated the surface with a silicone called polydimethylsiloxane (PDMS), which acquires electrons upon contact. On the other wood panel, the researchers coated the surface with nanocrystals that contain a network of organic molecules and metal ions. Together, these two coatings increased the wood's capacity to generate triboelectricity. 

Their findings also revealed that spruce wood, in particular, had the best triboelectric properties for a nanogenerator. The researchers demonstrated this prototype's ability to power household lamps, electrochromic windows, and calculators by simply walking across the wood floor. 

 

The First-ever Wooden Satellite

Other recent examples of wood and electronics integration include the first-ever wooden satellite. The WISA Woodsat nanosatellite project intends to utilize plywood-based surface panels for its satellite design. In collaboration with other key players like the European Space Agency (ESA), OpenQCM, UPM Plywood, and Huld, Jari Makinen and his company, Arctic Astronautics, hope to launch this innovative satellite before the end of 2021.

Arctic Astronautics is interested in investigating wood as a significantly more affordable alternative to other common paneling materials. 

 

The WISA Woodsat is the world's first wooden satellite. Image used courtesy of ESA

 

ESA has pledged to donate a sensor suite to this Woodsat project and Huld intends to design the camera boom.

“Ordinary plywood is too humid for space uses, so we place our wood in a thermal vacuum chamber to dry it out," explains Woodsat chief engineer and Arctic Astronatics co-founder Samuli Nyman. "Then we also perform atomic layer deposition, adding a very thin aluminum oxide layer–typically used to encapsulate electronics. This should minimize any unwanted vapors from the wood, known as ‘outgassing’ in the space field, while also protecting against the erosive effects of atomic oxygen."

 

Wood Makes for Bendy Electronics

Last year, New Zealand researchers designed a flexible transparent wood film (TWF) with a two-step approach: the scientists first produced TWF substrate from balsa wood, then printed a circuit on the substrate with lignin-derived carbon fibers (LCF) conductive inks. 

The researchers say their design is suitable for a range of applications, from electronic packaging to automotive design. The flexible TWF approach, they claim, can potentially replace conventional petroleum-based polymer electronics.

 

TWF exhibits excellent flexibility. Image used courtesy of ACS Publications

 

After running several tests on their design, the team discovered the TWF exhibiting remarkable qualities, including a high tensile strength (41.6 ±9 GPa cm3g−1), significant electro-mechanical stability, and excellent flexibility. 

Because of these characteristics, the researchers hope to see this substrate open doors for rollable, twistable, and bendable electronics

 

Combining Wood With GaN 

Similarly, University of Wisconsin researchers achieved a communication-based electronic component on the substrate of a wood-based product. Specifically, the team constructed a microwave amplifier circuit on cellulose nanofibril paper.

 

Engineers at the University of Wisconsin developed a microwave amplifier circuit on a flexible wood fiber-based substrate. Image used courtesy of the University of Wisconsin

 

To achieve this solution, the research team disintegrated wood fibers into nanofibrils and recombined them into a strong, flexible, and biodegradable film. The team then layered the resulting films with a comparatively small amount of gallium nitride (GaN) substrates to boost their electronic capabilities. They found that the overall cost of fabrication was lower compared to pure GaN-based substrates. The final solution demonstrated up to 10 mW of power.

Their design opens up the possibility for flexible microwave components that may be of use for wireless communication (like 5G microwave arrays) and several other applications, including drones and wearable devices.

 

Limitations of Wood as an Electronics Substrate 

While wood seems an unlikely candidate to include in electronic devices, there seems to be an uptick in wood-based electronics research in the past few years. Scientists from the University of Oulu have even gone so far as to propose nano-cellulose as a suitable substrate for 6G technology radio lenses.

Even so, this technology faces a few common challenges. Compared to conventional silicon substrates, wood-based cellulose substrates have a high solubility in water, causing adverse effects when they are incorporated into various electronics. Wood also captures moisture, which may require a thermal vacuum chamber to address. It's these properties (among others) that may confine wood-based electronics to the research realm in the near future.