Flexible electronics made with wood-based nanotechnology
(Nanowerk Spotlight) Wood has been traditionally used as lumber or has been deconstructed into elemental fibrils before being reconstructed into various material such as paper, cardboard and artificial wood-based products.
More recently, nanocellulose from wood – a nanomaterial derived from breaking wood fibers down to the nanoscale – has found additional applications such as strength enhancers in paper and biocomposites, barriers for packaging, emulsifiers, in oil separation, substrates for printing electronics, filtration, and biomedicine.
"Wood nanotechnologies are not only associated with extraction and use of nanocellulose or lignin but also with tailoring and functionalizing the hierarchical nanostructure of bulk wood for functional materials," Qiliang Fu, a Wood and Fiber Scientist at Scion, a New Zealand government research institute, tells Nanowerk. "In our latest research we use a top-down approach with a mild chemical treatment that allows the preservation of the complex structure and the original orientation of the cellulose fibres. By conserving the wood structures, a highly aligned cellulosic material with excellent strength is achieved."
A recent report in ACS Nano, ("Wood-Based Flexible Electronics") first-authored by Fu, details a method to produce a wood-derived, fully bio-based, and environmentally friendly flexible electronic circuit.
In this work, the Scion team tailored the wood nanostructure to create a wood film with high transparency, flexibility, and strong mechanical properties. According to the researchers, this material compares favorably with previously published two-dimensional cellulose-based materials developed for electronics or structural applications.
Processing of flexible and transparent wood film (TWF) for flexible electronics application. (a) Illustration of the process. Lignin and half the of hemicellulose are removed from the wood tissue. The treated wood is then pressed and dried under ambient conditions. The collapsed cell walls are bound together via hydrogen bonding. The hierarchical structure of TWF consists of cellulose microfiber bundles, nanofibrils, and cellulose chains, which include crystalline and amorphous regions. The bio-based amyloid/lignin-derived carbon fibers (LCF) ink is printed in a tree-shaped circuit on the TWF substrate. (b) Photograph of original wood, treated wood, and TWF. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
This flexible circuit highlights the fact that wood can be used as a feedstock, with the potential to displace petroleum-based material for high-value products.
The main difference between this circuit and other nanocellulose-based circuits resides in the fact that substrate and conductive elements are both obtained via wood nanotechnologies.
The researchers electrospun lignin, an abundant byproduct from wood processing with high carbon content and then carbonized it into conductive carbon fibers.
Taking advantage of the strong adhesive properties of amyloid fibrils, the team then formulated a fully bio-based and renewable amyloid/lignin-derived carbon fiber conductive ink and printed it on the transparent wood film substrate to produce an electronic circuit.
"Our work demonstrates the possibility of producing a wood-derived circuit with the synergistic combination between a transparent, flexible wood film substrate and a conductive lignin-derived carbon fibres ink," Fu points out.
However, most transparent wood requires an impregnation of petroleum-based polymer (epoxy or PMMA) after delignification. This treatment makes the whole product non-biodegradable and mechanically brittle.
Dr. Fu did his PhD in Prof. Berglund’s group, during which time he pioneered the research on transparent wood. One day, he noticed that a thin delignified wood veneer sample that was stored in a beaker and dried in ambient conditions had become transparent and flexible.
Due to other commitments, this finding was put aside. However, it was always very clear in his mind that this observation had great potential. Therefore, upon starting his new position as a scientist at Scion, he built up the lab facilities to produce high-quality, highly transparent and flexible strong wood films. He quickly improved the delignification and compression methods to obtain samples with high tensile strength and very smooth surface.
In fabricating the first fully wood-based flexible electronics, Fu and his team demonstrated a prototype circuit and a strain sensor for bending tests as proof-of-concept. However, wood-based flexible electronics could be used in many other areas such as wearable devices, smart packaging and sensors. It also has great potential applications in designing energy storage devices, such as flexible batteries and supercapacitors.
Demonstration of TWF flexible electronics. (a) Photograph of flexible TWF electronic circuit. (b) Cross-sectional SEM image of the wood-based flexible electronics. (c) SEM image of LCF ink dispersed on surface of the flexible TWF. (d) SEM image of the TWF surface on the edge of a printed circuit, with an ink-coated and uncoated area. (e) and (f) Photographs of a printed flexible circuit connected to a 9 V battery powering a LED with a bent (e) and folded (f) electronic circuit (Reprinted with permission by American Chemical Society) (click on image to enlarge)
Due to their mechanical flexibility and full biocompatibility, this type of electronics is perfectly adapted to be integrated into food packaging to track environmental conditions. Another application could be for single-use circuits. More and more medical applications and event organizations use small electronic circuits for a definite time use (for instance as short-term monitoring device or entry ticket).
"Currently, the water sensitivity of the substrate and the susceptibility to shear forces applied perpendicular to the fibre direction are our main concerns," notes Fu. "However, we are now solving the film sensitivity to moisture. We are also exploring the production of environmentally friendly (biobased/biodegradable) wood composite with high hydrophobicity."
He adds that the electrical performance of wood based flexible electronics also need improvements compared with graphene-based conductive inks. The team are looking for post-treatment options to improve the electrical properties.
They are now mainly focusing on developing new applications by improving the above-mentioned weak points and adding new functionalities, such as luminescence and hydrophobicity to the transparent wood film substrate.
Furthermore, the ink formulation's rheology is being adapted to various printers, through improvments to its adhesiveness, strengthening its mechanical robustness and increasing the electrical conductivity of lignin-derived carbon ink itself.
"We are already exploring the possibilities to scale up our process and have a continuous method to produce the substrate by using a roll-to-roll system, for example," Fu concludes. "There is great interest in developing this technology for the production of transparent wood film substrate using a diverse range of wood species. Scion is exploring the possibility to commercialize this bio-based ink."