Programmable and flexible wood-based origami electronics

The growing demand for high-tech electronics, such as smartphones and computers, has led to an increase in the consumption of non-renewable and non-biodegradable resources, resulting in significant e-waste and environmental concerns. Flexible electronic devices offer a promising solution due to their customized size, multifunctionality, and biocompatibility, finding applications in microfluidics, wearables, circuits, sensors, and soft robotics. However, the environmental impact of current flexible electronics remains a major challenge. This study focuses on developing sustainable and environmentally friendly flexible electronics using wood-based materials. Wood, particularly cellulose, offers a sustainable alternative, and researchers have explored various bottom-up and top-down strategies for utilizing wood and its derivatives in optoelectronics, smart displays, electrochromic devices, and sensors. The top-down strategy, utilizing bulk wood or veneer, offers the potential for direct conversion into substrates for wood-based functional electronics. However, current wood-based electronics are limited in their ability to undergo complex shape deformation, restricting their application in scenarios requiring custom sizes and foldable structures. This research addresses this limitation by integrating origami techniques with wood-based electronics to create programmable and editable devices. Origami's ability to transform 2D sheets into 3D structures with arbitrary shapes, combined with 3D printing technology, provides a pathway to creating flexible electronics with customizable shapes. Previous research on origami electronics has focused on soft elastomers, stretch polymers, liquid crystals, and isotropic paper substrates. This study aims to develop flexible origami electronics based on thin wood films, addressing the challenge of creating wood-based devices with shape programmability and editability.

Extensive research has been conducted on flexible electronics using various materials and techniques. The use of natural polymers as substrates for flexible electronics has gained traction due to their potential to combine high performance, intricate shape deformation, and environmental sustainability. Several studies have explored the use of cellulose and its derivatives in creating flexible and conductive materials, demonstrating their suitability for applications such as optoelectronics and sensors. The development of transparent wood films (TWF) as substrates for electronics has also shown promise. However, there are limitations in creating electronics with complex shape deformations, hindering broader applications. Origami-inspired designs have been incorporated into other materials, including elastomers and polymers, to create reconfigurable devices. However, the application of origami principles to wood-based electronics is relatively unexplored. Pencil-on-paper electronics have been studied, but their conductive durability under folding and deformation is a concern. This research aims to improve on these existing methods by combining the sustainability of wood with the flexibility and programmability of origami and the precision of 3D printing.

The study involved several key steps: 1. **Transparent Wood Film (TWF) Fabrication:** Balsa wood veneers were treated through delignification and alkali treatments to remove lignin and partial hemicellulose, resulting in a porous wood structure. This structure was then densified and dehydrated to create a thin, transparent wood film (TWF) approximately 50 ± 10 µm thick, characterized by highly aligned cellulose fibers. The morphology and properties of the TWF at different processing stages were analyzed using various techniques, such as scanning electron microscopy (SEM), confocal fluorescence microscopy, Fourier transform infrared spectroscopy (FTIR), and wide-angle X-ray diffraction (WAXS). 2. **Conductive Ink Preparation:** A conductive gel-like ink was prepared by combining TEMPO-oxidized cellulose nanofibrils (TOCNFs) and multi-walled carbon nanotubes (CNTs). The rheological properties of the ink were carefully characterized to optimize its printability. 3. **3D Printing:** The conductive ink was printed onto the TWF substrate using direct ink writing (DIW) technology, enabling the creation of various patterns and circuits. The adhesion between the ink and the TWF substrate was investigated using techniques such as sellotape peel-off tests and physical scraping. The surface morphology and thickness of the printed layer were analyzed using SEM and atomic force microscopy (AFM). 4. **Origami Design and Fabrication:** Origami crease patterns were pre-designed using computer software, incorporating different types of creases (mountain, valley, border, flexible, and faceted). Finite element method (FEM) simulations were performed to analyze the stress and strain distribution during the folding process, helping to optimize the design for different folding angles and directions. 5. **Characterization of Flexible Wood-Based Origami Electronics:** The mechanical and electrical properties of the resulting wood-based origami electronics were evaluated. This included tensile testing, measurement of electrical conductivity, and assessment of the stability of conductivity under repeated folding and unfolding cycles, different humidity conditions and elevated temperatures. The performance of a wood-based origami sensor for detecting human motion was tested and analyzed. 6. **Recyclability Assessment:** The recyclability of the TWF substrate was also evaluated by repeating the manufacturing process multiple times, characterizing the mechanical properties of the recycled materials.

The study yielded several key findings: 1. **Transparent Wood Film (TWF) Properties:** The TWF substrate demonstrated excellent mechanical properties, including a high tensile strength of 393 MPa and Young's modulus of 43.68 GPa in the longitudinal direction, significantly surpassing those of natural wood, delignified wood, printer paper, and PET. The high strength is attributed to the highly aligned cellulose fibers and the formation of numerous intermolecular hydrogen bonds between them. The TWF also exhibited a high transmittance of 78% and a haze of 88% at 550 nm, confirming its transparency. 2. **Conductive Ink and Adhesion:** The conductive CNT/TOCNFs ink showed strong adhesion to the TWF substrate due to hydrogen bonding and van der Waals forces. The printed electronics exhibited an average electrical conductivity of 0.24 S cm⁻¹ and maintained stable conductivity even after 500 folding-unfolding cycles at 180°. Furthermore, the conductivity remained stable under various humidity levels and temperatures up to 95°C. 3. **Origami Design and Function:** The origami design, leveraging the anisotropy of the TWF substrate, allowed for precise and repeatable folding into various pre-designed shapes without cracking or damage to the electronics. FEM simulations helped predict and optimize the folding behavior. A proof-of-concept human motion sensor demonstrated the functionality of the wood-based origami electronics, with the sensor successfully detecting and recording stable electrical signals during knee and elbow bending, indicating its potential for various wearable applications. 4. **Recyclability:** The TWF substrate exhibited good recyclability with mechanical properties comparable to those of randomly constructed cellulose nanopapers even after multiple recycling cycles.

This research successfully demonstrates the feasibility of creating programmable and flexible wood-based origami electronics. The integration of TWF substrates, 3D printing, and origami design offers a sustainable and innovative approach to flexible electronics. The high tensile strength and transparency of the TWF, coupled with the strong adhesion of the conductive ink and the predictable folding behavior, enabled the fabrication of highly flexible and stable electronic devices. The successful implementation of a human motion sensor showcases the potential of this technology in wearable applications. The superior mechanical properties and recyclability of the TWF materials further contribute to their sustainability. This work expands the possibilities of wood-based electronics beyond previously reported limitations in shape deformation, paving the way for complex and dynamic electronics. Future research could focus on further optimization of the material properties and exploration of different applications such as e-skins and soft robotics.

This study presents a novel approach to developing sustainable and flexible electronics using wood-based materials and origami principles. The resulting programmable and flexible wood-based origami electronics demonstrated excellent mechanical properties, stable conductivity, and shape programmability. The successful implementation of a human motion sensor demonstrates its potential in wearable applications. The inherent recyclability of the materials further enhances its environmental sustainability. Future research could explore more complex origami designs, integration with other functional materials, and the development of advanced applications in soft robotics and e-skins.

While the study demonstrates significant advancements, some limitations should be noted. The current conductivity of the TWF electronics, although sufficient for demonstrating the proof-of-concept sensor, could be further improved for high-performance applications. The long-term stability and durability of the devices under extreme environmental conditions warrant further investigation. The scalability of the fabrication process for mass production also requires future research.