Sustainable wood electronics by iron-catalyzed laser-induced graphitization for large-scale applications

The growing demand for sustainable electronics necessitates the development of devices made from renewable and biodegradable materials using eco-friendly manufacturing processes. Current green electronics often rely on nano-cellulose-based materials, but these can be limited by energy-intensive production methods and scalability challenges. Wood, as a readily available, renewable, and mechanically strong material, offers a promising alternative substrate for electronics. However, wood's lack of intrinsic electrical conductivity has hindered its use in this area. Previous attempts to create conductive wood have involved surface coatings or bulk impregnation with conductive materials, but these approaches often compromise the wood's structural integrity and are not always sustainable. Laser-induced graphitization (LIG) is a potentially cost-effective and scalable method for creating conductive patterns on various materials, but its application to wood has been limited due to thermal damage and ablation issues. This research addresses these limitations by introducing an innovative approach to LIG.

Existing literature demonstrates various methods to enhance the electrical conductivity of wood, including surface coatings with metal nanoparticles and carbon-based inks, as well as bulk impregnation with conductive materials. However, these approaches often result in conductivity values that are too low for most applications (≤ 0.5 S m⁻¹). Laser-induced graphitization (LIG) has shown promise in creating conductive patterns on various substrates, but applying this technique to wood presents challenges due to wood's low thermal conductivity, leading to significant thermal damage and ablation. Previous attempts to use LIG on wood have often employed multi-step processes, inert atmospheres, and fire retardants to mitigate these problems, but these methods are not ideal for large-scale, sustainable applications. Research on iron-polyphenol complexes, inspired by historical iron gall ink, suggests that these bio-compatible and sustainable materials may have potential applications in conductive coatings.

The researchers developed a novel iron-catalyzed laser-induced graphitization (IC-LIG) method. This involves coating wood veneers with a bio-based ink containing iron(III) citrate and tannic acid, inspired by historical iron-gall ink. The ink, also containing gum arabic and glycerol for stability and crack prevention, protects the wood during laser treatment and acts as a catalyst for graphitization. A conventional CO₂ laser is then used to engrave conductive patterns onto the wood surface in a single step, under ambient atmosphere. The process parameters (laser power, scan speed) were optimized for different wood species. The researchers characterized the resulting IC-LIG structures using a range of techniques, including four-point probe measurements and contactless eddy-current measurements for electrical conductivity, Raman spectroscopy for structural analysis, optical and electron microscopy for morphological analysis, and X-ray diffraction and X-ray photoelectron spectroscopy for compositional and structural characterization. Tensile tests were performed to evaluate the effect of the IC-LIG process on the wood's mechanical properties. To showcase the applications of the IC-LIG technique, proof-of-concept devices were fabricated, including strain sensors, flexible electrodes, a capacitive touch panel, and an electroluminescent device.

The IC-LIG method successfully produced large-scale (100 cm²), highly conductive (≥2500 S m⁻¹), and homogeneous conductive patterns on wood veneers as thin as 450 μm, using a single laser pass in ambient atmosphere. The iron-tannic acid ink played a crucial role in protecting the wood from thermal damage and promoting efficient graphitization. The resulting IC-LIG structures exhibited excellent mechanical properties, comparable to untreated wood. Raman spectroscopy confirmed the formation of graphite-like structures, with the presence of iron within the carbon matrix. The conductivity values achieved were significantly higher (up to 2500 S m⁻¹) compared to those reported in previous studies using LIG on wood (400 S m⁻¹). The fabricated strain sensors exhibited high durability, withstanding over 69,000 cycles without significant performance degradation. The flexible electrodes showed excellent resilience to bending and twisting. The capacitive touch panel successfully demonstrated functionality as a human-machine interface. The electroluminescent device achieved light emission comparable to that of a device using a copper foil electrode, highlighting the potential of IC-LIG as a replacement for metallic electrodes in sustainable electronics.

The results demonstrate the significant advantages of the IC-LIG method for creating sustainable wood electronics. The single-step process, ambient-atmosphere operation, and avoidance of hazardous fire retardants make it a highly scalable and eco-friendly technique compared to conventional LIG methods. The high conductivity and excellent mechanical properties of the IC-LIG structures open up numerous possibilities for applications in various fields, including structural health monitoring, flexible electronics, and human-machine interfaces. The use of bio-based materials and the absence of toxic chemicals align with the principles of green electronics. This work provides a significant advancement in the field of sustainable electronics, paving the way for the development of large-scale, environmentally friendly electronic devices.

This research successfully demonstrates a novel, sustainable method for fabricating large-scale conductive patterns on wood using iron-catalyzed laser-induced graphitization. The single-step, ambient-atmosphere process produces high-conductivity materials with excellent mechanical properties. The resulting devices, including strain sensors, flexible electrodes, and a touch panel, showcase the potential of this approach for creating a new generation of sustainable wood electronics. Future research could explore further optimization of the ink formulation, investigating other wood species, and exploring applications in energy storage devices.

While the study demonstrates the effectiveness of the IC-LIG method, further investigation could be conducted to explore the long-term stability of the conductive patterns under different environmental conditions (e.g., humidity, temperature). The study focused on a limited set of wood species; expanding the investigation to a wider range of species would improve the generalizability of the findings. A detailed cost-benefit analysis comparing IC-LIG to other methods would be valuable to assess its potential for large-scale industrial applications.