Applying electric field to assemble nanostructured filaments for sustainable electronics

Flexible electronics can withstand bending and folding which makes them ideal for wearables, foldable smartphones, and other portable devices that require a high degree of durability. They can be integrated into clothing to monitor health signs, embedded into flexible displays that can roll up or fold, or even used in smart packaging that can communicate information about the product’s condition. Another significant advantage is the potential for reduced size and weight. Moreover, flexible electronics can be made thinner and lighter than their rigid counterparts, which is important for aerospace and implantable medical devices applications. As technologies move towards the next generation of electronics, several advancements and challenges lie ahead, developing new materials that are more flexible, durable, and capable of conducting electricity efficiently is critical.  Additionally, as the demand for electronics continues to grow, so does the need for sustainable production practices and developing next generation of electronics that are recyclable will be crucial to minimize the environmental impact. Therefore, extensive research and development into novel polymers and nanomaterials, is ongoing to address these needs. To this account, a new study published in ACS Applied Materials & Interfaces conducted by Heather Wise and Professor Anthony Dichiara from the School of Environmental & Forest Sciences at University of Washington alongside Professor Hidemasa Takana from the Institute of Fluid Science at Tohoku University, the researchers investigated the process of assembling colloidal nanoparticles into macroscopic materials, focusing on the integration of cellulose nanofibrils (CNFs) and single-walled carbon nanotubes (SWNTs) to create high-performance functional filament nanocomposites.  This innovation is particularly timely and relevant given the growing need for sustainable and biodegradable systems in the context of the rapid proliferation of wearable devices and the corresponding increase in e-waste. Cellulose, being the most abundant renewable biomacromolecule, offers a sustainable alternative for the production of flexible electronics.

The team began by mixing CNFs and SWNTs in water without the need for external surfactants or binding agents. They optimized the dispersion process to achieve a stable colloidal mixture of CNFs and SWNTs, leveraging the natural properties of cellulose and carbon nanotubes to ensure uniform dispersion. The CNFs were extracted from bleached softwood pulp through TEMPO-mediated oxidation and mechanical defibrillation, while the SWNTs were carboxyl-functionalized to enhance their dispersibility and interaction with CNFs. The researchers used a field-assisted double flow focusing system aligned the dispersed CNFs and SWNTs within an alternating electric field combined with extensional sheath flows. This method allowed them to control the orientation of the nanoparticles precisely, ensuring uniform alignment. The aligned nanocolloids underwent a liquid-gel transition, locking in the nanoscale orientational anisotropy as the material assembled into macroscopic filaments. Structural characterization using scanning electron microscopy and transmission electron microscopy analyses revealed that the filament nanocomposites possessed a tightly packed core structure with evenly distributed nanoparticles. This uniformity and alignment were critical for the enhanced properties of the filaments. When the authors performed tensile testing and demonstrated that the filament nanocomposites exhibited significantly improved mechanical properties compared to those made from CNFs alone. The addition of SWNTs and the precise alignment achieved through the field-assisted process contributed to higher tensile strength and modulus, showcasing the material’s potential for structural applications.  The researchers evaluated the electrical conductivity of the filament nanocomposites and found that by incorporating SWNTs, the filaments displayed enhanced electrical properties, making them suitable for resistive-sensing applications. This aspect was particularly notable, as it highlighted the composites’ potential in the development of wearable electronics and sensors.

The researchers further explored the liquid sensing capabilities of the filament nanocomposites. The filaments exhibited ultra-sensitive resistive responses to liquid water, demonstrating potential applications in environmental monitoring and wearable health devices. This sensitivity was attributed to the structural and compositional optimization of the CNF/SWNT filaments. The authors’ findings demonstrate that the combination of CNFs and SWNTs, facilitated by the field-assisted flow focusing method, results in filament nanocomposites with significantly improved properties compared to those fabricated with CNFs alone. The method’s ability to preserve the nanoscale orientational anisotropy of the CNF/SWNT colloids during the transition to macroscopic filaments is critical for achieving the observed enhancements in performance. Moreover, the successful incorporation of SWNTs into the filament composition imparted conductivity and enabled the fabrication of resistive-sensing applications which opens up new possibilities for the development of advanced wearable devices and sensors with improved functionality and sustainability. The work also highlighted the potential of leveraging the unique properties of CNFs and SWNTs in colloidal form, which allows for a high degree of control over the material’s structure and properties at the nanoscale. In conclusion, the new study by Wise, Takana and Dichiara developed a sustainable, scalable, environmentally friendly and efficient method for manufacturing high-performance filament nanocomposites with multifunctional applications. Their work is a significant contribution to the development of next-generation wearable devices and sensors and addresses the critical challenge of reducing the environmental impact of electronic waste.

In a statement to Advances in Engineering, the authors said “In this research, we turned tiny wood particles and carbon nanotubes into high-performance filaments for sensing applications, all without harsh chemicals. This paves the way for new types of sustainable fibers with exciting possibilities!