Researchers Combine Carbon Nanotubes and Nanocellulose to 3D Print Conductive Microfibers

3D printing technology can only go as far as the different materials that are able to be printed…but from metal and plastic to sand and even food, I don’t think we have to worry about running out of possible 3D printing materials. Nanofibrillated cellulose (nanocellulose or NFC), a novel biomaterial with multiple industrial and scientific uses, is a protein that’s been used in wound care, and carbon nanotubes (CNTs), which are small tubes of carbon created on the nanoscale, have applications in industrial safety. Other than being 3D printable, what do these materials have in common? Engineers at the University of Maryland (UMD) have answered that question, by combining CNTs and NFC to 3D print strong, conductive microfibers.

(a) Programmable printed 3-layered conductive lines with designed shape. (b) SEM image of the cross section of the 3D printed CNF-NFC pattern. (c-d) SEM images of printed pattern, showing good adhesion between layers.

This creates a whole new use for NFC, and greatly improves the 3D printability of CNTs for use in wearables. The scalable 3D printed wood NFC-CNT microfibers have high mechanical strength and electrical conductivity – these qualities could bring down the cost of fabrication, and increase the performance, of wearable electronics like capacitors and batteries.

Associate Professor Liangbing Hu of the university’s Energy Research Center told Nanowerk, “Conventional methods to disperse carbon nanotube in aqueous solution include carbon nanotube surface covalent modifications and organic surfactants. This either leads to low mechanical strength or poor conductivity. We used nanocellulose particles as both dispersing agent for the carbon nanotubes and as reinforcement in the composite fibers.”

Wu and the rest of his team published a paper on their work, titled “Cellulose-Nanofiber-Enabled 3D Printing of a Carbon-Nanotube Microfiber Network,” in the journal Small Methods; in addition to Hu, co-authors include Yuanyuan Li, Hongli Zhu, Yibo Wang, Upamanyu Ray, Shuze Zhu, Jiaqi Dai, Chaoji Chen, Kun Fu, Soo-Hwan Jang, Doug Henderson, and Teng Li.

Schematic showing the CNT–NFC microfiber network formation through 3D printing. Solvent exchange was performed once the NFC-dispersed CNT solution was extruded into the coagulation bath of ethanol, leading to the formation of a stable gel fiber. A dry CNT–NFC microfiber can be obtained after pulling the gel fiber out to dry under tension for tens of seconds. During the process, all the building blocks are highly aligned along the fiber direction. The alignment of both CNT and NFC in microfibers leads to the combination of excellent mechanical strength and electrical conductivity.

According to the paper’s abstract, “Highly conductive and mechanically strong microfibers are attractive in energy storage, thermal management, and wearable electronics. Here, a highly conductive and strong carbon nanotube/nanofibrillated cellulose (CNT–NFC) composite microfiber is developed via a fast and scalable 3D-printing method. CNTs are successfully dispersed in an aqueous solution using 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) oxidated NFCs, resulting in a mixture solution with an obvious shear-thinning property. Both NFC and CNT fibers inside the all-fiber-based microfibers are well aligned, which helps to improve the interaction and percolation between these two building blocks, leading to a combination of high mechanical strength (247 ± 5 MPa) and electrical conductivity (216.7 ± 10 S cm⁻¹). Molecular modeling is applied to offer further insights into the role of CNT–NFC fiber alignment for the excellent mechanical strength. The combination of high electrical conductivity, mechanical strength, and the fast yet scalable 3D-printing technology positions the CNT–NFC composite microfiber as a promising candidate for wearable electronic devices.”

What’s interesting is that the team uses 3D printing to achieve a one-dimensional fiber formation, based on a solvent exchange between water and ethanol. The researchers used the cellulose nanofibers to disperse the CNTs in the water – these are not normally dispersible in either liquid. The team also tried to disperse the CNTs in ethanol using the NFCs, but it didn’t work.

“Using NFC as a dispersant for nanotube dispersion is easy to scale up, and it holds more advantages than CNT dispersion through chemical modification. Firstly, NFC can be extracted from abundant cheap resources, such as wood, cotton, and wheat straw. It is also bioactive because the NFC is biocompatible, and organic solvent is avoided. This makes the dispersion suitable for applications in life science and the disposal is a non issue,” explained Li, the first author on the paper. “Secondly, the dispersion of CNT is mainly based on absorption, wrapping, and fluctuation of counter ions between the CNT and the NFC. Thus, the electronic structure and conductivity of individual CNT are preserved compared with dispersion by chemical modification. In addition, the NFC remaining in the dispersion is an excellent building block to make strong, lightweight CNT nanocomposites.”

a) Schematic of microfiber fabrication, where the building blocks were pre-aligned by extruding into the coagulation and further aligned by tension during drying. b) A knot made from a microfiber showing the flexibility. c) Schematic of square-wave structure for 3D printing. d) 3D printed square-wave structure. e) Designed “UMD” shape structure. f) Optical image of the 3D printed “UMD” pattern. g) Programmable printed three-layered conductive lines with designed shapes; the scale bars in (d), (f), and (g) represent 10 mm. h) SEM image of the 3D printed pattern showing good adhesion between layers. The yellow arrows point the interface between two layers.

The NFC-dispered CNT solution was extruded into a coagulation ethanol bath. Then, researchers perform a solvent exchange to get a stable gel fiber, which was then removed and dried, under tension, at room temperature. The building blocks were able to align along the direction of the fiber length, due to the shear force that was introduced during extrusion and the applied tension during the drying, resulting in “a highly aligned microfiber.” This process is illustrated in the image above.

Hu said, “By dramatically improving the 3D printability and mechanical/electrical properties of CNT microfibers, we demonstrated that nanofibrillated cellulose is an excellent enabling material for high-performance microfibers for wearable electronics.”

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