Stretchable Carbon and Silver Inks for Wearable Applications

Abstract

For wearable electronic devices to be fully integrated into garments, without restricting or impeding movement, requires flexible and stretchable inks and coatings, which must have consistent performance and recover from mechanical strain. Combining Carbon Black (CB) and ammonia plasma functionalized Graphite Nanoplatelets (GNPs) in a Thermoplastic Polyurethane (TPU) resin created a conductive ink that could stretch to substrate failure (>300% nominal strain) and cyclic strains of up to 100% while maintaining an electrical network. This highly stretchable, conductive screen-printable ink was developed using relatively low-cost carbon materials and scalable processes making it a candidate for future wearable developments. The electromechanical performance of the carbon ink for wearable technology is compared to a screen-printable silver as a control. After initial plastic deformation and the alignment of the nano carbons in the matrix, the electrical performance was consistent under cycling to 100% nominal strain. Although the GNP flakes are pulled further apart a consistent, but less conductive path remains through the CB/TPU matrix. In contrast to the nano carbon ink, a more conductive ink made using silver flakes lost conductivity at 166% nominal strain falling short of the substrate failure strain. This was attributed to the failure of direct contact between the silver flakes.

Printed Nanocarbon Heaters for Stretchable Sport and Leisure Garments

“…The carbon inks have a significantly higher sheet resistance despite their similar thickness to the silver counterparts, because of the significantly higher bulk conductivity properties of silver. The sheet resistance of both the carbon and silver ink are in good agreement with the authors' previous work demonstrating the repeatability of the ink production and the screen-printing process [22]. This allows for the design of a printed heater where the significantly more conductive silver ink is used to uniformly deliver current to the heat generating resistive carbon heater block.…”

“…The white light images of single layer carbon (Figure 3a) and the single layer silver (Figure 3c) show no significant patterning on the surface of the coatings, suggesting that both inks were able to flow through the screen and relax to form a smooth coating, showing their suitability for screen printing. In a previous work by the authors Scanning Electron Microscope images to examine the microstructure of the coatings produced by the inks used in this study showed a near homogeneous surface, with the GNP flake size shown to be 4.05 ± 0.89 µm the smaller silver flakes appear to form a less homogenous microstructure, with voids present in the structure (Figure 3e,f) [22]. This relatively poorer particle distribution is a potential cause for the higher roughness in the silver print.…”

“…The heaters consisting of two printed layers of carbon with two printed layer silver busbars and a voltage of 15 V was selected for further mechanical robustness testing as at 15 V the heaters reached temperatures >60 °C and showed a fast temperature response, heating to 48.7 °C within 20 s of switch on. Straining conductive inks has also been shown to increase the electrical resistance of conductive prints [22], therefore, it was hypothesised The heaters consisting of two printed layers of carbon with two printed layer silver busbars and a voltage of 15 V was selected for further mechanical robustness testing as at 15 V the heaters reached temperatures >60 • C and showed a fast temperature response, heating to 48.7 • C within 20 s of switch on. Straining conductive inks has also been shown to increase the electrical resistance of conductive prints [22], therefore, it was hypothesised that under mechanical strain the resistance of the heaters would increase and the joule heating power of the heaters would decrease, decreasing the output temperature.…”

“…The authors have previously reported a stretchable screen printable nanocarbon ink with a printed sheet resistance of 230 Ω/ capable of withstanding strains of >100% and cyclic strains of 10 and 100% [22]. This paper examines the viability of using this high conductivity stretchable nanocarbon ink to create lightweight, printed large area, heaters for use in wearable technology.…”

Flexible Fiber Membrane Based on Carbon Nanotube and Polyurethane with High Thermal Conductivity

“…Thermoplastic polyurethane (TPU) is widely used in the electronic industry owing to its outstanding chemical resistance, excellent flexibility, elasticity and processability [1][2][3][4]. Therefore, TPU has great application potential in thermal interface materials (TIMs), which play a crucial role in efficient thermal management [5][6][7][8][9].…”