Next Generation Conductive Ink

Authors: Alan Brown, Dr. Jay R Dorfman, Zac Hudson, Brandon Peters

Contact: abrown@nagasechemtex.com, Nagase ChemteX America, LLC

In the rapidly evolving field of Printed Electronics, the demand for robust, durable, and highly stretchable conductive inks is paramount for advancing applications in wearable technology and hybrid electronics. While existing conductive inks offer some degree of stretchability, they often exhibit a trade-off between mechanical resilience and electrical performance, leading to a significant increase in resistivity under strain. This limitation poses a critical challenge for engineers designing products that require consistent electrical conductivity across a wide range of motion. Nagase ChemteX America, LLC. (NCU) has developed a next-generation conductive ink, CI-1096, that is formulated to address this gap by providing superior stretchability while maintaining excellent and stable electrical resistivity. Our work constitutes a novel material solution that enables the development of more reliable and versatile stretchable electronic devices, paving the way for future innovations in biomedical sensors, smart textiles, and flexible displays.

Stretch Ink

Wearable Electrode

NCU began this project with the goals of developing a Ag ink with increased stretchability, superior durability and high conductivity compared to CI-1036, an NCU Ag ink that has been on the market >10 years. This paper will document some of the challenges incurred during development in addition to sharing the data which supports CI-1096 meeting the development goals.

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Development Challenges

As with any R&D product, developing the CI-1096 did present challenges. To meet the goals of maximum stretch/ crease properties, while optimizing both resistivity and cost, it was imperative to balance the Ag:Binder ratio to meet these goals. In Figure 1, below, the impact of Resistivity vs.% Stretch based on three different Ag: Binder Ratios is measured:

Figure 1 demonstrates that A<B=C in terms of Resistivity vs. % Stretch, in particular, up to 20% stretch. While the samples made with Ag:B ratio C might have slightly lower resistivity than sample B, there is not enough of a difference to justify the cost impact of adding the Ag in sample C just to potentially get marginally lower resistivity.

Crease performance was also optimized at Ag:Binder ratio B. This can be observed in Figure 2 which details Resistivity vs. # Creases.

Figure 2 Creasibility was tested over various trace widths. CI-1096 on the narrowest trace width (0.381 mm) shows excellent creasibility.

Comparing to CI-1036

With Level B being the optimized Ag:Binder ratio, CI-1096 was created, and further testing was completed to show that CI-1096 was superior to CI-1036. Comparing the two products, testing includes stretchability, durability and washability.

For stretchability both CI-1096 and CI-1036 were screen printed on a commercially available TPU substrate. The silver inks were cured for 8 minutes at 130°C. Figure 3 shows that CI-1096 can stretch to at least 60% prior to the trace breaking whereas the CI-1036 starts to exhibit open circuits at 46%. It should also be noted that as the circuits are stretched the rate of change in resistivity for CI-1096, is not as significant as CI-1036. Because CI-1096 is more consistent as stretched it enables a more robust circuit design.

Figure 3 CI-1096 and CI-1036 printed on TPU and stretched to 60% (maximum stretch for test pattern). Again, CI-1096 exhibits excellent elongation.

Additionally, CI-1096 was also assessed over five cycles. Figure 4 shows the stability of both stretch and hysteresis over five cycles, again a benefit to designing robust circuits. Samples were stretched to 20% elongation then returned to the original position.

Figure 4 Cycle testing for CI-1096. CI-1096 stretched to 20% and returned to 0% over five cycles.

As already stated, durability was another critical property where CI-1096 performs extremely well. While the CI-1036 is known and is used in applications where there can be multiple creases, it was discovered that as traces become narrower the CI-1096 well outperforms the CI-1036. This is seen in Figure 5 where the testing was done by printing both CI-1096 and CI-1036 on print treated polyester and cured for 8 minutes at 130°C.

Figure 5 CI-1096, regardless of trace width, maintains excellent resistivity after 10 creases. Crease testing was performed in accordance with ASTM F2750.

Finally, as some of the intended applications for using the CI-1096 are in the wearable/ stretchable realm, it was critical to also perform wash testing. Previous testing done by NCU with CI-1036 as the Ag circuit had proven that a tri-layer circuit construction of inks consisting of C/Ag/C (Figure 7) provided users with a garment that could be washed ~20 times. While the tri-layer did perform best, with CI-1096 being new development, a dual layer (Ag coated with C) was also included to compare to historical data. The conductive inks, including CI-1096, were printed and cured on TPU, according to what is stated on the TDS (8 minutes @ 130°C for CI-1096). After curing, the TPU film containing the tri-layer construction was then heat laminated to fabric. The samples were then placed in a Laundrometer washing temperature set to 30°C. Resistance was measured after every five wash cycles. Once again, samples constructed with CI-1096 outperform those constructed with CI-1036 and show that fifty wash cycles can be achieved! This can be seen in Figure 6:

Figure 6 Compares washability of conductive ink stackups on two different TPU films. The combination of CI-2078/ CI-1096/ CI-2078 provides data that supports the conductive inks surviving fifty wash cycles. 

Figure 7 Example of triple layer conductive circuit construction. Carbon is printed on TPU followed by Ag ink with a final layer of carbon ink.

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Summary

In summary, NCU achieved the desired goal of developing a next-generation conductive ink. With superior stretchability, exceptional durability and excellent washability, CI-1096 is optimized for use in a diverse range of applications. Whether those applications are intended to print on PET or TPU the data supports CI-1096 being an excellent choice for any engineer. For more information please contact Alan Brown at abrown@nagasechemtex.com or visit the Nagase ChemteX America website: www.nagasechemtex.com  

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