Smart Ink for Flexo: 2015 Rossini Scholarship Winner Examines Printed Electronics

Editor’s Note: Bilge Nazli Altay took second place in the 2015 FFTA Rossini North America Flexographic Research Scholarship, and her research is ongoing. As such, the information contained in this article should be considered her preliminary results.

Smart inks are essentially inks with a purpose completely beyond mere coloring. These inks contain conductive pigments and polymers that allow them to conduct electricity. In recent years, developments and advancements in printing materials and press technologies have made possible the fabrication of electronic components in addition to traditional media. This addition generated its own discipline within the printing science field, named printed electronics (PE).

Although its name may seem intimidating, PE has a basic concept: It is a set of methods that are used to fully or partially create printed electronics devices on various substrates. There are many alternative terms for PE. Flexible hybrid electronics manufacturing is considered to be one of the more common; here are some others:

Figure 1: A waterproof textile EL lighting on a high visibility vest (Cetemmsa) and formable stretchable ink for 3-D circuitry (DuPont)
  • Functional printing
  • Flexible electronics
  • Organic polymer electronics
  • Printable electronics
  • Solution process electronics
  • Thin film electronics
  • Bendable electronics
  • Washable electronics

It might be fair to ask: Why would we even print electronics? First of all, it is really the idea that electronics can finally be printed on really thin, lightweight and flexible materials (see Figure 1 and Figure 2). That brings down not only the production cost, but also the production time. The products can get out faster and brand new products that could have never existed before can now be manufactured. We can make electronics that bend, electronics that roll, electronics that stretch—even electronics we can throw in the washing machine with our dirty socks. Finally, it is really the idea that these electronics are environmentally friendly in comparison to the traditional subtractive electronic fabrication methods. These are electronics that are lightweight and not made with extremely harsh etching chemicals; they will therefore not occupy a massive amount of space in landfills. If we are aiming to be sustainable, then PE is certainly the future.

Figure 2: Electronic devices fabricated by PE

There are three major ways to manufacture electronics (Figure 3). Printing goes under the additive technology, which selectively deposits material where it is needed. This greatly reduces the number of required steps as well as the amount of waste generated. On the other hand, traditional electronic manufacturing is a subtractive process that requires a rinsing step between each etching process.

Functional Materials

Since electrodes are the basic building blocks of most electronic components, formulation of conductive inks with different electrical properties is an emerging area of interest. There are PE components that have been already researched and fabricated, like solar cells, supercapacitors, batteries, electroluminescent devices and RFID tags. These components require certain ink functionalities like:

  • Conductivity
    Figure 3
  • Resistivity
  • Semi conductivity
  • Colorlessness/color changed by heat, light, moisture or pressure

Silver and gold are certainly the common conductors that have been studied, but the problem is they cost a lot of money. Copper is common as well and has a price advantage over silver and gold, but it can corrode or oxidize.

The conductive ink market is projected to be worth $3.36 billion by 2018, according to a report from IDTechEx. There is a big market demand, yet there is not enough variety in ink formulations and case studies to make the production more cost effective. Further, $735 million will be based on nano silver and nano copper—less than onethird of this money is focused on current technology. There is a huge amount of room for brand new ink formulations to seize the market and become profitable.

Figure 4: Flexible packaging, by printing process and region
Data courtesy of DuPont Cyrel

Nickel (Ni) is a lower cost metallic element with high conductivity and is also appropriate to use in ink formulation. It has high resistance to oxidation and corrosion. It can withstand both high and low temperatures. It has magnetic properties, good for proximity sensors, micro transformers and inductive charging. It is typically used in multilayer ceramic capacitors, magnetic devices, printed energy storage devices (as a current collector layer to enhance the use of stored power and to reduce overall energy consumption) and as electrodes and conductive interconnects for supercapacitors and flexible electronic devices.

Large pigment particles require higher temperatures for drying/sintering and they are prone to settle over time. They also limit the ability to obtain fine features. In contrast, nano particles have smaller diameters and higher surface area. They can be kept in solution without significant sedimentation due to their Brownian motion. As the diameter of the particle decreases, the melting temperature decreases as well. This allows sintering at lower temperatures, which is beneficial for substrates, especially polymer films. Therefore, nano Ni particles are the interest of this research.

Figure 5: SEM images of copper based ink, A–B as deposited and C–D photonically cured. Image courtesy of NovaCentrix

Conductive Ni inks are available in the market for screen printing applications from different vendors and the range of bulk resistivity is about 5 to 300 ohms/sq./mil. Nano Ni ink for inkjet and aerosol applications is available, though the products are still in the experimental phase. Ni ink for flexography and/or gravure printing has not been studied yet.