Revolution in electronics ink

Revolution in electronics ink

7:12 AM, 21st June 2018
Dr. Mohammad Vaseem, Post-Doctoral Fellow at KAUST.
Dr. Mohammad Vaseem, Post-Doctoral Fellow at KAUST.

In an interview, Dr. Mohammad Vaseem with Chemical Today Magazine opens up on the opportunities that printed electronics ink has in the age of wearable electronics.

Dr. Vaseem is a Post-Doctoral Fellow at Computer, Electrical and Mathematical Sciences and Engineering Division of King Abdullah University Of Science & Technology (KAUST), KSA.

Potential of printable electronics.

Printed electronics (or additive manufacturing) is revolutionizing the way electronics are manufactured. Inspired by the printing of newspapers and magazines, the roll-to-roll and reel-to reel printing capability is being considered as the future for large volume manufacturing of flexible and wearable electronics. With applications such as Internet of Things (IoT) emerging, billions of wireless sensors need to be manufactured which could be mounted on non-planar objects or be worn by humans. This is where printing the electronics can be a game changer. Printing does not require expensive masks, does not waste any material as it deposits material only at the required places, and can be used to print on unconventional substrates such as plastics, papers and even textiles. Due to this surge in printed electronics, numerous conductive inks have become commercial recently and are being used to realize electronics. Some dielectric and semi-conductor inks, though in their infancy, have also emerged. However, there is a dearth of functional inks, those which can bring smartness or control in the electronics.

Role of special inks in the research.

Developing such a functional ink that their material properties can be activated by external stimuli such as light, heat, electric, magnetic or pressure can be a significant advancement in low cost printable smart devices. Further, we have used iron-based reagents, iron (ii) and iron (iii) chloride with the combination of sodium hydroxide and acetic acid in the research.

Sectors that will benefit from this research on printable electronics.

In the wireless industry, there is a big desire to have tunable and re-configurable radio-frequency (RF) components (such as antenna and filters) which are required to cover multiple frequency bands or to be able to tune to a different wireless standard in a different zone of the world. We have many frequency bands in a typical cell phone, GSM (900 MHZ, 1800 MHZ), 3G/4G (2 to 3 GHz typically), GPS (1.5GHz), WiFi (2.4 and 5 GHz) etc and all of these bands require RF components such as antennas and filters. Also, these bands vary slightly in different parts of the world. This means that an antenna designed for Asian frequency bands may not work for the North American bands. Thus, there is a requirement that these antennas or filters can be tuned or reconfigured to different bands for different standards. Also, a big advantage can be that a single antenna or filter may work for multiple bands, reducing the number of components and eventually the cost and size of a wireless device.

Typically, these antennas and filters are made through metallic patterns realized on a dielectric/insulator substrate which is non-magnetic. If this dielectric substrate can be replaced with a magnetic substrate, the antennas, filters and other RF components can be made tunable and reconfigurable. And if that magnetic substrate can be printed with the flexibility of varying thicknesses and magnetic properties, there can be (not only) huge cost savings but also a number of other performance metrics of these components can be optimized, which are not presently possible with fixed thickness and material properties based substrates.

Preparing magnetic iron-oxide nanoparticles.

As our target was to prepare smaller size nanoparticles, we have adopted “Hot-injection” solution method. With typical iron chloride based precursor and the combination of acid (acetic acid) and base (sodium hydroxide), one can easily prepare smaller size nanoparticles. The presence of acetic acid and addition of sodium hydroxide at heating temperature played an important role in the formation of small iron oxide NPs. If sodium hydroxide was added to the boiling solution with the presence of acetic acid, higher temperatures generally caused faster reaction rates, generating large amounts of nuclei in a short time and leading to the formation of small (15-20 nm) iron oxide nanoparticles.

Resins used to strengthen magnetic ink.

We have utilized SU8-2002 polymeric resin purchased from MicroChem to strengthen the magnetic iron oxide nanoparticles ink. However, to make iron oxide nanoparticles compatible with SU8, iron oxide was required functionalization on the surfaces of nanoparticles. SU8 2002 is usually composed of an epoxy that is dissolved into an organic solvent (eg, cyclopentanone).

Herein, we have utilized oleic acid that is well known and has been successfully used as a molecule for functionalization of iron oxide nanoparticles in many biological applications. Furthermore, the choice of SU8 was also due to its low wt percentage of resin with low viscosity solvent composition and its photocuring capability. A number of other photocurable polymeric resins are available but due to their high content of resin (>99 percent) and high viscosity, it is very challenging to embed the nanoparticles in those resins.

Similar ongoing inks materials research projects.

Ink-preparation for printed electronics is a hot area of research. There are number of inks material available but mostly qualified as a metallic ink.

However, functional ink such as magnetic, ferroelectric, or piezoelectric inks are still lacking. As stated in our article, there is only a commercial magnetic ink solution available from Sigma Aldrich but it has a low concentration (<1 wt percentage) of iron oxide nanoparticles and is not suitable for these RF applications. However, our work demonstrated with 10wt percentage loading of iron oxide nanoparticles for ink-jet printing and achieved highest tunability so far.

Addressing the brittle nature of nanoparticles.

Usually magnetic iron oxide nanoparticles are brittle in nature so it was very difficult to create free standing substrate made of iron oxide nanoparticles only. We realized that we need some binder or polymer which can uphold the iron oxide nanoparticles to make it a free-standing substrate. In this work, photo-curable SU8 polymeric resin appeared to be the best choice but due to its solvent nature, blending iron oxide nanoparticles into SU8 solution was quite challenging. We functionalized iron oxide nanoparticles with Oleic acid to make it compatible with SU8 resin.

Another challenge that we encountered was the weight percentage loading of nanoparticles in SU8 solution. The 50:50 wt percentage ratios of iron oxide nanoparticles and SU8 resin turned out to be the optimum combination to get perfect free standing magnetic substrate.

Commercializing the technology.

We plan to commercialize our technology and are targeting our research specifically in “printed electronics ink” industry so as to provide the solution for many sensor and RF electronics related problems. We have also filed the patent in United State Patent and Trademark Office (USPTO).

Plans for future research.

We would like to extend the process to a multilayer design that can use embedded bias windings just like LTCC technology. In this case, no external magnets will be required for the generation of the bias magnetic field. Thus, a miniaturized module can be developed. Of course, we need more sophisticated printer with photo-curable unit that can be compatible with such kind of ink which can help to produce much smoother surfaces in a multilayer printing process.

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