Hybrid inks: printed electronics without sintering

Interview with Prof. Tobias Kraus, Head of the Structure Formation Program Division, INM – Leibniz Institute for New Materials

07/03/2017

Printed electronics play an ever-increasing role, both in medical technology and other sectors: it is a flexible and space-saving solution and can be manufactured in large quantities at low cost. Right now, newly developed hybrid inks simplify the production of printed electronics and open up new applications thanks to their biocompatible properties.

Image: Smiling man with short black hair and suit - Prof. Tobias Kraus; Copyright: Uwe Bellhäuser

Prof. Tobias Kraus; ©Uwe Bellhäuser

In this interview with COMPAMED-tradefair.com, Prof. Tobias Kraus explains hybrid inks, how they differ from other printable materials and he also describes where printed electronics could be used in medical technology of the future.

Prof. Kraus, what are the hybrid inks you developed made of?

Prof. Tobias Kraus: As a key component, they contain metal nanoparticles that are encased in organic conductive polymer. Gold particles are the most interesting particles in a range of medical applications because they are considered biocompatible. We are also able to implement the enclosure with polymers that are approved by the FDA and are thus most likely to have no unwanted toxic side effects.

Which carrier materials can you imprint with these inks?

Kraus: There are special paper types that absorb the ink with great definition and do not soak it up too much. We can also use polymer films that occasionally need a specific pretreatment, or we can use glass.

Image: Collage of several images that show the function of printed, flexible electronics; Copyright: Leibniz INM

This collage shows the flexibility of electronics printed with hybrid inks. Below, its function is shown - the metal nanoparticles are joined together; ©Leibniz INM

Where are printed circuits or printed electronics used today?

Kraus: At the moment, there appears to be an emerging market for the production of RFID tags, those being antenna systems to wirelessly read labels. What’s more, one future application might be the integration of electronics into printed products made out of paper by embedding additional electronic functions.

Needless to say, wearables and smart textiles with integrated electronics are exciting for their potential in medical technology. Colleagues at a neighboring Institute are presently working on “electronic tattoos“, that being thin polymer mediums you can apply and stick on skin. Right now there are several companies and research activities that look into how specific vital signs could be measured this way and transmitted to mobile devices for example.

There are also early leads in the field of implants. This pertains to electrode implants to facilitate diagnostics and monitoring of data, for instance, blood glucose levels. Other approaches are concerned with increasing the life of electronic implants by printing them on flexible thin films. However, questions concerning the long-term effects and the reaction to body fluids and tissue need to first be addressed and answered.

What makes your hybrid ink different from other materials?

Kraus: Our ink does not require a sintering step. It can print at room temperature and is immediately conductive and stabilized after drying. Both effects are generated by linking the polymers that coat the nanoparticles. When the solvent in the ink evaporates – we typically use aqueous or alcoholic mixtures – the nanoparticles draw ever closer together. Once the solvent is completely gone, the polymers have bonded together and thus also connect the metal cores. The composite conducts electricity and has a stable effect to withstand mechanical stress.

Other types of inks have to either be sintered or illuminated with intense light after printing to create conductivity. However, sintering is not possible with all carrier materials.

How does the printing process work?

Kraus: Right now we are able to apply the ink with a nozzle, though this does not yield a very high resolution. We are working on printing with inkjet printheads, the same way they are already being applied in organic electronics today. You might have to draw on imprint processes for very high resolutions. In this case, you print with a stamp that has tiny structures. This is also something we are currently working on.

What differences have you discerned in the end product compared to other inks?

Kraus: The conductivity of our ink is higher than conductivity of simple and pure, conductive polymers. However, it is not as high as with pure metal particles. Based on our preliminary measurements, the stability is better than with pure polymers, also as it pertains to the effects of humidity and oxygen. Having said that, we still have to conduct long-term measurements and compare different types of polymers.

Mechanical flexibility is very important and depends on the polymers and ink formulations. We are able to bend and stretch our ink after the printing process while maintaining electrical conductivity. Meanwhile, metal on its own is very sensitive, especially when stretched.

How do you plan to continue this development?

Kraus: We are currently further enhancing the ink because it needs to be adapted to concrete applications. For example, one question is whether gold is the right core in all cases. Maybe we can make the material less expensive by using silver for certain applications outside the realm of medicine. In medicine, it is crucial to use non-toxic polymers. What’s more, you always have to determine the best process to print a specific product. In this case, we are already working on several inquiries from interested companies.

Photo: Timo Roth; Copyright: B. Frommann

© B. Frommann

The interview was conducted by Timo Roth and translated from German by Elena O'Meara.
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