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3D Interconnection Technology for Futuristic Bioelectronics

Wearing a smartwatch no longer makes one look cool. This trend has long gone and the wearable biotech sector has recently publicized its ravenous hunger for futuristic products. Vital sign monitoring stickers, pain relief goggles that track brain waves, and even mind-reading glasses.

Graphical summary of the work. Carbon nanotubes with platinum (Pt) decoration show high affinity with liquid metals (left), and it results in the uniform dispersion of carbon nanotubes in liquid metal, forming stretchable metal composite (middle). The stretchable metal composite has superior mechanical properties than pristine liquid metal, and thus fits to be patterned as consistently-fine (that is, high resolution), 3D structures (right).

These are just a few of the newest products talked about at the 2019 Wearable Tech, Digital Health, and Neurotech Silicon Valley conferences. Not to be sure if all of these wearable models may catch on, but one thing is obvious: there are more emerging in the field of wearable technology. This unlimited potential has been, however, held back by a technical limitation: these wearables have never truly felt “wearable” to their users.

Although they were supposed to feel like a second skin of the wearer, it has been, in theory, challenging to develop “wearable” devices that are easy to bend and stretch and also maintain good data recording capabilities on soft and curved skin. Wearable smart devices collect a person’s bio measurements by linking electrodes to the surface of the skin. The device has 3D-shaped electrode wirings (namely, interconnects) that convey electrical signals.

Thus far, not only can the wirings be formed on a hard surface, but also the parts of such interconnects delicate and hardly stretchable metals like copper, gold, and aluminum.

In a paper published recently in the journal Nano Letters, the collaborative research team led by Prof. Jang-Ung Park at the Center for Nanomedicine within the Institute for Basic Science (IBS) in Daejeon, South Korea, and Prof. Chang Young Lee at the Ulsan National Institute of Science and Technology (UNIST) in Ulsan, South Korea reported completely-transformable electrode materials that also possess a high electric conductivity.

Particularly, this novel composite is extremely thin, 5 µm in diameter, which is half of the width of conventional wire bonding. By allowing ever-slimmer 3D interconnects, this research can help to transform the physical form of smart gadgets, besides strengthening their technical functions.

The study team used liquid metals (LM) as the key substrate since LMs are very stretchable and have comparatively high conductivities akin to solid metals. To enhance the mechanical stability of the metal liquid, carbon nanotubes (CNT) were spread evenly. “To have a uniform and homogeneous dispersion of CNTs in liquid metal, we selected platinum (Pt), for having a strong affinity to both CNT and LM, as the mixer and it worked,” said Young-Geun Park, the first author of the study.

This research also showed a new interconnection technology that can develop a very conductive 3D structure at room temperature: For possessing a high conductivity, the new system does not need any heating or compressing process. Furthermore, the soft and stretchable nature of the new electrode makes it easy to pass via the nozzle in a fine diameter. The team used a nozzle for the direct printing of different 3D patterning structure.

Park explains, “Forming high-conductivity 3D interconnections at room temperature is an essential technology that enables the use of various flexible electronic materials. The wire bonding technology used in existing electronic devices forms interconnects using heat, pressure, or ultrasonic waves that can damage soft, skin-like devices. They have been a great challenge in the manufacturing process of high-performance electronic devices.”

He observed that the pointed nozzle also permits reshaping of the preprinted pattern into different 3D structure, thus having an electrode function like a “switch” to switch power on and off.

Using the direct printing technique, the high-resolution 3D printing of this composite develops free-standing, wire-like interconnects. This new stretchable 3D electrical interconnections particularly comprises super-thin wires, as fine as 5 µm. Earlier studies on stretchable metals have only been able to deliver wire lines of several hundred micrometers in diameter. Compared to the interconnect of conventional wire bonding, the new system is even thinner.

We may soon be able to say goodbye to those bulky skin-based interfaces as this freely-transformable, super-thin 3D interconnection technology will come as a big breakthrough to the industry’s efforts to produce ever compact and slim gadgets.

Jang-Ung Park, Study Corresponding Author and Professor, Center for Nanomedicine, IBS

Jang-Ung Park added, “Blurring the boundary between the human body and electric devices, this new technology will facilitate the production of more integrated and higher-performing semiconductor components for use in existing computers and smartphones, as well as for flexible and stretchable electronic devices.”


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