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JST Press Release

Jul. 25, 2013
Japan Science and Technology Agency (JST)
(General Affairs, Public Relations Division)

The University of Tokyo
School of Engineering
(Public Relations Office)

“Imperceptible electronics that are lighter than a feather”
The world's lightest and thinnest flexible integrated circuits will produce stress-free wearable healthcare sensors

TOKYO, JAPAN - Within the framework of the Japan Science and Technology Agency's (JST) Exploratory Research for Advanced Technology (ERATO) project [Note 1], Professor Takao Someya, Associate Professor Tsuyoshi Sekitani, Dr. Martin Kaltenbrunner, University of Tokyo, and their coworkers have succeeded in developing the world's lightest (3 g/m2) and, simultaneously, the world's thinnest (2 m; one micrometer (m) is 1/1,000,000 of a meter) mechanically flexible integrated circuits and touch sensor system [Note 2].

Sensors and electronic circuits for healthcare and medical applications are generally fabricated using silicon and other rigid electronic materials. To minimize the discomfort of wearing rigid sensors, it is highly desirable to use soft electronic materials particularly for devices that come directly into contact with the skin. In this regard, electronics manufactured on thin polymeric films are very attractive: in general, a thinner substrate will provide better mechanical flexibility. However, directly manufacturing sensors or electronic circuits on ultrathin polymeric films with thicknesses of several micrometers or less is a difficult task if conventional semiconductor processes are used.

The international research team has manufactured the world's thinnest and lightest soft organic transistor integrated circuits (ICs) on ultrathin polymeric films with a thickness of only 1.2 m. This was possible because the team developed a novel technique to form a high-quality 19-nm-thick (one nanometer (nm) is 1/1,000,000,000 of a meter) insulating layer on the rough surface of the 1.2-m-thick polymeric film.

The organic transistor ICs exhibit extraordinary robustness in spite of being super-thin. Indeed, the electrical properties and mechanical performance of the transistor ICs were practically unchanged (no degradation was seen) even when squeezed to a bending radius of 5 m, dipped in physiological saline, or stretched to up to double their original size. Finally, these organic transistor ICs have been utilized to develop a flexible touch sensor system prototype.

Imperceptible electronics, namely, extremely thin, lightweight electronics whose presence cannot be perceived when worn, will open up a wide range of new applications in fields ranging from healthcare and biomedicine to welfare. Many new applications will emerge including wearable healthcare sensor systems, stress-free (free of discomfort) input units for welfare machines such as wheelchairs, sensors for medical electronic equipment, and tough sensors for sports usage.

This project was carried out in collaboration with Jonathan Reeder, Dr. Tomoyuki Yokota, Kazunori Kuribara, and Takeyoshi Tokuhara at the University of Tokyo, Tokyo, Japan; and Professor Siegfried Bauer, Michael Drack, Dr. Reinhard Schwdiauer, Assistant Professor Ingrid Graz, and Dr. Simona Bauer-Gogonea at Johannes Kepler University, Linz, Austria.

This achievement was published in Nature on July 25, 2013.

Background

Japan's birth rate remains one of the lowest in the world, and the population is aging rapidly. It is thus an urgent issue to figure out how to improve the quality of life of all citizens, including the elderly, and to reduce the rapid increase in medical costs associated with an aging society. In the last decade, there have been enormous advances in electronics and information technology (IT) including the establishment of high-speed optical communication networks and widespread use of mobile telecommunication devices such as smart phones, and Japan's leading IT infrastructure and electronic technologies are expected to play a role in resolving some of the various problems of its aging society with fewer children.

Thus far, electronics and IT devices have relied on rigid materials, mainly silicon, which have formed the basis of healthcare and medical sensors and electronic circuits. To realize next-generation electronics that harmonize technology with humankind, it is highly desirable to replace these rigid electronic components with soft materials, particularly for device parts that are placed directly on the skin.

To introduce mechanical flexibility into health-monitoring sensors and electronic circuits, some approaches that employ very thin silicon membranes and/or chips embedded in thin polymer films have been proposed and demonstrated. However, one of the major obstacles in these approaches is the fragile nature of the components during manufacturing and use.

Against this background, organic transistors—inherently soft electronic switches made of organic conducting materials—have been emerging and studied intensively. Since these transistors can be manufactured on polymeric films by solution processes such as printing methods at room temperature, their unique features such as a large area, low cost, lightness, and mechanical flexibility can be utilized easily all at once.

In terms of flexible organic transistor ICs, which possess an electrical performance as high as that on glass substrates, it was widely believed that 10 m represented the minimum possible thickness of the substrate and that any further reduction in the thickness was almost impossible.

The major limiting factor is the surface roughness of ultrathin (about 1 m-thick) polymeric films. Owing to the roughness, it has been impossible to form high-quality insulating layers of nanometer thicknesses uniformly on ultrathin polymeric films, without creating any pinholes (tiny holes that form undesirable conductive paths). While coating the polymeric material may reduce the surface roughness, it also significantly increases the total thickness.

For this reason, it is important to develop a manufacturing technology that can fabricate soft organic transistor ICs on ultrathin flexible polymeric films, particularly on those less than 10 m thick.

Research in detail

The international team has successfully developed the world's lightest (3 g/m2) and thinnest (2 m) high-performance organic transistor ICs on ultrathin (1.2 m) polyethylenenaphthalate films. To put these values into perspective, the thickness is about 1/5 of that of plastic kitchen wrap, and the weight is about 1/30 of that of standard weight office paper. The organic transistor IC can be used to manufacture a prototype touch sensor system consisting of 144 (12 12) sensor cells arranged in an effective area of 4.8 4.8 cm2 with an intercell spacing of 4 mm.

The key to the development was the successful establishment of a process technique to form a very uniform ultrathin insulating layer on a 1-m-thick polymeric film that has a fairly rough surface. More precisely, the team established a method to form a 19-nm-thick aluminum oxide layer with excellent adhesion to the substrate based on anodic oxidation that is carried out at room temperature. High-energy oxygen plasma treatment, which has been commonly employed to form aluminum oxide layers on plastic films, damages ultrathin polymer foils and creates pinholes. The here-developed methods avoid such high-energy processes and are fully compatible with the distinct processing requirements of ultrathin and conformable foil substrates.

As far as the robustness of the organic transistor ICs is concerned, they are amazingly tough in spite of being the world's thinnest. Indeed, the ICs are mechanically and electrically unbreakable, even when squeezed to reduce the bending radius to 5 m, crumpled like paper, or dropped from a height of a meter (or more). Furthermore, amazing durability was demonstrated in the study: after immersing the ICs in physiological saline (with components that are the same as bodily fluids or sweat) for more than two weeks, no obvious deterioration in the electrical properties was observed. Furthermore, the electric and mechanical performances of the organic transistor ICs were practically unchanged even when stretched by up to 233%.

The soft sensor system can be applied to freely curved surfaces like human skin to continuously measure body temperature, blood pressure, and many other vital signs.

In 2011, the same team succeeded in fabricating high-quality organic solar cells on 1-m-thick polymeric films, but in contrast, this development is the world's first report of the successful fabrication of organic transistor ICs on 1-m-thick polymeric films.

Outlook for the future

Imperceptible electronics, namely, extremely thin, lightweight electronics whose presence cannot be perceived, will realize a wide range of new healthcare, biomedical, and welfare applications. For instance, as sensor systems become thinner and lighter, the sensors will become so light that they will not be perceived when worn. This will enable sensors to measure biological information without causing any discomfort to the wearer. In other words, wearable health-monitoring systems to measure vital information, for example, body temperature and heart rate, over a 24 hour period will allow people to continue their daily activities as usual in a stress-free manner. The shock resistance of the sensors will enable vital information to be monitored continuously even during sport and exercise. If the present wearable bio-monitoring sensors are combined with the world's lightest solar cells, then new types of healthcare sensors such as a self-contained, semi-permanent health-monitoring system that generates electricity directly from indoor and outdoor light will be possible.

[Note 1] This result is an accomplishment of the Exploratory Research for Advanced Technology (ERATO) research funding program.

Project Name Someya Bio-Harmonized Electronics Project
Project Director Takao Someya, Professor
The University of Tokyo, School of Engineering
Group Leader Tsuyoshi Sekitani, Associate Professor
The University of Tokyo, School of Engineering
Project Period August 2011 - March 2017
Project Objective The aim of the project is to realize brand new electronic devices that seamlessly merge biological tissues and electronics together, by making the best use of the unique features of soft and bio-harmonized organic materials, and to subsequently open up new bio-harmonized electronics markets that are closed to conventional electronics relying on inorganic rigid materials represented by silicon.

[Note 2] The weight of 3 g/m2 and the thickness of 2 m account for the majority of the components, i.e., the substrate, semiconductor device, encapsulation layer, and sensing electrodes. The power supply and display unit, which are connected by wiring, are external components and not included in these weight and thickness specifications.

Figures

Figures 1

Figure 1: The world's lightest (3 g/m2) and, simultaneously, the world's thinnest (2 m) mechanically flexible touch sensor system It is lighter than a feather and used as a stress-free health-monitoring system when worn.

Figures 2

Figure 2: A schematic image of soft touch sensor.

Figures 3

Figure 3: The sensor film that is attached on the surface of a hand. It is extraordinarily flexible and can follow the shape of the complex surface such as wrinkles of the hand.

Movie

Contact information

Regarding this research:
Dr. Takao Someya, Professor
Department of Electrical Engineering and Information Systems
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Tel: (+81)-3-5841-0411
Fax: (+81)-3-5841-6709
E-mail:
URL:http://www.ntech.t.u-tokyo.ac.jp/

Dr. Tsuyoshi Sekitani, Associate Professor
Department of Electrical Engineering and Information Systems
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Tel: (+81)-3-5841-0413
Fax: (+81)-3-5841-6709
E-mail:

Dr. Martin Kaltenbrunner
Department of Electrical Engineering and Information Systems
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Tel: (+81)-3-5841-0411
Fax: (+81)-3-5841-6709
E-mail:

Regarding the JST project:
Tsuyoshi Nakamura
Research Project Group, Tokyo Headquarters
The Japan Science and Technology Agency (JST)
Tokyo Headquarters K's Gobancho
7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan
Tel: (+81)-3-3512-3528
Fax: (+81)-3-3222-2068
E-mail:

Regarding University of Tokyo:
Public Relations Office
School of Engineering
The University of Tokyo, Tokyo, Japan
Tel: (+81)-3-5841-1790
Fax: (+81)-3-5841-0529
E-mail:

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