Imperceptible Electronic Skin
The authors describe recent progress, bottlenecks, and future applications for extra-light and flexible interfaces such as electronic skin.
by Tsuyoshi Sekitani, Martin Kaltenbrunner, Tomoyuki Yokota, and Takao Someya
ELECTRONIC SKIN (E-Skin) is a flexible, stretchable sensor array that can essentially computerize a surface, including that of robots and human beings. The ideal E-Skin is still under development, but it will be sensitive to heat and pressure and also be so light that a user or wearer is unaware of its presence. It will stretch and conform to a variety of surfaces, including over large areas. Such a bionic skin applied directly to the human body could be used to monitor medical conditions or to provide more sensitive and life-like prosthetics with sensing “skins.”
Our research group at the University of Tokyo first developed E-Skin about a decade ago.1,2 We wanted to create large sheets of this material and embed them with enough sensors to at least roughly mimic the abilities of human skin and to do that economically. We dreamed of making an electronic skin embedded with temperature and pressure sensors that a robot could wear, so that if a robot health aide shook hands with a human patient, its sensor-clad skin would be able to measure some of the person’s vital signs at the same time. In this article, we describe recent research in terms of progress, bottlenecks, and future prospects for electronic-device–human interfaces such as E-Skin.
Uses of E-Skin
Flexible and stretchable electronics using organic transistors could serve a wide range of biomedical applications. As just one example, we have experimented with electromyography, the monitoring and recording of the electrical activity produced by muscles. We distributed organic-transistor-based amplifiers throughout a 2-µm-thick film made of polyethylene-naphthalate or polyethylene-terephthalate, and this allowed us to detect muscle signals very close to the source, which is key to improving the signal-to-noise ratio and thus the accuracy of the measurements. This was done by attaching electrodes with adhesive gels directly to the muscle surface. Conventional electromyography techniques typically use long wires to connect sensors on the skin with amplifier circuits, causing the signal-to-noise ratio to be pretty poor.9,10 In another example, in collaboration with the medical school at the University of Tokyo, we are working on an experiment that will place one of our amplifier matrices directly on the surface of a rat’s heart. We expect that we will be able to detect electrical signals from the heart with high spatial resolution and superb signal-to-noise ratios. Another example of E-Skin used internally is a catheter with a surface integrated with electronic circuits that can measure the pressure distribution within blood vessels.
Real biological skin has the critical ability to sense many variables at once. Our early generations of E-Skin have adopted an integrated system that simultaneously detects pressure and temperature and maps those stimuli to particular locations on the skin’s surface. An ultrasonic skin covering a robot’s entire body could work as a 360° proximity sensor, measuring the distance between the robot and obstacles. This would prevent the robot from crashing into walls or objects and or even allow it to interact safely with our relatively soft and fragile human bodies. In a similar fashion, for humans, electronic skin could enable prosthetics or garments that are hyper-aware of their surroundings.
Requirements and Raw Materials for E-Skin
Stretchability is an important key to realizing imperceptible E-Skin. The stretchability of the human epidermis is around 20%. To create similarly stretchable electronics , the type of electronic skin that can curve around an elbow or knee requires a thin material that can flex and stretch without destroying its conductive abilities. In our lab, we have focused on making TFTs that use various types of semiconductor materials that can be deposited in thin layers, such as amorphous silicon, low-temperature polycrystalline silicon, organic semiconductors, and carbon nanotubes.
The semiconducting materials described above have varying properties and should be chosen based on application and required performance. For example, if high electronic performance is desired, carbon-nanotube semiconductors are the best choice, and if flexibility is the main goal, organic semiconductors are better than other materials because of their inherent flexibility.
Another key challenge in realizing stretchable electronics is to simultaneously maintain electrodes with excellent electrical and mechanical characteristics. Highly conductive materials such as metals and conductive polymers are generally hard and not stretchable. In contrast, highly stretchable flexible materials such as rubber have poor electrical characteristics. Thus far, the development of stretchable electronics has been actively carried out worldwide by transferring highly conductive materials such as metals and graphene onto rubber sheets for use in stretchable interconnections and by machining metal-evaporated films into a mesh structure to make them stretchable.
After much experimentation, we have come to the conclusion that plastic films are the best for substrates. They are rugged and hold up well to mechanical strain; they cost very little and are compatible with new manufacturing processes that can produce large, flexible sheets of electronic materials – such as roll-to-roll manufacturing. To print TFTs on a plastic film like one made of polyethylene terephthalate, the processing temperature needs to be kept low enough to prevent the plastic from deforming. TFTs made with organic semiconductors can be easily printed on plastic at room temperature.
TFTs do not just allow the electronics to be flexible – they can also help E-Skin mimic the sensitivity of real human skin. There are more than 2 million pain receptors and 30,000 thermal receptors in a person’s entire skin, which is equivalent to the number of pixels found in a typical high-definition television. A major obstacle for E-Skin is figuring out how this many sensors can be integrated into electronic sheets. Two-million sensors cannot be directly wired up to the driver circuits that control them because this would require cramming 2-million contact pads onto a silicon chip. (Of course, it will not usually be necessary to create a piece of E-Skin this large.)
The solution is to do exactly what display manufacturers do for controlling the transistors in their TV screens. They use wiring layouts that allow the central processing unit to send commands to the transistors attached to individual pixels on the basis of where they lie in a big conductive grid. Each pixel’s address is designated by its column and row numbers, just as is done for active-matrix displays.
Developing Flexible Transistors and Their Integrated Circuits
In 2003, Sigurd Wagner of Princeton University and colleagues reported inverter circuits and transistor arrays that were stretchable by up to approximately 10% by mounting specially prepared amorphous silicon in silicone rubber and forming transistors consisting of wavy gold electrodes.11 This is the first reported example of stretchable electronics comprising active elements to the best of our knowledge.
In 2003, we successfully fabricated high-performance organic transistors by using the low-molecular-weight organic-semiconductor pentacene for the channel and by forming polyimide gate insulating layers on plastic substrates.3 When we systematically examined the effects of bending strain on electrical conduction properties by freely bending the organic transistors, we found that, unfortunately, the mobility increased or decreased by at least 10% upon the application of a strain of approximately only 1% depending on the bending direction.4 It was demonstrated that changes in the channel current upon the application of strain were independent of the relationship between the current and strain directions, but resulted from an electrical conduction phenomenon unique to polycrystalline organic semiconductors. In 2005, we also found that the minimum bending radius of the transistors was 1 mm or lower when we used a neutral-strain structure on 12.5-µm-thick thin plastic substrates. This revealed that the transistor characteristics did not deteriorate upon being subjected to a large bending strain. In bending tests, the transistors were confirmed to operate even after at least 100,000 repetitions of bending.5
In 2005, we fabricated a high-sensitivity organic thermal sensor using the property that a positive bias current greatly changes when an organic pn-junction fabricated using the organic p-type semiconductor fluorine copper phthalocyanine and the organic n-type semiconductor perylene tetracarboxylic diimide (PTCDI) is heated. We also successfully developed a thermal-sensor sheet that can measure temperature distribution over a large area by integrating the above organic thermal-sensor cell with an organic-transistor active matrix.2 Thus, a fused sensor that behaved similarly to human skin was realized through simultaneous flexible pressure and thermal-sensor sheets.
In 2010, we succeeded in fabricating organic-transistor-based complementary metal-oxide-semiconductor (CMOS) integrated circuits on 12.5-mm-thick plastic substrates with a drive voltage of 2 V that can maintain their electrical characteristics while being crushed.6 In addition, we developed a neutral-strain structure using organic polymers that do not damage organic semiconductors, thus enabling the fabrication of integrated circuits that can operate without any degradation in electrical characteristics while being bent to a radius of curvature of less than 0.1 mm. An aforementioned example of an application of this technique was a catheter with a surface integrated with electronic circuits that could measure the pressure distribution within blood vessels. This was fabricated by wrapping a transistor active matrix and a piece of pressure-sensitive conductive rubber around a 1-mm-diameter medical catheter in a spiral pattern. We also integrated and arranged organic non-volatile memory arrays and organic flexible pressure sensors in a two-dimensional lattice to form a 26 × 26 active-matrix sensor pixel that can store pressure data as an image within a sheet, as reported in the journal Science.7
Also in 2010, we developed a flexible ultrasonic imaging sheet that can detect objects without contact by integrating organic ferroelectric polymers and organic transistors.8 Ultrasound is radiated from a source, and the ultrasound reflected at the material is received by the polymeric receiving elements. By arranging the receiving elements in a two-dimensional array, not only the distance from the objects but also the shape of the objects can be obtained as three-dimensional information. We believe this particular technology could be commercially available within 10 years.
In 2013, we fabricated high-performance organic transistors and tactile sensors on an ultra-thin polymer sheet that measured 1 µm thick – thinner than a human hair and light enough to drift through the air like a feather (Fig. 1).
Fig. 1: (a) A 1-µm-thick organic electronic system in which organic transistors and tactile sensors were fabricated on an ultra-thin polymer sheet light enough to drift through the air like a feather. (b) A 64-channel organic active-matrix amplifier array can be placed on the human body for imperceptible electromyogram monitoring. (c) Shown is a rendering of the manufacturing flow of a stretchable organic transistor and the corresponding photographs. Adapted from Refs. 9, 10, and 16. Copyright 2013, IEEE, and 2013, Nature Publishing Group.
Even so, this material can withstand repeated bending, can be crumpled like paper, and can accommodate stretching of up to 230%. This material also works at high temperatures and in aqueous environments, meaning that it can function inside the human body.
Further Successes
In 2005, we integrated an organic-transistor active matrix with thermal and pressure sensors on a plastic film and then processed the device using a punching machine and numerical control (NC) cutter to form a net-shaped structure, realizing a stretchability of 25% (Fig. 2).
Fig. 2: (a) Shown is a 25% stretched plastic film with organic transistors, pressure-sensitive rubber, and thermal sensors processed mechanically to form unique net-shaped structures and (b) its magnified picture. (c) Demonstrated is the spatial distribution of pressure and thermal information using stretchable E-Skin. The saturation current or drain-source current of organic TFTs in an active matrix is measured at various temperatures under application of pressure (30 kPa) and release (0 kPa). A copper block (15 × 37 mm2) whose temperature is maintained at 50°C is positioned at the center of the array marked by the dotted line. The sensing area dimensions are 44 × 44 mm2. In (d), the pressure-sensor matrix is spread over an egg. Adapted from Ref. 2. Copyright 2005, National Academy of Sciences.
In addition, we implemented the device on the surface of a robot and successfully read out the spatial distribution of temperature and pressure. As an advantage of this method, we found that stretchable devices can be obtained by drilling holes into devices after their fabrication and that the electrical characteristics of these fabricated devices are stable because their active elements do not deform, even while they are being stretched. The applications of stretchable devices go beyond sensors and displays. In 2010, stretchable thin-film batteries were reported by Siegfried Bauer of the Johannes Kepler University of Linz in Austria and colleagues.12
To this point, conventional stretchable conductive materials had been given their elastic characteristics by machining highly conductive materials such as metals into a wavy or net-shaped structure.11,19 However, these materials are essentially not stretchable. In addition, materials obtained by this approach exhibit high electrical performance but are extremely difficult to apply to large-area electronics such as displays and sensors because of their low scalability (i.e., difficulties in increasing the area) due to the use of photolithography for transferring thin-film electrodes and patterning interconnections. As a result, conventional stretchable electronics had dimensions of a few tens of millimeters at most.
In 2008, we successfully developed intrinsically stretchable conductors that can be stretched similarly to rubber [Figs. 3(a) and 3(b)] and fabricated the world’s first stretchable large-area electronics using stretchable conductors as electrodes [Fig. 3(c)].
By applying a material-process method, we succeeded in uniformly dispersing nanotube gels within rubber materials . Although the stretchability and conductivity depended on the volume ratio of the nanotubes to rubber, we realized a new stretchable conductive material that exceeded a conductivity of 100 S/cm and had a stretchability up to 140%.13–15 This stretchable conductor, which is in the form of a paste before drying, can be patterned by printing and applied to large-area stretchable electronics. We also succeeded in fabricating an organic-transistor active matrix that can maintain its electrical characteristics, even when stretched 70–80% by integrating the stretchable conductors and organic-transistor pixels on a rubber sheet. Furthermore, by integrating the stretchable conductors with OLED elements, a stretchable organic active-matrix LED display was realized [Fig. 3(d)].
Fig. 3: (a) Shown is a photograph of printed elastic conductors on a poly(dimethylsiloxane) (PDMS) sheet. (b) Shown is a magnified SEM image of the elastic conductor. Finer or exfoliated bundles of single-walled carbon nanotubes (SWNTs) were uniformly dispersed in the rubber, where they formed well-developed conducting networks. (c) A large-area stretchable active matrix comprises 15 × 15 organic transistors and wiring using SWNT elastic conductors. The printed organic transistors function as active components, while the SWNT elastic conductors function as word lines and bit lines for the interconnections among the transistors. (d) Depicted is a rubber-like stretchable active-matrix organic LED display. Adapted from Refs.13 and 14. Copyright 2008, American Association for the Advancement of Science (AAAS), and 2009, Nature Publishing Group.
Parallel Progress
The next technical challenges for achieving imperceptible E-Skin that is very similar to real human skin are responsiveness, cost, environmental stability, energy management, reliability under long-term continued use, and more.16 Here, we would like to show several approaches to further realizing electronic skin. For example, commercial pressure-sensitive rubber exhibits a maximum sensitivity of 30 g/cm3, which is not sufficient to detect a gentle touch. To improve the E-Skin responsiveness to such stimuli, researchers are experimenting with a number of different techniques.
Zhenan Bao and her co-workers at Stanford University created a flexible membrane with extraordinarily touch sensitivity by using a type of precisely molded pressure-sensitive rubber sandwiched between electrodes.17 The thin rubber layer has a novel design that uses micrometer-sized pyramid-like structures that expand when compressed, allowing the material to detect the weight of a fly resting on its surface. With such sensors embedded, a bionic skin could sense a breath or perhaps a gentle breeze. In the most recent application of Bao’s technology, her team turned around the pressure sensors so that instead of detecting external stimuli, they measured a person’s internal attributes. The researchers developed a flexible pulse monitor that responds to each subtle surge of blood through an artery. It is meant to be worn on the inner wrist under a band-aid and could be used to keep track of a patient’s pulse and blood pressure.
Ali Javey and his co-workers at the University of California, Berkeley, first figured out how to make flexible, large-area electronic sheets by printing semiconducting nanowires onto pliable substrates such as plastics or paper. Then they added strain sensors to the material, which could endow their bionic skin with more native tactile sensitivity. A prosthetic hand wrapped in this sensitive material might be able to handle a delicate object like an egg with exquisite care.
Coupling sensors with radio-frequency communication technology within the E-Skin allows the information it is measuring to be transmitted wirelessly to computers – or conceivably even to other E-Skinned people. At the University of Illinois at Urbana-Champaign, John Rogers’ team has taken the first step toward this goal. His latest version of an “electrical epidermis” can be laminated onto your skin in the same fashion as a temporary tattoo18,19 The circuits are first printed onto a water-soluble plastic sheet, so the backing can be washed away after the circuit is pressed onto the skin. These tiny devices could be used to monitor a patient’s vital signs without the need for wires and bulky contact pads and could also be discretely used by people beyond the confines of the hospital. Rogers’ circuits use serpentine squiggles of semiconductors that can be stretched or squished without interfering with their function. He and his colleagues applied circuitry studded with sensors to a person’s throat, where it could detect the muscular activity involved in speech. By simply monitoring the signals, researchers were able to differentiate between several words spoken by the test subject. The user was even able to control a voice-activated video game.
A Range of Prospects for E-Skin
Flexible and stretchable electronics are a next-generation technology that will make people’s lives safer, more secure, and more comfortable. In addition to the applications described above, sensors could be embedded into objects that come in contact with people, such as clothes, seats, handles, and seatbelts, in order to help monitor their physical health. Large-area pressure sensors placed on the floor of homes could differentiate trespassers from residents by using information such as the size and shape of feet, weight, and the stride length. Large-area pressure sensors embedded in beds could monitor the heartbeat and cardiac motion of patients without causing them stress.
We have not realized all the necessary milestones yet and a great deal of our strategy for improving this device remains proprietary, but we believe that E-Skin will eventually provide humans with not only a new type of man–machine interface but an astonishing range of new applications, many of them yet to be imagined.
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