The field of electronic skin (e-skin) has blossomed in recent years, with researchers demonstrating a range of practical healthcare applications for these wearable devices, ranging from detecting inflammation around wounds to reading the electrical activity of the brain. Various research groups are now working toward incorporating stretchable light-emitting diodes (LEDs) into their devices, which would allow users to read their vitals straight from their skin.

In a step toward this goal, materials scientists at The University of Tokyo have created an ultrathin and ultraflexible optoelectronic skin with organic photodetectors and highly efficient polymer light-emitting diodes (PLEDs). In a practical test of the technology, published recently in Science Advances (doi:10.1126/sciadv.1501856), the team demonstrated a skin-laminated device that could serve as an accurate pulse oximeter to measure oxygen concentration of blood, and that is stable enough to survive in ambient air.

“Future technology is growing from wearable (e.g., smartbands, smart watches) to skin-attachable sensors and devices,” says Hyunhyub Ko, a chemical engineer at Ulsan National Institute of Science and Technology in South Korea who was not involved in the study. “The advantage of skin-attachable e-skins is the improvement of biosignal detection accuracy and unobtrusive monitoring of daily healthcare signals. In this regard, I think that this work [has] opened a pathway to skin-attachable organic optoelectronic skins.”

A key aspect of e-skin is its ability to flex and stretch with the skin without becoming damaged. Previously, researchers successfully created stretchable devices using very thin inorganic LEDs, but the fabrication process was unsuitable for creating inexpensive, large-area devices. Organic LEDs are an attractive alternative. In addition to the natural flexibility of organic materials, “we can fabricate the device using [a] low-temperature process and printing,” says the study's first author and University of Tokyo engineer Tomoyuki Yokota, adding that this reduces fabrication costs and allows for the use of very thin substrates.

In a previous study, the research group of Takao Someya, lead author of the current paper, developed ultrathin (2-μm-thick), highly flexible, stretchable PLEDs, but these devices were driven in a nitrogen atmosphere and were not stable in air. For the new work, Yokota, Someya, and their colleagues sought to solve this issue by optimizing the fabrication of their PLEDs—while still employing materials and processes currently used in the production of organic LEDs—and developing a protective, transparent film, or “passivation layer.”

To create their PLEDs, the team began by creating a parylene substrate, a process that involved depositing a 1-μm-thick parylene layer onto a glass plate surface using chemical vapor dep-osition (CVD). They then spin-coated a 500-nm-thick polyimide planarization layer, which created a very flat, smooth surface to attach a transparent indium tin oxide (ITO) electrode. They further created an active layer with blue, green, or red light-emitting polymer materials, and completed their PLEDs by depositing a NaF/Al cathode.

After fabricating their PLEDs, the researchers added a flexible and thin passivation layer comprised of five alternating inorganic (SiON) and organic (parylene) layers deposited with plasma-enhanced CVD and CVD, respectively. Altogether, the PLED was just 3-μm thick, but still exhibited high flexibility and brightness, even when crumpled up or bent over the tip of a 100-μm-thick razor. Furthermore, the new PLED was over six times more efficient than previously reported ultrathin PLEDs, and did not degrade after 1000 cycles with 60% stretching. The passivation layer also extended the in-air operational half-life of the PLEDs from 2 hours to 29 hours. These PLEDs could also be laminated onto skin as an analog or a seven-segment numerical digital display (similar to clock displays).

“Air-stable organic light-emitting devices are harder to realize because some of the materials are usually air-sensitive,” says Zhenan Bao of Stanford University, who was not involved in the research. “The ultrathin device here is impressive to have such a good air stability. We can imagine bright flashing tattoos.”

Yokota and his colleagues similarly fabricated organic photodetectors on 1-μm-thick parylene substrates with an active layer of poly(3-hexylthiophene):(6,6)-phenyl-C61-butyric acid methyl ester and a 1-μm-thick parylene passivation layer. Like the PLEDs, these photodetectors were also hardy, showing little current and voltage differences after being compressed by 40%. They also did not degrade after 300 stretching cycles.

In a real-world test, the team laminated three layers—one green PLED, one red PLED, and one organic photodetector—on skin, creating an ultraflexible reflective pulse oximeter that was used to measure a person’s blood oxygen concentration and pulse rate. When wrapped around a finger, the PLEDs emitted light into the body, and the reflected light was picked up by the photodetector. Pulses in the signal correspond to heart rate, and the amount of light absorbed by the body differs depending on how much oxygen is in the blood, allowing the device to read blood-oxygen concentrations.

The device performed as well as previous organic pulse oximeters that use glass and thick plastic substrates, but had the added benefit of being ultrathin and being strongly adhesive. “Our device is very thin, so we can achieve [a] very stable signal,” Yokota says.

“The flexibility of organic semiconductors makes them a natural technology for wearable sensors,” says Ifor Samuel of the Organic Semiconductor Optoelectronics Group at the University of St. Andrews, who was also not involved in the study. “This work shows a promising advance in flexible encapsulation of such devices.”

Dae-Hyeong Kim, a researcher in flexible electronics at Seoul National University, adds that the work is “extremely important” for the field of deformable optoelectronics used in long-term health monitoring due to the softness of the device. As mobile electronics become more prevalent, “this technology would become one of [the] good technological solutions toward fully integrated wearable electronics and bioelectronics,” Kim says.

The researchers are now looking to fabricate a full-color e-skin display and further improve the stability of the device in air. “If we succeed [in improving] stability, we want to use our device in water,” Yokota says.

Reprinted from MRS Bulletin