Ultraflexible organic photonic skin

Abstract

Thin-film electronics intimately laminated onto the skin imperceptibly equip the human body with electronic components for health-monitoring and information technologies. When electronic devices are worn, the mechanical flexibility and/or stretchability of thin-film devices helps to minimize the stress and discomfort associated with wear because of their conformability and softness. For industrial applications, it is important to fabricate wearable devices using processing methods that maximize throughput and minimize cost. We demonstrate ultraflexible and conformable three-color, highly efficient polymer light-emitting diodes (PLEDs) and organic photodetectors (OPDs) to realize optoelectronic skins (oe-skins) that introduce multiple electronic functionalities such as sensing and displays on the surface of human skin. The total thickness of the devices, including the substrate and encapsulation layer, is only 3 μm, which is one order of magnitude thinner than the epidermal layer of human skin. By integrating green and red PLEDs with OPDs, we fabricate an ultraflexible reflective pulse oximeter. The device unobtrusively measures the oxygen concentration of blood when laminated on a finger. On-skin seven-segment digital displays and color indicators can visualize data directly on the body.

Keywords: PLED; flexible electronics; organic photo detector; pulse oximeter.

Fig. 1. Smart e-skin system comprising health-monitoring sensors, displays, and ultraflexible PLEDs.

(A) Schematic illustration of the optoelectronic skins (oe-skins) system. (B) Photograph of a finger with the ultraflexible organic optical sensor attached. (C) Photographs of a human face with a blue logo of the University of Tokyo and a two-color logo. The brightness can be changed by the operation voltage. (D) Photograph of a red seven-segment PLEDs displayed on a hand.

Fig. 2. Characteristics of ultraflexible PLEDs and OPDs.

Note that all the measurements were performed in air. (A) Structure of the ultraflexible PLED. The passivation layer was composed of alternating organic (500-nm-thick Parylene) and inorganic (200-nm-thick SiON) layers. (B) Picture of the ultraflexible green PLED that was crumpled. (C) The EQE of the OPD (black line) with the normalized electroluminescence (EL) spectra of blue (blue line), green (green line), and red (red line) PLEDs. a.u., arbitrary unit. (D) Current density–dependent EQE characteristics of the ultraflexible PLEDs. The inset figure shows L-V curves of ultraflexible PLEDs. (E) Picture of the freestanding ultraflexible OPD. (F) Light intensity–dependent J-V characteristics of the OPD under simulated solar illumination. Red, orange, green, light blue, blue, purple, gray, and black represent the light intensity of 1000, 706, 502, 400, 297, 199, and 99 W/m² and the dark condition, respectively. (G) Characteristics of the ultraflexible OPD, measured using a solar simulator. Light intensity–dependent V_oc of the OPD (red) and light intensity–dependent J_sc of the OPD (green).

Fig. 3. Demonstrations of extreme flexibility of ultrathin optical devices.

(A) Images of an ultrathin red PLED adhered to a prestretched elastomer. The images from right to left represent the transition from the wrinkled state to the flat state. (B) Three-dimensional image of the wrinkled PLED state. The prestretch value was 60%. (C) Cyclic stretching test of the green PLED. After 1000 stretching cycle tests, the light intensity was decreased by only 10%. (D) V_oc of the wrinkled OPD. The inset figure shows the flat and compressed OPD. (E) Cyclic stretching test of the OPD. After 300 stretching cycle tests, the characteristics did not show any degradation. The black circles and red triangles represent the V_oc and normalized J_sc, respectively. The inset figure shows the J-V characteristics of the OPD (dot line, dark state; solid line, irradiated by green light). Black and red lines represent the initial state and the state after 300 stretching cycles, respectively.

Fig. 4. Ultraflexible organic pulse oximeter.

(A) Device structure of the pulse oximeter. (B) Operation principle of the reflective pulse oximeter. (C) Light intensity–dependent J-V characteristics of the OPD when irradiated by a green PLED. (D) Light intensity–dependent J-V characteristics of the OPD when irradiated by a red PLED. (E) Long-term measurement of the pulse wave. The pulse wave was measured using a red PLED and OPD. (F) Air stability of the PPG signal. The PPG signal was measured using a red PLED and OPD. (G) Output signal from OPD with 99% oxygenation of blood. The green and red lines represent the signals when the green and red PLEDs, respectively, were operated. (H) Output signal from OPD with 90% of oxygenation of blood. The green and red lines represent the signals when the green and red PLEDs, respectively, were operated.