Ultraflexible optical devices have been attracting great attention as next-generation electronics. This is thanks to their excellent flexibility and lightweight, which not only allow them to possess firm adhesion properties, but also grants an engineering freedom in shape of excellent conformity. Such a combination of properties makes for a perfect foundation of skin-applicable measurement devices, which further enables continuous monitoring of one’s vital signs. Heart rate sensors using pulse plethysmography (PPG) are of particular interest because of their non-invasiveness and their accuracy granted by the usage of optical light. In order to guarantee continuous monitoring of the pulse (heart rate) with an ultraflexible PPG sensor, such a sensor is in need of an ultraflexible source to power it without the restriction of having to exchange an external battery, hence to make the device self-powered. However, system-level integration of ultraflexible PPG sensors with power sources is quite challenging, due to insufficient air operational stability of ultraflexible polymer light-emitting diodes (PLED).
In this work, we developed an ultraflexible self-powered PPG sensors by combining air-operation-stable PLED, organic solar cells (OPVs), and organic photodetectors (OPDs) (Fig.1 a, b). The OPV is made as a module composed of in series connected single cells, to optimally powers the PLED (Fig. 1c) while PPG sensor consists of PLED and OPD as light source and detector respectively (Fig. 1d). An electrical circuit schematic is shown in Fig. 1e. The OPV module directly powers the PLED, and the emitted light reflects in the finger. The thus reflected light is detected by OPD and generates a measurable voltage, which can be correlated to the pulse. Changes in voltage over time allows thus for knowledge of changes in pulse at any given time.
Furthermore, with adopting an air-stable inverted structure and a doped polyethylenimine ethoxylated layer, ultraflexible polymer light-emitting diodes maintain 70% of the initial luminance even after 11.3 h of constant operation under air (Fig. 2a). Compared to the conventional Aluminum/Sodium Fluoride cathode devices, our inverted PLED shows an improvement by a factor of 7 during measurements at a constant voltage of 8 V under air (Fig. 2b).
With our air-stable ultraflexible PLED in combination with our OPV module, we created ourselves a foundation for a self-powered, air-stable ultrafleixble photoplethysmogram pulse sensors. The PPG sensor is constructed by stacking the ultraflexible OPD layer on top of the PLED one (Fig. 3a). In Fig, 3b, the voltage output of our OPD is shown using an ultraflexible PLED powered by an OPV module. The OPV module was irradiated with simulated sunlight in order to power the setup. As a result, the ultraflexible PPG sensor monitors 77 beats per minute for the pulse. This data shows that our device enables us to monitor the pulse continuously, without any invasive measure just by simply applying it on the skin all while being independently powered. Hence our device is not only promising for daily medical care application, especially for remote medical care in private household, but is also low in maintenance work due to the self-powered nature of it.
For more information, please refer to our article published in Nature Communications:
H.Jinno, et al. Self-powered ultraflexible photonic skin for continuous bio-signal detection via air-operation-stable polymer light-emitting diodes. Nat. Commun. 12, 2234, (2021).