Wearable and flexible electronics have come along in leaps and bounds in the last few years with devices like biomedical electronic patches, soft optogenetic devices and electronic skin. Organic materials are better than inorganic ones for use in such applications since they are flexible, strong and can accommodate strain (unlike inorganic materials) without suffering any change in their electronic properties. They can also be made ultrathin, allowing them to be applied directly and conformally onto skin.

A team of researchers led by Dae-Hyeong Kim has now developed a new cardiac monitor containing stretchable gold electrodes that detect electrocardiogram (ECG) signals that are then amplified by a p-MOS inverter based on four p-type carbon nanotube (CNT) transistors.

"These ECG signals are filtered and analysed using a data acquisition unit, and for normal and healthy ECG signals, our programme delivers voltages that produce colour changes of red, pink and white in the colour-tuneable organic light-emitting diodes (CTOLEDs)," explains Kim. "For abnormal ECG signals (like those associated with medical conditions like arrhythmia and ischemia, for example), the programme delivers voltages corresponding to colour changes of blue, sky blue and white.

"This process occurs in real time so that users can check their heart rate and condition immediately thanks to the colour changes of the CTOLEDs," he tells our sister site nanotechweb.org.

Testing out the device

The researchers fabricated their CTOLEDs using an ultrathin exciton-blocking layer made from a simple phosphine-oxide compound (DPEPO) sandwiched between two light-emitting materials – a blue phosphorescent layer and an undoped red phosphorescent layer. They used 1-bromopropane (NPB) as a hole-transporting layer, TPBI as an electron-transporting layer, lithium fluoride as an electron-injection layer, and aluminium as a cathode. "The DPEPO between the light-emitting layers acts as a selective charge- and exciton-blocking layer and is responsible for the colour tuneability," says Kim.

The researchers tested out their device on a 32-year-old healthy male and placed the sensor on his wrist (see image). They first used a pure ECG sensor alone, and then used their p-MOS inverter as the signal amplifier. "We estimate that the ECG signals are amplified by the p-MOS inverter with an effective gain of at least 60," adds Kim. "We also observed that distinguishable ECG signals could be measured when we employed the amplifier but we only acquired a noise-like signal when we didn't employ the amplifier."

The team, reporting its work in ACS Nano DOI: 10.1021/acsnano.7b04292, says that it would now like to make multiplexable, active matrix arrays using its device for use in larger stretchable displays.

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