Self-powered Biosensors for health monitoring As a follow-up development from prior work on thin-film organic photovoltaics, researchers from the University of Tokyo have improved their photovoltaic cell design to make it viable to power adjacent thin-film organic electrochemical transistors used as biosensors. (a) Structure of the double-grating-patterned organic photovoltaic film. (b) The OPV wrapped over a spatula rod and pulled by tweezers. Their paper "Self-powered ultra-flexible electronics via nanograting-patterned organic photovoltaics" published in Nature emphasizes on a novel development, a high-throughput roomtemperature moulding process to form nano-gratings on the photovoltaic cell's charge transporting layers. Schematic of a double-grating-patterned OPV integrated with an OECT The self-powered integrated biosensor attached to a finger. Designed at a 760nm periodicity, the nano-gratings are reported to substantially increase the efficiency of the 3pm-thin organic photovoltaics, yielding a power-conversion efficiency up to 10.5% which for the ultralight-weight device translated into a high power-per-weight value (11.46W/g). The nano-grating also reduced the light angle dependency of the cell's output voltage. The cells fared well under repetitive compression tests, withstanding wrinkling at a radius as low as 3pm. As proof of a wearable biological sensing application, the researchers co-designed the conformable and light-weight solar cells with organic electrochemical transistors (OECTs) to create a self-powered cardiac sensor. In this scheme, the potential difference between a gel electrode on the chest and the OECT channel on a fingertip acts as the gate bias, affecting the PEDOT:PSS channel conductance. The device had a responsivity above 1 kHz under physiological conditions, enabling the recording of clear biological signal curves under ambient light. The paper reports a peak intensity of the cardiac signal at 0.47pA, with a signal-to-noise ratio of 40.02 decibels for cardiac signal detection. These new findings open the way to the integration of ultra-flexible organic power sources with functional electronic devices for the precise, sensitive and continuous data acquisition of biological signals without external power connections, conclude the authors. October 2018

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