2, Osaka University, Ibaraki, Osaka, Japan
3, Advanced Photonics and Biosensing Open Innovation Laboratory, National Institute of Advanced Industrial Science and Technology (AIST), Suita, Osaka, Japan
In this study, we have developed a noise reduction technique utilizing an ultraflexible and bio-conformable organic differential amplification circuit, which has a capability of amplifying differential input signal while suppressing common-mode noise. The organic circuit was fabricated on an ultraflexible 1-μm-thick parylene foil with p-channel organic thin-film transistors (OTFTs) and thin-film capacitors, and therefore, showing highly mechanical flexibility to attach onto a soft human skin surface. To demonstrate the feasibility of improving signal integrity for biosignal monitoring, we have recorded human electrocardiogram (ECG) signal. As a result, the input signal to noise ratio of -1.7 dB was improved to 20.6 dB utilizing the differential amplification circuit.
Flexible and wearable electronic system with OTFTs is expected to play an important role in realizing next-generation medical and healthcare devices because of their mechanical flexibility and lightweight properties for a curvilinear and soft human body. With bio-conformable electronic applications, various kinds of human activities can be monitored more continuously and imperceptibly. Moreover, an essential component for such flexible electronic sensor is a signal integrity (the quality of a recorded signal) to establish reliable medical applications. For monitoring electrical signals generated by human organs such as nerves, a heart, and a brain, signal monitoring with high signal integrity is necessary. However, it is still difficult because these biosignals are on the order of microvolts to millivolts, which can be easily disturbed by noise artifacts from surrounding environments. Therefore, we have developed a noise reduction technique utilizing an ultraflexible and bio-conformable organic differential amplification circuit. With a developed organic amplifier, a differential input signal of 10 mVpp sin-wave at 1 Hz was amplified to a 500 mVpp output signal, while a common-mode input signal of 100 mVpp sin-wave at 1 Hz was attenuated to a 25 mVpp output signal. These results mean the differential to the common-mode gain ratio of 200. To demonstrate the potential of improving signal integrity for biosignal monitoring, we have recorded human electrocardiogram (ECG) signal using the ultraflexible organic amplifier. As a result, 60 Hz harmonic noise, which was superimposed on input ECG signal obtained from human skin near a heart, was suppressed: the input signal to noise ratio of -1.7 dB was improved to 20.6 dB. Our flexible organic differential amplifier circuit will demonstrate the strategy for achieving high signal integrity, realized by simultaneous signal amplification and noise reduction at the end of signal collection point. This idea has relevance not only to biomedical electronics but also to many growing fields such as structural health monitoring and smart agricultures.