Numerous wearable devices were developed to measure bioelectric signals for continuous healthcare monitoring. The electrode, which interconnects electronics and the human body, significantly affects the signal quality. Although Ag/AgCl electrodes have been commonly used, noble-metal electrodes are more promising in terms of long-term reusability and flexibility. However, the signal-to-noise ratio (SNR) of noble metals is still insufficient for highly accurate biosignal acquisition. In this study, we propose an approach to enhance the electrical characteristics of a noble-metal skin electrode by surface modification using gold nanoparticles. The process parameters for nanoparticle deposition were optimized to maximize the surface area, thereby significantly improving the SNR of the electrode. The SNR value was increased by 51% in electrocardiogram and by 63% in electromyogram (EMG). We also propose an approach to quantify the motion artifact by spectral analysis, and the high flexibility of our electrode reduced the motion noise by 95% compared to the conventional Ag/AgCl electrode. The enhanced electrode interface paves the way for analyzing complex biosignals such as EMG and electroencephalogram in wearable applications.
Numerous wearable sensors have been developed for a variety of needs in medical/healthcare/wellness/sports applications, but there are still doubts about their usefulness due to uncomfortable fit or frequent battery charging. Because the size or capacity of battery is the major factor affecting the convenience of wearable sensors, power consumption must be reduced. We developed a method that can significantly reduce the power consumption by introducing a signal repeater and a special switch that provides power only when needed. Antenna radiation characteristics are an important factor in wireless wearable sensors, but soft material encapsulation for comfortable fit results in poor wireless performance. We improved the antenna radiation characteristics by a local encapsulation patterning. In particular, ultra-low power operation enables the use of paper battery to achieve a very thin and flexible form factor. Also, we verified the human body safety through specific absorption rate simulations. With these methods, we demonstrated a wearable infant sleep position sensor. Infants are unable to call for help in unsafe situations, and it is not easy for caregivers to observe them all the time. Our wearable sensor detects infants' sleep positions in real time and automatically alerts the caregivers when needed.
Muscle fatigue is required to be assessed in real-time to maintain the best physical condition, especially for sports and rehabilitation areas. In recent years, numerous studies proposed muscle fatigue estimation methods with non-invasive surface electromyography (sEMG). However, the previous approaches were limited to discerning whether muscle fatigue occurs and were unable to quantify the fatigue level due to individual differences in muscle characteristics. In this study, we propose a novel method for quantitative muscle fatigue estimation that is applicable for various people without individual calibration. Because muscle mass is closely related to muscular endurance, it is utilized as a standard parameter in our assessment process. We introduce a new concept of muscle fatigue score (MFS), based on the cosine similarity between muscle mass and representative fatigue indicators. The MFS exhibits a high correlation coefficient (|R| = 0.7398) with key muscle characteristics compared to previous representative muscle fatigue indicators calculated from sEMG: mean frequency (|R| = 0.2848), median frequency (|R| = 0.1972), and low-frequency ratio (|R| = 0.0346).
Numerous wearable biomedical devices are developed for the continuous monitoring of personal health or condition. Biosignals acquisition with high sensitivity is important for designing wearable biomedical devices. A sensing electrode between the human body and wearable electronics significantly affects the sensitivity of the sensors. In this study, we fabricated hierarchically structured flexible electrodes on polyimide substrate (HSFE-PI) using micro-casting technique and gold nanoparticles electrodeposition. Polyimides provides robust and outstanding electrical characteristics, and the reliability of HSFE-PI was verified with a cyclic bending test. The integration of hierarchical structures significantly increased the surface area of the electrode by 2.06 times. We applied the HSFE-PI for electromyogram (EMG) and glucose sensing applications and achieved high sensitivity enhancement in both applications. The signal-to-noise ratio (SNR) of measured EMG signals was increased by 2.48 times, and the sensitivity of the glucose detection was increased by 1.42 times compared to the planar counterpart.
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