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.

Flexible materials with sufficient mechanical endurance under bending or folding is essential for flexible electronic devices. Conventional rigid materials such as metals and ceramics are mostly brittle so that their properties can deteriorate under a certain amount of strain. In order to utilize high-performance, but brittle conventional materials in flexible electronics, we propose a novel flexible substrate structure with a low-modulus interlayer. The low-modulus interlayer reduces the surface strain, where active electronic components are placed. The bending results with indium tin oxide (ITO) show that a critical bending radius, where the conductivity starts to deteriorate, can be reduced by more than 80% by utilizing the low-modulus layer. We demonstrate that even rigid electrodes can be used in flexible devices by manipulating the structure of flexible substrate.

We achieved the lowest contact resistance between a-IGZO and a metal electrode for >30 GHz operation of an oxide semiconductor device. For high-resolution display and high-speed electronic devices, both bulk and contact resistances need to be reduced. In this study, hydrogen plasma was used to lower the contact resistance significantly by modifying the surface of the a-IGZO thin film. The potential barrier width at the interface was decreased by increasing the carrier concentration, and weak M−OH bonds were sufficiently diffused out with optimized plasma process. The minimum contact resistance was measured to be 1.33 × 10 −6 Ω•cm 2 by the transfer line method, which is the lowest reported value to the best of our knowledge. Utilizing this enhanced contact property between a-IGZO and metal, the metal−insulator−semiconductor varactor was fabricated, and its operating frequency was measured to be higher than 30 GHz.

during data transfer between processor and memory. [6] To overcome this limitation, various research activities on computing in-memory have been conducted to optimize neural network computations. [7] Especially, numerous non-volatile memory devices have been proposed to update synaptic weights and perform matrix-vector multiplications (MVM). [8] However, most previous artificial synapse devices cannot satisfy all the requirements for high-performance neural network operations, for example, a large number of weight levels, wide conductance range, linear/symmetric weight update, outstanding device-to-device uniformity, low operating power, long retention, and good endurance. Resistive random access memory (RRAM) and phase change memory (PCM) exhibit a wide range of conductance changes, but show nonlinear weight modulation and poor device-todevice uniformity due to the abrupt transitions during device operation. [9][10][11] Spin-transfer torque magnetic random access memory shows extremely low programming delay and power consumption, but its conductance range is limited. [12] Flash memory and ferroelectric field-effect transistor (FeFET) exhibit outstanding CMOS compatibility and a wide conductance range; however, flash memory requires high operation voltage, and FeFET has poor fatigue characteristics, which results in cycle-tocycle variation. [13][14][15] To overcome the limitations of non-volatilememory-based synapse devices, metal-oxide-semiconductor This work presents an analog neuromorphic synapse device consisting of two oxide semiconductor transistors for high-precision neural networks. One of the two transistors controls the synaptic weight by charging or discharging the storage node, which leads to a conductance change in the other transistor. The programmed weight maintains for more than 300 s as electrons in the storage node are well preserved due to the extremely low off current of the oxide transistor. Ideal synaptic behaviors are achieved by utilizing superior properties of oxide transistors such as a high on/off ratio, low off current, and large-area uniformity. To further improve the synaptic performance, self-assembled monolayer treatment is applied for reducing the transistor conductance. The reduction of on current reduces the power consumption, and the reduced off current improves the retention characteristics. There is no noticeable decrease in simulated neural network accuracy even when the measured device-to-device variation is intentionally increased by 200%, indicating the possibility of large-array operation with the synapse device.

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.