Jong-Hoon Kim

Polyimide (PI) is widely used in flexible electronics because of its excellent mechanical and thermal stability; however, its high imidization temperature (>200 °C) is incompatible with low‐cost polymer substrates such as polyethylene terephthalate (PET), limiting scalable roll‐to‐roll manufacturing. The objective of this work is to enable screen printing and low‐temperature curing of PI on PET without compromising mechanical or electrical performance. To achieve this, a polyethylene glycol (PEG)‐modified PI precursor is developed, in which PEG acts as a plasticizer to enhance chain mobility and promote imidization at reduced temperatures. Using this approach, effective imidization is achieved at 140 °C, well below the thermal tolerance of PET, allowing fully screen‐printed fabrication of flexible electrodes without rigid carrier substrates. The printed electrodes demonstrate robust mechanical performance, with stable electrical resistance under cyclic bending and stretching up to 30% strain. Furthermore, the electrodes enable reliable real‐time electrocardiogram and electromyogram signal acquisition, confirming their suitability for wearable biomedical applications. This work establishes a scalable and cost‐effective pathway for integrating PI‐based electronics with low‐temperature polymer substrates, advancing roll‐to‐roll manufacturing of flexible wearable devices.

Flexible wearable electronics are promising for continuous health monitoring, particularly in electromyography (EMG) applications. A critical factor in their performance is electrode design, which affects mechanical resilience and electrical stability. Here, this study develops multiple electrode geometries: open-mesh, closed-mesh, and island-bridge, fabricated from gold-coated polyimide substrates to offer the best performance in EMG detection. Under standardized bending and stretching tests, the island-bridge design shows the lowest resistance variation (±1.61%), while the closed-mesh design provides balanced performance across various strains. EMG tests indicate that the closed-mesh electrodes deliver the highest signal-to-noise ratios (up to 14.83 dB) with minimal motion artifacts. Although the open-mesh design is flexible, it has lower electrical stability. In summary, the closed-mesh performs best overall, the open-mesh is better for handling motion artifacts, and the island-bridge is ideal for areas with minimal movement.

Skin-based wearable devices have gained significant attention due to advancements in soft materials and thin-film technologies. Nevertheless, traditional wearable electronics often entail expensive and intricate manufacturing processes and rely on metal-based substrates that are susceptible to corrosion and lack flexibility. In response to these challenges, this paper has emerged with an alternative substrate for wearable electrodes due to its cost-effectiveness and scalability in manufacturing. Paper-based electrodes offer an attractive solution with their inherent properties of high breathability, flexibility, biocompatibility, and tunability. In this study, we introduce carbon nanotube-based paper composites (CPC) electrodes designed for the continuous detection of biopotential signals, such as electrooculography (EOG), electrocardiogram (ECG), and electroencephalogram (EEG). To prevent direct skin contact with carbon nanotubes, we apply various packaging materials, including polydimethylsiloxane (PDMS), Eco-flex, polyimide (PI), and polyurethane (PU). We conduct a comparative analysis of their signal-to-noise ratios in comparison to conventional gel electrodes. Our system demonstrates real-time biopotential monitoring for continuous health tracking, utilizing CPC in conjunction with a portable data acquisition system. The collected data are analyzed to provide accurate heart rates, respiratory rates, and heart rate variability metrics. Additionally, we explore the feasibility using CPC for sleep monitoring by collecting EEG signals.

DNA sensing is critical in various applications such as the early diagnosis of diseases and the investigation of forensic evidence, food processing, agriculture, environmental protection, etc. As a wide-bandgap semiconductor with excellent chemical, physical, electrical, and biocompatible properties, silicon carbide (SiC) is a promising material for DNA sensors. In recent years, a variety of SiC-based DNA-sensing technologies have been reported, such as nanoparticles and quantum dots, nanowires, nanopillars, and nanowire-based field-effect-transistors, etc. This article aims to provide a review of SiC-based DNA sensing technologies, their functions, and testing results.

Recent advances in wearable technologies have enabled ways for people to interact with external devices, known as human-machine interfaces (HMIs). Among them, electrooculography (EOG), measured by wearable devices, is used for eye movement-enabled HMI. Most prior studies have utilized conventional gel electrodes for EOG recording. However, the gel is problematic due to skin irritation, while separate bulky electronics cause motion artifacts. Here, we introduce a low-profile, headband-type, soft wearable electronic system with embedded stretchable electrodes, and a flexible wireless circuit to detect EOG signals for persistent HMIs. The headband with dry electrodes is printed with flexible thermoplastic polyurethane. Nanomembrane electrodes are prepared by thin-film deposition and laser cutting techniques. A set of signal processing data from dry electrodes demonstrate successful real-time classification of eye motions, including blink, up, down, left, and right. Our study shows that the convolutional neural network performs exceptionally well compared to other machine learning methods, showing 98.3% accuracy with six classes: the highest performance till date in EOG classification with only four electrodes. Collectively, the real-time demonstration of continuous wireless control of a two-wheeled radio-controlled car captures the potential of the bioelectronic system and the algorithm for targeting various HMI and virtual reality applications.

Recent advances in soft materials and nano-microfabrication have enabled the development of flexible wearable electronics. At the same time, printing technologies have been demonstrated to be efficient and compatible with polymeric materials for manufacturing wearable electronics. However, wearable device manufacturing still counts on a costly, complex, multistep, and error-prone cleanroom process. Here, we present fully screen-printable, skin-conformal electrodes for low-cost and scalable manufacturing of wearable electronics. The screen printing of the polyimide (PI) layer enables facile, low-cost, scalable, high-throughput manufacturing. PI mixed with poly(ethylene glycol) exhibits a shear-thinning behavior, significantly improving the printability of PI. The premixed Ag/AgCl ink is then used for conductive layer printing. The serpentine pattern of the screen-printed electrode accommodates natural deformation under stretching (30%) and bending conditions (180°), which are verified by computational and experimental studies. Real-time wireless electrocardiogram monitoring is also successfully demonstrated using the printed electrodes with a flexible printed circuit. The algorithm developed in this study can calculate accurate heart rates, respiratory rates, and heart rate variability metrics for arrhythmia detection.

Eye movements show primary responses that reflect humans’ voluntary intention and conscious selection. Because visual perception is one of the fundamental sensory interactions in the brain, eye movements contain critical information regarding physical/psychological health, perception, intention, and preference. With the advancement of wearable device technologies, the performance of monitoring eye tracking has been significantly improved. It also has led to myriad applications for assisting and augmenting human activities. Among them, electrooculograms, measured by skin-mounted electrodes, have been widely used to track eye motions accurately. In addition, eye trackers that detect reflected optical signals offer alternative ways without using wearable sensors. This paper outlines a systematic summary of the latest research on various materials, sensors, and integrated systems for monitoring eye movements and enabling human-machine interfaces. Specifically, we summarize recent developments in soft materials, biocompatible materials, manufacturing methods, sensor functions, systems’ performances, and their applications in eye tracking. Finally, we discuss the remaining challenges and suggest research directions for future studies.