Jihyun Bae

Highly Stretchable Resistive Pressure Sensors Using a Conductive Elastomeric Composite on a Micropyramid Array

A stretchable resistive pressure sensor is achieved by coating a compressible substrate with a highly stretchable electrode. The substrate contains an array of microscale pyramidal features, and the electrode comprises a polymer composite. When the pressure-induced geometrical change experienced by the electrode is maximized at 40% elongation, a sensitivity of 10.3 kPa(-1) is achieved.

A Flexible Bimodal Sensor Array for Simultaneous Sensing of Pressure and Temperature

Diverse signals generated from the sensing elements embedded in flexible electronic skins (e-skins) are typically interfered by strain energy generated through processes such as touching, bending, stretching or twisting. Herein, we demonstrate a flexible bimodal sensor that can separate a target signal from the signal by mechanical strain through the integration of a multi-stimuli responsive gate dielectric and semiconductor channel into the single field-effect transistor (FET) platform.

Flutter-driven triboelectrification for harvesting wind energy

Technologies to harvest electrical energy from wind have vast potentials because wind is one of the cleanest and most sustainable energy sources that nature provides. Here we propose a flutter-driven triboelectric generator that uses contact electrification caused by the self-sustained oscillation of flags. We study the coupled interaction between a fluttering flexible flag and a rigid plate. In doing so, we find three distinct contact modes: single, double and chaotic. The flutter-driven triboelectric generator having small dimensions of 7.5 × 5 cm at wind speed of 15 ms(-1) exhibits high-electrical performances: an instantaneous output voltage of 200 V and a current of 60 μA with a high frequency of 158 Hz, giving an average power density of approximately 0.86 mW. The flutter-driven triboelectric generation is a promising technology to drive electric devices in the outdoor environments in a sustainable manner.

Automatic Recognition of Woven Fabric Patterns by an Artificial Neural Network

A neural network and image processing technology are introduced for classifying woven fabric patterns. An autocorrelation function is used to determine one weave repeat of the fabric. The reflected fabric image is captured and digitized by the computer system. The learning vector quantization algorithm as a learning rule of the artificial neural network enables recognition of woven fabric types more effectively. The results demon strate that three fundamental weave types can be classified accurately, and structural parameters such as yarn spacing, its variance, and the ratio of warp spacing to weft spacing can also be obtained.

Layer by layer assembly of ultrathin V2O5 anchored MWCNTs and graphene on textile fabrics for fabrication of high energy density flexible supercapacitor electrodes

Among transition metal oxides, vanadium oxides have received relatively modest attention for supercapacitor applications. Yet, this material is abundant, relatively inexpensive and offer several oxidation states which can provide a broad range of redox reactions suitable for supercapacitor operation. Electrochemical supercapacitors based on nanostructured vanadium oxide (V₂O₅) suffer from relatively low energy densities as they have low surface area and poor electrical conductivities. To overcome these problems, we developed a layer by layer assembly (LBL) technique in which a graphene layer was alternatively inserted between MWCNT films coated with ultrathin (3 nm) V₂O₅. The insertion of a conductive spacer of graphene between the MWCNT films coated with V₂O₅ not only prevents agglomeration between the MWCNT films but also substantially enhances the specific capacitance by 67%, to as high as ∼2590 F g(-1). Furthermore, the LBL assembled multilayer supercapacitor electrodes exhibited an excellent cycling performance of >97%, capacitance retention over 5000 cycles and a high energy density of 96 W h kg(-1) at a power density of 800 W kg(-1). Our approach clearly offers an exciting opportunity for enhancing the device performance of metal oxide-based electrochemical supercapacitors suitable for next-generation flexible energy storage devices by employing a facile LBL assembly technique.

Effects of Substrate on Piezoelectricity of Electrospun Poly(vinylidene fluoride)-Nanofiber-Based Energy Generators

We report the effects of various substrates and substrate thicknesses on electrospun poly(vinylidene fluoride) (PVDF)-nanofiber-based energy harvesters. The electrospun PVDF nanofibers showed an average diameter of 84.6 ± 23.5 nm. A high relative β-phase fraction (85.2%) was achieved by applying high voltage during electrospinning. The prepared PVDF nanofibers thus generated considerable piezoelectric potential in accordance with the sound-driven mechanical vibrations of the substrates. Slide glass, poly(ethylene terephthalate), poly(ethylene naphthalate), and paper substrates were used to investigate the effects of the intrinsic and extrinsic substrate properties on the piezoelectricity of the energy harvesters. The thinnest paper substrate (66 μm) with a moderate Youngs modulus showed the highest voltage output (0.4885 V). We used high-performance 76, 66, and 33 μm thick papers to determine the effect of paper thickness on the output voltage. The thinnest paper substrate resulted in the highest voltage output (0.7781 V), and the numerical analyses of the sound-driven mechanical deformation strongly support the hypothesis that substrate thickness has a considerable effect on piezoelectric performance.

Conformal coating of ultrathin Ni(OH)2 on ZnO nanowires grown on textile fiber for efficient flexible energy storage devices

A highly flexible supercapacitor is fabricated through a simple solution-based method in which conformal ultrathin (2 nm) nickel hydroxide (Ni(OH)2) layer is deposited on vertically grown zinc oxide (ZnO) nanowires on a three-dimensional, highly conductive textile substrate. The conformal ultrathin Ni(OH)2 layer enables a fast and reversible redox reaction which improves the specific capacitance by utilizing the maximum number of active sites for the redox reaction, while vertically grown ZnO nanowires on wearable textile fibers effectively transport electrolytes and shorten the ion diffusion path. The Ni(OH)2 coated ZnO nanowire electrodes show a high specific capacitance of 3150 F g−1 in a 1 M LiOH aqueous solution. Moreover, the asymmetric electrochemical capacitors with Ni(OH)2-coated ZnO nanowires as the positive electrode and multiwall carbon nanotubes-textile as the negative electrode exhibit promising characteristics with a maximum power density of 110 kW kg−1, an energy density of 54 W h kg−1, and excellent cycling performance of ∼96% capacitance retention over 5000 cycles.

A Sensor Array Using Multi-functional Field-effect Transistors with Ultrahigh Sensitivity and Precision for Bio-monitoring

Mechanically adaptive electronic skins (e-skins) emulate tactition and thermoception by cutaneous mechanoreceptors and thermoreceptors in human skin, respectively. When exposed to multiple stimuli including mechanical and thermal stimuli, discerning and quantifying precise sensing signals from sensors embedded in e-skins are critical. In addition, different detection modes for mechanical stimuli, rapidly adapting (RA) and slowly adapting (SA) mechanoreceptors in human skin are simultaneously required. Herein, we demonstrate the fabrication of a highly sensitive, pressure-responsive organic field-effect transistor (OFET) array enabling both RA- and SA- mode detection by adopting easily deformable, mechano-electrically coupled, microstructured ferroelectric gate dielectrics and an organic semiconductor channel. We also demonstrate that the OFET array can separate out thermal stimuli for thermoreception during quantification of SA-type static pressure, by decoupling the input signals of pressure and temperature. Specifically, we adopt piezoelectric-pyroelectric coupling of highly crystalline, microstructured poly(vinylidene fluoride-trifluoroethylene) gate dielectric in OFETs with stimuli to allow monitoring of RA- and SA-mode responses to dynamic and static forcing conditions, respectively. This approach enables us to apply the sensor array to e-skins for bio-monitoring of humans and robotics.

Preparation and properties of multi-functionalized cotton fabrics treated by extracts of gromwell and gallnut

Extracts from the root of Lithospermum erythrorhizon (gromwell) have been used for many centuries as a natural red dye and crude drugs with excellent performance for wound healing. In particular, shikonin a major constituent of the gromwell colorant is reported to possess antibacterial, antioxidant, anti-inflammatory, antitumor activities, etc. Therefore, we tried to manufacture antimicrobial and antioxidant cotton fabrics for biocompatible clothing materials using the natural stuff and environmentally friendly process. In this study, cotton fabrics were dyed with the colorant extracted from the root of gromwell, and their properties were closely investigated. Consequently, we discovered that the cotton fabrics dyed with the extracts from the root of gromwell showed excellent antioxidant performance as well as antibacterial ability. Also, the functionalities and color fastness were improved by gallnut mordanting and cationization of the cotton fabrics.

Electrical properties of conductive fabrics for operating capacitive touch screen displays

Electrically conductive textiles have many potential applications, such as sensors, static charge dissipation, and electro-magnetic interference shields. In this study, two different types of core spun yarns were produced with silver-plated nylon filaments as the conductive material. The electrical characteristics of the core spun yarns and the fabric samples knitted with these yarns were investigated. It was clear that the surface resistance of each type of knitted fabric depends on the surface exposure of the conductive materials. However, both knit types exhibited reasonable features for application as a touching operator for capacitive touch screen panels.