improved-polydimethylsiloxane-pdms-double-casting-via-silicone-oil

Improved Polydimethylsiloxane (PDMS) Double Casting via Silicone Oil Treatment for Densely Packed Microstructure Replication

Polydimethylsiloxane (PDMS) double casting is widely used for replicating microscale structures, including microfluidic channels and biomedical ...

residue-free-fabrication-van-der-waals-heterostructures-two

Residue-Free Fabrication of van der Waals Heterostructures of Two-Dimensional Materials

In this work, a unique methodology is presented that utilizes van der Waals (vdW) interactions to fabricate residue-free single flakes of two-dimensional ...

Gwangju Institute of Science and Technology

While sensors that allow for high-throughput enumeration of microorganisms within drinking water are useful for water quality monitoring, it is particularly challenging to accurately quantify microorganisms that are present in low numbers (<100 CFU/mL) in a high-throughput manner. Negative dielectrophoresis (nDEP) is typically utilized in DEP-based cell focusing methods; however, due to its low conductivity, drinking water cannot be analyzed by this approach. Here, we report a positive DEP (pDEP)-based Escherichia coli detection system that is integrated with a focusing and sensing electrode. By incorporating a passivation layer, we avoided issues with adhesion of E. coli to the electrode, and achieved efficient cell focusing under high flow rate conditions (1500 μL/h). The resulting focused E. coli cells were then trapped on the sensor electrode, resulting in changes in impedance. The proposed system was evaluated using four different E. coli populations (150-1500 CFU/mL). We successfully enumerated populations as low as 300 CFU/mL within 1 min, and the signal variation was 1.13±0.37%. The device introduced in this study provides the basis for the development of portable, highly sensitive microorganism sensors that enable rapid detection of bacteria in drinking water.

A microfluidic device for label-free detection of Escherichia coli in drinking water using positive dielectrophoretic focusing, capturing, and impedance measurement.

We report high performance flexible piezoelectric nanogenerators (PENGs) by employing vanadium (V)-doped ZnO nanosheets (NSs) and the polydimethylsiloxane (PDMS) composite structure. The V-doped ZnO NSs were synthesized to overcome the inherently low piezoelectric properties of intrinsic ZnO. Ferroelectric phase transition induced in the V-doped ZnO NSs contributed to significantly improve the performance of the PENGs after the poling process. Consequently, the PENGs exhibited high output voltage and current up to ∼32 V and ∼6.2 μA, respectively, under the applied strain, which are sufficient to directly turn on a number of light emitting diodes (LEDs). The composite approach for PENG fabrication is scalable, robust, and reproducible during periodic bending/releasing over extended cycles. The approach introduced here extends the performance limits of ZnO-based PENGs and demonstrates their potential as energy harvesting devices.

A vanadium-doped ZnO nanosheets-polymer composite for flexible piezoelectric nanogenerators.

We measured the thermal conductivity of Araneus ventricosus' spider dragline silk using a suspended microdevice. The thermal conductivity of the silk fiber was approximately 0.4WmK at room temperature and gradually increased with an increasing temperature in a manner similar to that of other disordered crystals or proteins. In order to elucidate the effect of β-sheet crystals in the silk, thermal denaturation was used to reduce the quantity of the β-sheet crystals. A calculation with an effective medium approximation supported this measurement result showing that the thermal conductivity of β-sheet crystals had an insignificant effect on the thermal conductivity of SDS. Additionally, the enhancement of bonding strength in a glycine-rich matrix by atomic layer deposition did not increase the thermal conductivity. Thus, this study suggests that the disordered part of the glycine-rich matrix prevented the peptide chains from being coaxially extended via the cross-linking covalent bonds.

Effects of β-sheet crystals and a glycine-rich matrix on the thermal conductivity of spider dragline silk.

The thermal conductivity enhancement of neat poly(vinyl alcohol) and poly(vinyl alcohol) (PVA)/cellulose nanocrystal (CNC) composite was attempted via electrospinning. The suspended microdevice technique was applied to measure the thermal conductivity of electrospun nanofibers (NFs). Neat PVA NFs and PVA/CNC NFs with a diameter of approximately 200 nm showed thermal conductivities of 1.23 and 0.74 W/m-K, respectively, at room temperature, which are higher than that of bulk PVA by factors of 6 and 3.5, respectively. Material characterization by Fourier transform infrared spectroscopy, differential scanning calorimetry, and thermogravimetric analysis confirmed that the thermal conductivity of the PVA/CNC NFs was enhanced by the reinforcement of their backbone rigidity, while that of the neat PVA NFs was attributed to the increase in their crystallinity that occurred during the electrospinning.

Thermal conductivity enhancement in electrospun poly(vinyl alcohol) and poly(vinyl alcohol)/cellulose nanocrystal composite nanofibers.

The thermal conductivity () of poly(acrylic acid) (PAA) nanofibers, which were electrospun at various electrospinning voltages, was measured using suspended microdevices. While the thermal conductivities of the as-spun PAA nanofibers varied depending on the electrospinning voltages, the most pronounced 3.1-fold increase in thermal conductivity in comparison to that of bulk PAA was observed at the electrospinning voltage of 14 kV. On the other hand, a reduction in the thermal conductivity of the nanofibers was observed when the as-spun nanofibers were either thermally annealed at the glass transition temperature of PAA or thermally crosslinked. It is notable that the thermal conductivity of crosslinked PAA nanofibers was comparable to that of crosslinked bulk PAA. Polarized Raman spectroscopy and Fourier transform infrared spectroscopy verified that the  enhancement via electrospinning and the  reduction by the thermal treatments could be attributed to the conformational changes between gauche and trans states, which may be further related to the orientation of molecular chains. In contrast, hydrogen bonds did not contribute significantly to the  enhancement. Additionally, the suppression of  observed for the crosslinked PAA nanofibers might result from the shortening of single molecular chains via crosslinking.

Crosslinking Effect on Thermal Conductivity of Electrospun Poly(acrylic acid) Nanofibers.

Recent advances in soft materials and mechanics activate development of many new types of electrical medical implants. Electronic implants that provide exceptional functions, however, usually require more electrical power, resulting in shorter period of usages although many approaches have been suggested to harvest electrical power in human bodies by resolving the issues related to power density, biocompatibility, tissue damage, and others. Here, we report an active photonic power transfer approach at the level of a full system to secure sustainable electrical power in human bodies. The active photonic power transfer system consists of a pair of the skin-attachable photon source patch and the photovoltaic device array integrated in a flexible medical implant. The skin-attachable patch actively emits photons that can penetrate through live tissues to be captured by the photovoltaic devices in a medical implant. The wireless power transfer system is very simple, e.g., active power transfer in direct current (DC) to DC without extra circuits, and can be used for implantable medical electronics regardless of weather, covering by clothes, in indoor or outdoor at day and night. We demonstrate feasibility of the approach by presenting thermal and mechanical compatibility with soft live tissues while generating enough electrical power in live bodies through  animal experiments. We expect that the results enable long-term use of currently available implants in addition to accelerating emerging types of electrical implants that require higher power to provide diverse convenient diagnostic and therapeutic functions in human bodies.

Active photonic wireless power transfer into live tissues.

A light-weight, flexible electromagnetic interference (EMI) shield was prepared by creating a layer-structured metal-polymer composite film consisting of electrospun nylon 66 nanofibers with silver films. The EMI shielding effectiveness (SE), specific SE, and absolute SE of the composite were as high as 60.6 dB, 67.9 dB cm/g, and 6792 dB cm/g in the X- and K-bands, respectively. Numerical and analytical calculations suggest that the energy of EM waves is predominantly absorbed by inter-layer multiple reflections. Because the absorbed EM energy is dissipated as heat, the thermal conductivity of absorption-dominant EMI shields is highly significant. Measured thermal conductivity of the composite was found to be 4.17 WmK at room temperature, which is higher than that of bulk nylon 66 by a factor of 16.7. The morphology and crystallinity of the composite were examined using scanning electron microscopy and differential scanning calorimetry, respectively. The enhancement of thermal conductivity was attributed to an increase in crystallinity of the nanofibers, which occurred during the electrospinning and subsequent hot pressing, and to the high thermal conductivity of the deposited silver films. The contribution of each fabrication process to the increase in thermal conductivity was investigated by measuring the thermal conductivity values after each fabrication process.

Electromagnetic Interference Shield of Highly Thermal-Conducting, Light-Weight, and Flexible Electrospun Nylon 66 Nanofiber-Silver Multi-Layer Film.

Creating a high-frequency electron system demands a high saturation velocity (υ). Herein, we report the high-field transport properties of multilayer van der Waals (vdW) indium selenide (InSe). The InSe is on a hexagonal boron nitride substrate and encapsulated by a thin, noncontinuous In layer, resulting in an impressive electron mobility reaching 2600 cm/(V s) at room temperature. The high-mobility InSe achieves υ exceeding 2 × 10 cm/s, which is superior to those of other gapped vdW semiconductors, and exhibits a 50-60% improvement in υ when cooled to 80 K. The temperature dependence of υ suggests an optical phonon energy (ω) for InSe in the range of 23-27 meV, previously reported values for InSe. It is also notable that the measured υ values exceed what is expected according to the optical phonon emission model due to weak electron-phonon scattering. The superior υ of our InSe, despite its relatively small ω, reveals its potential for high-frequency electronics, including applications to control cryogenic quantum computers in close proximity.

High-Field Electron Transport and High Saturation Velocity in Multilayer Indium Selenide Transistors.

Terahertz polarimetric imaging, capable of capturing not only intensity profiles but also the polarization states of the incident pattern, is an essential technique with promising applications such as security scans and medical diagnoses. Recently, a novel approach for terahertz imaging has been proposed using a metasurface absorber that converts terahertz light into a temperature profile. However, polarization remains indistinguishable in the imaging process due to the isotropic geometry of the metasurface. To address this issue, this study introduces an all-dielectric, polarization-sensitive metasurface absorber and showcases its suitability for terahertz polarimetric imaging. Optical and thermal simulations confirm that the polarization dependence of our metasurface is translated into the thermal domain, allowing us to distinguish both intensity and polarization states in the incoming image. Additionally, we demonstrate that polarimetric imaging under general, elliptical polarization is attainable. This metasurface facilitates terahertz polarimetric imaging, eliminating the need for complex setups or bulky components, thereby reducing the form factor and enabling widespread use.

All-dielectric polarization-sensitive metasurface for terahertz polarimetric imaging.

Phonon transport in two-dimensional materials has been the subject of intensive studies both theoretically and experimentally. Recently observed unique phenomena such as Poiseuille flow at low temperature in graphene nanoribbons (GNRs) initiated strong interest in similar effects at higher temperatures. Here, we carry out massive molecular dynamics simulations to examine thermal transport in GNRs at room temperature (RT) and demonstrate that non-diffusive behaviors including Poiseuille-like local thermal conductivity and second sound are obtained, indicating quasiballistic thermal transport. For narrow GNRs, a Poiseuille-like thermal conductivity profile develops across the nanoribbon width, and wider GNRs exhibit a mixed nature of phonon transport in that diffusive transport is dominant in the middle region whereas non-uniform behavior is observed near lateral GNR boundaries. In addition, transient heating simulations reveal that the driftless second sound can propagate through GNRs regardless of the GNR width. By decomposing the atomic motion into out-of-plane and in-plane modes, it is further shown that the observed quasiballistic thermal transport is primarily contributed by the out-of-plane motion of C atoms in GNRs.

Quasiballistic thermal transport in submicron-scale graphene nanoribbons at room-temperature.

2D materials have garnered considerable attention due to their distinctive properties, prompting diverse applications across various domains. Beyond their inherent qualities, the significance of 2D materials extends into the fabrication processes that can lead to the degradation of intrinsic performance through undesirable mechanical defects and surface contaminations. Herein, a novel fabrication technique to achieve residue-free 2D materials using van der Waals (vdW) interactions, primarily employing molybdenum disulfide (MoS) is proposed. Optical and electrical characterizations confirm the absence of residues, mechanical defects, oxidation, and strain, along with a prominent field-effect mobility of up to 60 cm V s and an on/off ratio of ≈10. Furthermore, the utilization of residue-free material as a stamp enables various manipulations of flakes transferred on substrates in advance, including pick-up and release, stacking, exfoliation, wiping-out, flipping, and smoothing-out processes. Additionally, the manipulation techniques also facilitate the fabrication of vdW heterostructures with precise positioning and the desired stacking order. In this regard, the feasibility of applying this method to hexagonal boron nitride and graphite is demonstrated. It is expected that this method will offer a versatile and effective approach to enhancing the qualities of 2D material-based electronic and optoelectronic devices.

ArticleTotal : 2	Year
Publication title	2025
Publication title	2025

Article	Year
A microfluidic device for label-free detection of Escherichia coli in drinking water using positive dielectrophoretic focusing, capturing, and impedance measurement. Biosensors & bioelectronics\| PubMed ID: 26264268	2015
A vanadium-doped ZnO nanosheets-polymer composite for flexible piezoelectric nanogenerators. Nanoscale\| PubMed ID: 26681551	2016
Effects of β-sheet crystals and a glycine-rich matrix on the thermal conductivity of spider dragline silk. International journal of biological macromolecules\| PubMed ID: 28013005	2017
Thermal conductivity enhancement in electrospun poly(vinyl alcohol) and poly(vinyl alcohol)/cellulose nanocrystal composite nanofibers. Scientific reports\| PubMed ID: 30816257	2019
Crosslinking Effect on Thermal Conductivity of Electrospun Poly(acrylic acid) Nanofibers. Polymers\| PubMed ID: 31083478	2019
Active photonic wireless power transfer into live tissues. Proceedings of the National Academy of Sciences of the United States of America\| PubMed ID: 32632002	2020
Electromagnetic Interference Shield of Highly Thermal-Conducting, Light-Weight, and Flexible Electrospun Nylon 66 Nanofiber-Silver Multi-Layer Film. Polymers\| PubMed ID: 32796775	2020
High-Field Electron Transport and High Saturation Velocity in Multilayer Indium Selenide Transistors. ACS nano\| PubMed ID: 38451218	2024
All-dielectric polarization-sensitive metasurface for terahertz polarimetric imaging. Scientific reports\| PubMed ID: 38555396	2024
Quasiballistic thermal transport in submicron-scale graphene nanoribbons at room-temperature. Nanoscale advances\| PubMed ID: 38817424	2024
Residue-Free Fabrication of 2D Materials Using van der Waals Interactions. Advanced materials (Deerfield Beach, Fla.)\| PubMed ID: 40171775	2025

Jae Hun Seol

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