Sungchul Shin

Properties of dc magnetron sputtered indium tin oxide films on polymeric substrates at room temperature

Indium tin oxide (ITO) thin films were deposited on polymeric substrates at room temperature by dc reactive magnetron sputtering from an In–Sn (90–10 wt%) alloy target. The electrical, optical, and mechanical properties of ITO films on various substrates such as polycarbonate, acrylic, polyethylene terephthalate, and glass are influenced sensitively by sputtering parameters. Therefore, the dependence of these properties on dc power, working pressure, and partial oxygen content has been systematically investigated. Low dc power was applied to avoid the deformation of polymeric substrates. The electrical resistivity of as-deposited ITO films decreases initially and then increases as oxygen partial pressure (PO2) increases. The optical transmittance at visible wavelength of 550 nm was as much as 85%. The friction force of as-deposited ITO films on various substrates is increased with an increase of dc power, and behaves similarly to the optimum curve of resistivity with increasing PO2.

Effect of negatively charged cellulose nanofibers on the dispersion of hydroxyapatite nanoparticles for scaffolds in bone tissue engineering

Nanofibrous 2,2,6,6-tetramethylpiperidine-1-oxyl(TEMPO)-oxidized bacterial cellulose (TOBC) was used as a dispersant of hydroxyapatite (HA) nanoparticles in aqueous solution. The surfaces of TOBC nanofibers were negatively charged after the reaction with the TEMPO/NaBr/NaClO system at pH 10 and room temperature. HA nanoparticles were simply adsorbed on the TOBC nanofibers (HA-TOBC) and dispersed well in DI water. The well-dispersed HA-TOBC colloidal solution formed a hydrogel after the addition of gelatin, followed by crosslinking with glutaraldehyde (HA-TOBC-Gel). The chemical modification of the fiber surfaces and the colloidal stability of the dispersion solution confirmed TOBC as a promising HA dispersant. Both the Youngs modulus and maximum tensile stress increased as the amount of gelatin increased due to the increased crosslinking of gelatin. In addition, the well-dispersed HA produced a denser scaffold structure resulting in the increase of the Youngs modulus and maximum tensile stress. The well-developed porous structures of the HA-TOBC-Gel composites were incubated with Calvarial osteoblasts. The HA-TOBC-Gel significantly improved cell proliferation as well as cell differentiation confirming the material as a potential candidate for use in bone tissue engineering scaffolds.

Deterministic domain formation observed in ferroelectrics by electrostatic force microscopy

The effect of crystalline defects on the formation of ferroelectric domains in triglycine sulfate (TGS) crystals has been investigated using a dynamic contact electrostatic force microscopy. This detection method, based on the Coulomb interaction between the charge on the tip and the surface charge on the sample, yields a good spatial resolution as well as a complete separation between the topography and the domain image. The crystal imperfections, degraded surface, and strain in the crystal affect strongly the static domain configuration. Comparison of the topography to the domain image reveals a deterministic behavior in the formation of the equilibrium domain configuration, which is determined by twin boundaries in the crystal. Surface imperfections appear different depending on the polarization direction. Positively polarized regions show circular islands, whereas negatively polarized region shows circular holes as well as islands of various shapes. When TGS samples were cooled below the Curie tempera...

Flexible conductive nanocellulose combined with silicon nanoparticles and polyaniline

Here we describe a unique conductive bacterial cellulose (BC) composite with silicon nanoparticles (SiNPs) and polyaniline. BC was used as a template for binding SiNPs resulting in a very promising anode material for Li-ion rechargeable batteries that showed a high specific capacity. The surfaces of the SiNPs were modified with phytic acid to enhance the binding of aniline monomer to the surface. A conformal coating of polyaniline (PANi) was formed on the modified SiNPs by in situ polymerization of aniline monomers. We also found that the phytic acid on the SiNPs was critical to ensure encapsulation of SiNPs with PANi. In addition, the phosphoric acid-tagged surface of the SiNPs enhanced the adhesion of SiNPs to the BC fibers. The resulting three dimensional network of BC was flexible and provided stress dissipation in the conductive BC composites. Flexural testing of conductive BC composites showed stable electrical conductivity even after repetitive bending over 100 times.

Investigation on hydrogen annealing effect for various ferroelectric films by electrostatic force microscope

Abstract Scanning probe microscope with a dc bias and an ac modulation signal applied to the probing tip has been quite successful for investigating the characteristics in a sub-micron scale for the high density ferroelectric memory application field. The degradation of ferroelectric films—PbZr 0.4 Ti 0.6 O 3 (PZT) and Bi 3.25 La 0.75 Ti 3 O 12 (BLT)—caused by the hydrogen forming gas annealing is investigated in a microscopic scale by using an electrostatic force microscope (EFM). From the first harmonic signal of EFM, we obtained different polarization behaviors from as-grown and hydrogen-annealed ferroelectric films. We found that the hydrogen forming gas annealing is degrading the ferroelectric film with no catalyst top electrode on top of the film. It is believed that the annealing process causes the diffusion of hydrogen into the ferroelectric film resulting in the destruction of polarization in these materials. We speculate that the different degradation behavior among these materials (PZT and BLT) is due to the different cohesivity of hydrogen in these materials.

Nanocellulose based asymmetric composite membrane for the multiple functions in cell encapsulation

We describe the nanocomposite membrane for cell encapsulation using nanocelluose hydrogels. One of the surfaces of bacterial cellulose (BC) pellicles was coated with collagen to enhance cell adhesion and the opposite side of the BC pellicles was coated with alginate to protect transplanted cells from immune rejection by the reduced pore size of the composite membrane. The morphology of nanocomposite membrane was observed by scanning electron microscopy and the permeability of the membrane was estimated by the release test using different molecular weights of polymer solution. The nanocomposite membrane was permeable to small molecules but impermeable to large molecules such as IgG antibodies inferring the potential use in cell implantation. In addition, the BC-based nanocomposite membrane showed a superior mechanical property due to the incorporation of compared with alginate membranes. The cells attached efficiently to the surface of BC composite membranes with a high level of cell viability as well as bioactivity. Cells grown on the BC composite membrane kit released dopamine freely to the medium through the membrane, which showed that the BC composite membrane would be a promising cell encapsulation material in implantation.

Matrix-Assisted Three-Dimensional Printing of Cellulose Nanofibers for Paper Microfluidics

A cellulose nanofiber (CNF), one of the most attractive green bioresources, was adopted for construction of microfluidic devices using matrix-assisted three-dimensional (3D) printing. CNF hydrogels can support structures printed using CAD design in a 3D hydrogel environment with the appropriate combination of rheological properties between the CNF hydrogel and ink materials. Amazingly, the structure printed freely in the bulky CNF hydrogels was able to retain its highly resolved 3D features in an ultrathin two-dimensional (2D) paper using a simple drying process. The dimensional change in the CNF hydrogels from 3D to 2D resulted from simple dehydration of the CNFs and provided transparent, stackable paper-based 3D channel devices. As a proof of principle, the rheological properties of the CNF hydrogels, the 3D structure of the ink, the formation of channels by evacuation of the ink, and the highly localized selectivity of the devices are described.

Melanin Nanoparticle-Incorporated Silk Fibroin Hydrogels for the Enhancement of Printing Resolution in 3D-Projection Stereolithography of Poly(ethylene glycol)-Tetraacrylate Bio-ink

It is not easy to design structures with transparent solutions, especially in light projection three-dimensional (3D) printing, since the penetration of light in solution is limitless. Here, silk fibroin incorporated with melanin nanoparticles (SFM) is used as a transparency modifier of poly(ethylene glycol)-tetraacrylate (PEG4A) solution. The incorporation of melanin into the SF hydrogel is performed in the range of 0.05-0.2% (w/v), and the SFM was added to the PEG4A precursor solution at 0.25-1.0% (w/v). The printing accuracy was examined by comparing the printed and designed feature sizes. The addition of 1.0% (w/v) SFM to a 4% (w/v) PEG4A (PEG4A/SFM) precursor solution effectively reduces the transparency of the solution and improves the printing resolution by confining the light beam to a designed region. This enables the fabrication of hard-to-express features such as hollow blood vessels or vacant tubes. Furthermore, the elastic modulus of the printed PEG4A/SFM composite hydrogel increases 2.5-fold higher than the PEG4A hydrogel without SFM. For the bio-ink, PEG4A/SFM-containing cells show non-cytotoxicity and improve the proliferation rate of embedded cells, confirming the high biocompatibility of PEG4A/SFM hydrogels.