Seung Sang Hwang

A scaffold design for tendon regeneration has been proposed, which mimics the microstructural features of tendons and provides appropriate mechanical properties. We synthesized a temperature-triggered shape-memory polymer (SMP) using the ring-opening polymerization of polycaprolactone (PCL) with polyethylene glycol (PEG) as a macroinitiator. We fabricated a micropatterned patch using SMP via capillary force lithography, which mimicked a native tendon, for providing physical cues and guiding effects. The SMP patches (the SMP-flat patch is referred to as SMP-F, and the SMP-patterned patch is referred to as SMP-P) were surface-modified with 3,4-dihydroxy-L-phenylalanine (L-DOPA, referred to as D) for improving cell adhesion. We hypothesized that SMP patches could be applied in minimally invasive surgery and the micropatterned structure would improve tendon regeneration by providing geometrical cues. The SMP patches exhibited excellent shape-memory properties, mechanical performance, and biocompatibility in vitro and in vivo. Especially, SMP-DP demonstrated enhanced cell behaviors in vitro, including cell orientation, elongation, migration, and tenogenic differentiation potential. The in vivo data showed notable biomechanical functionality and histological morphometric findings in various analyses of SMP-DP in the ruptured Achilles tendon model.

Cross-linkable organosilsesquioxanes were synthesized for application as catalyst binders in high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Four different organic functional groups were examined including methyl, phenyl, fluoroalkyl, and fluorophenyl and their chemical, physical, surface, and electrochemical properties were characterized. The effect of surface hydrophobicity on a HT-PEMFC membrane electrode assembly was elucidated, showing that organosilsesquioxanes with lower surface tension or higher hydrophobicity towards water and phosphoric acid could be considered as a key parameter for HT-PEMFC performance. Fuel cell tests showed that the pentafluorophenyl-functionalized organosilsesquioxane showed improved H₂/air performance (a peak power density of 527 mW cm⁻² at 0.4 V) compared to the MEA with PTFE (a peak power density of 425 mW cm⁻² at 0.4 V). Short term durability tests for 500 h showed that membrane electrode assemblies with alternative binders were stable and the developed organosilsesquioxane binders are a viable alternative to PTFE-based binders, all the while having additional advantages in vastly simplified ink slurry preparation through increased dispersibility in alcohol–water mixtures.

A facile methodology of fabricating alkaline exchange membranes was developed by blending an alkaline stable, ammonium network forming precursor with an ion-solvating polymer, poly(ethylene-co-vinyl alcohol) matrix, followed by thermal curing under mild conditions. The alkaline stable, ammonium network forming precursor was synthesized through the Menshutkin reaction of a multifunctional alkyl halide and methyldiallylamine, yielding an ammonium polymer precursor that under radical initiated thermal cyclo-polymerization forms pyrrolidinium cationic networks. The fabricated membranes exhibited tunable IEC, conductivity, and physical properties through the control of the poly(ethylene-co-vinyl alcohol) ion-solvating polymer content. The alkaline stability of the membranes was examined under 70 °C and 1 M KOH conditions for over 300 h, which showed impeccable retention of ion exchange capacity (IEC) and conductivity. The membrane electrode assembly (MEA) of anion exchange membrane electrolyzers with the fabricated membranes showed excellent performance (1.57 A cm⁻² at 2.0 V) and a short-term durability test showed improved durability over a commercial anion exchange membrane.

Ladder-type polysilsesquioxanes (LPSQs) containing phenyl as a high refractive index unit and cyclic epoxy as a curable unit were found to be excellent candidates for a transparent color conversion layer for displays due to being miscible with organic solvents and amenable to transparent film formation. Therefore, the LPSQs were combined with luminescent lanthanide metals, europium Eu(III), and terbium Tb(III), to fabricate transparent films with various emission colors, including red, orange, yellow, and green. The high luminescence and transmittance properties of the LPSQs–lanthanide composite films after thermal curing were attributed to chelating properties of hydroxyl and polyether side chains of LPSQs to lanthanide ions, as well as a light sensitizing effect of phenyl side chains of the LPSQs. Furthermore, Fourier-transform infrared (FT-IR) and X-ray photoelectron spectroscopy and nanoindentation tests indicated that the addition of the nanoparticles to the LPSQs moderately enhanced the epoxy conversion rate and substantially improved the wear resistance, including hardness, adhesion, and insusceptibility to atmospheric corrosion in a saline environment. Thus, the achieved LPGSG–lanthanide hybrid organic–inorganic material could effectively serve as a color conversion layer for displays.

The ability to completely absorb an electromagnetic (EM) wave with a material much thinner than the wavelength is a prerequisite for achieving ultracompact, flexible, and lightweight EM devices. Herein, a metamaterial microwave perfect absorber is demonstrated as thin as 1/1250 of the target wavelength by coupling an array of metal patches to inverse cross metal antennas through a dielectric separator and with a reflector placed underneath. Compared to a conventional three‐layer absorber, the modified design reduces the resonance frequency by up to tenfolds without altering the patch and dielectric layers, but by reconfiguring the induced current pathways that reorient the dipolar magnetic fields and dramatically boost the magnetic flux. With the reflector, the stack is framed for perfect absorption at the critical coupling condition, which is identified by monitoring the reflection phase shift across the resonance frequency. The study provides guidelines for shrinking the thickness further and can be extended to other frequencies.