Yong-Cheol Jeong

Control of liquid crystal pretilt angles by using organic/inorganic hybrid interpenetrating networks

A new photoalignment method of controlling the pretilt angle of liquid crystals (LCs) by using organic/inorganic hybrid interpenetrating polymer networks (IPNs) is proposed and demonstrated. In the hybrid IPN alignment layer system, the competition between poly(vinyl cinnamate) (PVCi) favoring planar alignment and poly(dimethyl siloxane) (PDMS) favoring vertical alignment made it possible to achieve pretilt angle in a wide range from 0 degrees to 90 degrees, and adjust pretilt angle as a function of PDMS content. In addition, we achieved the high azimuthal anchoring energy at the intermediate pretilt angle by using PDMS as the vertical-aligning component.

Facile fabrication of close-packed microlens arrays using photoinduced surface relief structures as templates

We demonstrate the cost-effective and facile method of fabricating close-packed microlens arrays using photoinduced two-dimensional (2-D) surface relief structures as original templates. 2-D surface relief structures are produced by successive inscription of two beams interference patterns with different grating vectors on azopolymer films. The employed exposure dose of 1st inscription stage and 2nd inscription stage are optimized to obtain symmetrical modulation heights. These photoinduced 2-D surface relief structures on azopolymer films are used directly to mold PDMS, and PDMS molds were then transferred onto photopolymer to imprint microlens arrays. Using this method, tetragonally and hexagonally close-packed microlens arrays are successfully fabricated in rapid and cost-effective way.

Holographic diffraction gratings with enhanced sensitivity based on epoxy-resin photopolymers

Photopolymers are interesting materials to obtain high-quality performance for the volume holographic data storage with a low noise and high diffraction efficiency. In this paper, the recording of holographic diffraction gratings with a spatial frequency of 1285lines/mm in photopolymerizable epoxy resin materials is experimentally demonstrated. Diffraction efficiency near 92% and an energetic sensitivity of 11.7 x 10-3cm2/J are achieved by designing the proper structure of matrix and also optimizing photopolymer compositions. The effect of photopolymer compositions on the fundamental optical properties is also discussed.

Organic-inorganic hybrid photopolymer with reduced volume shrinkage

An organic-inorganic hybrid photopolymer containing surface-photoreactive nanoparticles, which exhibits a significant enhancement in optical properties is reported. The photopolymer containing surface-photoreactive nanoparticles showed high diffraction efficiency near 90% and reduced volume shrinkage without affecting the energetic sensitivity and optical quality of the photopolymer.

Nanoparticle-induced refractive index modulation of organic-inorganic hybrid photopolymer

An organic-inorganic photopolymers have been studied for their potential in of reducing the volume shrinkage during photopolymerization and enhancing the dimensional stability of photopolymers. We demonstrate the diffraction efficiency of photopolymers could be significantly enhanced by the interfacial interactions induced at the surface of inorganic nanoparticles.

Tumor-binding prodrug micelles of polymer–drug conjugates for anticancer therapy in HeLa cells

An interesting phytosphingosine (PHS), a natural sphingolipid metabolite comprising ceramides, is believed to play a vital role in strong anticancer therapeutic efficacy for various types of cancer cells, which is based on a programmed cellular death mechanism, so called apoptosis. However, extremely low water-solubility has been an obstacle to its usage as an anticancer drug via the systemic administration route. To utilize the benefits of PHS, we developed tumor-targeting polymer–drug conjugates wherein hydrophobic PHS was connected to the folate-grafted hydrophilic and biocompatible polymer through pH-sensitive linkages. The polymer–drug conjugates formed nano-sized (10–20 nm) spherical micelles spontaneously in aqueous media and they were found to have high drug contents (10.3 wt%). We present a systematic study of in vitro anticancer therapy in HeLa cells treated by tumor-targeting micelles in terms of anti-proliferation. The anticancer effect was analyzed by apoptotic cell death as well as the preferential distribution of loaded drug in the cancer cells. The synergistic effect by loading the commercial drug of doxorubicin was also studied.

Unusual surface reliefs from photoinduced creeping and aggregation behavior of azopolymer

We report the spontaneous formation of unusual surface reliefs, in which two sets of sinusoidal gratings were hierarchically structured, merely by single-step holographic inscription on amorphous azopolymer films. By monitoring of growth behavior of surface reliefs during holographic inscription, we found that the formation of additional grating is caused by the creeping and resulting aggregation of dome structures. Our direct observation of creeping and aggregation behavior is expected to contribute to enhancing the understanding of unusual surface reliefs, and also in fabricating complex surface reliefs.

Multifunctional photoreactive inorganic cages for three-dimensional holographic data storage

We demonstrate a holographic photopolymer based on multifunctional photoreactive inorganic cages, polyhedral oligomeric silsesquioxane (POSS). It is shown that a second photopolymerizable monomer, POSS, for the photopolymer, contributes to significantly enhance photosensitivity as well as refractive index modulation (Deltan). We also found that during the formation of holographic gratings, polymerization of POSS could effectively suppress volume shrinkage of photopolymer resin, owing to its filler-strengthening effect of inorganic cages accompanied with interpenetrating effect.

Holographic photopolymers of organic/inorganic hybrid interpenetrating networks for reduced volume shrinkage

We demonstrated that holographic photopolymers of organic/inorganic hybrid interpenetrating networks (IPNs) showed both enhanced monomer diffusion and reduced volume shrinkage, simultaneously. Hybrid IPNs were developed from simple in-situ sol-gel reaction: the alkoxysilane of (3-aminopropyl)triethoxysilane (APTES) leads to the formation of inorganic networks, while simultaneously partial introduction of the two amines of APTES into epoxy resins reduced the overall crosslink density of organic networks. In particular, the morphology of hybrid IPNs can be easily controlled by simply adjusting the ratio of APTES to epoxy resins. When the ratio of APTES to oxirane was 7.0 wt %, interpenetrated hard inorganic networks were homogeneously distributed within soft organic networks without aggregation. The present hybrid IPNs were very effective for high performance holographic properties including diffraction efficiency and volume shrinkage. Soft organic networks with low overall crosslink density enabled fast monomer diffusion for photopolymerization, and consequently diffraction efficiency was enhanced. In addition to fast monomer diffusion, locally dispersed inorganic networks dramatically suppressed the volume shrinkage, due to their filler strengthening effect accompanied by an interpenetrating effect. This synergetic effect of hybrid IPNs allows the enhancement of diffraction efficiency without sacrificing volume shrinkage.

Medial preoptic circuit induces hunting-like actions to target objects and prey

As animals forage, they must obtain useful targets by orchestrating appropriate actions that range from searching to chasing, biting and carrying. Here, we reveal that neurons positive for the α subunit of Ca2+/calmodulin-dependent kinase II (CaMKIIα) in the medial preoptic area (MPA) that send projections to the ventral periaqueductal gray (vPAG) mediate these target-directed actions in mice. During photostimulation of the MPA–vPAG circuit, mice vigorously engaged with 3D objects and chased moving objects. When exposed to a cricket, they hunted down the prey and bit it to kill. By applying a head-mounted object control with timely photostimulation of the MPA–vPAG circuit, we found that MPA–vPAG circuit-induced actions occurred only when the target was detected within the binocular visual field. Using this device, we successfully guided mice to navigate specified routes. Our study explains how the brain yields a strong motivation to acquire a target object along the continuum of hunting behavior.This study finds the key neurons that respond to 3D objects in the medial preoptic area (MPA). Their photostimulation induces hunting-like behaviors towards toys and prey, showing how the brain organizes behaviors to acquire useful resources.