Moon Kyu Kwak

Rational design and enhanced biocompatibility of a dry adhesive medical skin patch.

This work was supported by National Research Foundation of Korea (NRF) grant (No. 20110017530), WCU (World Class University) program (R31-2008-000-10083-0) on multiscale mechanical design, and Basic Science Research Program (2010-0027955) funded by the Ministry of Education, Science, and Technology (MEST). This work was supported in part by the Award No KUK-F1-037-02, made by King Abdullah University of Science and Technology (KAUST) and Institute of Advanced Machinery and Design (IAMD) of Seoul National University.

Stretchable, Adhesion-Tunable Dry Adhesive by Surface Wrinkling

We introduce a simple yet robust method of fabricating a stretchable, adhesion-tunable dry adhesive by combining replica molding and surface wrinkling. By utilizing a thin, wrinkled polydimethyl siloxane (PDMS) sheet with a thickness of 1 mm with built-in micropillars, active, dynamic control of normal and shear adhesion was achieved. Relatively strong normal (approximately 10.8 N/cm(2)) and shear adhesion (approximately 14.7 N/cm(2)) forces could be obtained for a fully extended (strained) PDMS sheet (prestrain of approximately 3%), whereas the forces could be rapidly reduced to nearly zero once the prestrain was released (prestrain of approximately 0.5%). Moreover, durability tests demonstrated that the adhesion strength in both the normal and shear directions was maintained over more than 100 cycles of attachment and detachment.

Synergistically enhanced osteogenic differentiation of human mesenchymal stem cells by culture on nanostructured surfaces with induction media.

We have examined the effects of surface nanotopography on in vitro osteogenesis of human mesenchymal stem cells (hMSCs). UV-assisted capillary force lithography was employed to fabricate a scalable (4x5 cm), well-defined nanostructured substrate of a UV curable polyurethane polymer with dots (150, 400, 600 nm diameter) and lines (150, 400, 600 nm width). The influence of osteogenic differentiation of hMSCs was characterized at day 8 by alkaline phosphatase (ALP) assay, RT-PCR, and real-time PCR analysis. We found that hMSCs cultured on the nanostructured surfaces in osteogenic induction media showed significantly higher ALP activity compared to unpatterned PUA surface (control group). In particular, the hMSCs on the 400 nm dot pattern showed the highest level of ALP activity. Further investigation with real-time quantitative RT-PCR analysis demonstrated significantly higher expression of core binding factor 1 (Cbfa1), osteopontin (OP), and osteocalcin (OC) levels in hMSCs cultured on the 400 nm dot pattern in osteogenic induction media. These findings suggest that surface nanotopography can enhance osteogenic differentiation synergistically with biochemical induction substance.

Bio-inspired slanted polymer nanohairs for anisotropic wetting and directional dry adhesion

Slanted polymer nanohairs possess a number of attractive properties in terms of anisotropic wetting and directional adhesion. This highlight provides an overview of the recent progress in the development of bio-inspired slanted polymer nanohairs and their applications towards anisotropic wetting and directional dry adhesion properties. With the advanced nano-fabrication techniques, it is possible to fabricate angled, directionally bent polymer nanohairs in a highly reproducible and geometry-controllable manner. The fabrication methods can be categorized into two streams: direct replica molding from a master with slanted structure or nanofabrication (photolithography or molding) with post treatment such as e-beam exposure, thermal annealing and mechanical compression. In this highlight, the fabrication methods for angled, high aspect ratio polymer nanohairs are briefly described along with their potential applications in anisotropic wetting and directional adhesion. Particular emphasis is given to recent achievements and future directions in biomimetic functional surfaces.

Spatial control of adult stem cell fate using nanotopographic cues

Adult stem cells hold great promise as a source of diverse terminally differentiated cell types for tissue engineering applications. However, due to the complexity of chemical and mechanical cues specifying differentiation outcomes, development of arbitrarily complex geometric and structural arrangements of cells, adopting multiple fates from the same initial stem cell population, has been difficult. Here, we show that the topography of the cell adhesion substratum can be an instructive cue to adult stem cells and topographical variations can strongly bias the differentiation outcome of the cells towards adipocyte or osteocyte fates. Switches in cell fate decision from adipogenic to osteogenic lineages were accompanied by changes in cytoskeletal stiffness, spanning a considerable range in the cell softness/rigidity spectrum. Our findings suggest that human mesenchymal stem cells (hMSC) can respond to the varying density of nanotopographical cues by regulating their internal cytoskeletal network and use these mechanical changes to guide them toward making cell fate decisions. We used this finding to design a complex two-dimensional pattern of co-localized cells preferentially adopting two alternative fates, thus paving the road for designing and building more complex tissue constructs with diverse biomedical applications.

Robust superomniphobic surfaces with mushroom-like micropillar arrays

We present a simple method for the fabrication of robust superomniphobic surfaces with high transmittance (>90%) and durability (<6 months). The method consists of direct micromolding of mushroom-like micropillars and C4F8 gas surface treatment. Such re-entrant structures were found to be highly resistant against wetting by various liquids and oils with a wide range of surface tensions from 22.3 (ethanol) to 72.1 mN m−1 (water). Optimal structural parameters were derived based on the measurements of static contact angle and contact angle hysteresis.

Continuous and scalable fabrication of flexible metamaterial films via roll-to-roll nanoimprint process for broadband plasmonic infrared filters

We demonstrate the continuous fabrication of large-area flexible metamaterial films via roll-to-roll (R2R) nanoimprint lithography (NIL) technique that can be conducted in an ambient environment at high speed. The plasmonic metal-insulator-metal structure is successfully fabricated by R2R NIL to continuously pattern the sub-wavelength scale metal disk array on flexible substrates. The patterned metal disks having varying diameters and sub-micron spacing with few defects lead to the desired broadband IR filtering performance at the designed dual-band, which correlates well with simulation analysis. Our method realizes a simple and high-throughput fabrication of plasmonic metamaterials for scalable and flexible optoelectronic and photonic applications.

Anisotropic Adhesion Properties of Triangular‐Tip‐Shaped Micropillars

Directional dry adhesive microstructures consisting of high-density triangular-tip-shaped micropillars are described. The wide-tip structures allow for unique directional shear adhesion properties with respect to the peeling direction, along with relatively high normal adhesion.

Photo-roll lithography (PRL) for continuous and scalable patterning with application in flexible electronics.

A novel nanofabrication methodology for continuous, scalable, and geometry-tunable lithography is developed, named photo-roll lithography (PRL), by integrating photolithography with rollable processing. As a flexible mask attached to a quartz cylinder containing a UV source rolls over a photoresistcoated substrate, PRL realizes continuous photolithographic fabrication of various micro/nanoscale patterns with geometry that is tunable by controlling mask-substrate motions.

Directed migration of cancer cells guided by the graded texture of the underlying matrix

Living cells and the extracellular matrix (ECM) can display complex interactions that define key developmental, physiological and pathological processes. Here, we report a new type of directed migration — which we term ‘topotaxis’ — by which cell movement is guided by the gradient of the nanoscale topographic features in the cells’ ECM environment. We show that the direction of topotaxis is reflective of the effective cell stiffness, and that it depends on the balance of the ECM-triggered signalling pathways PI3K-Akt and ROCK-MLCK. In melanoma cancer cells, this balance can be altered by different ECM inputs, pharmacological perturbations or genetic alterations, particularly a loss of PTEN in aggressive melanoma cells. We conclude that topotaxis is a product of the material properties of cells and the surrounding ECM, and propose that the invasive capacity of many cancers may depend broadly on topotactic responses, providing a potentially attractive mechanism for controlling invasive and metastatic behaviour.