How keratin cortex thickness affects iridescent feather colours.

The bright, saturated iridescent colours of feathers are commonly produced by single and multi-layers of nanostructured melanin granules (melanosomes), air and keratin matrices, surrounded by an outer keratin cortex of varying thicknesses. The role of the keratin cortex in colour production remains unclear, despite its potential to act as a thin film or absorbing layer. We use electron microscopy, optical simulations and oxygen plasma-mediated experimental cortex removal to show that differences in keratin cortex thickness play a significant role in producing colours. The results indicate that keratin cortex thickness determines the position of the major reflectance peak (hue) from nanostructured melanosomes of common pheasant (Phasianus colchicus) feathers. Specifically, the common pheasant has appropriate keratin cortex thickness to produce blue and green structural colours. This finding identifies a general principle of structural colour production and sheds light on the processes that shaped the evolution of brilliant iridescent colours in the common pheasant.

Bioinspired nanoscale hierarchical pillars for extreme superhydrophobicity and wide angular transmittance.

Hierarchical structures in nature provide unique functions for living organisms that can inspire technology. Nanoscale hierarchical structured surfaces are essential to realize the dual functions of non-wetting and transparency for applications such as cover glasses and windows; however, these structures are challenging to fabricate. In this study, nano-hierarchical structured glass surfaces were fabricated using multi-step colloidal lithography and etching to obtain tunable morphology. Nanostructured surfaces of mono-pillar structures of diameter 120 and 350 nm and hierarchical-pillar structures of their combinations exhibited superhydrophobicity after perfluoropolyether coating. In particular, the hierarchical nanosurfaces showed excellent non-wetting properties with the apparent, advancing, and receding water contact angles exceeding 177° and contact angle hysteresis below 1°. Water bouncing behaviors - contact time, spreading diameter, and shape of the bouncing motion were also evaluated according to the Weber number to examine the robustness of superhydrophobicity. Hierarchical nanosurfaces showed larger spreading diameters than mono-nanosurfaces with 14 bounces, indicating minimal energy loss from friction, as can be explained by the effective slip length. Furthermore, the nano-hierarchical structures exhibited better transmittance for wide angles of incidence up to 70° than mono-nanostructures owing to their reduced scattering area and multi-periodicity.

Broadband Enhancement of Anti-reflectivity for a High Angle of Incidence Using Nanocone Geometry.

The broadband antireflective (AR) effect for wide incident angles has a significant effect on the photoconversion efficiency of photovoltaics and visibility of large-format display panels. The fabrication of surface nanostructures has continued to attract research interest as an effective way to provide such optical performance. However, the effects of different nanostructure geometries are not fully understood, especially for wide-angle AR effects. In this work, we conduct a systematic analysis of the effect of periodic nanostructures such as nanocones (NCs) and inverted nanocones (INCs) on anti-reflectivity at high angles of incidence (AOIs) in terms of light scattering, guided-mode resonance (GMR), and internal reflections. NCs provide good coupling of light scattering and GMR because of their protruding geometry; hence, reduced reflectance can be obtained in the short wavelength region. Further, NCs exhibit relatively weaker GMR intensities and internal reflections, resulting in low reflectance in the long wavelength region. Therefore, NCs offer a superior broadband AR effect for high AOIs compared with INCs. Based on this analysis, we demonstrate an extremely low average reflectance (5.4%) compared to that of the bare substrate (34.7%) for the entire visible range at an AOI of 75° by fabricating NCs on both sides of the substrate.

Melanin-based structural coloration of birds and its biomimetic applications.

Melanin has been a widely researched pigment by scientists for decades as it is undoubtedly the most ubiquitous and ancient pigment found in nature. Melanin plays very significant roles in structural plumage colors in birds: it has visible light-absorbing capabilities, and nanoscale structures can be formed by self-assembling melanin granules. Herein, we review recent progress on melanin-based structural coloration research. We hope that this review will provide current understanding of melanin's structural and optical properties, natural coloration mechanisms, and biomimetic methods to implement artificial melanin-based structural colors.

Quantitative Correlation of Nanotopography with Cell Spreading via Focal Adhesions Using Adipose-Derived Stem Cells.

Nanotopography mimicking extracellular environments reportedly impact cell morphological changes; however, elucidating this relationship has been challenging. To control cellular responses using nanostructures, in this study, the quantitative relationship between nanotopography and cell spreading mediated by focal adhesions (FAs) is demonstrated using adipose-derived stem cells (ASCs). The spreading of ASCs and area of FAs are analyzed for the distribution of filamentous actin and vinculin, respectively, using fluorescent images. FAs require a specific area for adhesion (herein defined as effective contact area [ECA]) to maintain cell attachment on nanopillar arrays. An ECA is the area of FAs supported by nanopillars, multiplying the area fraction (AF) of their top surface. Regarding the spreading of cells, the mean area of ASCs linearly decreases as the mean area of FAs increases. Because the area of FAs is inversely correlated to the AF of the nanopillar arrays, the spreading of cells can be quantitatively correlated with nanotopography. The results provide a conceptual framework for controlling cell behaviors to design artificial substrates for tissue-engineering applications.

Stem cell behaviors on periodic arrays of nanopillars analyzed by high-resolution scanning electron microscope images.

The biocompatible polyurethane acrylate (PUA) nanopillars were fabricated by soft lithography using three different sizes of nanobeads (350, 500, and 1000 nm), and the human adipose-derived stem cells (hASCs) were cultured on the nanopillars. The hASCs and their various behaviors, such as cytoplasmic projections, migration, and morphology, were observed by high resolution images using a scanning electron microscope (SEM). With the accurate analysis by SEM for the controlled sizes of nanopillars, the deflections are observed at pillars fabricated with 350- and 500-nm nanobeads. These high-resolution images could offer crucial information to elucidate the complicated correlations between nanopillars and the cells, such as morphology and cytoplasmic projections.

Fluorinated low-viscosity copolymer with enhanced release property for roll-to-plate UV nanoimprint lithography.

A new fluorinated low-viscosity copolymer resin with enhanced release properties has been developed to provide higher fidelity for roll-based ultraviolet (UV) nanoimprint lithography (NIL) and simplify the process. The properties are optimized by blending (2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,9-heptadecafluorononyl) oxirane (EP-F8) and 1,4-butanediol divinyl ether (BDDVE) with 3,4-epoxycyclohexylmethyl-3,4-epoxy cyclohexane carboxylate. The cured resin renders the hydrophobicity and allows demolding from the stamp mold without an anti-stick layer (ASL), as further verified by the contact angle and the work of adhesion measurements. Due to the addition of BDDVE, our resin also has much lower viscosity compared to commercially available low-viscosity resins. These properties enable the transfer of high-density dot patterns by roll-to-plate (R2P) UV NIL in the newly developed resin without adding solvent or coating it with an ASL. These results demonstrate that our resin is a good option for high-quality, low-cost R2P UV NIL, as its use eliminates several steps in the imprinting process, making it applicable to the scalable fabrication of flexible devices.

Hierarchical Nanoflowers on Nanograss Structure for a Non-wettable Surface and a SERS Substrate.

Hierarchical nanostructures of CuO nanoflowers on nanograss were investigated for self-cleaning and surface plasmonic applications. We achieved the hierarchical nanostructures using one-step oxidation process by controlling the formation of flower-like nanoscale residues (nanoflowers) on CuO nanograss. While the nanograss structure of CuO has a sufficient roughness for superhydrophobic characteristics, the additional hierarchy of nanoflowers on nanograss leads to a semi-reentrant structure with a high repellency even for a very small droplet (10 nL) of low surface tension liquid such as 25 % ethanol (~35 mN/m), thus providing non-wettable and self-cleaning properties. Furthermore, the CuO hierarchical nanostructure serves as a substrate for surface-enhanced Raman spectroscopy (SERS). Both of the CuO nanograss and nanoflower provide many nanoscale gaps that act as hot-spots for surface-enhanced Raman signal of 4-mercaptopyridine (4-Mpy), thus enabling a non-destructive detection in a short analysis time with relatively simple preparation of sample. Especially, the CuO nanoflower has larger number of hot-spots at the nanogaps from floral leaf-like structures, thus leading to three times higher Raman intensity than the CuO nanograss. These multifunctional results potentially provide a path toward cost-effective fabrication of a non-wettable surface for self-maintenance applications and a SERS substrate for sensing applications.

Optimal moth eye nanostructure array on transparent glass towards broadband antireflection.

Broadband antireflection (AR) is essential for improving the photocurrent generation of photovoltaic modules or the enhancement of visibility in optical devices. Beyond conventional AR coating methods, moth eye mimicking nanostructures give new directions to enhance broadband antireflection through the selection of geometrical parameters, such as height, periodic distance, shape, and arrangement. This study numerically and experimentally investigates the behavior of light on complex nanostructures designed to mimic the surface of the moth eye with mixed shapes and various arrangements. To obtain broadband AR, we rigorously study the design parameters, such as height, periodic distance, shape, and arrangement, on a transparent quartz substrate. Several kinds of nanopillar arrays are elaborately fabricated including mixed nanostructures comprising pointy and round shapes in ordered and random arrangements via colloidal lithography. The optimal morphology of moth eye nanostructure arrays for broadband antireflection is suggested in view of reflectance and average weight transmittance.

Improved antireflection properties of moth eye mimicking nanopillars on transparent glass: flat antireflection and color tuning.

The sub-wavelength structures in moth eyes exhibit fascinating antireflective properties over the broadband wavelength region and at large incident angle by generating an air-mixed heterogeneous optical interface. In this work, antireflective behavior of transparent glass is observed with the elaborate controls of the nanopillar morphology. The reflectance spectrum shows a red shift and a notable light scattering with increase of the height of the nanopillars. The nanopillar arrays with a pointed cone shape have better optical performance in visible range than the rounded cone shape which is typical antireflective nanostructures in nature. Based on the observed antireflective behaviors, the flat and low value reflectance spectrum in the visible wavelength range is demonstrated by moth eye mimicking nanostructures on both sides of a glass surface. It is a unique strategy to realize a flat and broadband spectrum in the visible range showing 99% transparency via the appropriate matching of nanopillar height on the front and back sides of glass. The controlled reflection based color tuning on the antireflective and transparent glass is also obtained by adjusting the height of the nanopillar arrays on both sides. The visibility and self-cleaning ability of moth eye mimicking glass are examined for practical applications such as antireflection and self-cleaning.

A quartz nanopillar hemocytometer for high-yield separation and counting of CD4(+) T lymphocytes.

We report the development of a novel quartz nanopillar (QNP) array cell separation system capable of selectively capturing and isolating a single cell population including primary CD4(+) T lymphocytes from the whole pool of splenocytes. Integrated with a photolithographically patterned hemocytometer structure, the streptavidin (STR)-functionalized-QNP (STR-QNP) arrays allow for direct quantitation of captured cells using high content imaging. This technology exhibits an excellent separation yield (efficiency) of ~95.3 ± 1.1% for the CD4(+) T lymphocytes from the mouse splenocyte suspensions and good linear response for quantitating captured CD4(+) T-lymphoblasts, which is comparable to flow cytometry and outperforms any non-nanostructured surface capture techniques, i.e. cell panning. This nanopillar hemocytometer represents a simple, yet efficient cell capture and counting technology and may find immediate applications for diagnosis and immune monitoring in the point-of-care setting.

Formation of (111)-oriented nickel nanocrystals by thermal annealing of nickel thin films.

We have produced Ni nanocrystals with face centered cubic structure by thermally annealing Ni films deposited on SiO2-covered Si(001) substrates in a flow of mixed hydrogen and argon gas. Ni films thicker than 5 nm self-assemble into highly (111)-oriented Ni nanocrystals on a flat and continuous SiO2 interlayer during the thermal annealing, while Ni films of 5 nm thickness aggregate to the irregularly shaped nanoparticles. The lateral width of the nanocrystals ranges from tens of nanometers to hundreds of nanometers, and the crystal height is under 100 nm. The nanocrystals have wide (111) top facets of hexagonal shape and narrow (100) sidewalls of truncated pyramidal shape, as a result of each crystal minimizing its total surface energy. Our results demonstrate that the formation of nanocrystals during thermal annealing is strongly affected by the morphology of the SiO2 interlayer, the Ni film thickness, the annealing temperature, and the partial pressure of hydrogen gas.