[Show abstract][Hide abstract] ABSTRACT: An ideal black material absorbs light perfectly over all wavelengths and is totally nonreflective. Material and structural design are crucial to the management of reflectivity. Here, we report a three-dimensionally designed (3D) silicon structure consisting of silicon pillars. To our knowledge, this 3D hierarchical surface has the lowest specular reflectance among silicon-based materials reported to date.
Chemical Communications 10/2014; · 6.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Possible mechanisms of overlimiting current in unsupported electrolytes,
exceeding diffusion limitation, have been intensely studied for their
fundamental significance and applications to desalination, separations,
sensing, and energy storage. In bulk membrane systems, the primary physical
mechanism is electro-convection, driven by electro-osmotic instability on the
membrane surface. It has recently been predicted that confinement by charged
surfaces in microchannels or porous media favors two new mechanisms,
electro-osmotic flow (EOF) and surface conduction (SC), driven by large
electric fields in the depleted region acting on the electric double layers on
the sidewalls. Here, we provide the first direct evidence for the transition
from SC to EOF above a critical channel height, using in situ particle tracking
and current-voltage measurements in a micro/nanofluidic device. The dependence
of the over-limiting conductance on channel depth (d) is consistent with
theoretical predictions, scaling as d^-1 for SC and d^4/5 for EOF with a
transition around d=8um. This complete picture of surface-driven over-limiting
current can guide engineering applications of ion concentration polarization
phenomena in microfluidics and porous media.
[Show abstract][Hide abstract] ABSTRACT: A stretchable humidity sensor which has mechanical stability has remained a challenging issue in stretchable electronics. Here, we report a novel stretchable nanostructured polyaniline humidity sensor. The sensor was fabricated by simple fabrication methods such as a pre-stretching process and self-assembly texturing process. The periodically wrinkled structure supported by microstructured elasto-meric substrates provides an excellent stretchability. The micro-textured substrate was introduced for solving the crack problem by the Poisson effect as a stretchable sensor. The sensor maintains its humidity sensitivity well at different elongations. To the best of our knowledge, this is the first report of a stretchable humidity sensor. Stretchable electronics is a challenging eld and so is the development of sensory skins for robotics, structural health monitors, and wearable communication devices, beyond ex-ible electronics. 1 Recently, there have been numerous stretch-able electronic devices that use components such as elastic conductors, 2,3 light emission diodes, 4,5 eld effect transistors 6 and temperature sensors. 7 One of the most critical issues in stretchable sensors is realizing wearable and electronic skins, keeping their sensitivity reliable level. Humidity sensors have gained great attention for their practical applications in industrial elds, laboratory environ-ment, and our daily life. 8 Flexible humidity sensors especially attract attention where moisture sensing under deformation is required. Recently, deformable humidity sensors have been developed that use only exible devices. 9–12 However, the oper-ation of such devices was feasible for only small deformations as there would be a rupture of the sensing material at high deformations. Here, we report a highly stretchable nanostructured poly-aniline humidity sensor. The sensor maintains its humidity sensitivity stably at the each differed elongations. To our best knowledge, this is the rst report for a stretchable humidity sensor. The stretchable sensor consisted of polydimethylsiloxane (PDMS) as an elastic substrate and polyaniline (PANI) layer as a sensing material as shown in the Fig. 1. PANI which is a kind of conducting polymers has an excellent electric property and Fig. 1 Fabrication of a stretchable nanostructured polyaniline humidity sensor. (a and b) Preparation of pre-strained elastic substrate. (c) PANI nanostructures coating by a dilute polymerization. (d) Wrin-kled PANI nanostructured surface after removing of the pre-strain.
[Show abstract][Hide abstract] ABSTRACT: Nanofabrication technologies have been a strong advocator for new scientific fundamentals that have never been described by traditional theory, and have played a seed role in ground-breaking nano-engineering applications. In this study, we fabricated ultra-high-aspect (∼10(6) with O(100) nm nanochannel opening and O(100) mm length) orthogonal nanochannel array using only polymeric materials. Vertically aligned nanochannel arrays in parallel can be stacked to form a dense nano-structure. Due to the flexibility and stretchability of the material, one can tune the size and shape of the nanochannel using elongation and even roll the stack array to form a radial-uniformly distributed nanochannel array. The roll can be cut at discretionary lengths for incorporation with a micro/nanofluidic device. As examples, we demonstrated ion concentration polarization with the device for Ohmic-limiting/overlimiting current-voltage characteristics and preconcentrated charged species. The density of the nanochannel array was lower than conventional nanoporous membranes, such as anodic aluminum oxide membranes (AAO). However, accurate controllability over the nanochannel array dimensions enabled multiplexed one microstructure-on-one nanostructure interfacing for valuable biological/biomedical microelectromechanical system (BioMEMS) platforms, such as nano-electroporation.
[Show abstract][Hide abstract] ABSTRACT: MEMS gyroscopes have favorable characteristics, including small size, high throughput, and low cost. The performance of MEMS gyroscopes depends on the displacement sensitivity of the capacitors. In this paper, we describe the fabrication of 300-?m-thick gyroscopes that can provide high displacement sensitivity and are robust to fabrication tolerances, i.e. deep reactive ion etch (DRIE) rate uniformity. When thick structures are perforated using DRIE to achieve high-aspect-ratio features, footing is commonly observed. However, we describe a fabrication method that circumvents problems associated with footing and side-wall etching, so that the gyroscopes can have uniform dimensions and small variations across the wafer. Using a post-fabrication translation approach, the position of capacitors is modified following DRIE, and the gap in the gyroscopes can be reduced to 3??m, which leads to an aspect ratio of 100. Using this method, we fabricated MEMS gyroscopes that can overcome the DRIE aspect ratio limit and have capacitors with higher sensitivities than those of other gyroscopes, which typically employ substrates that are less than 100??m thick. The gyroscope had a resonant frequency of 9.91?kHz, a quality factor of 2500 and a sensitivity of 23?mV/[deg/s].
Journal of Micromechanics and Microengineering 06/2014; 24(7):075013. · 1.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recently, tremendous application utilizing electrospun nanofibers have been actively reported due to its several advantages, such as high surface to volume ratio, simple fabrication and high-throughput manufacturing. In this paper, we developed highly sensitive and consistent nanofiber humidity sensor by electrospinning. The humidity sensor was fabricated by rapid electrospinning (~2 sec) /PVP/LiCl mixed solution on the micro-interdigitated electrode. In order to evaluate the humidity sensing performances, we measured current response using DC bias voltage under various relative humidity levels. The results show fast response / recovery time and marginal hysteresis as well as long-term stability. In addition, with the aid of micro-interdigitated electrode, we can reduce a total resistance of the sensor and increase the total reaction area of nanofibers across the electrodes resulting in high sensitivity and enhanced current level. Therefore, we expect that the electrospun nanofiber array for humidity sensor can be feasible and promising for diverse humidity sensing application.
Journal of Sensor Science and Technology. 01/2014; 23(1).
[Show abstract][Hide abstract] ABSTRACT: Microbes have been used extensively in various fields of researches and industries but has not been used widely for microfluidic biosensor applications because it is difficult to immobilize properly to a small space. Therefore, we developed a microbial immobilization method for microfluidic devices using single-walled nanotubes and dielectrophoretic force. Single-walled nanotubes and Escherichia coli were aligned between two cantilever electrodes by a positive dielectrophoretic force resulting in a film of single-walled nanotubes with attached Escherichia coli. The optimal condition of film formation without a cell lysis was investigated. Diameter of single-walled nanotubes and electric field (intensity and duration of application) had an effect on the cell viability. On the other hand, the cell concentration of the suspension did not affect the cell viability. Paraoxon was detected using single-walled nanotubes film with attached Escherichia coli that expressed organophosphorus hydrolase. This film which is suspended from the substrate showed faster response time than sensors that are not suspended from the substrate.
Journal of Sensor Science and Technology. 01/2014; 23(1).
[Show abstract][Hide abstract] ABSTRACT: We present a droplet-manipulation method using opto-thermal flows in oils. The flows are originated from Marangoni and buoyancy effects due to temperature gradient, generated by the adsorption of light on an amorphous silicon thin film. Using this method, we can transport, merge and mix droplets in an extremely simple system. Since the temperature rise during the operation is small, this method can be used for biological applications without the damage on cell viability.
Journal of the Korean Society for Precision Engineering. 01/2014; 31(1).
[Show abstract][Hide abstract] ABSTRACT: Conventional machining technologies such as a milling process have limitations in accuracy to fabricate microstructures. Deep X-ray lithography using the synchrotron radiation is a promising micromachining process with an excellent accuracy, whereas there are difficulties in the fabrication of multi-layered structures. Therefore, it is mainly used for fabricating simple mono-layered microstructures with a high aspect ratio. In this study, a novel technology for fabricating multi-layered microstructures is proposed by combining two processes. In advance, an X-ray resist material is cut and machined into various shapes and heights by the micro milling process. Subsequent X-ray irradiation process facilitates the fabrication of multi-layered microstructures. The proposed technology can overcome the limitation of the pattern accuracy in conventional milling process and the difficulty of the multi-layered machining in x-ray process. The usefulness of the proposed technology is demonstrated in this study by applying the technique in the realization of various multi-layered microstructures.
Journal of the Korean Society for Precision Engineering. 01/2014; 31(3).
[Show abstract][Hide abstract] ABSTRACT: The extracellular matrix provides mechanical support and affects cell behaviour. Nanoscale structures have been shown to have functions similar to the extracellular matrix. In this study, we fabricated nanoprotrusion structures with polyaniline as cell culture plates using a simple method and determined the effects of these nanoprotrusion structures on cells.
Nanoscale Research Letters 01/2014; 9(1):566. · 2.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We proposed a novel separation method, which is the first report using ion concentration polarization (ICP) to separate particles continuously. We analyzed the electrical forces that cause the repulsion of particles in the depletion region formed by ICP. Using the electrical repulsion, micro- and nano-sized particles were separated based on their electrophoretic mobilities. Because the separation of particles was performed using a strong electric field in the depletion region without the use of internal electrodes, it offers the advantages of simple, low-cost device fabrication and bubble-free operation compared with conventional continuous electrophoretic separation methods, such as miniaturizing free-flow electrophoresis (μ-FFE). This separation device is expected to be a useful tool for separating various biochemical samples, including cells, proteins, DNAs and even ions.
[Show abstract][Hide abstract] ABSTRACT: The fabrication of a stable, anti-wetting surface is a very challenging issue in surface chemistry. In general, superhydrophobicity highly depends on the surface structure. Moreover, mechanical deformation of the surface structure can produce dramatic changes in the surface wetting state, and in some cases, may even result in a complete loss of the surface's unique wettability. However, the study of stable surfaces under mechanical deformation conditions has been limited to flexible surfaces or small strain. Here, a mechanically stable superhydrophobic membrane is presented, which possesses high stretchability and gas breathability. The membrane, which consists of an elastic polyurethane fibrous matrix coated with polyaniline hairy nanostructures and polytetrafluoroethylene, exhibites excellent superhydrophobic properties under ≥300% strain. The breathability and wettability of the membrane is examined by examining various static and dynamic wetting parameters. The robust membrane maintaines its anti-wettability (water contact angle ≈160°, hysteresis ≈10°) for 1000 stretching cycles. It is also determined that the stretchable and superhydrophobic surface suppresses the fragmentation and rebound of impact droplets, compared with rigid superhydrophobic surfaces. Finally, underwater gas sensing is demonstrated as a novel application.
[Show abstract][Hide abstract] ABSTRACT: Flow over slanted grooves can generate a high degree of lateral transport by means of cross-movement of fluids along the groove. However, the quantitative relationship between the groove features and the cross-movement of fluid is rarely reported. We investigate the geometric effects of the groove on the cross-movement by computing the lateral displacements of particles in the cross section of the channel for varying geometric parameters of the groove, such as the depth, width, and angle. We identify the region of the cross-movement in the cross section of the channel, discuss the mechanism of material folding by the grooves, and touch on the validity of the effective slip model. The experimental results show the effect of the groove depth on the lateral transport of dye solutions, which confirms the numerical results. At high groove angles, the lateral transport can be characterized by complex swirling structures, and the swirls are quantified by the characteristic number of revolutions inside the groove.
Chemical Engineering Science 12/2013; 104:82–92. · 2.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We developed a novel fabrication method of polydimethylsioxane (PDMS) lens, which can easily control the shapes of the lens using soft lithography with common photolithography and water droplet molding. A mold for PDMS lens was prepared by patterning of hydrophobic photoresist on the hydrophilic substrate and dispensing small water droplets onto the predefined hydrophilic patterns. The size of patterns determined the dimension of the lens and the dispensed volume of the water droplet decided the radius of curvature of the PDMS lens independently. The water droplet with photoresist pattern played a robustly fixed mold for lens due to difference in wettability. The radius of curvature could be calculated theoretically because the water droplets could approximate spherical cap on the substrate. Finally, concave and convex PDMS lenses which could reduce or magnify optically were fabricated by curing of PDMS on the prepared mold. The measured radii of the fabricated PDMS lenses were well matched with the estimated values. We believe that our simple and efficient fabrication method can be adopted to PDMS microlens and extended to micro optical device, lab on a chip, and sensor technology.
Journal of Sensor Science and Technology. 09/2013; 22(5).
[Show abstract][Hide abstract] ABSTRACT: We developed a one-step method for fabrication of addressable suspended SWNT films and demonstrate excellent detection performance of paraoxon based on OPH-immobilized SWNT films for environmental monitoring. For dispersed SWNT suspension, COOH-SWNT was prepared by the oxidation of carbon nanotubes using acid treatment and sonication. Suspended SWNT-film was fabricated between cantilever electrodes by dielectrophoretic force and surface tension of the water meniscus. After that, OPH were immobilized on suspended SWNT-films by nonspecific binding for enzymatic hydrolysis of paraoxon. The electrical properties of the SWNT films were measured in real time at room temperature. Structurally suspended SWNT films from substrate surface made possible rapid and highly sensitive detection of target molecules with increased convectional and diffusional fluxes of the molecules and with a large binding surface area. SWNT film FET resulted in a real-time, label-free, and electrical detection of paraoxon to the concentration of ca. with a step-wise rapid response time of several seconds.
Journal of Sensor Science and Technology. 09/2013; 22(5).
[Show abstract][Hide abstract] ABSTRACT: Microelectrodes are widely used for monitoring neural activities in various neurobiological studies. The size of the neural electrode is an important factor in determining the signal-to-noise ratio (SNR) of recorded neural signals and, thereby, the recording sensitivity. Here, it is demonstrated that commercial tungsten microelectrodes can be modified with carbon nanotubes (CNTs), resulting in a highly sensitive recording ability. The impedance with the respect to surface area of the CNT-modified electrodes (CNEs) is much less than that of tungsten microelectrodes because of their large electrochemical surface area (ESA). In addition, the noise level of neural signals recorded by CNEs is significantly less. Thus, the SNR is greater than that obtained using tungsten microelectrodes. Importantly, when applied in a mouse brain invivo, the CNEs can detect action potentials five times more efficiently than tungsten microelectrodes. This technique provides a significant advance in the recording of neural signals, especially in brain regions with sparse neuronal densities.
[Show abstract][Hide abstract] ABSTRACT: Fabrication of one-dimensional nanostructures is a key issue for optical devices, fluidic devices, and solar cells because of their unique functionalities such as antireflection and superhydrophobicity. Here, we report a novel one-step process to fabricate patternable hierarchical structures consisting of microstructures and one-dimensional nanostructures using a sacrificial layer. The layer plays a role as not only a micromask for producing microstructures but also as a nanomask for nanostructures according to the etching time. Using this method, we fabricated patterned hierarchical structures, with the ability to control the shape and density of the nanostructure. The various architectures provided unique functionalities. For example, our sacrificial-layer etching method allowed nanostructures denser than what would be attainable with conventional processes to form. The dense nanostructure resulted in a very low reflectance of the silicon surface (less than 1%). The nanostructured surface and hierarchically structured surface also exhibited excellent antiwetting properties, with a high contact angle (>165°) and low sliding angle (<1°). We believe that our fabrication approach will provide new insight into functional surfaces, such as those used for antiwetting and antireflection surface applications.
Journal of Nanomaterials 07/2013; 2013. · 1.61 Impact Factor