[Show abstract][Hide abstract] ABSTRACT: In this paper, we introduce pressure-driven flow-induced miniaturizing free-flow electrophoresis (PDF-induced μ-FFE), a novel continuous separation method. In our separation system, the external flow and electric field are applied to particles, such that particle movement is affected by pressure-driven flow, electroosmosis, and electrophoresis. We then analyzed the hydrodynamic drag force and electrophoretic force applied to the particles in opposite directions. Based on this analysis, micro- and nano-sized particles were separated according to their electrophoretic mobilities with high separation efficiency. Because the separation can be achieved in a simple T-shaped microchannel, without the use of internal electrodes, it offers the advantages of low-cost, simple device fabrication and bubble-free operation, compared with conventional μ-FFE methods. Therefore, we expect the proposed separation method to have a wide range of filtering/separation applications in biochemical analysis.
[Show abstract][Hide abstract] ABSTRACT: Electrospinning technology is a versatile method for fabricating three-dimensional (3D) nanofibrous scaffolds using a wide range of polymeric materials for tissue engineering and regenerative medicine. However, a major concern regarding 3D electrospun scaffolds is that the densely packed layers hinder an even cellular distribution and in-depth infiltration. Here, we describe a new ‘all-at-once’ method enabling scaffold fabrication and cell seeding simultaneously, in which the medium bath containing cells is rotated eccentrically at high speed (>1500 rpm). The unstable flow of culture medium under hydrodynamic conditions resulted in a skein-shaped 3D structure and enhanced the even cellular distribution and in-depth infiltration. Cellular distribution and infiltration analyses confirmed that our method was superior to static and dynamic seeding methods. Moreover, we showed that our method facilitated long-term (14 days) proliferation.
No preview · Article · Nov 2015 · Sensors and Actuators B Chemical
[Show abstract][Hide abstract] ABSTRACT: Liquid pumping can occur along the outer surface of an electrode under a DC electric field. For biological applications, a better understanding of the ionic solution pumping mechanism is required. Here, we fabricated CNT wire electrodes (CWEs) and tungsten wire electrodes (TWEs) of various diameters to assess an ionic solution pumping. A DC electric field created by a bias of several volts pumped the ionic solution in the direction of the negatively biased electrode. The resulting electro-osmotic flow was attributed to the movement of an electric double layer near the electrode, and the flow rates along the CWEs were on the order of picoliters per minute. According to electric field analysis, the z-directional electric field around the meniscus of the small electrode was more concentrated than that of the larger electrode. Thus, the pumping effect increased as the electrode diameter decreased. Interestingly in CWEs, the initiating voltage for liquid pumping did not change with increasing diameter, up to 20 μm. We classified into three pumping zones, according to the initiating voltage and faradaic reaction. Liquid pumping using the CWEs could provide a new method for biological studies with adoptable flow rates and a larger 'Recommended pumping zone'.
[Show abstract][Hide abstract] ABSTRACT: In whole-cell based biosensors, spectrophotometry is one of the most commonly used methods for detecting organophosphates due to its simplicity and reliability. The sensor performance is directly affected by the cell immobilization method because it determines the amount of cells, the mass transfer rate, and the stability. In this study, we demonstrated that our previously-reported microbe immobilization method, a microbe-attached single-walled carbon nanotube film, can be applied to whole-cell-based organophosphate sensors. This method has many advantages over other whole-cell organophosphate sensors, including high specific activity, quick cell immobilization, and excellent stability. A device with circular electrodes was fabricated for an enlarged cell-immobilization area. Escherichia coli expressing organophosphorus hydrolase in the periplasmic space and single-walled carbon nanotubes were attached to the device by our method. Paraoxon was hydrolyzed using this device, and detected by measuring the concentration of the enzymatic reaction product, p-nitrophenol. The specific activity of our device was calculated, and was shown to be over 2.5 times that reported previously for other whole-cell organophosphate sensors. Thus, thismethod for generation of whole-cell-based OP biosensors might be optimal, as it overcomes many of the caveats that prevent the widespread use of other such devices.
[Show abstract][Hide abstract] ABSTRACT: The spontaneously generated electrical charge of a droplet dispensed from conventional pipetting is undesirable and unpredictable for most experiments that use pipetting. Hence, a method for controlling and removing the electrical charge needs to be developed. In this study, by using the electrode-deposited pipet tip (E-pipet tip), the charge-controlling system is newly developed and the electrical charge of a droplet is precisely controlled. The effect of electrolyte concentration and volume of the transferred solution to the electrical charge of a dispensed droplet is theoretically and experimentally investigated by using the equivalent capacitor model. Furthermore, a proof-of-concept example of the self-alignment and self-assembly of sequentially dispensed multiple droplets is demonstrated as one of the potential applications. Given that the electrical charge of the various aqueous droplets can be precisely and simply controlled, the fabricated E-pipet tip can be broadly utilized not only as a general charge-controlling platform of aqueous droplets but also as a powerful tool to explore fundamental scientific research regarding electrical charge of a droplet, such as the surface oscillation and evaporation of charged droplets.
[Show abstract][Hide abstract] ABSTRACT: Optical-resolution photoacoustic microscopy (OR-PAM) is an imaging tool to provide in vivo optically sensitive images in biomedical research. To achieve a small size, fast imaging speed, wide scan range, and high signal-to-noise ratios (SNRs) in a water environment, we introduce a polydimethylsiloxane (PDMS)-based 2-axis scanner for a flexible and waterproof structure. The design, theoretical background, fabrication process and performance of the scanner are explained in details. The designed and fabricated scanner has dimensions of 15 × 15 × 15 mm along the X, Y and Z axes, respectively. The characteristics of the scanner are tested under DC and AC conditions. By pairing with electromagnetic forces, the maximum scanning angles in air and water are 18° and 13° along the X and Y axes, respectively. The measured resonance frequencies in air and water are 60 and 45 Hz along the X axis and 45 and 30 Hz along the Y axis, respectively. Finally, OR-PAM with high SNRs is demonstrated using the fabricated scanner, and the PA images of micro-patterned samples and microvasculatures of a mouse ear are successfully obtained with high-resolution and wide-field of view. OR-PAM equipped with the 2-axis PDMS based waterproof scanner has lateral and axial resolutions of 3.6 μm and 26 μm, respectively. This compact OR-PAM system could potentially and widely be used in preclinical and clinical applications.
[Show abstract][Hide abstract] ABSTRACT: The application of nanomaterials for biosensors and fuel cells is becoming more common, but it requires an understanding of the relationship between the structure and electrochemical characteristics of the materials at the nanoscale. Herein, we report the development of scanning electrochemical microscopy–atomic force microscopy (SECM–AFM) nanoprobes for collecting spatially resolved data regarding the electrochemical activity of nanomaterials such as carbon nanotube (CNT) networks. The fabrication of the nanoprobe begins with the integration of CNT-bundle wire into a conventional AFM probe followed by the deposition of an insulating layer and cutting of the probe end. In addition, a protrusive insulating tip is integrated at the end of the insulated CNT-bundle wire to maintain a constant distance between the nanoelectrode and the substrate; this yields an L-shaped nanoprobe. The resultant nanoprobes produced well-fitted maps of faradaic current data with less than 300 nm spatial resolution and topographical images of CNT networks owing to the small effective distance (on the order of tens of nanometers) between the electrode and substrate. Electrochemical imaging using the L-shaped nanoprobe revealed that the electrochemical activity of the CNT network is not homogeneous and provided further understanding of the relationship between the topography and electrochemical characteristics of CNT networks.
[Show abstract][Hide abstract] ABSTRACT: Ion concentration polarization (ICP) is a distinctive electrochemical phenomenon that occurs near an ion-exchange membrane with an applied DC electric field, generating a significant concentration gradient in back and forth on the membrane. To date, however, there have been only a few attempts to introduce unconventional materials for ion transport in micro-nanofluidic systems. Here, we describe the development of a novel ICP system using an entangled single-wall carbon nanotube (SWNT) film as an ion-selective membrane instead of a Nafion membrane, for investigating the detailed relationship between electrical properties, i.e., ionic conductance through nanojunctions, and nonlinear electrokinetic behavior.
No preview · Article · Mar 2015 · Japanese Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Cartilage regeneration is a major challenge for researchers because cartilage tissue has limited innate regenerative ability. Encapsulation within an alginate gel has been used widely for 3D scaffolds to generate cartilage-like tissue, but alginate gels have limitations such as poor mechanical properties. In this study, we fabricated alginate microfibers for human septal chondrocyte (HSC) encapsulation and identified the conditions that result in the optimal mechanical properties of the alginate microfibers. In vitro experiments showed that HSCs encapsulated within alginate microfibers maintained >90% viability for 7 days, and the 140μm condition was more effective in terms of HSC proliferation than the 330 and 520μm conditions. In vivo, HSCs differentiated gradually into cartilage tissue over 4 weeks in immunocompetent mice. Importantly, the alginate-encapsulated HSCs were isolated and protected from the host immune response despite xenograft implantation.
[Show abstract][Hide abstract] ABSTRACT: Optical-resolution photoacoustic microscopy (OR-PAM) is a novel label-free microscopic imaging tool to provide in vivo optical absorbing contrasts. Specially, it is crucial to equip a real-time imaging capability without sacrificing high signal-to-noise ratios (SNRs) for identifying and tracking specific diseases in OR-PAM. Herein we demonstrate a 2-axis water-proofing MEMS scanner made of flexible PDMS. This flexible scanner results in a wide scanning range (9 × 4 mm(2) in a transverse plane) and a fast imaging speed (5 B-scan images per second). Further, the MEMS scanner is fabricated in a compact footprint with a size of 15 × 15 × 15 mm(3). More importantly, the scanning ability in water makes the MEMS scanner possible to confocally and simultaneously reflect both ultrasound and laser, and consequently we can maintain high SNRs. The lateral and axial resolutions of the OR-PAM system are 3.6 and 27.7 μm, respectively. We have successfully monitored the flow of carbon particles in vitro with a volumetric display frame rate of 0.14 Hz. Finally, we have successfully obtained in vivo PA images of microvasculatures in a mouse ear. It is expected that our compact and fast OR-PAM system can be significantly useful in both preclinical and clinical applications.
Full-text · Article · Jan 2015 · Scientific Reports
[Show abstract][Hide abstract] ABSTRACT: Electrospinning of polyurethane (PU) is widely used to fabricate breathable fabrics for applications in outdoor sportswear, and can be used to produce a highly porous structure, which is an essential property of a breathable fabric. To increase the breathability of the fabric, herein we describe the use of a metal mesh as the ground electrode in place of the conventional planar electrode during electrospinning. This electrode geometry results in an electric field that leads to the electrospun fibers being predominantly stacked over the metal wires of the mesh, with fewer fibers over the holes, resulting in larger pores in the membrane. The average pore size and thickness of the membrane were compared with those of a membrane fabricated using a conventional planar electrode. A quantitative analysis performed according to the Korean Industrial Standards (KS) indicated improved breathability.
Full-text · Article · Jan 2015 · Sensors and Materials
[Show abstract][Hide abstract] ABSTRACT: Dispensed small droplets are widely used in analyses of small organisms in various bioapplications. The generating power used to induce "flying beads" of dispensed small droplets should be sufficiently low to guarantee the safety of the organisms. In this study, we fabricated carbon nanotube (CNT) nanobundles electroplated with gold nanoparticles. Small droplets were generated by the repulsive force in an ion-concentrated zone; in this region, the droplets were generated at lower voltages due to the higher ion concentration. The generating power was examined as a function of electrode diameter (0.6, 20, and 500 μm) and decreased significantly with electrode size, specifically 0.007 mW for the 0.6-μm-diameter electrode compared with 0.017 W for the 20-μm-diameter electrode. The beads expelled from the mother droplet at 0.007 mW had an initial velocity of ∼2 m min-1. This technique is expected to be particularly useful for the analysis of very small analytes.
No preview · Article · Jan 2015 · Sensors and Materials
[Show abstract][Hide abstract] ABSTRACT: Non-axisymmetric drops impacting on a solid surface can alter impact dynamics significantly, thereby resulting in rebound suppression. Here, we present a method to control the bounce height of drops impacting on heated surfaces with ellipsoidal shaping. Experimental and numerical studies are used to investigate the effects of the geometrical aspect ratio (AR) of the drop on bouncing dynamics, which shows that maximum bounce heights of ellipsoidal drops can be reduced below spherical cases to nearly 40%. Control of bounce height can be explained in terms of a non-axial kinetic energy distribution during retraction. Interestingly, the non-axisymmetric hydrodynamics allows us to reduce contact time below this theoretical limit, which is explored both experimentally and numerically as a function of AR. This work may provide an understanding of bouncing dynamics on non-wetting surfaces for applications in surface cooling and cleaning.
No preview · Article · Dec 2014 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: We report a novel passive frequency-trimming method for microelectromechanical system (MEMS) resonators using carbon nanotube (CNT) rope synthesis on the side of the MEMS resonator. The method has a number of advantages over conventional methods, including low cost, reduced processing time, and greater applicability. First, the method requires only an ac voltage source and a dispersed CNT suspension to synthesize CNT ropes using dielectrophoresis. The method can be implemented in <;3 min at room temperature and atmospheric pressure. The resonant frequency of the MEMS resonator can be shifted by 0.5%-24%. In addition, the method can restore the original frequency by cutting the CNT ropes by passing a current through them, so that trimming can be carried out repeatedly by connecting and disconnecting the CNT ropes until the desired frequency is achieved.
No preview · Article · Dec 2014 · Journal of Microelectromechanical Systems
[Show abstract][Hide abstract] ABSTRACT: We present a novel method of fabricating ultra-precise patterns using multiple x-ray irradiations and precision stage movement. As the typical deep x-ray mask by ultraviolet (UV) lithography can have a minimum several-microns-scale pattern, fabrication of smaller patterns using general deep x-ray lithography with such a UV-process-based x-ray mask has limitations. In the present study, a substrate was loaded onto a precision stage allowing independent motion in the horizontal and vertical directions. The vertical stage, during x-ray irradiation, moves only up and down; after the initial x-ray irradiation, the horizontal stage moves the substrate in the horizontal direction in preparation for the next x-ray irradiation, which subsequently is carried out. The horizontal movement distance, crucially, can be adjusted to control the fabricated pattern size. By these combinations of precision stage movements and multiple x-ray irradiations, a pattern smaller than the original can be fabricated. The experimental results show in fact that this simple technique can be easily utilized for sub-micron-scale pattern fabrication using the typical UV-based x-ray mask
No preview · Article · Nov 2014 · International Journal of Precision Engineering and Manufacturing
[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.
Full-text · Article · Oct 2014 · Chemical Communications