A novel method combining microfluidic channels and holey poly(dimethylsiloxane) PDMS membranes is presented for patterning multiple types of cells on a large variety of substrate, including substrate bearing micro- and nanometer-scale structures. Different kinds of cell are patterned on diverse flat substrates composed of various materials. Further, different cells are patterned on substrate with microgrooved substrate, and the influence of cell-cell and cell-substrate interactions about cell group behaviors is shown. This method provides a new approach for studying many processes in vitro caused by cell-cell and/or cell-substrate interactions. In addition, the method is straightforward, easy to access, convenient, and may be useful for a broad set of biological studies.

We fabricated, in a microfluidic channel, dye molecule doped microspheres with a remarkable size monodispersivity and interesting optical properties. The microspheres were obtained by spontaneous formation of droplets and photo-polymerization of a low viscosity and UV sensitive resin. By varying the channel width and the flow rate, microspheres of radius in the range of 20–80 μm could be obtained with a size dispersivity of about 2%. When pumped with a pulsed laser at 532 nm, multi-mode laser emissions around 610 nm were detected from R6G doped microspheres at a threshold of 16 μJ/mm2.

We have developed a microturbidostat for long time bacterial culture at constant density controlled by optical detection and integrated pneumatic valves. The device was fabricated by multilayer soft lithography and in-situ formation of an agarose filter. The culture chamber of bacteria was connected in one side to a single bacterial input-output channel and in another side to a nutrient channel in which the agarose filter was formed to ensure the diffusion of nutrients and metabolites without bacterial loss. The bacterial number in the culture chamber was determined by measuring the fluorescence intensity of GFP proteins of the bacteria and the redundant bacteria could be exported automatically through the input-output channel with integrated micro-valves. In order to optimize the operation performance, we investigated the bacterial exportation efficiency with different input-output channel widths. As expected, the bacterial sorting coefficient was proportional to the input-output channel width. The results also showed that with a 20 µm channel-width, a long time culture was possible with a constant bacterial number in the chamber in the range from 400 to 700.

We report on results of fabrication and characterization of a conductive gel based pressure sensor which can be easily integrated into the commonly used microfluidic devices. The gel elements of the sensor are obtained by casting a viscous mixture of polydimethylsiloxane (PDMS) and metal powders on a patterned template. After solidification, the gel becomes conductive, showing piezoresistive effects that can be used for low pressure sensing. Our fabrication process of the gel elements is fully compatible to the integration requirement of multi-functional PDMS devices and the fabricated pressure sensors can be repeatedly used with direct current readout.

We present a simple and high-throughput microfluidic system for diffusion-based monolayer yeast cell culture monitoring. Yeast cells are patterned into the micro-cavity array with a suitable height (4 μm) that keeps the cells fixed in monolayer during the cell division. Different sizes of cavities and different repeating times of injection were tested in order to realize as many single-cell/cavity as possible. Single-cell/cavity has been achieved in about 40% of 100 parallel cavities. As a demonstration, we apply this technology to investigate budding yeast and fission yeast cultures and show that it permits single-cell resolution over many cellular generations. Our results show that the technique provides an easy way to study the phenotype of single yeast cell cycle or cell-cell communication in high-throughput microfluidic system.

We present a simple technique for cell loading, culturing, and phenotypic study in a multi-chamber microfluidic device made of polydimethylsiloxane (PDMS). This technique is based on the use of degassing induced aspiration of PDMS which allows loading cells into micro-cavities within 1 min. A large number of triangle cavities are patterned aside main flow channels with narrow connections so that cells can be loaded by aspirating into each cavity. In our device, high throughput and long-term monitoring can be done with minimum shear force of the flow. As a demonstration, we show a controlled loading at single cell level and the phenotypic variation of gene expression of the yeast strain w303 as a function of copper ion concentration of the medium.

We have demonstrated an improvement of light extraction from GaN based flip-chip LEDs by patterning encapsulant. Two dimensional (2D) micron-scale patterns of encapsulant were realized by using imprint technique of thermosetting polymer. This approach has several advantages such as technical simplification, low cost and freedom of material choice. In this work, we fabricated 2D micron-scale patterns with the triangular or sinusoidal profiles on the polymer encapsulated GaN-based flip-chip LEDs. The enhancement factors of light extraction of GaN LEDs with the patterned encapsulant comparing to the flat encapsulated LEDs are about 32% and 47% corresponding to the triangular and sinusoidal profiles, respectively. To evaluate the concept of a diffraction grating in enhancement of light extraction, we performed a simulation of diffraction based on simplified one-dimensional (1D) rigorous coupled wave analysis (RCWA). The calculation reveals that the grating of sinusoidal profile has greater transmittance than that of triangular profile which is in the same trend with the experimental results. These results provide a guideline for improvement of the LED light extraction.

We report a simple and inexpensive method to enhance the light output efficiency of a GaN-based light-emitting diode (LED). The method employs polydimethylsiloxane (PDMS) films prepared by nanoimprinting on the surface. Two kinds of PDMS films were prepared: one without a pattern and the other with a triangular pattern. After covering with a PDMS film, the light output was increased by about 25% for the LED with a no-pattern film, and about 33% for that with a triangular pattern film. These results can be explained by the surface roughening between the PDMS/air interfaces, which give more opportunity for photons generated in the LED active layer to escape. The research suggests that it is applicable to prepare patterned films by a nanoimprint technique to improve the light output of a GaN- based LED.

A microfluidic device combined with the microwell array and optical tweezers was set up for cell manipulation, localization and cultivation. Yeast cells were manipulated by a 1,064 nm laser and transferred to microwell array as a demonstration. The flow velocities at which the yeast cell can be confined in microwells of different sizes are charactered. The simulation of the cell's flow trace in the microwell at different flow velocities is consisting with our experiment result. And we also proved a trapping laser power of 0.30 W is harmless for yeast cell cultivation. As a simple approach, this method can push forward the cell cultivation, cell interaction and other cell biology or biomedical studies in microfluidic system.

Modification of silicon nitride nanopores under electron beam (e-beam) irradiation was investigated using a scanning electron microscope (SEM). Under e-beam irradiation, all pores with diameters ranging from 40to200nm undergo shrinkage, and the shrinkage rate increases with the rate of energy deposition. By using the selected-area scanning tool in the SEM, the silicon nitride nanopores can be selectively reshaped based on localized e-beam irradiation, with a characteristic dimension smaller than 10nm. A selected-area shaping technique was proposed to controllably shrink and shape the nanopores to a special structure.

To keep bacteria growing in a monolayer is important for studying the cellular dynamics of single cells in growing population. In this paper we report on a microfluidic system for long-term '2-Dimension' culture of bacteria, which allows the bacteria to grow freely in a micro-channel without forming multilayer structure. The culture environment is well controlled to be constant and it can be easily changed from one to another if necessary. This system offers a versatile platform for many research fields such as gene expression dynamics, nois-induced diversity between individuals and familial phylogenetics.

This report describes a method for patterning multiple types of adherent cells on the same substrate by electrochemical desorption of self-assembled monolayers (SAMs) in localized areas defined by a microfluidic system. Several groups have previously reported techniques that allow the patterning of two different types of cells. None of these reported techniques, however, could both confine two or more types of cells to specific locations on surfaces without the presence of physical constraints and control the motility of these different types of cells. 4, 5] The technique presented herein will be useful for a number of biological systems, such as in the studies of neuronal development and in the control of tumor growth. Our method employs a commercially available thiol (HS(CH2)11(OCH2CH2)6OH, abbreviated as “EG6”) to form a SAM on the gold surface, which resists adsorption of proteins and adhesion of cells (for convenience, we call this surface the “inert surface”). A poly(dimethylsiloxane) (PDMS) stamp with embedded microfluidic channels is used to carry out selective electrochemical desorption of EG6 from the gold substrate (Figure 1). [7] This procedure allows parts of the inert surface to promote the adsorption of proteins and the adhesion of cells (we call this transformation “activation of the inert surface”). Each of these individually addressable microchannels can deliver one type of cell to activated regions of the surface, resulting in a pattern of multiple types of cells on the surface. Because an electrochemical reaction can take place only in areas exposed to microfluidic channels, patterned cells are confined to activated regions, which are defined by these channels upon removal of the stamp that carries the fluidic system. As there is no physical barrier between these cells, there is a free exchange of substances between these different types of cells through the liquid medium. This exchange allows the studies of cell–cell interactions when different types of cells are confined to separate locations on the surface. A second step of electrochemical desorption can “turn on” motility of cells and allow them to move under the influence of each other. We illustrate this approach by patterning two types of cells (NIH 3T3 and Hela cells) in stripes. Fabrication of an inert surface is accomplished by coating a gold-covered glass substrate with EG6. To selectively activate the inert surface, we first coated the inert substrate with a PDMS stamp with embedded microfeatures (see the Supporting Information for its fabrication) to form enclosed microchannels. The features embedded in the PDMS stamp formed the ceilings and vertical walls and the gold substrate formed the floors of the channels; the channels were reversibly sealed. We filled the channels with solutions of the extracellular matrix (ECM) protein fibronectin (100 mgmL 1 in a phosphate-buffered Figure 1. Strategy for patterning different types of cells. a) To obtain “inert” surfaces, we formed SAMs on gold-coated coverslips with EG6. b) A PDMS stamp with an embedded microfluidic system was brought into contact with the substrate, and the channels were filled with solutions of fibronectin. Application of a cathodic potential on the gold substrates desorbed SAMs inside the channels. c)–f) Magnified views of the main functional locations of the channel system. c) Adsorption of proteins inside the microchannels after electrochemical activation of the surface. d) Adhesion of cells on the floors of the channels. e) After the PDMS stamp was peeled off, a pattern of different types of cells was formed. f) A second step of electrochemical desorption enabled cells that were previously confined in patterns to spread across the previously inert surface.

In this report, a further improvement of surface light extraction from flip-chip GaN-based LED was obtained by the micro patterning of encapsulation on the sapphire. The two dimensional taper arrays with period from 6 to 10 micron were successfully realized on polymer encapsulation by a simple and low cost technique so called embossing of thermosetting polymers. As a preliminary demonstration, at least 1.74 enhancement of the surface output intensity was achieved in the 1 mm x 1 mm GaN-based LED device under the injection current of 350 mA. (c) 2007 WILEYNCH Veriag GmbH & Co. KGaA, Weinheim.

This paper discusses a membraneless microfluidic fuel cell using the multi-stream laminar flow to keep the fuel and oxidant streams separated when they are flowing at a low Reynolds number state. The system we used consists of a microfluidic channel with three inlets. The fuel, oxidant, and the electrolyte solution enter the channel from the inlets and continue to flow in parallel without turbulent mixing (diffusive exchange occurs across the interface at a micro scale). During the working process of the fuel cell, controllable output potential and current of the fuel cell can be obtained and studied by changing the flow rate of the stream between the fuel and the oxidant. Results indicate that this novel design can be used in microscopic scale power source systems such as the lab-on-chip control systems because it is easy to integrate with other micro functional systems.

Asymmetrical properties of ion transport have been found in single conical nanopores and partly charged nano-channels. Recently, nanofluidic diodes based on this novel phenomenon have been fabricated. To generally understand the mechanism of the ionic current rectification, we study the ionic electric behaviours in several kinds of nanopores based on Poisson-Nernst-Planck equations. The calculated results show that for a partly charged nanopore, the geometry of the uncharged section, which might have been overlooked previously, has a substantial influence on current rectification. In addition, surface charge distribution is also an influential factor in current rectification. In particular, for a long homogeneously charged conical nanopore, the electrical and geometric properties of the section near the nanopore tip with a length of hundreds of nanometres are mainly responsible for the ionic current rectification. This result is consistent with the results of recent experiments on nanofluidic diodes.

How water ascends in tall trees is far from fully understood. At present, each theory suffers accusations of being inadequate and misleading. They share, none the less, one consensus that many air bubbles are p resent in xylem vessels, a feature believed to interrupt water lifting due to embolisms. Here, using a microfluidic technology to mimic xylem vessels in trees, we show that air bubbles are formed spontaneously when water ascends in artificial vessels with super-hydrophilic surface and end-wall pits. Contrary to the traditional believe, we show that air bubbles together with end-wall structures of xylem help water to ascend to a level that cannot be achieved in a continuous water column; the water ascent persists even in the presence of a small pressure gradient despite large numbers of air bubbles. The result may contribute a new explanation for water movement in higher plants.

Recently, the experimentally observed asymmetric properties of ion transport in charged conical nanopores (CCNs) that resemble those in biological ion channels have attracted a lot of attention in theoretical studies in nanotechnology research. In this paper, we report several tactics to study this effect by directly solving the Poisson-Nernst-Planck (PNP) equations. The result shows that PNP equations can indeed quantitatively describe the properties of these nanopores. Based on our numerical solutions, we contribute the rectification effect to ion-enrichment and ion-depletion. A detailed study of length dependence of current indicates that a relatively long length is indispensable for the CCNs to have rectification effect. We suggest that PNP equations and the calculation method could be further used to study other shapes of nanopores.

Optical properties of GaN-based light emission diode (LED) cooled with microfluidics have been studied as a function of injection currents at different water flow rates. Without cooling, the quantum efficiency of the LED emission degrades for large injection currents because of the limited heat dissipation. With microfluidic cooling, the maximum achievable injection current and the maximum light emission intensity increase with the water flow rate. Correspondingly, the red-shifts of the emission peaks are reduced in the whole range of the injection currents when the LED is cooled. Such a simple technique should also be applicable to other types of LEDs or high density LED arrays.

Analysis of the bending modulus of individual silicon nitride nanobelts in elastic regime is reported here. The nanobelts have the size between 200∼800 nm in width, and thickness 20∼50 nm. Atomic force microscopy was used to image and to perform measurements of force versus bending displacement on individual nanobelts suspending over strips. The bending modulus Eb is deduced by comparison of the measured force curves on the substrate and on the suspending nanobelts. It is shown that the elastic modulus of the silicon nitride nanobelts is about 570 GPa, which is much larger than that of bulk and film of the silicon nitride material. The larger elastic modulus is ascribed to the fact there are less structural defects in the silicon nitride nanobelts.

An electrochemical detection method was introduced for aqueous droplet analysis in oil phase of microfluidic devices. This method is based on the electrochemical signal difference between aqueous and oil. Applying a low alternating current (AC) voltage to a couple of Au microelectrodes, this method can offer size information and ion concentration range from 0.02 mmol/L to 1 mol/L of tens of picoliter to nanoliter aqueous droplets. Alternatively, applying a relative high AC voltage (18 Vpp) at a frequency of 1 kHz leads to electroporation of yeast cells encapsulated into picoliter droplets. We believe that this simple technique is useful for a number of aqueous droplet-based chemical and biological analyses as well as cell electroporation.