Tuning microchannel wettability and fabrication of multiple-step Laplace valves

Enhancement Mechanism of Fish-Scale Surface Texture on Flow Switching and Mixing Efficiency in Microfluidic Chips

Surface textures have a significant influence on surface-functional properties, which provide an alternative solution to create an accurate control of microfluidics flow. This paper studies the modulation ability of fish-scale surface textures on microfluidics flowing behavior on the ground of the early research on vibration machining-induced surface wettability variation. A microfluidic directional flow function is proposed by modifying the wall of the microchannel at the T-junction with different surface textures. The retention force caused by the surface tension difference between the two outlets in the T-junction is studied. In order to investigate the influence of fish-scale textures on the performance of the directional flowing valve and micromixer, T-shaped and Y-shaped microfluidic chips were fabricated. The experimental results indicated that with the aid of the fish-scale surface textures generated by vibration-assisted micromilling, directional liquid flow can be achieved at a specific input pressure range and the mixing efficiency of microfluidics can be improved dramatically.

A Review of Capillary Pressure Control Valves in Microfluidics

Microfluidics offer microenvironments for reagent delivery, handling, mixing, reaction, and detection, but often demand the affiliated equipment for liquid control for these functions. As a helpful tool, the capillary pressure control valve (CPCV) has become popular to avoid using affiliated equipment. Liquid can be handled in a controlled manner by using the bubble pressure effects. In this paper, we analyze and categorize the CPCVs via three determining parameters: surface tension, contact angle, and microchannel shape. Finally, a few application scenarios and impacts of CPCV are listed, which includes how CPVC simplify automation of microfluidic networks, work with other driving modes; make extensive use of microfluidics by open channel, and sampling and delivery with controlled manners. The authors hope this review will help the development and use of the CPCV in microfluidic fields in both research and industry.

Cytokine analysis on a countable number of molecules from living single cells on nanofluidic devices

Analysis of proteins released from living single cells is strongly required in the fields of biology and medicine to elucidate the mechanism of gene expression, cell-cell communication and cytopathology. However, as living single-cell analysis involves fL sample volumes with ultra-small amounts of analyte, comprehensive integration of entire chemical processing for single cells and proteins into spaces smaller than single cells (pL) would be indispensable to prevent dispersion-associated analyte loss. In this study, we proposed and developed a living single-cell protein analysis device based on micro/nanofluidics and demonstrated analysis of cytokines released from living single B cells by enzyme-linked immunosorbent assay. Based on our integration method and technologies including top-down nanofabrication, surface modifications and pressure-driven flow control, we designed and prepared the device where pL-microfluidic- and fL-nanofluidic channels are hierarchically allocated for cellular and molecular processing, respectively, and succeeded in micro/nanofluidic control for manipulating single cells and molecules. 13-unit operations for pL-cellular processing including single-cell trapping and stimulation and fL-molecular processing including fL-volumetry, antigen-antibody reactions and detection were entirely integrated into a microchip. The results suggest analytical performances for countable interleukin (IL)-6 molecules at the limit of detection of 5.27 molecules and that stimulated single B cells secrete 3.41 IL-6 molecules per min. The device is a novel tool for single-cell targeted proteomics, and the methodology of device integration is applicable to other single-cell analyses such as single-cell shotgun proteomics. This study thus provides a general approach and technical breakthroughs that will facilitate further advances in micro/nanofluidics, single-cell life science research, and other fields.

Inner Surface Design of Functional Microchannels for Microscale Flow Control

Fluidic flow behaviors in microfluidics are dominated by the interfaces created between the fluids and the inner surface walls of microchannels. Microchannel inner surface designs, including the surface chemical modification, and the construction of micro-/nanostructures, are good examples of manipulating those interfaces between liquids and surfaces through tuning the chemical and physical properties of the inner walls of the microchannel. Therefore, the microchannel inner surface design plays critical roles in regulating microflows to enhance the capabilities of microfluidic systems for various applications. Most recently, the rapid progresses in micro-/nanofabrication technologies and fundamental materials have also made it possible to integrate increasingly complex chemical and physical surface modification strategies with the preparation of microchannels in microfluidics. Besides, a wave of researches focusing on the ideas of using liquids as dynamic surface materials is identified, and the unique characteristics endowed with liquid-liquid interfaces have revealed that the interesting phenomena can extend the scope of interfacial interactions determining microflow behaviors. This review extensively discusses the microchannel inner surface designs for microflow control, especially evaluates them from the perspectives of the interfaces resulting from the inner surface designs. In addition, prospective opportunities for the development of surface designs of microchannels, and their applications are provided with the potential to attract scientific interest in areas related to the rapid development and applications of various microchannel systems.

Pressure-controlled microfluidic sub-picoliter ultramicro-volume syringes based on integrated micro-nanostructure arrays

Ultramicro-volume syringes were fabricated by integrating micro-nanostructure arrays in microchannels for quantitatively dispensing sub-picoliter volumes of liquids. Using this system, liquids were dispensed in volume increments as low as 0.5 pL with 96% accuracy. Specifically, the controllable synthesis of nanocrystals was achieved using a lab-on-chip platform that was integrated with the syringes.

Femtoliter nanofluidic valve utilizing glass deformation

In the field of micro/nanofluidics, the channel open/close valves are among the most important technologies for switching and partitioning actions and integration of various operations into fluidic circuits. While several types of valves have been developed in microfluidics, few are capable in nanofluidics. In this study, we proposed a femtoliter (fL) volume nanochannel open/close valve fabricated in glass substrates. The valve consists of a shallow, circular and stepped-bottom valve chamber connected to nanochannels and an actuator. Even with tiny deformation occurring at the nanolevel in glass, an open/closed state of a nanochannel (10-1000 nm) can be achieved. We designed a fL-valve based on an analytical material deformation model, and developed a valve fabrication process. We then verified the open/closed state of the valve using a 308 fL-valve chamber with a four-stepped nanostructure fitting an arc-shape of deflected glass, confirmed its stability and durability over 50 open/close operations, and succeeded in stopping/flowing an aqueous solution at 209 fL s-1 under a 100 kPa pressure in a 900 nm nanochannel with a fast response of ∼0.65 s. A leak flow from the closed valve was sufficiently small even at a 490 kPa pressure-driven flow. Since the developed fL-valve can be applied to various nanofluidic devices made of glass and other rigid materials such as plastic, it is expected that this work will contribute significantly to the development of novel integrated micro/nanofluidics chemical systems for use in various applications, such as single cell/single molecule analysis.

A review on the design and development of photocatalyst synthesis and application in microfluidic reactors: challenges and opportunities

Microfluidics is an emerging branch of science that has significant applications in various fields. In this review paper, after a brief introduction to the concept of photocatalysis, nanoparticle preparation methods and film formation techniques have been studied. Nanoparticle synthesis in microfluidic systems and microreactor types for on-chip photocatalyst synthesis and challenges of nanoparticles handling in microsystems have been reviewed. To resolve particle polydispersity and microchannel clogging, a good suggestion can be the use of droplet-based microreactors. The configurative designs for the microfluidic reactor with immobilized photocatalysts, their applications, and their challenges have been comprehensively addressed. The three main challenges ahead the immobilized photocatalytic microfluidic reactors are optimal light distribution, prevention of the recombination of photogenerated electrons and holes, and improved mass transfer. Internal light-emitting diodes with a waveguide can resolve the number one challenge of photocatalysis application in optofluidic reactors, that is, light distribution.

Transport of a Micro Liquid Plug in a Gas-Phase Flow in a Microchannel

Micro liquid droplets and plugs in the gas-phase in microchannels have been utilized in microfluidics for chemical analysis and synthesis. While higher velocities of droplets and plugs are expected to enable chemical processing at higher efficiency and higher throughput, we recently reported that there is a limit of the liquid plug velocity owing to splitting caused by unstable wetting to the channel wall. This study expands our experimental work to examine the dynamics of a micro liquid plug in the gas phase in a microchannel. The motion of a single liquid plug, 0.4⁻58 nL in volume, with precise size control in 39- to 116-m-diameter hydrophobic microchannels was investigated. The maximum velocity of the liquid plug was 1.5 m/s, and increased to 5 m/s with splitting. The plug velocity was 20% of that calculated using the Hagen-Poiseuille equation. It was found that the liquid plug starts splitting when the inertial force exerted by the fluid overcomes the surface tension, i.e., the Weber number (ratio of the inertial force to the surface tension) is higher than 1. The results can be applied in the design of microfluidic devices for various applications that utilize liquid droplets and plugs in the gas phase.

Continuous purification of active pharmaceutical ingredients utilizing polymer membrane surface wettability

Liquid–liquid extraction followed by dual membrane based phase separation in flow enables fully continuous purification of active pharmaceutical ingredients.

A Wettability Metric for Characterization of Capillary Flow on Textured Superhydrophilic Surfaces

Surface wettability is typically characterized by measuring the static contact angle of a sessile droplet placed on the surface. For extremely wetting surfaces on which liquid spontaneously spreads into a thin liquid film, the near-zero static contact angle is not amenable to measurement and does not fully describe the wetting behavior. There are unmet needs in microfluidics, boiling heat transfer enhancement, and antifogging applications for a metric to characterize highly wetting (i.e., superhydrophilic) textured surfaces based on their capillary-driven liquid pumping performance, as a supplement to the contact angle for this highly wetting regime. To describe the wetting behavior, the textured surface can be approximated as a thin porous layer through which the liquid spreads. An analytical model is developed for the volumetric flow in this layer, which reveals a single superhydrophilicity metric that captures the wetting behavior for a given liquid. A simple experimental approach is proposed to characterize this metric by measuring the volumetric liquid intake into the surface from a filled capillary tube. This approach is validated by characterizing micropillared superhydrophilic surfaces of known geometry; the predicted and measured wetting behaviors show good agreement. The metric proposed in this study offers a simple approach for accurately characterizing and differentiating highly wetting surfaces based on their liquid pumping ability.

Poly(methyl methacrylate) Surface Modification for Surfactant-Free Real-Time Toxicity Assay on Droplet Microfluidic Platform

Microfluidic droplet reactors have many potential uses, from analytical to synthesis. Stable operation requires preferential wetting of the channel surface by the continuous phase which is often not fulfilled by materials commonly used for lab-on-chip devices. Here we show that a silica nanoparticle (SiNP) layer coated onto a Poly(methyl methacrylate) (PMMA) and other thermoplastics surface enhances its wetting properties by creating nanoroughness, and allows simple grafting of hydrocarbon chains through silane chemistry. Using the unusual stability of silica sols at their isoelectric point, a dense SiNP layer is adsorbed onto PMMA and renders the surface superhydrophilic. Subsequently, a self-assembled dodecyltrichlorosilane (DTS) monolayer yields a superhydrophobic surface that allows the repeatable generation of aqueous droplets in a hexadecane continuous phase without surfactant addition. A SiNP-DTS modified chip has been used to monitor bacterial viability with a resazurin assay. The whole process involving sequential reagents injection, and multiplexed droplet fluorescence intensity monitoring is carried out on chip. Metabolic inhibition of the anaerobe Enterococcus faecalis by 30 mg L^-1 of NiCl₂ was detected in 5 min.

A 2.5-D glass micromodel for investigation of multi-phase flow in porous media

We developed a novel method for fabrication of glass micromodels with varying depth (2.5-D) with no additional complexity over the 2-D micromodels' fabrication. Compared to a 2-D micromodel, the 2.5-D micromodel can better represent the 3-D features of multi-phase flow in real porous media, as demonstrated in this paper with three different examples. Physically realistic capillary snap-off and the formation of isolated residual oil droplets were realized, which is not possible in 2-D micromodels. Droplet size variation during an emulsion flooding was investigated on the 2.5-D micromodel, showing that the droplet size decreases sharply at the inlet, with little change in size downstream of the micromodel. Displacement of light oil with ultra-low interfacial tension (IFT) surfactant was conducted in the 2.5-D micromodel, where we were able to visualize the generation and flowing of a microemulsion phase, which agrees with, and explains observations in experiments of more complex porous media.

High-Pressure Acceleration of Nanoliter Droplets in the Gas Phase in a Microchannel

Microfluidics has been used to perform various chemical operations for pL–nL volumes of samples, such as mixing, reaction and separation, by exploiting diffusion, viscous forces, and surface tension, which are dominant in spaces with dimensions on the micrometer scale. To further develop this field, we previously developed a novel microfluidic device, termed a microdroplet collider, which exploits spatially and temporally localized kinetic energy. This device accelerates a microdroplet in the gas phase along a microchannel until it collides with a target. We demonstrated 6000-fold faster mixing compared to mixing by diffusion; however, the droplet acceleration was not optimized, because the experiments were conducted for only one droplet size and at pressures in the 10–100 kPa range. In this study, we investigated the acceleration of a microdroplet using a high-pressure (MPa) control system, in order to achieve higher acceleration and kinetic energy. The motion of the nL droplet was observed using a high-speed complementary metal oxide semiconductor (CMOS) camera. A maximum droplet velocity of ~5 m/s was achieved at a pressure of 1–2 MPa. Despite the higher fluid resistance, longer droplets yielded higher acceleration and kinetic energy, because droplet splitting was a determining factor in the acceleration and using a longer droplet helped prevent it. The results provide design guidelines for achieving higher kinetic energies in the microdroplet collider for various microfluidic applications.

Interfacial Phenomena and Fluid Control in Micro/Nanofluidics

Fundamental aspects of rapidly advancing micro/nanofluidic devices are reviewed from the perspective of liquid interface chemistry and physics, including the influence of capillary pressure in microfluidic two-phase flows and phase transitions related to capillary condensation.

Spatial Site-Patterning of Wettability in a Microcapillary Tube

Substrate functionalization is of great importance in successfully manipulating flows and liquid interfaces in microdevices. Herein, we propose an alternative approach for spatial patterning of wettability in a microcapillary tube. The method combines a photolithography process with self-assembled monolayer formation. The modified microcapillaries show very sharp boundaries between the alternating hydrophilic/hydrophobic segments with an achieved smallest domain dimension down to 60 μm inside a 580 μm inner diameter capillary. Our two-step method allows us to pattern multiple types of functional groups in an enclosed channel. Such structures are promising regarding the manipulation of segmented flows inside capillaries.

Biomimetic water-collecting materials inspired by nature

Nowadays, water shortage is a severe issue all over the world, especially in some arid and undeveloped areas. Interestingly, a variety of natural creatures can collect water from fog, which can provide a source of inspiration to develop novel and functional water-collecting materials. Recently, as an increasingly hot research topic, bioinspired materials with the water collection ability have captured vast scientific attention in both practical applications and fundamental research studies. In this review, we summarize the mechanisms of water collection in various natural creatures and present the fabrications, functions, applications, and new developments of bioinspired materials in recent years. The theoretical basis related to the phenomenon of water collection containing wetting behaviors and water droplet transportations is described in the beginning, i.e., the Young's equation, Wenzel model, Cassie model, surface energy gradient model and Laplace pressure equation. Then, the water collection mechanisms of three typical and widely researched natural animals and plants are discussed and their corresponding bioinspired materials are simultaneously detailed, which are cactus, spider, and desert beetles, respectively. This is followed by introducing another eight animals and plants (butterfly, shore birds, wheat awns, green bristlegrass, the Cotula fallax plant, Namib grass, green tree frogs and Australian desert lizards) that are rarely reported, exhibiting water collection properties or similar water droplet transportation. Finally, conclusions and outlook concerning the future development of bioinspired fog-collecting materials are presented.

Biomimetic microchannels of planar reactors for optimized photocatalytic efficiency of water purification

This paper reports a biomimetic design of microchannels in the planar reactors with the aim to optimize the photocatalytic efficiency of water purification. Inspired from biology, a bifurcated microchannel has been designed based on the Murray's law to connect to the reaction chamber for photocatalytic reaction. The microchannels are designed to have a constant depth of 50 μm but variable aspect ratios ranging from 0.015 to 0.125. To prove its effectiveness for photocatalytic water purification, the biomimetic planar reactors have been tested and compared with the non-biomimetic ones, showing an improvement of the degradation efficiency by 68%. By employing the finite element method, the flow process of the designed microchannel reactors has been simulated and analyzed. It is found that the biomimetic design owns a larger flow velocity fluctuation than that of the non-biomimetic one, which in turn results in a faster photocatalytic reaction speed. Such a biomimetic design paves the way for the design of more efficient planar reactors and may also find applications in other microfluidic systems that involve the use of microchannels.

Bioinspired superhydrophobic surfaces, fabricated through simple and scalable roll-to-roll processing

A simple, scalable, non-lithographic, technique for fabricating durable superhydrophobic (SH) surfaces, based on the fingering instabilities associated with non-Newtonian flow and shear tearing, has been developed. The high viscosity of the nanotube/elastomer paste has been exploited for the fabrication. The fabricated SH surfaces had the appearance of bristled shark skin and were robust with respect to mechanical forces. While flow instability is regarded as adverse to roll-coating processes for fabricating uniform films, we especially use the effect to create the SH surface. Along with their durability and self-cleaning capabilities, we have demonstrated drag reduction effects of the fabricated films through dynamic flow measurements.

Simple bilayer on-chip valves using reversible sealability of PDMS

Simple bilayer on-chip valves exploiting the reversible sealability of PDMS were realized by patterning the non-covalent area between two parallel microchannels.

Controlled splitting and focusing of a stream of nanoparticles in a converging-diverging microchannel

We demonstrate the potential of a converging-diverging microchannel to split a stream of nanoparticles towards the interfacial region of the dispersed and the carrier phases, introduced through the middle inlet and through the remaining two inlets respectively, while maintaining a low Reynolds number limit (<10) for the flow of both phases. In addition to the splitting of passive tracer particles, such as polystyrene beads as used herein, the present setup has the potential to be utilized for a controlled reaction and thereby the separation of products towards an intended location, as observed from the experimentation with silver-nanoparticles and hydrogen-peroxide solution. Moreover, the microscale dimension of the channel allows controlled deposition of the reaction product over the bottom surface of the channel, allowing the possibility of bottom-up fabrication of microscale features. We unveil the underlying hydrodynamics that lead to such behaviours through numerical simulations, which are consistent with the experimental observations. The phenomenological features are found to be guided by the splitting of the intrinsic streamlines conforming to the flow geometry under consideration.

Anisotropic Janus Si nanopillar arrays as a microfluidic one-way valve for gas-liquid separation

In this paper, we demonstrate a facile strategy for the fabrication of a one-way valve for microfluidic (MF) systems. The micro-valve was fabricated by embedding arrays of Janus Si elliptical pillars (Si-EPAs) with anisotropic wettability into a MF channel fabricated in poly(dimethylsiloxane) (PDMS). Two sides of the Janus pillar are functionalized with molecules with distinct surface energies. The ability of the Janus pillar array to act as a valve was proved by investigating the flow behaviour of water in a T-shaped microchannel at different flow rates and pressures. In addition, the one-way valve was used to achieve gas-liquid separation. We believe that the Janus Si-EPAs modified by specific surface functionalization provide a new strategy to control the flow and motion of fluids in MF channels.

UV-driven microvalve based on a micro-nano TiO₂/SiO₂ composite surface for microscale flow control

This paper presents a novel ultraviolet (UV)-driven microvalve based on the concept of inserting a trimethyl chlorosilane (CTMS) modified TiO₂/SiO₂ composite patch of switchable wettability in a microfluidic system. A unique micro-nano hierarchical structure was designed and used to enhance the overall wetting contrast with the aim of improving the wetting-based valve performances. Field-emission scanning electron microscopy (FE-SEM) and x-ray photoelectron spectroscopy (XPS) were used to characterize the morphology and chemical composition of the surface. UV-driven wettability conversion on the patched microchannel was investigated using water column relative height tests, and the results confirmed the significant improvement of the hierarchical structure with the surface hydrophobic/hydrophilic conversion, which produced enhancements of 276% and 95% of the water-repellent and water-sucking pressures, respectively, compared with those of the single-scale TiO₂ nanopatterned structure. Accordingly, a good reversible and repeated on-off performance was identified by the valve tests, highlighting the potential application of the novel microvalve in the efficient control of microscale flow.

Single-step design of hydrogel-based microfluidic assays for rapid diagnostics

For the first time we demonstrate a microfluidic platform for the preparation of biosensing hydrogels by in situ polymerization of polyethyleneglycol diacrylate (PEG-DA) in a single step. Capillary pressure barriers enable the precise formation of gel microstructures for fast molecule diffusion. Parallel arrangement of these finger structures allows for macroscopic and standard equipment readout methods. The analyte automatically fills the space in between the gel fingers by the hydrophilic nature of the gel. Introducing the functional structures in the chip fabrication allows for rapid assay customization by making surface treatment, gel curing mask alignment and washing steps obsolete. Simple handling and functionality are illustrated by assays for matrix metalloproteinase, an important factor in chronic wound healing. Assays for total protein concentration and cell counts are presented, demonstrating the possibilities for a wide range of fast and simple diagnostics.

The future of microfluidic assays in drug development

BACKGROUND

Microfluidic technology emerges as a convenient route to applying automated and reliable assays in a high-throughput manner with low cost.

OBJECTIVE

This review aims to answer questions related to the capabilities and potential applications of microfluidic assays that can benefit the drug development process and extends an outlook on its future trends.

METHODS

This article reviews recent publications in the field of microfluidics, with an emphasis on novel applications for drug development.

RESULTS/CONCLUSION

Microfluidics affords unique capabilities in sample preparation and separation, combinatorial synthesis and array formation, and incorporating nanotechnology for more functionalities. The pharmaceutical industry, facing challenges from limited productivity and accelerated competition, can thus greatly benefit from applying new microfluidic assays in various drug development stages, from target screening and lead optimization to absorption distribution metabolism elimination and toxicity studies in preclinical evaluations, diagnostics in clinical trials and drug formulation and manufacturing process optimization.

Femtoliter droplet handling in nanofluidic channels: a Laplace nanovalve

Analytical technologies of ultrasmall volume liquid, in particular femtoliter to attoliter liquid, is essential for single-cell and single-molecule analysis, which is becoming highly important in biology and medical diagnosis. Nanofluidic chips will be a powerful tool to realize chemical processes for such a small volume sample. However, a technical challenge exists in fluidic control, which is femtoliter to attoliter liquid generation in air and handling for further chemical analysis. Integrating mechanical valves fabricated by MEMS (microelectric mechanical systems) technology into nanofluidic channels is difficult. Here, we propose a nonmechanical valve, which is a Laplace nanovalve. For this purpose, a nanopillar array was embedded in a nanochannel using a two-step electron beam lithography and dry-etching process. The nanostructure allowed precise wettability patterning with a resolution below 100 nm, which was difficult by photochemical wettability patterning due to the optical diffraction. The basic principle of the Laplace nanovalve was verified, and a 1.7 fL droplet (water in air) was successfully generated and handled for the first time.

Reversible superhydrophobic-superhydrophilic transition of ZnO nanorod/epoxy composite films

Tuning the surface wettability is of great interest for both scientific research and practical applications. We demonstrated reversible transition between superhydrophobicity and superhydrophilicity on a ZnO nanorod/epoxy composite film. The epoxy resin serves as an adhesion and stress relief layer. The ZnO nanorods were exposed after oxygen reactive ion etching of the epoxy matrix. A subsequent chemcial treatment with fluoroalkyl and alkyl silanes resulted in a superhydrophobic surface with a water contact angle up to 158.4° and a hysteresis as low as 1.3°. Under UV irradiation, the water contact angle decreased gradually, and the surface eventually became superhydrophilic because of UV induced decomposition of alkyl silanes and hydroxyl absorption on ZnO surfaces. A reversible transition of surface wettability was realized by alternation of UV illumination and surface treatment. Such ZnO nanocomposite surface also showed improved mechanical robustness.

Engineering superlyophobic surfaces as the microfluidic platform for droplet manipulation

We propose robust engineering superlyophobic surfaces (SLS) as a universal microfluidic platform for droplet manipulation enabling electric actuation, featured with characteristics of highly nonwetting, low adhesion, and low friction for various liquids including water and oil. To functionalize SLS with embedded electrodes, two configurations with continuous and discrete topologies have been designed and compared. The discrete configuration is found to be superior upon comparison of their fabrication, microstructures and nonwetting performances. We also present new formulation of SLS pressure stability for linear, square and hexagonal pattern layouts, and propose a criterion for three wetting states (the Cassie-Baxter, partial Cassie-Baxter and Wenzel states) by introducing two dimensionless parameters, which are supported by our experimental data. Droplet manipulation experiments including deformation and transport on electrode-embedded SLS were performed, showing that present SLS reduce adhesion and flow resistance of oil droplets respectively by 98% and 73% compared with a smooth hydrophobic surface, and the excellent hydrodynamic performances are applicable for a wide range of droplet velocity. Simulation of an oil droplet electrically actuated on SLS predicts the significantly increased droplet motion for a low solid fraction and a relatively large droplet size.

Biosensors in microfluidic chips

A biosensor is a sensing device that incorporates a biological sensing element and a transducer to produce electrochemical, optical, mass, or other signals in proportion to quantitative information about the analytes in the given samples. The microfluidic chip is an attractive miniaturized platform with valuable advantages, e.g., low cost analysis requiring low reagent consumption, reduced sample volume, and shortened processing time. Combination of biosensors and microfluidic chips enhances analytical capability so as to widen the scope of possible applications. This review provides an overview of recent research activities in the field of biosensors integrated on microfluidic chips, focusing on the working principles, characteristics, and applicability of the biosensors. Theoretical background and applications in chemical, biological, and clinical analysis are summarized and discussed.

Surface patterning of bonded microfluidic channels

Microfluidic channels in which multiple chemical and biological processes can be integrated into a single chip have provided a suitable platform for high throughput screening, chemical synthesis, detection, and alike. These microchips generally exhibit a homogeneous surface chemistry, which limits their functionality. Localized surface modification of microchannels can be challenging due to the nonplanar geometries involved. However, chip bonding remains the main hurdle, with many methods involving thermal or plasma treatment that, in most cases, neutralizes the desired chemical functionality. Postbonding modification of microchannels is subject to many limitations, some of which have been recently overcome. Novel techniques include solution-based modification using laminar or capillary flow, while conventional techniques such as photolithography remain popular. Nonetheless, new methods, including localized microplasma treatment, are emerging as effective postbonding alternatives. This Review focuses on postbonding methods for surface patterning of microchannels.

Constructing a superhydrophobic surface on polydimethylsiloxane via spin coating and vapor-liquid sol-gel process

In this study, a superhydrophobic surface on polydimethylsiloxane (PDMS) substrate was constructed via the proposed vapor-liquid sol-gel process in conjunction with spin coating of dodecyltrichlorosilane (DTS). Unlike the conventional sol-gel process where the reaction takes place in the liquid phase, layers of silica (SiO(2)) particles were formed through the reaction between the reactant spin-coated on the PDMS surface and vapor of the acid solution. This led to the SiO(2) particles inlaid on the PDMS surface. Followed by subsequent spin coating of DTS solution, the wrinkle-like structure was formed, and the static contact angle of the water droplet on the surface could reach 162 degrees with 2 degrees sliding angle and less than 5 degrees contact angle hysteresis. The effect of layers of SiO(2) particles, concentrations of DTS solution and surface topography on superhydrophobicity of the surface is discussed.

Fabrication of porous hierarchical polymer/ceramic composites by electron irradiation of organic/inorganic polymers: route to a highly durable, large-area superhydrophobic coating

Polymer/ceramic composite films with micro- and nanocombined hierarchical structures are fabricated by electron irradiation of poly(methyl methacrylate) (PMMA) microspheres/silicone grease. Electron irradiation induces volume contraction of PMMA microspheres and simultaneously transforms silicone grease into a ceramic material of silicon oxycarbide with many nanobumps. As a result, highly porous structures that consist of micrometer-sized pores and microparticles decorated with nanobumps are created. The fabricated films with the porous hierarchical structure exhibit good superhydrophobicity with excellent self-cleaning and antiadhesion properties after surface treatment with fluorosilane. In addition, the porous hierarchical structures are covered with silicon oxycarbide, and thus the superhydrophobic coatings have high hardness and strong adhesion to the substrate. The presented technique provides a straightforward route to producing large-area, mechanically robust superhydrophobic films on various substrate materials.

Facile approach in fabricating superhydrophobic and superoleophilic surface for water and oil mixture separation

Metal copper mesh with superhydrophobic and superoleophilic surface had been successfully fabricated via a facile solution-immersion process. The hierarchical structure was prepared on the commercial copper mesh surface by etching with the nitric acid. After being modified by 1-hexadecanethiol (HDT), the as-prepared mesh indicated both superhydrophobic and superoleophilic property simultaneously. This as-prepared metal mesh could then be applied for oil and water mixture separation. The unusual wettability of the as-prepared mesh was stable in corrosive conditions, such as acidic, basic, and salt solutions. The solution-immersion method was simple, time-saving, and inexpensive and therefore exhibited great potential application.

Parallel multiphase microflows: fundamental physics, stabilization methods and applications

Parallel multiphase microflows, which can integrate unit operations in a microchip under continuous flow conditions, are discussed. Fundamental physics, stabilization methods and some applications are shown.

Droplet handling

When quantitative analysis or quantitative chemical synthesis is performed using a micrototal analysis system (microTAS), the technologies for precise metering, transporting, and mixing of droplets are required. In this chapter, several technologies for the handling of droplets are described. For metering, dispensing and transporting of droplets, pneumatic and electrokinetic forces are used. Separation of cells and particles is also performed by electrical operation. Other handling technique, such as ultrasonic or centrifugal force applications, are also reviewed. Robotic synthesis devices or high throughput screening devices are promising applications for these technologies.

Serial DNA immobilization in micro- and extended nanospace channels

That focused arrays, even with a small set of ligands, provide more data than single point experiments is well established in the DNA microarray research field, but microarray technology has yet to be transferred to fused silica microchips. Fused silica microchips have several attractive features such as stability to pressure, solvents, acids and bases, and can be fabricated with minute dimensions, making them good candidates for nanofluidic research. However, due to harsh bonding conditions, DNA ligands must be immobilized after fabrication, thus preventing standard microarray spotting techniques from being used. In this paper, we provide tools for serial DNA immobilization in fused silica microchips using UV. We report the synthesis of a new UV-linker which was used to covalently couple functional DNA oligos to the inside of channels in fused silica microchips. With some simple modifications to our mask aligner, we were able to transfer OHP mask patterns, which allows the creation of basically any pattern in the channels. The functionality of the oligos was measured through the binding of fluorophore-labeled complementary target oligos. We examined parameters influencing DNA immobilization, and carry-over between spots after consecutive immobilizations inside the same channel. We also report the first successful multiple immobilizations of functional DNA oligos inside single channels of extended nanospace depth (460 nm).

Silver mirror reaction as an approach to construct superhydrophobic surfaces with high reflectivity

Superhydrophobic surfaces with high reflectivity might provide a promising self-cleaning approach in a wide variety of optical applications ranging from traffic to solar energy industries. However, the contradiction between the hierarchical micronanostructure and the high reflectivity is a challenge for superhydrophobic materials with high reflectivity. Here we report a facile method to fabricate a superhydrophobic silver film with reflectivity as high as that of polished silicon by carefully controlling the seed-induced silver mirror reaction.