Suppression of non-specific adsorption using sheath flow

Microfluidic biosensing of circulating tumor cells (CTCs): Recent progress and challenges in efficient diagnosis of cancer

Cancer metastasis is one of the foremost causes of cancer incidence and fatality in the whole of the world. Circulating tumor cells (CTC) have been confirmed to be among the most significant stimuli of metastasis in recent years and presently are the subject of extensive research aiming to be accurately identified by using biological and physical properties. Among the various studies conducted for isolation, identification, and characterization of CTCs, microfluidic systems have aroused great attention owing to their unique advantages such as low-cost, simplicity, reduction in reagent consumption, miniaturization, fast and precise control. The purpose of this review is to provide an overview of current state of the microfluidic biosensors for the screening of CTCs. Additionally, given the recent progress in this field, future outlook for the development of the microfluidics biosensing is briefly discussed.

A vascular-liver chip for sensitive detection of nutraceutical metabolites from human pluripotent stem cell derivatives

Human pluripotent stem cell (hPSC) is a great resource for generating cell derivatives for drug efficiency testing. Metabolites of nutraceuticals can exert anti-inflammatory effects on blood vessels. However, the concentration of nutraceutical metabolites produced in hPSC-derived hepatocytes (hPSC-HEPs) is usually low. To enable the detection of these metabolites under the in vitro environment, we have developed a co-culture model consisting of parallel co-culture chambers and a recirculating microfluidic system with minimum fluid volume, optimal cell culture environment. The model allows cells to be exposed continuously to nutraceutical metabolites. In this perfused culturing model, hPSC-derived endothelial cells and hPSC-HEPs are co-cultured without physical contact. When an anti-inflammatory nutraceutical, quercetin, was administrated to the co-culture, higher levels of quercetin metabolites were detected on-chip compared with static control. We further induced inflammation with Interleukin-1β in the co-culture model and measured interleukin 8 (IL-8) generation. The IL-8 level was suppressed more significantly by quercetin metabolites in the perfusion co-culture, as compared to static culture. This is due to enhanced metabolites production on-chip. This microfluidic co-culture model enables in vitro screening of nutraceuticals using hPSC-derived cells.

Non-Specific Adsorption Reduction Methods in Biosensing

Non-specific adsorption (NSA) is a persistent problem that negatively affects biosensors, decreasing sensitivity, specificity, and reproducibility. Passive and active removal methods exist to remedy this issue, by coating the surface or generating surface forces to shear away weakly adhered biomolecules, respectively. However, many surface coatings are not compatible or effective for sensing, and thus active removal methods have been developed to combat this phenomenon. This review aims to provide an overview of methods of NSA reduction in biosensing, focusing on the shift from passive methods to active methods in the past decade. Attention is focused on protein NSA, due to their common use in biosensing for biomarker diagnostics. To our knowledge, this is the first review to comprehensively discuss active NSA removal methods. Lastly, the challenges and future perspectives of NSA reduction in biosensing are discussed.

Two-layer Lab-on-a-chip (LOC) with passive capillary valves for mHealth medical diagnostics

There is a new potential to address needs for medical diagnostics in Point-of-Care (PoC) applications using mHealth (Mobile computing, medical sensors, and communications technologies for health care), a mHealth based lab test will require a LOC to perform clinical analysis. In this work, we describe the design of a simple Lab-on-a-chip (LOC) platform for mHealth medical diagnostics. The LOC utilizes a passive capillary valve with no moving parts for fluid control using channels with very low aspect ratios cross sections (i.e., channel width ≫ height) achieved through transitions in the channel geometry via that arrest capillary flow. Using a CO2 laser in raster engraving mode, we have designed and fabricated an eight-channel LOC for fluorescence signal detection fabricated by engraving and combining just two polymer layers. Each of the LOC channels is capable of mixing two reagents (e.g., enzyme and substrate) for various assays. For mHealth detection, we used a mobile CCD detector equipped with LED multispectral illumination in the red, green, blue, and white range. This technology enables the development of low-cost LOC platforms for mHealth whose fabrication is compatible with standard industrial plastic fabrication processes to enable mass production of mHealth diagnostic devices, which may broaden the use of LOCs in PoC applications, especially in global health settings.

CMOS spectrally-multiplexed FRET-on-a-chip for DNA analysis

A spectral-multiplexed fluorescence contact imaging microsystem for DNA analysis is presented. The microsystem integrates a filterless CMOS Color PhotoGate (CPG) sensor that exploits the polysilicon gate as an optical filter, and therefore does not require an external color filter. The CPG is applied to fluorescence-based transduction in a spectrally multiplexed format by differentiating among multiple emission bands, hence replacing the functionality of a bank of emission filters. A microsystem has been quantitatively modeled and prototyped based on the CPG fabricated in a standard 0.35 μm CMOS technology. The multi-color imaging capability of the microsystem in analyzing DNA targets has been validated in the detection of marker gene sequences for spinal muscular atropy disease and Escherichia coli (E. coli). Spectral-multiplexing enables the two DNA targets to be simultaneously detected with a measured detection limits of 240 nM and 210 nM for the two target concentrations at a sample volume of 10 μL for the green and red transduction channels, respectively.

Charged-coupled device (CCD) detectors for Lab-on-a Chip (LOC) optical analysis

A critical element of any Lab-on-a-Chip (LOC) is a detector; among the many detection approaches, optical detection is very widely used for biodetection. One challenge for advancing the development of LOC for biodetection has been to enhance the portability and lower the cost for Point-of-Care diagnostics, which has the potential to enhance the quality of healthcare delivery for underserved populations and for global health. We describe a simple and relatively low cost charged-coupled device (CCD)-based detector that can be integrated with a conventional microtiter plate or a portable LOC assay for various optical detection modalities including fluorescence, chemiluminescence, densitometry, and colorimetric assays. In general, the portable battery-operated CCD-based detection system consists of four modules: (1) a cooled CCD digital camera to monitor light emission, (2) a LOC or microtiter plate to perform assays, (3) a light source to illuminate the assay (such as electroluminescence (EL) or light emitting diode (LED)), and (4) a portable computer to acquire and analyze images. The configuration of the fluorescence detector presented here was designed to measure fluorogenic excitation at 490 nm and to monitor emission at 523 nm used for FITC detection.The LOC used for this detection system was fabricated with laminated object manufacturing (LOM) technology, and was designed to detection activity of botulinum neurotoxin serotype A (BoNT-A) using a fluorogenic peptide substrate (SNAP-25) for botulinum neurotoxin serotype A (BoNT-A) labeled with FITC. The limit of detection (LOD) for the CCD detector is 0.5 nM (25 ng/ml). The portable system is small and is powered by a 12 V source. The modular detector was designed with easily interchangeable LEDs, ELs, filters, lenses, and LOC, and can be used and adapted for a wide variety of densitometry, florescence and colorimetric assays.

An ELISA Lab-on-a-Chip (ELISA-LOC)

Laminated object manufacturing (LOM) technology using polymer sheets is an easy and affordable method for rapid prototyping of Lab-on-a-Chip (LOC) systems. It has recently been used to fabricate a miniature 96 sample ELISA lab-on-a-chip (ELISA-LOC) by integrating the washing step directly into an ELISA plate. LOM has been shown to be capable of creating complex 3D microfluidics through the assembly of a stack of polymer sheets with features generated by laser micromachining and by bonding the sheets together with adhesive. A six layer ELISA-LOC was fabricated with an acrylic (poly(methyl methacrylate) (PMMA)) core and five polycarbonate layers micromachined by a CO(2) laser with simple microfluidic features including a miniature 96-well sample plate. Immunological assays can be carried out in several configurations (1 × 96 wells, 2 × 48 wells, or 4 × 24 wells). The system includes three main functional elements: (1) a reagent loading fluidics module, (2) an assay and detection wells plate, and (3) a reagent removal fluidics module. The ELISA-LOC system combines several biosensing elements: (1) carbon nanotube (CNT) technology to enhance primary antibody immobilization, (2) sensitive ECL (electrochemiluminescence) detection, and (3) a charge-coupled device (CCD) detector for measuring the light signal generated by ECL. Using a sandwich ELISA assay, the system detected Staphylococcal enterotoxin B (SEB) at concentrations as low as 0.1 ng/ml, a detection level similar to that reported for conventional ELISA. ELISA-LOC can be operated by a syringe and does not require power for operation. This simple point-of-care (POC) system is useful for carrying out various immunological assays and other complex medical assays without the laboratory required for conventional ELISA, and therefore may be more useful for global healthcare delivery.

Microfluidic-based biosensors toward point-of-care detection of nucleic acids and proteins

This article reviews state-of-the-art microfluidic biosensors of nucleic acids and proteins for point-of-care (POC) diagnostics. Microfluidics is capable of analyzing small sample volumes (10^-9-10^-18 l) and minimizing costly reagent consumption as well as automating sample preparation and reducing processing time. The merger of microfluidics and advanced biosensor technologies offers new promises for POC diagnostics, including high-throughput analysis, portability and disposability. However, this merger also imposes technological challenges on biosensors, such as high sensitivity and selectivity requirements with sample volumes orders of magnitude smaller than those of conventional practices, false response errors due to non-specific adsorption, and integrability with other necessary modules. There have been many prior review articles on microfluidic-based biosensors, and this review focuses on the recent progress in last 5 years. Herein, we review general technologies of DNA and protein biosensors. Then, recent advances on the coupling of the biosensors to microfluidics are highlighted. Finally, we discuss the key challenges and potential solutions for transforming microfluidic biosensors into POC diagnostic applications.

ELISA-LOC: lab-on-a-chip for enzyme-linked immunodetection

A miniature 96 sample ELISA-lab-on-a-chip (ELISA-LOC) was designed, fabricated, and tested for immunological detection of Staphylococcal Enterotoxin B (SEB). The chip integrates a simple microfluidics system into a miniature ninety-six sample plate, allowing the user to carry out an immunological assay without a laboratory. Assay reagents are delivered into the assay plate without the need for separate devices commonly used in immunoassays. The ELISA-LOC was constructed using Laminated Object Manufacturing (LOM) technology to assemble six layers with an acrylic (poly(methyl methacrylate) (PMMA)) core and five polycarbonate layers micromachined by a CO(2) laser. The ELISA-LOC has three main functional elements: reagent loading fluidics, assay and detection wells, and reagent removal fluidics, a simple "surface tension" valve used to control the flow. To enhance assay sensitivity and to perform the assay without a lab, ELISA-LOC detection combines several biosensing elements: (1) carbon nanotube (CNT) technology to enhance primary antibody immobilization, (2) sensitive ECL (electrochemiluminescence) detection, and (3) a charge-coupled device (CCD) detector for measuring the light signal generated by ECL. Using a sandwich ELISA assay, the system detected SEB at concentrations as low as 0.1 ng ml(-1), which is similar to the reported sensitivity of conventional ELISA. The fluidics system can be operated by a syringe and does not require power for operation. This simple point-of-care (POC) system is useful for carrying out various immunological assays and other complex medical assays without a laboratory.

Lab-On-a-Chip for carbon nanotubes based immunoassay detection of Staphylococcal Enterotoxin B (SEB)

We describe a new eight channel Lab-On-a-Chip (LOC) for a Carbon Nanotube (CNT) based immunoassay with optical detection of Staphylococcal Enterotoxin B (SEB) for food safety applications. In this work, we combined four biosensing elements: (1) CNT technology for primary antibody immobilization, (2) Enhanced Chemiluminescence (ECL) for light signal generation, (3) a cooled charge-coupled device (CCD) for detection and (4) polymer lamination technology for developing a point of care immunological assay for SEB detection. Our concept for developing versatile LOCs, which can be used for many different applications, is to use a modular design with interchangeable recognition elements (e.g. various antibodies) to determine the specificity. Polymer lamination technology was used for the fabrication of a six layer, syringe operated LOC capable of analyzing eight samples simultaneously. An anti-SEB antibody-nanotube mixture was immobilized onto a polycarbonate strip, to serve as an interchangeable ligand surface that was then bonded onto the LOC. SEB samples are loaded into the device and detected by an ELISA assay using Horse Radish Peroxidase (HRP) conjugated anti-SEB IgG as a secondary antibody and ECL, with detection by a previously described portable cooled CCD detector. Eight samples of SEB in buffer or soy milk were assayed simultaneously with a limit of detection of 0.1 ng mL(-1). CNT immobilization of the antibody increased the sensitivity of detection six fold. Use of a simple interchangeable immunological surface allows this LOC to be adapted to any immunoassay by simply replacing the ligand surface. A syringe was used to move fluids for this assay so no power is needed to operate the device. Our versatile portable point-of-care CCD detector combined with the LOC immunoassay method described here can be used to reduce the exposure of users to toxins and other biohazards when working outside the lab, as well as to simplify and increase sensitivity for many other types of immunological diagnostics and detection assays.

Imaging of surfaces by concurrent surface plasmon resonance and surface plasmon resonance-enhanced fluorescence

Surface plasmon resonance imaging and surface plasmon induced fluorescent are sensitive tools for surface analysis. However, existing instruments in this area have provided limited capability for concurrent detection, and may be large and expensive. We demonstrate a highly cost-effective system capable of concurrent surface plasmon resonance microscopy (SPRM) and surface plasmon resonance-enhanced fluorescence (SPRF) imaging, allowing for simultaneous monitoring of reflectivity and fluorescence from discrete spatial regions. The instrument allows for high performance imaging and quantitative measurements with surface plasmon resonance, and surface plasmon induced fluorescence, with inexpensive off-the-shelf components.

Lab-on-a-chip for botulinum neurotoxin a (BoNT-A) activity analysis

A Lab-on-a-chip (LOC) was designed, fabricated and tested for the in vitro detection of botulinum neurotoxin serotype A (BoNT-A) activity using an assay that measures cleavage of a fluorophore-tagged peptide substrate specific for BoNT-A (SNAP-25) by the toxin light chain (LcA). LcA cleavage was detected by Förster Resonance Energy Transfer (FRET) fluorescence. FRET fluorescence was measured by a newly developed portable charge-coupled device (CCD) fluorescent detector equipped with multi-wavelength light-emitting diodes (LED) illumination. An eight V-junction microchannel device for BoNTs activity assays was constructed using Laminated Object Manufacturing (LOM) technology. The six-layer device was fabricated with a Poly(methyl methacrylate (PMMA) core and five polycarbonate (PC) layers micromachined by CO2 laser. The LOC is operated by syringe and is equipped with reagents, sample wells, reaction wells, diffusion traps (to avoid cross contamination among channels) and waste reservoirs. The system was detected LcA at concentrations as low as 0.5 nM, which is the reported sensitivity of the SNAP-25 in vitro cleavage assay. Combined with our CCD detector, the simple point of care system enables the detection of BoNTs activity and may be useful for the performance of other complex medical assays without a laboratory. This approach may realize the potential to enhance the quality of health care delivery for underserved populations.

Dynamic bioprocessing and microfluidic transport control with smart magnetic nanoparticles in laminar-flow devices

In the absence of applied forces, the transport of molecules and particulate reagents across laminar flowstreams in microfluidic devices is dominated by the diffusivities of the transported species. While the differential diffusional properties between smaller and larger diagnostic targets and reagents have been exploited for bioseparation and assay applications, there are limitations to methods that depend on these intrinsic size differences. Here a new strategy is described for exploiting the sharply reversible change in size and magnetophoretic mobility of "smart" magnetic nanoparticles (mNPs) to perform bioseparation and target isolation under continuous flow processing conditions. The isolated 5 nm mNPs do not exhibit significant magnetophoretic velocities, but do exhibit high magnetophoretic velocities when aggregated by the action of a pH-responsive polymer coating. A simple external magnet is used to magnetophorese the aggregated mNPs that have captured a diagnostic target from a lower pH laminar flowstream (pH 7.3) to a second higher pH flowstream (pH 8.4) that induces rapid mNP disaggregation. In this second dis-aggregated state and flowstream, the mNPs continue to flow past the magnet rather than being immobilized at the channel surface near the magnet. This stimuli-responsive reagent system has been shown to transfer 81% of a model protein target from an input flowstream to a second flowstream in a continuous flow H-filter device.

Monolayers of 3-mercaptopropyl-amino acid to reduce the nonspecific adsorption of serum proteins on the surface of biosensors

Monolayers prepared with polar or ionic amino acids with short side chains have a reduced nonspecific adsorption of serum proteins compared to that of hydrophobic amino acids and organic monolayers immobilized on the gold surface of surface plasmon resonance (SPR) biosensors. Proteins contained in biological samples adsorb on most surfaces, which in the case of biosensors causes a nonspecific response that hinders the quantification of biomarkers in these biological samples. To circumvent this problem, self-assembled monolayers (SAM) of N-3-mercaptopropyl-amino acids (3-MPA-amino acids) were prepared from 19 natural amino acids. These SAM were investigated to limit the nonspecific adsorption of proteins contained in biological fluids and to immobilize molecular receptors (i.e., antibodies) that are necessary in the construction of biosensors. SPR and Ge attenuated total reflection (GATR) FTIR spectroscopy were employed to characterize the formation of the amino acid SAMs. Monolayers of 3-MPA-amino acids densely packed on the surface of the SPR biosensors result in a surface concentration of approximately 10 (15) molecules/cm (2). SPR also quantifies the surface concentration of serum proteins nonspecifically adsorbed on 3-MPA-amino acids following the exposure of the biosensor to undiluted bovine serum. The concentration of nonspecifically bound proteins ranged from approximately 400 ng/cm (2) with polar and ionic amino acids to approximately 800 ng/cm (2) with amino acids of increased hydrophobicity. The nonspecific adsorption of serum proteins on the 3-MPA-amino acids increases in the following order: Asp < Asn < Ser < Met < Glu < Gln < Thr < Gly < His < Cys < Arg < Phe < Trp < Val < Pro < Ile < Leu < Ala < Tyr. The analysis of the adsorption and desorption curves for serum proteins on the SPR sensorgram has demonstrated the strong irreversibility of the protein adsorption on each surface. The effective hydrophilicity of the SAMs was measured from the contact angle with a saline buffer and has demonstrated that surfaces minimizing the contact angle with PBS performed better in serum. The antibody for beta-lactamase was immobilized on a 3-MPA-glycine SAM, and beta-lactamase was detected in the nanomolar range. The presence of beta-lactamase is an indicator of antibiotic resistance.

Micro-fluidic and lab-on-a-chip technology

By reducing the operational dimensions of a conventional macrofluidic-based system down to the micron scale, one can not only reduce the sample volume, but also access a range of unique characteristics, which are not achievable in conventional macro-scale systems. This chapter will discuss the unique properties of miniaturised systems based on micro-fluidic and Lab-on-a-Chip technology and consider how these may influence the overall performance associated with chemical and biological processing. Some consideration will also be given to the selection of materials and/or surface modifications that will be proactive in exploiting the high surface area and thermal and mass transfer properties, to enhance process performance.

Lamination-based rapid prototyping of microfluidic devices using flexible thermoplastic substrates

Transposing highly sensitive DNA separation methods (such as DNA sequencing with high read length or the detection of point mutations) to microchip format without loss of resolution requires fabrication of relatively long (approx. 10 cm) microchannels along with sharp injection bands. Conventional soft lithography methods, such as mold casting or hot-embossing in a press, are not convenient for fabricating long channels. We have developed a lamination-based replication technique for rapid fabrication of sealed microfluidic devices with a 10 cm long, linear separation channel. These devices are fabricated in thin cyclo-olefin copolymer (COC) plastic substrates, thus making the device flexible and capable of assuming a range of 3-D configurations. Due to the good optical properties of COC, this new family of devices combines multiple advantages of planar microfluidics and fused-silica capillaries.

A method for characterizing adsorption of flowing solutes to microfluidic device surfaces

We present a method for characterizing the adsorption of solutes in microfluidic devices that is sensitive to both long-lived and transient adsorption and can be applied to a variety of realistic device materials, designs, fabrication methods, and operational parameters. We have characterized the adsorption of two highly adsorbing molecules (FITC-labeled bovine serum albumin (BSA) and rhodamine B) and compared these results to two low adsorbing species of similar molecular weights (FITC-labeled dextran and fluorescein). We have also validated our method by demonstrating that two well-known non-fouling strategies [deposition of the polyethylene oxide (PEO)-like surface coating created by radio-frequency glow discharge plasma deposition (RF-GDPD) of tetraethylene glycol dimethyl ether (tetraglyme, CH(3)O(CH(2)CH(2)O)(4)CH(3)), and blocking with unlabeled BSA] eliminate the characteristic BSA adsorption behavior observed otherwise.

Microfluidic Flow-Flash: Method for Investigating Protein Dynamics

We report a new method, microfluidic flow-flash, for measuring protein reaction kinetics. The method couples a microscope imaging detection system with a microfluidic flow cell to reduce data acquisition times and sample consumption. This combination allows for the simultaneous collection of spectral and temporal information. The microfluidic flow cell design utilizes three-dimensional sheath flow to reduce sample dispersion and minimize sample consumption. The ability to alter the flow rates in the microfluidic flow cells allows a variety of time scales to be studied with submillisecond time resolution. The imaging detection system can be coupled with several spectroscopic probes including fluorescence and UV/visible absorbance spectroscopy. Here, we utilize the microfluidic flow-flash method to probe the kinetics of CO recombination or O2 binding to myoglobin after the laser-induced photolysis of CO from myoglobin by UV/visible absorbance spectral imaging.

Achieving Uniform Mixing in a Microfluidic Device: Hydrodynamic Focusing Prior to Mixing

We describe a microfluidic mixer that is well-suited for kinetic studies of macromolecular conformational change under a broad range of experimental conditions. The mixer exploits hydrodynamic focusing to create a thin jet containing the macromolecules of interest. Kinetic reactions are triggered by molecular diffusion into the jet from adjacent flow layers. The ultimate time resolution of these devices can be restricted by premature contact between co-flowing solutions during the focusing process. Here, we describe the design and characterization of a mixer in which hydrodynamic focusing is decoupled from the diffusion of reactants, so that the focusing region is free from undesirable contact between the reactants. Uniform mixing on the microsecond time scale is demonstrated using a device fabricated by imprinting optical-grade plastic. Device characterization is carried out using fluorescence correlation spectroscopy (FCS) and two-photon microscopy to measure flow speeds and to quantify diffusive mixing by monitoring the collisional fluorescence quenching, respectively. Criteria for achieving microsecond time resolution are described and modeled.

Controlled microfluidic interfaces

The microfabrication technologies of the semiconductor industry have made it possible to integrate increasingly complex electronic and mechanical functions, providing us with ever smaller, cheaper and smarter sensors and devices. These technologies have also spawned microfluidics systems for containing and controlling fluid at the micrometre scale, where the increasing importance of viscosity and surface tension profoundly affects fluid behaviour. It is this confluence of available microscale engineering and scale-dependence of fluid behaviour that has revolutionized our ability to precisely control fluid/fluid interfaces for use in fields ranging from materials processing and analytical chemistry to biology and medicine.

Diffusion based analysis in a sheath flow microchannel: the sheath flow T-sensor

This paper describes a microfluidic channel that allows for diffusion-based analysis of adsorbing species without passivation of the channel surfaces. The sheath flow configuration was used to measure the diffusion coefficient of fluorescently labeled species from their spatial distribution within the microchannel by analyzing the derivative of the intensity profile at the interface between two distinct core fluids. Measurements for both a small molecule (rhodamine B) and an intermediate-sized protein (wheat germ agglutinin) were made, demonstrating the utility of the sheath flow T-sensor.

Imaging of peptide adsorption to microfluidic channels in a plastic compact disc using a positron emitting radionuclide

A method for studying peptide-surface interactions within microfluidic channels by radionuclide imaging is described. With the high surface area-to-volume ratio of channels in miniaturised devices, combined with low amounts of analyte, non-specific peptide adsorption is a critical issue. The objective of the study was therefore to develop a method capable of direct detection of adsorbed peptide within microfluidic channels. A micro-device consisting of channels moulded in a plastic compact disc was chosen for the study, together with two selected peptides of different lengths and isoelectric point (pI) values. A bifunctional chelator, DOTA, was attached to the peptide by conjugation and labelled with the short-lived positron emitting radionuclide 68Ga. Quantitative images of radiotracer distribution within the microfluidic channels were obtained using a PhosphorImager system. The power of the method was demonstrated by the ability to clearly measure changes in adsorption when varying a number of parameters that typically affect peptide adsorption. These included surface modifications, analyte concentration, pH, and ionic strength. Additionally, two quantification methods were developed and compared. Radionuclide imaging also permitted visualisation of adsorption and release processes in microchannel chromatographic columns. The results suggest that radionuclide imaging is a suitable tool not only for the study of peptide adsorption to the microchannels presented in this study but also as a versatile tool to measure peptide-surface interactions in a wide variety of miniaturised structures and devices.