Capillary-driven Microfluidics Research Articles

Smartphone-enabled green anthocyanin sensor for Fe(III) sensing on paper using capillary-driven microfluidics

Chemical wastes, such as synthetic dyes and coloring agents, pose significant concerns due to their detrimental impact on human health and ecosystems. As an eco-friendly alternative, the study of natural dyes and coloring agents has gained momentum over the last decade, owing to their biodegradability and reduced environmental and health risks. Here, we report a novel approach employing a butterfly pea anthocyanin paper-based sensor to quantify Fe(III) ions in water. Compared to previous green sensing techniques reliant on heavy instrumentations, this approach offers enhanced accessibility, cost-effectiveness, and user-friendliness. Notably, this study is the first example of natural dye sensing on a capillary microfluidics system. The capillary-driven microfluidic system ensures user-friendly operation and allows the sensing platform to be directly immersed in water without reagent loss. The device's design involves drying the butterfly pea-derived anthocyanin reagent within a pH-controlled paper substrate embedded in the capillary-driven microfluidic system. Smartphone-based colorimetric signal generation and quantification was achieved within seconds for a working range of 50–800 mg/L, with the device's detection limit of 43 mg/L using ImageJ and 55 mg/L using the smartphone app. This green sensing methodology has the potential to be used for groundwater sources with elevated Fe(III) levels. Additionally, this platform can be used in chemistry educational settings as an effective means to understand fundamental concepts such as acid-base reactions, base strength, ionic equilibrium, and metal complexation in a user-friendly microfluidic system.

Diffusion-free valve for preprogrammed immunoassay with capillary microfluidics

By manipulating the geometry and surface chemistry of microfluidic channels, capillary-driven microfluidics can move and stop fluids spontaneously without external instrumentation. Furthermore, complex microfluidic circuits can be preprogrammed by synchronizing the capillary pressures and encoding the surface tensions of microfluidic chips. A key component of these systems is the capillary valve. However, the main concern for these valves is the presence of unwanted diffusion during the valve loading and activation steps that can cause cross-contamination. In this study, we design and validate a novel diffusion-free capillary valve: the π-valve. This valve consists of a 3D structure and a void area. The void acts as a spacer between two fluids to avoid direct contact. When the valve is triggered, the air trapped within the void is displaced by pneumatic suction induced from the capillary flow downstream without introducing a gas bubble into the circuit. The proposed design eliminates diffusive mixing before valve activation. Numerical simulation is used to study the function and optimize the dimensions of the π-valve, and 3D printing is used to fabricate either the mould or the microfluidic chip. A comparison with a conventional valve (based on a constriction-expansion valve) demonstrates that the π-valve eliminates possible backflow into the valve and reduces the mixing and diffusion during the loading and trigger steps. As a proof-of-concept, this valve is successfully implemented in a capillary-driven circuit for the determination of benzodiazepine, achieving the successive release of 3 solutions in a 3D-printed microfluidic chip without external instrumentation. The results show a 40% increase in the fluorescence intensity using the π-valve relative to the conventional value. Overall, the π-valve prevents cross-contamination, minimizes sample use, and facilitates a sophisticated preprogrammed release of fluids, offering a promising tool for conducting automated immunoassays applicable at point-of-care testing.

Capillary-driven microfluidics: impacts of 3D manufacturing on bioanalytical devices.

Over decades, decentralized diagnostics continues to move towards rapid and cost-effective testing at the point-of-care (POC). Although microfluidics has become a key enabling technology for POC testing, the need for robust peripheral equipment has been a key limiting factor in reaching an ideal device. Manufacturing technologies are now reaching a level of maturity that allows the definition of 3D features down to the sub-millimeter scale. Employing three-dimensional (3D) features and surface chemistry allows the possibility to pre-program sophisticated control of the capillary flow avoiding bulky peripheral equipment. By designing a sequence of steps, like elution of reagents, washing, mixing, and sensing, capillary valves have become a powerful tool for POC applications. These valves use capillary force to stop and then release flows within pre-programmed capillary circuits without any moving part. Without their 3D structure, the feasibility of creating pre-programmed bioanalytical devices would be nearly impossible. Besides, the advent of smart materials and their variety of surface properties permitted the unprecedented ability to fabricate reliable flow control with a range of capillary driving forces. The classification of such capillary elements is presented in two functional steps - stop and actuation. This review includes the advances in 3D microfabrication, design, and surface chemistry for manufacturing bioanalytical devices. These developments are critically reviewed, focusing on the process and considering phenomena such as timing, reproducibility, unwanted diffusion, and cross-contaminations.

Microfluidic paper-based wearable electrochemical biosensor for reliable cortisol detection in sweat

Herein, we report the first paper-based microfluidic device encompassing the filter paper to manage the flow and to load the reagents for a reagent-free competitive magnetic-bead-based immunosensor for cortisol analysis in sweat. The paper-based microfluidic pattern was made using wax printing and laser-cutter techniques, for the delivery of capillary-driven microfluidics. The presence of magnetic beads functionalized with monoclonal antibodies for the recognition of the cortisol in the reaction zone allows for the specific measurement of the target analyte. The competitive reaction between the target cortisol and the labeled cortisol with acetylcholinesterase enzyme gives a response inversely proportional to the target cortisol in the range of 10 and 140 ng/mL, by simply folding the pad loaded with the enzymatic substrate. The paper-based microfluidic device was successively combined with a Near-Field Communication wireless module to develop a flexible integrated wearable analytical tool for cortisol detection in sweat. The paper-based integrated device was successfully applied to determine the level of cortisol in sweat in one volunteer during cycling activities at two different times, demonstrating the reliability of this sustainable paper-based device.

Microscale Interfacial Polymerization on a Chip.

Forming hydrogels with precise geometries is challenging and mostly done using photopolymerization, which involves toxic chemicals, rinsing steps, solvents, and bulky optical equipment. Here, we introduce a new method for in situ formation of hydrogels with a well‐defined geometry in a sealed microfluidic chip by interfacial polymerization. The geometry of the hydrogel is programmed by microfluidic design using capillary pinning structures and bringing into contact solutions containing hydrogel precursors from vicinal channels. The characteristics of the hydrogel (mesh size, molecular weight cut‐off) can be readily adjusted. This method is compatible with capillary‐driven microfluidics, fast, uses small volumes of reagents and samples, and does not require specific laboratory equipment. Our approach creates opportunities for filtration, hydrogel functionalization, and hydrogel‐based assays, as exemplified by a rapid, compact competitive immunoassay that does not require a rinsing step.

Application of Microfluidics in Immunoassay: Recent Advancements.

In recent years, point-of-care testing has played an important role in immunoassay, biochemical analysis, and molecular diagnosis, especially in low-resource settings. Among various point-of-care-testing platforms, microfluidic chips have many outstanding advantages. Microfluidic chip applies the technology of miniaturizing conventional laboratory which enables the whole biochemical process including reagent loading, reaction, separation, and detection on the microchip. As a result, microfluidic platform has become a hotspot of research in the fields of food safety, health care, and environmental monitoring in the past few decades. Here, the state-of-the-art application of microfluidics in immunoassay in the past decade will be reviewed. According to different driving forces of fluid, microfluidic platform is divided into two parts: passive manipulation and active manipulation. In passive manipulation, we focus on the capillary-driven microfluidics, while in active manipulation, we introduce pressure microfluidics, centrifugal microfluidics, electric microfluidics, optofluidics, magnetic microfluidics, and digital microfluidics. Additionally, within the introduction of each platform, innovation of the methods used and their corresponding performance improvement will be discussed. Ultimately, the shortcomings of different platforms and approaches for improvement will be proposed.

Electro-actuated valves and self-vented channels enable programmable flow control and monitoring in capillary-driven microfluidics.

Microfluidics are essential for many lab-on-a-chip applications, but it is still challenging to implement a portable and programmable device that can perform an assay protocol autonomously when used by a person with minimal training. Here, we present a versatile concept toward this goal by realizing programmable liquid circuits where liquids in capillary-driven microfluidic channels can be controlled and monitored from a smartphone to perform various advanced tasks of liquid manipulation. We achieve this by combining electro-actuated valves (e-gates) with passive capillary valves and self-vented channels. We demonstrate the concept by implementing a 5-mm-diameter microfluidic clock, a chip to control four liquids using 100 e-gates with electronic feedback, and designs to deliver and merge multiple liquids sequentially or in parallel in any order and combination. This concept is scalable, compatible with high-throughput manufacturing, and can be adopted in many microfluidics-based assays that would benefit from precise and easy handling of liquids.

Microfluidics as an Emerging Platform for Tackling Antimicrobial Resistance (AMR): A Review

Background: Antimicrobial resistance (AMR) occurs when microbes become resistant to antibiotics causing complications and limited treatment options. AMR is more significant where antibiotics use is excessive or abusive and the strains of bacteria become resistant to antibiotic treatments. Current technologies for bacteria and its resistant strains identification and antimicrobial susceptibility testing (AST) are mostly central-lab based in hospitals, which normally take days to weeks to get results. These tools and procedures are expensive, laborious and skills based. There is an ever-increasing demand for developing point-of-care (POC) diagnostics tools for rapid and near patient AMR testing. Microfluidics, an important and fundamental technique to develop POC devices, has been utilized to tackle AMR in healthcare. This review mainly focuses on the current development in the field of microfluidics for rapid AMR testing. Method: Due to the limitations of conventional AMR techniques, microfluidic-based platforms have been developed for better understandings of bacterial resistance, smart AST and minimum inhibitory concentration (MIC) testing tools and development of new drugs. This review aims to summarize the recent development of AST and MIC testing tools in different formats of microfluidics technology. Results: Various microfluidics devices have been developed to combat AMR. Miniaturization and integration of different tools has been attempted to produce handheld or standalone devices for rapid AMR testing using different formats of microfluidics technology such as active microfluidics, droplet microfluidics, paper microfluidics and capillary-driven microfluidics. Conclusion: Current conventional AMR detection technologies provide time-consuming, costly, labor-intensive and central lab-based solutions, limiting their applications. Microfluidics has been developed for decades and the technology has emerged as a powerful tool for POC diagnostics of antimicrobial resistance in healthcare providing, simple, robust, cost-effective and portable diagnostics. The success has been reported in research articles; however, the potential of microfluidics technology in tackling AMR has not been fully achieved in clinical settings.

Immuno-gold silver staining assays on capillary-driven microfluidics for the detection of malaria antigens.

Accurate and affordable rapid diagnostic tests (RDTs) are indispensable but often lacking for many infectious diseases. Specifically, there is a lack of highly sensitive malaria RDTs that can detect low antigen concentration at the onset of infection. Here, we present a strategy to improve the sensitivity of malaria RDTs by using capillary-driven microfluidic chips and combining sandwich immunoassays with electroless silver staining. We used 5μm fluorescent beads functionalized with capture antibodies (cAbs). These beads are self-assembled by capillary action in recessed "bead lanes", which cross the main flow path of chips microfabricated in Si and SU-8. The binding of analytes to detection antibodies (dAbs) and secondary antibodies (2ndAbs) conjugated to gold nanoparticles (NPs) allows the formation of a silver film on the beads. Such silver film masks the fluorescent core of the bead inversely proportional to the concentration of antigen in a sample. We illustrate this method using the recombinant malaria antigen Plasmodium falciparum histidine-rich-protein 2 (rPfHRP2) spiked in human serum. This antigen was a recombinant HRP2 protein expressed in Escherichia coli, which is also the standard reference material. The limit of detection (LOD) of our immunoassay was found to be less than 6ngmL-1 of rPfHRP2 within 20min, which is approaching the desired sensitivity needed in the Target Product Profile (TPP) for malaria elimination settings. The concept presented here is flexible and may also be utilized for implementing fluorescence immunoassays for the parallel detection of biomarkers on capillary-driven microfluidic chips.

A bead-based immunogold-silver staining assay on capillary-driven microfluidics.

Point-of-care (POC) diagnostics are critically needed for the detection of infectious diseases, particularly in remote settings where accurate and appropriate diagnosis can save lives. However, it is difficult to implement immunoassays, and specifically immunoassays relying on signal amplification using silver staining, into POC diagnostic devices. Effective immobilization of antibodies in such devices is another challenge. Here, we present strategies for immobilizing capture antibodies (cAbs) in capillary-driven microfluidic chips and implementing a gold-catalyzed silver staining reaction. We illustrate these strategies using a species/anti-species immunoassay and the capillary assembly of fluorescent microbeads functionalized with cAbs in "bead lanes", which are engraved in microfluidic chips. The microfluidic chips are fabricated in silicon (Si) and sealed with a dry film resist. Rabbit IgG antibodies in samples are captured on the beads and bound by detection antibodies (dAbs) conjugated to gold nanoparticles. The gold nanoparticles catalyze the formation of a metallic film of silver, which attenuates fluorescence from the beads in an analyte-concentration dependent manner. The performance of these immunoassays was found comparable to that of assays performed in 96 well microtiter plates using "classical" enzyme-linked immunosorbent assay (ELISA). The proof-of-concept method developed here can detect 24.6ngmL-1 of rabbit IgG antibodies in PBS within 20min, in comparison to 17.1ngmL-1 of the same antibodies using a ~140-min-long ELISA protocol. Furthermore, the concept presented here is flexible and necessitate volumes of samples and reagents in the range of just a few microliters.

Engineering finger-operated peristaltic pumps

Peristaltic pumping has been widely adopted in microfluidic systems over the past decade. Most applications lie, however, in the regime where fluids or biospecimens are continuously pumped through the entire fluidic system, leaving the initial filling stage of fluid into an air-filled channel underexplored. We propose a compact, quasi-1D, lumped element model for describing the initial filling process of liquid into microfluidic channels driven by peristaltic pumps with discrete diaphragm valves. In addition, we experimentally demonstrated that the liquid penetration length into the fluid channel could be decently controlled (~ 0.3 mm/cycle) using human fingers as the source of actuation pressures. Moreover, we show from our experiments the possibility of controlling the profiles liquid penetration lengths to be other than Lucas–Washburn’s $$ l\,\sim\,t^{{\frac{1}{2}}} $$ dependence: Linear liquid penetration length profiles can be achieved via liquid channels with exponentially decaying cross-sectional areas. In the above ways, our model has successfully demonstrated that finger-operated peristaltic pumps can serve as an upgraded alternative for capillary-driven microfluidics, offering extra runtime adjustment capability in understanding and engineering of microfluidics in the initial filling stages.

Precision Diagnostics for Mobile Health Using Capillary-driven Microfluidics.

Precision Diagnostics for Mobile Health Using Capillary-driven Microfluidics.

Liquid Transport in Bio-Inspired Capillary-Driven Open-Air Channels

ABSTRACTWe fabricated a novel open-air channel with high efficient capillary-driven system inspired by a coastal animal “wharf roach”. The animal has open-air channels on its legs to transport water spontaneously using by capillary force. We abstracted principles controlling this phenomenon and applied it to artificial open-air channels, aiming at manipulating liquids without external energies. After surface modification for high surface free energy, the inspired open-air channels were able to transport water against gravity as well as the open-air channels of wharf roach by capillary effect of surface microstructures and chemistries. Topographical variation in micrometer-scaled patterns induced transport velocities improvement due to the enhancement of spreading intervals between the microstructure and wettability. Considering topography of micropatterned surfaces, the open-air channels with controllable transport velocity are applicable to capillary-driven microfluidics and lab-on-a-chips.

Numerical and experimental study of capillary-driven flow of PCR solution in hybrid hydrophobic microfluidic networks.

Capillary-driven microfluidics is essential for development of point-of-care diagnostic micro-devices. Polymerase chain reaction (PCR)-based micro-devices are widely developed and used in such point-of-care settings. It is imperative to characterize the fluid parameters of PCR solution for designing efficient capillary-driven microfluidic networks. Generally, for numeric modelling, the fluid parameters of PCR solution are approximated to that of water. This procedure leads to inaccurate results, which are discrepant to experimental data. This paper describes mathematical modeling and experimental validation of capillary-driven flow inside Poly-(dimethyl) siloxane (PDMS)-glass hybrid micro-channels. Using experimentally measured PCR fluid parameters, the capillary meniscus displacement in PDMS-glass microfluidic ladder network is simulated using computational fluid dynamic (CFD), and experimentally verified to match with the simulated data.

Microfluidic on CMOS with Laser Cut Adhesive Tape

CMOS integrated circuits are an ideal platform for microfluidic-based sensing because they offer small feature size at low cost and can incorporate control and sensing circuitry1. Fabricating microfluidic devices on top of CMOS presents two challenges: bonding the channel to the CMOS surface and working with the small area that is usually available. This paper presents and compares two techniques for building microfluidic networks on CMOS. Objectives The goal of this project is develop an efficient way of prototyping microfluidic devices on CMOS that can be translated into production. New Results Poly(dimethylsiloxane) (PDMS) has been widely used for making microfluidic devices on glass substrates1. Conventional microfluidic devices are formed by pouring uncured PDMS over a master. The cured slab is the peeled off from the master to punch ports and laid on a glass slide. The chamber is bonded by exposing the slide to oxygen plasma2. This technique works when the sample is analyzed under a microscope, but cannot be used on a CMOS chip. In this work, this technique was modified by using uncured PDMS as a glue to bond the cured PDMS chambers to the CMOS. To make the device, the cured chamber with punched ports was placed on the sensor and held down to hold it in place for a few seconds. Uncured PDMS was deposited on the outer edges of the channel so that the bottom of the channel is a clean CMOS sensor. The PDMS was then flash cured with a hot air gun. Tubing was then inserted at the ports before imaging. This set up was capable of imaging fluorescence sample but fabrication could not be consistently repeated. Images of the master, chamber, and the imager is shown in Fig 1. For the second method, Pressure Sensitive Adhesive (PSA) tape was used to make the microfluidic device3. For testing, we used a CO2 laser to cut the microfluidic channels out of the tape. The tape defines a channel which is 46.26 µm. In production, a die cutting process could be used. For conventional microfluidic device, the tape was stuck on a glass slide and the top layer was formed by placing a piece of clear acrylic plastic or a slab of PDMS. The thickness of the top layer serves to hold the inlet and outlet tubing. We have observed that PDMS forms a tighter seal around the tubing than acrylic plastic.When working on CMOS chips, the tape was stuck to the sensor and then the punched PDMS slab was placed on top to seal the chamber. Apart from occasional leaks at the input and output ports, acrylic plastic also filtered most of the UV light during fluorescence imaging. An image of the PDMS, PSA tape, and the imager is shown in Fig 2.Figure1: Microfluidic device made from PDMS (A) Master made from copper plates (B) Cured PDMS microfluidic chamber (C) Sealed chamber on CMOS imagerFigure2: Microfluidic device made from PSA tape (A) Top layer made of PDMS (B) Patterned PSA tape (C) Sealed chamber on CMOS imager Conclusion This microfluidic fabrication technique is repeatable and can be used to make microfluidic chambers with any dimensions and patterns. These on-sensor lensless imaging devices can be used for sample detection during fluorescence imaging and white light imaging, studying sample interaction, fluid dynamics, and for implementation of a low cost, portable, flow cytometer. References 1 D Psaltis, S Quake, C Yang, “Development optofluidic technology through the fusion of microfluidics and optics”, Nature, Vol 442, 20062 J Friend, A Yeo, “Fabrication of microfluidic device using polydimethylsiloxane”, Biomicrofluidics, 4, 20103 T Merian, F He, H Yan, D Chu, J Talbert, J Goddard, S Nugen, “Development and surface characterization of an electrowetting valve for capillary-driven microfluidics”, Colloids and Surfaces A: Physiochem. Eng Aspects 414, pp. 251-258, 2012

Development and surface characterization of an electrowetting valve for capillary-driven microfluidics

This article presents the development of a microfluidic valve to be used in capillary flow microfluidic devices. The valve used the principle of electrowetting and was able to be actuated at low voltage. The valve consisted of two inkjet-printed silver electrodes which were encountered in series within a microfluidic channel. The second electrode was modified with a hydrophobic monolayer resulting in a cessation of capillary flow. A potential of 4V resulted in a 70° reduction in apparent water contact angle within 10s which allowed capillary flow to continue. The electrode surface chemistry was investigated prior to monolayer deposition, after monolayer deposition and following the application of the 4V potential. The flexible microfluidic channel consisted of two layers of polyethylene terephthalate bonded by a pressure sensitive adhesive layer which was patterned with a laser ablated microfluidic channel. The final device represented a microfluidic valve for capillary flow microfluidics realized on a flexible substrate. The valve was designed to allow timed fluid delivery for low-cost lab-on-a-chip applications.

Capillary soft valves for microfluidics

Capillary-driven microfluidics are simple to use and provide the opportunity to perform fast biological assays with nanogram quantities of reagents and microliters of sample. Here we describe capillary soft valves (CSVs) as a simple-to-implement and -actuate approach for stopping liquids in capillary-driven microfluidics. CSVs are inserted between wettable microstructures and work to block liquids owing to a capillary pressure barrier of a few kPa. This barrier is suppressed by pressing down the soft cover of the CSV using, for example, the tip of a pen. CSVs comprise a hard layer (in silicon or polymer) with wettable microstructures and a soft cover made of poly(dimethylsiloxane) (PDMS) here. CSVs have a footprint as small as 0.6 mm(2). We illustrate how these valves work in the context of detecting DNA analytes. Specifically, a dsDNA target (997 bp PCR product, non-purified) was detected at concentrations of 20 and 200 nM in a sample volume of 0.7 μL and within 10 min. The assay includes melting of the dsDNA at 95 °C, annealing of a 30-base biotinylated probe at 50 °C, and intercalation of a fluorescent dye into the re-hybridized dsDNA at 25 °C. Actuation of the CSV allows the DNA target-probe-dye complexes to flow over 100 μm wide, streptavidin receptor lines. This work suggests that CSVs can fulfil the requirements set by complex assays, in which elevated temperatures and reaction with probes, dyes and capture species are needed. CSVs therefore greatly complement capillary-driven microfluidics without adding significant design, fabrication and actuation issues.

Controlled release of reagents in capillary-driven microfluidics using reagent integrators

The integration and release of reagents in microfluidics as used for point-of-care testing is essential for an easy and accurate operation of these promising diagnostic devices. Here, we present microfluidic functional structures, which we call reagent integrators (RIs), for integrating and releasing small amounts of dried reagents (ng quantities and less) into microlitres of sample in a capillary-driven microfluidic chip. Typically, a RI is less than 1 mm(2) in area and has an inlet splitting into a central reagent channel, in which reagents can be loaded using an inkjet spotter, and two diluter channels. During filling of the microfluidic chip, spotted reagents reconstitute and exit the RI with a dilution factor that relates to the relative hydraulic resistance of the channels forming the RI. We exemplify the working principle of RIs by (i) distributing ∼100 pg of horseradish peroxidase (HRP) in different volume fractions of a 1 μL solution containing a fluorogenic substrate for HRP and (ii) performing an immunoassay for C-reactive protein (CRP) using 450 pg of fluorescently labeled detection antibodies (dAbs) that reconstitute in ∼5 to 30% of a 1 μL sample of human serum. RIs preserve the conceptual simplicity of lateral flow assays while providing a great degree of control over the integration and release of reagents in a stream of sample. We believe RIs to be broadly applicable to microfluidic devices as used for biological assays.

Toward one-step point-of-care immunodiagnostics using capillary-driven microfluidics and PDMS substrates

Point-of-care diagnostics will strongly benefit from miniaturization based on microfluidics because microfluidics integrate functions that can together preserve valuable samples and reagents, increase sensitivity of a test, and accelerate mass transport limited reactions. But a main challenge is to incorporate reagents into microfluidics and to make microfluidics simple to use. Here, we integrate microfluidic functional elements, some of which were developed earlier, and reagents such as detection antibodies (dAbs), capture antibodies (cAbs) and analyte molecules for making one-step immunoassays: the integrated device only requires the addition of sample to trigger a cascade of events powered by capillary forces for effecting a sandwich immunoassay that is read using a fluorescence microscope. The microfluidic elements comprise a sample collector, delay valves, flow resistors, a deposition zone for dAbs, a reaction chamber sealed with a polydimethylsiloxane (PDMS) substrate, and a capillary pump and vents. Parameters for depositing 3.6 nL of a solution of dAb on the chip using an inkjet are optimized and the PDMS substrate is patterned with analytes, which provide a positive control, and cAbs. Various storage conditions of the patterned PDMS are investigated for up to 6 months revealing that storage with a desiccant preserved at least 51% of the activity of the cAbs. C-reactive protein (CRP), a general inflammation and cardiac marker, is detected using this one-step chip using only 5 microL of human serum by measuring fluorescent signals from 30 x 100 microm(2) areas of the PDMS substrate in the wet reaction chamber. The one-step chip can detect CRP at a concentration of 10 ng mL(-1) in less than 3 min and below 1 ng mL(-1) within 14 min. The work presented here may spur the adoption of fluorescence immunoassays using capillary driven microfluidics and PDMS substrates for point-of-care diagnostics.

Cellular microarrays for use with capillary-driven microfluidics

We present a method for the facile arraying of cells on microstructured substrates which should be suitable for cellular assays in autonomous microfluidic capillary systems (CSs). The CSs, which were designed and microfabricated in Si, have various microfluidic functional elements including reaction chambers wherein cellular arrays are located. Two methods for arraying the cells were explored. In the first method, a hydrophobic alkanethiol was microcontact-printed on the bottom surface of a microfluidic reaction chamber. The subsequent adsorption of protein-repellent alkanethiols around the printed areas and the deposition from solution of fibronectin (FN) on the hydrophobic areas resulted in an adhesive pattern for the attachment of living human breast cancer cells. This method was limited by the formation of cellular clusters, which proved difficult to remove selectively. The second method employed a poly(dimethylsiloxane) elastomer having oval recessed microstructures. The selective coating of the inner walls of the ovals with FN and the blocking of the mesas around the ovals with bovine serum albumin (BSA) permitted single or multiple cells to be arrayed depending on the size of the ovals. The possibility of sealing CSs with cells arrayed on poly(dimethylsiloxane) may provide a versatile platform for high-throughput experimentation down to the single-cell level.