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.

Printed Capillary Microfluidic Devices and Their Application in Biosensing.

Microfluidic devices with a free-standing structure were printed directly on polymer films using the functional materials that form interconnected pores. The printed devices can transport fluids by capillary action in the same fashion as paper-based microfluidic devices, and they can handle much smaller sample volumes than typical paper-based devices. Detection of glucose was performed using both colorimetric and electrochemical methods, and the observed limits of detection (LOD) were similar to those obtained with paper-based microfluidic devices under comparable testing conditions. It is demonstrated that printed microfluidic devices can be fabricated using printing processes that are suitable for high-volume and low-cost production and that the integration of microfluidic channels with electrodes is straightforward with printing. Several materials that are printable and form interconnected pores are presented.

Effect of TiO2 Nanoparticles on Capillary-Driven Flow in Water Nanofilters Based on Chitosan Cellulose and Polyvinylidene Fluoride Nanocomposites: A Theoretical Study.

In this study, a novel concept of nanofiltration process of drinking water based on capillary-driven nanofiltration is demonstrated using a bio-based nanocomposites' nanofilter as free power: a green and sustainable solution. Based on Lifshitz and Young-Laplace theories, we show that the chitosan (CS), cellulose acetate (CLA), and Polyvinylidene fluoride (PVDF) polymer matrixes demonstrate hydrophobic behavior, which leads to the draining of water from nanopores when negative capillary pressure is applied and consequently prevents the capillary-driven nanofiltration process. By incorporating 10%, 20%, and 30% volume fraction of titanium dioxide (TiO2) nanoparticles (NPs) to the polymers' matrixes, we demonstrate a wetting conversion from hydrophobic to hydrophilic behavior of these polymer nanocomposites. Subsequently, the threshold volume fraction of the TiO2 NPs for the conversion from draining (hydrophobic) to filling (hydrophilic) by capillary pressure were found to be equal to 5.1%, 10.9%, and 13.9%, respectively, for CS/TiO2, CLA/TiO2, and PVDF/TiO2 nanocomposites. Then, we demonstrated the negligible effect of the gravity force on capillary rise as well as the capillary-driven flow for nanoscale pore size. For nanofilters with the same effective nanopore radius, porosity, pore shape factor, and tortuosity, results from the modified Lucas-Washburn model show that the capillary rise as well as the capillary-driven water volume increase with increased volume fraction of the TiO2 NPs for all nanocomposite nanofilter. Interestingly, the capillary-driven water volume was in range (5.26-6.39) L/h·m2 with 30% volume fraction of TiO2 NPs, which support our idea for capillary-driven nanofiltration as zero energy consumption nano-filtration process. Correspondingly, the biodegradable CS/TiO2 and CLA/TiO2 nanocomposites nanofilter demonstrate capillary-driven water volume higher, ~1.5 and ~1.2 times, respectively, more than the synthetic PVDF/TiO2 nanocomposite.

Capillary-Driven Flow Microfluidics Combined with Smartphone Detection: An Emerging Tool for Point-of-Care Diagnostics.

Point-of-care (POC) or near-patient testing allows clinicians to accurately achieve real-time diagnostic results performed at or near to the patient site. The outlook of POC devices is to provide quicker analyses that can lead to well-informed clinical decisions and hence improve the health of patients at the point-of-need. Microfluidics plays an important role in the development of POC devices. However, requirements of handling expertise, pumping systems and complex fluidic controls make the technology unaffordable to the current healthcare systems in the world. In recent years, capillary-driven flow microfluidics has emerged as an attractive microfluidic-based technology to overcome these limitations by offering robust, cost-effective and simple-to-operate devices. The internal wall of the microchannels can be pre-coated with reagents, and by merely dipping the device into the patient sample, the sample can be loaded into the microchannel driven by capillary forces and can be detected via handheld or smartphone-based detectors. The capabilities of capillary-driven flow devices have not been fully exploited in developing POC diagnostics, especially for antimicrobial resistance studies in clinical settings. The purpose of this review is to open up this field of microfluidics to the ever-expanding microfluidic-based scientific community.

Design and Fabrication of Capillary-Driven Flow Device for Point-Of-Care Diagnostics.

Point-of-care (POC) diagnostics enables the diagnosis and monitoring of patients from the clinic or their home. Ideally, POC devices should be compact, portable and operatable without the requirement of expertise or complex fluid mechanical controls. This paper showcases a chip-and-dip device, which works on the principle of capillary-driven flow microfluidics and allows analytes' detection by multiple microchannels in a single microchip via smartphone imaging. The chip-and-dip device, fabricated with inexpensive materials, works by simply dipping the reagents-coated microchip consisting of microchannels into a fluidic sample. The sample is loaded into the microchannels via capillary action and reacts with the reagents to produce a colourimetric signal. Unlike dipstick tests, this device allows the loading of bacterial/pathogenic samples for antimicrobial testing. A single device can be coated with multiple reagents, and more analytes can be detected in one sample. This platform could be used for a wide variety of assays. Here, we show the design, fabrication and working principle of the chip-and-dip flow device along with a specific application consisting in the determination of ß-lactamase activity and cortisol. The simplicity, robustness and multiplexing capability of the chip-and-dip device will allow it to be used for POC diagnostics.

Electricity Generation from Capillary-Driven Ionic Solution Flow in a Three-Dimensional Graphene Membrane.

Harvesting energy from the ambient environment provides great promise in the applications of micro/nanodevices and self-powered systems. Herein, we report a novel energy-scavenging method where an ionic solution infiltrating into a three-dimensional graphene (3DG) membrane can spontaneously generate electricity under ambient conditions. A constructed 3DG nanogenerator (3DGNG) with an effective size of 0.5 × 2 cm can produce a continuous voltage of ∼0.28 V and a remarkable output current of ∼62 µA. The voltage is higher than those generated from the interaction between water and carbon nanomaterials in previous research typically in the range of microvolts to millivolts. Moreover, we demonstrate the potential application of the 3DGNG by illuminating a liquid crystal display (LCD) directly with 10 3DGNGs in series. These results present a novel avenue for energy harvesting and show bright potential applications in small devices and self-powered systems.

Capillary-Driven Microfluidic Chips for Miniaturized Immunoassays: Efficient Fabrication and Sealing of Chips Using a "Chip-Olate" Process.

The fabrication of silicon-based microfluidic chips is invaluable in supporting the development of many microfluidic concepts for research in the life sciences and in vitro diagnostic applications such as the realization of miniaturized immunoassays using capillary-driven chips. While being extremely abundant, the literature covering microfluidic chip fabrication and assay development might not have addressed properly the challenge of fabricating microfluidic chips on a wafer level or the need for dicing wafers to release chips that need then to be further processed, cleaned, rinsed, and dried one by one. Here, we describe the "chip-olate" process wherein microfluidic structures are formed on a silicon wafer, followed by partial dicing, cleaning, and drying steps. Then, integration of reagents (if any) can be done, followed by lamination of a sealing cover. Breaking by hand the partially diced wafer yields individual chips ready for use.

Enhancing Capillary-Driven Flow for Paper-Based Microfluidic Channels.

Paper-based microfluidic devices have received considerable interest due to their benefits with regards to low manufacturing costs, simplicity, and the wide scope of applications. However, limitations including sample retention in paper matrix and evaporation as well as low liquid flow rates have often been overlooked. This paper presents a paper-based capillary-driven flow system that speeds up flow rates by utilizing narrow gap geometry between two parallel surfaces separated by a spacer. The top surface is hydrophobic, while the bottom surface is a hydrophobic paper substrate with a microfluidic channel defined by a hydrophilic pathway, leaving sides of the channel open to air. The liquid flows on the hydrophilic path in the gap without spreading onto the hydrophobic regions. The closed-channel flow system showed higher spreading distances and accelerated liquid flow. An average flow rate increases of 200 and 100% were obtained for the nanoparticle-coated paperboard and the blotting papers used, respectively. Fast liquid delivery to detection zones or reaction implies rapid results from analytical devices. In addition, liquid drying and evaporation can be reduced in the proposed closed-channel system.

A reliable aptamer array prepared by repeating inkjet-spotting toward on-site measurement.

A preparation protocol is proposed for a reliable aptamer array utilizing an ink-jet spotter. We accumulated streptavidin and biotinylated-aptamer in this order on a biotinylated-polyethylene glycol-coated gold substrate to prepare an aptamer array. The aptamer array was prepared with an alternate spotting structure where each aptamer spot was placed between reference spots formed with blocking solution thus suppressing contamination from neighboring spots during the blocking and washing processes. Four aptamer spots were prepared in a small area of 1×4.8mm(2) with five reference spots made of blocking solution. We evaluated the thrombin binding ability of the spotted aptamer array using a multi-analysis surface plasmon resonance sensor. We prepared a disposable capillary-driven flow chip designed for on-site measurement (Miura et al., 2010) with our aptamer array and detected thrombin from phosphate-buffered saline at concentrations of 50ngmL(-1) and 1µgmL(-1) (equivalent to 1.35 and 27nM, respectively). A correlation was observed between the refractive index shift and thrombin concentration. This implies that our array preparation protocol meets the requirement for the preparation of a one-time-use chip for on-site measurement.

The dynamics of capillary-driven two-phase flow: the role of nanofluid structural forces.

Capillary-driven flows are fundamental phenomena and are involved in many key technological processes, such as oil recovery through porous rocks, ink-jet printing, the bubble dynamics in a capillary, microfluidic devices and labs on chips. Here, we discuss and propose a model for the oil displacement dynamics from the capillary by the nanofluid (which is composed of a liquid suspension of nanoparticles); we elucidate the physics of the novelty of the phenomenon and its application. The oil displacement by the nanofluid flow is a multi-stage phenomenon, first leading to the oil film formation on the capillary wall, its break-up, and retraction over the capillary wall; this lead to the formation of the oil double concave meniscus. With time, the process repeats itself, leading to the formation of a regular "necklace" of oil droplets inside the capillary. Finally, the oil droplets are separated by the nanofluid film from the capillary wall. The light reflected differential interferometry technique is applied to investigate the nanofluid interactions with the glass wall. We find nanoparticles tend to self-structure into multiple layers close to the solid wall, which cause the structural forces to arise that lead to the oil displacement from the capillary. This research is expected to benefit the understanding of nanofluid phenomena in a capillary and promote their use in technological applications.

Visualization and measurement of capillary-driven blood flow using spectral domain optical coherence tomography.

Capillary-driven flow (CD-flow) in microchannels plays an important role in many microfluidic devices. These devices, the most popular being those based in lateral flow, are becoming increasingly used in health care and diagnostic applications. CD-flow can passively pump biological fluids as blood, serum or plasma, in microchannels and it can enhance the wall mass transfer by exploiting the convective effects of the flow behind the meniscus. The flow behind the meniscus has not been experimentally identified up to now because of the lack of high-resolution, non-invasive, cross-sectional imaging means. In this study, spectral-domain Doppler optical coherence tomography is used to visualize and measure the flow behind the meniscus in CD-flows of water and blood. Microchannels of polydimethylsiloxane and glass with different cross-sections are considered. The predictions of the flow behind the meniscus of numerical simulations using the power-law model for non-Newtonian fluids are in reasonable agreement with the measurements using blood as working fluid. The extension of the Lucas-Washburn equation to non-Newtonian power-law fluids predicts well the velocity of the meniscus of the experiments using blood.