Capillary-Driven Toner-Based Microfluidic Devices for Clinical Diagnostics with Colorimetric Detection

Toward the Development of Rapid, Specific, and Sensitive Microfluidic Sensors: A Comprehensive Device Blueprint

Recent advances in nano/microfluidics have led to the miniaturization of surface-based chemical and biochemical sensors, with applications ranging from environmental monitoring to disease diagnostics. These systems rely on the detection of analytes flowing in a liquid sample, by exploiting their innate nature to react with specific receptors immobilized on the microchannel walls. The efficiency of these systems is defined by the cumulative effect of analyte detection speed, sensitivity, and specificity. In this perspective, we provide a fresh outlook on the use of important parameters obtained from well-characterized analytical models, by connecting the mass transport and reaction limits with the experimentally attainable limits of analyte detection efficiency. Specifically, we breakdown when and how the operational (e.g., flow rates, channel geometries, mode of detection, etc.) and molecular (e.g., receptor affinity and functionality) variables can be tailored to enhance the analyte detection time, analytical specificity, and sensitivity of the system (i.e., limit of detection). Finally, we present a simple yet cohesive blueprint for the development of high-efficiency surface-based microfluidic sensors for rapid, sensitive, and specific detection of chemical and biochemical analytes, pertinent to a variety of applications.

Paper-Based Multiplexed Colorimetric Device for the Simultaneous Detection of Salivary Biomarkers

In this study, we describe a monolithic and fully integrated paper-based device for the simultaneous detection of three prognostic biomarkers in saliva. The pattern of the proposed multiplexed device is designed with a central sample deposition zone and three identical arms, each containing a pre-treatment and test zone. Its one-step fabrication is realized by CO₂ laser cutting, providing remarkable parallelization and rapidity (ca. 5 s/device). The colorimetric detection is based on the sensitive and selective target-induced reshaping of plasmonic multibranched gold nanoparticles, which exhibit a clear spectral shift (and blue-to-pink color change) in case of non-physiological concentrations of the three salivary biomarkers. A rapid and multiplexed naked-eye or smartphone-based readout of the colorimetric response is achieved within 10 min. A prototype kit for POCT testing is also reported, providing robustness and easy handling of the device.

Flow control in a laminate capillary-driven microfluidic device

Capillary-driven microfluidic devices are of significant interest for on-site analysis because they do not require external pumps and can be made from inexpensive materials. Among capillary-driven devices, those made from paper and polyester film are among the most common and have been used in a wide array of applications. However, since capillary forces are the only driving force, flow is difficult to control, and passive flow control methods such as changing the geometry must be used to accomplish various analytical applications. This study presents several new flow control methods that can be utilized in a laminate capillary-driven microfluidic device to increase available functionality. First, we introduce push and burst valve systems that can stop and start flow. These valves can stop flow for >30 min and be opened by either pressing the channel or inflowing other fluids to the valve region. Next, we propose flow control methods for Y-shaped channels that enable more functions. In one example, we demonstrate the ability to accurately control concentration to create laminar, gradient, and fully mixed flows. In a second example, flow velocity in the main channel is controlled by adjusting the length of the inlet channel. In addition, the flow velocity is constant as the inlet length increases. Finally, the flow velocity in the Y-shaped device as a function of channel height and fluid properties such as viscosity and surface tension was examined. As in previous studies on capillary-driven channels, the flow rate was affected by each parameter. The fluidic control tools presented here will enable new designs and functions for low cost point of need assays across a variety of fields.

Study on Functionality and Surface Modification of a Stair-Step Liquid-Triggered Valve for On-Chip Flow Control

Distinctive from other forms of microfluidic system, capillary microfluidics is of great interest in autonomous micro-systems due to its well-engineered fluidic control based on capillary force. As an essential component of fluidic control in capillaric circuits, micro-valves enable sequential fluidic operations by performing actions such as stopping and triggering. In this paper, we present a stair-step liquid-triggered valve; the functionality of the valve and its dependencies on geometry and surface modification are studied. The surface contact angle of the microfabricated valves that are coated by polyethylene glycol (PEG) or (3-Aminopropyl) triethoxysilane (APTES) is evaluated experimentally, and the corresponding reliability of the valve structure is discussed. Moreover, the variation in the surface contact angle over time is investigated, indicating the shelf time of the device. We further discuss the overall fluidic behavior in such capillary valves, which benefits the capillaric circuit designs at the initial stage.

Advances in passively driven microfluidics and lab-on-chip devices: a comprehensive literature review and patent analysis

The development of passively driven microfluidic labs on chips has been increasing over the years. In the passive approach, the microfluids are usually driven and operated without any external actuators, fields, or power sources. Passive microfluidic techniques adopt osmosis, capillary action, surface tension, pressure, gravity-driven flow, hydrostatic flow, and vacuums to achieve fluid flow. There is a great need to explore labs on chips that are rapid, compact, portable, and easy to use. The evolution of these techniques is essential to meet current needs. Researchers have highlighted the vast potential in the field that needs to be explored to develop rapid passive labs on chips to suit market/researcher demands. A comprehensive review, along with patent analysis, is presented here, listing the latest advances in passive microfluidic techniques, along with the related mechanisms and applications.

Different approaches employed in the passively driven microfluidics and LOC devices.

A transparent paper-based platform for multiplexed bioassays by wavelength-dependent absorbance/transmittance

The present work describes the rational design of a paper-based biosensing platform for multi-target detection with low cost and high sensitivity by wavelength-dependent absorbance/transmittance.

Printed low-cost microfluidic analytical devices based on a transparent substrate

This work describes the development of a microfluidic analytical device prepared on a transparent OHP film substrate, named the microfluidic transparent film-based analytical device (μTFAD). Printing technologies including wax printing for microchannel patterning and inkjet printing for chemical assay component deposition have been employed for the μTFAD fabrication. The fully printed μTFAD allowed gravity-assisted pump-free transportation of the sample liquid (50 μL) and an absorbance measurement-based iron ion (Fe2+) assay using nitroso-PSAP as the colorimetric reagent within a wax-patterned microfluidic structure. By measuring absorbance values at the Fe2+-nitroso-PSAP complex-specific wavelength (756 nm), a response curve with a linear range of 0-200 μM was obtained. The limit of detection (1.18 μM) obtained with the proposed μTFADs was comparable to the results achieved with a conventional 96-well microplate assay (0.92 μM) and lower than that in the case of digital colour analysis-assisted filter paper spot tests (7.71 μM) or the absorbance analysis of refractive index-matched translucent filter paper spots (37.2 μM). In addition, highly selective Fe2+ detection has been achieved in the presence of potentially interfering metal ions (Cu2+, Co2+, Ni2+) without the use of any masking reagents, owing to the selection of the target complex-specific wavelength in the absorbance measurement on μTFADs.

An Effective Capillary Valve Based on Micro-hole Array for Microfluidic Systems

With the development of microfluidic systems for portable devices where the capillary force as the driving force drove the liquid in a microchannel. An effective capillary microvalve based on a micro-hole array is proposed in this paper for the purpose of good performance, and programmable control, and easy-to-fabricate. The Wenzel model was used to slow down the velocity of fluid flow. The microvalve was made by a hot-embossing and hot-bonding process. The valve function was assessed by liquid flowing experiments. The results showed that the valve employs an abruptly changing contact angle to slow down the fluid flow. The controlling time is approximately 177 - 213 s. The best control effect was obtained when the contact angle equaled to 90°. The effect of the roughness of the microvalve was also evaluated. Therefore, this work has a great potential and broad prospect for both research and applications in microfluidic systems, such as the portable devices in the medical testing field.

Microfluidic chip with movable layers for the manipulation of biochemicals

A simple and effective platform that can conglomerate various microfluidic functions in a single chip is essential for many bioassays, especially for point-of-care testing applications. Here, a chip that exploits surface tension in solutions with movable top and bottom layers is presented, for use in fluid transport, mixing, maintaining metered volumes, and biomolecule capture and release. The chip has open chambers in vertically mobile top layers and rotationally mobile bottom layers to exploit surface tension in biochemical solutions, and implements control over fluid motion. To manipulate biomolecules, a vertically mobile tip with a permanent magnet at the top layer performs collection, transport, release, and dispersion of magnetic beads. Thus, the chip orchestrates various fluidic control functions without using on-chip valves and pumps that increase operational and structural complexity. To demonstrate its utility, the chip performs automated DNA extraction by obtaining genomic DNA from a sample containing cells. Our approach provides a useful and effective alternative to numerous platforms that use active and passive on-chip components for bioassays.

A pressure-driven gas-diffusion/permeation micropump for self-activated sample transport in an extreme micro-environment

The pressure-driven gas-diffusion/permeation micropump is highlighted for stable microdroplet/liquid delivery under extreme conditions,e.g.high temperature, and a three-dimensional, long-distance and complex-topology microchannel.

Real-time measurement of cholesterol secreted by human hepatocytes using a novel microfluidic assay

The use of microfluidics has become widespread in recent years because of the use of lesser resources such as small size, low volume of reagents, and physiological representation of mammalian cells. One of the advantages of microfluidic-based cell culture is the ability to perfuse culture media which tends to improve cellular health and function. Although measurement of cellular function conventionally is carried out using well-plates and plate readers, these approaches are insufficient to carry out in-line analysis of perfused cell cultures because of mismatch between volumes and sensitivity. We report the development of a novel microfluidic device and assay that is carried out under perfusion, in-line to measure the cholesterol secreted from a human hepatocyte tissue-chip. The heart of the assay is the unique implementation of enzymatic chemistry that is carried out on a polystyrene bead. Using this approach, we successfully measured cholesterol secreted by the perfused human hepatocytes.

Endothelial Cell Culture Under Perfusion On A Polyester-Toner Microfluidic Device

This study presents an inexpensive and easy way to produce a microfluidic device that mimics a blood vessel, serving as a start point for cell culture under perfusion, cardiovascular research, and toxicological studies. Endpoint assays (i.e., MTT reduction and NO assays) were used and revealed that the components making up the microchip, which is made of polyester and toner (PT), did not induce cell death or nitric oxide (NO) production. Applying oxygen plasma and fibronectin improved the adhesion and proliferation endothelial cell along the microchannel. As expected, these treatments showed an increase in vascular endothelial growth factor (VEGF-A) concentration profiles, which is correlated with adherence and cell proliferation, thus promoting endothelialization of the device for neovascularization. Regardless the simplicity of the device, our “vein-on-a-chip” mimetic has a potential to serve as a powerful tool for those that demand a rapid microfabrication method in cell biology or organ-on-a-chip research.

A Review on Microfluidic Paper-Based Analytical Devices for Glucose Detection

Glucose, as an essential substance directly involved in metabolic processes, is closely related to the occurrence of various diseases such as glucose metabolism disorders and islet cell carcinoma. Therefore, it is crucial to develop sensitive, accurate, rapid, and cost effective methods for frequent and convenient detections of glucose. Microfluidic Paper-based Analytical Devices (μPADs) not only satisfying the above requirements but also occupying the advantages of portability and minimal sample consumption, have exhibited great potential in the field of glucose detection. This article reviews and summarizes the most recent improvements in glucose detection in two aspects of colorimetric and electrochemical μPADs. The progressive techniques for fabricating channels on μPADs are also emphasized in this article. With the growth of diabetes and other glucose indication diseases in the underdeveloped and developing countries, low-cost and reliably commercial μPADs for glucose detection will be in unprecedentedly demand.

Resolving Anomalies in Predicting Electrokinetic Energy Conversion Efficiencies of Nanofluidic Devices

We devise a new approach for capturing complex interfacial interactions over reduced length scales, towards predicting electrokinetic energy conversion efficiencies of nanofluidic devices. By embedding several aspects of intermolecular interactions in continuum based formalism, we show that our simple theory becomes capable of representing complex interconnections between electro-mechanics and hydrodynamics over reduced length scales. The predictions from our model are supported by reported experimental data, and are in excellent quantitative agreement with molecular dynamics simulations. The present model, thus, may be employed to rationalize the discrepancies between low energy conversion efficiencies of nanofluidic channels that have been realized from experiments, and the impractically high energy conversion efficiencies that have been routinely predicted by the existing theories.

Microfluidic toner-based analytical devices: disposable, lightweight, and portable platforms for point-of-care diagnostics with colorimetric detection

This chapter describes the development of microfluidic toner-based analytical devices (μTADs) to perform clinical diagnostics using a scanner or cell-phone camera. μTADs have been produced in a platform composed of polyester and toner by the direct-printing technology (DPT) in a matter of minutes. This technology offers simplicity and versatility, and it does not require any sophisticated instrumentation. Toner-based devices integrate the current generation of disposable analytical devices along paper-based chips. The cost of one μTAD has been estimated to be lower than $0.10. In addition, these platforms are lightweight and portable thus enabling their use for point-of-care applications. In the last 5 years, great efforts have been dedicated to spread out the use of μTADs in bioassays. The current chapter reports the fabrication of printed microplates and integrated microfluidic toner-based devices for dengue diagnostics and rapid colorimetric assays with clinically relevant analytes including cholesterol, triglycerides, total proteins, and glucose. The use of μTADs associated with cell-phone camera may contribute to the health care, in special, to people housed in developing regions or with limited access to clinics and hospitals.

Clinical chemistry measurements with commercially available test slides on a smartphone platform: Colorimetric determination of glucose and urea

BACKGKROUND

Rapidly increasing healthcare costs in economically advantaged countries are currently unsustainable, while in many developing nations, even 50-year-old technologies are too expensive to implement. New and unconventional technologies are being explored as solutions to this problem. In this study, we examined the use of a smartphone as the detection platform for 2 well-developed, relatively inexpensive, commercially available clinical chemistry assays as a model for rapid and inexpensive clinical diagnostic testing.

METHODS

An Apple iPhone 4 camera phone equipped with a color analysis application (ColorAssist) was combined with Vitros® glucose and urea colorimetric assays. Color images of assay slides at various concentrations of glucose or urea were collected with the iPhone 4 and quantitated in three different spectral ranges (red/green/blue or RGB) using the ColorAssist app. When the diffuse reflectance data was converted into absorbance, it was possible to quantitate glucose or blood urea nitrogen (BUN) over their clinically important concentration ranges (30-515mg/dl for glucose or 2-190mg/dl for BUN), with good linearity (R(2)=0.9994 or 0.9996, respectively [n=5]).

RESULTS

Data collected using the iPhone 4 and canine serum samples were in agreement with results from the instrumental "gold standard" (Beckman Coulter AU480 Chemistry System) (R(2)=0.9966 and slope=1.0001 for glucose; R(2)=0.9958 and slope=0.9454 for BUN). Glucose determinations of serum samples made using this smartphone method were as accurate as or more accurate than a commercial colorimetric dry slide analyzer (Heska® Element DC Chemistry Analyzer, Loveland, CO) and 2 glucometers: ReliOn® Ultima (Abbott Diabetes Care Inc) and Presto® (AgaMatrix Inc.H). BUN determinations made using the smartphone approach were comparable in accuracy to the Heska instrument.

CONCLUSION

This demonstration shows that smartphones have the potential to be used as simple, effective colorimetric detectors for quantitative diagnostic tests, and may be applicable for both point-of-care applications in the developed world and field deployment in developing nations.

Paper microfluidic-based enzyme catalyzed double microreactor

We describe a paper microfluidic-based enzyme catalyzed double microreactor assay using fluorescent detection. Here, solutions of lactate dehydrogenase (LDH) and diaphorase (DI) were directly spotted onto the microfluidic paper-based analytical device (μPAD). Samples containing lactic acid, resazurin, and nicotinamide adenine dinucleotide oxidized form (NAD(+) ), potassium chloride (KCl), and BSA, in MES buffer were separately spotted onto the μPAD and MES buffer flowed through the device. A cascade reaction occurs upon the sample spot overlapping with LDH to form pyruvate and nicotinamide adenine dinucleotide reduced form (NADH). Subsequently, NADH is used in the conversion of resazurin to fluorescent resorufin by DI. The μPAD avoids the need of surface functionalization or enzyme immobilization steps. These microreactor devices are low cost and easy to fabricate and effect reaction based solely on buffer capillary action.

Multiplex detection of nucleic acids using a low cost microfluidic chip and a personal glucose meter at the point-of-care

A simple assay for multiplex DNA detection has been developed using a low cost microfluidic chip and a personal glucose meter.

Fabricating millimeter-scale polymeric structures for biomedical applications via a combination of UV-activated materials and daily-use tools

Three-dimensional millimeter-scale polymeric structures can be fabricated by using UV-activated materials and daily-use tools for making PDMS-basedin vitrodiagnostic devices.

Quantitative Fluorescence Assays Using a Self-Powered Paper-Based Microfluidic Device and a Camera-Equipped Cellular Phone

Fluorescence assays often require specialized equipment and, therefore, are not easily implemented in resource-limited environments. Herein we describe a point-of-care assay strategy in which fluorescence in the visible region is used as a readout, while a camera-equipped cellular phone is used to capture the fluorescent response and quantify the assay. The fluorescence assay is made possible using a paper-based microfluidic device that contains an internal fluidic battery, a surface-mount LED, a 2-mm section of a clear straw as a cuvette, and an appropriately-designed small molecule reagent that transforms from weakly fluorescent to highly fluorescent when exposed to a specific enzyme biomarker. The resulting visible fluorescence is digitized by photographing the assay region using a camera-equipped cellular phone. The digital images are then quantified using image processing software to provide sensitive as well as quantitative results. In a model 30 min assay, the enzyme β-D-galactosidase was measured quantitatively down to 700 pM levels. This Communication describes the design of these types of assays in paper-based microfluidic devices and characterizes the key parameters that affect the sensitivity and reproducibility of the technique.

Effects of solvent-mediated nonelectrostatic ion-ion interactions on a streaming potential in microchannels and nanochannels

Here, we capture the consequences of solvent-mediated nonelectrostatic ion-ion interactions, coupled with the considerations of finite-sized effects of the ionic species, on electrokinetic transport in narrow fluidic confinements. We consider pressure-driven flow in microchannels and nanochannels in the presence of electrical double layer effects and analyze the establishment of a streaming potential as mediated by a Yukawa-like pair potential that integrates the ion specificity with the governing nonelectrostatic interactions. We bring out the influences of these interactions on electroviscous effects manifested due to the establishment of the streaming potential. Our considerations provide a plausible explanation for the gross overestimation of electrokinetic energy conversion efficiencies as predicted by classical electrical double layer theories that ignore nonelectrostatic interactions between the ionic species.