Abstract
Paper Microfluidics
1000-Fold Sample Focusing on Paper-Based Microfluidic Devices
Rosenfeld and Bercovici present an experimental and analytical study of a novel paper-based analytical device (µPAD) for isotachophoretic sample focusing. The authors share a wax printing fabrication technique to allow the creation of shallow channels, which are critical in providing sufficient heat dissipation, which is needed for high electric fields for isotachophoretic sample focusing. The device is self-contained on a simple piece of filter paper and does not require any specialized enclosures or cooling devices to combat evaporation at high temperatures. In addition, the authors provide an analytical model for isotachophoretic sample accumulation in porous media, introduce a simple figure of merit for evaluating and comparing the efficiency of such devices, and present experimental validation in both paper and glass channels. Using this device, the authors demonstrate the processing of 30 µL of sample achieving a 1000-fold increase in peak concentration in 6 min. The authors believe that this method and device can serve as a guide to the design of low-cost, rapid, and highly sensitive paper-based diagnostic platforms. (Rosenfeld, T., Bercovici, M., Lab Chip.
Evaporative Concentration on a Paper-Based Device to Concentrate Analytes in a Biological Fluid
Wong et al. report the first use of heat on a paper device to rapidly concentrate a clinically relevant analyte of interest from a biological fluid. This technology relies on the application of localized heat to a paper strip to evaporate off hundreds of microliters of liquid to concentrate the target analyte. It can be used to enrich a target analyte that is present at low concentrations within a biological fluid to enhance the sensitivity of downstream detection methods. The authors demonstrate the method by concentrating the tuberculosis-specific glycolipid, lipoarabinomannan (LAM), a promising urinary biomarker for the detection and diagnosis of tuberculosis. It is shown that the heat does not compromise the subsequent immunodetectability of LAM, and in 20 min, the tuberculosis biomarker is concentrated by nearly 20-fold in simulated urine. This method requires only 500 mW of power, and sample flow is self-driven via capillary action. Therefore, the authors believe it can be readily integrated into portable, battery-powered, instrument-free diagnostic devices intended for use in low-resource settings. (Wong, S. Y., et al., Anal. Chem.
Biosensors
Novel Integrated and Portable Endotoxin Detection System Based on an Electrochemical Biosensor
Zuzuarregui et al. report the design, implementation, and validation of a sensitive and integral portable sensor for endotoxin detection. This sensor is based on the electrochemical detection of endotoxins using a synthetic peptide immobilized on a thin-film electrode. The authors describe the fabrication and optimization of the sensor, the biofunctionalization protocol, and the design and implementation of the measuring units—a microfluidic chamber integrated with a portable potentiostat-galvanostat. The authors believe the use of thin-film technologies and the simple immobilization and detection methods enable a rapid, easy, and sensitive technique for in situ and real-time LPS detection. (Zuzuarregui, A., et al., Analyst.
Label-Free and Noncontact Optical Biosensing of Glucose with Quantum Dots
Khan et al. report a label-free, optical sensor for biomedical applications based on changes in the visible photoluminescence (PL) of quantum dots (QDs) in a thin polymer film. Using glucose as the target molecule, the screening of UV excitation due to preabsorption by the product of an enzymatic assay leads to quenching of the PL of QDs in a noncontact scheme. The irradiance changes in QD PL indicate quantitatively the level of glucose present. The noncontact nature of the assay prevents surface degradation of the QDs, which yields an efficient, waste-free, cost-effective, portable, and sustainable biosensor with attractive market features. The limit of detection of the demonstrated biosensor is ~3.5 µM, which is competitive with existing contact-based bioassays. In addition, the biosensor operates over the entire clinically relevant range of glucose concentrations of biological fluids, including urine and whole blood. The authors believe this sensing mechanism can be useful for low-cost, point-of-care biomedical diagnosis. (Khan, S. A., et al., Bios. Bioelectron.
Automation Technology
The Pumping Lid: Investigating Multimaterial 3D Printing for Equipment-Free, Programmable Generation of Positive and Negative Pressures for Microfluidic Applications
In microfluidic applications, particularly in resource-limited environments, pumping is a challenging problem and an active research area. Begolo et al. report an interesting integral pumping device named “the pumping lid” to achieve equipment-free pumping by controlled generation of pressure. In this device, pressure is generated using portable, lightweight, and disposable parts that can be integrated with existing microfluidic devices to simplify workflow and eliminate the need for pumping equipment. The authors use a multimaterial 3D printing technique to include composite parts that combine both rigid and elastic materials with different mechanical properties in the same part. Two types of pumping lids are introduced. The first type produces predictable positive or negative pressures via controlled compression or expansion of gases. Pressures can be preprogrammed by the geometry of the parts and can be tuned even further during use. Multiple pumping lids can work in parallel to enable simultaneous, independent control and pumping of multiple fluids. The second type is based on vapor-liquid equilibrium to generate pressure. The pumping lid method is tested in various application scenarios, including droplet generation, control of laminar flow profiles, and loading of SlipChip devices. The authors believe the pumping lid can be incorporated into existing microfluidic devices to enhance their use as portable diagnostic tools in resource-limited settings and to help accelerate adoption of microfluidics in laboratories. (Begolo, S., et al., Lab Chip
A Self-Powered One-Touch Blood Extraction System: A Novel Polymer-Capped Hollow Microneedle Integrated with a Prevacuum Actuator
Blood is the most important sample medium used in a wide variety of disease diagnoses. An easy, safe, and rapid blood extraction method is critical for developing portable point-of-care diagnosis. Li et al. introduce a novel self-powered one-touch blood extraction system based on a smart polymer-capped hollow microneedle in a prevacuum polydimethylsiloxane actuator. The hollow microneedle is designed for minimally invasive blood extraction and is fabricated using the drawing lithography method. The needle has a length of 1800 µm, an inner diameter of 60 µm, an outer diameter of 130 µm, and a bevel angle of 15°. The system requires only a single step for operation: a finger press activates the blood-sampling process using the negative pressure–driven force built into the prevacuum activated actuator. Each extraction yields approximately 30 µL of blood, sufficient for evaluation using a micro total analysis system. The system offers benefits of low cost, disposability, easy operation, simple mechanism, and small device size as well as no need for external power. The authors believe this device can be the solution to meet challenging requirements for point-of-care applications that require blood extraction. (Li, C. G., et al., Lab Chip
Fully Automated Sample Preparation Microsystem for Genetic Testing of Hereditary Hearing Loss Using Two-Color Multiplex Allele-Specific PCR
Zhuang et al. report a fully automated microsystem for genetic testing of hereditary hearing loss from human whole blood. The device consists of a disposable DNA extraction and PCR microchip, as well as a compact control instrument. DNA extraction and PCR are integrated into a single 15 µL reaction chamber, in which a piece of filter paper was embedded for capturing genomic DNA, followed by in situ PCR amplification without elution. Diaphragm microvalves actuated by external solenoids together with a one-way fluidic control strategy operated by a modular valve positioner and a syringe pump were employed to control the fluids and to seal the chamber during thermal cycling. Fully automated DNA extractions from as low as 0.3 µL human whole blood followed by amplifications of 59 bp β-actin fragments can be completed on the microsystem in about 100 min. The authors perform negative control tests between blood sample analyses and show that no contamination or carryover is found is the system. A two-color multiplex allele-specific PCR (ASPCR) assay is tested to showcase the capability of the device, with DNA extraction from blood and ASPCR performed on the automated microsystem and an electrophoretic analysis on an external portable microchip capillary electrophoresis system. Blood samples from both healthy donors and patients are tested, with accurate results obtained with only two steps in less than 2 h. (Zhuang, B., et al., Anal. Chem., DOI: 10.1021/ac5039303)
NMR-DMF: A Modular Nuclear Magnetic Resonance–Digital Microfluidics System for Biological Assays
Lei presents a modular nuclear magnetic resonance–digital microfluidics (NMR-DMF) system as a portable diagnostic platform for miniaturized biological assays. This device addresses the increasing needs for preparing customized NMR probes that target specific targets. Traditional probe preparation is laborious and time consuming. In this work, the authors propose a modular NMR-DMF system to allow electronic automation of multistep reaction-screening protocols. The authors introduce a figure 8–shaped coil enlarging the usable inner space of a portable magnet by 4.16 times, generating a radio frequency excitation field in the planar direction. By electronically managing the electro-wetting-on-dielectric effects over an electrode array, preloaded droplets with the inclusion of biological constituents and targets can be programmed to mix and be guided to the detection site for high-sensitivity NMR screening, with the result displayed in real time. The authors demonstrate the utility of the system with an automated real-time identification of 100 pM of avidin in a 14 µL droplet. The authors believe this robust and portable diagnostic device can be used in a variety of biological analyses and screening applications. (Lei, K.-M., Analyst.
Acoustofluidic Chemical Waveform Generator and Switch
Eliciting a cellular response to a changing chemical microenvironment is central to many biological processes including gene expression, cell migration, differentiation, apoptosis, and intercellular signaling. The nature and scope of the response are highly dependent on the spatiotemporal characteristics of the stimulus. To date, studies that investigate this phenomenon have been limited to digital or step chemical stimulation with little control over the temporal counterparts. Ahmed et al. demonstrate an acoustofluidic approach for generating programmable chemical waveforms that permits continuous modulation of the signal characteristics including the amplitude (i.e., sample concentration), shape, frequency, and duty cycle, with frequencies reaching up to 30 Hz. Furthermore, the authors demonstrate fast switching between multiple distinct stimuli, wherein the waveform of each stimulus is independently controlled. Using this device, frequency-dependent activation and internalization of the β2-adrenergic receptor using epinephrine is characterized. The authors believe the acoustofluidic-based programmable chemical waveform generation and switching method reported here can become a powerful tool for the investigation and characterization of the kinetics and other dynamic properties of many biological and biochemical processes. (Ahmed, D., et al., Anal. Chem.,
Printed Electronics and Wearable Devices
Tattoo-Based Noninvasive Glucose Monitoring
Bandodkar et al. introduce an all-printed temporary tattoo–based glucose sensor for noninvasive glycemic monitoring. The sensor is the first example of an easy-to-wear flexible tattoo-based epidermal diagnostic device combining reverse iontophoretic extraction of interstitial glucose and an enzyme-based amperometric biosensor. In vitro measurements show the tattoo sensor’s linear response toward physiologically relevant glucose levels with negligible interferences from common coexisting electroactive species. The sensor is then applied on human subjects and is shown to successfully detect variations in glycemic levels due to food consumption. Good correlation of the sensor response with that of a commercial glucose meter is reported, showing the promise of the tattoo sensor to detect glucose levels in a noninvasive fashion. These results provide proof that the tattoo-based iontophoresis-sensor platform holds promise for efficient diabetes management. The authors also believe the same platform can be extended toward noninvasive monitoring of other physiologically relevant analytes present in the interstitial fluid. (Bandodkar, A. J., Anal. Chem.,
Tactile Direction-Sensitive and Stretchable Electronic Skins Based on Human Skin–Inspired Interlocked Microstructures
Stretchable electronic skins with multidirectional force-sensing capabilities have many applications in robotics, prosthetics, and rehabilitation devices. The piezoresistive interlocked microdome array is a type of sensing module often used for stress-direction-sensitive, stretchable electronic skins, inspired by the interlocked microstructures found in epidermal-dermal ridges in human skin. Park et al. demonstrate that these arrays show highly sensitive detection capability of various mechanical stimuli including normal, shear, stretching, bending, and twisting forces. In addition, the unique geometry of interlocked microdome arrays enables the differentiation of various mechanical stimuli because the arrays exhibit different levels of deformation depending on the direction of applied forces, thus providing different sensory output patterns. The authors show that the electronic skins attached on human skin in the arm and wrist areas are capable of distinguishing various mechanical stimuli applied in different directions and can selectively monitor different intensities and directions of air flows and vibrations. (Park, J., et al., ACS Nano
Fully Printed Flexible Fingerprint-Like Three-Axis Tactile and Slip Force and Temperature Sensors for Artificial Skin
A three-axis tactile force sensor that determines the touch and slip/friction force is important for the development of artificial skin and robotic applications by fully imitating human skin. The ability to detect slip/friction and tactile forces simultaneously allows unknown objects to be held in robotic applications. However, the functionalities of flexible devices have been limited to a tactile force in one direction because of difficulties in fabricating devices on flexible substrates. Harada et al. demonstrate a fully printed fingerprint-like three-axis tactile force and temperature sensor for artificial skin applications. To achieve economic macroscale devices, these sensors are fabricated and integrated using only printing methods. Strain engineering enables the strain distribution to be detected upon applying a slip/friction force. By reading the strain difference at four integrated force sensors for a pixel, both the tactile and slip/friction forces can be analyzed simultaneously. As a proof of concept, the high sensitivity and selectivity for both force and temperature are demonstrated using a 3 × 3 array artificial skin that senses tactile, slip/friction, and temperature. The authors believe multifunctional sensing components for a flexible device represent an important advancement for both practical applications and basic research in flexible electronics. (Harada, S., et al., ACS Nano
Mobile Diagnostics
Smartphone-Based Simultaneous pH and Nitrite Colorimetric Determination for Paper Microfluidic Devices
Lopez-Ruiz et al. report an android application for measurement of nitrite concentration and pH determination in combination with a low-cost paper-based microfluidic device. The application uses seven sensing areas, containing the corresponding immobilized reagents, to produce selective color changes when a sample solution is placed in the sampling area. Under controlled conditions of light, using the flash of the smartphone as a light source, the image captured with the built-in camera is processed using a customized algorithm for multidetection of the colored sensing areas. The developed image processing allows for reducing the influence of the light source and the positioning of the microfluidic device in the picture. Then, the H (hue) and S (saturation) coordinates of the HSV color space are extracted and related to pH and nitrite concentration, respectively. A complete characterization of the sensing elements has been carried out as well as a full description of the image analysis for detection. The results show good use of a mobile phone as an analytical instrument. For the pH, the resolution obtained is 0.04 units of pH, 0.09 of accuracy, and a mean squared error of 0.167. With regard to nitrite, 0.51% at 4.0 mg L−1 of resolution and 0.52 mg L−1 as the limit of detection was achieved. (Lopez-Ruiz, N., et al., Anal. Chem.
Imaging and Sizing of Single DNA Molecules on a Mobile Phone
DNA imaging techniques using optical microscopy have are commonly used in biology, chemistry, and physics and are based on relatively expensive, bulky, and complicated setups that limit their use to advanced laboratory settings. Wei demonstrates that imaging and length quantification of single-molecule DNA strands can be accomplished with a compact, lightweight, and cost-effective fluorescence microscope installed on a mobile phone. In addition to an optomechanical attachment that creates a high-contrast dark-field imaging setup using an external lens, thin-film interference filters, a miniature dovetail stage, and a laser diode for oblique-angle excitation, the authors also introduce a computational framework and a mobile phone application connected to a server backend for measurement of the lengths of individual DNA molecules that are labeled and stretched using disposable chips. Using this mobile phone platform, the authors are able to image single DNA molecules of various lengths and demonstrate a sizing accuracy of <1 kilobase-pairs (kbp) for 10 kbp and longer DNA samples imaged over a field of view of 2 mm2. (Wei, Q., ACS Nano.
High-Throughput Screening
High-Throughput Gel Pad Array Chip for High-Throughput and Multianalyte Microbead-Based Immunoassays
Zhua et al. introduce a gel pad array chip for high-throughput and multianalyte microbead-based immunoassays. The chip is fabricated by photo-patterning of two polymeric gels, polyacrylamide gel and polyethylene glycol (PEG) gel, on a glass slide. The resulting chip consists of 40 polyacrylamide gel pad array units for the immobilization of microbeads, and each gel pad array is surrounded with a PEG micropillar ring to confine the samples within the microarray. The chip was tested for quantitative immunoassays for two model cancer markers, human chorionic gonadotropin (hCG) and prostate-specific antigen (PSA), in serum samples. Detection limits below the physiological threshold level for cancer diagnosis were achieved with good inter- and intrachip reproducibility. By using spatial encoded microbeads, simultaneous detection of both hCG and PSA on each gel pad array is achieved with single-filter fluorescence imaging. The authors believe this gel pad array chip is easy to use, easy to fabricate with low-cost materials and minimal equipment, and reusable, and it can be easily customized for various multianalyte immunoassays. (Zhua, Q., et al., Bios. Bioelectron.
Cell Streak Imaging Cytometry for Rare Cell Detection
Detection of rare cells, such as circulating tumor cells, has great implications in clinical applications. Balsam et al. have developed an imaging flow cytometer with a streak imaging mode capability to measure rare cells with increased sensitivity and improved data managements. The new streak mode imaging mode uses low-speed video to capture moving fluorescently labeled cells in a flow cell. Each moving cell is imaged on multiple pixels on each frame, where the cell path is marked as a streak line proportional to the length of the exposure. Finding rare cells (e.g., <1 cell/mL) requires measuring larger sample volumes to achieve higher sensitivity. To solve this problem, the authors combine streak mode imaging with a “wide” high-throughput flow cell in contrast to the conventional “narrow” hydrodynamic focusing cells typically used in cytometry that are inherently limited to low flow rates. To further increase sensitivity, the signal-to-noise ratio of the images was also enhanced by combining three imaging methods: (1) background subtraction, (2) pixel binning, and (3) CMOS color channel selection. The streaking mode cytometer is tested using SYTO-9–labeled THP-1 human monocytes, with extremely low abundance, in buffer and in blood, and successful measurement with high accuracy is demonstrated. The authors believe this new detection mechanism may meet the needs of many clinical applications, especially in resource-limited settings, because of its simplicity and low cost. (Balsam, J., et al., Bios. Bioelectron.