Next generation microfluidics: fulfilling the promise of lab-on-a-chip technologies

Microfluidic lab-on-a-chip technologies enable the analysis and manipulation of small fluid volumes and particles at small scales and the control of fluid flow and transport processes at the microscale, leading to the development of new methods to address a broad range of scientific and medical challenges. Microfluidic and lab-on-a-chip technologies have made a noteworthy impact in basic, preclinical, and clinical research, especially in hematology and vascular biology due to the inherent ability of microfluidics to mimic physiologic flow conditions in blood vessels and capillaries. With the potential to significantly impact translational research and clinical diagnostics, technical issues and incentive mismatches have stymied microfluidics from fulfilling this promise. We describe how accessibility, usability, and manufacturability of microfluidic technologies should be improved and how a shift in mindset and incentives within the field is also needed to address these issues. In this report, we discuss the state of the microfluidic field regarding current limitations and propose future directions and new approaches for the field to advance microfluidic technologies closer to translation and clinical use. While our report focuses on using blood as the prototypical biofluid sample, the proposed ideas and research directions can be extrapolated to other areas of hematology, oncology, biology, and medicine.

Emerging Approaches for Fluorescence-Based Newborn Screening of Mucopolysaccharidoses

Interest in newborn screening for mucopolysaccharidoses (MPS) is growing, due in part to ongoing efforts to develop new therapies for these disorders and new screening assays to identify increased risk for the individual MPSs on the basis of deficiency in the cognate enzyme. Existing tests for MPSs utilize either fluorescence or mass spectrometry detection methods to measure biomarkers of disease (e.g., enzyme function or glycosaminoglycans) using either urine or dried blood spot (DBS) samples. There are currently two approaches to fluorescence-based enzyme function assays from DBS: (1) manual reaction mixing, incubation, and termination followed by detection on a microtiter plate reader; and (2) miniaturized automation of these same assay steps using digital microfluidics technology. This article describes the origins of laboratory assays for enzyme activity measurement, the maturation and clinical application of fluorescent enzyme assays for MPS newborn screening, and considerations for future expansion of the technology.

Fluorimetric assay with a novel substrate for quantification of galactocerebrosidase activity in dried blood spot specimens

Background

Decreased galactocerebrosidase (GALC) enzyme activity is causative for Krabbe disease, a lysosomal storage disorder with devastating neurodegenerative consequences. Quantitative fluorimetric assays for GALC activity in isolated blood and skin cells have been described; however, no such assay has been described using dried blood spot (DBS) specimens.

Methods

GALC enzyme activity was measured quantitatively using fluorescence from a novel glycosidic substrate: carboxy derived from 6-hexadecanoylamino-4-methylumbelliferone. GALC activity was demonstrated on newborn DBS specimens, known Krabbe disease patient specimens, proficiency testing and quality control samples.

Results

We present data on characterization of the novel substrate and assay, including pH optimization and enzyme kinetics using a fluorimetric profile. Single and multi-day precision analyses revealed tight analytical measurements with %CV ranging from 5.2% to 14.1%. GALC enzyme activity was linear over the range of 0.31 - 12.04 μmol/l/h with a limit of detection of 0.066 μmol/l/h. Our results with this assay show a clear discrimination between GALC activities in samples from Krabbe disease patients versus presumed normal newborn samples.

Conclusions

A fluorimetric assay for GALC enzyme activity measurement on dried blood spot specimens is feasible. Improvements to the assay including novel substrate design, increased substrate concentration and removal of sodium chloride maximize the specificity of the assay and minimize interference from β-galactosidase.

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First fluorescent assay to measure GALC enzyme activity in dried blood spots.

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Linear range covers the activity normally seen in newborn dried blood spots.

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Successfully discriminates deficient from normal enzyme activity in DBS specimens.

The dynamics and stability of lubricating oil films during droplet transport by electrowetting in microfluidic devices

The operation of digital microfluidic devices with water droplets manipulated by electrowetting is critically dependent on the static and dynamic stability and lubrication properties of the oil films that separate the droplets from the solid surfaces. The factors determining the stability of the films and preventing surface fouling in such systems are not yet thoroughly understood and were experimentally investigated in this study. The experiments were performed using a standard digital microfluidic cartridge in which water droplets enclosed in a thin, oil-filled gap were transported over an array of electrodes. Stable, continuous oil films separated the droplets from the surfaces when the droplets were stationary. During droplet transport, capillary waves formed in the films on the electrode surfaces as the oil menisci receded. The waves evolved into dome-shaped oil lenses. Droplet deformation and oil displacement caused the films at the surface opposite the electrode array to transform into dimples of oil trapped over the centers of the droplets. Lower actuation voltages were associated with slower film thinning and formation of fewer, but larger, oil lenses. Lower ac frequencies induced oscillations in the droplets that caused the films to rupture. Films were also destabilized by addition of surfactants to the oil or droplet phases. Such a comprehensive understanding of the oil film behavior will enable more robust electrowetting-actuated lab-on-a-chip devices through prevention of loss of species from droplets and contamination of surfaces at points where films may break.

Novel application of digital microfluidics for the detection of biotinidase deficiency in newborns

OBJECTIVE

Newborn screening for biotinidase deficiency can be performed using a fluorometric enzyme assay on dried blood spot specimens. As a pre-requisite to the consolidation of different enzymatic assays onto a single platform, we describe here a novel analytical method for detecting biotinidase deficiency using the same digital microfluidic cartridge that has already been demonstrated to screen for five lysosomal storage diseases (Pompe, Fabry, Gaucher, Hurler and Hunter) in a multiplex format.

METHODS

A novel assay to quantify biotinidase concentration in dried blood spots (DBS) was developed and optimized on the digital microfluidic platform using proficiency testing samples from the Centers for Disease Control and Prevention. The enzymatic assay uses 4-methylumbelliferyl biotin as the fluorogenic substrate. Biotinidase deficiency assays were performed on normal (n=200) and deficient (n=7) newborn DBS specimens.

RESULTS

Enzymatic activity analysis of biotinidase deficiency revealed distinct separation between normal and affected DBS specimens using digital microfluidics and these results matched the expected activity.

CONCLUSIONS

This study has demonstrated performance of biotinidase deficiency assays by measurement of 4-methylumbelliferyl product on a digital microfluidic platform. Due to the inherent ease in multiplexing on such a platform, consolidation of other fluorometric assays onto a single cartridge may be realized.

Rapid assays for Gaucher and Hurler diseases in dried blood spots using digital microfluidics

OBJECTIVE

Easy tool for newborn screening of Gaucher and Hurler diseases.

METHODS

Method comparison between fluorometric enzymatic activity assay on a digital microfluidic platform and micro-titer plate bench assay was performed on normal (n = 100), Gaucher (n = 10) and Hurler (n = 7) dried blood spot samples.

RESULTS

Enzymatic activity analysis of glucocerebrosidase (Gaucher) and α-l-iduronidase (Hurler) revealed similar discrimination between normal and affected samples on both platforms.

CONCLUSIONS

Digital microfluidics is suitable for Gaucher and Hurler newborn screening.

Multiplex newborn screening for Pompe, Fabry, Hunter, Gaucher, and Hurler diseases using a digital microfluidic platform

PURPOSE

New therapies for lysosomal storage diseases (LSDs) have generated interest in screening newborns for these conditions. We present performance validation data on a digital microfluidic platform that performs multiplex enzymatic assays for Pompe, Fabry, Hunter, Gaucher, and Hurler diseases.

METHODS

We developed an investigational disposable digital microfluidic cartridge that uses a single dried blood spot (DBS) punch for performing a 5-plex fluorometric enzymatic assay on up to 44 DBS samples. Precision and linearity of the assays were determined by analyzing quality control DBS samples; clinical performance was determined by analyzing 600 presumed normal and known affected samples (12 for Pompe, 7 for Fabry and 10 each for Hunter, Gaucher and Hurler).

RESULTS

Overall coefficient of variation (CV) values between cartridges, days, instruments, and operators ranged from 2 to 21%; linearity correlation coefficients were ≥0.98 for all assays. The multiplex enzymatic assay performed from a single DBS punch was able to discriminate presumed normal from known affected samples for 5 LSDs.

CONCLUSIONS

Digital microfluidic technology shows potential for rapid, high-throughput screening for 5 LSDs in a newborn screening laboratory environment. Sample preparation to enzymatic activity on each cartridge is less than 3h.

A novel fluorometric enzyme analysis method for Hunter syndrome using dried blood spots

Mucopolysaccharidosis type II (MPS II) or Hunter syndrome is a lysosomal storage disease caused by deficiency of iduronate-2-sulfatase (IDS). A convenient single-step fluorometric microplate enzyme assay has been developed and validated for clinical diagnosis of MPS II using dried blood spots (DBS). The assay compared well with a recently reported digital microfluidic method, from which it was adapted. Results show that this DBS assay is robust and reproducible using both technologies.

Evaluation of a digital microfluidic real-time PCR platform to detect DNA of Candida albicans in blood

Species of Candida frequently cause life-threatening infections in neonates, transplant and intensive care unit (ICU) patients, and others with compromised host defenses. The successful management of systemic candidiasis depends upon early, rapid diagnosis. Blood cultures are the standard diagnostic method, but identification requires days and less than half of the patients are positive. These limitations may be eliminated by using real-time polymerase chain reaction (PCR) to detect Candida DNA in the blood specimens of patients at risk. Here, we optimized a PCR protocol to detect 5-10 yeasts in low volumes of simulated and clinical specimens. We also used a mouse model of systemic candidiasis and determined that candidemia is optimally detectable during the first few days after infection. However, PCR tests are often costly, labor-intensive, and inconvenient for routine use. To address these obstacles, we evaluated the innovative microfluidic real-time PCR platform (Advanced Liquid Logic, Inc.), which has the potential for full automation and rapid turnaround. Eleven and nine of 16 specimens from individual patients with culture-proven candidemia tested positive for C. albicans DNA by conventional and microfluidic real-time PCR, respectively, for a combined sensitivity of 94%. The microfluidic platform offers a significant technical advance in the detection of microbial DNA in clinical specimens.

Droplet-based pyrosequencing using digital microfluidics

The feasibility of implementing pyrosequencing chemistry within droplets using electrowetting-based digital microfluidics is reported. An array of electrodes patterned on a printed-circuit board was used to control the formation, transportation, merging, mixing, and splitting of submicroliter-sized droplets contained within an oil-filled chamber. A three-enzyme pyrosequencing protocol was implemented in which individual droplets contained enzymes, deoxyribonucleotide triphosphates (dNTPs), and DNA templates. The DNA templates were anchored to magnetic beads which enabled them to be thoroughly washed between nucleotide additions. Reagents and protocols were optimized to maximize signal over background, linearity of response, cycle efficiency, and wash efficiency. As an initial demonstration of feasibility, a portion of a 229 bp Candida parapsilosis template was sequenced using both a de novo protocol and a resequencing protocol. The resequencing protocol generated over 60 bp of sequence with 100% sequence accuracy based on raw pyrogram levels. Excellent linearity was observed for all of the homopolymers (two, three, or four nucleotides) contained in the C. parapsilosis sequence. With improvements in microfluidic design it is expected that longer reads, higher throughput, and improved process integration (i.e., "sample-to-sequence" capability) could eventually be achieved using this low-cost platform.

Applications of electrowetting-based digital microfluidics in clinical diagnostics

Digital microfluidics based on electrowetting is a type of microfluidic platform in which liquids are processed as individual unit-sized droplets that are dispensed from a source, merged together, split apart or transported between locations on demand. These devices are implemented using arrays of surface electrodes to control the shape and position of droplets through the electrowetting effect. A major thrust of digital microfluidics research has been the development of integrated lab-on-a-chip devices to perform clinical in vitro diagnostic assays. A variety of preparatory and analytical processes have been implemented and feasibility has been demonstrated for test types ranging from clinical chemistries to immunoassays, nucleic acid tests and cell-based assays. In this article, the current state and future potential of digital microfluidics for clinical diagnostic testing is reviewed and evaluated.

Digital microfluidic platform for multiplexing enzyme assays: implications for lysosomal storage disease screening in newborns

BACKGROUND

Newborn screening for lysosomal storage diseases (LSDs) has been gaining considerable interest owing to the availability of enzyme replacement therapies. We present a digital microfluidic platform to perform rapid, multiplexed enzymatic analysis of acid α-glucosidase (GAA) and acid α-galactosidase to screen for Pompe and Fabry disorders. The results were compared with those obtained using standard fluorometric methods.

METHODS

We performed bench-based, fluorometric enzymatic analysis on 60 deidentified newborn dried blood spots (DBSs), plus 10 Pompe-affected and 11 Fabry-affected samples, at Duke Biochemical Genetics Laboratory using a 3-mm punch for each assay and an incubation time of 20 h. We used a digital microfluidic platform to automate fluorometric enzymatic assays at Advanced Liquid Logic Inc. using extract from a single punch for both assays, with an incubation time of 6 h. Assays were also performed with an incubation time of 1 h.

RESULTS

Assay results were generally comparable, although mean enzymatic activity for GAA using microfluidics was approximately 3 times higher than that obtained using bench-based methods, which could be attributed to higher substrate concentration. Clear separation was observed between the normal and affected samples at both 6- and 1-h incubation times using digital microfluidics.

CONCLUSIONS

A digital microfluidic platform compared favorably with a clinical reference laboratory to perform enzymatic analysis in DBSs for Pompe and Fabry disorders. This platform presents a new technology for a newborn screening laboratory to screen LSDs by fully automating all the liquid-handling operations in an inexpensive system, providing rapid results.

Multiplexed real-time polymerase chain reaction on a digital microfluidic platform

This paper details the development of a digital microfluidic platform for multiplexed real-time polymerase chain reactions (PCR). Liquid samples in discrete droplet format are programmably manipulated upon an electrode array by the use of electrowetting. Rapid PCR thermocycling is performed in a closed-loop flow-through format where for each cycle the reaction droplets are cyclically transported between different temperature zones within an oil-filled cartridge. The cartridge is fabricated using low-cost printed-circuit-board technology and is intended to be a single-use disposable device. The PCR system exhibited remarkable amplification efficiency of 94.7%. To test its potential application in infectious diseases, this novel PCR system reliably detected diagnostic DNA levels of methicillin-resistant Staphylococcus aureus (MRSA), Mycoplasma pneumoniae , and Candida albicans . Amplification of genomic DNA samples was consistently repeatable across multiple PCR loops both within and between cartridges. In addition, simultaneous real-time PCR amplification of both multiple different samples and multiple different targets on a single cartridge was demonstrated. A novel method of PCR speed optimization using variable cycle times has also been proposed and proven feasible. The versatile system includes magnetic bead handling capability, which was applied to the analysis of simulated clinical samples that were prepared from whole blood using a magnetic bead capture protocol. Other salient features of this versatile digital microfluidic PCR system are also discussed, including the configurability and scalability of microfluidic operations, instrument portability, and substrate-level integration with other pre- and post-PCR processes.

Microfluidic platform versus conventional real-time polymerase chain reaction for the detection of Mycoplasma pneumoniae in respiratory specimens

Rapid, accurate diagnosis of community-acquired pneumonia (CAP) due to Mycoplasma pneumoniae is compromised by low sensitivity of culture and serology. Polymerase chain reaction (PCR) has emerged as a sensitive method to detect M. pneumoniae DNA in clinical specimens. However, conventional real-time PCR is not cost-effective for routine or outpatient implementation. Here, we evaluate a novel microfluidic real-time PCR platform (Advanced Liquid Logic, Research Triangle Park, NC) that is rapid, portable, and fully automated. We enrolled patients with CAP and extracted DNA from nasopharyngeal wash (NPW) specimens using a biotinylated capture probe and streptavidin-coupled magnetic beads. Each extract was tested for M. pneumoniae-specific DNA by real-time PCR on both conventional and microfluidic platforms using Taqman probe and primers. Three of 59 (5.0%) NPWs were positive, and agreement between the methods was 98%. The microfluidic platform was equally sensitive but 3 times faster and offers an inexpensive and convenient diagnostic test for microbial DNA.

Heterogeneous immunoassays using magnetic beads on a digital microfluidic platform

A digital microfluidic platform for performing heterogeneous sandwich immunoassays based on efficient handling of magnetic beads is presented in this paper. This approach is based on manipulation of discrete droplets of samples and reagents using electrowetting without the need for channels where the droplets are free to move laterally. Droplet-based manipulation of magnetic beads therefore does not suffer from clogging of channels. Immunoassays on a digital microfluidic platform require the following basic operations: bead attraction, bead washing, bead retention, and bead resuspension. Several parameters such as magnetic field strength, pull force, position, and buffer composition were studied for effective bead operations. Dilution-based washing of magnetic beads was demonstrated by immobilizing the magnetic beads using a permanent magnet and splitting the excess supernatant using electrowetting. Almost 100% bead retention was achieved after 7776-fold dilution-based washing of the supernatant. Efficient resuspension of magnetic beads was achieved by transporting a droplet with magnetic beads across five electrodes on the platform and exploiting the flow patterns within the droplet to resuspend the beads. All the magnetic-bead droplet operations were integrated together to generate standard curves for sandwich heterogeneous immunoassays on human insulin and interleukin-6 (IL-6) with a total time to result of 7 min for each assay.

An integrated digital microfluidic lab-on-a-chip for clinical diagnostics on human physiological fluids

Clinical diagnostics is one of the most promising applications for microfluidic lab-on-a-chip systems, especially in a point-of-care setting. Conventional microfluidic devices are usually based on continuous-flow in microchannels, and offer little flexibility in terms of reconfigurability and scalability. Handling of real physiological samples has also been a major challenge in these devices. We present an alternative paradigm--a fully integrated and reconfigurable droplet-based "digital" microfluidic lab-on-a-chip for clinical diagnostics on human physiological fluids. The microdroplets, which act as solution-phase reaction chambers, are manipulated using the electrowetting effect. Reliable and repeatable high-speed transport of microdroplets of human whole blood, serum, plasma, urine, saliva, sweat and tear, is demonstrated to establish the basic compatibility of these physiological fluids with the electrowetting platform. We further performed a colorimetric enzymatic glucose assay on serum, plasma, urine, and saliva, to show the feasibility of performing bioassays on real samples in our system. The concentrations obtained compare well with those obtained using a reference method, except for urine, where there is a significant difference due to interference by uric acid. A lab-on-a-chip architecture, integrating previously developed digital microfluidic components, is proposed for integrated and automated analysis of multiple analytes on a monolithic device. The lab-on-a-chip integrates sample injection, on-chip reservoirs, droplet formation structures, fluidic pathways, mixing areas and optical detection sites, on the same substrate. The pipelined operation of two glucose assays is shown on a prototype digital microfluidic lab-on-chip, as a proof-of-concept.

Rapid droplet mixers for digital microfluidic systems

The mixing of analytes and reagents for a biological or chemical lab-on-a-chip is an important, yet difficult, microfluidic operation. As volumes approach the sub-nanoliter regime, the mixing of liquids is hindered by laminar flow conditions. An electrowetting-based linear-array droplet mixer has previously been reported. However, fixed geometric parameters and the presence of flow reversibility have prevented even faster droplet mixing times. In this paper, we study the effects of varying droplet aspect ratios (height:diameter) on linear-array droplet mixers, and propose mixing strategies applicable for both high and low aspect ratio systems. An optimal aspect ratio for four electrode linear-array mixing was found to be 0.4, with a mixing time of 4.6 seconds. Mixing times were further reduced at this ratio to less than three seconds using a two-dimensional array mixer, which eliminates the effects of flow reversibility. For lower aspect ratio (</=0.2) systems, we present a split-and-merge mixer that takes advantage of the ability to perform droplet splitting at these ratios, resulting in a mixing time of less than two seconds.

Electrowetting-based droplet mixers for microfluidic systems

Mixing of analytes and reagents is a critical step in realizing a lab-on-a-chip. However, mixing of liquids is very difficult in continuous flow microfluidics due to laminar flow conditions. An alternative mixing strategy is presented based on the discretization of liquids into droplets and further manipulation of those droplets by electrowetting. The interfacial tensions of the droplets are controlled with the application of voltage. The droplets act as virtual mixing chambers, and mixing occurs by transporting the droplet across an electrode array. We also present an improved method for visualization of mixing where the top and side views of mixing are simultaneously observed. Microliters of liquid droplets are mixed in less than five seconds, which is an order of magnitude improvement in reported mixing times of droplets. Flow reversibility hinders the process of mixing during linear droplet motion. This mixing process is not physically confined and can be dynamically reconfigured to any location on the chip to improve the throughput of the lab-on-a-chip.