Publications by authors named "Kangning Ren"

37 Publications

"Barcode" cell sensor microfluidic system: Rapid and sample-to-answer antimicrobial susceptibility testing applicable in resource-limited conditions.

Biosens Bioelectron 2021 Nov 21;192:113516. Epub 2021 Jul 21.

Department of Chemistry, Hong Kong Baptist University. Waterloo Road, Kowloon, Hong Kong, China; HKBU Institute of Research and Continuing Education, Shenzhen, China; State Key Laboratory of Environmental and Biological Analysis, The Hong Kong Baptist University, Waterloo Road, Kowloon, Hong Kong, China. Electronic address:

Many rapid antimicrobial susceptibility testing (AST) methods have been proposed to contain clinical antimicrobial resistance (AMR) and preserve the effectiveness of remaining antimicrobials. However, far fewer methods have been proposed to test AMR in resource-limited conditions, such as for frequent safety screenings of water/food/public facilities, urgent surveys of massive samples during a pandemic, or AMR tests in low-income countries. Rapid AST methods realized thus far have a variety of drawbacks when used for such surveys, e.g., high cost and the requirement of expensive instruments such as microscopy. A more reasonable strategy would be to screen samples via onsite testing first, and then send any sample suspected to contain AMR bacteria for advanced testing. Accordingly, a cost-efficient AST is demanded, which can rapidly process a large number of samples without using expensive equipment. To this end, current work demonstrates a novel "barcode" cell sensor based on an adaptive linear filter array as a fully automatic and microscope-free method for counting very small volumes of cells (~1.00 × 10 cells without pre-incubation), wherein suspended cells concentrate into microbars with length proportional to the number of cells. We combined this sensor with an on-chip culture approach we had demonstrated for rapid and automated drug exposure and realized a low-cost and resource-independent platform for portable AST, from which results can be obtained simply through a cell phone. This method has a much shorter turnaround time (2-3 h) than that of standard methods (16-24 h). Thanks to its microscopy-free analysis, affordability, portability, high throughput, and user-friendliness, our "barcode" AST system has the potential to fulfill the various demands of AST when advanced facilities are not available, making it a promising new tool in the fight against AMR.
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http://dx.doi.org/10.1016/j.bios.2021.113516DOI Listing
November 2021

Defect-induced activity enhancement of enzyme-encapsulated metal-organic frameworks revealed in microfluidic gradient mixing synthesis.

Sci Adv 2020 01 29;6(5):eaax5785. Epub 2020 Jan 29.

Department of Chemistry, Hong Kong Baptist University, Hong Kong, P. R. China.

Mimicking the cellular environment, metal-organic frameworks (MOFs) are promising for encapsulating enzymes for general applications in environments often unfavorable for native enzymes. Markedly different from previous researches based on bulk solution synthesis, here, we report the synthesis of enzyme-embedded MOFs in a microfluidic laminar flow. The continuously changed concentrations of MOF precursors in the gradient mixing on-chip resulted in structural defects in products. This defect-generating phenomenon enables multimodal pore size distribution in MOFs and therefore allows improved access of substrates to encapsulated enzymes while maintaining the protection to the enzymes. Thus, the as-produced enzyme-MOF composites showed much higher (~one order of magnitude) biological activity than those from conventional bulk solution synthesis. This work suggests that while microfluidic flow synthesis is currently underexplored, it is a promising strategy in producing highly active enzyme-MOF composites.
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http://dx.doi.org/10.1126/sciadv.aax5785DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6989138PMC
January 2020

Crack engineering for the construction of arbitrary hierarchical architectures.

Proc Natl Acad Sci U S A 2019 11 7;116(48):23909-23914. Epub 2019 Nov 7.

Department of Chemistry, Hong Kong Baptist University, Kowloon, 999077 Hong Kong, China;

Three-dimensional hierarchical morphologies widely exist in natural and biomimetic materials, which impart preferential functions including liquid and mass transport, energy conversion, and signal transmission for various applications. While notable progress has been made in the design and manufacturing of various hierarchical materials, the state-of-the-art approaches suffer from limited materials selection, high costs, as well as low processing throughput. Herein, by harnessing the configurable elastic crack engineering-controlled formation and configuration of cracks in elastic materials-an effect normally avoided in various industrial processes, we report the development of a facile and powerful technique that enables the faithful transfer of arbitrary hierarchical structures with broad material compatibility and structural and functional integrity. Our work paves the way for the cost-effective, large-scale production of a variety of flexible, inexpensive, and transparent 3D hierarchical and biomimetic materials.
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http://dx.doi.org/10.1073/pnas.1915332116DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6883777PMC
November 2019

Reliable and reusable whole polypropylene plastic microfluidic devices for a rapid, low-cost antimicrobial susceptibility test.

Lab Chip 2019 09 1;19(17):2915-2924. Epub 2019 Aug 1.

Department of Chemistry, Hong Kong Baptist University, Waterloo Rd, Kowloon, Hong Kong, China. and HKBU Institute of Research and Continuing Education, Shenzhen, China and State Key Laboratory of Environmental and Biological Analysis, The Hong Kong Baptist University, Waterloo Rd, Kow-loon, Hong Kong, China.

Using an antimicrobial susceptibility test (AST) as an example, this work demonstrates a practical method to fabricate microfluidic chips entirely from polypropylene (PP) and the benefits for potential commercial use. Primarily caused by the misuse and abuse of antibiotics, antimicrobial resistance (AMR) is a major threat to modern medicine. The AST is a promising technique to help with the optimal use of antibiotics for reducing AMR. However, current phenotypic ASTs suffer from long turnaround time, while genotypic ASTs suffer from low reliability, and both are unaffordable for routine use. New microfluidics based AST methods are rapid but still unreliable as well as costly due to the PDMS chip material. Herein, we demonstrate a convenient method to fabricate whole PP microfluidic chips with high resolution and fidelity. Unlike PDMS chips, the whole PP chips showed better reliability due to their inertness; they are solvent-compatible and can be conveniently reused and recycled, which largely decreases the cost, and are environmentally friendly. We specially designed 3D chambers that allow for quick cell loading without valving/liquid exchange; this new hydrodynamic design satisfies the shear stress requirement for on-chip bacterial culture, which, compared to reported designs for similar purposes, allows for a simpler, more rapid, and high-throughput operation. Our system allows for reliable tracking of individual cells and acquisition of AST results within 1-3 hours, which is among the group of fastest phenotypic methods. The PP chips are more reliable and affordable than PDMS chips, providing a practical solution to improve current culture-based AST and benefiting the fight against AMR through helping doctors prescribe effective, narrow-spectrum antibiotics; they will also be broadly useful for other applications wherein a reliable, solvent-resistant, anti-fouling, and affordable microfluidic chip is needed.
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http://dx.doi.org/10.1039/c9lc00502aDOI Listing
September 2019

Microfluidic technologies for vasculature biomimicry.

Analyst 2019 Jul;144(15):4461-4471

Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China. and Division of Biomedical Engineering, The Hong Kong University of Science and Technology, Hong Kong, China.

Microfluidic technology has been extensively employed in biology and medicine since the field emerged in the 1990s. By utilizing microfluidic approaches, a variety of vascular system-related structures and functions have been mimicked on in vitro platforms. Herein, we begin by introducing microfluidic circulatory devices for the study of two-dimensional (2D) endothelial cells culture. Next, we focus on recent progress on on-chip mimicry of native vasculature, specifically generation of complex three-dimensional (3D) structures within cell-laden hydrogels using microfluidics and self-assembly-based methods. The utilization of microfluidic technology will facilitate the construction of progressively biomimetic in vitro models that have great potential in complementing existing animal models. We envision such platforms to be utilized in a wide range of applications involving vascular systems, including microphysiological studies, drug screening, and disease modeling.
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http://dx.doi.org/10.1039/c9an00421aDOI Listing
July 2019

A portable oligonucleotide-based microfluidic device for the detection of VEGF in a three-step suspended-droplet mode.

Dalton Trans 2019 Jul;48(26):9824-9830

Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China.

Vascular endothelial growth factor (VEGF165), an important glycosylated protein from the VEGF family, is a type of signal protein highly associated with the development and progression of cancers. In this work, we designed a G-quadruplex-based aptasensing platform for the sensitive and selective detection of VEGF165 in aqueous solution and red blood cell solution. A long-lived phosphorescence iridium(iii) complex (1) with promising photophysical properties and a large Stokes shift was chosen as a selective G-quadruplex probe. The platform could achieve a limit of detection (LOD) down to the picomolar level using a conventional fluorometer. Furthermore, we successfully applied the platform to a three-step suspended droplet (SD)-based microfluidic device for the monitoring of VEGF165. In contrast to the channel-based and digital microfluidic chips, SD-based chips allow easy introduction of liquid samples, valve-free manipulation of multiple reaction steps and flexible volume range. Importantly, polypropylene (PP), a hydrophobic and thermally stable material, was chosen as a substrate to fabricate the chip for the SD-based microfluidic device. The PP-based chip allows the combination of superhydrophobic force, gravity and surface tension for effective driving of the suspended droplet throughout the channel without reverse migration. After assembling all the major components, including a UV lamp, a rotatable chip holder, a filter and a camera into the portable device, we successfully demonstrated the applicability of the device to detect VEGF165 in aqueous solution with a LOD of 0.33 nM at a signal-to-noise ratio (S/N) of 3 and a linear range of 1-100 nM.
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http://dx.doi.org/10.1039/c9dt00427kDOI Listing
July 2019

Microfluidics for Combating Antimicrobial Resistance.

Trends Biotechnol 2017 12 15;35(12):1129-1139. Epub 2017 Nov 15.

Department of Chemistry, Hong Kong Baptist University, Hong Kong, China; Stanford University School of Medicine, Stanford, CA, United States; HKBU Institute of Research and Continuing Education, Shenzhen, China; State Key Laboratory of Environmental and Biological Analysis, Hong Kong Baptist University, Waterloo Road, Kowloon, Hong Kong, China. Electronic address:

The ever-growing threat of antimicrobial resistance (AMR) demands immediate countermeasures. With its novelty and enabling features including downscaled analysis, precisely controlled local environment, and enhanced speed, accuracy, and cost-efficiency, microfluidics has demonstrated potential in several key areas, including furthering our understanding of bacteria, developing better susceptibility testing tools, and overcoming obstacles in discovery and research of new antibiotics. While ample research results in the field of microfluidics are available, their transformation into practical application is still lagging far behind. We believe that the challenge of AMR will give microfluidics a much-needed opportunity to leap from research papers to true productivity, and gain wider acceptance as a mature technology.
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http://dx.doi.org/10.1016/j.tibtech.2017.07.008DOI Listing
December 2017

A suspending-droplet mode paper-based microfluidic platform for low-cost, rapid, and convenient detection of lead(II) ions in liquid solution.

Biosens Bioelectron 2018 Jan 1;99:361-367. Epub 2017 Aug 1.

Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China; HKBU Institute of Research and Continuing Education, Shenzhen, China; State Key Laboratory of Environmental and Biological Analysis, The Hong Kong Baptist University, Waterloo Rd, Kowloon, Hong Kong, China. Electronic address:

A paper-based microfluidic device based on unconventional principle was developed and used to detect lead ions through a two-step process including heated incubation and subsequent mixing. The device was made by generating a superhydrophobic pattern, which defines channel and reservoir barriers, on a water-impermeable paper substrate, followed by loading and drying the reagents in the defined reservoirs. Different from the conventional paper-based devices that are made of water-permeable paper, the as-prepared device holds water drops in discrete reservoirs, and the water drops will not move unless the device is titled along the direction of the predefined channels. In this way, the liquid samples applied onto the device are handled as individual drops and could be stored, transported, and mixed on demand. Different from the conventional paper-based devices that use capillary force to drive liquid, our new device uses wetting and gravity as driving force. We name this operation principle suspending-droplet mode paper-based device (SD-μPAD). The use of a Teflon contact-printing stamp makes the production of such devices rapid, cost efficient, and mass productive. Utilizing a G-quadruplex-based luminescence switch-on assay, we demonstrated rapid, convenient, highly sensitive, and low cost detection of lead(II) ions in water samples, using a custom made battery-powered portable device, and a smart phone as the detector.
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http://dx.doi.org/10.1016/j.bios.2017.07.073DOI Listing
January 2018

A Multiplexed, Gradient-Based, Full-Hydrogel Microfluidic Platform for Rapid, High-Throughput Antimicrobial Susceptibility Testing.

Chempluschem 2017 May;82(5):792-801

Department of Chemistry, Hong Kong Baptist University, Waterloo Rd, Kowloon, Hong Kong, P. R China.

Antimicrobial resistance has become an immediate threat to modern healthcare systems as it continues to spread across the globe. As development of novel antibiotics stalls, preserving the effectiveness of existing agents has become a priority. One of the major driving forces behind antimicrobial resistance is the misuse and overuse of antibiotics, often a result of data on the susceptibility of pathogens not being obtained in a convenient and timely manner, a need that conventional antimicrobial susceptibility testing struggles to meet. Here, a hydrogel microfluidic platform is reported for antimicrobial susceptibility testing purposes, capable of handling real samples and yielding results within 2.5 h of culture. By using a multiplayer design with channels crossing overhead of each other, multiple experiments, either one- or two-dimensional, can be staged on the same device. Bacteria grown on the surface of the hydrogel can be easily visualized with standard Gram staining after being transferred onto a glass slide. Coupled with software-based image analysis, the system can yield a variety of useful information on bacterial susceptibility and the effects of drugs, such as minimum inhibitory concentration and morphological changes in bacteria, either individually or in combination. Compared to conventional testing methods, this system requires less labor, reagents, and equipment to operate, and has significantly higher speed and efficiency.
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http://dx.doi.org/10.1002/cplu.201600654DOI Listing
May 2017

The application of a G-quadruplex based assay with an iridium(iii) complex to arsenic ion detection and its utilization in a microfluidic chip.

J Mater Chem B 2017 Jan 21;5(3):479-484. Epub 2016 Dec 21.

Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China.

In this work, the iridium(iii) complex 1 was synthesized and employed in constructing an assay which is based on a G-quadruplex for detecting arsenic ions in aqueous solution. The assay achieved a detection limit of 7.6 nM (ca. 0.57 μg L) and showed high selectivity towards arsenic ions over other metal ions. Additionally, the assay could function in natural water and a simple microfluidic chip was used to investigate the potential of this platform for real-time detection.
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http://dx.doi.org/10.1039/c6tb02656gDOI Listing
January 2017

A one-step strategy for ultra-fast and low-cost mass production of plastic membrane microfluidic chips.

Lab Chip 2016 10;16(20):3909-3918

Department of Chemistry, Hong Kong Baptist University, Waterloo Rd, Kowloon, Hong Kong, China. and HKBU Institute of Research and Continuing Education, Shenzhen, China and State Key Laboratory of Environmental and Biological Analysis, The Hong Kong Baptist University, Waterloo Rd, Kowloon, Hong Kong, China.

An ultra-fast, extremely cost-effective, and environmentally friendly method was developed for fabricating flexible microfluidic chips with plastic membranes. With this method, we could fabricate plastic microfluidic chips rapidly (within 12 seconds per piece) at an extremely low cost (less than $0.02 per piece). We used a heated perfluoropolymer perfluoroalkoxy (often called Teflon PFA) solid stamp to press a pile of two pieces of plastic membranes, low density polyethylene (LDPE) and polyethylene terephthalate (PET) coated with an ethylene-vinyl acetate copolymer (EVA). During the short period of contact with the heated PFA stamp, the pressed area of the membranes permanently bonded, while the LDPE membrane spontaneously rose up at the area not pressed, forming microchannels automatically. These two regions were clearly distinguishable even at the micrometer scale so we were able to fabricate microchannels with widths down to 50 microns. This method combines the two steps in the conventional strategy for microchannel fabrication, generating microchannels and sealing channels, into a single step. The production is a green process without using any solvent or generating any waste. Also, the chips showed good resistance against the absorption of Rhodamine 6G, oligonucleotides, and green fluorescent protein (GFP). We demonstrated some typical microfluidic manipulations with the flexible plastic membrane chips, including droplet formation, on-chip capillary electrophoresis, and peristaltic pumping for quantitative injection of samples and reagents. In addition, we demonstrated convenient on-chip detection of lead ions in water samples by a peristaltic-pumping design, as an example of the application of the plastic membrane chips in a resource-limited environment. Due to the high speed and low cost of the fabrication process, this single-step method will facilitate the mass production of microfluidic chips and commercialization of microfluidic technologies.
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http://dx.doi.org/10.1039/c6lc00957cDOI Listing
October 2016

Freestanding 3-D microvascular networks made of alginate hydrogel as a universal tool to create microchannels inside hydrogels.

Biomicrofluidics 2016 Jul 29;10(4):044112. Epub 2016 Aug 29.

Department of Chemistry, The Hong Kong University of Science and Technology , Clear Water Bay, Kowloon, Hong Kong, China.

The diffusion of molecules such as nutrients and oxygen through densely packed cells is impeded by blockage and consumption by cells, resulting in a limited depth of penetration. This has been a major hurdle to a bulk (3-D) culture. Great efforts have been made to develop methods for generating branched microchannels inside hydrogels to support mass exchange inside a bulk culture. These previous attempts faced a common obstacle: researchers tried to fabricate microchannels with gels already loaded with cells, but the fabrication procedures are often harmful to the embedded cells. Herein, we present a universal strategy to create microchannels in different types of hydrogels, which effectively avoids cell damage. This strategy is based on a freestanding alginate 3-D microvascular network prepared by in-situ generation of copper ions from a sacrificial copper template. This alginate network could be used as implants to create microchannels inside different types of hydrogels. This approach effectively addresses the issue of cell damage during microfabrication and made it possible to create microchannels inside different types of gels. The microvascular network produced with this method is (1) strong enough to allow handling, (2) biocompatible to allow cell culturing, and (3) appropriately permeable to allow diffusion of small molecules, while sufficiently dense to prevent blocking of channels when embedded in different types of gels. In addition, composite microtubules could be prepared by simply pre-loading other materials, e.g., particles and large biomolecules, in the hydrogel. Compared with other potential strategies to fabricate freestanding gel channel networks, our method is more rapid, low-cost and scalable due to parallel processing using an industrially mass-producible template. We demonstrated the use of such vascular networks in creating microchannels in different hydrogels and composite gels, as well as with a cell culture in a nutrition gradient based on microfluidic diffusion. In this way, the freestanding hydrogel vascular network we produced is a universal functional unit that can be embedded in different types of hydrogel; users will be able to adopt this strategy to achieve vascular mass exchange in the bulk culture without changing their current protocol. The method is readily implementable to applications in vascular tissue regeneration, drug discovery, 3-D culture, etc.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5010556PMC
http://dx.doi.org/10.1063/1.4961969DOI Listing
July 2016

Cell-on-hydrogel platform made of agar and alginate for rapid, low-cost, multidimensional test of antimicrobial susceptibility.

Lab Chip 2016 08;16(16):3130-8

Department of Chemistry,, Hong Kong Baptist University, Waterloo Rd, Kowloon, Hong Kong, China. and State Key Laboratory of Environmental and Biological Analysis, The Hong Kong Baptist University, Waterloo Rd, Kowloon, Hong Kong, China and HKBU Institute of Research and Continuing Education, Shenzhen, China.

Antimicrobial resistance (AMR) is a rapidly increasing threat to the effective treatment of infectious diseases worldwide. The two major remedies include: (1) using narrow-spectrum antibiotics based on rapid diagnosis; and (2) developing new antibiotics. A key part of both remedies is the antimicrobial susceptibility test (AST). However, the current standard ASTs that monitor colony formation are costly and time-consuming and the new strategies proposed are not yet practical to be implemented. Herein, we report a strategy to fabricate whole-hydrogel microfluidic chips using alginate-doped agar. This agar-based microfabrication makes it possible to prepare inexpensive hydrogel devices, and allows a seamless link between microfluidics and conventional agar-based cell culture. Different from common microfluidic systems, in our system the cells are cultured on top of the device, similar to normal agar plate culture; on the other hand, the microfluidic channels inside the hydrogel allow precise generation of linear gradient of drugs, thus giving a better performance than the conventional disk diffusion method. Cells in this system are not exposed to any shear flow, which allows the reliable tracking of individual cells and AST results to be obtained within 2-3 hours. Furthermore, our system could test the synergistic effect of drugs through two-dimensional gradient generation. Finally, the platform could be directly implemented to new drug discovery and other applications wherein a fast, cost-efficient method for studying the response of microorganisms upon drug administration is desirable.
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http://dx.doi.org/10.1039/c6lc00417bDOI Listing
August 2016

G-quadruplex-based logic gates for Hg and Ag ions employing a luminescent iridium(iii) complex and extension of metal-mediated base pairs by polymerase.

J Mater Chem B 2015 Jun 18;3(24):4780-4785. Epub 2015 May 18.

Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China.

We report herein the synthesis of a series of cyclometallated iridium(iii) complexes as luminescent G-quadruplex-selective probes, which were used to construct AND, OR and INHIBIT logic gates for the detection of Hg and Ag ions. To our knowledge, this is the first time that the C-Ag-T mismatched base pair has been used for the construction of luminescent assays or logic gates.
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http://dx.doi.org/10.1039/c5tb00718fDOI Listing
June 2015

LprG-mediated surface expression of lipoarabinomannan is essential for virulence of Mycobacterium tuberculosis.

PLoS Pathog 2014 Sep 18;10(9):e1004376. Epub 2014 Sep 18.

Department of Pathology, Stanford University, Stanford, California, United States of America; Division of Infectious Diseases and Geographic Medicine, Department of Medicine, Stanford University, Stanford, California, United States of America.

Mycobacterium tuberculosis employs various virulence strategies to subvert host immune responses in order to persist and cause disease. Interaction of M. tuberculosis with mannose receptor on macrophages via surface-exposed lipoarabinomannan (LAM) is believed to be critical for cell entry, inhibition of phagosome-lysosome fusion, and intracellular survival, but in vivo evidence is lacking. LprG, a cell envelope lipoprotein that is essential for virulence of M. tuberculosis, has been shown to bind to the acyl groups of lipoglycans but the role of LprG in LAM biosynthesis and localization remains unknown. Using an M. tuberculosis lprG mutant, we show that LprG is essential for normal surface expression of LAM and virulence of M. tuberculosis attributed to LAM. The lprG mutant had a normal quantity of LAM in the cell envelope, but its surface was altered and showed reduced expression of surface-exposed LAM. Functionally, the lprG mutant was defective for macrophage entry and inhibition of phagosome-lysosome fusion, was attenuated in macrophages, and was killed in the mouse lung with the onset of adaptive immunity. This study identifies the role of LprG in surface-exposed LAM expression and provides in vivo evidence for the essential role surface LAM plays in M. tuberculosis virulence. Findings have translational implications for therapy and vaccine development.
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http://dx.doi.org/10.1371/journal.ppat.1004376DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4169494PMC
September 2014

Recent developments in microfluidics for cell studies.

Adv Mater 2014 Aug 17;26(31):5525-32. Epub 2014 Feb 17.

Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.

As a technique for precisely manipulating fluid at the micrometer scale, the field of microfluidics has experienced an explosive growth over the past two decades, particularly owing to the advances in device design and fabrication. With the inherent advantages associated with its scale of operation, and its flexibility in being incorporated with other microscale techniques for manipulation and detection, microfluidics has become a major enabling technology, which has introduced new paradigms in various fields involving biological cells. A microfluidic device is able to realize functions that are not easily imaginable in conventional biological analysis, such as highly parallel, sophisticated high-throughput analysis, single-cell analysis in a well-defined manner, and tissue engineering with the capability of manipulation at the single-cell level. Major advancements in microfluidic device fabrication and the growing trend of implementing microfluidics in cell studies are presented, with a focus on biological research and clinical diagnostics.
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http://dx.doi.org/10.1002/adma.201305348DOI Listing
August 2014

New materials for microfluidics in biology.

Curr Opin Biotechnol 2014 Feb 5;25:78-85. Epub 2013 Oct 5.

Department of Chemistry and Division of Biomedical Engineering, The Hong Kong University of Science and Technology, Hong Kong. Electronic address:

With its continuous progress, microfluidics has become a key enabling technology in biological research. During the past few years, the major growth of microfluidics shifted to the introduction of new materials in making microfluidic chips, primarily driven by the demand of versatile strategies to interface microfluidics with biological cell studies. Although polydimethylsiloxane is still used as primary frame material, hydrogels have been increasingly employed in cell-culture related applications. Moreover, plastics and paper are attracting more attention in commercial device fabrication. Aiming to reflect this trend, current review focuses on the progress of microfluidic chip materials over the time span of January 2011 through June 2013, and provides critical discussion of the resulting major new tools in biological research.
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http://dx.doi.org/10.1016/j.copbio.2013.09.004DOI Listing
February 2014

Oil-water biphasic parallel flow for the precise patterning of metals and cells.

Biomed Microdevices 2014 Apr;16(2):245-53

Institute of Basic Medical Sciences Chinese Academy of Medical Sciences, School of Basic Medicine Peking Union Medical College, 100005, Beijing, People's Republic of China.

Fluidic patterning is a convenient and versatile tool for the patterning of materials, cells and microstructures on surface and in microchannels. However, its performance is usually limited by transverse diffusion between fluid streams. It would blur the boundary and deteriorate the precision of patterns. In this paper, we adopted geometric confinement to generate biphasic parallel flow that is constituted of oil and water. Since there is minimum transverse diffusion in biphasic parallel flow, the performance of fluid patterning is expected to be improved. The results show that the metal (Silver and Chromium) patterns have distinct boundary and well-controlled geometry in comparison with that by conventional laminar flow patterning. Furthermore, the high biocompatibility of oil phase (perfluorodecalin, PFD) enables the precise patterning of viable bacteria inside microchannels. Our work demonstrated a new route of using biphasic parallel flow to patterning, which would serve wide applications in prototyping and research settings.
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http://dx.doi.org/10.1007/s10544-013-9828-yDOI Listing
April 2014

Materials for microfluidic chip fabrication.

Acc Chem Res 2013 Nov 11;46(11):2396-406. Epub 2013 Jun 11.

Department of Chemistry, the Hong Kong University of Science and Technology , Clear Water Bay, Kowloon, Hong Kong, China.

Through manipulating fluids using microfabricated channel and chamber structures, microfluidics is a powerful tool to realize high sensitive, high speed, high throughput, and low cost analysis. In addition, the method can establish a well-controlled microenivroment for manipulating fluids and particles. It also has rapid growing implementations in both sophisticated chemical/biological analysis and low-cost point-of-care assays. Some unique phenomena emerge at the micrometer scale. For example, reactions are completed in a shorter amount of time as the travel distances of mass and heat are relatively small; the flows are usually laminar; and the capillary effect becomes dominant owing to large surface-to-volume ratios. In the meantime, the surface properties of the device material are greatly amplified, which can lead to either unique functions or problems that we would not encounter at the macroscale. Also, each material inherently corresponds with specific microfabrication strategies and certain native properties of the device. Therefore, the material for making the device plays a dominating role in microfluidic technologies. In this Account, we address the evolution of materials used for fabricating microfluidic chips, and discuss the application-oriented pros and cons of different materials. This Account generally follows the order of the materials introduced to microfluidics. Glass and silicon, the first generation microfluidic device materials, are perfect for capillary electrophoresis and solvent-involved applications but expensive for microfabriaction. Elastomers enable low-cost rapid prototyping and high density integration of valves on chip, allowing complicated and parallel fluid manipulation and in-channel cell culture. Plastics, as competitive alternatives to elastomers, are also rapid and inexpensive to microfabricate. Their broad variety provides flexible choices for different needs. For example, some thermosets support in-situ fabrication of arbitrary 3D structures, while some perfluoropolymers are extremely inert and antifouling. Chemists can use hydrogels as highly permeable structural material, which allows diffusion of molecules without bulk fluid flows. They are used to support 3D cell culture, to form diffusion gradient, and to serve as actuators. Researchers have recently introduced paper-based devices, which are extremely low-cost to prepare and easy to use, thereby promising in commercial point-of-care assays. In general, the evolution of chip materials reflects the two major trends of microfluidic technology: powerful microscale research platforms and low-cost portable analyses. For laboratory research, chemists choosing materials generally need to compromise the ease in prototyping and the performance of the device. However, in commercialization, the major concerns are the cost of production and the ease and reliability in use. There may be new growth in the combination of surface engineering, functional materials, and microfluidics, which is possibly accomplished by the utilization of composite materials or hybrids for advanced device functions. Also, significant expanding of commercial applications can be predicted.
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http://dx.doi.org/10.1021/ar300314sDOI Listing
November 2013

Monolithic integration of fine cylindrical glass microcapillaries on silicon for electrophoretic separation of biomolecules.

Biomicrofluidics 2012 Sep 20;6(3):36501. Epub 2012 Jul 20.

Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.

We demonstrate monolithic integration of fine cylindrical glass microcapillaries (diameter ∼1 μm) on silicon and evaluate their performance for electrophoretic separation of biomolecules. Such microcapillaries are achieved through thermal reflow of a glass layer on microstructured silicon whereby slender voids are moulded into cylindrical tubes. The process allows self-enclosed microcapillaries with a uniform profile. A simplified method is also described to integrate the microcapillaries with a sample-injection cross without the requirement of glass etching. The 10-mm-long microcapillaries sustain field intensities up to 90 kV/m and limit the temperature excursions due to Joule heating to a few degrees Celsius only.
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http://dx.doi.org/10.1063/1.4739075DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3411555PMC
September 2012

Sorting inactivated cells using cell-imprinted polymer thin films.

ACS Nano 2013 Jul 5;7(7):6031-6. Epub 2013 Jun 5.

Department of Chemistry, Stanford University, Stanford, California 94305-5080, USA.

Previous work showed that cell imprinting in a poly(dimethylsiloxane) film produced artificial receptors to cells by template-assisted rearrangement of functional groups on the surface of the polymer thin film which facilitated cell capture in the polymer surface indentations by size, shape, and, most importantly, chemical recognition. We report here that inactivation of cells by treatment with formaldehyde (4%), glutaraldehyde (2%), or a combination of the two leads to markedly improved capture selectivity (a factor of 3) when cells to be analyzed are inactivated in the same manner. The enhanced capture efficiency compared to living cells results from two factors: (1) rigidification of the cell surface through cross-linking of amine groups by the aldehyde; and (2) elimination of chemicals excreted from living cells which interfere with the fidelity of the cell-imprinting process. Moreover, cell inactivation has the advantage of removing biohazard risks associated with working with virulent bacteria. These results are demonstrated using different strains of Mycobacterium tuberculosis.
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http://dx.doi.org/10.1021/nn401768sDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3722270PMC
July 2013

Chemical recognition in cell-imprinted polymers.

ACS Nano 2012 May 6;6(5):4314-8. Epub 2012 Apr 6.

Department of Chemistry, Stanford University, Stanford, California 94305-5080, USA.

A glass slide covered with bacteria is pressed into another glass slide coated with partially cured polydimethylsiloxane (PDMS). The PDMS is hardened and the cells are removed to create a textured surface whose indentations preferentially capture the same type of bacteria when a mixture of bacteria is flowed over it. Overcoating the cell-imprinted PDMS with methylsilane groups causes the resulting surface to lose much of its ability to preferentially capture the imprinted bacteria, although the shapes of the imprints, measured by atomic force field microscopy, are shown to be hardly affected. We interpret this behavior as strong evidence that chemical recognition plays a dominant role in cell sorting with cell-imprinted PDMS polymer films.
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http://dx.doi.org/10.1021/nn300901zDOI Listing
May 2012

Convenient formation of nanoparticle aggregates on microfluidic chips for highly sensitive SERS detection of biomolecules.

Anal Bioanal Chem 2012 Feb 30;402(4):1601-9. Epub 2011 Nov 30.

Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.

Microfluidic chips combined with surface-enhanced Raman spectroscopy (SERS) offer an outstanding platform for rapid and high-sensitivity chemical analysis. However, it is nontrivial to conveniently form nanoparticle aggregrates (as SERS-active spots for SERS detection) in microchannels in a well-controlled manner. Here, we present a rapid, highly sensitive and label-free analytical technique for determining bovine serum albumin (BSA) on a poly(dimethylsiloxane) (PDMS) microfluidic chip using SERS. A modified PDMS pneumatic valve and nanopost arrays at the bottom of the fluidic microchannel are used for reversibly trapping gold nanoparticles to form gold aggregates, creating SERS-active spots for Raman detection. We fabricated a chip that consisted of a T-shaped fluidic channel and two modified pneumatic valves, which was suitable for fast loading of samples. Quantitative analysis of BSA is demonstrated with the measured peak intensity at 1,615 cm(-1) in the surface-enhanced Raman spectra. With our microfluidic chip, the detection limit of Raman can reach as low as the picomolar level, comparable to that of normal mass spectrometry.
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http://dx.doi.org/10.1007/s00216-011-5585-zDOI Listing
February 2012

A prototypic system of parallel electrophoresis in multiple capillaries coupled with microwell arrays.

Electrophoresis 2011 Nov 10;32(23):3324-30. Epub 2011 Nov 10.

Nano Science and Technology Program, Hong Kong University of Science and Technology, Kowloon, Hong Kong, P R China.

We present a microfluidic system that can be directly coupled with microwell array and perform parallel electrophoresis in multiple capillaries simultaneously. The system is based on an array of glass capillaries, fixed in a polydimethylsiloxane (PDMS) microfluidic scaffold, with one end open for interfacing with microwells. In this capillary array, every two adjacent capillaries act as a pair to be coupled with one microwell; samples in the microwells are introduced and separated by simply applying voltage between two electrodes that are placed at the other ends of capillaries; thus no complicated circuit design is required. We evaluate the performance of this system and perform multiple CE with direct sample introduction from microwell array. Also with this system, we demonstrate the analysis of cellular contents of cells lysed in a microwell array. Our results show that this prototypic system is a promising platform for high-throughput analysis of samples in microwell arrays.
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http://dx.doi.org/10.1002/elps.201100339DOI Listing
November 2011

Pumping-induced perturbation of flow in microfluidic channels and its implications for on-chip cell culture.

Lab Chip 2011 Jul 23;11(13):2288-94. Epub 2011 May 23.

Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.

We study the rate of response to changes in the rate of flow and the perturbations in flow in polydimethylsiloxane (PDMS) microfluidic chips that are subjected to several common flow-control systems. We find that the flow rate of liquid delivered from a syringe pump equipped with a glass syringe responds faster to the changes in the conditions of flow than the same liquid delivered from a plastic syringe; and the rate of flow delivered from compressed air responds faster than that from a glass syringe. We discover that the rate of flow that is driven by a syringe pump and regulated by an integrated pneumatic valve responds even faster, but this flow-control method is characterized by large perturbations. We also examine the possible effects of these large perturbations on NIH 3T3 cells in microfluidic channels and find that they could cause the detachment of NIH 3T3 cells in the microchannels.
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http://dx.doi.org/10.1039/c0lc00466aDOI Listing
July 2011

Single-cell assays.

Biomicrofluidics 2011 Apr 14;5(2):21501. Epub 2011 Apr 14.

Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People's Republic of China.

This review presents an overview of literature that describes the applications of microfluidics to assay individual cells. We quantify the content of an individual mammalian cell, so that we can understand what criteria a single-cell assay must satisfy to be successful. We put in context the justification for single-cell assays and identify the characteristics that are relevant to single-cell assays. We review the literature from the past 24 months that describe the methods that use microfabrication-conventional or otherwise-and microfluidics in particular to study individual cells, and we present our views on how an increasing emphasis on three-dimensional cell culture and the demonstration of the first chemically defined cell might impact single-cell assays.
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http://dx.doi.org/10.1063/1.3574448DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3089644PMC
April 2011

Whole-Teflon microfluidic chips.

Proc Natl Acad Sci U S A 2011 May 2;108(20):8162-6. Epub 2011 May 2.

Department of Chemistry, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.

Although microfluidics has shown exciting potential, its broad applications are significantly limited by drawbacks of the materials used to make them. In this work, we present a convenient strategy for fabricating whole-Teflon microfluidic chips with integrated valves that show outstanding inertness to various chemicals and extreme resistance against all solvents. Compared with other microfluidic materials [e.g., poly(dimethylsiloxane) (PDMS)] the whole-Teflon chip has a few more advantages, such as no absorption of small molecules, little adsorption of biomolecules onto channel walls, and no leaching of residue molecules from the material bulk into the solution in the channel. Various biological cells have been cultured in the whole-Teflon channel. Adherent cells can attach to the channel bottom, spread, and proliferate well in the channels (with similar proliferation rate to the cells in PDMS channels with the same dimensions). The moderately good gas permeability of the Teflon materials makes it suitable to culture cells inside the microchannels for a long time.
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http://dx.doi.org/10.1073/pnas.1100356108DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3100936PMC
May 2011

Fabrication of a microfluidic Ag/AgCl reference electrode and its application for portable and disposable electrochemical microchips.

Electrophoresis 2010 Sep;31(18):3083-9

Department of Chemistry, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P R China.

This report describes a convenient method for the fabrication of a miniaturized, reliable Ag/AgCl reference electrode with nanofluidic channels acting as a salt bridge that can be easily integrated into microfluidic chips. The Ag/AgCl reference electrode shows high stability with millivolt variations. We demonstrated the application of this reference electrode in a portable microfluidic chip that is connected to a USB-port microelectrochemical station and to a computer for data collection and analysis. The low fabrication cost of the chip with the potential for mass production makes it disposable and an excellent candidate for real-world analysis and measurement. We used the chip to quantitatively analyze the concentrations of heavy metal ions (Cd(2+) and Pb(2+)) in sea water. We believe that the Ag/AgCl reference microelectrode and the portable electrochemical system will be of interest to people in microfluidics, environmental science, clinical diagnostics, and food research.
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http://dx.doi.org/10.1002/elps.201000113DOI Listing
September 2010

Convenient method for modifying poly(dimethylsiloxane) to be airtight and resistive against absorption of small molecules.

Anal Chem 2010 Jul;82(14):5965-71

Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.

In this paper we present a simple and rapid method of modifying poly(dimethylsiloxane) (PDMS) surfaces with paraffin wax. PDMS that contains a layer of paraffin wax at its surface resists the absorption of hydrophobic molecules; we used fluorescence microscopy to confirm that paraffin-modified PDMS resists the absorption of rhodamine B. Furthermore, we demonstrated that microfluidic devices made from PDMS that contains a surface layer of paraffin wax prevent efficiently the transport of gas molecules through the bulk and into microchannels. We characterized the surface of PDMS that contains paraffin wax using the water contact angle, optical transmission, and X-ray photoelectron spectroscopy. We show that PDMS that contains paraffin wax can be substituted for native PDMS; specifically, we fabricated peristaltic valves in PDMS that contains paraffin wax, and the valves showed no degradation in performance after multiple open/close cycles. Finally, we show how to use PDMS that has been treated with paraffin wax as a mold for the fabrication of PDMS replicas; this approach avoids silanization of PDMS, which is a time-consuming step in soft lithography. The wax-modified PDMS channels also show performance superiro to that of bare PDMS in micellar electrokinetic chromatography for quantitative analysis.
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http://dx.doi.org/10.1021/ac100830tDOI Listing
July 2010
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