Microfluidic chip for combinatorial mixing and screening of assays

Automated and miniaturized screening of antibiotic combinations via robotic-printed combinatorial droplet platform

Antimicrobial resistance (AMR) has become a global health crisis in need of novel solutions. To this end, antibiotic combination therapies, which combine multiple antibiotics for treatment, have attracted significant attention as a potential approach for combating AMR. To facilitate advances in antibiotic combination therapies, most notably in investigating antibiotic interactions and identifying synergistic antibiotic combinations however, there remains a need for automated high-throughput platforms that can create and examine antibiotic combinations on-demand, at scale, and with minimal reagent consumption. To address these challenges, we have developed a Robotic-Printed Combinatorial Droplet (RoboDrop) platform by integrating a programmable droplet microfluidic device that generates antibiotic combinations in nanoliter droplets in automation, a robotic arm that arranges the droplets in an array, and a camera that images the array of thousands of droplets in parallel. We further implement a resazurin-based bacterial viability assay to accelerate our antibiotic combination testing. As a demonstration, we use RoboDrop to corroborate two pairs of antibiotics with known interactions and subsequently identify a new synergistic combination of cefsulodin, penicillin, and oxacillin against a model E. coli strain. We therefore envision RoboDrop becoming a useful tool to efficiently identify new synergistic antibiotic combinations toward combating AMR.

Utilisation of molecularly imprinting technology for the detection of glucocorticoids for a point of care surface plasmon resonance (SPR) device

Herein, we describe the synthesis and characterisation of four synthetic recognition materials (nanoMIPs) selective for the glucocorticoid steroids - prednisolone, prednisone, dexamethasone, and cortisone. Using a solid-phase synthesis approach, these materials were then applied in the development of a surface plasmon resonance (SPR) sensor for the detection of these four targets in doped urine, to mimic the routine testing of agricultural waste for possible environmental exposure. The synthesised particles displayed a range of sizes between 104 and 160 nm. Affinity studies were performed, and these synthetic materials were shown to display nanomolar affinities (15.9-62.8 nM) towards their desired targets. Furthermore, we conducted cross-reactivity studies to assess the materials selectivity towards their desired target and the materials showed excellent selectivity when compared to the non-desired target, with selectivity factors calculated. Furthermore, through the use of 3D visualisation it can be seen that small changes between structures (such as a hydroxyl to ketone transformation) there is excellent selectivity between the compounds in the ranges of 100 fold plus. Using Surine™ doped samples the materials offered comparable nanomolar affinities (10.7-75.7 nM) towards their targets when compared to the standardised buffer preparation. Detection levels in urine for all compounds was in the nanomolar range. The developed sensor offers potential for these devices to be used in the prevention of these pharmaceutical compounds to enter the surrounding environment through agricultural waste through monitoring at source. Likewise, they can be used to monitor use in clinical samples.

Microfluidic systems for particle capture and release: A review

Microfluidic technology has emerged as a promising tool in various applications, including biosensing, disease diagnosis, and environmental monitoring. One of the notable features of microfluidic devices is their ability to selectively capture and release specific cells, biomolecules, bacteria, and particles. Compared to traditional bulk analysis instruments, microfluidic capture-and-release platforms offer several advantages, such as contactless operation, label-free detection, high accuracy, good sensitivity, and minimal reagent requirements. However, despite significant efforts dedicated to developing innovative capture mechanisms in the past, the release and recovery efficiency of trapped particles have often been overlooked. Many previous studies have focused primarily on particle capture techniques and their efficiency, disregarding the crucial role of successful particle release for subsequent analysis. In reality, the ability to effectively release trapped particles is particularly essential to ensure ongoing, high-throughput analysis. To address this gap, this review aims to highlight the importance of both capture and release mechanisms in microfluidic systems and assess their effectiveness. The methods are classified into two categories: those based on physical principles and those using biochemical approaches. Furthermore, the review offers a comprehensive summary of recent applications of microfluidic platforms specifically designed for particle capture and release. It outlines the designs and performance of these devices, highlighting their advantages and limitations in various target applications and purposes. Finally, the review concludes with discussions on the current challenges faced in the field and presents potential future directions.

Thermally-actuated microfluidic membrane valve for point-of-care applications

Microfluidics has enabled low volume biochemistry reactions to be carried out at the point-of-care. A key component in microfluidics is the microfluidic valve. Microfluidic valves are not only useful for directing flow at intersections but also allow mixtures/dilutions to be tuned real-time and even provide peristaltic pumping capabilities. In the transition from chip-in-a-lab to lab-on-a-chip, it is essential to ensure that microfluidic valves are designed to require less peripheral equipment and that they are transportable. In this paper, a thermally-actuated microfluidic valve is presented. The valve itself is fabricated with off-the-shelf components without the need for sophisticated cleanroom techniques. It is shown that multiple valves can be controlled and operated via a power supply and an Arduino microcontroller; an important step towards transportable microfluidic devices capable of carrying out analytical assays at the point-of-care. It is been calculated that a single actuator costs less than $1, this highlights the potential of the presented valve for scaling out. The valve operation is demonstrated by adjusting the ratio of a water/dye mixture in a continuous flow microfluidic chip with Y-junction channel geometry. The power required to operate one microfluidic valve has been characterised both theoretically and experimentally. Cyclical operation of the valve has been demonstrated for 65 h with 585 actuations. The presented valve is capable of actuating rectangular microfluidic channels of 500 μm × 50 μm with an expected temperature increase of up to 5 °C. The fastest actuation times achieved were 2 s for valve closing (heating) and 9 s for valve opening (cooling).

Microfluidics for Peptidomics, Proteomics, and Cell Analysis

Microfluidics is the advanced microtechnology of fluid manipulation in channels with at least one dimension in the range of 1-100 microns. Microfluidic technology offers a growing number of tools for manipulating small volumes of fluid to control chemical, biological, and physical processes relevant to separation, analysis, and detection. Currently, microfluidic devices play an important role in many biological, chemical, physical, biotechnological and engineering applications. There are numerous ways to fabricate the necessary microchannels and integrate them into microfluidic platforms. In peptidomics and proteomics, microfluidics is often used in combination with mass spectrometric (MS) analysis. This review provides an overview of using microfluidic systems for peptidomics, proteomics and cell analysis. The application of microfluidics in combination with MS detection and other novel techniques to answer clinical questions is also discussed in the context of disease diagnosis and therapy. Recent developments and applications of capillary and microchip (electro)separation methods in proteomic and peptidomic analysis are summarized. The state of the art of microchip platforms for cell sorting and single-cell analysis is also discussed. Advances in detection methods are reported, and new applications in proteomics and peptidomics, quality control of peptide and protein pharmaceuticals, analysis of proteins and peptides in biomatrices and determination of their physicochemical parameters are highlighted.

On chip optofluidic low-pressure monitoring device

We present an on chip optofluidic surface deformable liquid Dove prism (LDP) based low-fluid flow pressure monitoring device. The unique design of the device in combination with liquid and soft solid enabled by the total internal reflection of light makes the sensor highly sensitive and compatible with the integration of a microfluidic and/or Lab-on-a-chip device. A layer-by-layer soft lithographic (LSL) and 3D printing technique are exploited to make the device. We have used Polydimethylsiloxane (PDMS) as the layer material and two variety of liquids (a) immersion oil (IO) and (b) di-iodomethane (DI) as refracting medium to construct the LDP sensor. Optical ray tracing simulation is performed to optimize the sensor. The pressure sensor shows sensitivity as high as ±28.5 mV per 50 Pa pressure with an error ± 2.5 mV and repeatability of ~99.56% at full scale. We have shown the applicability of the sensor by capturing and analyzing respiratory pressure signals of some human subjects at numerous conditions.

Fundamental Studies of Rapidly Fabricated On-Chip Passive Micromixer for Modular Microfluidics

Micromixers play an important role in many modular microfluidics. Complex on-chip mixing units and smooth channel surfaces ablated by lasers on polymers are well-known problems for microfluidic chip fabricating techniques. However, little is known about the ablation of rugged surfaces on polymer chips for mixing uses. This paper provides the first report of an on-chip compact micromixer simply, easily and quickly fabricated using laser-ablated irregular microspheric surfaces on a polymethyl methacrylate (PMMA) microfluidic chip for continuous mixing uses in modular microfluidics. The straight line channel geometry is designed for sequential mixing of nanoliter fluids in about 1 s. The results verify that up to about 90% of fluids can be mixed in a channel only 500 µm long, 200 µm wide and 150 µm deep using the developed micromixer fabricating method under optimized conditions. The computational flow dynamics simulation and experimental result agree well with each other.

How Nanophotonic Label-Free Biosensors Can Contribute to Rapid and Massive Diagnostics of Respiratory Virus Infections: COVID-19 Case

The global sanitary crisis caused by the emergence of the respiratory virus SARS-CoV-2 and the COVID-19 outbreak has revealed the urgent need for rapid, accurate, and affordable diagnostic tests to broadly and massively monitor the population in order to properly manage and control the spread of the pandemic. Current diagnostic techniques essentially rely on polymerase chain reaction (PCR) tests, which provide the required sensitivity and specificity. However, its relatively long time-to-result, including sample transport to a specialized laboratory, delays massive detection. Rapid lateral flow tests (both antigen and serological tests) are a remarkable alternative for rapid point-of-care diagnostics, but they exhibit critical limitations as they do not always achieve the required sensitivity for reliable diagnostics and surveillance. Next-generation diagnostic tools capable of overcoming all the above limitations are in demand, and optical biosensors are an excellent option to surpass such critical issues. Label-free nanophotonic biosensors offer high sensitivity and operational robustness with an enormous potential for integration in compact autonomous devices to be delivered out-of-the-lab at the point-of-care (POC). Taking the current COVID-19 pandemic as a critical case scenario, we provide an overview of the diagnostic techniques for respiratory viruses and analyze how nanophotonic biosensors can contribute to improving such diagnostics. We review the ongoing published work using this biosensor technology for intact virus detection, nucleic acid detection or serological tests, and the key factors for bringing nanophotonic POC biosensors to accurate and effective COVID-19 diagnosis on the short term.

A Single-Layer PDMS Chamber for On-Chip Bacteria Culture

On-chip cell culture devices have been actively developed for both mammalian cells and bacteria. Most designs are based on PDMS multi-layer microfluidic valves, which require complicated fabrication and operation. In this work, single-layer PDMS microfluidic valves are introduced in the design of an on-chip culture chamber for E. coli bacteria. To enable the constant flow of culturing medium, we have developed a (semi-)always-closed single-layer microfluidic valve. As a result, the growth chamber can culture bacteria over long duration. The device is applied for the whole-cell detection of heavy metal ions with genetically modified E. coli. The platform is tested with culturing period of 3 h. It is found to achieve a limit-of-detection (LoD) of 44.8 ppb for Cadmium ions.

An integrated microfluidic system for antimicrobial susceptibility testing with antibiotic combination

Polypharmacy is routinely administered to fight severe infections, though it has led to rampant multi-drug resistance in many bacterial strains. Preferably, antimicrobial susceptibility testing (AST) would be carried out prior to antibiotic prescription, though it is generally thought to be too complex and labor-intensive. In order to assist clinicians with better antibiotic administration for the effective treatment of bacterial infections, an integrated microfluidic system (IMS) capable of automating AST for 1-2 antibiotics against clinical bacterial pathogens was developed herein. Accurate determination of the minimum and fractional inhibitory concentrations of vancomycin, gentamicin, and linezolid were determined by assaying growth of two clinical methicillin-resistant Staphylococcus aureus isolates via a colorimetric assay on-chip. By applying various antibiotic combinations against a single pathogen in multiple chambers, the IMS could identify the optimal drug combination and the minimum effective dosage by evaluating the fractional inhibitory concentration index. This IMS possessed several advantages over conventional methods, including (1) a 50% reduction in bacterial sample and reagent volume (<50 μL per well), (2) less potential for human error due to its automatic nature, (3) faster liquid manipulation time by integrating the microfluidic components rather than labor-intensive process, and (4) straightforward result interpretation via colorimetric change instead of turbidity degree. Personalized medicine for treatment of bacterial infections may therefore be realized using this IMS.

A microfluidic approach for probing hydrodynamic effects in barite scale formation

Crystallization of mineral scale components ubiquitously plagues industrial systems for water treatment, energy production, and manufacturing. Chemical scale inhibitors and/or dissolvers are often employed to control scale formation, but their efficacy in flow conditions remains incompletely understood. We present a microfluidic platform to elucidate the time-resolved processes controlling crystallization and dissolution of barite, a highly insoluble and chemically resistant component of inorganic scale, in the presence of flow. In a growth environment, increasing the flow rate leads to a crossover from a transport-limited to a reaction-limited kinetic regime. In situ optical microscopy reveals that addition of diethylenetriaminepentaacetic acid (DTPA), a common dissolution agent, alters the morphology of barite crystals grown under flow. In a dissolution environment (i.e. alkaline solutions without barium sulfate), increasing the flux of DTPA, whether by increasing the flow rate or DTPA concentration, enhances the rate of dissolution of barite. Trends in the rate of barite dissolution with DTPA concentration and flow rate indicate an optimal combination of these parameters. The combined use of microfluidics and optical microscopy provides a robust and broadly-useful platform for capturing crystallization kinetics and morphological transformation under dynamic flow conditions.

Simultaneous Pumping and Mixing of Biological Fluids in a Double-Array Electrothermal Microfluidic Device

Transport and mixing of minute amounts of biological fluids are significantly important in lab-on-a-chip devices. It has been shown that the electrothermal technique is a suitable candidate for applications involving high-conductivity biofluids, such as blood, saliva, and urine. Here, we introduce a double-array AC electrothermal (ACET) device consisting of two opposing microelectrode arrays, which can be used for simultaneous mixing and pumping. First, in a 2D simulation, an optimum electrode-pair configuration capable of achieving fast transverse mixing at a microfluidic channel cross-section is identified by comparing different electrode geometries. The results show that by adjusting the applied voltage pattern and position of the asymmetrical microelectrodes in the two arrays, due to the resultant circular flow streamlines, the time it takes for the analytes to be convected across the channel cross-section is reduced by 95% compared to a diffusion-only-based transport regime, and by 80% compared to a conventional two-layer ACET device. Using a 3D simulation, the fluid transport (pumping and mixing) capabilities of such an electrode pair placed at different angles longitudinally relative to the channel was studied. It was found that an asymmetrical electrode configuration placed at an angle in the range of 30°≤θ≤45° can significantly increase transversal mixing efficiency while generating strong longitudinal net flow. These findings are of interest for lab-on-a-chip applications, especially for biosensors and immunoassays, where mixing analyte solutions while simultaneously moving them through a microchannel can greatly enhance the sensing efficiency.

Antibody recognition by a novel microgel photonic crystal

In this study, a easy-to-prepare biosensor for the sensitive detection of the antibody (Ab) protein was developed using a novel microgel photonic crystal (MPC). The MPC was fabricated by the spin-coated self-assembly method with the monodisperse Ab-sensitive poly (methyl methacrylate-acrylamide-glutaraldehyde-hapten) (P(MMA-AM-GA-HP)) microgels. Morphology characterization showed that the P(MMA-AM-GA-HP) microgels possessed round shapes and the large specific surface area, and the formed MPC had a highly ordered three dimensional (3D) periodically-ordered structure with the desired structural color. The Ab-response event of the P(MMA-AM-GA-HP) microgels can be directly transferred into a readable optical signal through a change in Bragg reflection of the periodic structure of the MPC. With the sensory system, the sensitive and selective detection of Ab was achieved without labeling techniques and expensive instruments. Therefore, this easy and sensitive detection system has great potential for next generation of the bioassay platform for clinical diagnosis and other applications.

Single-Layered Microfluidic Network-Based Combinatorial Dilution for Standard Simplex Lattice Design

In this paper, we presented a straightforward strategy to generate 15 combinations of three samples based on an experimental simplex lattice design using a single-layer microfluidic network. First, we investigated the performances of the plain structural and the groove structural combinatorial devices by computational simulation (CFD-ACE+). The simulated output concentrations were extremely close to the desirable values within an absolute error of less than 1%. Based on the simulated designs, polydimethylsiloxane (PDMS) devices were fabricated with soft lithography and tested with fluorescent dye (sodium salt). The mixing results for 15 combinations showed good performance, with an absolute error of less than 4%. We also investigated two liquid handling methods (bottom⁻up and top⁻down) for high-throughput screening and assay. The liquid-handling methods were successfully accomplished by adding the systematic structured groove sets on the mixing channels.

Challenges in paper-based fluorogenic optical sensing with smartphones

Application of optically superior, tunable fluorescent nanotechnologies have long been demonstrated throughout many chemical and biological sensing applications. Combined with microfluidics technologies, i.e. on lab-on-a-chip platforms, such fluorescent nanotechnologies have often enabled extreme sensitivity, sometimes down to single molecule level. Within recent years there has been a peak interest in translating fluorescent nanotechnology onto paper-based platforms for chemical and biological sensing, as a simple, low-cost, disposable alternative to conventional silicone-based microfluidic substrates. On the other hand, smartphone integration as an optical detection system as well as user interface and data processing component has been widely attempted, serving as a gateway to on-board quantitative processing, enhanced mobility, and interconnectivity with informational networks. Smartphone sensing can be integrated to these paper-based fluorogenic assays towards demonstrating extreme sensitivity as well as ease-of-use and low-cost. However, with these emerging technologies there are always technical limitations that must be addressed; for example, paper's autofluorescence that perturbs fluorogenic sensing; smartphone flash's limitations in fluorescent excitation; smartphone camera's limitations in detecting narrow-band fluorescent emission, etc. In this review, physical optical setups, digital enhancement algorithms, and various fluorescent measurement techniques are discussed and pinpointed as areas of opportunities to further improve paper-based fluorogenic optical sensing with smartphones.

Nanomaterial-based devices for point-of-care diagnostic applications

In this review, we have discussed the capabilities of nanomaterials for point-of-care (PoC) diagnostics and explained how these materials can help to strengthen, miniaturize and improve the quality of diagnostic devices.

An automated optofluidic biosensor platform combining interferometric sensors and injection moulded microfluidics

A primary limitation preventing practical implementation of photonic biosensors within point-of-care platforms is their integration with fluidic automation subsystems. For most diagnostic applications, photonic biosensors require complex fluid handling protocols; this is especially prominent in the case of competitive immunoassays, commonly used for detection of low-concentration, low-molecular weight biomarkers. For this reason, complex automated microfluidic systems are needed to realise the full point-of-care potential of photonic biosensors. To fulfil this requirement, we propose an on-chip valve-based microfluidic automation module, capable of automating such complex fluid handling. This module is realised through application of a PDMS injection moulding fabrication technique, recently described in our previous work, which enables practical fabrication of normally closed pneumatically actuated elastomeric valves. In this work, these valves are configured to achieve multiplexed reagent addressing for an on-chip diaphragm pump, providing the sample and reagent processing capabilities required for automation of cyclic competitive immunoassays. Application of this technique simplifies fabrication and introduces the potential for mass production, bringing point-of-care integration of complex automated microfluidics into the realm of practicality. This module is integrated with a highly sensitive, label-free bimodal waveguide photonic biosensor, and is demonstrated in the context of a proof-of-concept biosensing assay, detecting the low-molecular weight antibiotic tetracycline.

Optofluidic bioanalysis: fundamentals and applications

Over the past decade, optofluidics has established itself as a new and dynamic research field for exciting developments at the interface of photonics, microfluidics, and the life sciences. The strong desire for developing miniaturized bioanalytic devices and instruments, in particular, has led to novel and powerful approaches to integrating optical elements and biological fluids on the same chip-scale system. Here, we review the state-of-the-art in optofluidic research with emphasis on applications in bioanalysis and a focus on waveguide-based approaches that represent the most advanced level of integration between optics and fluidics. We discuss recent work in photonically reconfigurable devices and various application areas. We show how optofluidic approaches have been pushing the performance limits in bioanalysis, e.g. in terms of sensitivity and portability, satisfying many of the key requirements for point-of-care devices. This illustrates how the requirements for bianalysis instruments are increasingly being met by the symbiotic integration of novel photonic capabilities in a miniaturized system.

Photonic crystals: emerging biosensors and their promise for point-of-care applications

Biosensors are extensively employed for diagnosing a broad array of diseases and disorders in clinical settings worldwide. The implementation of biosensors at the point-of-care (POC), such as at primary clinics or the bedside, faces impediments because they may require highly trained personnel, have long assay times, large sizes, and high instrumental cost. Thus, there exists a need to develop inexpensive, reliable, user-friendly, and compact biosensing systems at the POC. Biosensors incorporated with photonic crystal (PC) structures hold promise to address many of the aforementioned challenges facing the development of new POC diagnostics. Currently, PC-based biosensors have been employed for detecting a variety of biotargets, such as cells, pathogens, proteins, antibodies, and nucleic acids, with high efficiency and selectivity. In this review, we provide a broad overview of PCs by explaining their structures, fabrication techniques, and sensing principles. Furthermore, we discuss recent applications of PC-based biosensors incorporated with emerging technologies, including telemedicine, flexible and wearable sensing, smart materials and metamaterials. Finally, we discuss current challenges associated with existing biosensors, and provide an outlook for PC-based biosensors and their promise at the POC.

A valve-free 2D concentration gradient generator

Our designed chip with a criss-cross 3D flow path realizes a valve-free 2D concentration gradient generator.

A Method of Targeted Cell Isolation via Glass Surface Functionalization

One of the limiting factors to the adoption and advancement of personalized medicine is the inability to develop diagnostic tools to probe individual nuances in expression from patient to patient. Current methodologies that try to separate cells to fill this niche result in disruption of physiological expression, making the separation technique useless as a diagnostic tool. In this protocol, we describe the functionalization and optimization of a surface for the cellular capture and release. This functionalized surface integrates biotinylated antibodies with a glass surface functionalized with an aminosilane (APTES), desthiobiotin and streptavidin. Cell release is facilitated through the introduction of biotin, allowing the recollection and purification of cells captured by the surface. This release is done through the targeting of the secondary moiety desthiobiotin, which results in a much more gentle release paradigm. This reduction in harsh reagents and shear forces reduces changes in cellular expression. The functionalized surface captures up to 80% of cells in a single cell mixture and has demonstrated 50% capture in a dual-cell mixture. Applications of this technology to xenografts and cancer separation studies are investigated. Quantification techniques for surface verification such as plate reader and ImageJ analyses are described as well.

Flexible optofluidic waveguide platform with multi-dimensional reconfigurability

Dynamic reconfiguration of photonic function is one of the hallmarks of optofluidics. A number of approaches have been taken to implement optical tunability in microfluidic devices. However, a device architecture that allows for simultaneous high-performance microfluidic fluid handling as well as dynamic optical tuning has not been demonstrated. Here, we introduce such a platform based on a combination of solid- and liquid-core polydimethylsiloxane (PDMS) waveguides that also provides fully functioning microvalve-based sample handling. A combination of these waveguides forms a liquid-core multimode interference waveguide that allows for multi-modal tuning of waveguide properties through core liquids and pressure/deformation. We also introduce a novel lifting-gate lightvalve that simultaneously acts as a fluidic microvalve and optical waveguide, enabling mechanically reconfigurable light and fluid paths and seamless incorporation of controlled particle analysis. These new functionalities are demonstrated by an optical switch with >45 dB extinction ratio and an actuatable particle trap for analysis of biological micro- and nanoparticles.

Design, microfabrication, and characterization of a moulded PDMS/SU-8 inkjet dispenser for a Lab-on-a-Printer platform technology with disposable microfluidic chip

In this paper, we present a disposable inkjet dispenser platform technology and demonstrate the Lab-on-a-Printer concept, an extension of the ubiquitous Lab-on-a-Chip concept, whereby microfluidic modules are directly integrated into the printhead. The concept is demonstrated here through the integration of an inkjet dispenser and a microfluidic mixer enabling control over droplet composition from a single nozzle in real-time during printing. The inkjet dispenser is based on a modular design platform that enables the low-cost microfluidic component and the more expensive actuation unit to be easily separated, allowing for the optional disposal of the former and reuse of the latter. To limit satellite droplet formation, a hydrophobic-coated and tapered micronozzle was microfabricated and integrated with the fluidics to realize the dispenser. The microfabricated devices generated droplets with diameters ranging from 150-220 μm, depending mainly on the orifice diameter, with printing rates up to 8000 droplets per second. The inkjet dispenser is capable of dispensing materials with a viscosity up to ∼19 mPa s. As a demonstration of the inkjet dispenser function and application, we have printed type I collagen seeded with human liver carcinoma cells (cell line HepG2), to form patterned biological structures.

Last Advances in Silicon-Based Optical Biosensors

We review the most important achievements published in the last five years in the field of silicon-based optical biosensors. We focus specially on label-free optical biosensors and their implementation into lab-on-a-chip platforms, with an emphasis on developments demonstrating the capability of the devices for real bioanalytical applications. We report on novel transducers and materials, improvements of existing transducers, new and improved biofunctionalization procedures as well as the prospects for near future commercialization of these technologies.

Solvent compatible microfluidic platforms for pharmaceutical solid form screening

The use of SIFEL in the crystallization fluid layers renders the microfluidic crystallization array compatible with solvents such as tetrahydrofuran, acetonitrile, chloroform, hexane, and toluene.

Secondary anchor targeted cell release

Personalized medicine offers the promise of tailoring therapy to patients, based on their cellular biomarkers. To achieve this goal, cellular profiling systems are needed that can quickly and efficiently isolate specific cell types without disrupting cellular biomarkers. Here we describe the development of a unique platform that facilitates gentle cell capture via a secondary, surface-anchoring moiety, and cell release. The cellular capture system consists of a glass surface functionalized with APTES, d-desthiobiotin, and streptavidin. Biotinylated mCD11b and hIgG antibodies are used to capture mouse macrophages (RAW 264.7) and human breast cancer (MCF7-GFP) cell lines, respectively. The surface functionalization is optimized by altering assay components, such as streptavidin, d-desthiobiotin, and APTES, to achieve cell capture on 80% of the functionalized surface and cell release upon biotin treatment. We also demonstrate an ability to capture 50% of target cells within a dual-cell mixture. This engineering advancement is a critical step towards achieving cell isolation platforms for personalized medicine.

A high-throughput device for size based separation of C. elegans developmental stages

Caenorhabditis elegans is a widely used model organism to study development, aging and behavior. Many of these biological studies require staging a large number of worms to assay a synchronized population of animals. Conventional synchronization techniques such as manual picking, gravity stratification and chemical bleaching are labor-intensive and could perturb animals' physiology. Thus, there is a need for a simple inexpensive technology to sort a mixed population of worms based on their developmental stages with minimal perturbation. Here we demonstrate a simple but accurate and high-throughput technique to sort based on animal size, which correlates well with developmental stages. The device consists of an array of geometrically optimized pillars that act as a sieve to allow worms of specific sizes to rapidly move through. With optimized chamber heights, pillar spacing and driving pressures, these binary separation devices are capable of independently separating a mixture of worms at two different stages at average efficiency of around 95%, and throughput of hundreds of worms per minute. In addition, when four devices are used sequentially, we demonstrate the ability to stratify a mixture of worms of all developmental stages with >85% overall efficiency.

Label-free direct visual analysis of hydrolytic enzyme activity using aqueous two-phase system droplet phase transitions

Dextran hydrolysis-mediated conversion of polyethylene glycol (PEG)-dextran (DEX) aqueous two-phase system droplets to a single phase was used to directly visualize Dextranase activity. DEX droplets were formed either by manual micropipetting or within a continuous PEG phase by computer controlled actuation of an orifice connecting rounded channels formed by backside diffused light lithography. The time required for the two-phase to one-phase transition was dependent on the Dextranase concentration, pH of the medium, and temperature. The apparent Michaelis constants for Dextranase were estimated based on previously reported catalytic constants, the binodal polymer concentration curves for PEG-DEX phase transition for each temperature, and pH condition. The combination of a microfluidic droplet system and phase transition observation provides a new method for label-free direct measurement of enzyme activity.

On-chip self-assembly of cell embedded microstructures to vascular-like microtubes

Currently, research on the construction of vascular-like tubular structures is a hot area of tissue engineering, since it has potential applications in the building of artificial blood vessels. In this paper, we report a fluidic self-assembly method using cell embedded microstructures to construct vascular-like microtubes. A novel 4-layer microfluidic device was fabricated using polydimethylsiloxane (PDMS), which contains fabrication, self-assembly and extraction areas inside one channel. Cell embedded microstructures were directly fabricated using poly(ethylene glycol) diacrylate (PEGDA) in the fabrication area, namely on-chip fabrication. Self-assembly of the fabricated microstructures was performed in the assembly area which has a micro well. Assembled tubular structures (microtubes) were extracted outside the channel into culture dishes using a normally closed (NC) micro valve in the extraction area. The self-assembly mechanism was experimentally demonstrated. The performance of the NC micro valve and embedded cell concentration were both evaluated. Fibroblast (NIH/3T3) embedded vascular-like microtubes were constructed inside this reusable microfluidic device.

Enhanced detection limit by dark mode perturbation in 2D photonic crystal slab refractive index sensors

We demonstrate for the first time a 300nm thick, 300μm × 300μm 2D dielectric photonic crystal slab membrane with a quality factor of 10,600 by coupling light to slightly perturbed dark modes through alternating nano-hole sizes. The newly created fundamental guided resonances greatly reduce nano-fabrication accuracy requirements. Moreover, we created a new layer architecture resulting in electric field enhancement at the interface between the slab and sensing regions, and spectral sensitivity of >800 nm/RIU, that is, >0.8 of the single-mode theoretical upper limit of spectral sensitivity.

Combinatorial generation of droplets by controlled assembly and coalescence

We describe a microfluidic system for generating a sequence of liquid droplets of multiple concentrations in a single experimental condition. The series of final droplets has the combination of the compositions varying periodically, with polydispersity of the size less than 8%. By utilizing the design of the microchannel geometry and the passive control of three immiscible fluids (oil, water, and air) including generation, breakup, separation and coalescence of droplets, we can change the system to generate diverse sets of combination of materials. The device can be used for testing different concentration of materials in picoliter volumes and developing a new way to deliver dynamic signals of chemicals with microfluidics.

Design, fabrication and characterization of drug delivery systems based on lab-on-a-chip technology

Lab-on-a-chip technology is an emerging field evolving from the recent advances of micro- and nanotechnologies. The technology allows the integration of various components into a single microdevice. Microfluidics, the science and engineering of fluid flow in microscale, is the enabling underlying concept for lab-on-a-chip technology. The present paper reviews the design, fabrication and characterization of drug delivery systems based on this amazing technology. The systems are categorized and discussed according to the scales at which the drug is administered. Starting with the fundamentals on scaling laws of mass transfer and basic fabrication techniques, the paper reviews and discusses drug delivery devices for cellular, tissue and organism levels. At the cellular level, a concentration gradient generator integrated with a cell culture platform is the main drug delivery scheme of interest. At the tissue level, the synthesis of smart particles as drug carriers using lab-on-a-chip technology is the main focus of recent developments. At the organism level, microneedles and implantable devices with fluid-handling components are the main drug delivery systems. For drug delivery to a small organism that can fit into a microchip, devices similar to those of cellular level can be used.

Laser-written photonic crystal optofluidics for electrochromatography and spectroscopy on a chip

Femtosecond laser processes were optimized for nonlinear interactions with various optical materials to develop a novel biophotonic lab-on-a-chip device that integrates laser-formed waveguides (WGs), microfluidic channels and photonic crystals (PCs). Such integration seeks the unique demonstration of dual PC functionalities: (1) efficient chromatographic separation and filtration of analytes through a porous PC embedded inside a microfluidic channel and (2) optofluidic spectroscopy through embedded WGs that probe PC stopband shifts as varying analyte concentrations flow and separate. The building blocks together with their integration were demonstrated, providing embedded porous PCs through which electrochromatography drove an accelerated mobile phase of analyte and an optical stopband was probed via integrated buried WGs. Together, these laboratory results underpin the promise of simultaneous chromatographic and spectroscopic capabilities in a single PC optofluidic device.

A microfluidic platform for evaporation-based salt screening of pharmaceutical parent compounds

We describe a microfluidic platform to screen for salt forms of pharmaceutical compounds (PCs) via controlled evaporation. The platform enables on-chip combinatorial mixing of PC and salt former solutions in a 24-well array (~200 nL/well), which is a drastic reduction in the amount of PC needed per condition screened compared to traditional screening approaches that require ~100 μL/well. The reduced sample needs enable salt screening at a much earlier stage in the drug development process, when only limited quantities of PCs are available. Compatibility with (i) solvents commonly used in the pharmaceutical industry, and (ii) Raman spectroscopy for solid form identification was ensured by using a hybrid microfluidic platform. A thin layer of elastomeric PDMS was utilized to retain pneumatic valving capabilities. This layer is sandwiched between layers of cyclic-olefin copolymer, a material with low air and solvent permeability and low Raman background to yield a physically rigid and Raman compatible chip. A solvent-impermeable thiolene layer patterned with evaporation channels permits control over the rate of solvent evaporation. Control over the rate of solvent evaporation (2-15 nL h(-1)) results in consistent, known rates of increase in the supersaturation levels attained on-chip, and increases the probability for crystalline solids to form. The modular nature of the platform enables on-chip Raman and birefringence analysis of the solid forms. Model compounds, tamoxifen and ephedrine, were used to validate the platform's ability to screen for salts. On-chip Raman analysis helped to identify six different salts each of tamoxifen and ephedrine.

Digitally programmable microfluidic automaton for multiscale combinatorial mixing and sample processing

A digitally programmable microfluidic Automaton consisting of a 2-dimensional array of pneumatically actuated microvalves is programmed to perform new multiscale mixing and sample processing operations. Large (μL-scale) volume processing operations are enabled by precise metering of multiple reagents within individual nL-scale valves followed by serial repetitive transfer to programmed locations in the array. A novel process exploiting new combining valve concepts is developed for continuous rapid and complete mixing of reagents in less than 800 ms. Mixing, transfer, storage, and rinsing operations are implemented combinatorially to achieve complex assay automation protocols. The practical utility of this technology is demonstrated by performing automated serial dilution for quantitative analysis as well as the first demonstration of on-chip fluorescent derivatization of biomarker targets (carboxylic acids) for microchip capillary electrophoresis on the Mars Organic Analyzer. A language is developed to describe how unit operations are combined to form a microfluidic program. Finally, this technology is used to develop a novel microfluidic 6-sample processor for combinatorial mixing of large sets (>2(6) unique combinations) of reagents. The digitally programmable microfluidic Automaton is a versatile programmable sample processor for a wide range of process volumes, for multiple samples, and for different types of analyses.

Direct-writing colloidal photonic crystal microfluidic chips by inkjet printing for label-free protein detection

Integrating photonic crystals (PC) into microfluidic systems has attracted immense interest for its novel functions. However, it is still a great challenge to fabricate PC microfluidic chips rapidly with complex functions. In this work, a direct-writing colloidal PC microchannel was firstly achieved by inkjet printing and was used for the surface-tension-confined microfluidic immune assay. PC channels with different structure colors have been successfully integrated on one chip. The fabricated chip has the advantages of rapid fabrication, quick fluidic transport and can monitor the fluidic fluxion using the naked eye. Utilizing this PC microfluidic chip, a colorimetric label-free immune assay was realized without nonspecific adsorption interference of the target.

Design considerations for electrostatic microvalves with applications in poly(dimethylsiloxane)-based microfluidics

Microvalves are critical in the operation of integrated microfluidic chips for a wide range of applications. In this paper, we present an analytical model to guide the design of electrostatic microvalves that can be integrated into microfluidic chips using standard fabrication processes and can reliably operate at low actuation potentials (<250 V). Based on the analytical model, we identify design guidelines and operational considerations for elastomeric electrostatic microvalves and formulate strategies to minimize their actuation potentials, while maintaining the feasibility of fabrication and integration. We specifically explore the application of the model to design microfluidic microvalves fabricated in poly(dimethylsiloxane), using only soft-lithographic techniques. We discuss the electrostatic actuation in terms of several microscale phenomena, including squeeze-film damping and adhesion-driven microvalve collapse. The actuation potentials predicted by the model are in good agreement with experimental data obtained with a microfabricated array of electrostatic microvalves actuated in air and oil. The model can also be extended to the design of peristaltic pumps for microfluidics and to the prediction of actuation potentials of microvalves in viscous liquid environments. Additionally, due to the compact ancillaries required to generate low potentials, these electrostatic microvalves can potentially be used in portable microfluidic chips.

Design of pressure-driven microfluidic networks using electric circuit analogy

This article reviews the application of electric circuit methods for the analysis of pressure-driven microfluidic networks with an emphasis on concentration- and flow-dependent systems. The application of circuit methods to microfluidics is based on the analogous behaviour of hydraulic and electric circuits with correlations of pressure to voltage, volumetric flow rate to current, and hydraulic to electric resistance. Circuit analysis enables rapid predictions of pressure-driven laminar flow in microchannels and is very useful for designing complex microfluidic networks in advance of fabrication. This article provides a comprehensive overview of the physics of pressure-driven laminar flow, the formal analogy between electric and hydraulic circuits, applications of circuit theory to microfluidic network-based devices, recent development and applications of concentration- and flow-dependent microfluidic networks, and promising future applications. The lab-on-a-chip (LOC) and microfluidics community will gain insightful ideas and practical design strategies for developing unique microfluidic network-based devices to address a broad range of biological, chemical, pharmaceutical, and other scientific and technical challenges.

Lifting gate polydimethylsiloxane microvalves and pumps for microfluidic control

We describe the development and characterization of pneumatically actuated "lifting gate" microvalves and pumps. A fluidic layer containing the gate structure and a pneumatic layer are fabricated by soft-lithography in PDMS and bonded permanently with an oxygen plasma treatment. The microvalve structures are then reversibly bonded to a featureless glass or plastic substrate to form hybrid glass-PDMS and plastic-PDMS microchannel structures. The break-through pressures of the microvalve increase linearly up to 65 kPa as the closing pressure increases. The pumping capability of these structures ranges from the nanoliter to microliter scale depending on the number of cycles and closing pressure employed. The micropump structures exhibit up to 86.2% pumping efficiency from flow rate measurements. The utility of these structures for integrated sample processing is demonstrated by performing an automated immunoassay. These lifting gate valve and pump structures enable facile integration of complex microfluidic control systems with a wide range of lab-on-a-chip substrates.

A microfluidic platform for pharmaceutical salt screening

We describe a microfluidic platform comprised of 48 wells to screen for pharmaceutical salts. Solutions of pharmaceutical parent compounds (PCs) and salt formers (SFs) are mixed on-chip in a combinatorial fashion in arrays of 87.5-nanolitre wells, which constitutes a drastic reduction of the volume of PC solution needed per condition screened compared to typical high throughput pharmaceutical screening approaches. Nucleation and growth of salt crystals is induced by diffusive and/or convective mixing of solutions containing, respectively, PCs and SFs in a variety of solvents. To enable long term experiments, solvent loss was minimized by reducing the thickness of the absorptive polymeric material, polydimethylsiloxane (PDMS), and by using solvent impermeable top and bottom layers. Additionally, well isolation was enhanced via the incorporation of pneumatic valves that are closed at rest. Brightfield and polarized light microscopy and Raman spectroscopy were used for on-chip analysis and crystal identification. Using a gold-coated glass substrate and minimizing the thickness of the PDMS control layer drastically improved the signal-to-noise ratio for Raman spectra. Two drugs, naproxen (acid) and ephedrine (base), were used for validation of the platform's ability to screen for salts. Each PC was mixed combinatorially with potential SFs in a variety of solvents. Crystals were visualized using brightfield polarized light microscopy. Subsequent on-chip analyses of the crystals with Raman spectroscopy identified four different naproxen salts and five different ephedrine salts.

Microfluidic sensing: state of the art fabrication and detection techniques

Here we introduce the existing fabrication techniques, detection methods, and related techniques for microfluidic sensing, with an emphasis on the detection techniques. A general survey and comparison of the fabrication techniques were given, including prototyping (hot embossing, inject molding, and soft lithography) and direct fabrication (laser micromachining, photolithography, lithography, and x-ray lithography) techniques. This is followed by an in-depth look at detection techniques: optical, electrochemical, mass spectrometry, as well as nuclear magnetic resonance spectroscopy-based sensing approaches and related techniques. In the end, we highlight several of the most important issues for future work on microfluidic sensing. This article aims at providing a tutorial review with both introductory materials and inspiring information on microfluidic fabrication and sensing for nonspecialists.

Multiplexed detection of nucleic acids in a combinatorial screening chip

Multiplexed diagnostic testing has the potential to dramatically improve the quality of healthcare. Simultaneous measurement of health indicators and/or disease markers reduces turnaround time and analysis cost and speeds up the decision making process for diagnosis and treatment. At present, however, most diagnostic tests only provide information on a single indicator or marker. Development of efficient diagnostic tests capable of parallel screening of infectious disease markers could significantly advance clinical and diagnostic testing in both developed and developing parts of the world. Here, we report the multiplexed detection of nucleic acids as disease markers within discrete wells of a microfluidic chip using molecular beacons and total internal reflection fluorescence microscopy (TIRFM). Using a 4 × 4 array of 200 pL wells, we screened for the presence of four target single stranded oligonucleotides encoding for conserved regions of the genomes of four common viruses: human immunodeficiency virus-1 (HIV-1), human papillomavirus (HPV), Hepatitis A (Hep A) and Hepatitis B (Hep B). Target oligonucleotides are accurately detected and discriminated against alternative oligonucleotides with different sequences. This combinatorial chip represents a versatile platform for the development of clinical diagnostic tests for simultaneous screening, detection and monitoring of a wide range of biological markers of disease and health using minimal sample size.

Integrated ionic liquid-based electrofluidic circuits for pressure sensing within polydimethylsiloxane microfluidic systems

This paper reports a novel pressure sensor with an electrical readout based on electrofluidic circuits constructed by ionic liquid (IL)-filled microfluidic channels. The developed pressure sensor can be seamlessly fabricated into polydimethylsiloxane (PDMS) microfluidic systems using the well-developed multilayer soft lithography (MSL) technique without additional assembly or sophisticated cleanroom microfabrication processes. Therefore, the device can be easily scaled up and is fully disposable. The pressure sensing is achieved by measuring the pressure-induced electrical resistance variation of the constructed electrofluidic resistor. In addition, an electrofluidic Wheatstone bridge circuit is designed for accurate and stable resistance measurements. The pressure sensor is characterized using pressurized nitrogen gas and various liquids which flow into the microfluidic channels. The experimental results demonstrate the great long-term stability (more than a week), temperature stability (up to 100 °C), and linear characteristics of the developed pressure sensing scheme. Consequently, the integrated microfluidic pressure sensor developed in this paper is promising for better monitoring and for characterizing the flow conditions and liquid properties inside the PDMS microfluidic systems in an easier manner for various lab on a chip applications.

Design rules for pumping and metering of highly viscous fluids in microfluidics

The use of fluids that are significantly more viscous than water in microfluidics has been limited due to their high resistance to flow in microscale channels. This paper reports a theoretical treatment for the flow of highly viscous fluids in deforming microfluidic channels, particularly with respect to transient effects, and discusses the implications of these effects on the design of appropriate microfluidic devices for highly viscous fluids. We couple theory describing flow in a deforming channel with design equations, both for steady-state flows and for the transient periods associated with the initial deformation and final relaxation of a channel. The results of this analysis allow us to describe these systems and also to assess the significance of different parameters on various deformation and/or transient effects. To exemplify their utility, we apply these design rules to two applications: (i) pumping highly viscous fluids for a nanolitre scale mixing application and (ii) precise metering of fluids in microfluidics.

Sensitivity enhancement in photonic crystal slab biosensors

Refractive index sensitivity of guided resonances in photonic crystal slabs is analyzed. We show that modal properties of guided resonances strongly affect spectral sensitivity and quality factors, resulting in substantial enhancement of refractive index sensitivity. A three-fold spectral sensitivity enhancement is demonstrated for suspended slab designs, in contrast to designs with a slab resting over a substrate. Spectral sensitivity values are additionally shown to be unaffected by quality factor reductions, which are common to fabricated photonic crystal nano-structures. Finally, we determine that proper selection of photonic crystal slab design parameters permits biosensing of a wide range of analytes, including proteins, antigens, and cells. These photonic crystals are compatible with large-area biosensor designs, permitting direct access to externally incident optical beams in a microfluidic device.

Comparison of label-free biosensing in microplate, microfluidic, and spot-based affinity capture assays

Using both experimental assays and fluid-dynamic finite element simulation models, we directly compared the achievable performance limits of four distinct assay configurations for label-free detection of an analyte from a test sample on a biosensor surface. The assay configurations studied in this work included a biosensor incorporated into the bottom surface of a microplate well and a microfluidic channel. For each configuration, we compared assay performance for the scenario in which the entire bottom surface of the fluid-handling vessel is coated with capture ligands with assay performance for the scenario in which the capture ligands are applied in the form of localized spots. As a model system, we used detection of the protein biomarker tumor necrosis factor-alpha (TNF-alpha) using immobilized TNF-alpha capture antibody. Results show that the microfluidic assay format dramatically reduces the time required to establish a stable equilibrium. Spot-based assays are advantageous for microplate-based detection for reducing the time required for equilibrium sensor response. The results derived are generally applicable to any label-free biosensor technology and any ligand-analyte system with adjustable variables that include sensor mass density sensitivity, analyte-ligand adsorption/desorption rate constants, immobilized ligand density, flow channel geometry, flow rate, and spot size.

A digital microfluidic platform for the automation of quantitative biomolecular assays

A digital microfluidic platform for the automation of quantitative, multi-step biomolecular assays is developed and optimized. The platform consists of a 2-dimensional array of microvalves that can be programmed to perform reagent routing, mixing, rinsing, serial dilution, and many other operations using nanolitre scale volumes of sample. Discrete transfer of fluid between microvalves is characterized using gravimetric flow analysis and optimized to achieve maximum efficiency. Protocols for on-chip reagent mixing and serial dilution are optimized to achieve linearity over a 1000-fold dilution range. These optimized programs are used to develop a rapid, quantitative assay for hydrogen peroxide, a biomarker of oxidative stress. A sub-micromolar limit of detection is demonstrated with an 8.5 min program runtime, thus establishing this platform as an effective tool for the automation of multi-step bioassays. The programmability of this system enables rapid development of diverse assay protocols on a common chip format.