An Innovative AI-based primer design tool for precise and accurate detection of SARS-CoV-2 variants of concern

As the COVID-19 pandemic winds down, it leaves behind the serious concern that future, even more disruptive pandemics may eventually surface. One of the crucial steps in handling the SARS-CoV-2 pandemic was being able to detect the presence of the virus in an accurate and timely manner, to then develop policies counteracting the spread. Nevertheless, as the pandemic evolved, new variants with potentially dangerous mutations appeared. Faced by these developments, it becomes clear that there is a need for fast and reliable techniques to create highly specific molecular tests, able to uniquely identify VOCs. Using an automated pipeline built around evolutionary algorithms, we designed primer sets for SARS-CoV-2 (main lineage) and for VOC, B.1.1.7 (Alpha) and B.1.1.529 (Omicron). Starting from sequences openly available in the GISAID repository, our pipeline was able to deliver the primer sets for the main lineage and each variant in a matter of hours. Preliminary in-silico validation showed that the sequences in the primer sets featured high accuracy. A pilot test in a laboratory setting confirmed the results: the developed primers were favorably compared against existing commercial versions for the main lineage, and the specific versions for the VOCs B.1.1.7 and B.1.1.529 were clinically tested successfully.

Freeze-Dried Microfluidic Monodisperse Microbubbles as a New Generation of Ultrasound Contrast Agents

We succeeded in freeze-drying monodisperse microbubbles without degrading their performance, that is, their monodispersity in size and echogenicity. We used microfluidic technology to generate cryoprotected highly monodisperse microbubbles (coefficient of variation [CV] <5%). By using a novel retrieval technique, we were able to freeze-dry the microbubbles and resuspend them without degradation, that is, keeping their size distribution narrow (CV <6%). Acoustic characterization performed in two geometries (a centimetric cell and a millichannel) revealed that the resuspended bubbles conserved the sharpness of the backscattered resonance peak, leading to CVs ranging between 5% and 10%, depending on the geometry. As currently observed with monodisperse bubbles, the peak amplitudes are one order of magnitude higher than those of commercial ultrasound contrast agents. Our work thus solves the question of storage and transportation of highly monodisperse bubbles. This work might open pathways toward novel clinical non-invasive measurements, such as local pressure, impossible to carry out with the existing commercial ultrasound contrast agents.

Optimal self-assembly of lipid nanoparticles (LNP) in a ring micromixer

Lipid nanoparticles (LNPs) for RNA and DNA delivery have attracted considerable attention for their ability to treat a broad range of diseases and to vectorize mRNA for COVID vaccines. LNPs are produced by mixing biomolecules and lipids, which self-assemble to form the desired structure. In this domain, microfluidics shows clear advantages: high mixing quality, low-stress conditions, and fast preparation. Studies of LNPs produced in micromixers have revealed, in certain ranges of flow rates, a degradation in performance in terms of size, monodispersity and encapsulation efficiency. In this study, we focus on the ring micromixer, which is well adapted to high throughput. We reveal three regimes, side-by-side, transitional and highly mixed, that control the mixing performance of the device. Furthermore, using cryo-TEM and biochemical analysis, we show that the mixing performances are strongly correlated to the characteristics of the LNPs we produce. We emphasize the importance of the flow-rate ratio and propose a physical criterion based on the onset of temporal instabilities for producing LNPs with optimal characteristics in terms of geometry, monodispersity and encapsulation yield. These criteria are generally applicable.

Dendritic growth of protein gel in the course of sweat pore plugging by aluminium salts under physiological conditions

Are aluminium ions unavoidable in antiperspirants? To answer this question, we present confocal microscopy images of dendritic plugs appearing in sweat flowing across a microfluidic channel in the presence of aluminium salts. By comparing with numerical simulations, we identify the mechanisms forming this structured protein gel inside the pore.

Real time observation of the interaction between aluminium salts and sweat under microfluidic conditions

Aluminium salts such as aluminium chlorohydrate (ACH) are the active ingredients of antiperspirant products. Their mechanism of action involves a temporary and superficial plugging of eccrine sweat pores at the skin surface. We developed a microfluidic system that allows the real time observation of the interactions between sweat and ACH in conditions mimicking physiological sweat flow and pore dimensions. Using artificial sweat containing bovine serum albumin as a model protein, we performed experiments under flowing conditions to demonstrate that pore clogging results from the aggregation of proteins by aluminium polycations at specific location in the sweat pore. Combining microfluidic experiments, confocal microscopy and numerical models helps to better understand the physical chemistry and mechanisms involved in pore plugging. The results show that plugging starts from the walls of sweat pores before expanding into the centre of the channel. The simulations aid in explaining the influence of ACH concentration as well as the impact of flow conditions on the localization of the plug. Altogether, these results outline the potential of both microfluidic confocal observations and numerical simulations at the single sweat pore level to understand why aluminium polycations are so efficient for sweat channel plugging.

Unraveling driving regimes for destabilizing concentrated emulsions within microchannels

Coalescence is the most widely demonstrated mechanism for destabilizing emulsion droplets in microfluidic chambers. However, we find that depending on the channel wall surface functionalization, surface zeta potential, type of surfactant, characteristics of the oil as a dispersed phase, or even the presence of externally-induced stress, other different destabilization mechanisms can occur in subtle ways. In general, we observe four regimes leading to destabilization of concentrated emulsions: (i) coalescence, (ii) emulsion bursts, (iii) a combination of the two first mechanisms, attributed to the simultaneous occurrence of coalescence and emulsion bursts; and (iv) compaction of the droplet network that eventually destabilizes to fracture-like behavior. We correlate various physico-chemical properties (zeta potential, contact angle, interfacial tension) to understand their respective influence on the destabilization mechanisms. This work provides insights into possible ways to control or inflict emulsion droplet destabilization for different applications.

Performing point-of-care molecular testing for SARS-CoV-2 with RNA extraction and isothermal amplification

To respond to the urgent need for COVID-19 testing, countries perform nucleic acid amplification tests (NAAT) for the detection of SARS-CoV-2 in centralized laboratories. Real-time RT-PCR (Reverse transcription-Polymerase Chain Reaction), used to amplify and detect the viral RNA., is considered, as the current gold standard for diagnostics. It is an efficient process, but the complex engineering required for automated RNA extraction and temperature cycling makes it incompatible for use in point of care settings [1]. In the present work, by harnessing progress made in the past two decades in isothermal amplification and paper microfluidics, we created a portable test, in which SARS-CoV-2 RNA is extracted, amplified isothermally by RT-LAMP (Loop-mediated Isothermal Amplification), and detected using intercalating dyes or fluorescent probes. Depending on the viral load in the tested samples, the detection takes between twenty minutes and one hour. Using a set of 16 pools of naso-pharyngal swab eluates, we estimated a limit of detection comparable to real-time RT-PCR (i.e. 1 genome copies per microliter of clinical sample) and no cross-reaction with eight major respiratory viruses currently circulating in Europe. We designed and fabricated an easy-to-use portable device called "COVIDISC" to carry out the test at the point of care. The low cost of the materials along with the absence of complex equipment will expedite the widespread dissemination of this device. What is proposed here is a new efficient tool to help managing the pandemics.

Micrometric Monodisperse Solid Foams as Complete Photonic Bandgap Materials

Solid foams with micrometric pores are used in different fields (filtering, 3D cell culture, etc.), but today, controlling their foam geometry at the pore level, their internal structure, and the monodispersity, along with their mechanical properties, is still a challenge. Existing attempts to create such foams suffer either from slow speed or size limitations (above 80 μm). In this work, by using a temperature-regulated microfluidic process, 3D solid foams with highly monodisperse open pores (PDI lower than 5%), with sizes ranging from 5 to 400 μm and stiffnesses spanning 2 orders of magnitude, are created for the first time. These features open the way for exciting applications, in cell culture, filtering, optics, etc. Here, the focus is set on photonics. Numerically, these foams are shown to open a 3D complete photonic bandgap, with a critical index of 2.80, thus compatible with the use of rutile TiO₂. In the field of photonics, such structures represent the first physically realizable self-assembled FCC (face-centered cubic) structure that possesses this functionality.

Stable liquid foams from a new polyfluorinated surfactant

Liquid foams exhibiting long-term stability are a key-challenge in material design. Based on this perspective, new pyridinium polyfluorinated surfactants were synthesized from simple building blocks enabling unusually stable liquid foams. While the batch-generated foams were used for qualitative foaming evaluation, microfluidics allowed a quantitative insight into the aging effects of monodisperse foams.

Microfluidics Mediated Production of Foams for Biomedical Applications

Within the last decade, there has been increasing interest in liquid and solid foams for several industrial uses. In the biomedical field, liquid foams can be used as delivery systems for dermatological treatments, for example, whereas solid foams are frequently used as scaffolds for tissue engineering and drug screening. Most of the foam functionalities are largely correlated to their mechanical properties and their structure, especially bubble/pore size, shape, and interconnectivity. However, the majority of conventional foaming fabrication techniques lack pore size control which can induce important inhomogeneities in the foams and subsequently decrease their performance. In this perspective, new advanced technologies have been introduced, such as microfluidics, which offers a highly controlled production, allowing for design customization of both liquid foams and solid foams obtained through liquid-templating. This short review explores both the fabrication and the characterization of foams, with a focus on solid polymer foams, and sheds the light on how microfluidics can overcome some existing limitations, playing a crucial role in their production for biomedical applications, especially as scaffolds in tissue engineering.

Colloidal particle deposition on microchannel walls, for attractive and repulsive surface potentials

When both surfaces possess opposite charges, particle deposition increases at low ionic strengths due to van der Waals forces assisted by electrostatic attraction.

Deposition kinetics of bi- and tridisperse colloidal suspensions in microchannels under the van der Waals regime

We investigate the kinetics of irreversible adsorption under the van der Waals regime, i.e. weakly Brownian polydisperse colloidal suspensions injected into shallow microchannels at high ionic strengths, where each suspension is represented by populations of particles with different particle sizes. We find that each population size of the particle in the suspension can be treated independently using an analytical solution based on the advection-diffusion equation and that the distribution of the adsorbed particles along the channel axis behaves according to a power law. The experimental measurements agree with Langevin simulations and are well accounted for by theory valid in the van der Waals regime. Operating in the van der Waals regime permits the present study to confirm the use of microfluidics as an effective in situ method to measure the Hamaker constant of particles under aqueous conditions.

Fibrin-Targeted Polymerized Shell Microbubbles as Potential Theranostic Agents for Surgical Adhesions

The development of new therapies for surgical adhesions has proven to be difficult as there is no consistently effective way to assess treatment efficacy in clinical trials without performing a second surgery, which can result in additional adhesions. We have developed lipid microbubble formulations that use a short peptide sequence, CREKA, to target fibrin, the molecule that forms nascent adhesions. These targeted polymerized shell microbubbles (PSMs) are designed to allow ultrasound imaging of early adhesions for diagnostic purposes and for evaluating the success of potential treatments in clinical trials while acting as a possible treatment. In this study, we show that CREKA-targeted microbubbles preferentially bind fibrin over fibrinogen and are stable for long periods of time (∼48 h), that these bound microbubbles can be visualized by ultrasound, and that neither these lipid-based bubbles nor their diagnostic-ultrasound-induced vibrations damage mesothelial cells in vitro. Moreover, these bubbles show the potential to identify adhesionlike fibrin formations and may hold promise in blocking or breaking up fibrin formations in vivo.

Optimizing Hyperuniformity in Self-Assembled Bidisperse Emulsions

We study long range density fluctuations (hyperuniformity) in two-dimensional jammed packings of bidisperse droplets. Taking advantage of microfluidics, we systematically span a large range of size and concentration ratios of the two droplet populations. We identify various defects increasing long range density fluctuations mainly due to organization of local particle environment. By choosing an appropriate bidispersity, we fabricate materials with a high level of hyperuniformity. Interesting transparency properties of these optimized materials are established based on numerical simulations.

Microfluidic step-emulsification in axisymmetric geometry

Biphasic step-emulsification (Z. Li et al., Lab Chip, 2015, 15, 1023) is a promising microfluidic technique for high-throughput production of μm and sub-μm highly monodisperse droplets. The step-emulsifier consists of a shallow (Hele-Shaw) microchannel operating with two co-flowing immiscible liquids and an abrupt expansion (i.e., step) to a deep and wide reservoir. Under certain conditions the confined stream of the disperse phase, engulfed by the co-flowing continuous phase, breaks into small highly monodisperse droplets at the step. Theoretical investigation of the corresponding hydrodynamics is complicated due to the complex geometry of the planar device, calling for numerical approaches. However, direct numerical simulations of the three dimensional surface-tension-dominated biphasic flows in confined geometries are computationally expensive. In the present paper we study a model problem of axisymmetric step-emulsification. This setup consists of a stable core-annular biphasic flow in a cylindrical capillary tube connected co-axially to a reservoir tube of a larger diameter through a sudden expansion mimicking the edge of the planar step-emulsifier. We demonstrate that the axisymmetric setup exhibits similar regimes of droplet generation to the planar device. A detailed parametric study of the underlying hydrodynamics is feasible via inexpensive (two dimensional) simulations owing to the axial symmetry. The phase diagram quantifying the different regimes of droplet generation in terms of governing dimensionless parameters is presented. We show that in qualitative agreement with experiments in planar devices, the size of the droplets generated in the step-emulsification regime is independent of the capillary number and almost insensitive to the viscosity ratio. These findings confirm that the step-emulsification regime is solely controlled by surface tension. The numerical predictions are in excellent agreement with in-house experiments with the axisymmetric step-emulsifier.

Paper microfluidics for nucleic acid amplification testing (NAAT) of infectious diseases

The diagnosis of infectious diseases is entering a new and interesting phase. Technologies based on paper microfluidics, coupled to developments in isothermal amplification of Nucleic Acids (NAs) raise opportunities for bringing the methods of molecular biology in the field, in a low setting environment. A lot of work has been performed in the domain over the last few years and the landscape of contributions is rich and diverse. Most often, the level of sample preparation differs, along with the sample nature, the amplification and detection methods, and the design of the device, among other features. In this review, we attempt to offer a structured description of the state of the art. The domain is not mature and there exist bottlenecks that hamper the realization of Nucleic Acid Amplification Tests (NAATs) complying with the constraints of the field in low and middle income countries. In this domain however, the pace of progress is impressively fast. This review is written for a broad Lab on a Chip audience.

Particle Deposition Kinetics of Colloidal Suspensions in Microchannels at High Ionic Strength

Despite its considerable practical importance, the deposition of real Brownian particles transported in a channel by a liquid, at small Reynolds numbers, has never been described at a comprehensive level. Here, by coupling microfluidic experiments, theory, and numerics, we succeed in unravelling the problem for the case of straight channels at high salinity. We discover a broad regime of deposition (the van der Waals regime) in which particle-wall van der Waals interactions govern the deposition mechanism. We determine the range of existence of the regime, for which we calculate the concentration profiles, retention profiles, and deposition kinetics analytically. The retention profiles decay as the inverse of the square root of the distance from the entry, and the deposition kinetics are given by the expression [Formula: see text], where S is a dimensionless deposition function, A is the Hamaker constant, and ξ_L is a dimensionless parameter characterizing fluid flow properties. These findings are well supported by numerics. Experimentally, we find that the retention profiles behave as x^-0.5±0.1 (where x is the distance from the channel entry) over three decades in scale, as predicted theoretically. By varying the flow conditions (speed, geometry, surface properties, and concentration) so as to cover four decades in ξ_L and taking the Hamaker constant as a free parameter, we accurately confirm the theoretical expression for the deposition kinetics. Operating in the van der Waals regime enables control of the deposition rates via surface chemistry. From a surface science perspective, working in the van der Waals regime enables us to measure the Hamaker constants of thousands of particles in a few minutes, a task that would take a much longer time to perform with standard AFM.

Paper-based RNA detection and multiplexed analysis for Ebola virus diagnostics

The most performing techniques enabling early diagnosis of infectious diseases rely on nucleic acid detection. Today, because of their high technicality and cost, nucleic acid amplification tests (NAAT) are of benefit only to a small fraction of developing countries population. By reducing costs, simplifying procedures and enabling multiplexing, paper microfluidics has the potential to considerably facilitate their accessibility. However, most of the studies performed in this area have not quit the lab. This letter brings NAAT on paper closer to the field, by using clinical samples and operating in a resource-limited setting. We first performed isothermal reverse transcription and Recombinase Polymerase Amplification (RT-RPA) of synthetic Ribonucleic Acid (RNA) of Ebola virus using paper microfluidics devices. We further applied this method in Guinea to detect the presence of Ebola virus in human sample RNA extracts, with minimal facilities (carry-on detection device and freeze-dried reagents on paper). RT-RPA results were available in few minutes and demonstrate a sensitivity of 90.0% compared to the gold-standard RT-PCR on a set of 43 patient samples. Furthermore, the realization of a nine-spot multilayered device achieving the parallel detection of three distinct RNA sequences opens a route toward the detection of multiple viral strains or pathogens.

Tailoring Patterns of Surface-Attached Multiresponsive Polymer Networks

A new strategy for the fabrication of micropatterns of surface-attached hydrogels with well-controlled chemistry is reported. The "grafting onto" approach is preferred to the "grafting from" approach. It consists of cross-linking and grafting preformed and functionalized polymer chains through thiol-ene click chemistry. The advantage is a very good control without adding initiators. A powerful consequence of thiol-ene click reaction by UV irradiation is the facile fabrication of micropatterned hydrogel thin films by photolithography. It is achieved either with photomasks using common UV lamp or without photomasks by direct drawing due to laser technology. Our versatile approach allows the fabrication of various chemical polymer networks on various solid substrates. It is demonstrated here with silicon wafers, glass and gold surfaces as substrates, and two responsive hydrogels, poly(N-isopropylacrylamide) for its responsiveness to temperature and poly(acrylic acid) for its pH-sensitivity. We also demonstrate the fabrication of stable hydrogel multilayers (or stacked layers) in which each elementary layer height can widely range from a few nanometers to several micrometers, providing an additional degree of freedom to the internal architecture of hydrogel patterns. This facile route for the synthesis of micrometer-resolute hydrogel patterns with tailored architecture and multiresponsive properties should have a strong impact.

Designing Colloidal Molecules with Microfluidics

The creation of new colloidal materials involves the design of functional building blocks. Here, a microfluidic method for designing building blocks one by one, at high throughput, with a broad range of shapes is introduced. The method exploits a coupling between hydrodynamic interactions and depletion forces that controls the configurational dynamics of droplet clusters traveling in microfluidic channels. Droplet clusters can be solidified in situ with UV. By varying the flow parameters, clusters are prescribed a given size, geometry, chemical and/or magnetic heterogeneities enabling local bonding. Compact structures (chains, triangles, diamonds, tetrahedrons,...) and noncompact structures, such as crosses and T, difficult to obtain with current techniques are produced. Size dispersions are small (2%) and throughputs are high (30 000 h^-1). The work opens a new pathway, based on microfluidics, for designing colloidal building blocks with a potential to enable the creation of new materials.

Multiscale Surface-Attached Hydrogel Thin Films with Tailored Architecture

A facile route for the fabrication of surface-attached hydrogel thin films with well-controlled chemistry and tailored architecture on wide range of thickness from nanometers to micrometers is reported. The synthesis, which consists in cross-linking and grafting the preformed and ene-reactive polymer chains through thiol-ene click chemistry, has the main advantage of being well-controlled without the addition of initiators. As thiol-ene click reaction can be selectively activated by UV-irradiation (in addition to thermal heating), micropatterned hydrogel films are easily synthesized. The versatility of our approach is illustrated by the possibility to fabricate various chemical polymer networks, like stimuli-responsive hydrogels, on various solid substrates, such as silicon wafers, glass, and gold surfaces. Another attractive feature is the development of new complex hydrogel films with targeted architecture. The fabrication of various architectures for polymer films is demonstrated: multilayer hydrogel films in which single-networks are stacked one onto the other, interpenetrating networks films with mixture of two networks in the same layer, and nanocomposite hydrogel films where nanoparticles are stably trapped inside the mesh of the network. Thanks to its simplicity and its versatility this novel approach to surface-attached hydrogel films should have a strong impact in the area of polymer coatings.

Correction: Step-emulsification in a microfluidic device

Correction for 'Step-emulsification in a microfluidic device' by Z. Li et al., Lab Chip, 2015, 15, 1023-1031.

A fast and switchable microfluidic mixer based on ultrasound-induced vaporization of perfluorocarbon

Mixing two fluids together within a microfluidic device still remains a challenging operation today. In order to achieve this goal, a number of effective micromixers have been developed over the years based on the use of either passive or active systems. Typically, passive mixers require no external energy, are more robust, and are easy to manufacture albeit they are poorly flexible. Active mixers, on the other hand, rely on external disturbance and are thus more difficult to use but are proven to have greater efficacy. Here, we report a particularly effective, remotely induced and switchable microfluidic mixer, which relies on the concomitant use of ultrasound and a perfluorocarbon (PFC) phase, with the latter benefiting from its immiscibility with most fluids and its low boiling point. More specifically, our approach is based on localized vaporization of a PFC phase at the focal zone of a transducer leading to efficient mixing of two adjacent fluids. The results show that mixing occurs ~100 ms following the delivery of the acoustic pulse, while a laminar flow is re-established on roughly the same time scale. Overall, this method is simple and effective, does not require tailored channel geometries, is compatible with both hydrophilic and hydrophobic microfluidic systems, and is applicable to a wide range of Reynolds numbers (10(-4) < Re < 2.10(0)), and the PFC phase can be easily separated from the mixed phase at the end of the run.

Step-emulsification in a microfluidic device

We present a comprehensive study of the step-emulsification process for high-throughput production of colloidal monodisperse droplets. The 'microfluidic step emulsifier' combines a shallow microchannel operating with two co-flowing immiscible fluids and an abrupt (step-like) opening to a deep and wide reservoir. Based on Hele-Shaw hydrodynamics, we determine the quasi-static shape of the fluid interface prior to transition to oscillatory step-emulsification at low capillary numbers. The theoretically derived transition threshold yields an excellent agreement with experimental data. A closed-form expression for the size of the droplets generated in the step-emulsification regime and derived using geometric arguments also shows a very good agreement with the experiment.

Droplet-based microfluidics at the femtolitre scale

We have built a toolbox of modules for droplet-based microfluidic operations on femtolitre volume droplets. We have demonstrated monodisperse production, sorting, coalescence, splitting, mixing, off-chip incubation and re-injection at high frequencies (up to 3 kHz). We describe the constraints and limitations under which satisfactory performances are obtained, and discuss the physics that controls each operation. For some operations, such as internal mixing, we obtained outstanding performances: for instance, in 75 fL droplets the mixing time was 45 μs, 35-fold faster than previously reported for a droplet microreactor. In practice, in all cases, a level of control comparable to nanolitre or picolitre droplet manipulation was obtained despite the 3 to 6 order of magnitude reduction in droplet volume. Remarkably, all the operations were performed using devices made using standard soft-lithography techniques and PDMS rapid prototyping. We show that femtolitre droplets can be used as microreactors for molecular biology with volumes one billion times smaller than conventional microtitre plate wells: in particular, the Polymerase Chain Reaction (PCR) was shown to work efficiently in 20 fL droplets.

Physics and technological aspects of nanofluidics

From a physical perspective, nanofluidics represents an extremely rich domain. It hosts many mechanisms acting on the nanoscale, which combine together or interact with the confinement to generate new phenomena. Superfast flows in carbon nanotubes, nonlinear electrokinetic transport, slippage over smooth surfaces, nanobubble stability, etc. are the most striking phenomena that have been unveiled over the past few years, and some of them are still awaiting an explanation. One may anticipate that new nanofluidic effects will be discovered in the future, but at the moment, the technological barrier is high. Fabrication of nanochannels is most often a tour de force, slow and costly. However, with the accumulation of technological skills along with the use of new nanofluidic materials (like nanotubes), nanofluidics is becoming increasingly accessible to experimentalists. Among the technological challenges faced by the field, fabricating devices mimicking natural nanometric systems, such as aquaporins, ionic pumps or kidney osmotic filtering, seems the most demanding in terms of groundbreaking ideas. Nanoflow characterization remains delicate, although considerable progress has been achieved over the past years. The targeted application of nanofluidics is not only in the field of genomics and membrane science--with disruptive developments to be expected for water purification, desalination, and energy harvesting--but also for oil and gas production from unconventional reservoirs. Today, in view of the markets that are targeted, nanofluidics may well impact the industry more than microfluidics; this would represent an unexpected paradox. These successes rely on using a variety of materials and technologies, using state-of-the-art nanofabrication, or low-tech inexpensive approaches. As a whole, nanofluidics is a fascinating field that is facing considerable challenges today. It possesses a formidable potential and offers much space for creative groundbreaking ideas.

Cyclic olefin copolymer plasma millireactors

The novelty of this paper lies in the development of a multistep process for the manufacturing of plasma millireactors operating at atmospheric pressure. The fabrication process relies on the integration of metallic electrodes over a cyclic olefin copolymer chip by a combination of photopatterning and sputtering. The developed plasma millireactors were successfully tested by creating air discharges in the gas volume of the millichannel. A sputtered silica layer was deposited on the channel walls to provide a barrier between the plasma and the polymer in order to prevent the alteration of polymer surfaces during the plasma treatment. Interest in this process of employing plasma millireactor as a high reactive environment is demonstrated here by the degradation of a volatile organic compound (acetaldehyde) in ambient air. In this miniaturized device, we obtained a high acetaldehyde conversion (98%) for a specific input energy lower than 200 J L(-1).

Dynamical role of slip heterogeneities in confined flows

We demonstrate that flows in confined systems are controlled by slip heterogeneities below a certain size. To show this we image the motion of soft glassy suspensions in microchannels whose inner walls impose different slip velocities. As the channel height decreases, the flow ceases to have the symmetric shape expected for yield-stress fluids. A theoretical model accounts for the role of slip heterogeneities and captures the velocity profiles. We generalize these results by introducing a length scale, valid for all fluids, below which slip heterogeneities dominate the flow in confined systems. General implications of this notion, concerning the interplay between slip and confinement, are presented.

Recent progress in the physics of microfluidics and related biotechnological applications

Since the mid-nineties, the physical understanding of microfluidic flows has reached a level sufficiently elaborate for envisaging applications in all sorts of domains. As the domain expanded, the existence of new situations where fluid dynamics at small or moderate Reynolds numbers combines with confinement, interfaces, transport, particles along with disordered substrates raised new challenges. The present review is restricted to three domains in which progress in the physical description has been made recently (droplet-based, inertial and paper-based microfluidics) and for which biotechnological applications are foreseeable.

Determining phase diagrams of gas-liquid systems using a microfluidic PVT

A novel microfluidic device designed for analyzing phase diagrams of gas-liquid systems (PVT or pressure-volume-temperature measurements) is described. The method mimics the phase transition of a reservoir fluid as it travels through the wellbore from the formation to the surface. The device consists of a long serpentine microchannel etched in a silicon substrate. The local pressure inside the channel is measured using membrane-based optical pressure sensors positioned along the channel. Geometrical restrictions are placed along the microchannel in order to nucleate bubbles when nucleation conditions are met, thus preventing the development of a supersaturation state in the channel. We point out that a local equilibrium state between gas and liquid phases is achieved, which implies that equilibrium properties can be directly measured on the chip. We analyze different mixtures of hydrocarbon systems and, consistently with the preceding analysis, obtain excellent agreement between our technique and conventional measurements. From a practical viewpoint (important for the relevance of the technology), we observe that the measurement time of thermodynamic properties of gas-liquid systems is reduced from hours to minutes with the present device without compromising the measurement accuracy.

In vivo targeted delivery of large payloads with an ultrasound clinical scanner

PURPOSE

Performing drug-delivery with an ultrasonic imaging scanner in situ could drastically simplify treatment and improve its specificity. Our objective is to deliver large amounts of an encapsulated agent in vivo using a clinical ultrasound scanner with a millimetric resolution. This study describes the encapsulation of fluorescein within ultrasound-inducible composite droplets and its targeted release in predefined zones in the liver of rats.

METHODS

An aqueous solution of fluorescein was encapsulated within perfluorocarbon liquid in 4 μm monodisperse droplets using a microfluidic system. The agent was then injected within the femoral vein of 12 rats. After exploratory ultrasound imaging, the sonographer defined five zones in the liver and a release sequence was initiated on the same apparatus. The surface of the liver was observed under fluorescence macroscopy and intraoperative fluorescence camera in vivo, before liver samples were sliced for pathology.

RESULTS

Following the conversion of the droplets, a 25 dB increase in contrast was observed in the zones selected by the sonographer. These hyperechoic regions were colocalized with the bright fluorescent spots observed on the surface of the liver. A minimum peak-negative pressure of 2.6 MPa, which is within regulations for imaging pulses, was required for the delivery of the content of the droplets. The tissue and cellular structures were not affected by the exposure to the release sequence.

CONCLUSIONS

Since composite droplets can carry various therapeutic and imaging agents, they could deliver such agents specifically in any organ accessible to ultrasound.

Obstructed breakup of slender drops in a microfluidic T junction

In this Letter we present a theoretical analysis of the droplet breakup with "permanent obstruction" in a microfluidic T junction [M.-C. Jullien et al., Phys. Fluids 21, 072001 (2009)]. The proposed theory is based on a simple geometric construction for the interface shape combined with Tanner's law for the local contact angle. The resulting scaling of the droplet deformation with time and capillary number is in excellent agreement with the results of direct numerical simulations and prior experiments. More rigorous analysis based on the lubrication approximation reveals a self-similar behavior analogous to the classical problem of a droplet spreading over a preexisting liquid film.

Analysis of gene expression at the single-cell level using microdroplet-based microfluidic technology

In the present work, we have measured the messenger RNA expression of specific genes both from total RNA and cells encapsulated in droplets. The microfluidic chip introduced includes the following functionalities: RNA∕cell encapsulation, lysis, reverse transcription and real-time polymerase chain reaction. We have shown that simplex and duplex gene expression measurements can be carried out over a population of 100 purified RNA samples encapsulated simultaneously in 2 nl droplets in less than 2 h. An analysis of 100 samples containing one to three cells has shown excellent consistency with standard techniques regarding average values. The cell-to-cell distributions of the E-cadherin expression suggest fluctuations on the order of 80% in the number of transcripts, which is highly consistent with the general findings from the literature. A mathematical model has also been introduced to strengthen the interpretation of our results. The present work paves the way for the systematic acquisition of such information in biological and biomedical studies.

Ultrasound internal tattooing

PURPOSE

The ability of remotely tagging tissues in a controlled and three-dimensional manner during preoperative imaging could greatly help surgeons to identify targets for resection. The authors' objective is to selectively and noninvasively deposit markers under image guidance for such internal tattooing.

METHODS

This study describes the production of new ultrasound-inducible droplets carrying large payloads of fluorescent markers and the in vivo proof of concept of their remote and controlled deposition via focused ultrasound. The droplets are monodispersed multiple emulsions produced in a microfluidic system, consisting of aqueous fluorescein in perfluorocarbon in water. Their conversion (either by vaporization or cavitation) is performed remotely using a clinical ultrasonic imaging probe.

RESULTS

When submitted to 5 MHz imaging pulses, the droplets vaporize in vitro at 1.4 MPa peak-negative pressure and eject their content. After several seconds, a brightly fluorescent spot (0.5 mm diameter) is observed at the focus of the transducer. Experiments in the chorioallantoique membrane of chicken eggs and chicken embryo demonstrate that the spot is stable and is easily seen by naked eye.

CONCLUSIONS

These ultrasound-inducible multiple emulsions could be used to deliver large amounts of contrast agents, chemotherapy, and genetic materials in vivo using a conventional ultrasound scanner.

Microfluidics and complex fluids

In this paper, we describe four experimental studies we carried out over the last four years in the MMN lab, regarding the dynamical behaviour of complex fluids in microfluidic systems. The topics are: (1) Polymer breakup in microfluidic systems. (2) Flows of polymer solutions in microchannels close to a smooth wall. (3) Shear banding flows in microchannels (rheology, instabilities). (4) Flows of concentrated solutions of microgel particles through microchannels. Depending on the situation, we exploit the duality low Reynolds numbers/high Weissenberg numbers (for instance, by working at high shear rates without generating turbulence), use visualization windows naturally offered by the microfluidic environment or take advantage of the integration of various functionalities on the chip. In all cases, new information, hardly accessible to non-miniaturized approaches, could be obtained by using microfluidic technology.

Benzophenone absorption and diffusion in poly(dimethylsiloxane) and its role in graft photo-polymerization for surface modification

Following the great success of traditional microfluidic devices across many disciplines, a new class of microfluidic systems emerged in recent years, which features finely tuned, localized surface modifications within the microstructures in order to keep up with the demand for devices of ever increasing complexity (lab on chip, assay on chip, etc.). Graft photopolymerization has become a powerful tool for such localized surface modifications particularly in combination with poly(dimethylsiloxane) (PDMS) devices, as it is compatible with many functional monomers and allows for high spatial resolution. However, application within enclosed PDMS microstructures and in particular well-controlled surface-directed polymerization remains challenging. Detailed understanding of the interaction between photoinitiator, benzophenone (BP), and polymer matrix is needed. We have developed a visualization technique, which allows for observation of reacted BP in situ within the PDMS matrix. We present a detailed study on solvent-driven BP diffusion providing results essential to successful surface treatment. We also identified and investigated photoinitiator inhibition by oxygen and provide appropriate mitigation strategies.

Interfacially driven instability in the microchannel flow of a shear-banding fluid

Using microparticle image velocimetry, we resolve the spatial structure of the shear-banding flow of a wormlike micellar surfactant solution in a straight microchannel. We reveal an instability of the interface between the shear bands, associated with velocity modulations along the vorticity direction. We compare our results with a detailed theoretical study of the diffusive Johnson-Segalman model. The quantitative agreement obtained favors an instability scenario previously predicted theoretically but hitherto unobserved experimentally, driven by a normal stress jump across the interface between the bands.

Monodisperse colloids synthesized with nanofluidic technology

Limitations in the methods employed to generate micrometric colloidal droplets hinder the emergence of key applications in the fields of material science and drug delivery. Through the use of dedicated nanofluidic devices and by taking advantage of an original physical effect called capillary focusing, we could circumvent some of these limitations. The nanofluidic (i.e., submicrometric) devices introduced herein are made of soft materials, and their fabrication relies upon rapid technologies. The objects that we have generated are simple droplets, multiple droplets, particles, and Janus particles whose sizes lie between 900 nm and 3 microm (i.e., within the colloidal range). Colloidal droplets have been assembled on-chip into clusters and crystals, yielding discrete diffraction patterns. We illustrate potential applications in the field of drug delivery by demonstrating the ability of multiple droplets to be phagocytosed by murine macrophage-type cells.

Synthesizing microcapsules with controlled geometrical and mechanical properties with microfluidic double emulsion technology

Using lithography-based microfluidic technology, we produce monodisperse single-core microcapsules with UV-cured TPGDA (triprophylene glycol diacrylate) shells. We show that the geometrical and mechanical characteristics of the microcapsules can be predicted on a quantitative basis and tuned by varying the flow conditions. Shell thicknesses are varied by changing the flow rates of the inner or intermediate phases, according to mass conservation constraint. Off-centering of the core with respect to the shell is controlled by varying the shell phase viscosity. The mechanical properties of the capsules can be varied by changing the flow conditions and are quantitatively predicted by a numerical simulation. The simulation moreover provides a correct qualitative description of their rupture. As a whole, the work carried out in the present paper shows, on a quantitative basis, that microfluidic technology allows to finely control the geometrical and mechanical properties of microcapsules generated on chip. The level of control we reach here is not accessible, by far, to conventional technologies. Combined with parallelization, the present work opens routes toward the production of novel families of monodisperse microcapsules with tunable properties.

Equilibrium and nonequilibrium states in microfluidic double emulsions

We describe experimental and theoretical studies dedicated to establishing the physics of formation of double droplets in microfluidic systems. We show that the morphologies (complete engulfing, partial engulfing, and nonengulfing) obtained at late times minimize the interfacial energy of the system. We explain that nonequilibrium morphologies generated in the system can have long lifetimes. Remarkably, the physics of formation of the double droplets with microfluidics allows the synthesis of particles with new morphologies.

Nanofluidics in the Debye layer at hydrophilic and hydrophobic surfaces

By using evanescent waves, we study equilibrium and dynamical properties of liquid-solid interfaces in the Debye layer for hydrophilic and hydrophobic surfaces. We measure velocity profiles and nanotracer concentration and diffusion profiles between 20 and 300 nm from the walls in pressure-driven and electro-osmotic flows. We extract electrostatic and zeta potentials and determine hydrodynamic slip lengths with 10 nm accuracy. The spectacular amplification of the zeta potential resulting from hydrodynamic slippage allows us to clarify for the first time the dynamic origin of the zeta potential.

Using surface force apparatus, diffusion and velocimetry to measure slip lengths

Determining the slip lengths for liquids flowing close to smooth walls is challenging. The reason lies in the fact that the scales that must be addressed range between a few and hundreds of nanometres. Several techniques have been used over the last few years. Here, we consider three of them based on surface force apparatus, diffusion and velocimetry, respectively. The descriptions offered here incorporate recent instrumental progress made in the field.

Synthesis of iron oxide nanoparticles in a microfluidic device: preliminary results in a coaxial flow millichannel

A millimetric coaxial flow device operating under laminar flow has been designed to study the synthesis of iron oxide nanoparticles in a millichannel where the flow rate of the different reagents has been adjusted all over the experiments so that the magnetic and stable colloidal iron oxide particles with a size less than 7 nm have been prepared continuously.