Microfluidic Network Research Articles

Abstract P1046: An Oxygen Gradient Microphysiological System To Model The Myocardial Infarct Border Zone

After a myocardial infarction, the border zone is characterized by an oxygen (O 2 ) gradient located at the interface between the injured, hypoxic cardiac tissue and adjacent normoxic, viable tissue. Yet, the impact of an O 2 gradient on cardiac function is not well understood because there is a lack of in vitro experimental platforms for generating such biomimetic O 2 landscapes. This unmet need has prevented systematic investigation into how hypoxic-normoxic intercellular communication affects cardiac function. In this study, a microphysiological system was engineered to generate and expose engineered cardiac tissues to a stable O 2 gradient as a “Border-Zone-on-a-Chip” model. Briefly, a microfluidic gas supply channel network was overlaid with a thin, gas-permeable polydimethylsiloxane membrane on which engineered cardiac tissue composed of neonatal rat ventricular myocytes was cultured. Constant perfusion of compressed gases through separate microfluidic channels and rapid diffusion of the gas conditions across the thin membrane produced an O 2 gradient. Functional and transcriptomic analysis revealed biological responses were uniquely regulated by an O 2 gradient when compared to engineered tissues exposed to global, homogeneous O 2 levels. In the gradient, there was a significant increase in the calcium transient time to peak (85%, p=0.0007) and tau time constant of decay (49%, p=0.006). There was a significant decrease in the longitudinal calcium wave propagation velocity (35%, p=0.03), diastolic stress (47%, p=0.01), and peak systolic stress (47%, p=0.01). Overall, the gradient exhibited hallmarks of arrhythmogenesis, hypocontractility, and activation of inflammation after 4 h of O 2 modulation. Importantly, these changes were distinct from those observed in tissues exposed to uniform normoxia or hypoxia, demonstrating distinct regulation of cardiac tissue phenotypes by an O 2 gradient. Studies are ongoing to elucidate a possible paracrine mechanism involved in the observed effects. By gaining functional and mechanistic insight into intercellular communication activated in O 2 gradients, microphysiological systems can direct future therapeutic strategies aimed at minimizing or preventing cardiac hypoxic injury.

Life sciences discovery and technology highlights

Life sciences discovery and technology highlights

A fluidic relaxation oscillator for reprogrammable sequential actuation in soft robots

A fluidic relaxation oscillator for reprogrammable sequential actuation in soft robots

3D in-plane integrated micro reflectors enhancing signal capture in lab on a chip applications.

The integration of micro-optics in lab on a chip (LOCs) devices is crucial both for increasing the solid angle of acquisition and reducing the optical losses, aiming at improving the signal-to-noise ratio (SNR). In this work, we present the thriving combination of femtosecond laser irradiation followed by chemical etching (FLICE) technique with CO2 laser polishing and inkjet printing to fabricate in-plane, 3D off-axis reflectors, featuring ultra-high optical quality (RMS ∼3 nm), fully integrated on fused silica substrates. Such micro-optic elements can be used both in the excitation path, focusing an incoming beam in 3D, and in the acquisition branch, harvesting the optical signal coming from a specific point in space. The flexibility of the manufacturing process allows the realization of micro-optics with several sizes, shapes and their integration with photonic circuits and microfluidic networks.

On the efficiency of convective mixing in a Y-shaped channel

Continuous-flow microfluidic devices are widely used in microbiology, fine organic synthesis, pharmaceuticals, biomedicine, etc. Most applications require rapid mixing of the fluids that pass through the microfluidic chip. The mechanism of natural diffusion is not always efficient due to limitations on the length of the channel. In this work, we numerically study the efficiency of using various mechanisms of natural convection for the mixing of fluids entering the microfluidic chip. Solutions typically differ in buoyancy and diffusion rates of dissolved components, making them sensitive to gravity-dependent instabilities such as Rayleigh-Taylor convection, double diffusion and diffusion layer convection. We consider a Y-shaped microchannel, which is, on the one hand, the simplest, and, on the other hand, a typical element of a microfluidic network. We assume that two miscible solutions independently flow into a common channel where they come into contact. For each type of instability, we numerically estimated the characteristic channel length, after which complete mixing of the solutions occurs. The simulations were performed in the framework of both 2D and 3D models. Finally, we compare the numerical results with the experimental data obtained recently.

In-plane Extended Nano-coulter Counter (XnCC) for the Label-free Electrical Detection of Biological Particles.

We report an in-plane extended nanopore Coulter counter (XnCC) chip fabricated in a thermoplastic via imprinting. The fabrication of the sensor utilized both photolithography and focused ion beam milling to make the microfluidic network and the in-plane pore sensor, respectively, in Si from which UV resin stamps were generated followed by thermal imprinting to produce the final device in the appropriate plastic (cyclic olefin polymer, COP). As an example of the utility of this in-plane extended nanopore sensor, we enumerated SARS-CoV-2 viral particles (VPs) affinity-selected from saliva and extracellular vesicles (EVs) affinity-selected from plasma samples secured from mouse models exposed to different ionizing radiation doses.

A miniaturized 3D printed pressure regulator (µPR) for microfluidic cell culture applications

Well-defined fluid flows are the hallmark feature of microfluidic culture systems and enable precise control over biophysical and biochemical cues at the cellular scale. Microfluidic flow control is generally achieved using displacement-based (e.g., syringe or peristaltic pumps) or pressure-controlled techniques that provide numerous perfusion options, including constant, ramped, and pulsed flows. However, it can be challenging to integrate these large form-factor devices and accompanying peripherals into incubators or other confined environments. In addition, microfluidic culture studies are primarily carried out under constant perfusion conditions and more complex flow capabilities are often unused. Thus, there is a need for a simplified flow control platform that provides standard perfusion capabilities and can be easily integrated into incubated environments. To this end, we introduce a tunable, 3D printed micro pressure regulator (µPR) and show that it can provide robust flow control capabilities when combined with a battery-powered miniature air pump to support microfluidic applications. We detail the design and fabrication of the µPR and: (i) demonstrate a tunable outlet pressure range relevant for microfluidic applications (1–10 kPa), (ii) highlight dynamic control capabilities in a microfluidic network, (iii) and maintain human umbilical vein endothelial cells (HUVECs) in a multi-compartment culture device under continuous perfusion conditions. We anticipate that our 3D printed fabrication approach and open-access designs will enable customized µPRs that can support a broad range of microfluidic applications.

A Novel Fluidic Platform for Semi-Automated Cell Culture into Multiwell-like Bioreactors.

In this work, we developed and characterized a novel fluidic platform that enables long-term in vitro cell culture in a semi-automated fashion. The system is constituted by a control unit provided with a piezoelectric pump, miniaturized valves, and a microfluidic network for management and fine control of reagents’ flow, connected to a disposable polymeric culture unit resembling the traditional multiwell-like design. As a proof of principle, Human Umbilical Vein Endothelial Cells (HUVEC) and Human Mesenchymal Stem Cells (hMSC) were seeded and cultured into the cell culture unit. The proliferation rate of HUVEC and the osteogenic differentiation of hMSC were assessed and compared to standard culture in Petri dishes. The results obtained demonstrated that our approach is suitable to perform semi-automated cell culture protocols, minimizing the contribution of human operators and allowing the standardization and reproducibility of the procedures. We believe that the proposed system constitutes a promising solution for the realization of user-friendly automated control systems that will favor the standardization of cell culture processes for cell factories, drug testing, and biomedical research.

Analytical model describing the nonlinear behavior of an elastomeric pump membrane in a microfluidic network

Analytical model describing the nonlinear behavior of an elastomeric pump membrane in a microfluidic network

Numerical investigation of continuous acoustic particle separation using electrothermal pumping in a point of care microfluidic device

Acoustic microfluidics has many advantages; however, the method has an integration limitation due to the pumping mechanism in which fluids could only be pumped using equipment which lays outside the whole system. This limits the footprint of the microfluidic network to have integrated devices. In this study, a new solver is developed in open-source code Open-FOAM to apply dielectrophoresis, electrothermal and standing-surface-acoustic-waves force to the fluid and particles. This study presents a novel pumping mechanism for acoustic microfluidics utilizing electrothermal force. The pumping section not only creates forward movement, but also focuses the particles in the middle of the micro-channel removing the need for a separate IDT module for particle focusing. The new configuration of electrodes increases fluid velocity up to 400% in comparison with conventional electrodes arrangement. The new configuration let 87% of particles to pass through the electrodes toward the outlet eliminating the problem in particle passage through the electrodes. The appropriate arrangement of the electrodes is also investigated for efficient particle movement in the micro-channel. In the acoustic separation, the power of 0.0015 W is considered efficient with a separation efficiency of 96% and purity of 94.1% for 10 micrometer and 95.9% for 5 micrometer polystyrene particles.

Lab-on-PCB: One step away from the accomplishment of μTAS?

The techniques, protocols, and advancements revolving around printed circuit boards (PCBs) have been gaining sustained attention in the realm of micro-total analysis systems (μTAS) as more and more efforts are devoted to searching for standardized, highly reliable, and industry-friendly solutions for point-of-care diagnostics. In this Perspective, we set out to identify the current state in which the field of μTAS finds itself, the challenges encountered by researchers in the implementation of these technologies, and the potential improvements that can be targeted to meet the current demands. We also line up some trending innovations, such as 3D printing and wearable devices, along with the development of lab-on-PCB to increase the possibility of multifunctional biosensing activities propelled by integrated microfluidic networks for a wider range of applications, anticipating to catalyze the full potential of μTAS.

Microfluidics-based fabrication of flexible ionic hydrogel batteries inspired by electric eels

Electric eels in nature can generate high voltage with hundreds of volts based on the mechanism of gradient-induced ion flux, which provides an excellent prototype to inspire the exploration of more efficient and green energy generation strategies in artificial systems. Here we developed a novel microfluidics-based strategy to efficiently fabricate flexible ionic hydrogel batteries by mimicking ion-concentration gradients in biological organs. Microfluidic networks were designed to simultaneously transport four types of ionic hydrogel solutions into pre-defined well arrays and enable the perfusion of 801 hydrogel precursor wells within 1.53 min. A specifically-designed photomask was used to selectively solidify these precursors into separate hydrogel particles, which were further assembled into a flexible ionic hydrogel battery pack by using negative pressure. The voltage output of the resultant ionic hydrogel battery pack was found to increase linearly with the increase of battery unit number, and a maximum voltage of 73.33 V can be achieved by connecting eight ionic hydrogel battery packs in series. The resultant ionic hydrogel battery exhibited unique capability in stably working in wide temperature ranges (0–90°C) and large deformation conditions, and can be further preserved for a long period after dehydration and instantly recover its original discharge capacity once rehydrated. PEDOT:PSS was introduced to the ionic hydrogel battery for reduced internal resistance, resulting in a 1.5-fold improvement in electrical current output. The presented microfluidics-based and high-efficient fabrication strategy exhibits great promise to translate the biomimetic flexible power-generating systems into a viable soft power source for various electrically-driven applications.

Micro-particle entrapment dynamics in microfluidic pulmonary capillary networks

The journey of vascular targeted carriers (VTC) in the circulatory system is highly intricate and includes navigation through different vessel structures, such as the vast pulmonary capillary network (PCN) in the lungs where particles can get entrapped and lead to blockage. Here, we leverage microfluidic PCN models to explore, for the first time, micro-particle capillary entrapment, in a well-controlled biophysical environment mimicking human physiological hemodynamics at true scale. This in vitro strategy mimics the challenges of vascular carrier transport during their journey in the smallest capillaries of the body (∼5 µm). Specifically, we explore in the PCN model entrapment dynamics of spherical micro-particles of different diameters (i.e. 3, 4 and 4.5 µm) at different concentrations, comparing their motion in cell-free buffer to that in the presence of red blood cells (RBCs). Notably, while 3 µm particles exhibit undisturbed transport in all of the examined concentrations, both in cell-free buffer and in the presence of RBCs, particles of 4 and 4.5 µm exhibit a concentration-dependent transport where the presence of RBCs leads in fact to reduced entrapment. Our experiments suggest that collisions of micro-particles with RBCs can facilitate their navigability, allowing for carrier transport that would lead otherwise to rapid entrapment in a cell-free environment. Altogether, the proposed preclinical in vitro assays offer rapid screening opportunities for design optimization of VTC transport in capillary networks.

Reinforcement-Learning designs droplet microfluidic networks

Droplet microfluidics provides a platform for realizing programmable and flexible devices for performing diverse process operations on a single chip. Although recent developments have incorporated basic functionalities in droplet-based devices such as droplet sorting, sequencing, synchronization, etc., the design of microfluidic networks for specific functionalities remains to be of research interest. This requires the development of integrated devices that can perform complex operations and incorporate multiple functionalities on a single microfluidic chip. In this work, a state-of-the-art Reinforcement-learning (RL) algorithm based on Temporal-difference Q-learning is used to identify discrete microfluidic networks for a given functionality. This is the first time that a machine learning-based approach is deployed for discovering integrated network designs. The implementation of the algorithm has been illustrated through a combinatorial sorting problem, where the objective is to sort all the incoming droplet sequences at the inlet for two distinct droplet types. The algorithm identifies unapparent network designs using RL-based decisions in a microfluidic network space. The superiority of the RL-based algorithm is established against random search based on execution time, and against Genetic Algorithms (GA) based on solution quality. This work could be a first step towards achieving the ultimate goal of developing an unified droplet microfluidic design framework that can cater to a large number of drops and varying functionalities.

Nonlinear Phenomena in Microfluidics.

This review focuses on experimental work on nonlinear phenomena in microfluidics, which for the most part are phenomena for which the velocity of a fluid flowing through a microfluidic channel does not scale proportionately with the pressure drop. Examples include oscillations, flow-switching behaviors, and bifurcations. These phenomena are qualitatively distinct from laminar, diffusion-limited flows that are often associated with microfluidics. We explore the nonlinear behaviors of bubbles or droplets when they travel alone or in trains through a microfluidic network or when they assemble into either one- or two-dimensional crystals. We consider the nonlinearities that can be induced by the geometry of channels, such as their curvature or the bas-relief patterning of their base. By casting posts, barriers, or membranes─situated inside channels─from stimuli-responsive or flexible materials, the shape, size, or configuration of these elements can be altered by flowing fluids, which may enable autonomous flow control. We also highlight some of the nonlinearities that arise from operating devices at intermediate Reynolds numbers or from using non-Newtonian fluids or liquid metals. We include a brief discussion of relevant practical applications, including flow gating, mixing, and particle separations.

Droplet microfluidic networks as hybrid dynamical systems: Inlet spacing optimization for sorting of drops

AbstractThe ability to manipulate drops is essential for integrating multiple processes in droplet microfluidics‐based lab‐on‐a‐chip devices. Examples of such droplet manipulation operations include sorting, sequencing, synchronization, and so forth. Microfluidic networks are promising platforms for performing these functionalities. This work explores the design of entry times of drops, a set of operating parameters, in a microfluidic network to achieve a desired functionality. Here, we specifically focus on sorting of drops at the exit of the network. For the first time, droplet microfluidic networks are perceived as hybrid dynamical systems. This viewpoint allows us to develop a methodology for designing the entry times of drops in microfluidic networks to achieve desired functionalities through the notion of constraint satisfaction. The solution to this constraint satisfaction problem provides entry times that result in the desired functionality. The proposed method allows one to establish the existence/nonexistence of entry time solutions for a desired functionality.

Surface Charge Affecting Fluid–Fluid Displacement at Pore Scale

AbstractEfficiency in fluid–fluid displacement is drastically reduced by viscous fingering, limiting the overall effectiveness in enhanced oil recovery, membrane science, and lateral flow devices used in biomedical applications. Local instabilities at the fluid–fluid interface lead to finger‐like patterns when a less viscous fluid displaces an immiscible fluid of higher viscosity. This widely observed phenomenon in multiphase flow inside porous media is infamously intricate to control, especially for given geometry and viscosity ratio. The presented study uses a highly controlled microfluidic porous network structure with tailored ionic surface strength. The direct correlation of viscous fingering evolution on the porous structure's zeta potential at a pore‐scale level is demonstrated via polyelectrolyte coatings using a layer‐by‐layer technique. Displacement patterns are tuned from vigorous viscous fingering over stable displacement to corner flow events across a broad range of capillary numbers depending on the applied coatings. The experimental data show an increasing trend of oil recovery with increasing surface wettability, consistent with several previous findings. Furthermore, the results reveal that surface zeta potential correlates positively with recovery rate but negatively with the displacement stability quantified by the fractal dimension. These insights enable a more targeted porous media design to obtain optimal multiphase flow control.

Early stage of externally driven filling of viscous fluids within a microfluidic pore-doublet network

The early stage of active capillary filling of viscous fluids in a pore-doublet network is studied experimentally. The effects of operating conditions and fluid properties on the filling process are revealed. In the straight channel, the evolution of the meniscus with time transferred from a linear stage to a power-law stage is due to the interplay among the inertial force, capillary force, and viscous force. An expression of the filling rate is proposed at the present startup of the capillary filling flow. With the addition of surfactant sodium dodecyl sulfate (SDS), the power-law stage of the penetration process turns into another linear stage. In symmetrical Y-shaped microchannels, increasing the capillary number or liquid viscosity or adding surfactant SDS to the solutions effectively reduces the difference in liquid position between the branches. A larger Ohnesorge number leads to more uniform and stable penetration.

Designable microfluidic ladder networks from backstepping microflow analysis for mass production of monodisperse microdroplets.

Controllable mass production of monodisperse droplets plays a key role in numerous fields ranging from scientific research to industrial application. Microfluidic ladder networks show great potential in mass production of monodisperse droplets, but their design with uniform microflow distribution remains challenging due to the lack of a rational design strategy. Here an effective design strategy based on backstepping microflow analysis (BMA) is proposed for the rational development of microfluidic ladder networks for mass production of controllable monodisperse microdroplets. The performance of our BMA rule for rational microfluidic ladder network design is demonstrated by using an existing analogism-derived rule that is widely used for the design of microfluidic ladder networks as the control group. The microfluidic ladder network designed by the BMA rule shows a more uniform flow distribution in each branch microchannel than that designed by the existing rule, as confirmed by single-phase flow simulation. Meanwhile, the microfluidic ladder network designed by the BMA rule allows mass production of droplets with higher size monodispersity in a wider window of flow rates and mass production of polymeric microspheres from such highly monodisperse droplet templates. The proposed BMA rule provides new insights into the microflow distribution behaviors in microfluidic ladder networks based on backstepping microflow analysis and provides a rational guideline for the efficient development of microfluidic ladder networks with uniform flow distribution for mass production of highly monodisperse droplets. Moreover, the BMA method provides a general analysis strategy for microfluidic networks with parallel multiple microchannels for rational scale-up.

Optimization of Biosynthesized Ninps Production Using a Precise Gradient Generation from a Microfluidic Network

Optimization of Biosynthesized Ninps Production Using a Precise Gradient Generation from a Microfluidic Network