Dynamics of Microvalve Operations in Integrated Microfluidics

Toward continuous production of high-quality nanomaterials using microfluidics: nanoengineering the shape, structure and chemical composition

Over the last decade, a multitude of synthesis strategies has been reported for the production of high-quality nanoparticles. Wet-chemical methods are generally the most efficient synthesis procedures since high control of crystallinity and physicochemical properties can be achieved. However, a number of challenges remain from inadequate reaction control during the nanocrystallization process; specifically variability, selectivity, scalability and safety. These shortcomings complicate the synthesis, make it difficult to obtain a uniform product with desired properties, and present serious limitations for scaling the production of colloidal nanocrystals from academic studies to industrial applications. Continuous flow reactors based on microfluidic principles offer potential solutions and advantages. The reproducibility of reaction conditions in microfluidics and therefore product quality have proved to exceed those obtained by batch processing. Considering that in nanoparticles' production not only is it crucial to control the particle size distribution, but also the shape and chemical composition, this review presents an overview of the current state-of-the-art in synthesis of anisotropic and faceted nanostructures by using microfluidics techniques. The review surveys the available tools that enable shape and chemical control, including secondary growth methods, active segmented flow, and photoinduced shape conversion. In addition, emphasis is placed on the available approaches developed to tune the structure and chemical composition of nanomaterials in order to produce complex heterostructures in a continuous and reproducible fashion.

Microvalve with Trapezoid-Shaped Cross-Section for Deep Microchannels

Simple microfluidic systems for handling large particles such as three-dimensional (3D) cultured cells, microcapsules, and animalcules have contributed to the advancement of biology. However, obtaining a highly integrated microfluidic device for handling large particles is difficult because there are no suitable microvalves for deep microchannels. Therefore, this study proposes a microvalve with a trapezoid-shaped cross-section to close a deep microchannel. The proposed microvalve can close a 350 μm deep microchannel, which is suitable for handling hundreds of micrometer-scale particles. A double-inclined lithography process was used to fabricate the trapezoid-shaped cross-section. The microvalve was fabricated by bonding three polydimethylsiloxane layers: a trapezoid-shaped liquid channel layer, a membrane, and a pneumatic channel layer. The pneumatic balloon, consisting of the membrane and the pneumatic channel, was located beneath a trapezoid-shaped cross-section microchannel. The valve was operated by the application of pneumatic pressure to the pneumatic channel. We experimentally confirmed that the expansion of the pneumatic balloon could close the 350 μm deep microchannel.

A Shearless Microfluidic Device Detects a Role in Mechanosensitivity for AWCON Neuron in Caenorhabditis elegans

AWC olfactory neurons are fundamental for chemotaxis toward volatile attractants in Caenorhabditis elegans. Here, it is shown that AWC^ON responds not only to chemicals but also to mechanical stimuli caused by fluid flow changes in a microfluidic device. The dynamics of calcium events are correlated with the stimulus amplitude. It is further shown that the mechanosensitivity of AWC^ON neurons has an intrinsic nature rather than a synaptic origin, and the calcium transient response is mediated by TAX-4 cGMP-gated cation channel, suggesting the involvement of one or more "odorant" receptors in AWC^ON mechano-transduction. In many cases, the responses show plateau properties resembling bistable calcium dynamics where neurons can switch from one stable state to the other. To investigate the unprecedentedly observed mechanosensitivity of AWC^ON neurons, a novel microfluidic device is designed to minimize the fluid shear flow in the arena hosting the nematodes. Animals in this device show reduced neuronal activation of AWC^ON neurons. The results observed indicate that the tangential component of the mechanical stress is the main contributor to the mechanosensitivity of AWC^ON . Furthermore, the microfluidic platform, integrating shearless perfusion and calcium imaging, provides a novel and more controlled solution for in vivo analysis both in micro-organisms and cultured cells.

A design and optimization of a high throughput valve based microfluidic device for single cell compartmentalization and analysis

The need for high throughput single cell screening platforms has been increasing with advancements in genomics and proteomics to identify heterogeneity, unique cell subsets or super mutants from thousands of cells within a population. For real-time monitoring of enzyme kinetics and protein expression profiling, valve-based microfluidics or pneumatic valving that can compartmentalize single cells is advantageous by providing on-demand fluid exchange capability for several steps in assay protocol and on-chip culturing. However, this technique is throughput limited by the number of compartments in the array. Thus, one big challenge lies in increasing the number of microvalves to several thousand that can be actuated in the microfluidic device to confine enzymes and substrates in picoliter volumes. This work explores the design and optimizations done on a microfluidic platform to achieve high-throughput single cell compartmentalization as applied to single-cell enzymatic assay for protein expression quantification. Design modeling through COMSOL Multiphysics was utilized to determine the circular microvalve’s optimized parameters, which can close thousands of microchambers in an array at lower sealing pressure. Multiphysical modeling results demonstrated the relationships of geometry, valve dimensions, and sealing pressure, which were applied in the fabrication of a microfluidic device comprising of up to 5000 hydrodynamic traps and corresponding microvalves. Comparing the effects of geometry, actuation media and fabrication technique, a sealing pressure as low as 0.04 MPa was achieved. Applying to single cell enzymatic assay, variations in granzyme B activity in Jurkat and human PBMC cells were observed. Improvement in the microfluidic chip’s throughput is significant in single cell analysis applications, especially in drug discovery and treatment personalization.

Adoption of reinforcement learning for the intelligent control of a microfluidic peristaltic pump

We herein report a study on the intelligent control of microfluidic systems using reinforcement learning. Integrated microvalves are utilized to realize a variety of microfluidic functional modules, such as switching of flow pass, micropumping, and micromixing. The application of artificial intelligence to control microvalves can potentially contribute to the expansion of the versatility of microfluidic systems. As a preliminary attempt toward this motivation, we investigated the application of a reinforcement learning algorithm to microperistaltic pumps. First, we assumed a Markov property for the operation of diaphragms in the microperistaltic pump. Thereafter, components of the Markov decision process were defined for adaptation to the micropump. To acquire the pumping sequence, which maximizes the flow rate, the reward was defined as the obtained flow rate in a state transition of the microvalves. The present system successfully empirically determines the optimal sequence, which considers the physical characteristics of the components of the system that the authors did not recognize. Therefore, it was proved that reinforcement learning could be applied to microperistaltic pumps and is promising for the operation of larger and more complex microsystems.

3D high-density microelectrode array with optical stimulation and drug delivery for investigating neural circuit dynamics

Investigation of neural circuit dynamics is crucial for deciphering the functional connections among regions of the brain and understanding the mechanism of brain dysfunction. Despite the advancements of neural circuit models in vitro, technologies for both precisely monitoring and modulating neural activities within three-dimensional (3D) neural circuit models have yet to be developed. Specifically, no existing 3D microelectrode arrays (MEAs) have integrated capabilities to stimulate surrounding neurons and to monitor the temporal evolution of the formation of a neural network in real time. Herein, we present a 3D high-density multifunctional MEA with optical stimulation and drug delivery for investigating neural circuit dynamics within engineered 3D neural tissues. We demonstrate precise measurements of synaptic latencies in 3D neural networks. We expect our 3D multifunctional MEA to open up opportunities for studies of neural circuits through precise, in vitro investigations of neural circuit dynamics with 3D brain models.

Actuation Mechanism of Microvalves: A Review

The microvalve is one of the most important components in microfluidics. With decades of development, the microvalve has been widely used in many industries such as life science, chemical engineering, chip, and so forth. This paper presents a comprehensive review of the progress made over the past years about microvalves based on different actuation mechanisms. According to driving sources, plenty of actuation mechanisms are developed and adopted in microvalves, including electricity, magnetism, gas, material and creature, surface acoustic wave, and so on. Although there are currently a variety of microvalves, problems such as leakage, low precision, poor reliability, high energy consumption, and high cost still exist. Problems deserving to be further addressed are suggested, aimed at materials, fabrication methods, controlling performances, flow characteristics, and applications.

Design and Fabrication of a PDMS-Based Manual Micro-Valve System for Microfluidic Applications

In this study, a manual micro-valve system (dimension: w × h: 1000 × 50 μm with 8-integrated channel valves was designed for controlling up to 8 different flows of agents (including magnetic nanoparticle flow) injected into the mixture zone of the microfluidics. The working parts of the micro-valve and microfluidic channel were fabricated from Poly(dimethyl siloxane) materials. The aperture of each channel valve was manually manipulated by a screw and a support kit (made of Plexiglas® materials). This valve system was connected to a microfluidic device with two important modules: a multi-liquid mixing component and a micro-reactor (~5 μL of volume). The study on controlling liquid flows proved that this valve system was effective for the experiments on the flow mixing and delivering the reactants into the micro-reaction chamber in order. The results are the first step for the fabrication of liquid flow controllers in integrated microfluidic systems towards biological analysis applications.

Real-time pressure monitoring system for microfluidic devices using deformable colloidal crystal membrane

Understanding the hydrodynamic behavior in microfluidic devices is crucial for utilization of these systems in lab-on-a-chip applications. However, hydrodynamic capacitance in microfluidic devices causes a delay and mismatch between the applied pressure and actual pressure in the microchannel. Therefore, real-time monitoring of the site-specific internal pressure in microchannels is important for designing the operating conditions of the system. We introduce a deformable colloidal crystal membrane composed of polystyrene (PS) colloidal crystals and poly(dimethylsiloxane) (PDMS) and its integration into a PDMS microfluidic device. As pressure is applied to the device, the optical intensity reflected back from the colloidal crystal membrane changes due to membrane deformation. The optical signal originating from structural deformation of the colloidal crystal membrane allows the realization of a real-time pressure monitoring system, as no external apparatus or powering devices are required for signal processing. The internal pressure in microfluidic devices was also monitored to investigate the effect of the hydrodynamic capacitance during pressure application.

Surface Modification of Glass/PDMS Microfluidic Valve Assemblies Enhances Valve Electrical Resistance

Microfluidic instrumentation offers unique advantages in biotechnology applications including reduced sample and reagent consumption, rapid mixing and reaction times, and a high degree of process automation. As dimensions decrease, the ratio of surface area to volume within a fluidic architecture increases, which gives rise to some of the unique advantages inherent to microfluidics. Thus, manipulation of surface characteristics presents a promising approach to tailor the performance of microfluidic systems. Microfluidic valves are essential components in a number of small volume applications and for automated microfluidic platforms, but rigorous evaluation of the sealing quality of these valves is often overlooked. In this work, the glass valve seat of hybrid glass/PDMS microfluidic valves was surface modified with hydrophobic silanes, octyldimethylchlorosilane (ODCS) or (tridecafluoro-1,1,2,2-tetrahydrooctyl)dimethylchlorosilane (PFDCS), to investigate the effect of surface energy on electrical resistance of valves. Valves with ODCS- or PFDCS-modified valve seats both exhibited >70-fold increases in electrical resistance (>500 GΩ) when compared to the same valve design with unmodified glass valve seats (7 ± 3 GΩ), indicative of higher sealing capacity. The opening times for valves with ODCS- or PFDCS-modified valve seats was ca. 5× shorter compared to unmodified valve seats, whereas the closing time was up to 8× longer for modified valve seats, although the total closing time was ≤1.5 s, compatible with numerous microfluidic valving applications. Surface modified valve assemblies offered sufficient electrical resistance to isolate sub-pA current signals resulting from electrophysiology measurement of α-hemolysin conductance in a suspended lipid bilayer. This approach is well-suited for the design of novel microfluidic architectures that integrate fluidic manipulations with electrophysiological or electrochemical measurements.

Innovative Hydrophobic Valve Allows Complex Liquid Manipulations in a Self-Powered Channel-Based Microfluidic Device

We present an innovative, simple, and versatile hydrophobic valve enabling all-important complex liquid manipulations on self-powered, channel-based microfluidic devices and as such being extremely valuable for the design of highly demanding point-of-care (POC) platforms. The presented hydrophobic valve is made of filter paper treated with a fluorinated compound (i.e., Aquapel) and shows both superhydrophobic properties (contact angle up to 155°) and high resistance to liquid pressure (up to 9 kPa), while retaining gas permeability and utter fabrication simplicity. Whereas this valve can be integrated in any channel-based system and can be used both as a vent, to delay liquid displacement on chip, or as a barrier, to stop the liquid flow in a certain direction, in this work we demonstrate some of its capacities by combining it with our in house developed self-powered SIMPLE and iSIMPLE platforms. First, we integrated it with the infusion iSIMPLE pump, thus generating completely fail-proof activation regardless of how the operator is actuating the system. Second, we used hydrophobic valves as both barrier and vent in the same microfluidic chip, which allowed the combination of two SIMPLE pumps for splitting one sample in two parallel channels. This attribute is fundamental for achieving multiplex analysis on completely autonomous microfluidic platforms. Finally, we achieved an unprecedented liquid manipulation for a self-powered microfluidic platform, namely, shuttling of liquid, after a single user activation by combining for the first time SIMPLE and iSIMPLE with the developed hydrophobic vent and barrier, all in a single chip. These results convincingly demonstrated that the developed hydrophobic valve combined with SIMPLE/iSIMPLE presents an essential building block for an ideal POC system, which is self-powered, inexpensive, and robust and can perform complex bioassays upon a single user activation.

Tunable Confinement for Bridging Single-Cell Manipulation and Single-Molecule DNA Linearization

DNA linearization by nanoconfinement has offered a new avenue toward large-scale genome mapping. The ability to smoothly interface the widely different length scales from cell manipulation to DNA linearization is critical to the development of single-cell genomic mapping or sequencing technologies. Conventional nanochannel technologies for DNA analysis suffer from complex fabrication procedures, DNA stacking at the nanochannel entrance, and inefficient solution exchange. In this work, a dynamic and tunable confinement strategy is developed to manipulate and linearize genomic-length DNA molecules from a single cell. By leveraging pneumatic microvalve control and elastomeric collapse, an array of nanochannels with confining dimension down to 20 nm and length up to sub-millimeter is created and can be dynamically tuned in size. The curved edges of the microvalve form gradual transitions from microscale to nanoscale confinement, smoothly facilitating DNA entry into the nanochannels. A unified micro/nanofluidic device that integrates single-cell trapping and lysis, DNA extraction, purification, labeling, and linearization is developed based on dynamically controllable nanochannels. Mbp-long DNA molecules are extracted directly from a single cell and in situ linearized in the nanochannels. The device provides a facile and promising platform to achieve the ultimate goal of single-cell, single-genome analysis.

Fabrication of Pneumatic Microvalve for Tall Microchannel Using Inclined Lithography

We used inclined lithography to fabricate a pneumatic microvalve for tall microchannels such as those used to convey large cells. The pneumatic microvalve consists of three layers. The upper layer is the actual liquid microchannel, which has a parallelogram-shaped cross section of width 500 μm, height 100 μm, and an acute angle of 53.6°. The lower layer is a pneumatic microchannel that functions as an actuator, and the middle layer is a thin polydimethylsiloxane membrane between the upper and lower layers. The operation of the pneumatic microchannel actuator causes the thin membrane to bend, resulting in the bending of the liquid microchannel and its closure. It was confirmed that the closure of the liquid microchannel completely stopped the flow of the HeLa cell suspension that was used to demonstrate the operation of the microvalve. The HeLa cells that passed through the microchannel were also observed to retain their proliferation and morphological properties.

Characterizing the Deformation of the Polydimethylsiloxane (PDMS) Membrane for Microfluidic System through Image Processing

Polydimethylsiloxane (PDMS) membranes have been widely used in the microfluidic community to achieve various functions such as control, sensing, filter, etc. In this paper, an experimental process was proposed to directly characterize the deformation of the on-chip PDMS membrane at large deformation based on the image processing method. High precision pressures were applied on the surface of the PDMS membrane with fixed edges and a series deformation of the PDMS membrane were captured by the imaging system. The Chan and Vese (CV) level set method was applied to segment the images of the deformed membrane. The volumes wrapped by the deformed membranes were obtained, and pressure-volumes relationships of the PDMS membranes with different geometry parameters were also calculated. Then the membrane capacitance can be derived by differentiating the curve of pressure-volumes. In addition, the theoretical estimation of the capacitance of the PDMS membrane at large deformation was also obtained through finite element simulation (FEM), which was in good agreement with the experimental results. These results are expected to be significant for designing and on-chip measuring of such PDMS membrane based microfluidic components in our future work.

Leak-tight vertical membrane microvalves

Pneumatic microvalves are fundamental control components in a large range of microfluidic applications. Their key performance parameters are small size, i.e. occupying a minimum of microfluidic real estate, low flow resistance in the open state, and leak-tight closing at limited control pressures. In this work we present the successful design, realization and evaluation of the first leak-tight, vertical membrane, pneumatic microvalves. The realization of the vertical membrane microvalves is enabled by a novel dual-sided molding method for microstructuring monolithic 3D microfluidic networks in PDMS in a single step, eliminating the need for layer-to-layer alignment during bonding. We demonstrate minimum lateral device features down to 20-30 μm in size, and vertical via density of ∼30 000 per cm(2), which provides significant gains in chip real estate compared to previously reported PDMS manufacturing methods. In contrast to horizontal membrane microvalves, there are no manufacturing restrictions on the cross-sectional geometry of the flow channel of the vertical membrane microvalves. This allows tuning the design towards lower closing pressure or lower open state flow resistance compared to those of horizontal membrane microvalves.

Optofluidic Device Based Microflow Cytometers for Particle/Cell Detection: A Review

Optofluidic devices combining micro-optical and microfluidic components bring a host of new advantages to conventional microfluidic devices. Aspects, such as optical beam shaping, can be integrated on-chip and provide high-sensitivity and built-in optical alignment. Optofluidic microflow cytometers have been demonstrated in applications, such as point-of-care diagnostics, cellular immunophenotyping, rare cell analysis, genomics and analytical chemistry. Flow control, light guiding and collecting, data collection and data analysis are the four main techniques attributed to the performance of the optofluidic microflow cytometer. Each of the four areas is discussed in detail to show the basic principles and recent developments. 3D microfabrication techniques are discussed in their use to make these novel microfluidic devices, and the integration of the whole system takes advantage of the miniaturization of each sub-system. The combination of these different techniques is a spur to the development of microflow cytometers, and results show the performance of many types of microflow cytometers developed recently.

Microfluidic cell chips for high-throughput drug screening

The current state of screening methods for drug discovery is still riddled with several inefficiencies. Although some widely used high-throughput screening platforms may enhance the drug screening process, their cost and oversimplification of cell-drug interactions pose a translational difficulty. Microfluidic cell-chips resolve many issues found in conventional HTS technology, providing benefits such as reduced sample quantity and integration of 3D cell culture physically more representative of the physiological/pathological microenvironment. In this review, we introduce the advantages of microfluidic devices in drug screening, and outline the critical factors which influence device design, highlighting recent innovations and advances in the field including a summary of commercialization efforts on microfluidic cell chips. Future perspectives of microfluidic cell devices are also provided based on considerations of present technological limitations and translational barriers.