Sheathless hydrophoretic particle focusing in a microchannel with exponentially increasing obstacle arrays

We present a novel microfluidic device with exponentially increasing obstacle arrays to enable sheathless particle focusing. The anisotropic fluidic resistance of slant obstacles generates transverse flows, along which particles are focused to one sidewall. In the successive channel with exponentially increasing widths, bent obstacles extended from the slant obstacles increase the focusing efficiency of the particles. With the device, we achieved the focusing efficiency of 76%, 94%, and 98% for 6, 10, and 15 microm beads, respectively. The focusing efficiency of the particles can be further improved in the devices with more extension steps. In addition, using the microfluidic devices with the symmetric structure of the slant and bent obstacles, we achieved complete focusing of biological cells to the centerline of a channel within 1.7% coefficient of variation. The results demonstrated the sheathless hydrophoretic focusing of microparticles and cells with the advantages of a sheathless method, passive operation, single channel, and flow rate independence.     

Sheathless focusing of microbeads and blood cells based on hydrophoresis

This paper presents a microfluidic device for sheathless focusing of microbeads and blood cells based on a hydrophoretic platform comprising a V-shaped obstacle array (VOA). The VOA generates lateral pressure gradients that induce helical recirculations. Following the focusing flow particles passing through the VOA are focused in the center of the channel. In the device, the focusing pattern can be modulated by varying the gap height of the VOA. To achieve complete focusing within 4.4% coefficient of variation, the relative size differences between the gap and the particle were 3 and 4 microm for 10 and 15 microm beads, respectively. Red blood cells were used to study the hydrophoretic focusing pattern of biconcave, disk-shaped particles.     

Microfluidic Generation of All‐Aqueous Double and Triple Emulsions

Higher order emulsions are used in a variety of different applications in biomedicine, biological studies, cosmetics, and the food industry. Conventional droplet generation platforms for making higher order emulsions use organic solvents as the continuous phase, which is not biocompatible and as a result, further washing steps are required to remove the toxic continuous phase. Recently, droplet generation based on aqueous two‐phase systems (ATPS) has emerged in the field of droplet microfluidics due to their intrinsic biocompatibility. Here, a platform to generate all‐aqueous double and triple emulsions by introducing pressure‐driven flows inside a microfluidic hybrid device is presented. This system uses a conventional microfluidic flow‐focusing geometry coupled with a coaxial microneedle and a glass capillary embedded in flow‐focusing junctions. The configuration of the hybrid device enables the focusing of two coaxial two‐phase streams, which helps to avoid commonly observed channel‐wetting problems. It is shown that this approach achieves the fabrication of higher‐order emulsions in a poly(dimethylsiloxane)‐based microfluidic device, and controls the structure of the all‐aqueous emulsions. This hybrid microfluidic approach allows for facile higher‐order biocompatible emulsion formation, and it is anticipated that this platform will find utility for generating biocompatible materials for various biotechnological applications.     

High Throughput‐Per‐Footprint Inertial Focusing

Matching the scale of microfluidic flow systems with that of microelectronic chips for realizing monolithically integrated systems still needs to be accomplished. However, this is appealing only if such re‐scaling does not compromise the fluidic throughput. This is related to the fact that the cost of microelectronic circuits primarily depends on the layout footprint, while the performance of many microfluidic systems, like flow cytometers, is measured by the throughput. The simple operation of inertial particle focusing makes it a promising technique for use in such integrated flow cytometer applications, however, microfluidic footprints demonstrated so far preclude monolithic integration. Here, the scaling limits of throughput‐per‐footprint (TPFP) in using inertial focusing are explored by studying the interplay between theory, the effect of channel Reynolds numbers up to 1500 on focusing, the entry length for the laminar flow to develop, and pressure resistance of the microchannels. Inertial particle focusing is demonstrated with a TPFP up to 0.3 L/(min cm²) in high aspect‐ratio rectangular microfluidic channels that are readily fabricated with a post‐CMOS integratable process, suggesting at least a 100‐fold improvement compared to previously demonstrated techniques. Not only can this be an enabling technology for realizing cost‐effective monolithically integrated flow cytometry devices, but the methodology represented here can also open perspectives for miniaturization of many biomedical microfluidic applications requiring monolithic integration with microelectronics without compromising the throughput.     

Two-dimensional oscillatory motion of inertially focused particles in microfluidic flows

Inertially focused particles flowing in microchannels form an evenly spaced streamline on each channel face due to hydrodynamic interaction. Previous studies of this interaction have only reported the oscillatory pairwise dynamics of focused particles, which was limited to the one-dimensional (1D) streamwise direction. Thus, despite its practical and intellectual importance, there remains a lack of comprehensive research on the pairwise oscillation, due to the difficulty of high-resolution observation. Here, I explore the hydrodynamic interaction between inertially focused particles in microfluidic flows to determine the ordering mechanism. Direct numerical simulation (DNS) is applied to a pressure-driven flow of a pair of particles due to the lack of established formulas for the inertial focusing of finite-sized particles; in particular, only DNS allows the author to simulate the microscale flow structures. I describe the unique periodic oscillations of the pairwise particles as they flow downstream. Upon the formation of a train structure in the steady state, the following particle shows periodic oscillations on a two-dimensional (2D) limit cycle around its equilibrium position, whereas the leading particle exhibits 1D oscillation at a specific distance downstream. The 2D oscillatory motion of the following particle is produced by a combination of the lift forces and the disturbance flow induced by the leading particle, coupled with forward/backward transport by the main flow. Thus, the spacing of the particle train is a function of the particle size and flow conditions, leading to even spacing between inertially focused particles. The finding of the asymmetric oscillatory dynamics of the pairwise system provides direct evidence for the self-assembly mechanism of inertially focused particles. I highlight a mostly overlooked aspect of the lift forces: that they stabilize focused streamlines that might otherwise break apart due to finite-particle-induced disturbance flows.     

A Counter Propagating Lens‐Mirror System for Ultrahigh Throughput Single Droplet Detection

Fluorescence‐based detection schemes provide for multiparameter analysis in a broad range of applications in the chemical and biological sciences. Toward the realization of fully portable analysis systems, microfluidic devices integrating diverse functional components have been implemented in a range of out‐of‐lab environments. That said, there still exits an unmet and recognized need for miniaturized, low‐cost, and sensitive optical detection systems, which provide not only for efficient molecular excitation, but also enhanced photon collection capabilities. To this end, an optofluidic platform that is adept at enhancing fluorescence light collection from microfluidic channels is presented. The central component of the detection module is a monolithic parabolic mirror located directly above the microfluidic channel, which acts to enhance the number of emitted photons reflected toward the detector. In addition, two‐photon polymerization is used to print a microscale‐lens below the microfluidic flow channel and directly opposite the mirror, to enhance the delivery of excitation radiation into the channel. Using such an approach, it is demonstrated that fluorescence signals can be enhanced by over two orders of magnitude, with component parallelization enabling the detection of pL‐volume droplets at rates up to 40 000 droplets per second.     

A Hierarchical 3D Nanostructured Microfluidic Device for Sensitive Detection of Pathogenic Bacteria

Efficient capture and rapid detection of pathogenic bacteria from body fluids lead to early diagnostics of bacterial infections and significantly enhance the survival rate. We propose a universal nano/microfluidic device integrated with a 3D nanostructured detection platform for sensitive and quantifiable detection of pathogenic bacteria. Surface characterization of the nanostructured detection platform confirms a uniform distribution of hierarchical 3D nano‐/microisland (NMI) structures with spatial orientation and nanorough protrusions. The hierarchical 3D NMI is the unique characteristic of the integrated device, which enables enhanced capture and quantifiable detection of bacteria via both a probe‐free and immunoaffinity detection method. As a proof of principle, we demonstrate probe‐free capture of pathogenic Escherichia coli (E. coli) and immunocapture of methicillin‐resistant‐Staphylococcus aureus (MRSA). Our device demonstrates a linear range between 50 and 10⁴ CFU mL^?1, with average efficiency of 93% and 85% for probe‐free detection of E. coli and immunoaffinity detection of MRSA, respectively. It is successfully demonstrated that the spatial orientation of 3D NMIs contributes in quantifiable detection of fluorescently labeled bacteria, while the nanorough protrusions contribute in probe‐free capture of bacteria. The ease of fabrication, integration, and implementation can inspire future point‐of‐care devices based on nanomaterial interfaces for sensitive and high‐throughput optical detection.     

Compartmentalized Jet Polymerization as a High‐Resolution Process to Continuously Produce Anisometric Microgel Rods with Adjustable Size and Stiffness

In the past decade, anisometric rod‐shaped microgels have attracted growing interest in the materials‐design and tissue‐engineering communities. Rod‐shaped microgels exhibit outstanding potential as versatile building blocks for 3D hydrogels, where they introduce macroscopic anisometry, porosity, or functionality for structural guidance in biomaterials. Various fabrication methods have been established to produce such shape‐controlled elements. However, continuous high‐throughput production of rod‐shaped microgels with simultaneous control over stiffness, size, and aspect ratio still presents a major challenge. A novel microfluidic setup is presented for the continuous production of rod‐shaped microgels from microfluidic plug flow and jets. This system overcomes the current limitations of established production methods for rod‐shaped microgels. Here, an on‐chip gelation setup enables fabrication of soft microgel rods with high aspect ratios, tunable stiffness, and diameters significantly smaller than the channel diameter. This is realized by exposing jets of a microgel precursor to a high intensity light source, operated at specific pulse sequences and frequencies to induce ultra‐fast photopolymerization, while a change in flow rates or pulse duration enables variation of the aspect ratio. The microgels can assemble into 3D structures and function as support for cell culture and tissue engineering.     

High‐Density Microfluidic Particle‐Cluster‐Array Device for Parallel and Dynamic Study of Interaction between Engineered Particles

A high‐density and high‐performance microfluidic particle‐cluster‐array device utilizing a novel hydrodynamically tunable pneumatic valve (HTPV) is reported for parallel and dynamic monitoring of the interactions taking place in particle clusters. The key concept involves passive operation of the HTPV through elastic deformation of a thin membrane using only the hydrodynamic force inherent in microchannel flows. This unique feature allows the discrete and high‐density (≈30 HTPVs mm^?2) arrangement of numerous HTPVs in a microfluidic channel without any pneumatic connection. In addition, the HTPV achieves high‐performance clustering (≈92%) of three different particles in an array format through the optimization of key design and operating parameters. Finally, a contamination‐free, parallel, and dynamic biochemical analysis strategy is proposed, which employs a simple one‐inlet–one‐outlet device operated by the effective combination of several techniques, including particle clustering, the interactions between engineered particles, two‐phase partitioning and dehydration control of aqueous plugs, and shape/color‐based particle identification.     

Microfluidic scaffolds for tissue engineering

Most methods to culture cells in three dimensions depend on a cell-seedable biomaterial to define the global structure of the culture and the microenvironment of the cells. Efforts to tailor these scaffolds have focused on the chemical and mechanical properties of the biomaterial itself. Here, we present a strategy to control the distributions of soluble chemicals within the scaffold with convective mass transfer via microfluidic networks embedded directly within the cell-seeded biomaterial. Our presentation of this strategy includes: a lithographic technique to build functional microfluidic structures within a calcium alginate hydrogel seeded with cells; characterization of this process with respect to microstructural fidelity and cell viability; characterization of convective and diffusive mass transfer of small and large solutes within this microfluidic scaffold; and demonstration of temporal and spatial control of the distribution of non-reactive solutes and reactive solutes (that is, metabolites) within the bulk of the scaffold. This approach to control the chemical environment on a micrometre scale within a macroscopic scaffold could aid in engineering complex tissues.     

Three-dimensional fluidic self-assembly by axis translation of two-dimensionally fabricated microcomponents in railed microfluidics

A method for high-throughput 3D self-assembly of 2D photopatterned microstructures using railed microfluidics is presented. Vertical device patterning of heterogeneous materials requires high-level integration using conventional microelectromechanical system (MEMS) technology; however, 3D railed assembly enables easy and fast self-assembly via a fluidic axis-translation process and simple material exchange in microfluidic channels. Individually photopatterned 2D microstructures are axis-translated from in-plane to out-of-plane and fluidically self-assembled, guided by side-rails in microfluidic channels to form a 3D morphology. Since the structures are fabricated in fluidic environments, there are no fixed initial points on the channel substrate allowing fluidic horizontal stacking of erected 2D structures. The guiding mechanism of railed microfluidics enables efficient fluidic handling and deterministic 3D self-assembly of heterogeneous components such as electronic components or polymeric microstructures using only fluidic force.     

Microfluidic Molding of Photonic Microparticles with Engraved Elastomeric Membranes

A microfluidic approach to prepare photonic microparticles by repeated molding of photocurable colloidal suspension is reported. An elastomeric membrane with negative relieves which vertically separates two microfluidic channels is integrated; bottom channel is used for suspension flow, whereas water‐filled top channel is used for pneumatic actuation of the membrane. Upon pressurization of the top channel, membrane is deformed to confine the suspension into its negative relieves, which is then polymerized by UV irradiation, making microparticles with mold shape. The microparticles are released from the mold by relieving the pneumatic pressure and flows through the bottom channel. This one cycle of molding, polymerization, and release can be repeatedly performed in microfluidic device of which pneumatic valves are actuated in a programmed manner. The microparticles exhibit structural colors when the suspension contains high concentration of silica nanoparticles; the nanoparticles form regular arrays and the microparticles reflect specific wavelength of light as a photonic crystals. The silica nanoparticles can be selectively removed to make pronounced structural colors. In addition, the microparticles can be further functionalized by embedding magnetic particles in the matrix of the microparticles, enabling the remote control of rotational motion of microparticles.     

Deconstructed Microfluidic Bone Marrow On‐A‐Chip to Study Normal and Malignant Hemopoietic Cell–Niche Interactions

Human hematopoietic niches are complex specialized microenvironments that maintain and regulate hematopoietic stem and progenitor cells (HSPC). Thus far, most of the studies performed investigating alterations of HSPC‐niche dynamic interactions are conducted in animal models. Herein, organ microengineering with microfluidics is combined to develop a human bone marrow (BM)‐on‐a‐chip with an integrated recirculating perfusion system that consolidates a variety of important parameters such as 3D architecture, cell–cell/cell–matrix interactions, and circulation, allowing a better mimicry of in vivo conditions. The complex BM environment is deconvoluted to 4 major distinct, but integrated, tissue‐engineered 3D niche constructs housed within a single, closed, recirculating microfluidic device system, and equipped with cell tracking technology. It is shown that this technology successfully enables the identification and quantification of preferential interactions—homing and retention—of circulating normal and malignant HSPC with distinct niches.     

Three-dimensional magnetic focusing of superparamagnetic beads for on-chip agglutination assays

We present a new method for three-dimensional (3D) magneticfocusing of magnetic microparticles in a microfluidic system. On-chip magnetic particle manipulation in the microchannel is achieved by a high-gradient magnetic field generated by means of a micromachined field concentrator. The system allows retention of functionalized beads in a dense plug while flowing through buffer or analyte. Slowly reducing the magnetic retention force in the presence of a flow results in controlled release of the particles into a fine streamline with regular longitudinal interparticle spacing. Alignment at half-height of the channel is readily obtained through the symmetry of the magnetic field. A single lateral sheath flow is required to provide full 3D focusing of the microparticles in the middle of the microchannel with a maximum deviation of ±5 μm from the center position. With the use of this system, a new approach for performing an immunoagglutination assay on-chip has been implemented. Three-dimensional focusing allowed reliable counting of singlets and agglutinated doublets. We demonstrate the potential of the agglutination assay in a microfluidic format using a streptavidin/biotinylated bovine serum albumin (bBSA) model system. A detection limit of about 400 pg/mL (6 pM) is achieved.     

Advection Flows‐Enhanced Magnetic Separation for High‐Throughput Bacteria Separation from Undiluted Whole Blood

A major challenge to scale up a microfluidic magnetic separator for extracorporeal blood cleansing applications is to overcome low magnetic drag velocity caused by viscous blood components interfering with magnetophoresis. Therefore, there is an unmet need to develop an effective method to position magnetic particles to the area of augmented magnetic flux density gradients while retaining clinically applicable throughput. Here, a magnetophoretic cell separation device, integrated with slanted ridge‐arrays in a microfluidic channel, is reported. The slanted ridges patterned in the microfluidic channels generate spiral flows along the microfluidic channel. The cells bound with magnetic particles follow trajectories of the spiral streamlines and are repeatedly transferred in a transverse direction toward the area adjacent to a ferromagnetic nickel structure, where they are exposed to a highly augmented magnetic force of 7.68 µN that is much greater than the force (0.35 pN) at the side of the channel furthest from the nickel structure. With this approach, 91.68% ± 2.18% of Escherichia coli (E. coli) bound with magnetic nanoparticles are successfully separated from undiluted whole blood at a flow rate of 0.6 mL h^?1 in a single microfluidic channel, whereas only 23.98% ± 6.59% of E. coli are depleted in the conventional microfluidic device.     

On-chip coupling of isoelectric focusing and free solution electrophoresis for multidimensional separations

We have developed an acrylic microfluidic device that sequentially couples liquid-phase isoelectric focusing (IEF) and free solution capillary electrophoresis (CE). Rapid separation (<1 min) and preconcentration (73x) of species were achieved in the initial IEF dimension. Using full-field fluorescence imaging, we observed nondispersive mobilization velocities on the order of 20 microm/s during characterization of the IEF step. This transport behavior allowed controlled electrokinetic mobilization of focused sample bands to a channel junction, where voltage switching was used to repeatedly inject effluent from the IEF dimension into an ampholyte-based CE separation. This second dimension was capable of analyzing all fluid volumes of interest from the IEF dimension, as IEF was 'parked' during each CE analysis and refocused prior to additional CE analyses. Investigation of each dimension of the integrated system showed time-dependent species displacement and band-broadening behavior consistent with IEF and CE, respectively. The peak capacity of the 2D system was approximately 1300. A comprehensive 2D analysis of a fluid volume spanning 15% of the total IEF channel length was completed in less than 5 min.     

Gravity-driven microfluidic particle sorting device with hydrodynamic separation amplification

This paper describes a simple microfluidic sorting system that can perform size profiling and continuous mass-dependent separation of particles through combined use of gravity (1 g) and hydrodynamic flows capable of rapidly amplifying sedimentation-based separation between particles. Operation of the device relies on two microfluidic transport processes: (i) initial hydrodynamic focusing of particles in a microchannel oriented parallel to gravity and (ii) subsequent sample separation where positional difference between particles with different mass generated by sedimentation is further amplified by hydrodynamic flows whose streamlines gradually widen out due to the geometry of a widening microchannel oriented perpendicular to gravity. The microfluidic sorting device was fabricated in poly(dimethylsiloxane), and hydrodynamic flows in microchannels were driven by gravity without using external pumps. We conducted theoretical and experimental studies on fluid dynamic characteristics of laminar flows in widening microchannels and hydrodynamic amplification of particle separation. Direct trajectory monitoring, collection, and post-analysis of separated particles were performed using polystyrene microbeads with different sizes to demonstrate rapid (<1 min) and high-purity (>99.9%) separation. Finally, we demonstrated biomedical applications of our system by isolating small-sized (diameter <6 microm) perfluorocarbon liquid droplets from polydisperse droplet emulsions, which is crucial in preparing contrast agents for safe, reliable ultrasound medical imaging, tracers for magnetic resonance imaging, or transpulmonary droplets used in ultrasound-based occlusion therapy for cancer treatment. Our method enables straightforward, rapid, real-time size monitoring and continuous separation of particles in simple stand-alone microfabricated devices without the need for bulky and complex external power sources. We believe that this system will provide a useful tool to separate colloids and particles for various analytical and preparative applications and may hold potential for separation of cells or development of diagnostic tools requiring point-of-care sample preparation or testing.     

Desktop‐Stereolithography 3D‐Printing of a Poly(dimethylsiloxane)‐Based Material with Sylgard‐184 Properties

The advantageous physiochemical properties of poly(dimethylsiloxane) (PDMS) have made it an extremely useful material for prototyping in various technological, scientific, and clinical areas. However, PDMS molding is a manual procedure and requires tedious assembly steps, especially for 3D designs, thereby limiting its access and usability. On the other hand, automated digital manufacturing processes such as stereolithography (SL) enable true 3D design and fabrication. Here the formulation, characterization, and SL application of a 3D‐printable PDMS resin (3DP‐PDMS) based on commercially available PDMS‐methacrylate macromers, a high‐efficiency photoinitiator and a high‐absorbance photosensitizer, is reported. Using a desktop SL‐printer, optically transparent submillimeter structures and microfluidic channels are demonstrated. An optimized blend of PDMS‐methacrylate macromers is also used to SL‐print structures with mechanical properties similar to conventional thermally cured PDMS (Sylgard‐184). Furthermore, it is shown that SL‐printed 3DP‐PDMS substrates can be rendered suitable for mammalian cell culture. The 3DP‐PDMS resin enables assembly‐free, automated, digital manufacturing of PDMS, which should facilitate the prototyping of devices for microfluidics, organ‐on‐chip platforms, soft robotics, flexible electronics, and sensors, among others.     

Numerical investigations of strong hydrodynamic interaction between neighboring particles inertially driven in microfluidic flows

Dispersed particles traveling at a high throughput in microchannels laterally migrate and focus into a streamline at each channel face. The focusing attractors within the cross-section are determined by the balance between the lift forces. However, particles in close proximity (e.g. due to high concentration and abrupt particle contact) suffer a breakdown of distinct focusing due to excessive hydrodynamic interaction. Here, I present numerical investigations into the effects of the strong hydrodynamic interaction on the inertial focusing. The direct numerical simulation is used to calculate the focusing/defocusing of particles, specifically since the particle-induced disturbance flows vary at the particle scale and hence affect the individual particle motion. The simulated defocusing of many-body systems prefer finite inter-particle separation, in contrast with sedimentation of two mobile particles, whereby the trailing particle catches up with the leading particle due to reduced drag in its wake. I numerically demonstrate that the finite separation between nearest neighbors is a consequence of hydrodynamic repulsive motion unique to wall-bound shear flows. The author further presents direct demonstrations of the effects of the strong hydrodynamic interaction on the inertial focusing in an experimentally unachievable manner. The excessive hydrodynamic interaction drastically dissipates the near-wall focusing attractors and thus causes irreversible defocusing by breaking the balance between the lift forces. Unexpectedly, I also find that moderate hydrodynamic interaction can alter focusing speed on specific conditions, suggesting that an optimum concentration may significantly boost the inertial focusing in microfluidic-based applications.     

Design of an optimized ECCA microchannel for particle manipulation utilizing dean flow coupled elasto-inertial method

In this paper, an Eulerian-Lagrangian simulation was conducted to achieve optimal Expanded-Contracted Cavity Arrays microchannel. First, a new code was developed to solve the viscoelastic flow field, and then the particles were solved by adding appropriate forces to the OpenFOAM Lagrangian solver. This code was then validated for both Eulerian and Lagrangian models. Subsequently, the effect of different parameters such as flow rate, distance from the inlet, cavity depth and distance, and particle size were also studied to obtain the proper geometry for particle focusing. Finally, the selected channel was integrated with a straight channel to separate 4.8 and 13 μm particles. The results of current research can be used to find a proper design of an Expanded-Contracted Cavity Arrays channel to achieve precise focusing and efficient, continuous, and sheathless particle/cell separation, which is much worthy for applications such as high-speed cytometry, cell counting, sorting, and many biological applications.