Pulsatile Flow in Microfluidic Systems

Advances in Microfluidic Systems and Numerical Modeling in Biomedical Applications: A Review

The evolution in the biomedical engineering field boosts innovative technologies, with microfluidic systems standing out as transformative tools in disease diagnosis, treatment, and monitoring. Numerical simulation has emerged as a tool of increasing importance for better understanding and predicting fluid-flow behavior in microscale devices. This review explores fabrication techniques and common materials of microfluidic devices, focusing on soft lithography and additive manufacturing. Microfluidic systems applications, including nucleic acid amplification and protein synthesis, as well as point-of-care diagnostics, DNA analysis, cell cultures, and organ-on-a-chip models (e.g., lung-, brain-, liver-, and tumor-on-a-chip), are discussed. Recent studies have applied computational tools such as ANSYS Fluent 2024 software to numerically simulate the flow behavior. Outside of the study cases, this work reports fundamental aspects of microfluidic simulations, including fluid flow, mass transport, mixing, and diffusion, and highlights the emergent field of organ-on-a-chip simulations. Additionally, it takes into account the application of geometries to improve the mixing of samples, as well as surface wettability modification. In conclusion, the present review summarizes the most relevant contributions of microfluidic systems and their numerical modeling to biomedical engineering.

A Lego-Like Reconfigurable Microfluidic Stabilizer System with Tunable Fluidic RC Constants and Stabilization Ratios

In microfluidic systems, it is important to maintain flow stability to execute various functions, such as chemical reactions, cell transportation, and liquid injection. However, traditional flow sources, often bulky and prone to unpredictable fluctuations, limit the portability and broader application of these systems. Existing fluidic stabilizers, typically designed for specific flow sources, lack reconfigurability and adaptability in terms of the stabilization ratios. To address these limitations, a modular and standardized stabilizer system with tunable stabilization ratios is required. In this work, we present a Lego-like modular microfluidic stabilizer system, which is fabricated using 3D printing and offers multi-level stabilization combinations and customizable stabilization ratios through the control of fluidic RC constants, making it adaptable to various microfluidic systems. A simplified three-element circuit model is used to characterize the system by straightforwardly extracting the RC constant without intricate calculations of the fluidic resistance and capacitance. By utilizing a simplified three-element model, the stabilizer yields two well-fitted operational curves, demonstrating an R-square of 0.95, and provides an optimal stabilization ratio below 1%. To evaluate the system's effectiveness, unstable input flow at different working frequencies is stabilized, and droplet generation experiments are conducted and discussed. The results show that the microfluidic stabilizer system significantly reduces flow fluctuations and enhances droplet uniformity. This system provides a new avenue for microfluidic stabilization with a tunable stabilization ratio, and its plug-and-play design can be effectively applied across diverse applications to finely tune fluid flow behaviors in microfluidic devices.

Accelerated corrosion of low carbon steel by oscillatory acidic streams generated with a bio-inspired claw device

A mechanical device inspired by the pistol shrimp snapper claw was developed. This technology features a claw characterized by a periodic opening/closing motion, at a controlled frequency, capable of producing oscillating flows at transitional Reynolds numbers. An innovative method was also proposed for determining the corrosion rate of carbon steel samples under oscillating acidic streams (aqueous solution of HCl). By employing very-thin carbon steel specimens (25 μm thickness), with one side coated with Zn and not exposed to the stream, it became possible to electrochemically sense the Zn surface once the steel sample was perforated, thus providing the average dissolution rate into the most relevant pit on the steel surface. Furthermore, a laser light positioned beneath the metallic sample, along with a camera programmed to periodically capture images of the steel surface, facilitated the accurate counting of the number of newly formed pits. The system consisting of the thin steel sample and the Zn coating can be seen as a type of corrosion sensor. Furthermore, the proposed laser illumination method allows corroborating the electrochemical detection of pits and also establishing their location. The techniques crafted in this study pave the way for developing alternative corrosion sensors that boast appealing attributes: affordability, compactness, and acceptable accuracy to detect in time and space localized damage.

Flowing Liquid-Based Triboelectric Nanogenerator Performance Enhancement with Functionalized Polyvinylidene Fluoride Membrane for Self-Powered Pulsating Flow Sensing Application

Pulsating flow, a common term in industrial and medical contexts, necessitates precise water flow measurement for evaluating hydrodynamic system performance. Addressing challenges in measurement technologies, particularly for pulsating flow, we propose a flowing liquid-based triboelectric nanogenerator (FL-TENG). To generate sufficient energy for a self-powered device, we employed a fluorinated functionalized technique on a polyvinylidene fluoride (PVDF) membrane to enhance the performance of FL-TENG. The results attained a maximum instantaneous power density of 50.6 µW/cm2, and the energy output proved adequate to illuminate 10 white LEDs. Regression analysis depicting the dependence of the output electrical signals on water flow revealed a strong linear relationship between the voltage and flow rate with high sensitivity. A high correlation coefficient R2 within the range from 0.951 to 0.998 indicates precise measurement accuracy for the proposed FL-TENG. Furthermore, the measured time interval between two voltage peaks precisely corresponds to the period of pulsating flow, demonstrating that the output voltage can effectively sense pulsating flow based on voltage and the time interval between two voltage peaks. This work highlights the utility of FL-TENG as a self-powered pulsating flow rate sensor.

Microfluidic investigation of the impacts of flow fluctuations on the development of Pseudomonas putida biofilms

Biofilms play critical roles in wastewater treatment, bioremediation, and medical-device-related infections. Understanding the dynamics of biofilm formation and growth is essential for controlling and exploiting their properties. However, the majority of current studies have focused on the impact of steady flows on biofilm growth, while flow fluctuations are common in natural and engineered systems such as water pipes and blood vessels. Here, we reveal the effects of flow fluctuations on the development of Pseudomonas putida biofilms through systematic microfluidic experiments and the development of a theoretical model. Our experimental results showed that biofilm growth under fluctuating flow conditions followed three phases: lag, exponential, and fluctuation phases. In contrast, biofilm growth under steady-flow conditions followed four phases: lag, exponential, stationary, and decline phases. Furthermore, we demonstrated that low-frequency flow fluctuations promoted biofilm growth, while high-frequency fluctuations inhibited its development. We attributed the contradictory impacts of flow fluctuations on biofilm growth to the adjustment time (T0) needed for biofilm to grow after the shear stress changed from high to low. Furthermore, we developed a theoretical model that explains the observed biofilm growth under fluctuating flow conditions. Our insights into the mechanisms underlying biofilm development under fluctuating flows can inform the design of strategies to control biofilm formation in diverse natural and engineered systems.

Integrating spheroid-on-a-chip with tubeless rocker platform: A high-throughput biological screening platform

Spheroid-on-a-chip platforms are emerging as promising in vitro models that enable screening of the efficacy of biologically active ingredients. Generally, the supply of liquids to spheroids occurs in the steady flow mode with the use of syringe pumps; however, the utilization of tubing and connections, especially for multiplexing and high-throughput screening applications, makes spheroid-on-a-chip platforms labor- and cost-intensive. Gravity-induced flow using rocker platforms overcomes these challenges. Here, a robust gravity-driven technique was developed to culture arrays of cancer cell spheroids and dermal fibroblast spheroids in a high-throughput manner using a rocker platform. The efficiency of the developed rocker-based platform was benchmarked to syringe pumps for generating multicellular spheroids and their use for screening biologically active ingredients. Cell viability, internal spheroid structure as well as the effect of vitamin C on spheroids' protein synthesis was studied. The rocker-based platform not only offers comparable or enhanced performance in terms of cell viability, spheroids formation, and protein production by dermal fibroblast spheroids but also, from a practical perspective, offers a smaller footprint, requires a lower cost, and offers an easier method for handling. These results support the application of rocker-based microfluidic spheroid-on-a-chip platforms for in vitro screening in a high-throughput manner with industrial scaling-up opportunities.

Tunable resins with PDMS-like elastic modulus for stereolithographic 3D-printing of multimaterial microfluidic actuators

Stereolithographic 3D-printing (SLA) permits facile fabrication of high-precision microfluidic and lab-on-a-chip devices. SLA photopolymers often yield parts with low mechanical compliancy in sharp contrast to elastomers such as poly(dimethyl siloxane) (PDMS). On the other hand, SLA-printable elastomers with soft mechanical properties do not fulfill the distinct requirements for a highly manufacturable resin in microfluidics (e.g., high-resolution printability, transparency, low-viscosity). These limitations restrict our ability to print microfluidic actuators containing dynamic, movable elements. Here we introduce low-viscous photopolymers based on a tunable blend of the monomers poly(ethylene glycol) diacrylate (PEGDA, Mw ∼ 258) and the monoacrylate poly(ethylene glycol methyl ether) methacrylate (PEGMEMA, Mw ∼ 300). In these blends, which we term PEGDA-co-PEGMEMA, tuning the PEGMEMA content from 0% to 40% (v/v) alters the elastic modulus of the printed plastics by ∼400-fold, reaching that of PDMS. Through the addition of PEGMEMA, moreover, PEGDA-co-PEGMEMA retains desirable properties of highly manufacturable PEGDA such as low viscosity, solvent compatibility, cytocompatibility and low drug absorptivity. With PEGDA-co-PEGMEMA, we SLA-printed drastically enhanced fluidic actuators including microvalves, micropumps, and microregulators with a hybrid structure containing a flexible PEGDA-co-PEGMEMA membrane within a rigid PEGDA housing. These components were built using a custom "Print-Pause-Print" protocol, referred to as "3P-printing", that allows for fabricating high-resolution multimaterial parts with a desktop SLA printer without the need for post-assembly. SLA-printing of multimaterial microfluidic actuators addresses the unmet need of high-performance on-chip controls in 3D-printed microfluidic and lab-on-a-chip devices.

Fluid flow to mimic organ function in 3D in vitro models

Many different strategies can be found in the literature to model organ physiology, tissue functionality, and disease in vitro; however, most of these models lack the physiological fluid dynamics present in vivo. Here, we highlight the importance of fluid flow for tissue homeostasis, specifically in vessels, other lumen structures, and interstitium, to point out the need of perfusion in current 3D in vitro models. Importantly, the advantages and limitations of the different current experimental fluid-flow setups are discussed. Finally, we shed light on current challenges and future focus of fluid flow models applied to the newest bioengineering state-of-the-art platforms, such as organoids and organ-on-a-chip, as the most sophisticated and physiological preclinical platforms.

Jacketed elastomeric tubes for passive self-regulation of pulsatile flow

Regulating pulsatile flow is important to achieve optimal separation and mixing and enhanced heat transfer in microfluidic devices, as well as maintaining homeostasis in biological systems. The human aorta, a composite and layered tube made (among others) of elastin and collagen, is an inspiration for researchers who seek an engineering solution for a self-regulation of pulsatile flow. Here, we present a bio-inspired approach showing that fabric-jacketed elastomeric tubes, manufactured using commercially available silicone rubber and knitted textiles, can be used to regulate pulsatile flow. Our tubes are evaluated via incorporation into a mock-circulatory 'flow loop' that replicates the pulsatile fluid flow conditions of an ex-vivo heart perfusion (EVHP) device, a machine used in heart transplants. Pressure waveforms measured near the elastomeric tubing clearly indicated an effective flow regulation. The 'dynamic stiffening' behavior of the tubes during deformation is analyzed quantitatively. Broadly, the fabric jackets allow for the tubes to experience greater magnitudes of pressure and distension without risk of asymmetric aneurysm within the expected operating time of an EVHP. Owing to its highly tunable nature, our design may serve as a basis for tubing systems that require passive self-regulation of pulsatile flow.

Influence of Non-Newtonian Viscosity on Flow Structures and Wall Deformation in Compliant Serpentine Microchannels: A Numerical Study

The viscosity of fluid plays a major role in the flow dynamics of microchannels. Viscous drag and shear forces are the primary tractions for microfluidic fluid flow. Capillary blood vessels with a few microns diameter are impacted by the rheology of blood flowing through their conduits. Hence, regenerated capillaries should be able to withstand such impacts. Consequently, there is a need to understand the flow physics of culture media through the lumen of the substrate as it is one of the vital promoting factors for vasculogenesis under optimal shear conditions at the endothelial lining of the regenerated vessel. Simultaneously, considering the diffusive role of capillaries for ion exchange with the surrounding tissue, capillaries have been found to reorient themselves in serpentine form for modulating the flow conditions while developing sustainable shear stress. In the current study, S-shaped (S1) and delta-shaped (S2) serpentine models of capillaries were considered to evaluate the shear stress distribution and the oscillatory shear index (OSI) and relative residual time (RRT) of the derivatives throughout the channel (due to the phenomena of near-wall stress fluctuation), along with the influence of culture media rheology on wall stress parameters. The non-Newtonian power-law formulation was implemented for defining rheological viscosity of the culture media. The flow actuation of the media was considered to be sinusoidal and physiological, realizing the pulsatile blood flow behavior in the circulatory network. A distinct difference in shear stress distributions was observed in both the serpentine models. The S1 model showed higher change in shear stress in comparison to the S2 model. Furthermore, the non-Newtonian viscosity formulation was found to produce more sustainable shear stress near the serpentine walls compared to the Newtonian formulation fluid, emphasizing the influence of rheology on stress generation. Further, cell viability improved in the bending regions of serpentine channels compared to the long run section of the same channel.

Peptide-Functionalized Electrospun Meshes for the Physiological Cultivation of Pulmonary Alveolar Capillary Barrier Models in a 3D-Printed Micro-Bioreactor

In vitro environments that realize biomimetic scaffolds, cellular composition, physiological shear, and strain are integral to developing tissue models of organ-specific functions. In this study, an in vitro pulmonary alveolar capillary barrier model is developed that closely mimics physiological functions by combining a synthetic biofunctionalized nanofibrous membrane system with a novel three-dimensional (3D)-printed bioreactor. The fiber meshes are fabricated from a mixture of polycaprolactone (PCL), 6-armed star-shaped isocyanate-terminated poly(ethylene glycol) (sPEG-NCO), and Arg-Gly-Asp (RGD) peptides by a one-step electrospinning process that offers full control over the fiber surface chemistry. The tunable meshes are mounted within the bioreactor where they support the co-cultivation of pulmonary epithelial (NCI-H441) and endothelial (HPMEC) cell monolayers at air-liquid interface under controlled stimulation by fluid shear stress and cyclic distention. This stimulation, which closely mimics blood circulation and breathing motion, is observed to impact alveolar endothelial cytoskeleton arrangement and improve epithelial tight junction formation as well as surfactant protein B production compared to static models. The results highlight the potential of PCL-sPEG-NCO:RGD nanofibrous scaffolds in combination with a 3D-printed bioreactor system as a platform to reconstruct and enhance in vitro models to bear a close resemblance to in vivo tissues.

Microfluidic systems for particle capture and release: A review

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

A User-Centric 3D-Printed Modular Peristaltic Pump for Microfluidic Perfusion Applications

Microfluidic organ-on-a-chip (OoC) technology has enabled studies on dynamic physiological conditions as well as being deployed in drug testing applications. A microfluidic pump is an essential component to perform perfusion cell culture in OoC devices. However, it is challenging to have a single pump that can fulfil both the customization function needed to mimic a myriad of physiological flow rates and profiles found in vivo and multiplexing requirements (i.e., low cost, small footprint) for drug testing operations. The advent of 3D printing technology and open-source programmable electronic controllers presents an opportunity to democratize the fabrication of mini-peristaltic pumps suitable for microfluidic applications at a fraction of the cost of commercial microfluidic pumps. However, existing 3D-printed peristaltic pumps have mainly focused on demonstrating the feasibility of using 3D printing to fabricate the structural components of the pump and neglected user experience and customization capability. Here, we present a user-centric programmable 3D-printed mini-peristaltic pump with a compact design and low manufacturing cost (~USD 175) suitable for perfusion OoC culture applications. The pump consists of a user-friendly, wired electronic module that controls the operation of a peristaltic pump module. The peristaltic pump module comprises an air-sealed stepper motor connected to a 3D-printed peristaltic assembly, which can withstand the high-humidity environment of a cell culture incubator. We demonstrated that this pump allows users to either program the electronic module or use different-sized tubing to deliver a wide range of flow rates and flow profiles. The pump also has multiplexing capability as it can accommodate multiple tubing. The performance and user-friendliness of this low-cost, compact pump can be easily deployed for various OoC applications.

Dynamics of flexible filaments in oscillatory shear flows

The fluid-structure interactions between flexible fibers and viscous flows play an essential role in various biological phenomena, medical problems, and industrial processes. Of particular interest is the case of particles freely transported in time-dependent flows. This work elucidates the dynamics and morphologies of actin filaments under oscillatory shear flows by combining microfluidic experiments, numerical simulations, and theoretical modeling. Our work reveals that, in contrast to steady shear flows, in which small orientational fluctuations from a flow-aligned state initiate tumbling and deformations, the periodic flow reversal allows the filament to explore many different configurations at the beginning of each cycle. Investigation of filament motion during half time periods of oscillation highlights the critical role of the initial filament orientation on the emergent dynamics. This strong coupling between orientation and deformation results in new deformation regimes and novel higher-order buckling modes absent in steady shear flows. The primary outcome of our analysis is the possibility of suppression of buckling instabilities for certain combinations of the oscillation frequency and initial filament orientation, even in very strong flows. We explain this unusual behavior through a weakly nonlinear Landau theory of buckling, in which we treat the filaments as inextensible Brownian Euler-Bernoulli rods whose hydrodynamics are described by local slender-body theory.

In silico analysis of a hierarchical microfluidic vascular network: Detecting the location of angiogenic sprouting

Lack of oxygen is one of the leading causes of failure in engineered tissue. Therefore, angiogenesis will be necessary for the survival of larger tissues in vivo. In addition, a proper lymphatic system that plays an essential role in relieving inflammation and maintaining tissue homeostasis is of great importance for tissue regeneration and repair. Many biomechanical parameters are involved in controlling angiogenesis and capillary network generation, which are challenging to study and control in experimental studies or in vitro. In the present study, using numerical modeling, the effect of various geometric and biomechanical parameters in creating suitable conditions for angiogenesis was investigated. Furthermore, sprouting points were predicted using flow dynamics. For this purpose, a porous scaffold, flow channels with parametric geometry that followed Murray's law, and a drainage channel were considered. Results suggested that the geometry of the microfluidic channels and the characteristics of the vessel wall and scaffold plays a complementary role in determining the transmural pressure. It was found that a twofold increase in the vascular hydraulic conductivity can reduce the minimum transmural pressure by up to 28% and increase the drainage flow rate by 44%. In addition, the minimum magnitude of transmural pressure tends to zero for scaffold's hydraulic conductivity values smaller than 10^-11  m³ s kg^-1 . The results of this study can be used in optimizing the design of the relevant microfluidic systems to induce angiogenesis and avoid leakage in the constructed implantable tissue.

Critical design parameters to develop biomimetic organ-on-a-chip models for the evaluation of the safety and efficacy of nanoparticles

INTRODUCTION

Organ-on-a-chip (OOC) models are based on microfluidics and can recapitulate the healthy and diseased microstructure of organs¹ and tissues and the dynamic microenvironment inside the human body. However, the use of OOC models to evaluate the safety and efficacy of nanoparticles (NPs) is still in the early stages.

AREAS COVERED

The different design parameters of the microfluidic chip and the mechanical forces generated by fluid flow play a pivotal role in simulating the human environment. This review discusses the role of different key parameters on the performance of OOC models. These include the flow pattern, flow rate, shear stress (magnitude, rate, and distribution), viscosity of the media, and the microchannel dimensions and shape. We also discuss how the shear stress and other mechanical forces affect the transport of NPs across biological barriers, cell uptake, and their biocompatibility.

EXPERT OPINION

We describe several good practices and design parameters to consider for future OOC research. We submit that following these recommendations will help realize the full potential of the OOC models in the preclinical evaluation of novel therapies, including NPs.

Microfluidic Systems for Blood and Blood Cell Characterization

A laboratory blood test is vital for assessing a patient's health and disease status. Advances in microfluidic technology have opened the door for on-chip blood analysis. Currently, microfluidic devices can reproduce myriad routine laboratory blood tests. Considerable progress has been made in microfluidic cytometry, blood cell separation, and characterization. Along with the usual clinical parameters, microfluidics makes it possible to determine the physical properties of blood and blood cells. We review recent advances in microfluidic systems for measuring the physical properties and biophysical characteristics of blood and blood cells. Added emphasis is placed on multifunctional platforms that combine several microfluidic technologies for effective cell characterization. The combination of hydrodynamic, optical, electromagnetic, and/or acoustic methods in a microfluidic device facilitates the precise determination of various physical properties of blood and blood cells. We analyzed the physical quantities that are measured by microfluidic devices and the parameters that are determined through these measurements. We discuss unexplored problems and present our perspectives on the long-term challenges and trends associated with the application of microfluidics in clinical laboratories. We expect the characterization of the physical properties of blood and blood cells in a microfluidic environment to be considered a standard blood test in the future.

Recent progress in the synthesis of all-aqueous two-phase droplets using microfluidic approaches

An aqueous two-phase system (ATPS) is a system with liquid-liquid phase separation and shows great potential for the extraction, separation, purification, and enrichment of proteins, membranes, viruses, enzymes, nucleic acids, and other biomolecules because of its simplicity, biocompatibility, and wide applicability [1-4]. The clear aqueous-aqueous interface of ATPSs is highly advantageous for their implementation, therefore making ATPSs a green alternative approach to replace conventional emulsion systems, such as water-in-oil droplets. All aqueous emulsions (water-in-water, w-in-w) hold great promise in the biomedical field as glucose sensors [5] and promising carriers for the encapsulation and release of various biomolecules and nonbiomolecules [6-10]. However, the ultralow interfacial tension between the two phases is a hurdle in generating w-in-w emulsion droplets. In the past, bulk emulsification and electrospray techniques were employed for the generation of w-in-w emulsion droplets and the fabrication of microparticles and microcapsules in the later stage. Bulk emulsification is a simple and low-cost technique; however, it generates polydisperse w-in-w emulsion droplets. Another technique, electrospray, involves easy experimental setups that can generate monodisperse but nonspherical w-in-w emulsion droplets. In comparison, microfluidic platforms provide monodisperse w-in-w emulsion droplets with spherical shapes, deal with the small volumes of solutions and short reaction times and achieve portability and versatility in their design through rapid prototyping. Owing to several advantages, microfluidic approaches have recently been introduced. To date, several different strategies have been explored to generate w-in-w emulsions and multiple w-in-w emulsions and to fabricate microparticles and microcapsules using conventional microfluidic devices. Although a few review articles on ATPSs emulsions have been published in the past, to date, few reviews have exclusively focused on the evolution of microfluidic-based ATPS droplets. The present review begins with a brief discussion of the history of ATPSs and their fundamentals, which is followed by an account chronicling the integration of microfluidic devices with ATPSs to generate w-in-w emulsion droplets. Furthermore, the stabilization strategies of w-in-w emulsion droplets and microfluidic fabrication of microparticles and microcapsules for modern applications, such as biomolecule encapsulation and spheroid construction, are discussed in detail in this review. We believe that the present review will provide useful information to not only new entrants in the microfluidic community wanting to appreciate the findings of the field but also existing researchers wanting to keep themselves updated on progress in the field.

Numerical investigation of flexible Purcell-like integrated microfluidic pumps

Integrating miniature pumps within microfluidic devices is crucial for advancing point-of-care diagnostics. Understanding the emergence of flow from novel integrated pumping systems is the first step in their successful implementation. A Purcell-like elasto-magnetic integrated microfluidic pump has been simulated in COMSOL Multiphysics and its performance has been investigated and evaluated. An elastic, cilia-like element contains an embedded magnet, which allows for actuation via a weak, uniaxial, sinusoidally oscillating, external magnetic field. Pumping performance is correlated against a number of variables, such as the frequency of the driving field and the proximity of the pump to the channel walls, in order to understand the emergence of the pumping behavior. Crucially, these simulations capture many of the trends observed experimentally and shed light on the key interactions. The proximity of the channel walls in the in-plane direction strongly determines the direction of net fluid flow. This characterization has important implications for the design and optimization of this pump in practical applications.

Influence of the solid fraction on the clogging by bridging of suspensions in constricted channels

Clogging can occur whenever a suspension of particles flows through a confined system. The formation of clogs is often correlated to a reduction in the cross-section of the channel. In this study, we consider the clogging by bridging, i.e., through the formation of a stable arch of particles at a constriction that hinders the transport of particles downstream of the clog. To characterize the role of the volume fraction of the suspension on the clogging dynamics, we study the flow of particulate suspensions through 3D-printed millifluidic devices. We systematically characterize the bridging of non-Brownian particles in a quasi-bidimensional system in which we directly visualize and track the particles as they flow and form arches at a constriction. We report the conditions for clogging by bridging when varying the constriction width to particle diameter ratio for different concentrations of the particles in suspension. We then discuss our results using a stochastic model to rationalize the influence of solid fraction on the probability of clogging. Understanding the mechanisms and conditions of clog formation is an important step for optimizing engineering design and developing more reliable dispensing systems.

Microfluidic Nanoparticles for Drug Delivery

Nanoparticles (NPs) have attracted tremendous interest in drug delivery in the past decades. Microfluidics offers a promising strategy for making NPs for drug delivery due to its capability in precisely controlling NP properties. The recent success of mRNA vaccines using microfluidics represents a big milestone for microfluidic NPs for pharmaceutical applications, and its rapid scaling up demonstrates the feasibility of using microfluidics for industrial-scale manufacturing. This article provides a critical review of recent progress in microfluidic NPs for drug delivery. First, the synthesis of organic NPs using microfluidics focusing on typical microfluidic methods and their applications in making popular and clinically relevant NPs, such as liposomes, lipid NPs, and polymer NPs, as well as their synthesis mechanisms are summarized. Then, the microfluidic synthesis of several representative inorganic NPs (e.g., silica, metal, metal oxide, and quantum dots), and hybrid NPs is discussed. Lastly, the applications of microfluidic NPs for various drug delivery applications are presented.

A numerical lift force analysis on the inertial migration of a deformable droplet in steady and oscillatory microchannel flows

Inertial migration of deformable particles has become appealing in recent years due to its numerous applications in microfluidics and biomedicine. The physics underlying the motion of these particles is contingent upon the presence of lift forces in microchannels. Therefore, in this work, we present a lift force analysis for such migration of a deformable droplet in steady and oscillatory flow regimes and identify the effects of varying Capillary number and oscillation frequency on its dynamics. We then propose an expression that mimics the lift force behavior in oscillatory flows accurately. Finally, we introduce a procedure to derive and predict a simple expression for the steady and averaged oscillatory lift for any given combination of Capillary number and oscillation frequency within a continuous range.

Optimization of Microchannels and Application of Basic Activation Functions of Deep Neural Network for Accuracy Analysis of Microfluidic Parameter Data

The fabrication of microflow channels with high accuracy in terms of the optimization of the proposed designs, minimization of surface roughness, and flow control of microfluidic parameters is challenging when evaluating the performance of microfluidic systems. The use of conventional input devices, such as peristaltic pumps and digital pressure pumps, to evaluate the flow control of such parameters cannot confirm a wide range of data analysis with higher accuracy because of their operational drawbacks. In this study, we optimized the circular and rectangular-shaped microflow channels of a 100 μm microfluidic chip using a three-dimensional simulation tool, and analyzed concentration profiles of different regions of the microflow channels. Then, we applied a deep learning (DL) algorithm for the dense layers of the rectified linear unit (ReLU), Leaky ReLU, and Swish activation functions to train and test 1600 experimental and interpolation of data samples which obtained from the microfluidic chip. Moreover, using the same DL algorithm, we configured three models for each of these three functions by changing the internal middle layers of these models. As a result, we obtained a total of 9 average accuracy values of ReLU, Leaky ReLU, and Swish functions for a defined threshold value of 6×10-5 using the trial-and-error method. We applied single-to-five-fold cross-validation technique of deep neural network to avoid overfitting and reduce noises from data-set to evaluate better average accuracy of data of microfluidic parameters.

Plant-inspired TransfOrigami microfluidics

The healthy functioning of the plants' vasculature depends on their ability to respond to environmental changes. In contrast, synthetic microfluidic systems have rarely demonstrated this environmental responsiveness. Plants respond to environmental stimuli through nastic movement, which inspires us to introduce transformable microfluidics: By embedding stimuli-responsive materials, the microfluidic device can respond to temperature, humidity, and light irradiance. Furthermore, by designing a foldable geometry, these responsive movements can follow the preset origami transformation. We term this device TransfOrigami microfluidics (TOM) to highlight the close connection between its transformation and the origami structure. TOM can be used as an environmentally adaptive photomicroreactor. It senses the environmental stimuli and feeds them back positively into photosynthetic conversion through morphological transformation. The principle behind this morphable microsystem can potentially be extended to applications that require responsiveness between the environment and the devices, such as dynamic artificial vascular networks and shape-adaptive flexible electronics.

Pulsatile flow in venous perforators of the lower limb

Teaching traditionally asserts that the arterial pressure pulse is dampened across the capillary bed to the extent that pulsatility is non-existent in the venous circulation of the lower limbs. Herein, we present evidence of transmission of arterial pulsations across the capillary network into perforator veins in the lower limbs of healthy, heat-stressed humans. Perforator veins are connections from the superficial veins that drain into the deep veins. When assessed using ultrasound at rest, they infrequently demonstrate flow and a pulsatile flow waveform is not described. We investigated perforator vein pulsatility in ten young, healthy volunteers who underwent passive heating by +2 ºC deep body temperature via a hot-water-perfused suit, and five who also underwent active heating by +2 ºC via low-intensity cycling while wearing the hot-water-perfused suit. At +0.5 ºC increments in temperature, blood velocity in an ankle perforator vein was measured using duplex ultrasound. In all perforators with heating, sustained flow was demonstrated, with a pulsatile waveform that was synchronous with the cardiac cycle. The maximum velocity was 29 ± 14 cm/s with passive heating and approximately half with active heating (P=0.04). The small veins of the skin at the ankle also demonstrated increased perfusion with pulsatility, seen with low-velocity microvascular imaging technology. We consider explanations for this pulsatility and conclude that it is propagated from the arterial inflow through the skin microcirculation as a result of increased dilatation and flow volume, and that this a normal response to increased skin blood flow.

Assessment of Blood Biophysical Properties Using Pressure Sensing with Micropump and Microfluidic Comparator

To identify the biophysical properties of blood samples consistently, macroscopic pumps have been used to maintain constant flow rates in a microfluidic comparator. In this study, the bulk-sized and expensive pump is replaced with a cheap and portable micropump. A specific reference fluid (i.e., glycerin solution [40%]) with a small volume of red blood cell (RBC) (i.e., 1% volume fraction) as fluid tracers is supplied into the microfluidic comparator. An averaged velocity (<Ur>) obtained with micro-particle image velocimetry is converted into the flow rate of reference fluid (Qr) (i.e., Qr = CQ × Ac × <Ur>, Ac: cross-sectional area, CQ = 1.156). Two control variables of the micropump (i.e., frequency: 400 Hz and volt: 150 au) are selected to guarantee a consistent flow rate (i.e., COV < 1%). Simultaneously, the blood sample is supplied into the microfluidic channel under specific flow patterns (i.e., constant, sinusoidal, and periodic on-off fashion). By monitoring the interface in the comparator as well as Qr, three biophysical properties (i.e., viscosity, junction pressure, and pressure-induced work) are obtained using analytical expressions derived with a discrete fluidic circuit model. According to the quantitative comparison results between the present method (i.e., micropump) and the previous method (i.e., syringe pump), the micropump provides consistent results when compared with the syringe pump. Thereafter, representative biophysical properties, including the RBC aggregation, are consistently obtained for specific blood samples prepared with dextran solutions ranging from 0 to 40 mg/mL. In conclusion, the present method could be considered as an effective method for quantifying the physical properties of blood samples, where the reference fluid is supplied with a cheap and portable micropump.

Microfluidic Systems for Cancer Diagnosis and Applications

Microfluidic devices have led to novel biological advances through the improvement of micro systems that can mimic and measure. Microsystems easily handle sub-microliter volumes, obviously with guidance presumably through laminated fluid flows. Microfluidic systems have production methods that do not need expert engineering, away from a centralized laboratory, and can implement basic and point of care analysis, and this has attracted attention to their widespread dissemination and adaptation to specific biological issues. The general use of microfluidic tools in clinical settings can be seen in pregnancy tests and diabetic control, but recently microfluidic platforms have become a key novel technology for cancer diagnostics. Cancer is a heterogeneous group of diseases that needs a multimodal paradigm to diagnose, manage, and treat. Using advanced technologies can enable this, providing better diagnosis and treatment for cancer patients. Microfluidic tools have evolved as a promising tool in the field of cancer such as detection of a single cancer cell, liquid biopsy, drug screening modeling angiogenesis, and metastasis detection. This review summarizes the need for the low-abundant blood and serum cancer diagnosis with microfluidic tools and the progress that has been followed to develop integrated microfluidic platforms for this application in the last few years.

Optimizing pressure-driven pulsatile flows in microfluidic devices

Unsteady and pulsatile flows receive increasing attention due to their potential to enhance various microscale processes. Further, they possess significant relevance for microfluidic studies under physiological flow conditions. However, generating a precise time-dependent flow field with commercial, pneumatically operated pressure controllers remains challenging and can lead to significant deviations from the desired waveform. In this study, we present a method to correct such deviations and thus optimize pulsatile flows in microfluidic experiments using two commercial pressure pumps. Therefore, we first analyze the linear response of the systems to a sinusoidal pressure input, which allows us to predict the time-dependent pressure output for arbitrary pulsatile input signals. Second, we explain how to derive an adapted input signal, which significantly reduces deviations between the desired and actual output pressure signals of various waveforms. We demonstrate that this adapted pressure input leads to an enhancement of the time-dependent flow of red blood cells in microchannels. The presented method does not rely on any hardware modifications and can be easily implemented in standard pressure-driven microfluidic setups to generate accurate pulsatile flows with arbitrary waveforms.

Goodbye fouling: a unique coaxial lamination mixer (CLM) enabled by two-photon polymerization for the stable production of monodisperse drug carrier nanoparticles

Poorly soluble drugs can be incorporated in lipid carrier nanoparticles to achieve sufficient bioavailability and open up diverse routes of administration. Preparation by antisolvent precipitation in microfluidic systems enables excellent control of lipid nanoparticle size. However, particle-containing flows bear the risk of material deposition on microchannel surfaces, limiting reproducibility, prolonged continuous processing and scale-up by parallelization as required for practical use. The coaxial lamination mixer (CLM) introduced in this study can fully eliminate contact of the organic phase with the channel walls while efficiently mixing organic and aqueous phases. This unique micromixer, including a nozzle for coaxial injection, a sequence of stretch-and-fold elements and inlet filters, cannot be realized by conventional 2.5D microfabrication but only by 3D two-photon polymerization. Hydrodynamic focusing of the organic phase and fast coaxial lamination were studied in simulations and flow visualization experiments. Different concentrations of castor oil or a hard fat and polysorbate 80 dissolved in ethanol were injected and combined with purified water. Total flow rates of 100 and 200 μL min-1 and flow rate ratios of 15% or less resulted in particle sizes between 67 and 153 nm and polydispersity indices of 0.04 to 0.10. Extended preparation time revealed stable particle sizes and displayed no fouling, indicating that CLMs will even allow high throughput parallelization. Stable castor oil nanoemulsions loaded with the poorly soluble drugs fenofibrate or cannabidiol were prepared. In conclusion, the unique 3D design of the CLM enables prolonged, stable and scalable production of small as well as very narrowly distributed, in most cases even monodisperse drug-loaded lipid nanoparticles.

AC Insulator-Based Dielectrophoretic Focusing of Particles and Cells in an "Infinite" Microchannel

It is often necessary to prefocus particles and cells into a tight stream for subsequent separation and/or analysis in microfluidic devices. A DC electric field has been widely used for particle and cell focusing in insulator-based dielectrophoretic (iDEP) microdevices, where a large field magnitude, a high constriction ratio, and/or a long microchannel are usually required to enhance the iDEP effect. We demonstrate, in this work, an AC iDEP focusing technique, which utilizes a low-frequency AC electric field to generate both an oscillatory electrokinetic flow of the particle/cell suspension and a field direction-independent dielectrophoretic force for particle/cell focusing in a virtually "infinite" microchannel. We also develop a theoretical analysis to evaluate this focusing in terms of the AC voltage frequency, amplitude, and particle size, which are each validated through both experimental demonstration and numerical simulation. The effectiveness of AC iDEP focusing increases with the second order of electric field magnitude, superior to DC iDEP focusing with only a first-order dependence. This feature and the "infinite" channel length together remove the necessity of large electric field and/or small constriction in DC iDEP focusing of small particles.

Nonlinear microfluidics: device physics, functions, and applications

The microfluidic flow is typically laminar due to the dominant viscous effects. At Reynolds numbers far below 1 (Re ≪ 1), the fluid inertia can be neglected. For the steady flow of incompressible Newtonian fluids, it approaches linear Stokes flow. At intermediate Re, there exists a weak-inertia flow regime where secondary flows such as Dean vortices are accessible for microfluidic manipulations. Apart from the fluid inertia, other nonlinear factors such as the non-Newtonian fluid properties, concurrent flow of dissimilar fluids, compliant fluidic structures and stimuli-responsive materials can also cause intriguing flow behaviours. Through proper designs, they can be applied for a variety of microfluidic components including mixers, valves, oscillators, stabilizers and auto-regulators etc., greatly enriching the microfluidic flow control and manipulation strategies. Due to its unique working characteristics and advantages, nonlinear microfluidics has increasingly attracted extensive attention. This review presents a systematic survey on this subject. The designs of typical nonlinear microfluidic devices, their working mechanisms, key applications, and the perspective of their future developments will be discussed. The nonlinear microfluidic techniques are believed to play an essential role in the next generation of highly-integrated, automated, and intelligent microfluidics.

Inertial migration of a deformable capsule in an oscillatory flow in a microchannel

Dynamics of a deformable capsule in an oscillatory flow of a Newtonian fluid in a microchannel has been studied numerically. The effects of oscillation frequency, capsule deformability, and channel flow rate have been explored by simulating the capsule within a microchannel. In addition, the simulation captures the effect of the type of imposed pressure oscillations on the migration pattern of the capsule. An oscillatory channel flow enables the focusing of extremely small biological particles by eliminating the need to design impractically long channels. The presented results show that the equilibrium position of the capsule changes not only by the addition of an oscillatory component to the pressure gradient but it also is influenced by the capsule deformability and channel flow rate. Furthermore, it has been shown that the amplitude of oscillation of capsules decreases as the channel flow rate and the rigidity of the capsule increases.

Tunable Harmonic Flow Patterns in Microfluidic Systems through Simple Tube Oscillation

Generation of tunable harmonic flows at low cost in microfluidic systems is a persistent and significant obstacle to this field, substantially limiting its potential to address major scientific questions and applications. This work introduces a simple and elegant way to overcome this obstacle. Harmonic flow patterns can be generated in microfluidic structures by simply oscillating the inlet tubes. Complex rib and vortex patterns can be dynamically modulated by changing the frequency and magnitude of tube oscillation and the viscosity of liquid. Highly complex rib patterns and synchronous vortices can be generated in serially connected microfluidic chambers. Similar dynamic patterns can be generated using whole or diluted blood samples without damaging the sample. This method offers unique opportunities for studying complex fluids and soft materials, chemical synthesis of various compounds, and mimicking harmonic flows in biological systems using compact, tunable, and low-cost devices.