Microfluidic viscometers for shear rheology of complex fluids and biofluids

Portable Microfluidic Viscometer for Formulation Development and in Situ Quality Control of Protein and Antibody Solutions

Viscosity of protein solutions is a critical product quality attribute for protein therapeutics such as monoclonal antibodies. Here we introduce a portable single-use analytical chip-based viscometer for determining the viscosity of protein solutions using low sample volumes of 10 μL. Through the combined use of a microfluidic viscometer, a smartphone camera for image capture, and an automated data processing algorithm for the calculation of the viscosity of fluids, we enable measurement of viscosity of multiple samples in parallel. We first validate the viscometer using glycerol-water mixtures and subsequently demonstrate the ability to perform rapid characterization of viscosity in four different monoclonal antibody formulations in a broad concentration (1 to 320 mg/mL) and viscosity (1 to 600 cP) range, showing excellent agreement with values obtained by a conventional cone-plate rheometer. Not only does the platform offer benefits of viscosity measurements using minimal sample volumes, but enables higher throughput compared to gold-standard methodologies owing to multiplexing of the measurement and single-use characteristics of the viscometer, thus showing great promise in developability studies. Additionally, as our platform has the capability of performing viscosity measurements at the point of sample collection, it offers the opportunity to employ viscosity measurement as an in situ quality control of therapeutic proteins and antibodies.

Microfluidic Rheology: An Innovative Method for Viscosity Measurement of Gels and Various Pharmaceuticals

Measuring the viscosity of pharmaceutical dosage forms is a crucial process. Viscosity provides information about the stability of the composition, the release rate of the drug, bioavailability, and, in the case of injectable drug formulations, even the force required for injection. However, measuring viscosity is a complex task with numerous challenges, especially for non-Newtonian materials, which include most pharmaceutical formulations, such as gels. Selecting the appropriate shear rate is critical. Since viscosity in many systems is highly temperature-dependent, stable temperature control is necessary during the measurement. Using microfluidics technology, it is now possible to perform rheological characterization and conduct fast and accurate measurements. Small sample volumes (even below 500 µL) are required, and viscosity determination can be carried out over a wide range of shear rates. Nevertheless, the pharmaceutical application of viscometers operating on the principle of microfluidics is not yet widespread. In our work, we compare the results of measurements taken with a microfluidic chip-based viscometer on different pharmaceutical forms (gels, solution) with those obtained using a traditional rotational viscometer, evaluating the relative advantages and disadvantages of the different methods. The microfluidics-based method enables time- and sample-efficient viscosity analysis of the examined pharmaceutical forms.

3D-printed ultra-small Brownian viscometers

Measuring viscosity in volumes smaller than a microliter is a challenging endeavor. A new type of microscopic viscometers is presented to assess the viscosity of Newtonian liquids. Micron-sized flexible polymer cantilevers are created by two-photon polymerization direct laser writing. Because of the low stiffness and high elasticity of the polymer material the microcantilevers exhibit pronounced Brownian motion when submerged in a liquid medium. By imaging the cantilever's spherically shaped end, these fluctuations can be tracked with high accuracy. The hydrodynamic resistance of the microviscometer is determined by fitting the power spectral density of the measured fluctuations with a theoretical frequency dependence. Validation measurements in water-glycerol mixtures with known viscosities reveal excellent linearity of the hydrodynamic resistance to viscosity, allowing for a simple linear calibration. The stand-alone viscometer structures have a characteristic size of a few tens of microns and only require a very basic external instrumentation in the form of microscopic imaging at moderate framerates (~ 100 fps). Thus, our results point to a practical and simple to use ultra-low volume viscometer that can be integrated into lab-on-a-chip devices.

A viscosity measurement technique for ultra-low sample volumes

We describe a unique method to measure the viscosity of liquids based on the fluid mechanics of thin films. A drop of sample is spread over a substrate by contacting a blade with the drop and moving the blade across the substrate. The thickness of the film is determined by the capillary number, which measures the competition between the viscous force that smears the liquid over the glass slide and the surface tension that resists the deformation of the interface. We show that the length of the smear for a fixed sample volume is also set by capillary number and can be used as a reliable measure of fluid viscosity. The technique is especially suitable for viscosity measurements of biological fluids where viscosities are low and sample sizes are small. The technique can detect small changes in blood viscosity enabling it to be used as a non-specific, screening tool for diseases and therapeutic interventions.

Molecular Rotors: Fluorescent Sensors for Microviscosity and Conformation of Biomolecules

The viscosity and crowding of biological environment are considered vital for the correct cellular function, and alterations in these parameters are known to underly a number of pathologies including diabetes, malaria, cancer and neurodegenerative diseases, to name a few. Over the last decades, fluorescent molecular probes termed molecular rotors proved extremely useful for exploring viscosity, crowding, and underlying molecular interactions in biologically relevant settings. In this review, we will discuss the basic principles underpinning the functionality of these probes and will review advances in their use as sensors for lipid order, protein crowding and conformation, temperature and non-canonical nucleic acid structures in live cells and other relevant biological settings.

Optimizing Sensitivity in a Fluid-Structure Interaction-Based Microfluidic Viscometer: A Multiphysics Simulation Study

Fluid-structure interactions (FSI) are used in a variety of sensors based on micro- and nanotechnology to detect and measure changes in pressure, flow, and viscosity of fluids. These sensors typically consist of a flexible structure that deforms in response to the fluid flow and generates an electrical, optical, or mechanical signal that can be measured. FSI-based sensors have recently been utilized in applications such as biomedical devices, environmental monitoring, and aerospace engineering, where the accurate measurement of fluid properties is critical to ensure performance and safety. In this work, multiphysics models are employed to identify and study parameters that affect the performance of an FSI-based microfluidic viscometer that measures the viscosity of Newtonian and non-Newtonian fluids using the deflection of flexible micropillars. Specifically, we studied the impact of geometric parameters such as pillar diameter and height, aspect ratio of the pillars, pillar spacing, and the distance between the pillars and the channel walls. Our study provides design guidelines to adjust the sensitivity of the viscometer toward specific applications. Overall, this highly sensitive microfluidic sensor can be integrated into complex systems and provide real-time monitoring of fluid viscosity.

Green Extraction of Antioxidant Fractions from Humulus lupulus Varieties and Microparticle Production via Spray-Drying

The formulation of polymeric microparticles to encapsulate bioactive compounds from two hop varieties (Nugget and Perle) using sequential green extraction processes was performed. The technologies used were ultrasound-assisted extraction (UAE) and pressurized hot water (PHW) extraction. Liquid phases were analyzed for total phenolic content (~2%), antioxidant activity (IC50, DPPH: 3.68 (Nugget); 4.46 (Perle) g/L, TEAC (~4-5%), FRAP (~2-3%), and reducing power (~4%)), protein content (~1%), oligosaccharide content (~45%), and for structural features. The fractions obtained from UAE were selected to continue with the drying process, achieving the maximum yield at 120 °C (Perle) and 130 °C (Nugget) (~77%). Based on these results, the formulation of polymeric microparticles using mannitol as the carrier was performed with these fractions. The production yield (~65%), particle size distribution (Perle: 250-750 µm and Nugget: ~100 µm), and rheological features (30-70 mPa s at 0.1 s-1) were the parameters evaluated. The UAE extracts from hop samples processed using a sustainable aqueous treatment allowed the formulation of microparticles with a suitable yield, and morphological and viscosity properties adequate for potential food and non-food applications.

Microfluidic viscometer by acoustic streaming transducers

Measurement of fluid viscosity represents a huge need for many biomedical and materials processing applications. Sample fluids containing DNA, antibodies, protein-based drugs, and even cells have become important therapeutic options. The physical properties, including viscosity, of these biologics are critical factors in the optimization of the biomanufacturing processes and delivery of therapeutics to patients. Here we demonstrate an acoustic microstreaming platform termed as microfluidic viscometer by acoustic streaming transducers (μVAST) that induces fluid transport from second-order microstreaming to measure viscosity. Validation of our platform is achieved with different glycerol content mixtures to reflect different viscosities and shows that viscosity can be estimated based on the maximum speed of the second-order acoustic microstreaming. The μVAST platform requires only a small volume of fluid sample (∼1.2 μL), which is 16-30 times smaller than that of commercial viscometers. In addition, μVAST can be scaled up for ultra-high throughput measurements of viscosity. Here we demonstrate 16 samples within 3 seconds, which is an attractive feature for automating the process flows in drug development and materials manufacturing and production.

Microfluidic techniques for mechanical measurements of biological samples

The use of microfluidics to make mechanical property measurements is increasingly common. Fabrication of microfluidic devices has enabled various types of flow control and sensor integration at micrometer length scales to interrogate biological materials. For rheological measurements of biofluids, the small length scales are well suited to reach high rates, and measurements can be made on droplet-sized samples. The control of flow fields, constrictions, and external fields can be used in microfluidics to make mechanical measurements of individual bioparticle properties, often at high sampling rates for high-throughput measurements. Microfluidics also enables the measurement of bio-surfaces, such as the elasticity and permeability properties of layers of cells cultured in microfluidic devices. Recent progress on these topics is reviewed, and future directions are discussed.

Ultrasonic Vitrectomy Performance Assessment Using Micro-Extensional Rheology

Purpose

The purpose of this study was to assess the performance of ultrasonic (US) vitrectomy devices by quantifying and comparing its impact on extracted vitreous properties to conventional pneumatic blade (PB) cutters using micro-extensional rheology. US vitrectomy is a new technology that offers an alternative to PB cutters used in vitreo-retinal surgeries.

Methods

Thirty-six porcine vitreous samples were extracted using US and PB cutters. Each sample was kept at 4°C and tested within 24 hours postmortem and 4 hours post-vitrectomy. A recently developed micro-extensional rheology technique is used to infer the relative protein fragment size of extracted vitreous by quantifying the extensional relaxation time.

Results

US-extracted vitreous exhibited extensional relaxation times orders of magnitude lower than PB-extracted vitreous (0.37 ms and 27.25 ms, respectively). Relaxation time is directly correlated to the fragment size of the collagen fibers in the vitreous. The formation of beads-on-a-string droplets within the PB samples indicates the presence of larger collagen fragments. These droplets were not seen on US samples.

Conclusions

This new micro-extensional rheology technique can identify significant differences in physical properties of extracted vitreous. Long relaxation times and beads-on-a-string droplets within the PB vitreous samples indicate larger protein fragments compared to the US samples.

Translational Relevance

Higher fragmentation of vitreous and lower extensional relaxation times may improve retina safety due to a reduction in vitreo-retinal traction resulting from the continuous shear action and aspiration applied by ultrasonic vitrectomy technology.

Biosensing of Haemorheological Properties Using Microblood Flow Manipulation and Quantification

The biomechanical properties of blood have been used to detect haematological diseases and disorders. The simultaneous measurement of multiple haemorheological properties has been considered an important aspect for separating the individual contributions of red blood cells (RBCs) and plasma. In this study, three haemorheological properties (viscosity, time constant, and RBC aggregation) were obtained by analysing blood flow, which was set to a square-wave profile (steady and transient flow). Based on a simplified differential equation derived using a discrete circuit model, the time constant for viscoelasticity was obtained by solving the governing equation rather than using the curve-fitting technique. The time constant (λ) varies linearly with respect to the interface in the coflowing channel (β). Two parameters (i.e., average value: <λ>, linear slope: dλdβ) were newly suggested to effectively represent linearly varying time constant. <λ> exhibited more consistent results than dλdβ. To detect variations in the haematocrit in blood, we observed that the blood viscosity (i.e., steady flow) is better than the time constant (i.e., transient flow). The blood viscosity and time constant exhibited significant differences for the hardened RBCs. The present method was then successfully employed to detect continuously varying haematocrit resulting from RBC sedimentation in a driving syringe. The present method can consistently detect variations in blood in terms of the three haemorheological properties.

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.

A micro-scale rheometer to study foam texture and flow resistance in planar fractures

We designed and fabricated a new microfluidic device to better enable study of foam microstructure and rheology in planar fractures. The design phase included stress-strain finite element analysis to enhance the pressure tolerance of the device. The optimized design is a 2 cm wide by 7.75 cm long rough fracture that includes 25 posts to anchor the glass cover plate. The posts simulate asperities and provide structural support during bonding of a glass cover plate to the device. Importantly, the new design illustrates improved ability to sustain large differential pressure compared to previous designs in the literature. The rheometer permits study of the relationship among foam bubble morphology, pressure drop, and flow rates. Our findings validated the previous, sparse microvisual studies mentioned in the literature and confirmed that small quality foam, ranging from 20 to 50% gas by volume, contains dispersed bubbles separated by liquid lenses. In this range, the distribution of bubble sizes was roughly 80-90% small uniform bubbles and only 10-20% of larger and more elongated bubbles. Additionally, our studies reveal that foam apparent viscosity is a strong function of foam quality, velocity, and texture (i.e., bubble size). Apparent viscosity of foam ranged from 100 to 600 cP for the conditions studied. High quality foams in fractures are independent of gas flow rates but very sensitive to liquid flow rates. On the other hand, low quality foams are sensitive to gas flow rates but independent of liquid flow rates.

Drop-of-sample rheometry of biological fluids by noncontact acoustic tweezing spectroscopy

Knowledge of rheological properties, such as viscosity and elasticity, is necessary for efficient material processing and transportation as well as biological analysis. Existing rheometers operate with large sample volume and induce sample contact with container or device walls, which are inadequate for rheological analysis of sensitive fluids limited in availability. In this work, we introduce acoustic tweezing spectroscopy (ATS), a novel noncontact rheological technique that operates with a single 4-6 μl drop of fluid sample. In ATS, a sample drop is acoustically levitated and then exposed to a modulated acoustic signal to induce its forced oscillation. The time-dependent sample viscosity and elasticity are measured from the resulting drop response. The ATS measurements of polymeric solutions (dextran, xanthan gum, gelatin) agree well with previously reported data. The ATS predicts that the shear viscosity of blood plasma increases from 1.5 cP at 1.5 min of coagulation onset to 3.35 cP at 9 min, while its shear elastic modulus grows from a negligible value to 10.7 Pa between 3.5 min and 6.5 min. Coagulation increases whole blood viscosity from 5.4 cP to 20.7 cP and elasticity from 0.1 Pa to 19.2 Pa at 15 min. In summary, ATS provides the opportunity for sensitive small-volume rheological analysis in biomedical research and medical, pharmaceutical, and chemical industries.

Rheological Properties of Different Graphene Nanomaterials in Biological Media

Carbon nanomaterials have received increased attention in the last few years due to their potential applications in several areas. In medicine, for example, these nanomaterials could be used as contrast agents, drug transporters, and tissue regenerators or in gene therapy. This makes it necessary to know the behavior of carbon nanomaterials in biological media to assure good fluidity and the absence of deleterious effects on human health. In this work, the rheological characterization of different graphene nanomaterials in fetal bovine serum and other fluids, such as bovine serum albumin and water, is studied using rotational and microfluidic chip rheometry. Graphene oxide, graphene nanoplatelets, and expanded graphene oxide at concentrations between 1 and 3 mg/mL and temperatures in the 25–40 °C range were used. The suspensions were also characterized by transmission and scanning electron microscopy and atomic force microscopy, and the results show a high tendency to aggregation and reveals that there is a protein–nanomaterial interaction. Although rotational rheometry is customarily used, it cannot provide reliable measurements in low viscosity samples, showing an apparent shear thickening, whereas capillary viscometers need transparent samples; therefore, microfluidic technology appears to be a suitable method to measure low viscosity, non-transparent Newtonian fluids, as it is able to determine small variations in viscosity. No significant changes in viscosity are found within the solid concentration range studied but it decreases between 1.1 and 0.6 mPa·s when the temperature raises from 25 to 40 °C.

Normalization of Blood Viscosity According to the Hematocrit and the Shear Rate

The rheological properties of blood depend highly on the properties of its red blood cells: concentration, membrane elasticity, and aggregation. These properties affect the viscosity of blood as well as its shear thinning behavior. Using an experimental analysis of the interface advancement of blood in a microchannel, we determine the viscosity of different samples of blood. In this work, we present two methods that successfully normalize the viscosity of blood for a single and for different donors, first according to the concentration of erythrocytes and second according to the shear rate. The proposed methodology is able to predict the health conditions of the blood samples by introducing a non-dimensional coefficient that accounts for the response to shear rate of the different donors blood samples. By means of these normalization methods, we were able to determine the differences between the red blood cells of the samples and define a range where healthy blood samples can be described by a single behavior.

Rapid Temperature-Dependent Rheological Measurements of Non-Newtonian Solutions Using a Machine-Learning Aided Microfluidic Rheometer

Biofluids such as synovial fluid, blood plasma, and saliva contain several proteins which impart non-Newtonian properties to the biofluids. The concentration of such protein macromolecules in biofluids is regarded as an important biomarker for the diagnosis of several health conditions, including cardiovascular disorders, joint quality, and Alzheimer's. Existing technologies for the measurements of macromolecules in biofluids are limited; they require a long turnaround time, or require complex protocols, thus calling for alternative, more suitable, methodologies aimed at such measurements. According to the well-established relations for polymer solutions, the concentration of macromolecules in solutions can also be derived via measurement of rheological properties such as shear-viscosity and the longest relaxation time. We here introduce a microfluidic rheometer for rapid simultaneous measurement of shear viscosity and longest relaxation time of non-Newtonian solutions at different temperatures. At variance with previous technologies, our microfluidic rheometer provides a very short turnaround time of around 2 min or less thanks to the implementation of a machine-learning algorithm. We validated our platform on several aqueous solutions of poly(ethylene oxide). We also performed measurements on hyaluronic acid solutions in the clinical range for joint grade assessment. We observed monotonic behavior with the concentration for both rheological properties, thus speculating on their use as potential rheo-markers, i.e., rheological biomarkers, across several disease states.

Microfluidics Approach to the Mechanical Properties of Red Blood Cell Membrane and Their Effect on Blood Rheology

In this article, we describe the general features of red blood cell membranes and their effect on blood flow and blood rheology. We first present a basic description of membranes and move forward to red blood cell membranes’ characteristics and modeling. We later review the specific properties of red blood cells, presenting recent numerical and experimental microfluidics studies that elucidate the effect of the elastic properties of the red blood cell membrane on blood flow and hemorheology. Finally, we describe specific hemorheological pathologies directly related to the mechanical properties of red blood cells and their effect on microcirculation, reviewing microfluidic applications for the diagnosis and treatment of these diseases.

A Novel Device for the Quantification of Synovial Fluid Viscosity via Magnetic Deflection

Changes in synovial fluid viscosity may be used to detect joint disease; however, methods to evaluate these changes at the point of care are rudimentary. Previously, we demonstrated magnetic particle translation through synovial fluid could serve as a surrogate marker of synovial fluid mechanics. In this work, the relationship of magnetic deflection of a stream of particles is examined in relation to synovial fluid mechanics. A flow device was designed to assess the deflection of a magnetic particle stream as it flows past a fixed permanent magnet. Deflection is recorded via a camera, measuring the deflection distance of the particle stream at a given distance. Theoretically, as fluid viscosity decreases, the deflection of the particle stream should increase. To validate this approach, particle deflection was first measured in Newtonian glycerol solutions of varying viscosity under different flow conditions. Next, the device was used to test synovial fluid viscosity in bovine synovial fluid that was progressively degraded using ultrasonication. A strong correlation was observed between the deflection of the magnetic particles and the viscosity of the glycerol solutions (R2=0.987) and the degradation of synovial fluid (R2=0.7045). Moreover, this approach does not require particles to be separated from the fluid for quantification and serves as a proof-of-principle for future lab-on-a-chip assessments of synovial fluid biomechanics.

Microfluidic Rheometry and Particle Settling: Characterizing the Effect of Polymer Solution Elasticity

The efficient transport of solid particles using polymeric fluids is an important step in many industrial operations. Different viscoelastic fluids have been designed for this purpose, however, the effects of elasticity have not been fully integrated in examining the particle-carrying capacity of the fluids. In this work, two elastic fluid formulations were employed to experimentally clarify the effect of elasticity on the particle drag coefficient as a proxy model for measuring carrying capacity. Fluids were designed to have a constant shear viscosity within a specific range of shear rates, γ˙<50(1/s), while possessing distinct (longest) relaxation times to investigate the influence of elasticity. It is shown that for dilute polymeric solutions, microfluidic rheometry must be practiced to obtain a reliable relaxation time (as one of the measures of viscoelasticity), which is on the order of milliseconds. A calibrated experimental setup, furnished with two advanced particle velocity measurement techniques and spheres with different characteristics, was used to quantify the effect of elasticity on the drag coefficient. These experiments led to a unique dataset in moderate levels of Weissenberg numbers, 0<Wi<8.5. The data showed that there is a subtle reduction in the drag coefficient at low levels of elasticity (Wi<1), and a considerable enhancement at high levels of elasticity (Wi>1). The experimental results were then compared with direct numerical simulation predictions yielding R2=0.982. These evaluations endorse the numerically quantified behaviors for the drag coefficient to be used to compare the particle-carrying capacity of different polymeric fluids under different flow conditions.

A Review of Microfluidic Devices for Rheological Characterisation

The rheological characterisation of liquids finds application in several fields ranging from industrial production to the medical practice. Conventional rheometers are the gold standard for the rheological characterisation; however, they are affected by several limitations, including high costs, large volumes required and difficult integration to other systems. By contrast, microfluidic devices emerged as inexpensive platforms, requiring a little sample to operate and fashioning a very easy integration into other systems. Such advantages have prompted the development of microfluidic devices to measure rheological properties such as viscosity and longest relaxation time, using a finger-prick of volumes. This review highlights some of the microfluidic platforms introduced so far, describing their advantages and limitations, while also offering some prospective for future works.

A small-volume microcapillary rheometer

We demonstrate a capillary device used to measure the shear rate-dependent viscosity of microliter scale volumes. Liquid samples are driven pneumatically through a microcapillary and partially fill a larger glass capillary. The glass capillary is mounted on an optical linear sensor to track the air-liquid meniscus in real time and trigger the reversal of flow direction by switching a pneumatic valve. Each transit provides a volumetric flow rate measurement, which is used with the pressure drop to determine viscosity as a function of shear rate. A given sample of at least 50 μL can be measured over at least 2 to 3 decades in shear rate, in the range of 10 to 10⁵ s^-1, and be essentially fully recovered. Validation by comparison to reference measurements is performed using samples of Newtonian and non-Newtonian fluid, with viscosity ranging from 1 to 100 mPa s. The range of operation and uncertainty arising from instrumentation, meniscus effects, and inertial effects are discussed. The performance of this rheometer is advantageous, especially for use and reuse of small volumes.

Soft hydraulics: from Newtonian to complex fluid flows through compliant conduits

Microfluidic devices manufactured from soft polymeric materials have emerged as a paradigm for cheap, disposable and easy-to-prototype fluidic platforms for integrating chemical and biological assays and analyses. The interplay between the flow forces and the inherently compliant conduits of such microfluidic devices requires careful consideration. While mechanical compliance was initially a side-effect of the manufacturing process and materials used, compliance has now become a paradigm, enabling new approaches to microrheological measurements, new modalities of micromixing, and improved sieving of micro- and nano-particles, to name a few applications. This topical review provides an introduction to the physics of these systems. Specifically, the goal of this review is to summarize the recent progress towards a mechanistic understanding of the interaction between non-Newtonian (complex) fluid flows and their deformable confining boundaries. In this context, key experimental results and relevant applications are also explored, hand-in-hand with the fundamental principles for their physics-based modeling. The key topics covered include shear-dependent viscosity of non-Newtonian fluids, hydrodynamic pressure gradients during flow, the elastic response (deformation and bulging) of soft conduits due to flow within, the effect of cross-sectional conduit geometry on the resulting fluid-structure interaction, and key dimensionless groups describing the coupled physics. Open problems and future directions in this nascent field of soft hydraulics, at the intersection of non-Newtonian fluid mechanics, soft matter physics, and microfluidics, are noted.

Microfluidics for Drug Development: From Synthesis to Evaluation

Drug development is a long process whose main content includes drug synthesis, drug delivery, and drug evaluation. Compared with conventional drug development procedures, microfluidics has emerged as a revolutionary technology in that it offers a miniaturized and highly controllable environment for bio(chemical) reactions to take place. It is also compatible with analytical strategies to implement integrated and high-throughput screening and evaluations. In this review, we provide a comprehensive summary of the entire microfluidics-based drug development system, from drug synthesis to drug evaluation. The challenges in the current status and the prospects for future development are also discussed. We believe that this review will promote communications throughout diversified scientific and engineering communities that will continue contributing to this burgeoning field.

Microrheometer for Biofluidic Analysis: Electronic Detection of the Fluid-Front Advancement

The motivation for this study was to develop a microdevice for the precise rheological characterization of biofluids, especially blood. The method presented was based on the principles of rheometry and fluid mechanics at the microscale. Traditional rheometers require a considerable amount of space, are expensive, and require a large volume of sample. A mathematical model was developed that, combined with a proper experimental model, allowed us to characterize the viscosity of Newtonian and non-Newtonian fluids at different shear rates. The technology presented here is the basis of a point-of-care device capable of describing the nonlinear rheology of biofluids by the fluid/air interface front velocity characterization through a microchannel. The proposed microrheometer uses a small amount of sample to deliver fast and accurate results, without needing a large laboratory space. Blood samples from healthy donors at distinct hematocrit percentages were the non-Newtonian fluid selected for the study. Water and plasma were employed as testing Newtonian fluids for validation of the system. The viscosity results obtained for the Newtonian and non-Newtonian fluids were consistent with pertinent studies cited in this paper. In addition, the results achieved using the proposed method allowed distinguishing between blood samples with different characteristics.

Review of nonlinear electrokinetic flows in insulator-based dielectrophoresis: From induced charge to Joule heating effects

Insulator-based dielectrophoresis (iDEP) has been increasingly used for particle manipulation in various microfluidic applications. It exploits insulating structures to constrict and/or curve electric field lines to generate field gradients for particle dielectrophoresis. However, the presence of these insulators, especially those with sharp edges, causes two nonlinear electrokinetic flows, which, if sufficiently strong, may disturb the otherwise linear electrokinetic motion of particles and affect the iDEP performance. One is induced charge electroosmotic (ICEO) flow because of the polarization of the insulators, and the other is electrothermal flow because of the amplified Joule heating in the fluid around the insulators. Both flows vary nonlinearly with the applied electric field (either DC or AC) and exhibit in the form of fluid vortices, which have been utilized to promote some applications while being suppressed in others. The effectiveness of iDEP benefits from a comprehensive understanding of the nonlinear electrokinetic flows, which is complicated by the involvement of the entire iDEP device into electric polarization and thermal diffusion. This article is aimed to review the works on both the fundamentals and applications of ICEO and electrothermal flows in iDEP microdevices. A personal perspective of some future research directions in the field is also given.

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.

Droplet Interfacial Tensions and Phase Transitions Measured in Microfluidic Channels

Measurements of droplet phase and interfacial tension (IFT) are important in the fields of atmospheric aerosols and emulsion science. Bulk macroscale property measurements with similar constituents cannot capture the effect of microscopic length scales and highly curved surfaces on the transport characteristics and heterogeneous chemistry typical in these applications. Instead, microscale droplet measurements ensure properties are measured at the relevant length scale. With recent advances in microfluidics, customized multiphase fluid flows can be created in channels for the manipulation and observation of microscale droplets in an enclosed setting without the need for large and expensive control systems. In this review, we discuss the applications of different physical principles at the microscale and corresponding microfluidic approaches for the measurement of droplet phase state, viscosity, and IFT.

The Effects of Stiffness, Fluid Viscosity, and Geometry of Microenvironment in Homeostasis, Aging, and Diseases: A Brief Review

Cells sense biophysical cues in the micro-environment and respond to the cues biochemically and biophysically. Proper responses from cells are critical to maintain the homeostasis in the body. Abnormal biophysical cues will cause pathological development in the cells; pathological or aging cells, on the other hand, can alter their micro-environment to become abnormal. In this minireview, we discuss four important biophysical cues of the micro-environment-stiffness, curvature, extracellular matrix (ECM) architecture and viscosity-in terms of their roles in health, aging, and diseases.

A Review of the Real-Time Monitoring of Fluid-Properties in Tubular Architectures for Industrial Applications

The real-time monitoring of fluid properties in tubular systems, such as viscosity and flow rate, is essential for industries utilizing liquid mediums. Nowadays, most studies of the fluid characteristics are performed off-line using laboratory facilities that can provide accurate results, yet they do not match the demanded industrial pace. Off-line measurements are ineffective and time-consuming. The available real-time monitoring sensors for fluid properties are generally destructive methods that produce significant and persistent damage to the tubular systems during the installation process. Others use huge and bulky invasive instrument methods that generate considerable pressure reduction and energy loss in tubular systems. For these drawbacks, industries centered their attention on non-invasive and non-destructive testing (NDT) methodologies, which are installed on the outer tubular surface to avoid flow disturbance and desist shutting down systems for installations. Although these sensors showed excellent achievement for monitoring and inspecting pipe health conditions, the performance was not convincing for monitoring the properties of fluids. This review paper presents an overview of the real-time monitoring of fluid properties in tubular systems for industrial applications, particularly for pipe monitoring sensors, viscosity, and flow measurements. Additionally, the different available sensing mechanisms and their advantages, drawbacks, and potentials are discussed.

Rheological Characterization of Pharmaceutical and Cosmetic Formulations for Cutaneous Applications

Rheology, the study of the flow and deformation of matter, can be a daunting subject for scientists new to this field. However, its importance in characterization and optimization of formulations is indisputable. This review intends to provide basic and practical rheological notions in order to better understand the key concepts such as shear stress, shear rate, viscosity, elastic and viscous moduli and phase angle, and learn to distinguish between flow and oscillation experiments. We will explain the different rheological behaviors such as shear thinning, thixotropy or viscoelasticity. Throughout this review, these concepts will be illustrated with examples taken from pharmaceutical and cosmetic formulations. Rheology is a broad subject and this review does not intend to be comprehensive, but rather to be concise and pedagogical.

Technology Advancements in Blood Coagulation Measurements for Point-of-Care Diagnostic Testing

In recent years, blood coagulation monitoring has become crucial to diagnosing causes of hemorrhages, developing anticoagulant drugs, assessing bleeding risk in extensive surgery procedures and dialysis, and investigating the efficacy of hemostatic therapies. In this regard, advanced technologies such as microfluidics, fluorescent microscopy, electrochemical sensing, photoacoustic detection, and micro/nano electromechanical systems (MEMS/NEMS) have been employed to develop highly accurate, robust, and cost-effective point of care (POC) devices. These devices measure electrochemical, optical, and mechanical parameters of clotting blood. Which can be correlated to light transmission/scattering, electrical impedance, and viscoelastic properties. In this regard, this paper discusses the working principles of blood coagulation monitoring, physical and sensing parameters in different technologies. In addition, we discussed the recent progress in developing nanomaterials for blood coagulation detection and treatments which opens up new area of controlling and monitoring of coagulation at the same time in the future. Moreover, commercial products, future trends/challenges in blood coagulation monitoring including novel anticoagulant therapies, multiplexed sensing platforms, and the application of artificial intelligence in diagnosis and monitoring have been included.

Viscosity Estimation of a Suspension with Rigid Spheres in Circular Microchannels Using Particle Tracking Velocimetry

Suspension flows are ubiquitous in industry and nature. Therefore, it is important to understand the rheological properties of a suspension. The key to understanding the mechanism of suspension rheology is considering changes in its microstructure. It is difficult to evaluate the influence of change in the microstructure on the rheological properties affected by the macroscopic flow field for non-colloidal particles. In this study, we propose a new method to evaluate the changes in both the microstructure and rheological properties of a suspension using particle tracking velocimetry (PTV) and a power-law fluid model. Dilute suspension (0.38%) flows with fluorescent particles in a microchannel with a circular cross section were measured under low Reynolds number conditions (Re ≈ 10^-4). Furthermore, the distribution of suspended particles in the radial direction was obtained from the measured images. Based on the power-law index and dependence of relative viscosity on the shear rate, we observed that the non-Newtonian properties of the suspension showed shear-thinning. This method will be useful in revealing the relationship between microstructural changes in a suspension and its rheology.

Contact-free determination of viscosity in multiple parallel samples

Viscosity is an inherent characteristic of fluids and is therefore an important parameter in many different processes. Current methods to measure viscosity involve direct contact with the liquid sample, which is often undesirable. Here we present a simple, precise and robust contact-free method to determine viscosity, using a single drive motor, inexpensive components and disposable sample vessels. The measurement principle involves the detection of viscosity-dependent angular positions in a rotating liquid relative to the direction of centrifugal acceleration in an orbitally shaken vessel. The signal can be detected using different optical methods, as shown here using fluorescence and transmitted light. The sensitivity of the system can be adjusted over a wide range by varying the sample volume or the shaking diameter, and multiple samples can be analysed in parallel. This novel viscometer is also applicable to characterize non-Newtonian shear rate-dependent fluids.

In vitro and ex vivo measurement of the biophysical properties of blood using microfluidic platforms and animal models

Haemorheologically impaired microcirculation, such as blood clotting or abnormal blood flow, causes interrupted blood flows in vascular networks. The biophysical properties of blood, including blood viscosity, blood viscoelasticity, haematocrit, red blood bell (RBC) aggregation, erythrocyte sedimentation rate and RBC deformability, have been used to monitor haematological diseases. In this review, we summarise several techniques for measuring haemorheological properties, such as blood viscosity, RBC deformability and RBC aggregation, using in vitro microfluidic platforms. Several methodologies for the measurement of haemorheological properties with the assistance of an extracorporeal rat bypass loop are also presented. We briefly discuss several emerging technologies for continuous, long-term, multiple measurements of haemorheological properties under in vitro or ex vivo conditions.

3D printed microfluidic viscometer based on the co-flowing stream

Considering the role of viscosity in the dynamics of physical, chemical, and biological systems, accurate measurement of viscosity is essential. Although many conventional viscometers have been widely used, these conventional viscometers suffer from some drawbacks. In this study, a three-dimensional (3D) printed microfluidic viscometer was proposed based on the estimation of the pressure between two fluids to easily measure viscosity with small samples. The 3D printed microfluidic viscometer can be fabricated through amine-epoxy bonding on 3D printed blocks. By separately delivering samples and reference fluids into two inlets, an interfacial line could be induced. Based on the relation between the pressure ratio and the width of the reference flow, the viscosity (μ) of the sample can be estimated by measuring the relative width of the reference flow. The relation between the pressure and interfacial width between test samples and reference flows in the 3D printed microfluidic viscometers was analyzed by experiment and simulation to determine the effects of the mesh-like pattern of the 3D printed viscometers on the pressure estimation. To validate the proposed method, the viscosity values of glycerol mixtures measured by the 3D printed viscometer were compared with those measured by a conventional viscometer. As an application of the 3D printed viscometer, the viscosity curves for blood samples collected from diabetic and non-diabetic patients depending on their shear rates were compared. As expected, a high blood viscosity in the diabetic group was observed. Based on the experimental demonstrations, the 3D printed viscometer has strong potential to develop portable viscometers that can be translated to commercial outcomes.

Design and Fabrication of a Microfluidic Viscometer Based on Electrofluidic Circuits

This paper reports a microfluidic viscometer based on electrofluidic circuits for measuring viscosities of liquid samples. The developed micro-device consists of a polydimethylsiloxane (PDMS) layer for electrofluidic circuits, a thin PDMS membrane, another PDMS layer for sample pretreatment, and a glass substrate. As the sample flows inside the microfluidic channel, its viscosity causes flow resistance and a pressure drop along this channel. This pressure drop, in turn, generates a hydraulic pressure which deforms the PDMS membrane, causing changes in the cross-sectional area and the electrical resistance of the electrofluidic resistor. This small resistance change is then measured via the electrofluidic Wheatstone bridge to relate the measured voltage difference to the fluidic viscosity. The performance of this viscometer was first tested by flowing nitrogen gas with controllable pressures into the device. The relationship between measured voltage difference and input gas pressure was analyzed to be linear in the pressure range of 0–15 psi. Another test using pure water indicated good linearity between measured voltage difference and flow rate in the rate range of 20–100 μL/min. Viscosities of glycerol/water solutions with volume/volume (v/v) concentrations ranging from 0 to 30% were measured, and these values were close to those obtained using commercially available viscometers. In addition, the sample-pretreatment layer can be used to mix and/or dilute liquid samples to desired concentrations. Therefore, this microfluidic device has potential for measurements of fluidic viscosity in a fast, accurate, and high-throughput manner.

3D-Printed Capillary Circuits for Calibration-Free Viscosity Measurement of Newtonian and Non-Newtonian Fluids

Measuring viscosity is important for the quality assurance of liquid products, as well as for monitoring the viscosity of clinical fluids as a potential hemodynamic biomarker. However, conventional viscometers and their microfluidic counterparts typically rely on bulky and expensive equipment, and lack the ability for rapid and field-deployable viscosity analysis. To address these challenges, we describe 3D-printed capillary circuits (3D-CCs) for equipment- and calibration-free viscosity measurement of Newtonian and non-Newtonian fluids. A syringe, modified with an air chamber serving as a pressure buffer, generates and maintains a set pressure to drive the pressure-driven flows of test fluids through the 3D-CCs. The graduated fluidic chambers of the 3D-CCs serve as a flow meter, enabling simple measurement of the flow rates of the test fluids flowing through the 3D-CCs, which is readable with the naked eye. The viscosities of the test fluids can be simply calculated from the measured flow rates under a set pressure condition without the need for peripheral equipment and calibration. We demonstrate the multiplexing capability of the 3D-CC platform by simultaneously measuring different Newtonian-fluid samples. Further, we demonstrate that the shear-rate dependence of the viscosity of a non-Newtonian fluid can be analyzed simultaneously under various shear-rate conditions with the 3D-CC platform.

Simultaneous Measurement of Viscosity and Optical Density of Bacterial Growth and Death in a Microdroplet

Herein, we describe a novel method for the assessment of droplet viscosity moving inside microfluidic channels. The method allows for the monitoring of the rate of the continuous growth of bacterial culture. It is based on the analysis of the hydrodynamic resistance of a droplet that is present in a microfluidic channel, which affects its motion. As a result, we were able to observe and quantify the change in the viscosity of the dispersed phase that is caused by the increasing population of interacting bacteria inside a size-limited system. The technique allows for finding the correlation between the viscosity of the medium with a bacterial culture and its optical density. These features, together with the high precision of the measurement, make our viscometer a promising tool for various experiments in the field of analytical chemistry and microbiology, where the rigorous control of the conditions of the reaction and the monitoring of the size of bacterial culture are vital.

Recent progress of particle migration in viscoelastic fluids

Recently, research on particle migration in non-Newtonian viscoelastic fluids has gained considerable attention. In a viscoelastic fluid, three dimensional (3D) particle focusing can be easily realized in simple channels without the need for any external force fields or complex microchannel structures compared with that in a Newtonian fluid. Due to its promising properties for particle precise focusing and manipulation, this field has been developed rapidly, and research on the field has been shifted from fundamentals to applications. This review will elaborate the recent progress of particle migration in viscoelastic fluids, especially on the aspect of applications. The hydrodynamic forces on the micro/nano particles in viscoelastic fluids are discussed. Next, we elaborate the basic particle migration in viscoelasticity-dominant fluids and elasto-inertial fluids in straight channels. After that, a comprehensive review on the applications of viscoelasticity-induced particle migration (particle separation, cell deformability measurement and alignment, particle solution exchange, rheometry-on-a-chip and others) is presented; finally, we thrash out some perspectives on the future directions of particle migration in viscoelastic fluids.

Microfluidic bypass manometry: highly parallelized measurement of flow resistance of complex channel geometries and trapped droplets

Current lithography methods allow facile fabrication of microfluidic conduits where not only the shape of the bounding walls can be arbitrarily varied but also the internal conduit space can be laden with a variety of microstructures and wetting properties. This virtually infinite design space of microfluidic geometries brings in the challenge of how to quantify fluid resistance in a large number of microfluidic conduits, while maintaining operational simplicity. We report a versatile experimental technique referred to as microfluidic bypass manometry for measurement of pressure drop versus flow rate (ΔP-Q) relations in a parallelized manner. The technique involves introducing co-flowing laminar streams into a microfluidic network that contains a series of loops, where each loop is comprised of a test geometry and a bypass channel as a flow-rate sensing element. We optimize the network geometry and present operational considerations for microfluidic bypass manometry. To demonstrate the power of our technique, we used single-phase fluids and measured ΔP-Q relations simultaneously for forty test geometries ranging from linear to contraction-expansion to serpentine to pillar-laden microchannels. To expand the capabilities of the method, we measured ΔP-Q relations for similar-sized oil droplets trapped in microcavities where the cavity geometry spans from prisms of 3-10 sides to circular disks. We found in all cases, the ΔP-Q relation is nonlinear and the flow resistance of droplets is sensitive to confinement. At high flow rates, the drop resistance depends on the cavity geometry and is higher in a triangular prism compared to a circular disk. We compared the measured flow resistance of single-phase fluids and droplets in different microfluidic geometries to that from computational fluid dynamics simulations and found them to be in excellent agreement. Given the simplicity and versatility of the microfluidic bypass manometry method, we anticipate that it may find broad application in several areas including design of lab-on-chip devices, laminar drag reduction and mechanics of deformable particles.

A robust, portable and backflow-free micromixing device based on both capillary- and vacuum-driven flows

A robust, portable and backflow-free micromixing device using capillary-driven bypassing and syringe-assisted vacuum-driven pumping shows great promise for a variety of blood typing assays, agglutination-based assays and point-of-care or lab-on-a-chip testing applications.

A simple capillary viscometer based on the ideal gas law

We report a simple, inexpensive and user-friendly capillary viscometer based on the measurement of pressure drop in capillary tubing using the principle of ideal gas law. Viscosity is an important physical property of a fluid that provides molecular information of the fluid's behavior under flow conditions. Measuring viscosity, however, generally requires relatively large fluid volume samples and is expensive with commercial viscometers. Microfluidic viscometers at different levels of complexity can measure fluids at different flow rates with a small sample volume but the cost of commercially available microfluidic viscometers is still high. The reported capillary viscometer is cost-effective, uses small amounts of sample fluid and can measure viscosity under various shear rates. According to the Hagen–Poiseuille equation, the pressure drop of laminar flows in a capillary at a given flow rate is proportional to the viscosity of the fluid. When an enclosed air volume is connected to the upstream of the capillary, the pressure drop can be calculated with the change of the connected air volume, which is reflected by the displacement change of the air–liquid interface in the connecting capillary to the enclosed air volume. Based on these principles, the viscometer was assembled with readily accessible materials, and required no internal sensors or extensive programming. Measurements were successfully performed for five liquids including water, acetone, 2% fat milk, glycerin 30% and glycerin 40%. Except for acetone, the difference between measured and known viscosity was within 4% and highly consistent, well within the 13% uncertainty errors of readily accessible laboratory materials. Overall, the simple viscometer was easily assembled with low cost materials, was portable and accurate, and provided an alternative to expensive commercial viscometers. Finally, the simple capillary viscometer was a good outreach project for K-12 students to understand fluid behavior.

Fluid viscosity proportional to pressure drop in a capillary (L) was reflected by the air–fluid interface displacement (ΔL) to enclosed air.