Rheology of human blood plasma: viscoelastic versus Newtonian behavior

Size-Tunable Micro-Nano Liposomes: Enhanced Lung Targeting and Tumor Penetration for Combination Treatment of Lung Cancer

The inefficient delivery of nanocarriers and drug resistance seriously limit therapeutic effects of lung cancer. Here, a size-tunable micro-nano liposome system, PCAL@TM, is designed for targeted delivery of paclitaxel (PTX) and oxygen to lung tumors. PTX-loaded corosolic acid (CA) nano-liposomes (PCAL, 100 nm) are anchored to the surface of oxygenated perfluorotributylamine (TBA)-loaded multivesicular liposomes (TM, 10 µm) via the biotin-avidin interactions with matrix metalloproteinase-9 (MMP-9) cleavable linker. After intravenous administration to lung tumor-bearing mice, the distribution amount of PCAL@TM in the lungs is extremely higher than that in the liver and spleen. The MMP-9-sensitive PCAL@TM can decouple into nano-PCAL and micro-TM in tumors; while, TMs enable breaking into smaller vesicles under vascular pressure, and release oxygen leading to the downregulation of HIF-1α and platelet-activated TGF-β. Meanwhile, PCAL can penetrate deeply into tumor by the tumor-targeted-penetrable CA liposomes, to promote the reduction of inflammation levels and enhance PTX-induced immunogenic cell death (ICD). Together, these results lead to the reversals of chemoresistance and tumor immunosuppressive, achieving significant improvement in PTX chemotherapy and α-PD-1 immunotherapy. The PCAL@TM system presents a novel strategy to enhance the efficiency of nano-drug delivery and the outcome of combined therapy for lung tumor.

Validation of a Microfluidic Device Prototype for Cancer Detection and Identification: Circulating Tumor Cells Classification Based on Cell Trajectory Analysis Leveraging Cell-Based Modeling and Machine Learning

Microfluidic devices (MDs) present a novel method for detecting circulating tumor cells (CTCs), enhancing the process through targeted techniques and visual inspection. However, current approaches often yield heterogeneous CTC populations, necessitating additional processing for comprehensive analysis and phenotype identification. These procedures are often expensive, time-consuming, and need to be performed by skilled technicians. In this study, we investigate the potential of a cost-effective and efficient hyperuniform micropost MD approach for CTC classification. Our approach combines mathematical modeling of fluid-structure interactions in a simulated microfluidic channel with machine learning techniques. Specifically, we developed a cell-based modeling framework to assess CTC dynamics in erythrocyte-laden plasma flow, generating a large dataset of CTC trajectories that account for two distinct CTC phenotypes. Convolutional neural network (CNN) and recurrent neural network (RNN) were then employed to analyze the dataset and classify these phenotypes. The results demonstrate the potential effectiveness of the hyperuniform micropost MD design and analysis approach in distinguishing between different CTC phenotypes based on cell trajectory, offering a promising avenue for early cancer detection.

Measurement of fluid viscosity based on pressure-driven flow digital-printed microfluidics

Viscosity is an important characteristic of fluids. Microfluidics has shown significant advantages in the viscosity measurement of biopharmaceuticals, especially in meeting the needs of low sample volumes and accurately controlling microscale fluids. However, the viscosity chip of the traditional straight channel structure has limitations, and the processing technology is also facing challenges. In this study, a variable cross-section microfluidic chip structure was designed and successfully manufactured by photocuring 3D printing technology. A digital-printed (DP) microfluidic viscometer was realized by a pressure-driven flow combined with optical imaging. The device measures the change in sample viscosity with shear rate by recording the change in pressure and flow velocity with time. The whole experiment requires only 25 μl of reagents per time, and the single experiment time is less than 2 minutes, which not only reduces the consumption of samples dramatically, but also improves the efficiency of the experiment significantly. Compared with commercial viscometers, our measurements are accurate and capable of supporting non-Newtonian fluids. The proposed platform provides good cost-effectiveness and operational simplicity and lays the methodological foundation for viscosity measurements of more complex properties of fluids.

Rheology of CHO Cell Suspensions and Its Effects on High-Density Cultivation Process and Bioreactor Design

With the rapidly growing demand for monoclonal antibodies (mAbs) worldwide, optimizing the high-density and ultra-high-density cultivation processes of Chinese hamster ovary (CHO) cells has become crucial for enhancing production efficiency. Shear stress and mass transfer have always been the vital operating parameters for the bioreactor in creating a suitable microenvironment for cell growth and antibody production. However, researchers have not actively focused on the rheology of CHO cell suspensions and its impact on these parameters in bioreactors. The factors influencing the rheology of suspensions were first investigated in this study. The findings demonstrated that the shear-thinning behavior of the suspension was primarily affected by the cell volume fraction (Φ). As Φ increases, the shear-thinning behavior gradually weakened, and the viscosity increased. The Sisko model was used to characterize rheology, while computational fluid dynamics simulations evaluated its impact on bioreactor performance. The simulation results revealed that the rheology of the suspensions caused a multiple increase in shear stress and a 10%-40% decrease in the volumetric mass transfer coefficient (kLa) in the bioreactor. Therefore, the effects of rheology cannot be ignored while designing operating parameters. This study established empirical correlations among Pg/V, Vg, Φ, and kLa, thus delivering guidance for selecting appropriate operating parameters in high-density and ultra-high-density cell cultivation processes. The findings provide a scientific foundation for optimizing CHO cell cultivation processes and quantifying suitable microenvironment parameters for cell growth and production. They also offer novel ideas and strategies for scaling up and optimizing the structural parameters of bioreactors.

Electro-Elastic Instability and Turbulence in Electro-osmotic Flows of Viscoelastic Fluids: Current Status and Future Directions

The addition of even minute amounts of solid polymers, measured in parts per million (ppm), into a simple Newtonian fluid like water significantly alters the flow behavior of the resulting polymer solutions due to the introduction of fluid viscoelasticity. This viscoelastic behavior, which arises due to the stretching and relaxation phenomena of polymer molecules, leads to complex flow dynamics that are starkly different from those seen in simple Newtonian fluids under the same conditions. In addition to polymer solutions, many other fluids, routinely used in various industries and our daily lives, exhibit viscoelastic properties, including emulsions; foams; suspensions; biological fluids such as blood, saliva, and cerebrospinal fluid; and suspensions of biomolecules like DNA and proteins. In various microfluidic platforms, these viscoelastic fluids are often transported using electro-osmotic flows (EOFs), where an electric field is applied to control fluid movement. This method provides more precise and accurate flow control compared to pressure-driven techniques. However, several experimental and numerical studies have shown that when either the applied electric field strength or the fluid elasticity exceeds a critical threshold, the flow in these viscoelastic fluids becomes unstable and asymmetric due to the development of electro-elastic instability (EEI). These instabilities are driven by the normal elastic stresses in viscoelastic fluids and are not observed in Newtonian fluids under the same conditions, where the flow remains steady and symmetric. As the electric field strength or fluid elasticity is further increased, these instabilities can transition into a more chaotic and turbulent-like flow state, referred to as electro-elastic turbulence (EET). This article comprehensively reviews the existing literature on these EEI and EET phenomena, summarizing key findings from both experimental and numerical studies. Additionally, this article presents a detailed discussion of future research directions, emphasizing the need for further investigations to fully understand and harness the potential of EEI and EET in various practical applications, particularly in microscale flow systems where better flow control and increased transport rates are essential.

Design, fabrication, and application of bioengineering vascular networks based on microfluidic strategies

Vascularization is a critical component of tissue engineering research and is essential for enhancing the success rate of tissue construction and function. Over the past decade, researchers have explored various methods to construct in vitro vascular networks, including 3D printing, cell sphere technology, and microfluidics. Microfluidic technology has garnered significant attention due to its notable advantages in precision, controllability, flexibility, and applicability. It can be primarily classified into two modes: (i) the pre-designed mode, which involves creating vascular networks by pre-designing vascular channels and seeding endothelial cells, encompassing microfluidic chips and microfluidic spinning technologies; and (ii) the self-assembly mode, where cell spheres are fabricated using microfluidic technology and subsequently self-assemble into vascular networks. In this review, we first provide a brief overview of the normal physiological and pathological characteristics of vascular networks, followed by a discussion of the factors to be considered in designing in vitro vascular networks, and conclude with an examination of the classification of technologies for the preparation of microfluidic vascular networks and recent advancements. It is anticipated that in vitro vascular network models will soon be successfully applied in regenerative medicine and drug development.

Correlation Analysis Between Echinocytosis Stages and Blood Viscosity During Oxygenator Perfusion: An In Vitro Study

The study aimed to investigate the effect of red blood cell (RBC) morphology on oxygenator perfusion, focusing on stages of echinocytosis and their correlation with blood viscosity. A test circuit with an oxygenator and human RBC mixtures was used to induce changes in RBC shape by increasing sodium salicylate concentrations (0, 10, 20, 30, 60, and 120 mmol/L), while hematocrit, blood temperature, and anticoagulation were maintained. Blood viscosity was measured using a continuous blood viscosity monitoring system based on pressure-flow characteristics. Under a scanning electron microscope, the percentages of discocytes, echinocytes I-III, spheroechinocytes, and spherocytes were determined from approximately 400 cells per RBC sample. Early echinocytes, mainly discocytes and echinocytes I and II in the range of 0-30 mmol/L were predominant, resulting in a gradual increase in blood viscosity from 1.78 ± 0.12 to 1.94 ± 0.12 mPa s. At 60 mmol/L spherocytes emerged, and at 120 mmol/L, spheroidal RBCs constituted 50% of the population, and blood viscosity sharply rose to 2.50 ± 0.15 mPa s, indicating a 40% overall increase. In conclusion, the presence of spherocytes significantly increases blood viscosity, which may affect oxygenator perfusion.

Scaling Effects of the Weissenberg Number in Electrokinetic Oldroyd-B Fluid Flow Within a Microchannel

This study attempts to extend previous research on electrokinetic turbulence (EKT) in Oldroyd-B fluid by investigating the relationship between the Weissenberg number (W i $Wi$ ) and the second-order velocity structure function (S v 2 $S_v^2$ ) under applied electric fields. Inspired by Sasmal's demonstration in Sasmal (2022) of how heterogeneous zeta potentials induce turbulence above a criticalW i $Wi$ , we develop a mathematical framework linkingW i $Wi$ to turbulent phenomena. Our analysis incorporates recent findings on AC (Zhao & Wang, 2017) and DC (Zhao & Wang 2019) EKT, which have defined scaling laws for velocity and scalar structure functions in the forced cascade region. Our finding shows thatS v 2 ( l ) ∼ λ 1 4 / 5 l 2 / 5 $S_v^2(l) \sim \lambda _1^{4/5} l^{2/5}$ andS σ 2 ( l ) ∼ λ 1 - 2 / 5 l 4 / 5 $S_\sigma ^2(l) \sim \lambda _1^{-2/5} l^{4/5}$ , for a length scale l $l$ , andW i = λ 1 u l l $Wi = \frac{\lambda _1 u_l}{l}$ , whereu l = S u 2 ( l ) $u_l = \sqrt {S_u^2(l)}$ is a velocity fluctuations quantity andλ 1 $\lambda _1$ denotes the time relaxation parameter. This work establishes a positive correlation betweenλ 1 $\lambda _1$ and turbulent flow phenomena through a rigorous analysis of velocity structure functions, thereby offering a mathematical foundation for building the design and optimization of EKT-based microfluidic devices.

Validation of a Microfluidic Device Prototype for Cancer Detection and Identification: Circulating Tumor Cells Classification Based on Cell Trajectory Analysis Leveraging Cell-Based Modeling and Machine Learning

Microfluidic devices (MDs) present a novel method for detecting circulating tumor cells (CTCs), enhancing the process through targeted techniques and visual inspection. However, current approaches often yield heterogeneous CTC populations, necessitating additional processing for comprehensive analysis and phenotype identification. These procedures are often expensive, time-consuming, and need to be performed by skilled technicians. In this study, we investigate the potential of a cost-effective and efficient hyperuniform micropost MD approach for CTC classification. Our approach combines mathematical modeling of fluid-structure interactions in a simulated microfluidic channel with machine learning techniques. Specifically, we developed a cell-based modeling framework to assess CTC dynamics in erythrocyte-laden plasma flow, generating a large dataset of CTC trajectories that account for two distinct CTC phenotypes. Convolutional Neural Network (CNN) and Recurrent Neural Network (RNN) were then employed to analyze the dataset and classify these phenotypes. The results demonstrate the potential effectiveness of the hyperuniform micropost MD design and analysis approach in distinguishing between different CTC phenotypes based on cell trajectory, offering a promising avenue for early cancer detection.

Diagnostic potential of blood plasma longitudinal viscosity measured using Brillouin light scattering

Blood plasma viscosity (PV) is an established biomarker for numerous diseases. Measurement of the shear PV using conventional rheological techniques is, however, time consuming and requires significant plasma volumes. Here, we show that Brillouin light scattering (BLS) and angle-resolved spectroscopy measurements of the longitudinal PV from microliter-sized plasma volumes can serve as a proxy for the shear PV measured using conventional viscometers. This is not trivial given the distinct frequency regime probed and the longitudinal viscosity, a combination of the shear and bulk viscosity, representing a unique material property on account of the latter. We demonstrate this for plasma from healthy persons and patients suffering from different severities of COVID-19 (CoV), which has been associated with an increased shear PV. We further show that the additional information contained in the BLS-measured effective longitudinal PV and its temperature scaling can provide unique insight into the chemical constituents and physical properties of plasma that can be of diagnostic value. In particular, we find that changes in the effective longitudinal viscosity are consistent with an increased suspension concentration in CoV patient samples at elevated temperatures that is correlated with disease severity and progression. This is supported by results from rapid BLS spatial-mapping, angle-resolved BLS measurements, changes in the elastic scattering, and anomalies in the temperature scaling of the shear viscosity. Finally, we introduce a compact BLS probe to rapidly perform measurements in plastic transport tubes. Our results open a broad avenue for PV diagnostics based on the high-frequency effective longitudinal PV and show that BLS can provide a means for its implementation.

Single-cell magnetotaxis in mucus-mimicking polymeric solutions

Magnetotactic bacteria (MTB) are promising candidates for use as biomicrorobots in biomedical applications due to their motility, self-propulsion, and the ability to direct their navigation with an applied magnetic field. When in the body, the MTB may encounter non-Newtonian fluids such as blood plasma or mucus. However, their motility and the effectiveness of directed navigation in non-Newtonian fluids has yet to be studied on a single-cell level. In this work, we investigate motility of Magnetospirillum magneticum AMB-1 in three concentrations of polyacrylamide (PAM) solution, a mucus-mimicking fluid. The swimming speeds increase from 44.0 ± 13.6 μm/s in 0 mg/mL of PAM to 52.73 ± 15.6 μm/s in 1 mg/mL then decreases to 24.51 ± 11.7 μm/s in 2 mg/mL and 21.23 ± 10.5 μm/s in 3 mg/mL. This trend of a speed increase in low polymer concentrations followed by a decrease in speed as the concentration increases past a threshold concentration is consistent with other studies of motile, flagellated bacteria. Past this threshold concentration of PAM, there is a higher percentage of cells with an overall trajectory angle deviating from the angle of the magnetic field lines. There is also less linearity in the trajectories and an increase in reversals of swimming direction. Altogether, we show that MTB can be directed in polymer concentrations mimicking biological mucus, demonstrating the influence of the medium viscosity on the linearity of their trajectories which alters the effective path that could be predefined in Newtonian fluids when transport is achieved by magnetotaxis.

Impact of anti-coagulant choice on blood elongational behavior

Blood is a highly complex fluid with rheological properties that have a significant impact on various flow phenomena. In particular, it exhibits a non-Newtonian elongational viscosity that is comparable to polymer solutions. In this study, we investigate the effect of three different anticoagulants, namely EDTA (ethylene diamine tetraacetic acid), heparin, and citrate, on the elongational properties of both human and swine blood. We observe a unique two stage thinning process and a strong dependency of the characteristic relaxation time on the chosen anticoagulant, with the longest relaxation time and thus the highest elongational viscosity being found for the case of citrate. Our findings for the latter are consistent with the physiological values obtained from a dripping droplet of human blood without any anticoagulant. Furthermore, our study resolves the discrepancy found in the literature regarding the reported range of characteristic relaxation times, confirming that the elongational viscosity must be taken into account for a full rheological characterization of blood. These results have important implications for understanding blood flow in various physiological, pathological and technological conditions.

Roadmap for Clinical Translation of Mobile Microrobotics

Medical microrobotics is an emerging field to revolutionize clinical applications in diagnostics and therapeutics of various diseases. On the other hand, the mobile microrobotics field has important obstacles to pass before clinical translation. This article focuses on these challenges and provides a roadmap of medical microrobots to enable their clinical use. From the concept of a "magic bullet" to the physicochemical interactions of microrobots in complex biological environments in medical applications, there are several translational steps to consider. Clinical translation of mobile microrobots is only possible with a close collaboration between clinical experts and microrobotics researchers to address the technical challenges in microfabrication, safety, and imaging. The clinical application potential can be materialized by designing microrobots that can solve the current main challenges, such as actuation limitations, material stability, and imaging constraints. The strengths and weaknesses of the current progress in the microrobotics field are discussed and a roadmap for their clinical applications in the near future is outlined.

Optimization of Microfluidics for Point-of-Care Blood Sensing

Blood tests are widely used in modern medicine to diagnose certain illnesses and evaluate the overall health of a patient. To enable testing in resource-limited areas, there has been increasing interest in point-of-care (PoC) testing devices. To process blood samples, liquid mixing with active pumps is usually required, making PoC blood testing expensive and bulky. We explored the possibility of processing approximately 2 μL of whole blood for image flow cytometry using capillary structures that allowed test times of a few minutes without active pumps. Capillary pump structures with five different pillar shapes were simulated using Ansys Fluent to determine which resulted in the fastest whole blood uptake. The simulation results showed a strong influence of the capillary pump pillar shape on the chip filling time. Long and thin structures with a high aspect ratio exhibited faster filling times. Microfluidic chips using the simulated pump design with the most efficient blood uptake were fabricated with polydimethylsiloxane (PDMS) and polyethylene oxide (PEO). The chip filling times were tested with 2 μL of both water and whole blood, resulting in uptake times of 24 s for water and 111 s for blood. The simulated blood plasma results deviated from the experimental filling times by about 35% without accounting for any cell-induced effects. By comparing the flow speed induced by different pump pillar geometries, this study offers insights for the design and optimization of passive microfluidic devices for inhomogenous liquids such as whole blood in sensing applications.

Forced capillary wetting of viscoelastic fluids

HYPOTHESIS

Achieving rapid capillary wetting is highly desirable in nature and industries. Previous endeavors have primarily concentrated on passive wetting strategies through surface engineering. However, these approaches are inadequate for high-viscosity fluids due to the significant viscous resistance, especially for non-Newtonian fluids. In contrast, forced wetting emerges as a promising method to address the challenges associated with achieving rapid wetting of non-Newtonian fluids in capillaries.

EXPERIMENTS

To investigate the forced wetting behavior of viscoelastic fluids in capillaries, we employ Xanthan Gum (XG) aqueous solutions as target fluids with the storage modulus significantly exceeding the loss modulus. We utilize smooth glass capillaries connected to a syringe pump to achieve high moving speeds of up to 1 m/s.

FINDINGS

Our experiments reveal a significant distinction in the power-law exponent that governs the scaling relationship between the dynamic contact angle and velocity for viscoelastic fluids compared to Newtonian fluids. This exponent is considerably smaller and varies based on the concentration of viscoelastic fluids and the diameter of the capillaries. We suggest that the viscosity dominates the wetting dynamics of viscoelastic fluids, manifested by the contact line morphology-dependent behavior. This insight has significant implications for microfluidics and drug injectability.

Numerical analysis of blood flow through stenosed microvessels using a multi-phase model

Blood flow in arterioles have attracted considerable research attention due to their clinical implications. However, the fluid structure interaction between red blood cells and plasma in the blood poses formidable difficulty to the computational efforts. In this contribution, we seek to represent the red blood cells in the blood as a continuous non-Newtonian phase and construct a multi-phase model for the blood flow in microvessels. The methods are presented and validated using a channel with sudden expansion. And the resulting blood flow inside a stenosed microvessel is investigated at different inlet velocity amplitudes and hematocrits. It is show that the increase of both inlet velocity amplitude and inlet hematocrit leads to longer and thicker cell-rich layer downstream the stenosis. Besides, it is found that the maximum values of wall shear stress scales up with inlet velocity amplitudes and hematocrits. These results show the validity of the proposed computational model and provide helpful insights into blood flow behaviors inside stenosed vessels.

Microrheological characterisation of Cyanoflan in human blood plasma

Naturally occurring polysaccharidic biopolymers released by marine cyanobacteria are of great interest for numerous biomedical applications, such as wound healing and drug delivery. Such polymers generally exhibit high molecular weight and an entangled structure that impact the rheology of biological fluids. However, biocompatibility tests focus not so much on rheological properties as on immune response. In the present study, the rheological behaviour of native blood plasma as a function of the concentration of a cyanobacterium biopolymer is investigated via multiple particle tracking microrheology, which measures the Brownian motion of probes embedded in a sample, and cryogenic scanning electron microscope microstructural characterisation. We use Cyanoflan as the biopolymer of choice, and profit from our knowledge of its chemical structure and its exciting potential for biotechnological applications. A sol-gel transition is identified using time-concentration superposition and the power-law behaviour of the incipient network's viscoelastic response is observed in a variety of microrheological data. Our results point to rheology-based principles for blood compatibility tests by facilitating the assignment of quantitative values to specific properties, as opposed to more heuristic approaches.

An Improved In Vitro Blood-Brain Barrier Model for the Evaluation of Drug Permeability Using Transwell with Shear Stress

The development of drugs targeting the central nervous system (CNS) is challenging because of the presence of the Blood-Brain barrier (BBB). Developing physiologically relevant in vitro BBB models for evaluating drug permeability and predicting the activity of drug candidates is crucial. The transwell model is one of the most widely used in vitro BBB models. However, this model has limitations in mimicking in vivo conditions, particularly in the absence of shear stress. This study aimed to overcome the limitations of the transwell model using immortalized human endothelial cells (hCMEC/D3) by developing a novel dish design for an orbital shaker, providing shear stress. During optimization, we assessed cell layer integrity using trans-endothelial electrical resistance measurements and the % diffusion of lucifer yellow. The efflux transporter activity and mRNA expression of junctional proteins (claudin-5, occludin, and VE-cadherin) in the newly optimized model were verified. Additionally, the permeability of 14 compounds was evaluated and compared with published in vivo data. The cell-layer integrity was substantially increased using the newly designed annular shaking-dish model. The results demonstrate that our model provided robust conditions for evaluating the permeability of CNS drug candidates, potentially improving the reliability of in vitro BBB models in drug development.

Sedimentation of a Charged Spherical Particle in a Viscoelastic Electrolyte Solution

When a charged particle translates through an electrolyte solution, the electric double layer around it deforms in response to the fluid motion and creates an electric force opposite the direction of motion, decreasing the settling velocity. This is a multidisciplinary phenomenon that combines fluid mechanics and electrodynamics, differentiating it from the classical problem of an uncharged sedimenting particle. It has many applications varying from mechanical to biomedical, such as in drug delivery in blood through charged microparticles. Related studies so far have focused on Newtonian fluids, but recent studies have proven that many biofluids, such as human blood plasma, have elastic properties. To this end, we perform a computational study of the steady sedimentation of a spherical, charged particle in human blood plasma due to the centrifugal force. We used the Giesekus model to describe the rheological behavior of human blood plasma. Assuming axial symmetry, the governing equations include the momentum and mass balances, Poisson's equation for the electric field, and the species conservation. The finite size of the ions is considered through the local-density approximation approach of Carnahan-Starling. We perform a detailed parametric analysis, varying parameters such as the ζ potential, the size of the ions, and the centrifugal force exerted upon the particle. We observe that as the ζ potential increases, the settling velocity decreases due to a stronger electric force that slows the particle. We also conduct a parametric analysis of the relaxation time of the material to investigate what happens generally in viscoelastic electrolyte solutions and not only in human blood plasma. We conclude that elasticity plays a crucial role and should not be excluded from the study. Finally, we examine under which conditions the assumption of point-like ions gives different predictions from the Carnahan-Starling approach.

Using symmetry to control viscoelastic waves in pillar arrays

Solutions of macromolecules exhibit viscoelastic properties and unlike Newtonian fluids, they may break time-reversal symmetry at low Reynolds numbers resulting in elastic turbulence. Furthermore, under some conditions, instead of the chaotic turbulence, the result is large-scale waves in the form of cyclic spatial and temporal concentration variations, as has been shown for macromolecular DNA flowing in microfluidic pillar arrays. We here demonstrate how altering the symmetry of the individual pillars can be used to influence the symmetry of these waves. We control the extent of instabilities in viscoelastic flow by leveraging the effects of the symmetry of the pillars on the waves, demonstrating suppressed viscoelastic fluctuations with relevance for transport and sorting applications, or conversely opening up for enhanced viscoelasticity-mediated mixing. The onset of waves, which changes flow resistance, occurs at different Deborah numbers for flow in different directions through the array of triangular pillars, thus breaking the symmetry of the flow resistance along the device, opening up for using the occurrence of the waves to construct a fluidic diode.

Anomalous crystalline ordering of particles in a viscoelastic fluid under high shear

Addition of particles to a viscoelastic suspension dramatically alters the properties of the mixture, particularly when it is sheared or otherwise processed. Shear-induced stretching of the polymers results in elastic stress that causes a substantial increase in measured viscosity with increasing shear, and an attractive interaction between particles, leading to their chaining. At even higher shear rates, the flow becomes unstable, even in the absence of particles. This instability makes it very difficult to determine the properties of a particle suspension. Here, we use a fully immersed parallel plate geometry to measure the high-shear-rate behavior of a suspension of particles in a viscoelastic fluid. We find an unexpected separation of the particles within the suspension resulting in the formation of a layer of particles in the center of the cell. Remarkably, monodisperse particles form a crystalline layer which dramatically alters the shear instability. By combining measurements of the velocity field and torque fluctuations, we show that this solid layer disrupts the flow instability and introduces a single-frequency component to the torque fluctuations that reflects a dominant velocity pattern in the flow. These results highlight the interplay between particles and a suspending viscoelastic fluid at very high shear rates.

Exploitation of elasto-inertial fluid flow for the separation of nano-sized particles: Simulating the isolation of extracellular vesicles

High throughput and efficient separation/isolation of nanoparticles such as exosomes remain a challenge owing to their small size. Elasto-inertial approaches have a new potential to be leveraged because of the ability to achieve fine control over the forces that act on extremely small particles. That is, the viscoelasticity of fluid that helps carry biological particles such as extracellular vesicles (EVs) and cells through microfluidic channels can be tailored to optimize how different-sized particles move within the chip. In this contribution, we demonstrate through computational fluid dynamics (CFD) simulations the ability to separate nanoparticles with a size comparable to exosomes from larger spheres with physical properties comparable to cells and larger EVs. Our current design makes use of an efficient flow-focusing geometry at the inlet of the device in which two side channels deliver the sample, while the inner channel injects the sheath flow. Such flow configuration results in an efficient focusing of all the particles near the sidewalls of the channel at the inlet. By dissolving a minute amount of polymer in the sample and sheath fluid, the elastic lift force arises and the initially focused particle adjacent to the wall will gradually migrate toward the center of the channel. This results in larger particles experiencing larger elastic forces, thereby migrating faster toward the center of the channel. By adjusting the size and location of the outlets, nanoparticles comparable to the size of exosomes (30-100 nm) will be effectively separated from other particles. Furthermore, the influence of different parameters such as channel geometry, flow rate, and fluid rheology on the separation process is evaluated by computational analysis.

Cell-free layer development and spatial organization of healthy and rigid red blood cells in a microfluidic bifurcation

Bifurcations and branches in the microcirculation dramatically affect blood flow as they determine the spatiotemporal organization of red blood cells (RBCs). Such changes in vessel geometries can further influence the formation of a cell-free layer (CFL) close to the vessel walls. Biophysical cell properties, such as their deformability, which is impaired in various diseases, are often thought to impact blood flow and affect the distribution of flowing RBCs. This study investigates the flow behavior of healthy and artificially hardened RBCs in a bifurcating microfluidic T-junction. We determine the RBC distribution across the channel width at multiple positions before and after the bifurcation. Thus, we reveal distinct focusing profiles in the feeding mother channel for rigid and healthy RBCs that dramatically impact the cell organization in the successive daughter channels. Moreover, we experimentally show how the characteristic asymmetric CFLs in the daughter vessels develop along their flow direction. Complimentary numerical simulations indicate that the buildup of the CFL is faster for healthy than for rigid RBCs. Our results provide fundamental knowledge to understand the partitioning of rigid RBC as a model of cells with pathologically impaired deformability in complex in vitro networks.

Alginate/xanthan gum hydrogels as forensic blood substitutes for bloodstain formation and analysis

Understanding the behaviour of human blood outside of the body has important implications in forensic research, especially related to bloodstain pattern analysis (BPA). The design of forensic blood substitutes (FBSs) can provide many advantages, including the incorporation of multiple physiological components for use as safe and reliable materials for forensic applications. In this work, we present the design of synthetic alginate and xanthan gum-based hydrogels that contain electrosprayed microparticles (MPs) with and without crosslinked DNA. In addition to the MPs, the alginate/xanthan gum FBS materials include fillers to alter the physical appearance and fluid properties of the material. The optimized FBS consisted of alginate (1% w/v) and xanthan gum (5.0 × 10^-3% w/v), 2 mM CaCl₂, ferric citrate (0.5% w/v), magnesium silicate (0.25% w/v), Allura Red dye (2% w/v), 0.025% v/v Tween 20 and 9.5% v/v MPs. The FBS was tested in passive dripping experiments relevant to BPA scenarios at various impact angles. The spreading ratio (D_s/D₀) was determined for 90° stains made on a paper surface and compared to bovine blood where the FBS was shown to simulate accurate and predictable spreading behaviour. In addition, we simulated other common BPA scenarios (e.g., impact patterns) and evidence processing potential. The FBS could be swabbed, and the DNA could be extracted, amplified, and genotyped analogous to human blood evidence. A stability test was also conducted which revealed a shelf-life of over 4 weeks where the material remains relevant to human blood at physiological temperature.

Dynamics of an oscillating microbubble in a blood-like Carreau fluid

A numerical model for cavitation in blood is developed based on the Keller-Miksis equation for spherical bubble dynamics with the Carreau model to represent the non-Newtonian behavior of blood. Three different pressure waveforms driving the bubble oscillations are considered: a single-cycle Gaussian waveform causing free growth and collapse, a sinusoidal waveform continuously driving the bubble, and a multi-cycle pulse relevant to contrast-enhanced ultrasound. Parameters in the Carreau model are fit to experimental measurements of blood viscosity. In the Carreau model, the relaxation time constant is 5-6 orders of magnitude larger than the Rayleigh collapse time. As a result, non-Newtonian effects do not significantly modify the bubble dynamics but do give rise to variations in the near-field stresses as non-Newtonian behavior is observed at distances 10-100 initial bubble radii away from the bubble wall. For sinusoidal forcing, a scaling relation is found for the maximum non-Newtonian length, as well as for the shear stress, which is 3 orders of magnitude larger than the maximum bubble radius.

The role of plasma in the yield stress of blood

BACKGROUND

Yielding and shear elasticity of blood are merely discussed within the context of hematocrit and erythrocyte aggregation. However, plasma might play a substantial role due its own viscoelasticity.

OBJECTIVE

If only erythrocyte aggregation and hematocrit would determine yielding, blood of different species with comparable values would present comparable yield stresses.

METHODS

rheometry (SAOS: amplitude and frequency sweep tests; flow curves) of hematocrit-matched samples at 37°C. Brillouin Light Scattering Spectroscopy at 38°C.

RESULTS

Yield stress for pig: 20mPa, rat: 18mPa, and human blood: 9mPa. Cow and sheep blood were not in quasi-stationary state supporting the role of erythrocyte aggregation for the development of elasticity and yielding. However, pig and human erythrocytes feature similar aggregability, but yield stress of porcine blood was double. Murine and ruminant erythrocytes both rarely aggregate, but their blood behavior was fundamentally different. Pig plasma was shear-thinning and murine plasma was platelet-enriched, supporting the role of plasma for triggering collective effects and gel-like properties.

CONCLUSIONS

Blood behavior near zero shear flow is not based solely on erythrocyte aggregation and hematocrit, but includes the hydrodynamic interaction with plasma. The shear stress required to break down elasticity is not the critical shear stress for dispersing erythrocyte aggregates, but the shear stress required to fracture the entire assembly of blood cells within their intimate embedding.

Simple Assessment of Red Blood Cell Deformability Using Blood Pressure in Capillary Channels for Effective Detection of Subpopulations in Red Blood Cells

Assessment of red blood cell (RBC) deformability as a biomarker requires expensive equipment to induce and monitor deformation. In this study, we present a simple method for quantifying RBC deformability. We designed a microfluidic channel consisting of a micropillar channel and a coflowing channel connected in series. When blood (loading volume = 100 μL) was injected continuously into the device under constant pressure (1 bar), we monitored the boundary position of the blood and the reference flow in the coflowing channel. A decrease in the deformability of RBCs results in a growing pressure drop in the micropillar channel, which is mirrored by a decrease in blood pressure in the coflowing channel. Analysis of this temporal variation in blood pressure allowed us to define the clogging index (CI) as a new marker of RBC deformability. As a result of the analytical study and numerical simulation, we have demonstrated that the coflowing channel may serve as a pressure sensor that allows the measurement of blood pressure with accuracy. We have shown experimentally that a higher hematocrit level (i.e., more than 40%) does not have a substantial influence on CI. The CI tended to increase to a higher degree in glutaraldehyde-treated hardened RBCs. Furthermore, we were able to resolve the difference in deformability of RBCs between two different RBC density subfractions in human blood. In summary, our approach using CI provides reliable information on the deformability of RBCs, which is comparable to the readouts obtained by ektacytometry. We believe that our microfluidic device would be a useful tool for evaluating the deformability of RBCs, which does not require expensive instruments (e.g., high-speed camera) or time-consuming micro-PIV analysis.

A particulate blood analogue based on artificial viscoelastic blood plasma and RBC-like microparticles at a concentration matching the human haematocrit

There has been enormous interest in the production of fluids with rheological properties similar to those of real blood over the last few years. Application fields range from biomicrofluidics (microscale) to forensic science (macroscale). The inclusion of flexible microparticles in blood analogue fluids has been demonstrated to be essential in order to reproduce the behaviour of blood flow in these fields. Here, we describe a protocol to produce a whole human blood analogue composed of a proposed plasma analogue and flexible spherical microparticles that mimic the key structural attributes of RBCs (size and mechanical properties), at a concentration matching the human haematocrit (∼42% by volume). Polydimethylsiloxane (PDMS) flexible microparticles were used to mimic RBCs, whose capability to deform is tunable by means of the mixing ratio of the PDMS precursor. Their flow through glass micronozzles allowed us to find the appropriate mixing ratio of PDMS to have approximately the same Young's modulus (E) as that exhibited by real RBCs. Shear and extensional rheology and microrheology techniques were used to match the properties exhibited by human plasma and whole blood at body temperature (37 °C). Finally, we study the flow of our proposed fluid through a microfluidic channel, showing the in vitro reproduction of the multiphase flow effects taking place in the human microcirculatory system, such as the cell-free layer (CFL) and the Fåhræus-Lindqvist effect. A macroscale application in the field of forensic science is also presented, concerning the impact of our blood analogue droplets on a solid surface for bloodstain pattern analysis.

Microfluidics geometries involved in effective blood plasma separation

The last two decades witnessed a significant advancement in the field of diluted and whole blood plasma separation. This is one of the common procedures used to diagnose, cure and treat numerous acute and chronic diseases. For this separation purpose, various types of geometries of microfluidic devices, such as T-channel, Y-channel, trifurcation, constriction-expansion, curved/bend/spiral channels, a combination of any of the two geometries, etc., are being exploited, and this is detailed in this review article. The evaluation of the performance of such devices is based on the several parameters such as separation efficiency, flow rate, hematocrits, channel dimensions, etc. Thus, the current extensive review article endeavours to understand how particular geometry influences the separation efficiency for a given hematocrit. Additionally, a comparative analysis of various geometries is presented to demonstrate the less explored geometric configuration for the diluted and whole blood plasma separation. Also, a meta-analysis has been performed to highlight which geometry serves best to give a consistent separation efficiency. This article also presents tabulated data for various geometries with necessary details required from a designer's perspective such as channel dimensions, targeted component, studied range of hematocrit and flow rate, separation efficiency, etc. The maximum separation efficiency that can be achieved for a given hematocrits and geometry has also been plotted. The current review highlights the critical findings relevant to this field, state of the art understanding and the future challenges.

Direct separation and enumeration of CTCs in viscous blood based on co-flow microchannel with tunable shear rate: a proof-of-principle study

Circulating tumor cells (CTCs), which have extremely low density in whole blood, are an important indicator of primary tumor metastasis. Isolation and enumeration of these cells are critical for clinical applications. Separation of CTCs from massive blood cells without labeling and addition of synthetic polymers is challenging. Herein, a novel well-defined co-flow microfluidic device is presented and used to separate CTCs in viscous blood by applying both inertial and viscoelastic forces. Diluted blood without any synthetic polymer and buffer solution were used as viscoelastic fluid and Newtonian fluid, respectively, and they were co-flowed in the designed chip to form a sheath flow. The co-flow system provides the function of particle pre-focusing and creates a tunable shear rate region at the interface to adjust the migration of particles or cells from the sample solution to the buffer solution. Successful separation of CTCs from viscous blood was demonstrated and enumeration was also conducted by image recognition after separation. The statistical results indicated that a recovery rate of cancer cells greater than 87% was obtained using the developed method, which proved that the direct separation of CTCs from diluted blood can be achieved without the addition of any synthetic polymer to prepare viscoelastic fluid. This method holds great promise for the separation of cells in viscous biological fluid without either complicated channel structures or the addition of synthetic polymers.

Shear stress and intravascular pressure effects on vascular dynamics: two-phase blood flow in elastic microvessels accounting for the passive stresses

We study the steady hemodynamics in physiological elastic microvessels proposing an advanced fluid-structure interaction model. The arteriolar tissue is modeled as a two-layer fiber-reinforced hyperelastic material representing its Media and Adventitia layers. The constitutive model employed (Holzapfel et al. in J Elast 61:1-48, 2000) is parametrized via available data on stress-strain experiments for arterioles. The model is completed by simulating the blood/plasma flow in the lumen, using the thixotropic elasto-viscoplastic model in its core, and the linear Phan-Thien and Tanner viscoelastic model in its annular part. The Cell-Free Layer (CFL) and the Fåhraeus and Fåhraeus-Lindqvist effects are considered via analytical expressions based on experimental data (Giannokostas et al. in Materials (Basel) 14:367, 2021b). The coupling between tissue deformation and blood flow is achieved through the experimentally verified pressure-shear hypothesis (Pries et al. Circ Res 77:1017-1023, 1995). Our calculations confirm that the increase in the reference inner radius produces larger expansion. Also, by increasing the intraluminal pressure, the thinning of the walls is more pronounced and it may reach 40% of the initial thickness. Comparing our predictions with those in rigid-wall microtubes, we conclude that apart from the vital importance of vasodilation, there is an up to 25% reduction in wall shear stress. The passive vasodilation contributes to the decrease in the tissue stress fields and affects the hemodynamic features such as the CFL thickness, reducing the plasma layer when blood flows in vessels with elastic walls, in quantitative agreement with previous experiments. Our calculations verify the correctness of the pressure-shear hypothesis but not that of the Laplace law.

Upstream wall vortices in viscoelastic flow past a cylinder

We report a novel inertia-less, elastic flow instability for a viscoelastic, shear-thinning wormlike micellar solution flowing past a microcylinder in a channel with blockage ratio B_R = 2R/W = 0.5 and aspect ratio α = H/W ≈ 5, where R ≈ 100 μm is the cylinder radius, W is the channel width, and H is the channel height. The instability manifests upstream of the cylinder and changes form with increasing Weissenberg number over the range 0.5 ≲ Wi = Uλ/R ≲ 900, where U is the average flow velocity and λ is the terminal relaxation time of the fluid. Beyond a first critical Wi, the instability begins as a bending of the streamlines near the upstream pole of the cylinder that breaks the symmetry of the flow. Beyond a second critical Wi, small, time-steady, and approximately symmetric wall-attached vortices form upstream of the cylinder. Beyond a third critical Wi, the flow becomes time dependent and pulses with a characteristic frequency commensurate with the breakage timescale of the wormlike micelles. This is accompanied by a breaking of the symmetry of the wall-attached vortices, where one vortex becomes considerably larger than the other. Finally, beyond a fourth critical Wi, a vortex forms attached to the upstream pole of the cylinder whose length fluctuates in time. The flow is highly time dependent, and the cylinder-attached vortex and wall-attached vortices compete dynamically for space and time in the channel. Our results add to the rapidly growing understanding of viscoelastic flow instabilities in microfluidic geometries.

Simulation of Mechanical Heart Valve Dysfunction and the Non-Newtonian Blood Model Approach

The mechanical heart valve (MHV) is commonly used for the treatment of cardiovascular diseases. Nonphysiological hemodynamic in the MHV may cause hemolysis, platelet activation, and an increased risk of thromboembolism. Thromboembolism may cause severe complications and valve dysfunction. This paper thoroughly reviewed the simulation of physical quantities (velocity distribution, vortex formation, and shear stress) in healthy and dysfunctional MHV and reviewed the non-Newtonian blood flow characteristics in MHV. In the MHV numerical study, the dysfunction will affect the simulation results, increase the pressure gradient and shear stress, and change the blood flow patterns, increasing the risks of hemolysis and platelet activation. The blood flow passes downstream and has obvious recirculation and stagnation region with the increased dysfunction severity. Due to the complex structure of the MHV, the non-Newtonian shear-thinning viscosity blood characteristics become apparent in MHV simulations. The comparative study between Newtonian and non-Newtonian always shows the difference. The shear-thinning blood viscosity model is the basics to build the blood, also the blood exhibiting viscoelastic properties. More details are needed to establish a complete and more realistic simulation.

Combined electromechanically driven pulsating flow of nonlinear viscoelastic fluids in narrow confinements

Controlled microscale transport is at the core of many scientific and technological advancements, including medical diagnostics, separation of biomolecules, etc., and often involves complex fluids. One of the challenges in this regard is to actuate flows at small scales in an energy efficient manner, given the strong viscous forces opposing fluid motion. We try to address this issue here by probing a combined time-periodic pressure and electrokinetically driven flow of a viscoelastic fluid obeying the simplified linear Phan-Thien-Tanner model, using numerical as well as asymptotic tools, in view of the fact that oscillatory fields are less energy intensive. We establish that the interplay between oscillatory electrical and mechanical forces can lead to complex temporal mass flow rate variations with short-term bursts and peaks in the flow rate. We further demonstrate that an oscillatory pressure gradient or an electric field, in tandem with another steady actuating force can indeed change the net throughput significantly-a paradigm that is not realized in Newtonian or other simpler polymeric liquids. Our results reveal that the extent of augmentation in the flow rate strongly depends on the frequency of the imposed actuating forces along with their waveforms. We also evaluate the streaming potential resulting from an oscillatory pressure-driven flow and illustrate that akin to the volume throughput, the streaming potential also shows complex temporal variations, while its time average gets augmented in the presence of a time-periodic pressure gradient in a nonlinear viscoelastic medium.

Passive high-frequency microrheology of blood

Indicative of various pathologies, blood properties are under intense scrutiny. The hemorheological characteristics are traditionally gauged by bulk, low-frequency indicators that average out critical information about the complex, multi-scale, and multi-component structure. In particular, one cannot discriminate between the erythrocytes contribution to global rheology and the impact of plasma. Nevertheless, in their fast stochastic movement, before they encounter each other, the erythrocytes probe the subtle viscoelasticity of their protein-rich environment. Thus, if these short time scales can be resolved experimentally, the plasma properties could be determined without having to separate the blood components; the blood is practically testing itself. This microrheological description of blood plasma provides a direct link between the composition of whole blood and its coagulability status. We present a parametric model for the viscoelasticity of plasma, which is probed by the erythrocytes over frequency ranges of kilohertz in a picoliter-sized volume. The model is validated both in vitro, using artificial hemo-systems where the composition is controlled, as well as on whole blood where continuous measurements provide real-time information. We also discuss the possibility of using this passive microrheology as an in vivo assay for clinically relevant situations where the blood clotting condition must be observed and managed continuously for diagnosis or during therapeutic procedures at different stages of hemostatic and thrombotic processes.

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.

A simple yet efficient approach for electrokinetic mixing of viscoelastic fluids in a straight microchannel

Many complex fluids such as emulsions, suspensions, biofluids, etc., are routinely encountered in many micro and nanoscale systems. These fluids exhibit non-Newtonian viscoelastic behaviour instead of showing simple Newtonian one. It is often needed to mix such viscoelastic fluids in small-scale micro-systems for further processing and analysis which is often achieved by the application of an external electric field and/or using the electroosmotic flow phenomena. This study proposes a very simple yet efficient  strategy to mix such viscoelastic fluids based on extensive numerical simulations. Our proposed setup consists of a straight microchannel with small patches of constant wall zeta potential, which are present on both the top and bottom walls of the microchannel. This heterogeneous zeta potential on the microchannel wall generates local electro-elastic instability and electro-elastic turbulence once the Weissenberg number exceeds a critical value. These instabilities and turbulence, driven by the interaction between the elastic stresses and the streamline curvature present in the system, ultimately lead to a chaotic and unstable flow field, thereby facilitating the mixing of such viscoelastic fluids. In particular, based on our proposed approach, we show how one can use the rheological properties of fluids and associated fluid-mechanical phenomena for their efficient mixing even in a straight microchannel.

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.

Oscillative Trapping of a Droplet in a Converging Channel Induced by Elastic Instability

Permanent trapping of an oscillating, nonwetting droplet is observed in a converging-diverging microchannel when aqueous, viscoelastic fluids are injected. Classical theories based on the balance between capillary and viscous forces suggest that the droplet size should decrease with increasing flow rates of a displacing Newtonian fluid, and the droplet should be completely displaced at high injection rates. However, droplets in viscoelastic fluids cannot be removed by increasing flow rates due to the oscillation. The oscillation amplitude linearly increases with the Deborah number (De), which further inhibits the droplet's passing through the constriction, "permanently." Our microfluidic experiments show that the onset of oscillation is determined by a critical De, which is near 1. We derive a linear relationship for the trapped droplet length with Ec^{1/3}, where Ec is the elastocapillary number, by introducing the elastic force into the force balance in addition to the capillary and viscous forces.

The effect of rigid cells on blood viscosity: linking rheology and sickle cell anemia

Sickle cell anemia (SCA) is a disease that affects red blood cells (RBCs). Healthy RBCs are highly deformable objects that under flow can penetrate blood capillaries smaller than their typical size. In SCA there is an impaired deformability of some cells, which are much stiffer and with a different shape than healthy cells, and thereby affect regular blood flow. It is known that blood from patients with SCA has a higher viscosity than normal blood. However, it is unclear how the rigidity of cells is related to the viscosity of blood, in part because SCA patients are often treated with transfusions of variable amounts of normal RBCs and only a fraction of cells will be stiff. Here, we report systematic experimental measurements of the viscosity of a suspension varying the fraction of rigid particles within a suspension of healthy cells. We also perform systematic numerical simulations of a similar mixed suspension of soft RBCs, rigid particles, and their hydrodynamic interactions. Our results show that there is a rheological signature within blood viscosity to clearly identify the fraction of rigidified cells among healthy deformable cells down to a 5% volume fraction of rigidified cells. Although aggregation of RBCs is known to affect blood rheology at low shear rates, and our simulations mimic this effect via an adhesion potential, we show that such adhesion, or aggregation, is unlikely to provide a physical rationalization for the viscosity increase observed in the experiments at moderate shear rates due to rigidified cells. Through numerical simulations, we also highlight that most of the viscosity increase of the suspension is due to the rigidity of the particles rather than their sickled or spherical shape. Our results are relevant to better characterize SCA, provide useful insights relevant to rheological consequences of blood transfusions, and, more generally, extend to the rheology of mixed suspensions having particles with different rigidities, as well as offering possibilities for developments in the field of soft material composites.

Recent advances in blood rheology: a review

Due to the potential impact on the diagnosis and treatment of various cardiovascular diseases, work on the rheology of blood has significantly expanded in the last decade, both experimentally and theoretically. Experimentally, blood has been confirmed to demonstrate a variety of non-Newtonian rheological characteristics, including pseudoplasticity, viscoelasticity, and thixotropy. New rheological experiments and the development of more controlled experimental protocols on more extensive, broadly physiologically characterized, human blood samples demonstrate the sensitivity of aspects of hemorheology to several physiological factors. For example, at high shear rates the red blood cells elastically deform, imparting viscoelasticity, while at low shear rates, they form "rouleaux" structures that impart additional, thixotropic behavior. In addition to the advances in experimental methods and validated data sets, significant advances have also been made in both microscopic simulations and macroscopic, continuum, modeling, as well as novel, multiscale approaches. We outline and evaluate the most promising of these recent developments. Although we primarily focus on human blood rheology, we also discuss recent observations on variations observed across some animal species that provide some indication on evolutionary effects.

Droplet impact of blood and blood simulants on a solid surface: Effect of the deformability of red blood cells and the elasticity of plasma

The impact of blood droplets onto a solid wall is of great importance for bloodstain pattern analysis in forensic science. Previous studies suggest that the behaviour of impacting blood is similar to that of a Newtonian fluid, which has a shear viscosity equivalent to that of blood at high shear rates. To understand this important fact, we conducted comparative experiments of droplet impact on a glass surface using whole blood and three solutions with a shear viscosity similar to that of blood. Specifically, we used dog's whole blood (deformable red blood cells dispersed in plasma, WB), plasma with non-deformable resin particles (PwP), glycerol and water with resin particles (GWwP), and a commercial blood simulant (hard particles dispersed in a water-based Newtonian solution, BS). Ranges of Reynolds and Weber numbers in our experiments were 550<Re<1700 and 120<We<860, respectively. Side and bottom views of droplet impact were simultaneously recorded by two high-speed cameras. The spreading radius of the impacting WB droplet in our experiments agreed well with that of Newtonian fluids with viscosity similar to that of WB at high shear rates. Splashing droplets of WB and Newtonian fluids form finger structures (finger-splashing). Although PwP has a viscosity similar to that of WB at high shear rates, an impacting PwP droplet exhibited typical characteristics of impacting suspension droplets, that is, a reduced spreading radius and splashing with ejection of particles. Such significant differences between impacting droplets of PwP and WB indicates that the high deformability of red blood cells in WB plays a crucial role in the Newtonian-like behaviour of blood droplets on impact. The finger-splashing of PwP and GWwP exhibited no significant difference, indicating that the effect of plasma elasticity on finger-splashing is negligible. Importantly, the impacting BS droplet behaved quite differently from WB in both spreading and splashing. Our results imply that the use of deformable particles rather than hard particles in a BS is essential for mimicking blood droplet impact.

Measuring the density and viscosity of culture media for optimized computational fluid dynamics analysis of in vitro devices

Culture medium is frequently modelled as water in computational fluid dynamics (CFD) analysis of in vitro culture systems involving flow, such as bioreactors and organ-on-chips. However, culture medium can be expected to have different properties to water due to its higher solute content. Furthermore, cellular activities such as metabolism and secretion of ECM proteins alter the composition of culture medium and therefore its properties during culture. As these properties directly determine the hydromechanical stimuli exerted on cells in vitro, these, along with any changes during culture must be known for CFD modelling accuracy and meaningful interpretation of cellular responses. In this study, the density and dynamic viscosity of DMEM and RPMI-1640 media supplemented with typical concentrations of foetal bovine serum (0, 5, 10 and 20% v/v) were measured to serve as a reference for computational design analysis. Any changes in the properties of medium during culture were also investigated with NCI-H460 and HN6 cell lines. The density and dynamic viscosity of the media increased proportional to the % volume of added foetal bovine serum (FBS). Importantly, the viscosity of 5% FBS-supplemented RPMI-1640 was found to increase significantly after 3 days of culture of NCI-H460 and HN6 cell lines, with distinct differences between magnitude of change for each cell line. Finally, these experimentally-derived values were applied in CFD analysis of a simple microfluidic device, which demonstrated clear differences in maximum wall shear stress and pressure between fluid models. Overall, these results highlight the importance of characterizing model-specific properties for CFD design analysis of cell culture systems.

Extensional Magnetorheology of Viscoelastic Human Blood Analogues Loaded with Magnetic Particles

This study represents a pioneering work on the extensional magnetorheological properties of human blood analogue fluids loaded with magnetic microparticles. Dynabeads M-270 particles were dispersed in Newtonian and viscoelastic blood analogue fluids at 5% wt. Capillary breakup experiments were performed, with and without the influence of an external magnetic field aligned with the flow direction. The presence of the particles increased the viscosity of the fluid, and that increment was larger when embedded within a polymeric matrix. The application of an external magnetic field led to an even larger increment of the viscosity of the working fluids, as the formation of small aggregates induced an increment in the effective volume fraction of particles. Regarding the liquid bridge stability, the Newtonian blood analogue fluid remained as a Newtonian liquid exhibiting a pinch-off at the breakup time in any circumstance. However, in the case of the viscoelastic blood analogue fluid, the presence of the particles and the simultaneous application of the magnetic field enhanced the formation of the beads-on-a-string structure, as the Ohnesorge number remained basically unaltered, whereas the time of the experiment increased due to its larger viscosity, which resulted in a decrease in the Deborah Number. This result was confirmed with fluids containing larger concentrations of xanthan gum.

Small-volume extensional rheology of concentrated protein and protein-excipient solutions

Limited studies measure extensional rheology in protein solutions due to volume constraints and measurement challenges. We developed a small-volume, dripping-onto-substrate (DoS) extensional rheology device to measure the capillary thinning of protein and protein-excipient solutions via DoS for the first time. Ovalbumin (OVA) was used as a model system, examined via DoS both with and without excipient poloxamer 188 (P188). Water and dilute OVA break apart rapidly and demonstrate inertiocapillary (IC) thinning behavior, where longer breakup times in OVA can be attributed to lower surface tension. Further increasing OVA content leads to longer breakup times and deviations from IC thinning at the start of thinning, however, no evidence of elastic behavior is observed. P188 more effectively lowers the droplet surface tension than OVA, transitioning from IC behavior in dilute solution to weakly elastic behavior at higher concentrations. Combined protein/excipient formulations act synergistically at low concentrations, where breakup times are identical to those of the individual components despite the higher total concentration. However concentrated protein/excipient formulations exhibit elasticity, where extensional rheology parameters depend on P188 content and total concentration. These findings imply that excipients intended to stabilize proteins in shear flow can cause undesirable behavior in extensional flows like injection.

Using the likelihood ratio in bloodstain pattern analysis

There is an apparent paradox that the likelihood ratio (LR) approach is an appropriate measure of the weight of evidence when forensic findings have to be evaluated in court, while it is typically not used by bloodstain pattern analysis (BPA) experts. This commentary evaluates how the scope and methods of BPA relate to several types of evaluative propositions and methods to which LRs are applicable. As a result of this evaluation, we show how specificities in scope (BPA being about activities rather than source identification), gaps in the underlying science base, and the reliance on a wide range of methods render the use of LRs in BPA more complex than in some other forensic disciplines. Three directions are identified for BPA research and training, which would facilitate and widen the use of LRs: research in the underlying physics; the development of a culture of data sharing; and the development of training material on the required statistical background. An example of how recent fluid dynamics research in BPA can lead to the use of LR is provided. We conclude that an LR framework is fully applicable to BPA, provided methodic efforts and significant developments occur along the three outlined directions.

Bifurcations in flows of complex fluids around microfluidic cylinders

Flow around a cylinder is a classical problem in fluid dynamics and also one of the benchmarks for testing viscoelastic flows. The problem is of wide relevance to understanding many microscale industrial and biological processes and applications, such as porous media and mucociliary flows. In recent years, we have developed model microfluidic geometries consisting of very slender cylinders fabricated in glass by selective laser-induced etching. The cylinder radius is small compared with the channel width, which allows the effects of the stagnation points in the flow to dominate over the effects of squeezing between the cylinder and the channel walls. Furthermore, the cylinders are contained in high aspect ratio microchannels that render the flow field approximately two-dimensional (2D) and therefore conveniently permit comparison between experiments and 2D numerical simulations. A number of different viscoelastic fluids including wormlike micellar and various polymer solutions have been tested in our devices. Of particular interest to us has been the occurrence of a striking, steady-in-time, flow asymmetry that occurs for certain non-Newtonian fluids when the dimensionless Weissenberg number (quantifying the importance of elastic over viscous forces in the flow) increases above a critical value. In this perspective review, we present a summary of our key findings related to this novel flow instability and present our current understanding of the mechanism for its onset and growth. We believe that the same fundamental mechanism may also underlie some important non-Newtonian phenomena observed in viscoelastic flows around particles, drops, and bubbles, or through geometries composed of multiple bifurcation points such as cylinder arrays and other porous media. Knowledge of the instability we discuss will be important to consider in the design of optimally functional lab-on-a-chip devices in which viscoelastic fluids are to be used.

Electrokinetics of polymeric fluids in narrow rectangular confinements

The flow of polymeric liquids in narrow confinements with a rectangular cross section, in the presence of electrical double layers is analyzed here. Our analysis is motivated by the fact that many of the previous studies on the flow of complex fluids tend to focus on highly idealized parallel plate channels, which are markedly different from the rectangular ducts, used in many experiments and devices. We consider the combined electroosmotic and pressure driven flows as well as the streaming potential resulting from a mechanically driven flow. We use two distinct constitutive relations to model the polymeric liquids, namely the simplified exponential Phan-Thien-Tanner (sePTT) model and the Giesekus model, both of which are non-linear viscoelastic models, capable of capturing the shear thinning behavior. We establish that the applied electric field may have a strong influence on the overall flow rate, which rapidly increases with the field strength as well as the extent of viscoelasticity of the fluid. Viscoelasticity and shear thinning behavior also enhance the streaming potential by several fold as compared to a Newtonian medium. We demonstrate that the aspect ratio of a channel has a bigger influence on the net throughput and the streaming potential, when the extent of viscoelasticity is relatively large. We illustrate that for sePTT fluids, the flow is strictly unidirectional, while for Giesekus fluids, secondary flows are inevitably present on account of their non-zero second normal stress coefficient. Although the electric field does not change the overall patterns of these secondary flows, their magnitude does depend on the imposed field strength for combined flows.