Extensional flow behavior of aqueous guar gum derivative solutions by capillary breakup elongational rheometry (CaBER)

The extensional rheological properties of aqueous ionic carboxymethyl hydroxypropyl guar gum (CMHPG) and non-ionic hydroxypropyl guar gum (HPG) solutions between the semi-dilute solution state and the concentrated network solution state were investigated by capillary breakup elongational rheometry (CaBER). Carboxymethylated guar gum derivatives show an instable filament formation in deionized water. The ratio of elongational relaxation time λE over the shear relaxation time λS follows a power law of λE/λS∼(c · [η])(-2). The difference of the relaxation times in shear and elongation can be related to the loss of entanglements and superstructures in elongational flows at higher strains.

A Continuum Model for the Unfolding of von Willebrand Factor

von Willebrand Factor is a mechano-sensitive protein circulating in blood that mediates platelet adhesion to subendothelial collagen and platelet aggregation at high shear rates. Its hemostatic function and thrombogenic effect, as well as susceptibility to enzymatic cleavage, are regulated by a conformational change from a collapsed globular state to a stretched state. Therefore, it is essential to account for the conformation of the vWF multimers when modeling vWF-mediated thrombosis or vWF degradation. We introduce a continuum model of vWF unfolding that is developed within the framework of our multi-constituent model of platelet-mediated thrombosis. The model considers two interconvertible vWF species corresponding to the collapsed and stretched conformational states. vWF unfolding takes place via two regimes: tumbling in simple shear and strong unfolding in flows with dominant extensional component. These two regimes were demonstrated in a Couette flow between parallel plates and an extensional flow in a cross-slot geometry. The vWF unfolding model was then verified in several microfluidic systems designed for inducing high-shear vWF-mediated thrombosis and screening for von Willebrand Disease. The model predicted high concentration of stretched vWF in key regions where occlusive thrombosis was observed experimentally. Strong unfolding caused by the extensional flow was limited to the center axis or middle plane of the channels, whereas vWF unfolding near the channel walls relied upon the shear tumbling mechanism. The continuum model of vWF unfolding presented in this work can be employed in numerical simulations of vWF-mediated thrombosis or vWF degradation in complex geometries. However, extending the model to 3-D arbitrary flows and turbulent flows will pose considerable challenges.

Poiseuille and extensional flow small-angle scattering for developing structure-rheology relationships in soft matter systems

As the rheoscattering community has grown, so has the breadth of experiments both in terms of materials systems, and also in terms of flow types and analysis approaches. In this review, we seek to highlight important recent developments in rheoscattering that go beyond simple shear measurements. In particular, this review will focus on Poiseuille flow and extensional flow small-angle scattering and results from recent experiments that lead to the development of structure-function relationships in a wide variety of soft materials.

Local Concentrating, Not Shear Stress, That May Lead to Possible Instability of Protein Molecules During Syringe Injection: A Fluid Dynamic Study with Two-Phase Flow Model

A two-phase flow computational fluid dynamic (CFD) model was developed to study the hydrodynamic forces and the protein concentration changes of a protein solution in a syringe injector. Proteins were assumed to be nanosized solid spheres commensurate with their molecular weight and suspended in an aqueous environment, passing through the rapidly constricted sections of the syringe. Interaction between the solid and the liquid phase was taken into account, and four laminar flow cases were studied under the extensional flow. Profiles of pressure, velocity, and shear stress of the different cases were examined and compared. Hydrodynamic forces on a single protein particle were further analyzed. Our results indicate that the hydrodynamic forces are too small to affect significant conformational changes in proteins. The plunger rate showed limited impact on the distribution of protein particles inside the syringe. Nonetheless, the larger velocity gradient at the connection section of the hub toward the needle resulted in considerable accumulation of proteins. Such a concentrating effect may lead to protein aggregation and subsequent structural changes and will be examined in future studies.LAY ABSTRACT: Concentrated protein in the liquid form has become a preferred formulation strategy for delivering protein products, but it suffers from a high possibility of aggregation and precipitation, which may trigger a structural change and denaturation of the protein molecules and eventually cause the loss of the therapeutic functions of the protein products. To understand the effect of hydrodynamic forces on the change in local protein concentration in a syringe injector, we developed a two-phase flow computational fluid dynamic (CFD) model in this work. It was found that the local concentration of protein strongly depends on the velocity gradient of the fluid. A higher amount of proteins accumulates at the connection section of the hub toward the needle where the maximum velocity gradient is exhibited. The model results also imply a limited effect by hydrodynamic forces on possible unfolding of protein molecules. The local concentrating effect may enhance the possibility of molecular collision, resulting in aggregation and structural change.

Non-contact microfluidic mechanical property measurements of single apoptotic bodies

BACKGROUND

Cells exchange information by secreting micro- and nanosized extracellular vesicles (EVs), ranging from exosomes (30-100 nm) to apoptotic bodies (ABs, 1-5 μm). There is still much to understand about fundamental EV biological, physical, and chemical properties before clinical applications can be developed. EV mechanical properties have only been measured with atomic force microscopy (AFM) with its problematic adhesion and hard substrate effects. To understand EV mechanical behavior in less extreme mechanical conditions relevant to blood flow and many soft tissue environments, a non-contact measurement technique is needed.

METHODS

We measured the mechanical properties of single microscale ABs derived from human blood plasma using non-contact microfluidics. EVs were gently stretched in extensional flow, similar to a traditional tensile test, and a linear mechanical model was applied to estimate mechanical stiffnesses from the observed stretching.

RESULTS

The effective shear elastic modulus of ABs in non-contact flow conditions is approximately 5.6 ± 0.5 Pa, 7 orders of magnitude lower than previously reported AFM-measured biological exosome stiffnesses and 200 times smaller than suspended cells.

CONCLUSIONS

Apoptotic bodies are very soft in fluid environments and exhibit lower effective stiffnesses than suspended cells. By measuring ABs in a natural fluid environment and low-force regime without hard probes and surfaces, we achieved closer agreement with linear mechanical theory and therefore more accurate stiffness measurements.

GENERAL SIGNIFICANCE

AFM manufacturers and users should consider implementing new mechanical models to interpret AFM force indentation curves so that accurate extracellular vesicle mechanical properties can be extracted.

Influence of erythrocyte density on aggregability as a marker of cell age: Dissociation dynamics in extensional flow

Blood rheology and microcirculation are strongly influenced by red blood cell (RBC) aggregation. The aggregability of RBCs can vary significantly due to factors such as their mechanical and membrane surface properties, which are affected by cell aging in vivo. In this study, we investigate RBC aggregability as a function of their density, a marker of cell age and mechanical properties, by separating RBCs from healthy donors into different density fractions using Percoll density gradient centrifugation. We examine the dissociation rates of aggregates in a controlled medium supplemented with Dextran, employing an extensional flow technique based on hyperbolic microfluidic constrictions and image analysis, assisted by a convolutional neural network (CNN). In contrast to other techniques, our microfluidic experimental approach highlights the behavior of RBC aggregates in dynamic flow conditions relevant to microcirculation. Our results demonstrate that aggregate dissociation is strongly correlated with cell density and that aggregates formed from the denser fractions of RBCs are significantly more robust than those from the average cell population. This study provides insight into the effect of RBC aging in vivo on their mechanical properties and aggregability, underscoring the importance of further exploration of RBC aggregation in the context of cellular senescence and its potential implications for hemodynamics. Additionally, it suggests that this technique can complement existing methods for improved evaluation of RBC aggregability in health and disease.