Sepsis, a life-threatening disease caused by an unregulated immune response to infection, is a significant global health concern, contributing to approximately one fifth of all worldwide deaths. The chances of a patient surviving sepsis decrease by 8% for every hour before treatment is started, making rapid diagnosis critical to improving patient outcomes. Current sepsis diagnosis methods may take 24 to 48 hours, highlighting the need for faster alternatives. To address this challenge, a novel sepsis detection technique (Guillou et al. 2021) has been developed that evaluates the behaviour of leukocytes as they traverse a microfluidic cross-slot junction. The cell behaviour, specifically the degree of cell stretching due to elongational stresses in the junction, and the rate at which the cell oscillates when interacting with a vortex present in the junction outlets, are measured and used in statistical models to interpret the results. This approach enables the rapid and accurate determination of an individual's risk of developing sepsis within 10 minutes, significantly improving patient outcomes.

Despite the technique's proven ability to ability to correlate leukocyte behaviour in the microfluidic junction with patient sepsis risk, there is no theoretical understanding that connects the measured cell behaviour with the pathophysiological changes that differentiate septic and healthy cells. Previous studies have simulated the behaviour of rigid spheres in the microfluidic cross-slot and have shown that the device is highly sensitive to both the initial position and the size of particles entering the junction. (Kechagidis et al. 2022) However, leukocytes have a complex mechanical structure which influences the way in which they interact with fluids, especially when deforming due to fluid stresses. This means that to build a full picture of the behaviour of cells within the device, considerations of the leukocyte mechanical structure must be included.We have developed a detailed leukocyte model capable of simulating cell membrane viscoelastic and non-linear behaviour, cytoplasm viscosity, and non-homogeneous considerations such as cell nuclei. The cell model employs the finite element method for tracking cell membranes, coupled via the immersed boundary method to an extensively validated custom lattice-Boltzmann fluid solver.

In this talk, I will present the results of a series of simulations that examine the sensitivity of sepsis detection metrics in the microfluidic device to the parameters employed in the leukocyte model, such as the viscosity or stiffness of cells and their nuclei. By analysing the interaction of the flow field with the leukocyte and its internal structure, I will discuss the mechanisms underlying this sensitivity.

References
Guillou, L., Sheybani, R., Jensen, A.E., Di Carlo, D., Caffery, T.S., Thomas, C.B., Shah, A.M., Tse, H.T.K., O’Neal, H.R., 2021. Development and validation of a cellular host response test as an early diagnostic for sepsis. PLoS ONE. https://doi.org/10.1371/journal.pone.0246980

Kechagidis K., Owen B., Guillou L., Tse H., Di Carlo D., Krüger T., 2022. Numerical investigation of the dynamics of a rigid spherical particle in a vortical cross-slot flow at moderate inertia. BioRxiv https://doi.org/10.1101/2022.12.19.520995

Elucidating the lymph fluid pumping mechanism is of crucial significance towards a better understanding of the lymphatic system related diseases, such as lymphedema and cancer, and subsequently the development of more efficient treatments. To this end, we develop numerical framework to model the interplay between the lymph fluid flow, the contraction-relaxation of the lymphatic vessel walls and the lymphatic two-leaflet valves opening-closing. Our numerical method accounts for the biochemical signals of calcium ions (Ca²⁺) and the nitric oxide (NO) regulating the vessel contraction-relaxation. We use the lattice Boltzmann method to compute the lymph fluid flow and the chemical species mass transport, a spring network model for the lymphatic vessel walls and valves, and the immersed boundary method to implement the two-way coupling of the fluid-structure interaction. Results on the effects of various fluid, geometrical and mechanical properties on the mechanical performance of the valves, the vessel and the lymphatic pumping mechanism will be presented.

Thrombotic deposition is a major consideration in the development of implantable cardiovascular devices. The process of thrombosis is governed by both biochemical and mechanical considerations. The mechanical contribution has traditionally been thought to involve platelet activation due to sufficiently elevated shear stress levels in blood. Recent experimental evidence, however, suggests that beyond the effect of shear stress alone, localized changes in the blood shear rate, i.e. shear gradients, play a critical role in thrombogenesis by controlling the process of platelet deposition. The goal of the present work is to develop a predictive computational model of platelet plug formation that incorporates for the first time the effects of shear gradients and that can be used to assess the thrombotic burden of cardiovascular devices. To this end, we have developed a comprehensive model of platelet-mediated thrombogenesis which includes platelet transport in blood flow, platelet activation and aggregation induced by both biochemical and mechanical factors, and the kinetics and mechanics of platelet adhesion. Moreover, we also consider the effect of thrombus growth on the local fluid dynamics and how these alterations in the fluid dynamic environment feed back into the process of thrombogenesis.

The two-dimensional computational model was developed using the commercial multi-physics finite element solver COMSOL 5.6. The biochemical component of the model can be described by a set of coupled convection-diffusion-reaction equations that account for resting platelets, activated platelets, and various pro- and anti-thrombotic chemical species. Platelet adhesion to the blood vessel surface was modeled via a flux boundary condition. By incorporating a moving mesh for the blood vessel wall into the model, thrombus growth and consequent alterations in blood flow were modeled. The mechanical component of the model includes the induction of platelet activation by sufficiently high levels of shear stress and the promotion of platelet deposition in zones of negative shear gradients. Two physiologically and pathologically relevant scenarios were studied. The first scenario involves stenoses of varying severity where the mechanical contribution includes platelet activation in the contraction zone and platelet deposition in the expansion zone downstream of the stenosis. The second scenario involves a bifurcation where local disturbances in the flow field dictate the localization and extent of the computed thrombi.

The results demonstrate the model’s ability to provide the spatial and temporal evolution of a platelet plug within a flow field. The computed platelet plug size evolution was validated against experimental data from the literature. The results confirm the importance of considering both the chemical and mechanical contributions to platelet aggregation and underscore the importance of accounting for the effects of shear gradients. The developed model represents a potentially useful tool for the optimization of the design of the cardiovascular device flow path.

Interendothelial slits in the spleen fulfill the major physiological function of continuously filtering red blood cells (RBCs) from the bloodstream to remove abnormal and aged cells. To date, the process of extreme deformation and passage of 8 µm RBCs through 0.3-µm wide slits remains enigmatic. Should the slits increase their caliber during RBC passage as sometimes proposed in the literature? The values of the mechanical quantities selected during the passage of RBCs in the splenic slits and the associated dynamics, such as deformation mechanisms and transit times, remain poorly known today. Recent numerical and/or theoretical approaches have suggested that the spleen may play an important role in defining the surface area-to-volume ratio of the RBCs circulating in the microvascular system, but, so far, numerical approaches are not quantitatively validated by experiments and no experimental direct observation of RBCs flowing in splenic-like slits supports this hypothesis.

Here, we couple a unique in-vitro microfluidic technique to a multiscale in-silico RBC model that enables a quantitative approach of the mechanisms of passage of RBCs through interendothelial slits. The in-vitro technique allows the observation of the dynamics of passage of RBCs in slits of submicron width under tunable external stresses. The in-silico RBC model is implemented in dynamic and quasi-static versions. The dynamic version is integrated with a boundary integral simulation of surrounding flows to resolve the full fluid-cell interactions during this passage process, while the quasi-static version is done in commercial software ABAQUS. Agreement between experiments are simulations is remarkable.

We show that RBCs are capable of amazing extreme deformations allowing them to pass through rigid slits as narrow as 0.28 μm under a pressure drop of 500 Pa at body temperature, but not at room temperature. To achieve this tour de force, they must meet two requirements. Geometrically, the surface area-to-volume ratio of individual cells must be sufficient to form two tether-connected equal spheres. Mechanically, they must be able to locally unfold their spectrin cytoskeleton inside the slits. In contrast, activation of the mechanosensitive PIEZO1 channel is not required. The RBC transit time through slits scales with in-slit pressure drop and slit width to the -1 and -3 power, respectively. This transit dynamics is similar to that of a Newtonian fluid in a 2D Poiseuille flow, thus showing that it is controlled by the RBC cytoplasmic viscosity. We quantitatively predict transit times as a function of the cell mechanical properties and external parameters: pressure drop, slit size, and temperature. Altogether, our results clearly show that filtration through submicron-wide slits is possible without further slit opening. Furthermore, our coupled experimental/simulation approach is quantitative and addresses the critical need for in-vitro evaluation of splenic clearance of diseased or engineered RBCs for transfusion and drug delivery.

A.Moreau, F. Yaya, H. Lu, A. Surendranath, A. Charrier, B. Dehapiot, E. Helfer, A. Viallat, Z. Peng, bioRxiv 2023.01.10.523245; doi: https://doi.org/10.1101/2023.01.10.523245