Session R08: Biofluids: General I

EVAPORATION OF BACTERIA-LADEN SURROGATE RESPIRATORY FLUID DROPLETS: SESSILE MODE V/S LEVITATED MODE

The mayhem caused by Covid-19 proves that intensive research is necessary to decode the infection pathways of respiratory ailments involving both bacteria and viruses as pathogens. Transmission of infectious bacteria or viruses occurs via four major modes, namely direct contact, indirect contact (fomites), large droplets, or fine aerosols. A droplet is an integral part of aerosol spray and exhalations from the host. Therefore, it is a holistic study to understand the desiccation dynamics of infected droplets coupled with the infection study of the crystal/settled droplet. Given the complexity of the experiment with actual bacteria/viruses in respiratory fluid, such studies have rarely been attempted. In our study, we have experimentally compared the contact-free environment precipitates (via the levitated mode of evaporation) and the precipitates on hydrophilic substrates (via the sessile mode of evaporation), maintaining the same initial volume of the bacterial solution, aiming to deduce the effect of evaporation on bacterial survival and its virulence. The study examines mass transport, the deposition pattern of bacteria in the precipitates, and their survival and virulence. The desiccation dynamics play a pivotal role in bacterial survival and virulence. The evaporation mode influences the bacteria's viability within the droplet, with the sessile precipitate harboring more non-viable, aged, and dead bacteria than the levitated case.   

Extracellular flow patterns of Red Blood Cells in microchannels under time-dependent shear rates

Red Blood Cell (RBC) is highly flexible with convoluted morphological deformations. These characteristics can be used to provide valuable insights into their membranes' mechanical properties and facilitate the identification of pathological changes. This study explores the dynamics of RBC, which is subjected to oscillatory flows in microchannels. To achieve this, we employed numerical simulations with a hybrid continuum-particle approach. The Dissipative Particle Dynamics (DPD) method is used to model the cell membrane and cytosol fluid. The Immersed Boundary Method (IBM) is employed to represent the interaction between the membrane surface and the blood plasma (an incompressible fluid). Our results demonstrate the controllability of the morphological modes of the RBC membrane by the transient shear rates. In particular, adjusting the oscillatory flow waveform at the channel inlet can control the transient dynamics of the RBC and induce the axial and lateral migration of the cell. Our findings suggest that flow oscillation can serve as a viable method for cell manipulation, potentially offering significant implications for cell separation and sorting.   

An experimental and numerical investigation of particle dynamics in idealized aorta models grafted with a ventricular assist device outflow

Understanding the dynamics of particle-laden flows in the intricate geometry of aorta that is characterized by high Reynolds numbers and large Stokes numbers poses a significant challenge to both physical and numerical studies. Building upon our previous work, we aimed to verify and generalize our observations in four patient-specific aorta models to a broader range of anatomies. Based on a critical review of healthy human aortic morphology and dimensions, we developed two idealized computer-aided design (CAD) models. While the geometries are simplified, they together are expected to capture the major anatomic features of the general population. Each CAD model is grafted with a heart-assist pump outflow to study the transport of inertial particles injected at its inlet. A set of experimental and computational studies is conducted to investigate the Lagrangian trajectories of beads ranging from 0.4 to 1.2 mm at two physiological flow rates. The particle tracking velocimetry (PTV) measurements in thin-wall 3D-printed models are complemented with computational fluid dynamics (CFD) simulations using the same particle sizes, and inflow and outflow boundary conditions as in the experiments. The mapping of the particle fate and identifying the influencing factors provides an insight into an optimal graft candidate that can be used to improve the clinical outcome such as reducing the risk of ischemic stroke.   

Integral Parameter Analysis Using 4D Flow MRI Data for Carotid Artery Stenosis

As cardiovascular and cerebrovascular diseases become more prevalent, the value of hemodynamic studies for predicting disease progression is increasingly recognized. These studies often use CFD and 4D-flow MRI to calculate critical parameters such as wall shear stress, oscillatory shear index, and relative residence time. However, these methods present challenges: CFD can yield inconsistent velocity information due to variable boundary conditions, while 4D-flow MRI suffers from low resolution, noise, and artifacts leading to inaccurate results, especially for differential parameters such as WSS and OSI. We propose a novel approach using integral hemodynamic parameters for more accurate disease analysis instead of relying solely on differential parameters. We prepared carotid artery phantoms and implemented patient-specific pulsatile flow to measure the phantom flows using 4D-flow MRI with relatively high spatial resolution. As a result, we computed integral hemodynamic parameters of stasis and recirculation and compared them against the conventional parameters of WSS, OSI, and RRT. Additionally, abnormal regions were redefined for the integral analysis, and the results were in good agreement with those obtained from the differential analysis. Our findings reveal that the integral analysis method demonstrates substantial alignment with the conventional approach, introducing a promising direction for enhanced, more reliable hemodynamic studies in carotid artery stenosis.   

Investigation of frequency coupling in restricted pulsatile flows modeling aortic stenosis

A quantitative understanding of the frequencies present in sound signals produced by aortic stenosis is crucial to assess the severity of the stenosis. Inspired by this problem, a parametric study is performed to correlate changes in the acoustic spectrum with the restriction percentage in a pulsatile flow. To simulate some of the conditions in an aortic valve, the pulsing frequency is set to 70 beats per minute and the geometric shape of the model valve opening mimics an open tricuspid valve. The dynamic conditions are set to match a range of Reynolds numbers relevant to human aortic valves. Eight different restrictions are tested at eight different Reynolds numbers to provide a parametric study of the relationship between the acoustic spectrum and narrowing severity. The sound signals measured with a contact microphone are analyzed with both the power spectrum and the bicoherence, which provides a measure of quadratic phase coupling between frequencies present in the signal. Across all Reynolds numbers, the energy present in high frequencies increases with increasing restriction percentage. Characteristic frequency bands are identified that uniquely correlate with the restriction, where 100 – 150 Hz appears significant for moderate to severe restrictions, and a band around 270 Hz increases in prominence with increasing severity. The level of phase coupling appears to decrease with increasing restriction, providing a unique quantitative measure that contrasts with the results in the power spectrum.   

Characterization of Pulsatile Flow Through A Perfusion Chamber for the Optimization of Spatial Shear Stress Distribution

A perfusion chamber (Hele-Shaw Cell) device is used to impose a pulsatile flow on endothelial cells to examine the cellular response of the cells under cyclic shear stress. The design of the device must guarantee that cells within the testing region are subject to a spatially uniform time-periodic shear stress. For the conditions typically found in applications, the viscous flow in the perfusion chamber exhibits order-unity values of the associated Womersley number. The associated unsteady lubrication problem was solved to determine the spatial distribution of shear stress in a prototypical device of hexagonal planform. Accompanying experiments using particle tracking velocimetry in a fabricated device were used to evaluate theoretical predictions and to assess the spatial shear stress distribution.   

Boundary element method for membranes

Motivated by recent experiments on the dynamics of colloidal membranes, we employ the boundary element method to determine the evolution of a fluid membrane in a viscous solvent sagging under its own weight. The membrane velocities are expressed using the boundary integral formulation in terms of a distribution of Green's functions on the membrane. We assume the associated force densities to be a combination of gravitational forces, surface tension, and bending stiffness. A surface incompressibility constraint is used to maintain the experimentally observed conservation of membrane area. This model recovers and recapitulates the experimentally observed neck and pendant-like drop formation.    

A Comprehensive Experimental Study on the Biomechanics of Deep Brain Stimulation

During Deep Brain Stimulation (DBS), a radiofrequency (RF) probe is inserted into select brain regions to treat Parkinson's disease. Understanding the interaction dynamics between the probe and brain tissue during insertion and removal is critical for improving accuracy. This comprehensive experimental study is designed to explore these interactions, focusing on the fluid-structure interaction (FSI) between the RF probes and a 1.5% w/w agar gel. A load cell-equipped RF probe is inserted into the gel block and then removed at varying speeds. The mechanical properties of the gel were evaluated using Magnetic Resonance Elastography (MRE). The reaction forces generated during the process were captured and analyzed. High-speed imaging was used to visualize the opening and closing of the probe channel. This revealed variations in channel diameter at different depths and removal speeds. Using Digital Image Correlation (DIC), agar deformation data was collected at different cross-sections. This investigation provides crucial insights into the FSI between RF probes and soft gels. The findings lay the groundwork for future DBS studies involving actual brain tissue. Ultimately, the knowledge gained from this research could improve DBS procedure accuracy, leading to better patient outcomes.