Fluorescent Tracer Particles Research Articles

Cytoplasmic fluidization contributes to breaking spore dormancy in fission yeast

The cytoplasm is a complex, crowded environment that influences myriad cellular processes including protein folding and metabolic reactions. Recent studies have suggested that changes in the biophysical properties of the cytoplasm play a key role in cellular homeostasis and adaptation. However, it still remains unclear how cells control their cytoplasmic properties in response to environmental cues. Here, we used fission yeast spores as a model system of dormant cells to elucidate the mechanisms underlying regulation of the cytoplasmic properties. By tracking fluorescent tracer particles, we found that particle mobility decreased in spores compared to vegetative cells and rapidly increased at the onset of dormancy breaking upon glucose addition. This cytoplasmic fluidization depended on glucose-sensing via the cyclic adenosine monophosphate-protein kinase A pathway. PKA activation led to trehalose degradation through trehalase Ntp1, thereby increasing particle mobility as the amount of trehalose decreased. In contrast, the rapid cytoplasmic fluidization did not require de novo protein synthesis, cytoskeletal dynamics, or cell volume increase. Furthermore, the measurement of diffusion coefficients with tracer particles of different sizes suggests that the spore cytoplasm impedes the movement of larger protein complexes (40 to 150 nm) such as ribosomes, while allowing free diffusion of smaller molecules (~3 nm) such as second messengers and signaling proteins. Our experiments have thus uncovered a series of signaling events that enable cells to quickly fluidize the cytoplasm at the onset of dormancy breaking.

Monitoring sediment transport pathways from an artificial nearshore berm, South Padre Island, Texas, USA, August 2018 to November 2019—Implications for coastal management

During August 2018 – November 2019, the transport pathways of dredge material from a specially constructed nearshore feeder berm were investigated as part of a collaborative study by the City of South Padre Island, U.S. Army Corps of Engineers–Galveston District, U.S. Geological Survey, Partrac GeoMarine Inc., and Texas A&M University, into the efficacy of beneficial use dredge material (BUDM) as a method of replenishing the beach profile and shoreface at South Padre Island, Texas with sediment. Dual-signature (fluorescent and ferrimagnetic) tracer particles, designed to be hydraulically equivalent to the dredge material, were placed on the berm, and a sampling program was initiated to monitor the spatiotemporal movement of tracer particles under the influence of the prevailing hydrodynamic regime. Wave and current data were collected and were used together with available metocean data to identify the forcing mechanisms of sediment transport; improved understanding garnered from the consideration of multiple datasets can be used to inform future coastal management decisions.Tracer analysis results indicated low magnitude, shoreward transport of dredge material from the berm and along shore transport in both northerly and southerly directions. Small amounts of tracer detected in beach face samples demonstrated connectivity between the constructed feeder berm and the shoreface. However, the generally low tracer concentration within beach face samples indicated low rates of sediment transport for berm sediments, and a low magnitude sediment transport pathway from the berm directly to the beach face, with possible storage of sediment in the nearshore bar system.Given that the berm was at or beyond the closure depth and considering the relatively weak prevailing near bottom hydrodynamic conditions, the potential for sediment to be mobilized and transported during the study period was generally low. Periods of higher energy waves, driven by north-northwest winds, were identified as a key driver of sediment transport but due to the infrequent nature of these events the sediment transport regime across the area of interest was temporally limited. Longshore movement of sediment was both north and south, mainly dependent on prevailing wind directions and resulting longshore currents.We discuss implications for coastal zone management and the use of feeder berms as an artificial beach nourishment mechanism to replenish sediments lost by coastal erosional processes. The insight into nearshore emplacement of material can be used by local governments and coastal managers to optimize the efficacy of berm emplacement and implement such schemes as a cost-effective nourishment option, or when onshore placement of material is not feasible.

A fluorescent PIV technique for turbomachinery flows

This paper presents the development and applications of a fluorescent Particle Image Velocimetry (PIV) technique as a countermeasure for undesired light reflections on surfaces of channel walls or obstacles. The employment of the fluorescent dye-doped tracer particle with a wavelength-specific optical filter enables a separation of the stokes-shifted particle light emission from reflections on the surfaces. Aiming at applying the technique to engine-like turbomachinery flows, the fluorescent tracer particles were made out of pyrromethene 567 and Di-Ethyl-Hexyl-Sebacat considering multiple aspects of characteristics. The developed particles were firstly tested in a low-speed wind tunnel with a metal turbine blade placed in the flow path and a laser sheet oriented to impinge the blade surface. With the installation of an adequate optical filter, the undesired light reflections were successfully removed, and reasonable vector calculations were enabled in proximity to the reflective blade surfaces. Secondly, the fluorescent PIV was performed on a high-pressure turbine stage hosted in a high-speed short-duration rotating turbine rig to demonstrate the feasibility of the technique in engine-representative turbomachinery flows.

Visualisation of clogging in green infrastructure growing media

ABSTRACT Growing media in green infrastructure (GI) designed for stormwater management plays a critical role in providing hydrological benefits. However, sediment deposition by urban stormwater can cause clogging and reduce the infiltration capacity. This study introduces a new approach to characterise the impact of clogging of GI that uses fluorescent tracer particles. Results are compared for two contrasting growing media: Grey to Green Substrate (G2G) and Marie Curie Substrate (MCS). Results showed that most sediment particles were retained on the surface of both growing media, and surface clogging in G2G caused a decrease in infiltration capacity. Sediment vertical movement was observed in both growing media, but particles travelled deeper in MCS. Vertical sediment movement is influenced by the growing media’s pore size distribution, and a high clogging risk is expected in growing media with a pore size distribution that is close to the particle size distribution of the incoming sediment particles.

Intricate network of nuclear morphology, chromatin dynamics and macromolecular crowding facilitates efficient nuclear functioning.

The highly crowded nuclear interior impacts molecular motion and assembly, and therefore influence sub-cellular organization and function. However, we have limited understanding of nuclear crowding at the mesoscale (10-100 nanometers), which corresponds to the size of regulatory molecules such as RNA polymerase II. Here, we leveraged nuclear Genetically Encoded Multimeric nanoparticles (nucGEMs) to characterize physical properties of nucleus and to study its impact on transcription. nucGEMs are self-assembling fluorescent tracer particles of 40-nm diameter whose motion can be tracked to infer biophysical properties. By tracking GEMs, we found that nucleus is more crowded than the cytoplasm. Within the nucleus, nucGEMs majorly populated euchromatic compartments, excluded from heterochromatin and the nucleolus. Changing nuclear volume by growing cells to confluency or by exposing them to hypo and hyper osmotic shock analogously altered nuclear crowding. Increasing heterochromatin using HDAC inhibitors or inhibiting transcription using actinomycin D led to an immediate decrease in nucGEM diffusion followed by slow and gradual recovery. Interestingly, transcriptional activity, as monitored by 5EU incorporation, also changed in-correlation with nuclear crowding changes. These findings indicate that nuclear morphology, chromatin dynamics and nuclear processes altogether shape the molecular crowding of the nucleus, which in turn affects transcription. Furthermore, through an osmotic-stress resistance screen, we discovered that MYC confers resistance to increased molecular crowding. We found that overexpression of key transcriptional regulator MYC, increased crowding and elevated transcription rate. We propose that MYC enables transcription even in a hyper-crowded molecular environment ensuring cell proliferation, shedding a new light on the function of this molecule. Our findings indicate a direct link between crowding and transcription suggesting that nuclear crowding may play a crucial role in several biological processes such as stem cell differentiation and oncogenic transformation of normal cells.

Fluorescent Tracer Particles For PIV Measurement In Gas Flows

This paper presents the development and employment of fluorescent tracer particles which allow removal of undesired strong light reflections in gas-phase Particle Image Velocimetry measurements. A spectroscopic investigation on fluorescent dye-doped solutions at different concentrations provided fluorescent emission spectra ranging their peak between 540 and 585 nm when excited by 527 nm wavelength Nd:YLF laser. The fluorescence intensity of the solutions in a form of sub-micron droplet was examined through comparison of statistical pixel intensities of raw particle images. Within the investigated concentration range, the intensity was found to be almost proportional to the dye concentration. The developed particle was tested in a wind tunnel at 5 m/s mean flow speed with a metal turbine blade placed in the channel and laser sheet oriented to impinge the blade surface. With the employment of an adequate optical band-stop filter, the undesired light reflections were successfully removed and reasonable vector calculations were enabled in proximity to the reflective surfaces. Lastly, the fluorescent PIV was performed in a high-speed short-duration rotating turbine test rig in order to demonstrate the effectiveness of the technique in industrial or extreme applications comprise severe conditions.

The Application Of Apertured Filter Method To The Simultaneous Measurement Of Particle-Turbulence Interaction In Dilute Suspension Layer Under Oscillatory Sheet Flows

One of the challenges confronted in particle image velocimetry (PIV) measurements applied in multi-phase flows is the discrimination of signals from different phases simultaneously registered in each single monochromatic camera. In the current sheet flow study, the sediment particles with a mean diameter of 240 μm are used and the diameter of tracer particles ranges from 25~50 μm. Under conventional imaging, the disparity in particle sizes would inevitably cause highly imbalanced reflected signals from the two phases and the image quality of one of the phases typically must be sacrificed. This manuscript introduces a low-cost and exceedingly convenient solution, named as the apertured filter method, which is characterized by the combined use of fluorescent tracer particles and an optical band pass filter with a round hole drilled in the center, with the hole diameter comparable to the smallest lens aperture size. It is demonstrated that the apertured filter method can provide balanced image intensities among the sediment and tracer particles and preserve the identifiability and separability of both phases. To compute the carrier phase velocity, median filter subtraction (Kiger & Pan, 2000) is applied to the acquired images, followed by direct implementation of stereoscopic PIV (SPIV) process. The cross-talk PIV errors due to the presence of sediment particles are assessed and found to be within 0.3 pixels up to a sediment concentration of C=1%. Some guidance on the fabrication of the apertured filter are also addressed to assist potential users. Enabled by the apertured filter method and the multi-camera single-plane (MCSP) method developed by Liu & Kiger (2022), a whole field, phase-locked time-resolved, particle-resolved and concurrent measurement of both the fluid and the sediment phase within the dilute regime (C≤1%) in a sinusoidal oscillatory sheet flow (period, T=5s, and peak free stream velocity, U_(o,p)=1 m/s) is achieved. Such detailed measurements in sheet flow have never been reported before to the author's best knowledge. The analysis of the acquired data focused upon three phase angles when the external flow is reversing the direction. The phase averaged sediment concentration, mean velocities for each phase and slip velocities between the two phases are evaluated.

Aerosol tracer testing in Boeing 767 and 777 aircraft to simulate exposure potential of infectious aerosol such as SARS-CoV-2.

The COVID-19 pandemic has reintroduced questions regarding the potential risk of SARS-CoV-2 exposure amongst passengers on an aircraft. Quantifying risk with computational fluid dynamics models or contact tracing methods alone is challenging, as experimental results for inflight biological aerosols is lacking. Using fluorescent aerosol tracers and real time optical sensors, coupled with DNA-tagged tracers for aerosol deposition, we executed ground and inflight testing on Boeing 767 and 777 airframes. Analysis here represents tracer particles released from a simulated infected passenger, in multiple rows and seats, to determine the exposure risk via penetration into breathing zones in that row and numerous rows ahead and behind the index case. We present here conclusions from 118 releases of fluorescent tracer particles, with 40+ Instantaneous Biological Analyzer and Collector sensors placed in passenger breathing zones for real-time measurement of simulated virus particle penetration. Results from both airframes showed a minimum reduction of 99.54% of 1 μm aerosols from the index source to the breathing zone of a typical passenger seated directly next to the source. An average 99.97 to 99.98% reduction was measured for the breathing zones tested in the 767 and 777, respectively. Contamination of surfaces from aerosol sources was minimal, and DNA-tagged 3 μm tracer aerosol collection techniques agreed with fluorescent methodologies.

Near-Wall Velocimetry in the Impingement-Zones of a Microdroplet and a Round Jet Stream

Abstract Near-wall fluid velocimetry in the impingement zone of a microdroplet stream on a flat surface is reported utilizing microparticle tracking velocimetry (μPTV). The results are then compared with the near-wall fluid velocimetry in the impingement region of a steady microjet stream. The presence of tracer particles in the fluid results in a small random movement of the droplets away from the orifice axis, causing a change in the location of the droplet impingement center. A new method to find the center of impingement is described, and an algorithm is developed to obtain the radial velocities in the impingement zone at three out-of-plane heights of 2, 7, and 10 ± 2 μm from the wall. Single-frame double-exposed images of low loading fluorescent tracer particles are used for the experiments. As the impingement frequency of the droplet stream is much higher than the image-capturing rate of the camera, each double-exposed image corresponds to a random instance within the impingement period of the droplets. The presented results show the occurrence of a higher normalized root-mean-square along with positive skewness of the measured radial velocity values for the droplet stream. These indicate higher velocity fluctuations or fluid mixing characteristics induced by the droplet-crown propagation for the droplet stream when compared to that of a jet stream. The near-wall velocity measurements support previously reported observations of the enhanced convection heat transfer characteristics for a droplet stream case.

Coupled measurements of interface topography and three-dimensional velocity field of a free surface flow

Complete energy balance evaluations for interfacial flow require simultaneous coupled estimations of the free surface topography and the flow velocity beneath the surface. We describe here an extension of the tomographic PIV and volumetric LPT technique for this coupled measurement. We applied this optical measurement technique to the study of three-dimensional (3D) flow at the air–water interface during the controlled fall of a droplet. Our original setup made it possible to monitor, in real time, the 3D position of fluorescent tracer particles, both on the surface and in the bulk flow. We overcame the shadow effect due to the presence of waves at the interface by illuminating the flow through the underside of a truncated squared pyramidal water tank. We chose to use a water tank of this shape to ensure that optical access could be established orthogonally through the walls of the tank. Four high-speed cameras were focused on the illuminated volume of the flow through the four lateral sides of the pyramidal water tank. The images of the four cameras were analyzed by 3D Lagrangian particle tracking velocimetry (Schanz et al. 2016). With this technique, we were able to track particles accurately at seeding densities compatible with the thresholds for tomographic PIV and to reduce considerably the number of ghost particles. We then obtained local 3D velocities by interpolating vector volumes from the discrete particles. The energy balance was determined by evaluating interface position and bulk flow velocity. Surface curvature and potential energy were determined from surface topography. The kinetic energy of the flow was derived from the bulk flow velocities. We then compared the energy balance thus obtained with that derived from direct visualization of the impact of the same drop. Our findings demonstrate the ability of this technique to characterize such complex interfacial flows in terms of their energetics.

Characteristics of the Irrigant Flow in a Simulated Lateral Canal Under Two Typical Laser-Activated Irrigation Regimens.

Photon-induced photoacoustic streaming (PIPS) and shockwave-enhanced emission photoacoustic streaming (SWEEPS) are two promising laser-activated irrigation (LAI) methods for root canal irrigation. Their performance in driving irrigant flush in a complex root canal system will be evaluated by microscale particle image velocimetry (μPIV) measurement and will be compared with that of ultrasonic-activated irrigation (UAI). A μPIV system with 7 μm fluorescent tracer particles was adopted to measure two-dimensional (2D) velocity fields around the junction region, with a size of 1.8 mm × 1.5 mm, between one main canal and one lateral canal in an endodontic training block, which was driven by SWEEPS (Er:YAG laser) operating at 15 Hz and 20 mJ. The flow field driven by PIPS (Er:YAG laser) at the same frequency and energy, as well as by UAI (with non-cutting insert) operating at 40% unit power, was also measured for a direct comparison. It was found that both SWEEPS and PIPS can activate a so-called "breath mode" during the irrigation. Namely, the induced irrigant flush presented a back-and-forth oscillation along both the main canal and the lateral one. The maximum flow speed in the lateral canal was observed to be up to 10 m/s in the SWEEPS modality, while reduced to around 7 m/s in the PIPS modality. The penetration length in the lateral canal in both modalities was estimated to be larger than 1 mm. In comparison, the flow field induced by UAI was characterized as irregular vortical structures, the maximum flow speed in the lateral canal was 0.15 m/s and significantly lower than LAI (P < 0.01), and the penetration length was less than 300 μm and lower than LAI (P < 0.05). Compared to UAI, PIPS, and SWEEPS are more capable of delivering the irrigant deeper into the lateral canal. Furthermore, the back-and-forth flush in the breath mode is ideal for removing debris during irrigation. Lasers Surg. Med. © 2020 Wiley Periodicals LLC.

Probing Intracellular Dynamics Using Fluorescent Carbon Dots Produced by Femtosecond Laser In Situ.

Fluorescent particle tracking is a powerful technique for studying intracellular transport and microrheological properties within living cells, which in most cases employs exogenous fluorescent tracer particles delivered into cells or fluorescent staining of cell organelles. Herein, we propose an alternative strategy, which is based on the generation of fluorescent species in situ with ultrashort laser pulses. Using mouse germinal vesicle oocytes as a model object, we demonstrate that femtosecond laser irradiation produces compact dense areas in the intracellular material containing fluorescent carbon dots synthesized from biological molecules. These dots have tunable persistent and excitation-dependent emission, which is highly advantageous for fluorescent imaging. We further show that tight focusing and tuning of irradiation parameters allow precise control of the location and size of fluorescently labeled areas and minimization of damage inflicted to cells. Pieces of the intracellular material down to the submicrometer size can be labeled with laser-produced fluorescent dots in real time and then employed as probes for detecting intracellular motion activity via fluorescent tracking. Analyzing their diffusion in the oocyte cytoplasm, we arrive to realistic characteristics of active forces generated within the cell and frequency-dependent shear modulus of the cytoplasm. We also quantitatively characterize the level of metabolic activity and density of the cytoskeleton meshwork. Our findings establish a new technique for probing intracellular mechanical properties and also promise applications in tracking individual cells in population or studies of spatiotemporal cell organization.

Microstreaming induced by acoustically trapped, non-spherically oscillating microbubbles

While numerous theoretical studies exist on microstreaming around acoustically excited, trapped gas bubbles, experimental approaches have mainly been conducted for bubbles attached to a solid boundary. One of the main difficulties lies in the positional stability of the microbubble. In the present work we trigger surface modes by bubble coalescence, with the advantage of limiting translational instabilities and controlling the orientation of the axisymmetric deformation. Furthermore, streaming is visualised by fluorescent tracer particles. In this way, bubble dynamics and streaming patterns can be studied together. Different types of streaming patterns are observed and correlated to the respective mode number. Besides the mode number, the bubble size and the phase difference between modal components are identified as important parameters in the definition of the pattern type.

Mass Transport Characterization in Redox Flow Batteries Using Fluorescence Microscopy in Operando

Significant advancements in aqueous organic redox flow battery (RFB) chemistries are generating greater anticipation for grid scale energy storage systems. However, low power densities, driven in part by mass transport limitations within the cell stack, limit the performance of these systems. A detailed understanding of mass transport and electrochemical activity within the porous electrode would benefit all RFB chemistries, particularly those using aqueous organic electrolytes. In this study we use fluorescence microscopy to image fluid flow and electrochemical activity in operando using fluorescent tracer particles and redox-active aqueous-soluble quinones respectively. The fluorescence of the reduced quinone is used as a direct measure of local state of charge and the tracer particles allow measurements of local velocities and fluid streamlines. The combined approach illuminates the influence of the electrode architecture on electrochemical-flow processes, unveiling surprisingly heterogeneous behavior over length scales significantly larger then the average pore diameter. Several electrode styles are compared including papers, cloths and felts with an emphasis on mass transport characteristics impacting their performance. We will discuss further applications for this in operando technique and suggest potential electrode architectures to improve RFB performance as grid scale energy storage systems. Figure: (left) Strobe imaging of fluorescent tracer particles highlighting fluid streamlines in a woven carbon fiber electrode. (center) Colorized image of a fluorescent quinone electrolyte undergoing electrochemical reduction in operando. Image width is 2 mm highlighting the strikingly heterogeneous mass transport process. (right) particle tracking of fluorescent tracer particles providing insight into local velocity fields and fluid dead zones. Figure 1

Development of Temperature and Velocity Measurement Method for Liquid Utilizing Fluorescent Tracer Particles

Development of Temperature and Velocity Measurement Method for Liquid Utilizing Fluorescent Tracer Particles

Impact of air-handling system exhaust failure on dissemination pattern of simulant pathogen particles in a clinical biocontainment unit.

Biocontainment units (BCUs) are facilities used to care for patients with highly infectious diseases. However, there is limited guidance on BCU protocols and design. This study presents the first investigation of how HVAC (heating, ventilation, air‐conditioning) operating conditions influence the dissemination of fluorescent tracer particles released in a BCU. Test conditions included normal HVAC operation and exhaust failure resulting in loss of negative pressure. A suspension of optical brightener powder and water was nebulized to produce fluorescent particles simulating droplet nuclei (0.5‐5 μm). Airborne particle number concentrations were monitored by Instantaneous Biological Analyzers and Collectors (FLIR Systems). During normal HVAC operation, fluorescent tracer particles were contained in the isolation room (average concentration = 1 × 104 ± 3 × 103/Lair). Under exhaust failure, the automated HVAC system maximizes airflow into areas adjacent to isolation rooms to attempt to maintain negative pressure differential. However, 6% of the fluorescent particles were transported through cracks around doors/door handles out of the isolation room via airflow alone and not by movement of personnel or doors. Overall, this study provides a systematic method for evaluating capabilities to contain aerosolized particles during various HVAC scenarios. Recommendations are provided to improve situation‐specific BCU safety.

A technique for measuring ensemble-averaged, three-component liquid velocity fields in two-phase, gas–liquid, intermittent pipe flows

Gas–liquid intermittent flows can be found in many engineering applications, nevertheless a detailed knowledge of this flow pattern is still not fully available. In the present work, an experimental study was conducted with the objective of developing a measurement procedure capable of providing ensemble-averaged three-component velocity fields in the liquid phase of a gas–liquid, intermittent, horizontal flow in a pipe. To this end, a high-frequency stereoscopic particle image velocimetry system (SPIV) was employed, combined with the laser induced fluorescence (LIF) technique to separate the light scattered by the liquid–gas interfaces from that emitted by the fluorescent tracer particles. A set of photogates was used to trigger the SPIV system, allowing for the measurement of velocity fields in the liquid plug, downstream of the elongated bubble, and in the liquid film, upstream of the elongated bubble nose position. The triggered measurements allowed the determination of ensemble-averaged three-component velocity fields at different positions in relation to the bubble nose, obtained from the replication of a sufficiently large number of bubble passage events. Contours of the liquid flow streamwise vorticity component in cross-stream planes upstream and downstream of the bubble nose tip were also obtained from the SPIV measurements. The photogate system was also employed to measure the bubble velocity. This information was used to transform time-based into space-based velocity field data. This allowed the construction of a three-dimensional representation of the ensemble-averaged structure of the gas bubble nose and the associated vortical structures induced in the liquid flow. The three-component velocity information obtained revealed the influence of the gas bubble motion on the liquid flow in the plug and liquid film regions.

Image processing techniques for the measurement of two-phase bubbly pipe flows using particle image and tracking velocimetry (PIV/PTV)

Particle Image Velocimetry (PIV) measurement in gas-liquid bubbly flows is a challenging task, mainly due to the dispersion of the laser light caused by the gas-liquid interfaces. A common solution adopted is the use of fluorescent seeding particles associated with a bandpass filter for the laser light. Therefore, the camera captures only the light fluoresced by the seeding particles, filtering the laser light dispersed by the gas-liquid interfaces and measuring a velocity field which corresponds to the liquid phase, seeded with the particles. However, even for relatively low gas fractions, the fluoresced light reflected by the interfaces distorts the measurements, as it illuminates particles out from the laser plane. In addition, the fluoresced light also reflects at the interfaces, distorting the measurements and trending to overshoot the measured liquid velocities In this work, PIV measurements of air-water bubbly flows in a small diameter pipe (D = 26.2 mm) are performed with fluorescent tracer particles (PIV/LIF). A new method for the phase discrimination in PIV was developed to overcome the problems caused by the presence of bubbles in the flow, which uses the pixels intensity information of each interrogation window, to identify if that window corresponds to liquid or bubble region. To validate the PIV procedure, a Particle Tracking Velocimetry (PTV) algorithm was also developed to measure the bubbles velocities, based on similar implementations found in literature, but some corrections were also proposed to overcome the bubble overlap phenomena which arises in bubbly pipe flows, when backward illumination is used. The PIV and the PTV methods were tested and validated by a series of distinct gas void fraction and bubble diameters experimental cases, confirming the accuracy and reliability of both methods. The proposed method was used to analyze a set of upward laminar and turbulent bubbly flows, showing that the averaged axial velocity and the dispersed gas bubbles modify the turbulence intensity profiles.

The Effects of Shear Force Transmission Across Vesicle Membranes.

We report a comprehensive study on mechanotransmission of shear forces across lipid bilayer membranes of giant unilamellar vesicles (GUVs). GUVs containing fluorescent tracer particles were immobilized on a microfluidic platform and exposed to shear flows. A method was developed for the visualization of three-dimensional flows at high precision by defocusing microscopy. We quantify the symmetry of external flow around the GUV and show its effects on vortex flows and luminal dynamics. With increasing asymmetry, luminal vortices merged while liquid exchange in between them increased. The effect of membrane composition was studied through addition of cholesterol. Mechanotransmission efficacy, quantified by the ratio of luminal flow to external flow, ranged from ε = 0.094 (0 mol % cholesterol) to ε = 0.043 (16 mol % cholesterol). Our findings give new cues to the mechanisms underlying the sensing of strength and spatial distribution of shear forces by cells and the impact of membrane composition.

A three-dimensional strain measurement method in elastic transparent materials using tomographic particle image velocimetry.

BackgroundThe mechanical interaction between blood vessels and medical devices can induce strains in these vessels. Measuring and understanding these strains is necessary to identify the causes of vascular complications. This study develops a method to measure the three-dimensional (3D) distribution of strain using tomographic particle image velocimetry (Tomo-PIV) and compares the measurement accuracy with the gauge strain in tensile tests.Methods and findingsThe test system for measuring 3D strain distribution consists of two cameras, a laser, a universal testing machine, an acrylic chamber with a glycerol water solution for adjusting the refractive index with the silicone, and dumbbell-shaped specimens mixed with fluorescent tracer particles. 3D images of the particles were reconstructed from 2D images using a multiplicative algebraic reconstruction technique (MART) and motion tracking enhancement. Distributions of the 3D displacements were calculated using a digital volume correlation. To evaluate the accuracy of the measurement method in terms of particle density and interrogation voxel size, the gauge strain and one of the two cameras for Tomo-PIV were used as a video-extensometer in the tensile test. The results show that the optimal particle density and interrogation voxel size are 0.014 particles per pixel and 40 × 40 × 40 voxels with a 75% overlap. The maximum measurement error was maintained at less than 2.5% in the 4-mm-wide region of the specimen.ConclusionsWe successfully developed a method to experimentally measure 3D strain distribution in an elastic silicone material using Tomo-PIV and fluorescent particles. To the best of our knowledge, this is the first report that applies Tomo-PIV to investigate 3D strain measurements in elastic materials with large deformation and validates the measurement accuracy.