Micro-particle image velocimetry for velocity profile measurements of micro blood flows

Capturing physiological hemodynamic flow and mechanosensitive cell signaling in vessel-on-a-chip platforms

Recent advances in organ chip (or, "organ-on-a-chip") technologies and microphysiological systems (MPS) have enabled in vitro investigation of endothelial cell function in biomimetic three-dimensional environments under controlled fluid flow conditions. Many current organ chip models include a vascular compartment; however, the design and implementation of these vessel-on-a-chip components varies, with consequently varied impact on their ability to capture and reproduce hemodynamic flow and associated mechanosensitive signaling that regulates key characteristics of healthy, intact vasculature. In this review, we introduce organ chip and vessel-on-a-chip technology in the context of existing in vitro and in vivo vascular models. We then briefly discuss the importance of mechanosensitive signaling for vascular development and function, with focus on the major mechanosensitive signaling pathways involved. Next, we summarize recent advances in MPS and organ chips with an integrated vascular component, with an emphasis on comparing both the biomimicry and adaptability of the diverse approaches used for supporting and integrating intravascular flow. We review current data showing how intravascular flow and fluid shear stress impacts vessel development and function in MPS platforms and relate this to existing work in cell culture and animal models. Lastly, we highlight new insights obtained from MPS and organ chip models of mechanosensitive signaling in endothelial cells, and how this contributes to a deeper understanding of vessel growth and function in vivo. We expect this review will be of broad interest to vascular biologists, physiologists, and cardiovascular physicians as an introduction to organ chip platforms that can serve as viable model systems for investigating mechanosensitive signaling and other aspects of vascular physiology.

Image-Based Experimental Measurement Techniques to Characterize Velocity Fields in Blood Microflows

Predicting blood microflow in both simple and complex geometries is challenging because of the composition and behavior of the blood at microscale. However, characterization of the velocity in microchannels is the key for gaining insights into cellular interactions at the microscale, mechanisms of diseases, and efficacy of therapeutic solutions. Image-based measurement techniques are a subset of methods for measuring the local flow velocity that typically utilize tracer particles for flow visualization. In the most basic form, a high-speed camera and microscope setup are the only requirements for data acquisition; however, the development of image processing algorithms and equipment has made current image-based techniques more sophisticated. This mini review aims to provide a succinct and accessible overview of image-based experimental measurement techniques to characterize the velocity field of blood microflow. The following techniques are introduced: cell tracking velocimetry, kymographs, micro-particle velocimetry, and dual-slit photometry as entry techniques for measuring various velocity fields either in vivo or in vitro.

Effect of drag-reducing polymer on blood flow in microchannels

Drag-reducing polymers (DRPs) can significantly improve blood circulation when added to blood at a nanomolar concentration, manifesting great potential for application in the biomedical field. In this work, hyaluronic acid (HA) was selected as a natural DRP, and its effects on blood microcirculation at different concentrations, flow rates, and channel geometry were studied in microchannels. The experimental results show that adding a small dose of HA can increase the velocity and shorten the thickness of the cell-free layer (CFL or cell depletion layer (CDL)) near the wall. After considering efficiency, our experiments determined 50 ppm addition of HA to be the most suitable amount for improving blood circulation. Our results demonstrate that HA has high efficiency in improving the circulation of blood flow and shed light on unveiling the mechanism of using natural DRPs to cure some cardiovascular diseases.

Effects of Cooling on the SNR and Contrast Detection of a Low-Light Event-Based Camera

Johnson-Nyquist noise is the electronic noise generated by the thermal agitation of charge carriers, which increases when the sensor overheats. Current high-speed cameras used in low-light conditions are often cooled down to reduce thermal noise and increase their signal to noise ratio. These sensors, however, record hundreds of frames per second, which takes time, requires energy, and heavy computing power due to the substantial data load. Event-based sensors benefit from a high temporal resolution and record the information in a sparse manner. Based on an asynchronous time-based image sensor, we developed another version of this event-based camera whose pixels were designed for low-light applications and added a Peltier-effect-based cooling system at the back of the sensor in order to reduce thermal noise. We show the benefits from thermal noise reduction and study the improvement of the signal to noise ratio in the estimation of event-based normal flow norm and angle and particle tracking in microscopy.

Controlled Microfluidic Environment for Dynamic Investigation of Red Blood Cell Aggregation

Blood, as a non-Newtonian biofluid, represents the focus of numerous studies in the hemorheology field. Blood constituents include red blood cells, white blood cells and platelets that are suspended in blood plasma. Due to the abundance of the RBCs (40% to 45% of the blood volume), their behavior dictates the rheological behavior of blood especially in the microcirculation. At very low shear rates, RBCs are seen to assemble and form entities called aggregates, which causes the non-Newtonian behavior of blood. It is important to understand the conditions of the aggregates formation to comprehend the blood rheology in microcirculation. The protocol described here details the experimental procedure to determine quantitatively the RBC aggregates in microcirculation under constant shear rate, based on image processing. For this purpose, RBC-suspensions are tested and analyzed in 120 x 60 µm poly-dimethyl-siloxane (PDMS) microchannels. The RBC-suspensions are entrained using a second fluid in order to obtain a linear velocity profile within the blood layer and thus achieve a wide range of constant shear rates. The shear rate is determined using a micro Particle Image Velocimetry (µPIV) system, while RBC aggregates are visualized using a high speed camera. The videos captured of the RBC aggregates are analyzed using image processing techniques in order to determine the aggregate sizes based on the images intensities.