Name: micro-particle image velocimetry for velocity profile measurements of micro blood flows
Uploaded: 2013-04-25T12:00:00
Description: Watch this Scientific Journal Video about Micro-particle Image Velocimetry for Velocity Profile Measurements of Micro Blood Flows at JoVE.com

The authors would like to thank NSERC (Natural Sciences and Engineering Council of Canada) for funding, Catherine Pagiatikis for her help in initial runs, Sura Abu-Mallouh and Frederick Fahim for testing the protocol, Richard Prevost of LaVision, Inc for technical support, and Guy Cloutier of the University of Montr&eacute;al for the loan of the Dalsa high-speed camera.

1. Microchip Fabrication
The first step is to create or purchase your microchannel. There are many options for microchip material.
One of the most common materials chosen is poly(dimethylsiloxane) (PDMS). There are many publications on directions for PDMS fabrication through soft lithography 16,17,18.
Once the PDMS channel is fabricated, there are several surface treatments available to reverse its natural hydrophobicity. Oxygena...

<ol>
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D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optically+sliced+micro-PIV+using+confocal+laser+scanning+microscopy+(CLSM).">Optically sliced micro-PIV using confocal laser scanning microscopy (CLSM).</a> Exp. Fluids. 37 (1), 105-119 (2004).</li><li>King, M. R., Bansal, D., Kim, M. B., Sarelius, I. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+effect+of+hematocrit+and+leukocyte+adherence+on+flow+direction+in+the+microcirculation.">The effect of hematocrit and leukocyte adherence on flow direction in the microcirculation.</a> Ann. of Biomed. 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T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Advances+and+applications+on+microfluidic+velocimetry.">Advances and applications on microfluidic velocimetry.</a> Microfluid. Nanofluid. 8 (6), 709-726 (2010).</li><li>Chiu, J. J., Chen, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effects+of+disturbed+flow+on+vascular+endothelium:+Pathophysiological+basis+and+clinical+perspectives.">Effects of disturbed flow on vascular endothelium: Pathophysiological basis and clinical perspectives.</a> Physiol. Rev. 91 (1), 327-387 (2011).</li><li>Secomb, T. W., Pries, A. 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F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Contact+angle+study+of+blood+dilutions+on+common+microchip+materials.">Contact angle study of blood dilutions on common microchip materials.</a> Journal of the Mechanical Behavior of Biomedical Materials. , (2012).</li><li>Hovav, T., Yedgar, S., Manny, N., Barshtein, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Alteration+of+red+blood+cell+aggregability+and+shape+during+blood+storage.">Alteration of red blood cell aggregability and shape during blood storage.</a> Transfusion. 39, 277-281 (1999).</li><li>Pitts, K. L., Mehri, R., Mavriplis, C., Fenech, M. 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Using &mu;PIV for blood flow measurements at the scale of the microcirculation can give insight into a great number of relevant biomedical, mechanical and chemical engineering processes. Some of the key factors to account for are the density of the RBC themselves, the aggregation and deformability of the RBC, aggregation or movement of the fluorescing micro particles, and the settling of the RBC in the channels. All of these can be accounted for if the general guidelines laid out above are followed. There is a basic chec...

The human body contains numerous vessels with diameters less than 50 &mu;m, which are the main exchange site between blood and tissues. The study of blood flow in these vessels represents a considerable challenge due to both the scale of the measurements and the fluid properties of blood. These measurements, including the pressure gradient, the shear at the wall, and velocity profiles in arterioles and venules, are key factors linked with physiological responses. There are now unprecedented opportunities to resolve these measurement challenges, thanks to new experimental techniques at the micro scale to study the microcirculation and solve this multiscale problem.
Micro-particle image velocimetry (&mu;PIV) is a particle-based flow visualization technique that is used to evaluate velocity profiles of blood flow in microchannels via cross correlation. &mu;PIV, first developed by Santiago et al., has been used with hemorheology studies since Sugii et al. in 2001 used the technique to measure blood flow in 100 &mu;m round glass tubes 1,2. Different approaches to &mu;PIV exist. High speed cameras can be used to correlate the movement of red blood cells (RBCs), and pulsed images can be used to correlate the movement of tracer particles. Either of these options can be coupled with an upright or inverted microscope, depending on the application. In both cases, the result is a 2D velocity profile. Another approach is to use a confocal microscope to achieve 2D and 3D profiles. This method has been applied to blood 3,4,5.
Micro scale PIV has several complications when compared with macro PIV. In macro PIV the data can be limited to a single plane through sheets of light, but at the micro scale volume illumination is necessary. Volume illumination is a greater problem for the imaging of micro blood flows, as the RBCs themselves are large in comparison to the channels, and using the RBCs as the tracer particles leads to a depth of correlation (DOC) which can significantly decrease the accuracy of the cross-correlation results 6,7,8. Following Wereley et al. (1998) the DOC for a 40 &mu;m tall channel with RBC as the tracers is 8.8 &mu;m, while with a 1 &mu;m tracer particle the DOC is 6.7 &mu;m. This difference becomes more pronounced when changing channel height and magnification. Additionally, RBCs are opaque, and increasing the density of the RBCs in the flow causes imaging difficulties. Fluorescing tracer particles, first used by Santiago et al. (1998), have been advocated as a tool to decrease the influence of out-of-focus particles, when using the smallest particles possible. Using 1 &mu;m diameter fluorescing microparticles coupled with a laser is one approach that may decrease the depth of focus problem in micro blood flow imaging 10. There are several current reviews of the state of &mu;PIV technology, each of which highlights the importance of &mu;PIV to blood flow studies 11,12. Several important considerations must be taken into account when using &mu;PIV for blood. At the micro level, the scale of the microcirculation, blood cannot be considered a homogeneous fluid, as it is a suspension of flexible red blood cells (RBCs), large white blood cells, platelets and other proteins suspended in a Newtonian fluid (plasma).
The velocity profiles measured here can be used to measure certain characteristics of the micro blood flows. The important factors in microhemorheology are the flow rate of the blood, the shape of the velocity profile, and the shear stress at the wall of the vessel. This information has clinical implications, as the microcirculation is the site for nutrient exchange in the body, and this exchange is shear-dependent. There are several current review studies on the state of research in the microcirculation as well 13,14,15.
Presented here is a protocol for &mu;PIV measurements of blood flows in polydimethylsiloxane (PDMS) microchannels. PDMS channels were fabricated in-house following the sources in section 1 of the protocol. Porcine blood samples were obtained from an accredited slaughterhouse and cleaned following section 2 of the protocol. All data was obtained using the LaVision MITAS &mu;PIV system, as described in section 3 of the protocol. The set-up consists of a Nd:YAG laser (New Wave Research, USA) and CCD camera (Image Intense, Lavision) controlled by a programmable triggering unit, a fluorescent microscope coupled with a stage moving in 3 axes, and a computer, in addition to a high speed camera (Dalsa 1M150, Netherlands) was added for visualization of the RBCs themselves. Both cameras are connected to a 2 port optic box (Custom by Zeiss, Germany). In typical in vivo measurements of blood flow, an upright microscope is used to track the RBCs themselves, while in typical in vitro applications an inverted microscope is used to track the tracer particles. In this unique dual set-up, the optics box allows both tracers to be imaged using the inverted microscope. Blood was introduced into the microchips via a high precision syringe pump (Nexus3000, Chemyx Inc., USA). A diagram of the system is shown in Figure 1, where the top portion of the figure represents the 140 &mu;m by 40 &mu;m rectangular channels fabricated of PDMS, and the bottom portion represents the entire system including both cameras, the laser, the syringe pump and the microscope.
Current &mu;PIV set-ups available, usually with proprietary software, include TSI, Dantec Dynamics, and LaVision. Standard cross-correlation algorithms can be achieved through numerous software options, including the MATLAB. The software is not the key, understanding what the dialogue boxes correspond to mathematically will serve the user much better. In this protocol DaVis, LaVision's proprietary software or MATLAB are utilized. The protocol is not software specific, but the menu options might be in different places in different software packages.

<table border="1">
	<tbody>
		<tr>
			<td>Name of Reagent/Material</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>poly(dimethylsiloxane) (PDMS), i.e. Sylgard-184</td>
			<td>Dow-Corning</td>
			<td>3097358-1004</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>ethylenediaminetetraacetic acid (EDTA)</td>
			<td>Sigma Aldrich</td>
			<td>E9884-100G</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>poshpate buffered saline (PBS)</td>
			<td>Sigma Aldrich</td>
			<td>P5368-10PAK</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>fluorescing micro particles</td>
			<td>Microgenics/FisherSci</td>
			<td>R900</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>glycerol (OPTIONAL)</td>
			<td>Sigma Aldrich</td>
			<td>G6279-500 ml</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>microcentrifuge, i.e. CritSpin</td>
			<td>FisherSci</td>
			<td>22-269-291</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>syringe, i.e. 50 &mu;l Gastight</td>
			<td>Hamilton</td>
			<td>80965</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>camera, i.e. Imager Intense, high speed</td>
			<td>LaVision, Dalsa</td>
			<td>Imager Intense</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>microscope, i.e. MITAS</td>
			<td>LaVision</td>
			<td>MITAS</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Nd:YAG laser</td>
			<td>New Wave Research</td>
			<td>Solo-II</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>syringe pump, i.e. Nexus3000</td>
			<td>Chemyx, Inc.</td>
			<td>Nexus-3000</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>flexible tubing, i.e. Tygon</td>
			<td>FisherSci</td>
			<td>14-169-1A</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>data processing software, i.e. DaVis</td>
			<td>LaVision</td>
			<td>DaVis</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>centrifuge, i.e. Thermo Scientific CL2</td>
			<td>Thermo Scientific</td>
			<td>004260F</td>
			<td>&nbsp;</td>
		</tr>
	</tbody>
</table>

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

Micro-particle image velocimetry (&mu;PIV) is used to visualize paired images of micro particles seeded in blood flows. The images are cross-correlated to give an accurate velocity profile. A protocol is presented for &mu;PIV measurements of blood flows in microchannels. At the scale of the microcirculation, blood cannot be considered a homogeneous fluid, as it is a suspension of flexible particles suspended in plasma, a Newtonian fluid. Shear rate, maximum velocity, velocity profile shape, and flow rate can be derived from these measurements. Several key parameters such as focal depth, particle concentration, and system compliance, are presented in order to ensure accurate, useful data along with examples and representative results for various hematocrits and flow conditions.

In all figures, flow is left to right in raw images, and upwards in calculated velocity profiles. An example of the raw data obtained with blood at hematocrit H=10 flowing at 10 &mu;l/hr is shown in Figure 2. Raw data may be cross-correlated without any data processing to achieve velocity profiles. The impact of pre-processing and data processing methods is discussed by Pitts, et al., (2012b). An example of a resultant velocity profiles from data similar to Figure 2 at hematorcr...

Watch this Scientific Journal Video about Micro-particle Image Velocimetry for Velocity Profile Measurements of Micro Blood Flows at JoVE.com

Micro-particle Image Velocimetry for Velocity Profile Measurements of Micro Blood Flows

Micro-particle image velocimetry (&mu;PIV) is used to visualize paired images of micro particles seeded in blood flows which are cross-correlated to give an accurate velocity profile. Shear rate, maximum velocity, velocity profile shape, and flow rate, each of which has clinical applications, can be derived from these measurements.

Micro-particle image velocimetry (&mu;PIV) is used to visualize paired images of micro particles seeded in blood flows. The images are cross-correlated to ...

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