Front Microrheology of biological Fluids

Abstract

Since Poiseuille times, several techniques has been developed to measure the viscosity of blood. During the 60's and 70's, with the appearance of the first rheomethers, the rheological properties of blood were accurately measured and a behavior dependent of the velocity gradient of the fluid flow was determined. As well, it was observed that blood had shear thinning properties, meaning, that if the velocity of the blood flow increases, the viscosity of blood decreases.

From a medical point of view, blood and its blood plasma are the most effective fluids to detect global pathologies in human and animals. These pathologies may be related to their viscosities, their plasma proteins, or the properties of its red blood cells, as their aggregation, deformability or the elastic properties of its cellular membrane.

Lately, with the birth of microfluidics at the beginnings of the 90’s, new techniques for the diagnostic of diseases has been developed. The avantages in the use of microfluidic devices for diagnostics are: the low amount of sample required to perform a measure, their portability, that they are easy to use and the low cost of its fabrication.

The aim of this thesis project was to extend the study of front microrheology through the development of a device and method that describes accurately the non-linear rheology of biofluids, mainly blood, by means of a simple optical detection method based on tracking the fluid-air interface moving inside a microchannel. We centered in the fluid front (interface fluid-air) since is a direct, easy and cheap method to study fluid flows. In order to achieve this, we first had to developed a microfluidic device and method which would allow us to obtain a clear image of the fluid front. This was made, using a microscope and a high speed camera. The images obtained with the camera were analyzed by means of a computational code developed in Wolfram Mathematica©.

The thesis work was mainly experimental comprinsing: fabrication of microfluidic devices and experiments with Newtonian and non-Newtonian fluids. Our results have been compared with theoretical and bibliographical results. The original results from this thesis are separated in two parts.

The first part of the research was dedicated to the study the interface fluid-air of flows of Newtonian fluids. In order, to achieve reliable viscosity results with our device and method, and prove our system as a viscometer, we tested several fluids, including blood plasma which is known to be Newtonian. Our results were compared with the results obtained with a different viscometer to prove their reliability. This part of the study states our microfluidic device and method as a viscometer.

The second part of this thesis was dedicated to extend the results for Newtonian fluid to non-linear hemorheology and comprises all the results for blood. Blood is essentially a difficult fluid to manipulate and study. In general, it presents non-Newtonian properties as shear thinning, meaning, that its viscosity decreases as the stress or the shear rate increases. This non-Newtonian properties are due to plasma proteins and especial characteristics of its red blood cells. With our device and method we were able to observe the non-Newtonian behavior of blood and to obtain its viscosity at different shear rates and stresses. As well, we related its viscosity to some of its red blood cells properties, as their tendency to form aggregates and the flexibility of their cellular membrane. The studies of blood were developed at different hematocrits, different dates from the extraction of the sample and with anemic blood and blood with alphathalessemia.

In general, our device and method is usefull as a viscometer and rheometer, as well as, it enables to establish a relation between blood viscosity and its red blood cells characteristics.

Desde los tiempos de Poiseuille, se han desarrollado variadas técnicas para medir la viscosidad de la sangre. Durante las décadas de los 60’s y 70’s con la aparición de los primeros reómetros las propiedades reológicas de la sangre fueron medidas y se determinó su comportamiento dependiente del gradiente de velocidad. Además se observó que posee un comportamiento pseudoplástico, es decir, que a medida que aumenta su velocidad su viscosidad disminuye.

Desde un punto de vista médico, la sangre y su plasma sanguíneo son los fluidos más eficaces para la detección de patologías globales. Estas patologías pueden estar relacionadas con su viscosidad, con las proteínas presentes en el plasma o con las propiedades de sus glóbulos rojos, como su agregación, deformabilidad o la capacidad elástica de su membrana celular.

En los últimos años, con el nacimiento de la microfluídica a principio de los 90’s, nuevas técnicas para el diagnóstico de enfermedades se han desarrollado. La ventaja del uso de la microfluídica en el diagnóstico de enfermedades viene dada por: el bajo requerimiento de muestra para realizar la detección, su portabilidad, la facilidad de uso y el bajo costo de su fabricación.

El objetivo de esta tesis ha sido el estudio de la interfase fluido-aire, por medio del desarrollo de un dispositivo microfluídico y método sencillo que permite obtener la viscosidad tanto de fluidos newtonianos e.g. plasma sanguíneo y sangre con un error no superior al 10%. Además de ser capaces de observar el comportamiento no-Newtoniano de la sangre, y a su vez, relacionar su viscosidad con características específicas de sus células rojas como la agregación y la flexibilidad de su membrana. Los estudios de sangre se realizaron a distintos hematocritos, distintos días desde la extracción de la muestra y muestras de anemia y alfa-talasemia.

La tesis ha sido desarrollada principalmente desde un punto de vista experimental y está separada en 2 partes. La primera contempla los resultados obtenidos en el estudio de frentes de fluidos Newtonianos. La segunda parte se centra en los resultados obtenidos para la sangre y su relación con las propiedades de sus células rojas.