Engineering of microfluidic devices for cell hydrodynamic manipulation

Abstract

In the recent years microfluidic technology has affirmed itself as a powerful tool in medical and biological research. Micro-flowcytometers, micromixers, cell sorters and cell analyzers are only few examples of the developed devices. Among these different applications, cell manipulation in microfluidics has been widely investigated. Various methods for cell manipulation have been proposed, such as hydrodynamic, magnetic, optical, mechanical, and electrical, in this way categorized according to the manipulating force employed. In particular, in manipulating cells by hydrodynamic forces, since there is no needing of applying external forces, the design and fabrication phases of the device are simplified, and at the same time undesired effects on the biological sample are avoided. This thesis has been focused on the design, fabrication and testing of microfluidic devices that allow to make a complete cell characterization by only exploiting hydrodynamic effects. The worldwide used technology for cell sample analysis is flowcytometry. Since cells focusing is the key point for the correct operation of a flowcytometer, several efforts have been made in order to reproducing it in a proper way on a micro-chip. In most cases, the traditional hydrodynamic focusing mechanism employing sheath fluid has been translated to the micro-scale. The main drawback in the approaches proposed till now, is that multiple inlets are needed. This brings to a high complexity if parallelization has to be introduced into the device. However, conventional flow cytometry measurements lose some information, if we consider that for the fluorescent signal only the information related to the total intensity is collected. This means that it is not possible to know the fluorescent signal distribution on the cell surface. This limitation has been recently overcome by introducing a new procedure, known as imaging flow cytometry, that allows to image each cell at high speed and therefore make available also the fluorescent distribution. Nevertheless, the traditional approach used for imaging cells in microfluidic systems allows to image cells by a single point of view. A way in which cells can be imaged by different sides, without the needing of moving the acquisition system, is to induce cell rotation as they flow through the channel. In most of the microfluidic devices for hydrodynamic focusing multiple inlets are needed, and this is a limitation if you want to introduce more parallel channels. Starting from this consideration, the first part of this thesis has been related to the realization of a microfluidic device that allows to integrate hydrodynamic focusing and parallelization in a single device thanks to the fact that only one inlet is required. This is achieved by introducing a cross-filter region at each one of the parallel channels. On the second hand, in this thesis a microfluidic device that allow to control cell rotation by only taking advantage of hydrodynamic forces has been realized. The basic idea started from the consideration that, in a pressure driven flow, the velocity profile is parabolic. In particular, the velocity is minimum at the channel walls and reaches its maximum value at the center of the channel. This means that, if a cell is close to a channel wall, it will experience a velocity gradient on its surface, and therefore a rotation will be induced. A microfluidic device that allows to focus cells at the channel wall has been developed, and the correct device operation in inducing cell rotation has been tested by using asymmetrical shaped cells.