Abstract : The invention of µTAS concept (micro total chemical analysis system) in the early nineties introduced fluids in the microsystems world. A lab on chip integrates all the functions of a macroscopic laboratory (handling, mixing, heating liquids, filtering, separating, detecting molecules etc.) on a small surface (typically a few square centimetres). The technological challenge lies on the coupling between a conventional microsystem and a microfluidic network. As silicon and glass processes have been widely used in the nineties, they demonstrated major drawbacks: incompatibility of silicon technologies with high electric fields required for electrophoretic separations and/or electroosmotic pumping, inadequate technologies for large surfaces processing, integration issues in a complete system, high cost of the materials and associated processes, etc. The proposed solution in this thesis consist on directly constructing the microfluidic network above the conventional microsystem in photosensitive resists (SU-8), making easier the integration and allowing 3D structures fabrication with an excellent level to level alignment. Microfluidic characterization tools developed and used during this work are presented. Surface effects becoming essential at this scale, a generic modification strategy of physicochemical properties of SU-8 is proposed and characterized.