Hydrodynamic focusing of flows can be used for a variety of applications. This paper presents the design of a microdevice that
performs 3D hydrodynamic flow focusing, with the advantage of being capable to be integrated into bidimensional arrays. The device is formed by two structures that conduct different fluids from different inlet ports to a common outlet. Near the outlet, a three-dimensional structure is built that performs the focusing of one fluid inside the other one. The device is built on silicon substrates. The paper describes the fabrication process in detail, and some preliminary tests made on a prototype are presented. Numerical simulations of the fluidic behavior are also discussed.