Microfluidic devices provide a unique tool to manipulate cells and perform complex tasks such as cell sorting. Their small size, in fact, enhances the nonlinear coupling between the fluid flow and the deformable cell membranes causing cells with different mechanical properties to behave differently. Our ability to model the fluid-structure interactions allows us to interpret the different behaviours, infer the cell mechanical characteristics and propose devices to achieve an effective sorting. I will discuss the modelling of a single-cell motion in microfluidic devices and I will show numerical results for proposed cell-sorting devices.