The liquid droplet spray device (5) has a micromachined substrate (8, 18, 18a; 8a, 18, 18a), a space (9) within the substrate for containing a liquid substance (4), an inlet channel for supplying (7) the liquid (4) to the space (9), and outlet structure (13, 14, 15) for ejecting the liquid as a mono-dispersive droplet spray when the liquid undergoes a vibration. The outlet structure comprises a cut-off pyramid-shaped tapered cavity (13) which is micromachined in the space (9). The outlet structure further comprises at least one output (14) and output channel (15) connecting the tapered cavity (13) to the outlet (14), the outlet (14) and the output channel (15) having a non-tapered vertical shape. Thanks to such a device, the exact amount ejected may be determined accurately which allows for a predicted lung deposition of the droplets by using appropriate control means adapted to the kind of patient.