Actuators at the tip of a sub-millimetric catheter could facilitatein vivointer-ventional procedures at cellular scales by enabling tissue biopsy, manipulationor supporting active micro-optics. However the dominance of frictional forcesat this scale makes classical mechanism problematic. In this paper, we reportthe design of a micro-scale piston, with a maximum dimension of 150μm,fabricated with two-photon lithography onto the tip of 140μm diameter cap-illaries. An oil drop method is used to create a seal between the piston andthe cylinder which prevents any leakage below 185 mbar pressure differencewhile providing lubricated friction between moving parts. This piston gener-ates forces that increase linearly with pressure up to 130μN without breakingthe liquid seal. The practical value of the design is demonstrated with its inte-gration with a micro-gripper that can grasp, move and release 50μm micro-spheres. Such a mechanism opens the way to micron-size catheter actuation.