A method for medical treatment includes bringing an applicator, having an array of protrusions disposed thereon, into proximity with a skin surface of a patient so as to maintain a space between a plurality of the protrusions and the skin surface. Radio-frequency (RF) electrical power is applied to the applicator so as to cause electrical discharges to be generated, in response to the RF electrical power, between the plurality of the protrusions and respective points on the skin surface via a fluid medium in the space.