Thin films from acrylic acid and Ar mixtures were deposited on polypropylene films, aluminum foils and silicon 100 wafers by radiofrequency (RF) plasma polymerization. Different deposition conditions were investigated, varying the gas-mixture feed composition and the reactor geometry. For every tested condition, stable coatings (as assessed by long time immersion in phosphate buffer saline) were obtained by varying the RF power input. The influence of the gas carrier on the layer stability was discussed. The films were further characterized by water contact angle measurements, attenuated total reflectance infrared spectroscopy, X-ray photoelectron spectroscopy and atomic force microscope. Moreover, retention of carboxylic acid groups in the stable layers were investigated by means of ion-exchange reaction with thionin acetate. Results show a strong influence of the gas feed composition and the reactor geometry on the chemical structure of the deposited coatings, especially in their carboxylic groups concentration.