Irradiation of a surface of a material with a gas cluster ion beam modifies the wettability of the surface. The wettability may be increased or decreased dependent on the characteristics of the gas cluster ion beam. Improvements in wettability of a surface by the invention exceed those obtained by conventional plasma cleaning or etching. The improvements may be applied to surfaces of medical devices, such as vascular stents for example, and may be used to enable better wetting of medical device surfaces with liquid drugs in preparation for adhesion of the drug to the device surfaces. A mask may be used to limit processing to a portion of the surface. Medical devices formed by using the methods of the invention are disclosed.