Implantable or insertable medical devices comprising a surface region that is modified by covalently coupling a molecular species (or a combination of molecular species) to the same. The molecular species are selected such that the resulting modified surface region has critical surface energy between 20 and 30 dynes/cm. In certain embodiments, the covalently coupled molecular species comprise one or more methyl groups. An advantage of the present invention is that novel medical devices are provided, which have a surface with a critical surface energy value that has been shown to display enhanced biocompatibility, including enhanced thromboresistance, relative to other surfaces.