Application of bio-functionalized coatings on bone implantable devices is a promising approach to direct rapid bone-implant integration. Plasma polymer (PP) films have become increasingly popular as platforms for surface bio-functionalization of implantable devices. However, the production of a reactive, yet stable PP film represents a technological challenge; as achieving a balance between the film's stability and functional group density is not trivial. Here we report the development of highly reactive and stable radical-functionalized PP films, using a combination of plasma polymerization and plasma immersion ion implantation. We provide new insights into the role of energetic ion bombardment on the growth mechanisms of plasma polymers by measuring the hydrogen content of PP structures using elastic recoil detection analysis. Nano-indentation and nano-scratch tests, as well as stability studies in simulated body fluid show a strong correlation between the degree of energetic ion bombardment and physico-chemical stability of the coatings. The potential of such ion-treated PP films to fabricate biofunctionalized implants that promote the functionality of primary osteoprogenitor cells is confirmed by studying cellular interactions after covalent attachment of fibronectin or bone morphogenetic protein (BMP)-2. We found that covalent attachment of fibronectin improved adhesion, spreading and proliferation of primary osteoblasts; whereas covalent attachment of BMP-2 enhanced the osteocalcin expression in bone-marrow isolated mesenchymal stem cells (MSC). These results present great promise for the fabrication of a new class of robust, biologically-functionalized interfaces for the surface engineering of biomaterials, particularly implants that need to be overgrown with bone-producing cells and thereby become firmly attached to host tissue.