Plasma Polymerization of TEMPO Yields Coatings Containing Stable Nitroxide Radicals for Controlling Interactions with Prokaryotic and Eukaryotic Cells

Stable organic nitroxide radicals have been shown to exhibit similar cell biology signaling properties as the well-known but short-lived small molecule nitric oxide, such as affecting intracellular redox states and cell proliferation behavior. Biological processes might thus be amenable to biointerfacial regulation via release of stable nitroxide molecules from coatings applied onto biomedical devices. In this study, we utilized the facile and technologically attractive process of plasma polymerization for the deposition of thin layers containing stable nitroxide radicals, using TEMPO (2,2,6,6-tetramethylpiperidin-1-yl)­oxyl as the “monomer” for creating a thin polymeric film. Coatings (TEMPOpps) produced under various conditions were characterized by ellipsometry, XPS, ToF-SIMS, and EPR as well as in vitro biological effects on bacteria (Staphylococcus epidermidis), fungi (Candida albicans), and human cancer cells (KG1a). TEMPOpps were compared with plasma coatings from three structurally related precursors that lack nitroxide groups. Surface characterization by XPS and ToF-SIMS confirmed the similarity of atomic composition and molecular fragments of the TEMPOpp films to the precursor molecule. Thin (241–312 nm) films were shown by EPR to contain stable nitroxide radicals, with a G-factor of 17 G typical of TEMPO. The plasma conditions modulated the density of radicals included in the films. On TEMPOpp surfaces, the microbial pathogens Staphylococcus epidermidis and Candida albicans exhibited reduced capacity to form biofilm, and fungal cells did not transition to hyphal forms. In addition, for the nonadherent human cancer cell line KG1a, we found that TEMPOpp coatings upregulated the cells’ intracellular reactive oxygen species (ROS) but were not cytotoxic. Thus, we demonstrate that TEMPOpp films with nitroxide radicals possess versatile promising biological activities, such as for coating biomedical devices to prevent infections.