Reagent-free biomolecule functionalization of atmospheric pressure plasma-activated polymers for biomedical applications: Pathways for covalent attachment

Atmospheric pressure plasma jets (APPJs) are advancing as a versatile dry technology for creating biofunctional structures. Recently, reagent-free, single-step covalent immobilization of bioactive molecules onto surfaces was demonstrated. Despite this, the mechanisms governing the covalent attachment process remain obscure. Here, we studied morphological changes, concentrations of radicals, and the formation of reactive species on APPJ-treated polymers to shed light on the underlying mechanisms of covalent attachment. The APPJ-treated polyethylene surfaces, prepared either in air or with controlled ambient gas composition, were analyzed using Fourier transform infrared, X-ray photoelectron, electron spin resonance, and fluorescence spectroscopies, as well as atomic force microscopy. It was demonstrated that only non-radical reactive oxygen species (ROS) could explain the attachment. This attachment was also demonstrated on silicone (PDMS), broadening the range of possible applications. Finally, to identify reaction pathways, fluorinated carbon brushes were used, each presenting a specific functional group. Attachment data indicated that molecules with amine or thiol groups can be covalently bound to treated surfaces. As a result, reactions involving the ROS, hydroxyl, carbonyl, carboxyl, and peroxides are potential pathways. These findings provide a means of optimizing treatment features such as binding site density in future studies and applications, thereby expanding the capabilities of APPJ treatment within 3D bioprinters.