Understanding the effect of hydrophobic protecting blocks on the stability and biopassivity of polymer brushes in aqueous environments: A Tiramisù for cell-culture applications

• Synthesis and characterization of diblock-copolymer brushes aimed at cell-culture experiments.
• Diblock copolymers had a hydrophobic, substrate-bound layer and a hydrophilic interfacial brush.
• Effect of the hydrophobic block on biopassivity and stability in cell-culture medium was determined.
• Higher stiffness and the hydrophobicity of substrate-bound block improved stability of system compared to brush alone.
• Grafting density of the substrate-bound layer was the main factor determining biopassivity.

Despite the wide range and versatility of polymer-grafting methods to produce biopassive interfaces, commonly applied polymer brushes suffer from an intrinsic lack of long-term stability when incubated in aqueous media under physiological conditions. The robustness of brush films in such environments can be greatly improved by applying a hydrophobic, “protecting” layer between the supporting substrate and the interfacial, biopassive brush. Block-copolymer brushes synthesized by sequential surface-initiated atom transfer radical polymerization (SI-ATRP) forming well-defined bilayered architectures can produce coatings with tunable interfacial properties and improved stability when subjected to cell-culture conditions. Three poly(n-alkyl methacrylates) (PAMA), exhibiting different mechanical properties, have been tested as protecting layers. A biopassive, hydrophilic brush is chosen to constitute the interfacial layer in all three cases. On the one hand, the structure of the interfacial layer clearly regulates film biopassivity, as well as its nanomechanical and nanotribological properties, as determined by atomic force microscopy (AFM) techniques. On the other hand, the composition and the mechanical properties of the polymer constituting the substrate-bound layer govern the stability of the entire bilayered film in the aqueous cell-culture environment. Precise control of bilayered brush architecture thus determines the film’s applicability for cell manipulation or biomaterials surface engineering.

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