Mechanical stability of phase-segregated multicomponent lipid bilayers enhanced by PS-b-PEO diblock copolymers

Abstract

Polymeric additives affect the mechanics of phospholipid vesicles, but little is known about the effect to supported lipid bilayers that are phase segregated on the submicron length scale. In this study, we use AFM-based force mapping, by means of breakthrough forces, to quantify the spreading pressures and line tensions of raft-forming lipid bilayers consisting of dioleoylphosphatidylcholine (DOPC), egg sphingomyelin (ESM), and cholesterol (Chol) in the presence of diblock copolymers comprised of polystyrene (PS) and poly(ethylene oxide) (PEO), PS-b-PEO. Varying molecular weights of PS-b-PEO were used in the experiments. The presence of the polymer leads to higher breakthrough forces when compared to pure DOPC/ESM/Chol bilayers. The lipid–polymer composite made with a PS block radius of gyration comparable to the bilayer thickness and a PEO block length that is the shortest exhibits the highest breakthrough forces and hence is the most stable mechanically. The breakthrough force distributions are analyzed to extract the spreading pressures and line tensions of the lipid–polymer composites. The spreading pressure is seen to increase with the addition of PS-b-PEO, and on average, increases with decreasing PEO block length. Based on the results, we propose the incorporation of the PS moiety into the bilayer core as the main mechanism of this enhanced resistance to bilayer breakthrough by the AFM tip.