Polymerization From Lipid Membranes

In living cells, lipid bilayer membranes can be asymmetrically functionalized, with brush-like layers of macromolecules. These coatings, like the glycocalyx, often originate from the membrane and feature high surface concentrations. Yet in vitro, synthetic methods used to functionalize lipid membranes with macromolecules typically graft presynthesized macromolecules to a lipid bilayer. This approach limits the density and thickness of polymer layers. Here, we describe a lipid membrane-initiated polymerization reaction that allows for the growth of thick and dense polymer brushes directly from lipid membranes. By incorporating a novel lipid-based polymerization initiator into lipid bilayers, we could grow poly(N-isopropylacrylamide) (PNIPAM) brushes from supported lipid bilayers (SLBs), small unilamellar vesicles (SUVs), and giant unilamellar vesicles (GUVs), via aqueous atom transfer radical polymerization (ATRP). We used quartz crystal microbalance with dissipation monitoring (QCM-D) and dynamic light scattering (DLS) to quantify growth kinetics from SLBs and SUVs. In both cases, the thickness of the polymer layer increased linearly with time for tens of minutes. The resulting polymer brushes were up to 70 nm in thickness, more than fifteen times the thickness of the bare lipid bilayer. Growth from GUVs was monitored using confocal fluorescence microscopy, and led to the spontaneous transformation of spheroidal vesicles into dense, bush-like networks of “strings of pearls”. Broadly speaking, these unusually thick and dense brushes could offer improved performance for biomedical applications, a valuable in vitro model for the biophysics of asymmetric lipid membranes, and a new mechanism for synthesizing bioinspired materials.