Giant Unilamellar Vesicles with Internalized Poly-(n-Isopropylacrylamide)-Vinyl Ferrocene Copolymer (PNIPAAm-VFC), a New Membrane-Interactive Thermoresponsive Material

Abstract

It is very challenging to develop artificial systems mimicking components of the biological cell, but offers rewards with respect to a better understanding of cell processes, function and complexity. Giant unilamellar vesicles are micro-sized biomimetic compartments. They possess a simple, very important structural feature of natural cells, the double layer membrane, but are otherwise limited in internal functionality. There are a few existing methods of introducing more complex internal structure into liposomes. Internalizing water soluble polymers like poly-N-isopropylacrylamide (PNIPAAm) or poly-ethylene glycol (PEG) is a suitable method to increase concentration, viscosity, and to achieve compartmentalization, thus approaching more complex artificial cell architectures [1-3].This time we want to present a new type of thermoresponsive polymer, PNIPAAm with co-polymerized vinyl ferrocene (VFc)[4]. The increased hydrophobicity achieved by 3% (mass) ferrocene, promised better dynamic properties, reduced equilibrium compartment size and more homogenous hydrogels. During our investigation, an exceptionally strong interaction between PNIPAam-VFc and the vesicle boundary was observed. Polymer chains are anchored in the bilayer membrane by means of the lipophilic metallocene groups, creating unexpectedly strong attachment points. After increasing the temperature above the lower critical solution temperature where gel formation sets in, numerous lipid nanotubes are pulled from the vesicle, connecting the vesicular membrane with the internal hydrogel compartment surface. This leads to spontaneous shape changes of the vesicle, associated with multiple protrusion formation, followed by rapid coarsening to a single liposome.1. Karlsson, A., et al., Journal of Physical Chemistry B, 2005. 109(4): p. 1609-1617.2. Cans, A.S., M. Andes-Koback, and C.D. Keating, Journal of the American Chemical Society, 2008. 130(23): p. 7400-7406.3. Markstrom, M., et al., Langmuir, 2008. 24(9): p. 5166-5171.4. Nagel, B., A. Warsinke, and M. Katterle, Langmuir, 2007. 23(12): p. 6807-6811.