  Understanding dynamic and complex interaction of biological membranes with
extracellular matrices plays a crucial role in controlling a variety of cell
behavior and functions, from cell adhesion and growth to signaling and
differentiation. Tremendous interest in tissue engineering has made it possible
to design polymeric scaffolds mimicking the topology and mechanical properties
of the native extracellular microenvironment; however, a fundamental question
remains unanswered: that is, how the viscoelastic extracellular environment
modifies the hierarchical dynamics of lipid membranes. In this work, we used
aqueous solutions of poly(ethylene glycol) (PEG) with different molecular
weights to mimic the viscous medium of cells and nearly monodisperse
unilamellar DMPC DMPG liposomes as a membrane model. Using small angle Xray
scattering (SAXS), dynamic light scattering, temperature modulated differential
scanning calorimetry, bulk rheology, and fluorescence lifetime spectroscopy, we
investigated the structural phase map and multiscale dynamics of the liposome
polymer mixtures. The results suggest an unprecedented dynamic coupling between
polymer chains and phospholipid bilayers at different length and time scales.
The microviscosity of the lipid bilayers is directly influenced by the
relaxation of the whole chain, resulting in accelerated dynamics of lipids
within the bilayers in the case of short chains compared to the polymer free
liposome case. At the macroscopic level, the gel to fluid transition of the
bilayers results in a remarkable thermal stiffening behavior of polymer
liposome solutions that can be modified by the concentration of the liposomes
and the polymer chain length.
