Metastability in pixelation patterns of coexisting fluid lipid bilayer phases imposed by e-beam patterned substrates

Abstract

We study the dynamic evolution of pixilation patterns of the liquid-ordered (L_o) phase in coexistence with the liquid-disordered phase in lipid multibilayers. The pixilation patterns were formed by imposing lattice patterns of localized high curvature on phase-separating multibilayers using curvature-patterned regions of an underlying support. The projected radius of underlying hemisphere-like features, that provided the local curvature, was varied from 60 nm to 100 nm and the square lattice spacing between the features was varied between 200 nm and 400 nm using standard electron (e)-beam lithography. Over time, the area fraction of the L_o phase on the patterned regions of the substrate decreased toward zero at room temperature. This apparent metastability of the pattern derives from the high line energy of a pixelation pattern where a Boltzmann distribution shows near zero equilibrium partitioning of the L_o phase in the patterned regions. Kinetic rate analysis identifies two pattern-dependent mechanisms that dominate the transition to zero L_o area fraction; diffusion limited dissolution of the L_o phase driven by an Ostwald ripening-type process or the cooperative formation of vesicles containing L_o phase lipids. Interestingly, we observed the spontaneous formation of tubules in the corners of the array due to the high local curvature applied to the membrane. Furthermore we show that it is possible to regenerate pixilation patterns on the curvature-patterned regions by cooling below room temperature. Regenerated area fractions are in agreement with a room-temperature composition of primarily L_d phase and the high degree of overlap with the original patterns is suggestive of fixed nucleation sites.