Substrate for Supported Lipid Bilayers Affects Domain Mobility and Phase Behaviour

Abstract

A wide range of biophysical techniques have enabled a detailed structure of the phospholipid bilayer to be established. In particular, Fluorescence Microscopy, AFM, NMR and XRD have been used to elucidate the lateral heterogeneity of phase separating membranes. Different techniques can use different model membranes, such as Giant Unilamellar Vesicles (GUVs), Black Lipid Membranes and Supported Lipid Bilayers (SLBs). Heterogeneity in SLBs can be investigated using many surface sensitive techniques, but different substrates are often used. AFM uses Mica, Fluorescence Microscopy/FCS/Fluorescence Recovery after Photobleaching (FRAP) generally use glass, and QCM-D uses Silicon Oxide. Another substrate of recent interest for SLBs is polydimethylsiloxane (PDMS). This cheap and relatively inert polymer is simple to mould into different patterns and can be used to design a wide-range of biotechnological devices. These include drug-screening chips and microfluidic devices. We are interested in how substrates affect the phase behaviour of SLBs. We have investigated how different substrates affect 1) the hydrophobic and thermodynamic drive for bilayers to form, 2) the diffusion of individual lipids, domain formation and hydrodynamic motion of domains, and 3) the lipid ordering and melting transition. We find that whilst molecular diffusion is hardly affected by changes in substrate, domain mobility is significantly hindered on glass and PDMS compared to mica, likely due to a combination of increased surface roughness and increased hydrophobicity. We also show that nanoscale domains on PDMS can only be observed using ‘Partial Penetration AFM’, where imaging force is controlled to selectively break through different phases.