Molecular diffusion and nano-mechanical properties of multi-phase supported lipid bilayers

Understanding the properties of cell membranes is important in the fields of fundamental and applied biology. While the characterization of simplified biological membrane mimics comprising liquid phase lipids has been routinely performed due to the ease of fabrication, the characterization of more realistic membrane mimics comprising multi-phase lipids remains challenging due to more complicated fabrication requirements. Herein, we report a convenient approach to fabricate and characterize multi-phase supported lipid bilayers (SLBs). We employed the solvent-assisted lipid bilayer (SALB) formation method to fabricate mixed lipid bilayers comprising liquid phase 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and gel phase 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipids at room temperature. The fabrication procedure was performed inside a newly designed microfluidic chamber, which facilitated the subsequent characterization of the SLBs without exposure to air. The SLBs were then characterized via fluorescence microscopy, fluorescence recovery after photobleaching (FRAP), atomic force microscopy (AFM) and AFM-based force-distance measurements. Interestingly, results from these characterization techniques revealed that regardless of the gel phase composition, the SALB formation method consistently yielded uniform SLBs at room temperature, even though the transition temperature of DPPC is considerably higher. Furthermore, the composition ratio of DOPC and DPPC in the precursor solution is well reproduced in the fabricated SLBs. We also identified from diffusivity measurements that a high ratio of gel phase lipid revitalizes lipid-lipid interactions, which led to reduced molecular fluidity and the suppression of thermal undulation within the SLBs. Taken together, our results highlight the robustness of the SALB formation method that allows the fabrication of complex lipid bilayers with a high degree of precision, which is suitable for functional studies of biological membranes.