Mechanical Parameters of Lipid Bilayers

we argued that a portion of a finite-sized membrane can close upon itself to remove edge effects. In this way, vesicles are formed. The thermodynamic stability of vesicles is still a topic of hot debate in the literature, primarily because there are so many scenarios. The SCF analysis of vesicles leads to information on the mechanical parameters for a particular membrane system. 

The first point to be made is that the structure of the bilayers changes due to an imposed curvature. These curvature effects are easily monitored in an SCF analysis. Unless one is willing to do a three-dimensional analysis, the method is restricted to homogeneously curved bilayers, i.e. cylindrical or spherically shaped vesicles. 

In the SCF analysis of curved bilayers, it was found that all results could be fitted with the Helfrich equation, without the need to invoke a nonzero Jo. This means that the vesicles are typically stabilised by translational and undulational entropic contributions. This result is consistent with results by Leermakers [114] for uncharged lipid bilayers, and can be rationalised by symmetry arguments as discussed above. 

Going in the opposite direction, i.e. when we consider the membrane stability with increasing ionic strength, we notice the approach of k towards zero. Going towards this value, the tendency of the bilayers to form saddle-shaped connections (also called stalks ) between bilayers increases. Saddle-shaped membrane structures also occur in processes like vesicle fusion, endo and exocytosis. The SCF predictions thus indicate that these events will occur with more ease at high ionic strength than at very low ionic strength. 

---