Entropic force between membranes reexamined

Lipid bilayers serve as model surfaces that are often used to understand the behavior of actual cell membranes. Cell adhesion (1, 2), membrane fusion (1), binding-unbinding transition (3), self-assembly, and related phenomena govern an astounding array of biological functions; for example, sperm-egg fusion is the basis of mammalian reproducion (1). These phenomena are dictated by a complex interplay between the various attractive and repulsive forces that mediate between biological membranes. The key role is played by a repulsive force termed “steric hindrance,” or simply entropic pressure, the origins of which lie in the thermally excited fluctuations of membranes. Nearly four decades ago, in a landmark paper, Wolfgang Helfrich (4) proposed both the concept and the quantitative nature of this force. Freund (5) reexamines this paradigm in PNAS, and finds that the entropic force is of a remarkably different character than hitherto believed. Ubiquitous van der Waals forces provide the weak attraction between biological surfaces; these vary as 1/c3 for close separations and transition to 1/c6 at larger distances (Fig. 1 A and B) (6, 7). Here, c is the mean distance between the interacting membranes. The notable aspect of the attractive force is that it is long-ranged. A catch-all phrase, “hydration forces” (8), is used to denote the repulsive force that becomes operative when membranes are nearly touching each other; this is quite short-ranged and drops off exponentially with distance. The underlying mechanisms of hydration forces are still debated and a subject of active research (9). A notable observation is that both the van der Waals and hydration interactions would be present even if membranes were perfectly rigid. The origins of a third interaction—the so-called “entropic force”—is predicated on the fact that biomembranes are (generally) quite flexible and the energetic cost of elastic bending …