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Binding and unbinding of lipid membranes: A Monte Carlo study.

The behavior of interacting fluid membranes is simulated using a vectorized Monte Carlo code. The Monte Carlo data give clear evidence for the existence of continuous unbinding transitions from a bound to an unbound state of the membranes. The location of the phase boundary and, thus, the macroscopic membrane state is found to be strongly affected by shape fluctuations on microscopic length scales. The observed critical behavior confirms theoretical predictions even though the accessible critical region is severely restricted by the rapid growth of relaxation times. PACS numbers: 82.70.-y Biological membranes form the surfaces of cells and organelles and, thus, give rise to an amazingly complex and diverse architecture.' Nevertheless, all biomembranes have the same basic structure: A lipid bilayer which is "decorated" by various macromolecules. The physical properties of such bilayers have been intensively studied during the last decade.' One useful concept which has emerged is that the shape of membranes is mainly controlled by bending elasticity and, thus, by curvature. Likewise, typical shape fluctuations are thermally excited bending modes which can be directly observed in the microscope. In this paper, we explore the possibility of studying fluctuating membranes by computer simulations. Such simulations should be useful for many membrane phenomena. Here, we study the shape fluctuations of a membrane which interacts with another membrane or surface. This interaction has an attractive part which leads to a bound state of the membrane and, thus, to a confinement of its fluctuations. Such a confinement turns out to be crucial for computer simulations since the relaxation times grow very rapidly when the membranes become unbound. Interactions of membranes play an essential role for many biological and biophysical phenomena. For example,-attractive interactions between cell membranes lead to mutual adhesion or binding of cells. Likewise, many transport processes involve the binding and unbinding of vesicles to and from the membrane surfaces of cells and organelles.' This latter process can be used for the delivery of drugs to specific cell types. Another example is the construction of biosensors which is often based on the binding of membranes to solid surfaces.