  Supported lipid membranes are useful and important model systems for studying
cell membrane properties and membrane mediated processes. One attractive
application of supported membranes is the design of phantom cells exhibiting
well defined adhesive properties and receptor densities. Adhesion of membranes
may be achieved by specific and non-specific interactions, and typically
requires the clustering of many adhesion bonds into "adhesion domains". One
potential mediator of the early stages of the aggregation process is the
Casimir-type forces between adhesion sites induced by the membrane thermal
fluctuations. In this review, I will present a theoretical analysis of
fluctuation induced aggregation of adhesion sites in supported membranes. I
will first discuss the influence of a single attachment point on the spectrum
of membrane thermal fluctuations, from which the free energy cost of the
attachment point will be deduced. I will then analyze the problem of a
supported membrane with two adhesion points. Using scaling arguments and Monte
Carlo simulations, I will demonstrate that two adhesion points attract each
other via an infinitely long range effective potential that grows
logarithmically with the pair distance. Finally, I will discuss the many-body
nature of the fluctuation induced interactions. I will show that while these
interactions alone are not sufficient to allow the formation of aggregation
clusters, they greatly reduce the strength of the residual interactions
required to facilitate cluster formation. Specifically, for adhesion molecules
interacting via a short range attractive potential, the strength of the direct
interactions required for aggregation is reduced by about a factor of two to
below the thermal energy $k_BT$.
