  Biological membranes by virtue of their elastic properties should be capable
of propagating localized perturbations analogous to sound waves. However, the
existence and the possible role of such waves in communication in biology
remains unexplored. Here we report the first observations of 2D solitary
elastic pulses in lipid interfaces, excited mechanically and detected by FRET.
We demonstrate that the nonlinearity near a maximum in the susceptibility of
the lipid monolayer results in solitary pulses that also have a threshold for
excitation. These experiments clearly demonstrate that the state of the
interface regulates the propagation of pulses both qualitatively and
quantitatively. We elaborate on the striking similarity of the observed
phenomenon to nerve pulse propagation and a thermodynamic basis of cell
signaling in general.
