Wave Propagation in Lipid Monolayers

TL;DR

This study measures sound wave propagation in lipid monolayers, revealing a phase-dependent velocity minimum and suggesting that nerve pulse propagation may involve adiabatic sound waves with physical property changes similar to biological signals.

Contribution

It provides experimental evidence linking lipid monolayer sound velocity to phase transitions and supports the hypothesis of nerve pulses as adiabatic sound waves.

Findings

Sound velocity exhibits a minimum at phase transition.

No significant attenuation indicates adiabatic propagation.

Physical property changes during wave propagation match nerve pulse measurements.

Abstract

Sound waves are excited on lipid monolayers using a set of planar electrodes aligned in parallel with the excitable medium. By measuring the frequency dependent change in the lateral pressure we are able to extract the sound velocity for the entire monolayer phase diagram. We demonstrate that this velocity can also be directly derived from the lipid monolayer compressibility and consequently displays a minimum in the phase transition regime. This minimum decreases from v0=170m/s for one component lipid monolayers down to vm=50m/s for lipid mixtures. No significant attenuation can be detected confirming an adiabatic phenomenon. Finally our data propose a relative lateral density oscillation of \Delta\rho/\rho ~ 2% implying a change in all area dependent physical properties. Order of magnitude estimates from static couplings therefore predict propagating changes in surface potential of 1-50mV, 1 unit in pH (electrochemical potential) and 0.01{\deg}K in temperature and fall within the same order of magnitude as physical changes measured during nerve pulse propagation. These results therefore strongly support the idea of propagating adiabatic sound waves along nerves as first thoroughly described by Kaufmann in 1989 and recently by Heimburg and Jackson, but claimed by Wilke already in 1912.