Structure of symmetric and asymmetric "ripple" phases in lipid bilayers
  We reproduce the symmetric and asymmetric ``rippled'' $P_{\beta'}$ states of lipid membranes by Monte Carlo simulations of a coarse-grained molecular model for lipid-solvent mixtures. The structure and properties compare favorably with experiments. The asymmetric ripple state is characterized by a periodic array of fully interdigitated ``defect'' lines. The symmetric ripple state maintains a bilayer structure, but is otherwise structurally similar. The formation of both ripple states is driven by the propensity of lipid molecules with large head groups to exhibit splay. 
Olaf Lenz and Friederike Schmid