  W. Goldman and V. Turaev defined a Lie bialgebra structure on the $\mathbb
Z$-module generated by free homotopy classes of loops of an oriented surface
(i.e. the conjugacy classes of its fundamental group). We develop a
generalization of this construction replacing homotopies by thin homotopies,
based on the combinatorial approach given by M. Chas. We use it to give a
geometric proof of a characterization of simple curves in terms of the
Goldman-Turaev bracket, which was conjectured by Chas.
