From the Tetrahedron Equation to Universal R-Matrices
  Modified universal R-matrices, associated with the central extension (through the Drinfeld's double construction) of the quantum groups U_q(sl_n), are realized through an infinite dimensional spectral parameter dependent solution for the tetrahedron equation, provided a certain identity on $q$-exponentials holds true. 
R. M. Kashaev, A. Yu. Volkov