[80][121][133][124] - Physics-based modeling is the most promising approach to achieving realistic facial deformations. Physical models of the skin have been proposed which make use of unilayer and multilayer spring meshes and finite element meshes. The multilayer models may include volumetric properties such as skin volume preservation. Deformations are induced by forces and simulated by solving the dynamic equations of the physical system. Several methods have been developed to define forces, including embedded muscle models. Physics-based models are also suitable for realistically modeling the dynamics of hair and the fluid dynamics of tears.