The main concept of this article is to allow for the optimized tree-like design of the flow distributors to actually define the shape of the fuel cell, thereby eliminating problems associated with the mismatch between a predecided rectangular shape and a functionally preferred channel distribution system. The work focuses on direct methanol fuel cells (DMFC). A one-dimensional across-the-cell model is extended to a two-dimensional along-the-channel model and numerically solved to predict the polarization curves of a direct methanol fuel cell with tree network channels and of a cell with traditional serpentine channels. For both flow configurations, pressure drop and pumping power are estimated. Net power densities are computed in terms of constructal parameters and operating conditions. In contrast to the traditional rectangular shape of fuel cells, the resulting "pyramidal" or "double-staircase" shape is based on the functionality of the fluid distribution system. It is found that tree network channels can provide substantially improved electric and net power densities compared to the traditional non-bifurcating single serpentine channels, as a result of their intrinsic advantage with respect to both mass transfer and pressure drop. For six (12) branching levels and inlet channel diameters of 0.05, 0.04, and 0.03 cm, the tree network channels allow for 14% (21%), 17% (26%), and 30% (46%) higher net power densities, respectively. (c) 2005 Elsevier Ltd. All rights reserved.