A microfluidic device containing a constructal-based branched flow structure with an externally controllable complexity was developed and tested in this study. The device was fabricated in the form of a 10 mm square by 9 mm high monolithic silicon/polydimethylsiloxane (PDMS) test section which contains three interconnected flow channels in a tee configuration. Before and after branching, the flow channels have widths of 464 and 249 pm respectively, and a uniform height of 195 pm. The complexity of the flow structure is changed by rerouting the flow of fluid using three pneumatically actuated microvalves that are incorporated into the test section. The microvalves, which are positioned in strategic locations within the test section, are made of a 48 pm thick PDMS membrane and have a diameter of 2 mm. Experiments were run aiming to characterize the pneumatic and mechanical properties of the valves. A separate series of experiments were performed to determine how the hydrodynamic and thermal performance of the test section was affected by the flow channel configuration, which was controlled by opening and/or closing the valves. The data collected showed that the valve design was robust and the mechanical integrity of the microvalve's membrane is far beyond what is required by the operational conditions (e.g., control pressures and deflections) considered in the present experiments. Also, as expected, the results show that the hydraulic losses and the amount of heat transferred through the channels is highly affected by the flow channel footprint. These results demonstrate, in principle, a means by which the complexity of a branched flow structure could be dynamically controlled in order to maintain an optimal configuration when operational conditions vary. (C) 2011 Elsevier Ltd. All rights reserved.