Conventional fabrication of microfluidic channels/devices faces challenges such as single use channels, material incompartibility, and/or significant time consumption. We propose a flexible platform for fabricating microfluidic channels simply through indentation on a smart composite-the so-called ST3R (Stiffness Tuning through Thermodynamic Relaxation) composite. The application of the ST3R composite allows rapid fabrication of microfluidic channels by hand or with a prefabricated stamp and precise prototyping of complex designs using a 2D plotter. Indenter geometry, applied stress, filler loading, and number of repeated indentations affect channel dimensions and/or shape. These channels further exhibit the following: (i) substantial improvement against swelling by organic solvent, in part due to the high modulus of the solidified metal network, and (ii) channel reconfigurability by heating the solidified undercooled metals. ST3R composite slabs have the potential to serve as microfluidic "breadboards", from which complex channels can be integrated in a flexible manner.