A composition made of at least 60 wt. % of a thermoplastic elastomer resin and additives that are solid at least from 0-50° C., that has a Shore A hardness that is less than about 50 bears a patterned surface, the pattern comprising at least one microfluidic channel having a cross-sectional dimension smaller than 100 microns is a substrate for forming a microfluidic device. The chief advantages of such compositions are: its ability to bond in a sealing manner to smooth surfaces of many different compositions, its ease of manufacture and microstructure patterning, and its general impermeability to liquids.