A 3-dimensional PDMS cell sorter having multiple passages in a PDMS layer that follow the same path in a DEP separation region and that are in fluid communication with each other within that region. The passages may differ in width transverse to the flow direction within the passages. Flat plates may sandwich the PDMS layer; each plate may have a planar electrode used to generate a DEP field within a sample fluid flowed within the passages. The DEP field may concentrate target cells or particulates within one of the passages within the DEP separation region. The passages may diverge after the DEP-separation region, leaving one passage with a high concentration of target cells or particulates. Techniques for manufacturing such structures, as well as other micro-fluidic structures, are also provided.