Enrichment of rare cell populations, such as Circulating Tumor Cells (CTCs) or parasitized Red Blood Cells (RBCs), is a tremendous challenge. Indeed, technologies must be able to reach high both purity and recovery rates while keeping the cells intact for downstream analysis. Two main approaches, affinity-based and label-free methods, have already been demonstrated in microfluidic format. Among them, the use of field gradients, electrical field for dielectrophoresis and magnetic field for immuno-magnetic separation, has gained a large interest. This paper demonstrates that PDMS-based composite materials present many advantages over metallic microstructures for the fabrication of active microfluidic devices. Indeed, we showed that these materials, combining properties of both the matrix and doping particles, allows the easy and fast integration of metallic microstructures using soft-lithography approach while preserving O 2 plasma bonding properties of PDMS substrate and avoiding cumbersome alignment procedure. Here, we focused on two different composites: (i) PDMS doped with carbon nanoparticles (C-PDMS) to obtain conductive polymer and (ii) PDMS doped with iron carbonyl microparticles (i-PDMS) to give this composite, ferromagnetic properties. We demonstrated that 3D conductive or ferromagnetic microstructures were easily obtained. C-PDMS electrodes showed dielectrophoresis capture of CTCs from a flowing model blood sample hence highlighting the interest of the composite material. Similarly, i-PDMS allowed the trapping of magnetically labelled cancer cells flowing at its vicinity, thanks to the generation of high magnetic field gradient.