Advances in nanotechnology, in particular the development of novel types of nanoparticles, will result in advanced tools for biomedical research and clinical practice. One exciting aspect of future nanomaterial research will be the possibility to combine therapy and imaging in multifunctional nanoparticle designs. In this context, anisotropic particles with subcellular dimensions may offer so far unattainable capabilities, because they could provide access to directional information with respect to nanoparticle-cell interactions. We have recently developed an electrified jetting process, which can produce water-stable polymer particles with two distinct phases. To address the first critical hurdle towards the application of these biphasic nanocolloids as imaging probes, short-term biocompatibility was evaluated using model cell culture systems. Exposure of human endothelial cells and murine fibroblasts to biphasic nanocolloids made of 0.5% polyacrylic acid and 4.5% poly(acrylamide-co-acrylic acid) did not affect cell proliferation as determined by a colorimetric proliferation assay. Moreover, double staining with Annexin V and propidium iodide and subsequent flow cytometric analysis indicated high cell viability, although slightly decreased viability was observed at the highest dose tested (1mg particles/10(6) seeded cells). Particle internalization as well as surface binding occurred simultaneously for both cell types, as evidenced by confocal laser scanning microscopy. Taken together, these results suggest excellent short-term biocompatibility in physiological systems for wide concentration ranges of the biphasic nanocolloids and open possibilities for future work investigating receptor- or surface marker-mediated targeting.