Mehmet Toner and his group at the BioMEMS Resource Center, Massachusetts General Hospital describe the illustration that appears on the cover of the current issue of Biophysical Journal. A longstanding interest in our lab is the sorting and separation of complex mixtures of cells, which is important for diagnostics, therapeutics, and basic cell biology. Our general approach has been based on the selective capture of cells using antibody-functionalized surfaces in microfluidic devices. However, interfacial phenomena at low Reynolds numberscan impose “speed limits” on how efficiently cells can be captured. For the cover image, we worked with medical illustrator Janet Sinn-Hanlon to visualize the two physical mechanisms utilized to circumvent these “speed limits.” Our microfluidic device incorporated a nanoporous, fluid-permeable membrane that allowed us to steer cells directly towards the bottom capture surface. To illustrate this, the background of the image shows a mixture of target cells (red) and other cells (green) entering the microfluidic channel. As these cells travel from the image background towards the viewer, we also emphasized their movement downward on the page. Janet conveyed this idea by progressively increasing the opacity of the cells and sharpening the focus. In addition, streamlines and bubbles were used to suggest a sense of movement in the fluid. After being transported to the membrane, the cells must also interact specifically with antibodies by “rolling” in order to be captured. Since the flow is primarily directed into the surface, the diminished flow along the surface promoted specific capture of target cells but also limited non-specific adhesion of other cells. This was illustrated by showing only the red cells rolling to a stop as they approached the foreground, while the green cells continued onward. This behavior is reminiscent of the in vivo “homing” of leukocytes and stem cells to porous vasculature during inflammation and injury. We are thrilled that our image was chosen for the cover of Biophysical Journal, in conjunction with the publication of this article. The fundamental ideas and issues we explored are highly relevant for the development of new microfluidic technologies to diagnose cancer, burns, and trauma, or HIV/AIDS. We hope this cover inspires public interest in the exciting research problems we investigate at the interface of the physical sciences, clinical biomedicine and technological development, which are described further at our website, http://cem.sbi.org.