A new class of nanofabricated membranes whose pores are about the size of the molecules being filtered could improve macromolecular separations. Philippe M. Fauchet, James L. McGrath, and coworkers at the University of Rochester describe porous, nanocrystalline silicon membranes that are approximately 10 nm thick with mean pore sizes between 5 and 25 nm (Nature 2007, 445, 749). The researchers use standard silicon deposition and etching techniques to make the membranes. They tune the size of the pores, which appear as white spots in the micrograph shown, by changing the temperature during fabrication. The membranes can separate proteins of different sizes and molecular weights. As an example, the researchers separated the common blood proteins bovine serum albumin (BSA) and immunoglobulin-γ (IgG) with such a membrane. BSA diffuses through the membrane more rapidly than does IgG, allowing for a higher degree of separation of BSA. The researchers expect to be able to fabricate a membrane with pore sizes that block IgG while letting BSA through. Such membranes could be easily incorporated into microfluidic devices.