We propose to develop a new approach for separating cells for cell-based screens. Separation of cells based upon biological differences-known as cell screening-is fundamental to both basic biology and biotechnology. These screens are an instrumental part of developing therapeutics and other biomolecles, and as such strongly impact human health. For both basic biology and biotechnology, one is interested in perturbing cells-to study or improve biosynthetic pathways-and then screening for desirable mutants in the population. These desirable mutants may produce new biomolecules or produce existing biomolecules more efficiently. In order to screen, one must have an experimental technique that can discern the phenotype of interest. The overall ability to study or engineer these organisms using screens is thus fundamentally dependent on our ability to measure. For biotechnology in particular, screening cells for biomolecule production typically requires development of a new assay for each biomolecule, even though the separation metric (e.g., production) is the same. Instead, we propose a new generic approach for separating cells based upon biomolecule production that is high-throughput, continuous, and real-time. This approach exploits the differences in electrical properties that exist between cells producing different amounts of biomolecule. Our hypothesis is that accumulating significant amounts of biomolecules will decrease cytoplasmic conductivity and permittivity in a way that we can exploit for particle separation. Our approach involves creating proportional spatial gradients of electric- field intensity and liquid conductivity, which causes cells to separate in space where their electrical properties match those of the liquid. We call this approach iso-dielectric separation (IDS). Our goal for this R21 proposal is to validate and characterize this approach with both test particles and bacteria. The relevance of this project to public health is that we are creating technology to better evaluate cells that produce biomolecules, including therapeutics. This has the potential to both increase the class of biomolecules available as well as more efficiently find & produce them.