Miniature Chip Detects Cancer Cells

Cancer cells that migrate into bloodstream are called circulating tumor cells or CTCs. In 2007, a team led by biochemical engineer Mehmet Toner of Massachussets General Hospital in Boston devised a method to trap and detect CTCs on a silicon chip the size of a microscope slide etched with micro channels each no wider than a hair. Toner pumped samples of whole blood through the channels, which were coated with an antibody designed to trap any cancer cell that carries a common surface protein, much as flypaper snags pesky insects. But cancer cells without that protein, such as melanoma slid past undetected.

The new device gets around that limitation. Called the CTC-iChip system (the "i" is for "inertial focusing"), it targets blood cells instead of cancer cells. Sorting by cell size, the first chip skims off small red blood cells and platelets, letting only CTCs and white blood cells flow past. Then, a second chip winds the cells through curving channels, channeling the remaining cells into a single-file line. Magnetic beads the size of a bacterium attach to specific surface proteins on white blood cells, and a magnetic field nudges these cells out of the stream of CTCs. That leaves just the CTCs, which can be collected in a vial and individually analyzed by conventional lab methods.