The 2-D microfluidic trap. a) Optical micrograph of a microfluidic manipulation device. Single particles are confined at a predetermined location within the junction of two perpendicular microchannels (trapping region). Two on-chip membrane valves (black) positioned above one inlet channel and one outlet channel are used as metering valves to control the relative flow rates through the opposing channels (red), thereby manipulating and trapping particles at the microchannel junction. b) Schematic of 2-D particle trapping. Two opposing laminar streams meet at the intersection of two perpendicular microchannels, creating a well-defined flow field containing a stagnation point where an object is trapped. c) The microfluidic manipulation device consists of a glass coverslip and a PDMS slab containing the microchannels and valves. Reprinted with permission 2013 American Chemical Society.