The potential of the device is demonstrated through the results of a micro-bead counting experiment. A 0.5 μl sample volume containing 10 μm polystyrene micro-beads in Dl water is deposited into an on-chip inlet reservoir from where it flows, under capillary action, through a buried flow cell. A laser on one side of the cell is forward biased and the opposing laser is operated as a photodiode (Fig. 2(a)). Both are pulsed to provide sub-μs time-resolution of bead transit 'events' with a 30 mV noise floor that affords a large dynamic range of over 1.2 V (Fig. 2(b)). After passing through the flow cell the sample fluid is drawn into a spiral patterned exit reservoir that provides sufficient pull through to sustain a continuous flow for over 30 s with flow rate of > 4mm/s.