Over the past decade,there has been an increased interest in the development of miniaturized analysis devices.One of the most expected features in these miniaturized devices is the integration and automation of liquid operations for biochemical analysis applications [1].In spite of intensive effort on microfluidics,there has been no practical solution for the integrated and easy-to-use liquid handling technique.Thus,a power-free device is proposed in the present study for nanoliter fluidic handling on a chip.Fig.1 shows the schematic of the microfluidic device based on the pump/microchip assembly.It is assembled by reversibly bonding a PDMS slab containing a group of microchambers on a PDMS chip containing a microchannel network.The PDMS chip contains 24 parallel microstructure units.All units are connected by a common feeding channel and each unit comprises a centre metering structure,a peripheral metering structure,a reaction chamber,a peripheral inlet port,and an air venting port.In the combination,the PDMS slab acts as a pump to provide pumping power,which is pre-degassed and then attached to the outlet ports of the microchip to create a negative pressure for driving fluid by absorbing the air in the channels [2].In addition,a sudden contract of cross-section is designed at the end of each metering channel,which acts as a geometrical stop-valve.During operation,a pre-degassed modular PDMS pump was firstly placed onto the outlet ports of the microfluidic chip,and then aliquots of liquid samples were dispensed into the inlet reservoirs.Due to negative pressure generated inside microchannels by the air dissolution into the pre-evacuated PDMS pump,the samples were automatically sucked into the feeding channels.When the advancing liquid fronts reached the joints between the metering channel and the connection channel,they stopped owing to capillary pressure barriers.Subsequently,air was introduced into the feeding channel to remove the excess liquid,and droplets of precise volumes were left in the metering channels.With a further absorption of the air in the closed channel-reservoir system,the negative pressure increased and pushed the metered droplets into the connection channel,and finally transported the mixed droplet into the reaction chamber.Fig.2 shows snapshots of the metering and mixing of two food dye solutions in the proposed microfluidic device.