A highly-integrated polymer-based microfluidic device for disposable applications is presented. This device is plastic injection molded and exhibits three levels of integration. First, fluidic interconnects are monolithically formed with the device, enabling robust manufacturing and high-pressure operation (>500 psi). Second, a metal layer is lithographically patterned in the form of microheaters. Finally, a thermally-sensitive hydrogel valve is integrated into the channel. The valve is normally closed at room temperature. Upon heating to above the lower critical solution temperature of 32°C, the polymer valve becomes hydrophobic, shrinks while forming large pores, and permits flow. The device has been actuated reliably over 100 times with no apparent degradation.

The device is polymer-based and therefore much less expensive than microfluidic chips based on traditional substrates. The fabrication process consists of a five-step process. First, the cyclic olefin copolymer (COC) chip is injection molded with a single 100 × 100 μm microchannel. Second, before enclosing the channel, 20nm of chrome and 100nm of gold are thermally evaporated onto a COC cover slide. The metal layers are then etched to define 25 μm heater traces using standard photolithographic procedures. Third, the two parts of the chip are bonded by exposing the structured-half of the chip to solvent vapor and applying pressure. Alignment between the channel and the heater is obtained using a custom alignment fixture under an optical microscope. Fourth, the walls of the channel are surface modified to ensure covalent attachment of the valve to the channel wall. Fifth, the valve is prepared in situ by filling the channel with a polymerization solution and exposing selected regions with UV light through a photomask. A complete working prototype can be produced in less than two hours, demonstrating exceptional manufacturability.

This level of integration affords many advantages. Compared with off-chip heating, the valves exhibit a 400% faster turn-off response using the integrated on-chip heaters. Reliable dosing of less than 5 nl aliquots is demonstrated. Additionally, higher spatial resolution is possible with on-chip heaters allowing for higher channel densities and peristaltic pumping.