Easy-disassembly bonding of PDMS used for leak-tight encapsulation of microfluidic devices

Microfluidic chips are widely used in the field of biopharmaceuticals, chemical synthesis, separation, and purification. Different requirements are asserted by different applications on the bonding processes of the microfluidic chips. In this paper, the easy-disassembly leak-tight encapsulation of the PDMS microfluidic chip was studied. A bonding method based on selective oxygen plasma treatment of PDMS was proposed. The size and distribution of the oxygen plasma treated area on the bonding surface could be controlled by shadow masks with certain patterns. Oxygen plasma treated and activated PDMS surface was irreversibly bonded to the substrate while the untreated bonding surface was mainly reversibly bonded through the Van der Waals force. Leak-tight encapsulation with high bonding strength has been realized with rather small irreversibly bonded area. Chips made by such method can not only be used with high-pressure fluid but also be disassembled easily without destroying the internal products in the microchannels afterwards. The failure mode and reliability of chips with different irreversibly bonded area were evaluated by the shear test and the pressure withstand test. During the shear test, peeling occurred first on the force-loaded side of the irreversibly bonded area of the PDMS. Cracks were then generated here and there through the edges of the bonded area. Propagation of the cracks finally resulted in the failure of the bond. The shape and size of the fracture surface resembled those of the irreversibly bonded area controlled by the shadow masks. The withstand pressure value tended to increase with the irreversibly bonded area under a rather large fluctuation. A more effective way to increase the withstand pressure was to enhance the uniformity of the irreversibly bonded area. With a proper treatment of substrates, the withstand pressure was increased by about 3 times. It was as large as 560 kPa when the irreversibly bonded area only accounted for 2% of the total bonded area. This met the requirements for pressure withstanding of most microfluidic chips. The uniformity rather than the irreversibly bonded area was the dominant factor in fabricating leak-tight microfluidic chips.