Corona discharge assisted thermal bonding of polymer microfluidic devices

We present a study on the use of corona discharge surface treatment to achieve a fast thermal diffusion bonding process for the creation of microfluidic chips. Wafer scale bonding at 100 mm diameter was attempted. Polymethyl methacrylate (PMMA) wafers were hot embossed to create microchannels before bonding to another blank PMMA wafer. Corona discharge treatment of the PMMA resulted in a more hydrophilic surface. The average water contact angle on PMMA surface decreased from 74.5° before treatment to 35.5° after the treatment. The optimized bonding condition was found to be: 108°C for 4 min at a contact pressure of 3.1 MPa. The bonded chips could withstand an internal gauge pressure in the microchannels of at least seven bars. The rectangular shape of the cross section of the microchannels was conserved with some contraction in the dimensions of 3.7% on the mean widths and 2.1% on the mean depths. Bonding strength was found to increase with the bonding temperature and time while the effect of bonding pressure is evident at lower pressures. At higher pressures, the effect of bonding pressure seemed to have reduced. These effects could be explained by the diffusion mechanisms of the process.