Plasma-activated Inter-layer Bonding of Thermoplastics for Micro- and nano-fluidic Manufacturing

Plasma-activated polymer–polymer bonding is a promising way of encapsulating microand nano-fluidic channels across large substrate areas, without the substantial distortion of channel geometries that can plague thermallyand solvent-assisted bonding. The process involves treating the surfaces to be bonded with an oxygen or air plasma, and then pressing the surfaces together to allow an irreversible chemical bond to form [1]. A convenient method is desired for measuring the toughness of such a bonded interface. Simple crack-opening tests (whereby a blade prizes apart the two bonded layers and the length of the interlayer crack determines the bond toughness [2]) are clumsy and hard to automate. We propose that built-in microscopic crackopening test sites be distributed across manufactured substrates [3]. At each test site, a polymeric film bonded over a step in the substrate would peel back from the step after bonding, by a distance depending on the toughness of the bond. The presence of a wedge-shaped air gap between the covering film and the substrate leads to visible interference fringes, the spacing of which can be used to extract the bond strength (Figure 1). Arrays of these in situ cracks might be imaged without removing the substrate from a production line and would allow us to monitor both substrate-tosubstrate and cross-substrate bond toughness variation.