Fabrication of SU-8 free-standing structures embedded in microchannels for microfluidic control

The application of cantilevered structures as check valves or flow sensors can provide new possibilities towards the integration of accurate sample preparation systems within a lab-on-a-chip. The cantilevers presented in this paper act as flaps enclosed within a channel in a direction perpendicular to the flow. This orientation allows simpler designs and easier integration of the valve or flow sensor within the microfluidic network. The cantilevers have been embedded in a microfluidic channel by low temperature full wafer adhesive bonding. In this way, electrodes, microchannels, microchambers and cantilevers can be fabricated and sealed at the same time at a wafer level. To the author's knowledge, this is the first example of flap cantilevers embedded in a polymeric microfluidic channel. The mobility of the structure and the leakage are dependent on the size of the sealing gaps between the cantilever and the enclosing channel. In this paper, we present three different fabrication methods for a range of bottom sealing gaps from the micro to the nanometer size. The top sealing gap is determined by the adhesive bonding and is 11 µm wide. Furthermore, various geometrical features have been introduced in order to optimize a valve or flow sensor. The characterization of the structures comprises measurements of the sensitivity of each cantilever design by obtaining their relative spring constant, measurements of their elastic and plastic working regimes and Young's modulus of the SU-8.