6I-1 Novel Echo Method Implementation to Locate Microdroplets Transported by SAW

Digitalized microfluidics is now seen as a relevant solution to implement programmable and addressable biology. It is based on liquid microdroplet manipulation for performing basic operations such as transporting, splitting, merging and mixing. In our aimed applications, the moving droplets contain proteins and must cross bio-active micropads to achieve specific operations (Renaudin et al., 2006). We thus need the droplet position for blind hitting these micropads. This paper addresses the issue of both transporting and locating liquid droplets by SAW. It is also known that SAW permit to locate a droplet from its transmitted signal. But implementation of this method requires a complex configuration of InterDigital Transducers (IDTs) and an arduous command of circuits (Alzuaga et al., 2003) or slanted finger IDTs working in a specific frequency bandwidth (Wu and Chang, 2005). We propose hereafter for the first time the droplet location from its SAW reflected signal. The implementation of this new echo method only requires a single standard IDT. The IDT can work alternately as a SAW emitter for actuation and as a receptor for the droplet reflected signal or echo. First a radiofrequency pulsed excitation RFPE_a is sent to displace the droplet. Once actuation is stopped a dedicated RFPE_l is sent for droplet location and the position is determined from the delay sigma between excitation and the corresponding reflected signal. A LiNbO₃ platform working at 20 MHz and fitted with four IDTs demonstrates the ability of this new method to achieve 5 mul droplet 2D displacement and location with submillimeter accuracy quite sufficient for biologic purpose in current lab-on-chips. The tested platform allows the tracking of water droplets running free or squeezed under a top cover. We have brought a new solution to the 2D-monitoring of microdroplets and thus hope helping to set a better coupling of SAW with microfluidics in lab-on-chips dedicated t- o biological applications so as to take advantage of SAW possibilities