Capacitive sensing for monitoring of microfluidic protocols using nanolitre dispensing and acoustic mixing.

The development of protocols for bio/chemical reaction requires alternate dispensing and mixing steps. Whilst most microfluidic systems use the opening of additional parts of the channel to allow the ingress of fixed volumes of fluid, this requires knowledge of the protocol before the design of the chip. Our approach of using a microfluidic valve to regulate the flow into an initially empty cavity allows for on-chip protocol development and refinement. Mixing is provided by way of surface acoustic wave excitation; this high-frequency vibration causes steady-state streaming flows. We show that capacitive sensing can be used to measure fluid levels, even if multiple fluid types are used, such that nanolitre dispensing accuracy is achieved. Also, the capacitive readout can be used to establish mixing quality and to monitor temperature fluctuations. These capabilities allow for protocols to be conducted without optical assessment, and so will allow for multiplexing, such that reactions could be conducted, simultaneously, in multiple chambers across a chip.