High precision control of gap height for enhancing principal digital microfluidics operations

A variable gap size actuation (VGSA) mechanism is integrated into the digital microfluidic (DMF) system. The VGSA mechanism serves to optimize the aspect ratio during performing different microfluidic operations by changing the gap height between the top and bottom plates. This in effect will have a direct impact on the four main DMF operations including droplet transport, splitting, dispensing and merging as they are greatly affected by changing the aspect ratio. Experimental results demonstrate that the VGSA mechanism significantly enhances the principal DMF operations by retaining the appropriate gap height for each operation which is also dependent on droplets volumes when using fixed electrode size. Specifically, varying the gap height precisely between the two plates will enable us to transport the droplets more reliably, control the volume of the dispensed droplet, carry out splitting and merging more effectively, facilitate motion of residual droplets resulting from splitting or partial evaporation, enhance mixing at faster rates, achieve accurate positioning of droplets regardless of their volume, and minimize evaporation without complicating the DMF system with the use of a filler medium. The proposed mechanism is realized by accurate positioning of the top plate over the fixed bottom plate, instead of maintaining a fixed gap height during the operation. In this work, an experimental setup is constructed for the proof of concept to meet precise alignment requirements of the two parallel plates using a feedback-controlled positioning system. Three different methods viz., visual, capacitance-based and encoder values, are used to measure the gap height between the two plates precisely. For practical lab-on-chip devices, micro-actuators in conjunction with capacitance measurement feedback can be used to position the top plate during the operation. In this way, the proposed VGSA mechanism will introduce a mean for optimizing the parameters controlling the DMF operations. (C) 2013 Elsevier B.V. All rights reserved.