Voltage Phase Control for Enhanced Addressability in Highly-Parallel Digital Microfluidic Architectures

Digital microfluidic architectures have been a source of great enthusiasm for on-chip fluid applications requiring precise control and reconfigurability. Droplet-based systems operating with exceedingly small volumes (pL) can make use of digital microfluidic control systems to direct fluid motion using voltages on cascaded electrode structures. The voltage on these electrodes can be adapted via software, thus the generalized templates offered by digital microfluidic systems can be tailored for numerous end-user applications. The work presented here addresses the two major challenges for implementing these digital microfluidics systems for end-user applications: parallel addressability and reduced input voltages. The challenges are overcome through dual-phase AC voltage routing in a 16×16 digital microfluidic multiplexer using low (10 V_rms) input voltages. The first challenge, related to parallel addressability, comes about because of the generalized template for digital microfluidics, with underlying square-grid electrodes forming a two-dimensional, M×N, plane. Such a structure cannot be readily scaled up for use in single-layered highly-parallel architectures as external address lines cannot be effectively contacted to internal square electrodes lying within a 2-dimensional. With this in mind, the work here introduces multiplexing with a cross-referenced architecture having only M+N input lines. Microdroplets lie between orthogonal overlying row electrodes and underlying column electrodes, and nonlinear threshold-voltage localization is used to initiate motion of the desired microdroplet in the two-dimensional plane. Microdroplet interference (motion of undesired microdroplets) along the activated row and column is avoided, as the applied voltage initiates motion only at the overlapped electrode region (where the voltage is doubled and above-threshold). A dual-phase AC voltage control system is used to address the above bi-layered cross-referenced electrode structure and simultaneously provides a natural solution to the second, reduced voltage, challenge of practical digital microfluidic architectures. Reduced input voltages can be achieved in the digital microfluidic system through an integrated centre-tap AC transformer (a dielectric layer in the digital microfluidic multiplexer limits the current and power consumption, allowing for step-up voltage transformation). The dual-phase outputs from this voltage transformer are 180° out-of-phase, and the AC signals from these outputs are routed to the appropriate row and column electrodes to bring about above-threshold motion. Controlled switching is demonstrated in this work for input voltages below 10 V_rms. Structural and electrical design issues for this dual-phase AC digital microfluidic integrated chip are addressed in this work, and results are presented for an integrated digital microfluidic multiplexer prototype.