A Scheduling and Routing Algorithm for Digital MicrofluidicRing Layouts with Bus-phase AddressingMegha Gupta and Srinivas AkellaAbstract— Digital microfluidic systems (DMFS) are a newclass of lab-on-a-chip systems for biochemical analysis. A DMFSuses electrowetting to manipulate discrete droplets on a planararray of electrodes. The chemical analysis is performed byrepeatedly moving, mixing, and splitting droplets on the elec-trodes. Recently, there has been a lot of interest in developingalgorithms and computational tools for the design, simulation,and performance evaluation of DMFS. In this paper, we presentan algorithm for coordinating droplet movement in batch modeoperations on ring layouts with bus-phase addressing. In bus-phase systems, each electrode is not individually addressable,instead a set of electrodes are all controlled by the same signal.Though this hardware design simplifies chip fabrication, itincreases the complexity of routing droplets. The presentedalgorithm allows multiple independent reactions, each withtwo reactants and one product, and chain reactions withmultiple stages, where each stage produces reactants for thenext stage, to take place simultaneously on the chip. Thisalgorithm is scalable to different number of reactions withina limit which depends on the size of the layout, placementof sources and number of phases used. It also addresses anysensor constraints under which droplets need to visit sensorlocations for specified amounts of time. We present simulationresults using our algorithm to coordinate droplet movementsfor example analyses on a ring layout.I. INTRODUCTIONLab-on-a-chip systems based on digital microfluidic tech-nology have recently generated a lot of interest in the field ofbiochemical analysis. Digital microfluidic systems (DMFS)manipulate discrete nanoliter-sized droplets on a planar arrayof electrodes. These systems can be used for rapid automatedbiochemical analysis, thus impacting a wide variety of ap-plications including biological research, genetic analysis, andbiochemical sensing. The biochemical analysis is performedby repeatedly moving, mixing, and splitting droplets on thearray. DMFS technology offers scalability, programmability,reconfigurability, and power reduction, and enables analysisof intermediate product droplets. These miniature systemsdrastically reduce the size of the equipment and the amountsof reagents used for the analysis. However, DMFS systemscurrently are programmed manually which greatly limits thescope of this technology, particularly when the number ofdroplets is large. Therefore, our goal is to design algorithmsfor the automated scheduling and routing of droplets in thesesystems.This work was supported in part by NSF under Award No. IIS-0093233and Award No. IIS-0541224.The authors are with the Department of Computer Science, Rens-selaer Polytechnic Institute, Troy, NY 12180 USA{guptam,sakella}@cs.rpi.eduOur focus in this paper is on DMFS systems that con-sist of ring layouts with bus-phase addressing [1]. In bus-phase systems, each electrode is not directly addressable.Instead, a set of electrodes are controlled by the same signaland are said to have the same phase. This design makesfabrication easier and minimizes the number of electricalcontacts. However, the complexity of droplet routing goes upsignificantly because a bus-phase system requires operationsto be synchronized and imposes constraints on the paral-lelism achievable in the analysis. Ring layouts with bus-phaseaddressing, described in detail in Section 3, have been usedto demonstrate droplet manipulation and pipelined glucoseassays1[1]. We are not aware of algorithms for automatedcontrol of droplets in these systems.We present an approach to completely automate batchmode operations on ring layouts. For batch mode, we takeone droplet each of all reactants to produce one dropleteach of all final products. Droplet routes are dynamicallychosen and the same set of electrodes is shared among alldroplets for transport, mixing and chemical reaction. Phaseassignment for a layout can be done at the fabrication stageand once that is done, different reactions can be carriedout on the same layout without changing the phases. Thealgorithm scales with the array size; it can handle a largernumber of droplets on larger layouts.II. RELATED WORKElectrowetting [2], where the interfacial tension of dropletsis modulated by a voltage, is an important method of actu-ation in a DMFS. A droplet moves to an adjacent electrodewhen the electrode is activated. Thus, by deactivating theelectrode the droplet is present on, and activating the adjacentelectrode simultaneously, a droplet can be manipulated on anarray of electrodes as desired.Droplet DropletControl ElectrodesTop ViewSide ViewHydrophobic InsulationTop PlateBottom PlateGround ElectrodeFiller FluidFig. 1. Droplets on an electrowetting array (side and top views). Thedroplets are in a medium (usually oil or air) between two glass plates.The gray and white droplets represent the same droplet in initial and finalpositions. A droplet moves to a neighboring activated electrode which isturned off when the droplet has completed its motion. Based on [2].Pollack, Fair, and Shenderov [2] first demonstrated thiskind of manipulation of discrete microdroplets (Fig. 1). Fair1Example videos of droplet manipulation on a ring layout may be seenat http://www.rsc.org/suppdata/lc/b4/b403341h/.Proceedings of the 2007 IEEE/RSJ InternationalConference on Intelligent Robots and SystemsSan Diego, CA, USA, Oct 29 - Nov 2, 2007ThA10.41-4244-0912-8/07/$25.00 ©2007 IEEE. 3144et al. [3] described experiments on dispensing, dilution,and mixing of samples in an electrowetting DMFS. Cho,Moon, and Kim [4] created, merged, split, and moveddroplets using electrodes covered with dielectrics in an airenvironment. Gong, Fan, and Kim [5] developed a portabledigital microfluidics lab-on-chip platform using electrowet-ting on dielectrics. Recent DMFS research has also focusedon applications. Pollack et al. [6] demonstrated the use ofelectrowetting-based microfluidics for real-time polymerasechain reaction (PCR) applications. Srinivasan et al. [7]demonstrated the use of a DMFS as a biosensor for glu-cose, lactate, glutamate and pyruvate assays, and for clinicaldiagnostics on human blood, urine, saliva, sweat, and tears.Ding, Chakrabarty, and Fair [8] described an architec-tural design and optimization methodology for schedulingbiochemical analysis. They
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