Abstract
Digital (droplet) microfluidics (DµF) is a powerful platform for automated lab-on-a-chip procedures, ranging from quantitative bioassays such as RT-qPCR to complete mammalian cell culturing. The simple MEMS processing protocols typically employed to fabricate DµF devices limit their functionality to two dimensions, and hence constrain the applications for which these devices can be used. This paper describes the integration of vertical functionality into a DµF platform by stacking two planar digital microfluidic devices, altering the electrode fabrication process, and incorporating channels for reversibly translating droplets between layers. Vertical droplet movement was modeled to advance the device design, and three applications that were previously unachievable using a conventional format are demonstrated: (1) solutions of calcium dichloride and sodium alginate were vertically mixed to produce a hydrogel with a radially symmetric gradient in crosslink density; (2) a calcium alginate hydrogel was formed within the through-well to create a particle sieve for filtering suspensions passed from one layer to the next; and (3) a cell spheroid formed using an on-chip hanging-drop was retrieved for use in downstream processing. The general capability of vertically delivering droplets between multiple stacked levels represents a processing innovation that increases DµF functionality and has many potential applications.
Highlights
Digital microfluidics (DμF) allows the dispensing, splitting, mixing, and translation of nanoliter- to microliter-sized droplets of liquid on a two-dimensional (2D) array of electrodes [1,2]
If the driving force for droplet insertion is greater than the minimum initial threshold but less than the final force required for injection into the bottom layer, vertical functionality can only be achieved if the height of the spherical cap protruding through the channel is large enough to allow the liquid to contact the opposing electrode to enable actuation
We evaluated the feasibility of utilizing vertical functionality to fabricate radially symmetric hydrogel gradients on a DμF device
Summary
Digital (droplet) microfluidics (DμF) allows the dispensing, splitting, mixing, and translation of nanoliter- to microliter-sized droplets of liquid on a two-dimensional (2D) array of electrodes [1,2]. The DμF device was relocated to an incubator after droplet manipulation, and after 24 to 48 h, the cells naturally aggregated into spheroids that were later used as tumor models in a drug screening assay. While this platform offers a new tool for 3D tissue development for automated drug screening, the difficulty in manually retrieving the spheroid limits downstream processing capabilities. A theoretical framework for droplet translation between stacked layers is presented and used to establish design parameters for DμF devices with vertical functionality. The bottom and middle plates contain driving electrodes and are used to translate discrete droplets of liquid during experiments. A voltage of approximately 110 VPP at 17 kHz was applied
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