Digital microfluidic (DMF) liquid handling includes active (electrostatic) and passive (surface tension) mechanisms for reagent dispensing. Here we implement a simple and straightforward Teflon-AF liftoff protocol for patterning hydrophilic sites on a two-plate device for precise passive dispensing of reagents forming virtual microwells--an analogy to the wells found on a microtitre plate. We demonstrate here that devices formed using these methods are capable of reproducible dispensing of volumes ranging from ~80 to ~800 nL, with CVs of 0.7% to 13.8% CV. We demonstrate that passive dispensing is compatible with DMF operation in both air and oil, and provides for improved control of dispensed nano- and micro- litre volumes when compared to active electrostatic dispensing. Further, the technique is advantageous for cell culture and we report the first example of reagent dispensing on a single-plate DMF device. We anticipate this method will be useful for a wide range of applications--particularly those involving adherent cell culture and analysis.

Three-dimensional (3D) hydrogel structures are finding use in fundamental studies of self-assembly, rheology, and 3D cell culture. Most techniques for 3D hydrogel formation are 'single pot', in which gels are not addressable after formation. For many applications, it would be useful to be able to form arrays of gels bearing mixtures of constituents and/or formed from composites of different gel materials. Here, in response to this challenge, we introduce a digital microfluidic method for 'on-demand' formation of arrays of microgels bearing arbitrary contents and shapes. On formation of the gels, each microgel is individually addressable for reagent delivery and analysis. We demonstrate the utility of the method for 3D cell culture and higher-order tissue formation by implementing the first sub-microlitre recapitulation of 3D kidney epithelialization. We anticipate this platform will enable new research that can exploit the flexible nature of this technique for forming and addressing arrays of hydrogels with unique geometries and contents.

Digital microfluidics (DMF) is a droplet-based liquid-handling technology that has recently become popular for cell culture and analysis. In DMF, picoliter- to microliter-sized droplets are manipulated on a planar surface using electric fields, thus enabling software-reconfigurable operations on individual droplets, such as move, merge, split, and dispense from reservoirs. Using this technique, multistep cell-based processes can be carried out using simple and compact instrumentation, making DMF an attractive platform for eventual integration into routine biology workflows. In this review, we summarize the state-of-the-art in DMF cell culture, and describe design considerations, types of DMF cell culture, and cell-based applications of DMF.