*Article* **A Portable, Negative-Pressure Actuated, Dynamically Tunable Microfluidic Droplet Generator**

**Martin Trossbach <sup>1</sup> , Marta de Lucas Sanz <sup>1</sup> , Brinton Seashore-Ludlow <sup>2</sup> and Haakan N. Joensson 1,\***

<sup>1</sup> KTH Royal Institute of Technology & Science for Life Laboratory, 17165 Solna, Sweden

<sup>2</sup> Karolinska Institutet & Science for Life Laboratory, 17165 Solna, Sweden

**\*** Correspondence: hakan.jonsson@scilifelab.se

**Abstract:** Droplet microfluidics utilize a monodisperse water-in-oil emulsion, with an expanding toolbox offering a wide variety of operations on a range of droplet sizes at high throughput. However, translation of these capabilities into applications for non-expert laboratories to fully harness the inherent potential of microscale manipulations is woefully trailing behind. One major obstacle is that droplet microfluidic setups often rely on custom fabricated devices, costly liquid actuators, and are not easily set up and operated by non-specialists. This impedes wider adoption of droplet technologies in, e.g., the life sciences. Here, we demonstrate an easy-to-use minimal droplet production setup with a small footprint, built exclusively from inexpensive commercially sourced parts, powered and controlled by a laptop. We characterize the components of the system and demonstrate production of droplets ranging in volume from 3 to 21 nL in a single microfluidic device. Furthermore, we describe the dynamic tuning of droplet composition. Finally, we demonstrate the production of droplet-templated cell spheroids from primary cells, where the mobility and simplicity of the setup enables its use within a biosafety cabinet. Taken together, we believe this minimal droplet setup is ideal to drive broad adoption of droplet microfluidics technology.

**Keywords:** droplet microfluidics; plug & play; portable microfluidics; spheroids; microtissues
