Droplet microfluidics has found use in many bio-
logical assay applications as a means of high-throughput sam-
ple processing. One of the challenges of the technology, how-
ever, is the ability to control and merge droplets on-demand as
they flow through the microdevices. It is in the interest of
developing lab-on-chip devices to be able to combinatorically
program additive mixing steps for more complex multistep
and multiplex assays. Existing technologies to merge droplets
are either passive in nature or require highly predictable drop-
let movement for feedforward control, making them vulnera-
ble to errors during high throughput operation. In this paper,
we describe and demonstrate a microfluidic valve-based de-
vice for the purpose of combinatorial droplet injection at any
stage in a multistep assay. Microfluidic valves are used to
robustly control fluid flow, droplet generation, and droplet
mixing in the device on-demand, while on-chip impedance
measurements taken in real time are used as feedback to ac-
curately time the droplet injections. The presented system is
contrasted to attempts without feedback, and is shown to be

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• Impedance feedback control of microfluidic valves for reliable post processing combinatorial droplet injection