Passive droplet control in microfluidic networks: A survey and new perspectives on their practical realization

Two-phase flow microfluidics is a promising platform for realizing Lab-on-a-Chip (LoC) devices, that perform different laboratory functions on a single chip. This is accomplished by processing droplets, containing biological/chemical samples, by different elements each performing specific operations. Usually, the sequence of elements in which droplets are processed is fixed, which limits the flexibility, effectiveness and reusability of such LoC devices. Recently, microfluidic networks for two-phase flow microfluidics have been introduced with the aim of realizing programmable and flexible LoC devices. In particular, the goal is to dynamically and passively assign the droplets' path through a microfluidic network, which enables to reuse the LoC devices for different laboratory functions. This paper presents the state-of-the-art and discusses various aspects of the practical realization. First, we provide a survey on microfluidic networking, including passive switching, network topologies and validation methods. Second, we propose a simple Droplet-on-Demand (DoD) system, which allows generating droplets at prescribed times and with prescribed volumes - a crucial accomplishment in order to exploit the potential of microfluidic networks for practical purposes. We verify its functionality through experimental results. Third, we describe two promising applications for microfluidic networks, namely fast and flexible drug screening and screening of waterborne pathogens. Finally, we discuss future research opportunities and challenges. (C) 2018 Elsevier B.V. All rights reserved.