Expanding the droplet microfluidic community – towards a modular active platform
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Microﬂuidics has progressed tremendously as a ﬁeld over the last two decades. Various areas of microﬂuidics developed in fully-ﬂedged domains of their own such as organ-on-a-chip, digital and paper microﬂuidics. Nevertheless, the technological advancement of microﬂuidics as a ﬁeld has not yet reached end-users such as chemists and biologists for independent use. A modular automated platform is envisioned to provide the stacking and modularity required to lower the knowledge barrier for end-users; hence, microﬂuidics will simply be considered as a tool for the application-focused researchers. The numerous advantages of droplet microﬂuidics–self-contained reactions, shorter reaction times, lower reagent consumption–will be leveraged by non-microﬂuidic researchers. However, the technological and knowledge gaps between the current state of the ﬁeld and the modular automated platform is prohibitively large. This thesis aims to signiﬁcantly advance the automated technology and to target key issues restricting droplet microﬂuidic accessibility. The main advancements are separated into two categories: technological and knowledge-focused. The technology-focused advancements (semi-automated droplet control, open-source pressure pump, critical system overview) are necessary stepping stones towards the development of a fully automated modular microﬂuidic platform. The knowledge-focused contribution (air tubing dynamics, droplet resistance, microﬂuidic chip compliance) enable the smarter development of the technology; better understanding and modelling the system is of special importance for active control. Furthermore, a deeper understanding of the system can be leveraged for other active platforms and passive microﬂuidic devices. The semi-automated droplet manipulations are implemented in an additional layer of the control algorithm. The functionality enables a user to set the droplet length or split ratio before automatically performing the manipulation. The droplet generation accuracy is ± 10 % of the length and a monodispersity of ± 1.3 % for 500-µm-long droplets. The splitting ratio resolution is limited by the channel width for the daughter droplet length; the accuracy is ± 4 % of the initial length. The droplets are merged on-demand. Finally, the eﬀective mixing of the droplet is demonstrated. The manipulations are leveraged in a qualitative drug screening assay that showcases the potential of the platform. µPump is an open-source pressure pump that targets microﬂuidic users. Researchers focusing on either passive or active microﬂuidics can beneﬁt from this system. A similar application performance (i.e. consistency in droplet volume) is achieved for a lesser price tag than comparable commercial systems. Furthermore, the open-source nature of the system enables a better understanding of the actuation limitation and the communication protocol. The air tubing connecting the pressure pump outlet to the reservoir holder is generally neglected. The dynamics of the tubing are investigated. For 1/16” inner diameter, the dynamic eﬀects are negligible for a length up to 60 cm and a pressure resolution of 2 mbar. For the 1 mm inner diameter tubing, the dynamic eﬀects are signiﬁcant. The dynamics are modelled as a ﬁrst-order system. The performance diﬀerence between the nonlinear and linear ﬁrst-order model is found to be negligible. Therefore, a simple ﬁrst-order model with a time constant depending on tubing length is deemed adequate. Passive and active microﬂuidic devices beneﬁt from a better understanding of the air tubing dynamics. Droplet resistance aﬀects the micro-channel network behaviour as they move through the channels. The uncertainties of their impact lengthen the iterative design process. Numerous studies investigated droplet resistance. However, a consensus is still pending. Most methods rely on passive principles. Oppositely, the method herein introduced relies on the active droplet control platform and grey-box system identiﬁcation. The preliminary results are promising and in agreement with the literature. Process improvement is envisioned to better the resolution and to enable apply the technique to many more conditions such as non-Newtonian ﬂuids. The model of the microﬂuidic chip compliance is improved and justiﬁed using experimental results. The better understanding of the dynamics is especially impactful for active microﬂuidics but also for passive microﬂuidics. A ﬁtting parameter (φ) is required to adjust for the diﬀerence in geometry and viscosity. The capacitance is on the order of 10−15. The previous formula predicted values around 10−20 whereas the other formula (without considering the ﬁtting parameter) predicts around 10−16. Moreover, the relationship of the ﬁtting parameter is unintuitively stronger with respect to the height-to-length ratio than with the width-to-length ratio. The length is related to the ﬂow rate. Shorter lengths mean larger ﬂow rates, and consequently, larger volume per unit pressure (i.e. capacitance). The path towards an automated modular platform that can easily be adopted by end-users as a tool relies on a shift towards a standalone system. The most important components to focus on are the actuation, feedback system, and the automation algorithm. The actuation through the current pressure pump is limiting due to its dependence on a pressured airline. The bulky and expensive microscope prohibits users from easily adopting the platform. Finally, the algorithm must be further developed to handle the procedure automatically such that minimal user input is required.
Cite this version of the work
Marie Hebert (2020). Expanding the droplet microfluidic community – towards a modular active platform. UWSpace. http://hdl.handle.net/10012/16556