Towards a Theory of Droplet-Mixing Graphs in Microfluidics

In this work, we study the problem of fluid mixing in microfluidic chips. The motivation for studying this problem comes from the process of sample preparation for chemical, biological, medical and environmental experiments, which often require preparation of fluid mixtures with desired concentrations. We assume that fluids are manipulated in discrete units called droplets. The input set of droplets consist of two distinct fluids: the reactant, which is the fluid of interest, and the buffer fluid that is used to dilute it. The goal is to produce a target set of droplets with prespecified reactant concentrations. In the model we study, the mixing process in a microfluidic chip can be abstractly represented as a mixing graph. A mixing graph is a collection of micro-mixers (nodes) connected by micro-channels (edges) that converts an input set of droplets I into a set of output droplets T, by applying a sequence of 1:1 mixing operations. This graph may also produce some waste, which are superfluous droplets of fluid not used in the target set. Computational complexity of most natural questions regarding such mixing graphs remain open. For example, it is not even known whether it is decidable for a given target set to be produced without waste. Current work in the literature contains only heuristic approaches that compute mixing graphs while attempting to optimize certain objectives, including minimizing waste, reactant usage, the depth of the graphs, and more.Our first contribution is an efficient algorithm for computing mixing graphs for single-droplet targets. Our algorithm produces significantly less waste than state-of-the-art algorithms in an experimental comparison. We also provide a bound on its worst-case performance that is significantly better than those for earlier algorithms. Our second result concerns the variant of the problem where the objective is to design a mixing graph that perfectly mixes a collection of input droplets with arbitrary concentrations. We provide a complete characterization of input sets for which such graphs exist, and an efficient algorithm to construct these graphs. In addition, we provide several other results about properties of mixing graphs and the computational complexity of computing mixing graphs of fixed depth

Microfluidics for Biosensing and Diagnostics

Efforts to miniaturize sensing and diagnostic devices and to integrate multiple functions into one device have caused massive growth in the field of microfluidics and this integration is now recognized as an important feature of most new diagnostic approaches. These approaches have and continue to change the field of biosensing and diagnostics. In this Special Issue, we present a small collection of works describing microfluidics with applications in biosensing and diagnostics

Magnetic Nanoparticle Enhanced Actuation Strategy for mixing, separation, and detection of biomolecules in a Microfluidic Lab-on-a-Chip System

Magnetic nanoparticle (MNP) combined with biomolecules in a microfluidic system can be efficiently used in various applications such as mixing, pre-concentration, separation and detection. They can be either integrated for point-of care applications or used individually in the area of bio-defense, drug delivery, medical diagnostics, and pharmaceutical development. The interaction of magnetic fields with magnetic nanoparticles in microfluidic flows will allow simplifying the complexity of the present generation separation and detection systems. The ability to understand the dynamics of these interactions is a prerequisite for designing and developing more efficient systems. Therefore, in this work proof-of-concept experiments are combined with advanced numerical simulation to design, develop and optimize the magnetic microfluidic systems for mixing, separation and detection. Different strategies to combine magnetism with microfluidic technology are explored; a time-dependent magnetic actuation is used for efficiently mixing low volume of samples whereas tangential microfluidic channels were fabricated to demonstrate a simple low cost magnetic switching for continuous separation of biomolecules. A simple low cost generic microfluidic platform is developed using assembly of readily available permanent magnets and electromagnets. Microfluidic channels were fabricated at much lower cost and with a faster construction time using our in-house developed micromolding technique that does not require a clean room. Residence-time distribution (RTD) analysis obtained using dynamic light scattering data from samples was successfully used for the first time in microfluidic system to characterize the performance. Both advanced multiphysics finite element models and proof of concept experimentation demonstrates that MNPs when tagged with biomolecules can be easily manipulated within the microchannel. They can be precisely captured, separated or detected with high efficiency and ease of operation. Presence of MNPs together with time-dependent magnetic actuation also helps in mixing as well as tagging biomolecules on chip, which is useful for point-of-care applications. The advanced mathematical model that takes into account mass and momentum transport, convection & diffusion, magnetic body forces acting on magnetic nanoparticles further demonstrates that the performance of microfluidic surface-based bio-assay can be increased by incorporating the idea of magnetic actuation. The numerical simulations were helpful in testing and optimizing key design parameters and demonstrated that fluid flow rate, magnetic field strength, and magnetic nanoparticle size had dramatic impact on the performance of microfluidic systems studied. This work will also emphasize the importance of considering magnetic nanoparticles interactions for a complete design of magnetic nanoparticle-based Lab-on-a-chip system where all the laboratory unit operations can be easily integrated. The strategy demonstrated in this work will not only be easy to implement but also allows for versatile biochip design rules and provides a simple approach to integrate external elements for enhancing mixing, separation and detection of biomolecules. The vast applications of this novel concept studied in this work demonstrate its potential of to be applied to other kinds of on-chip immunoassays in future. We think that the possibility of integrating magnetism with microfluidic-based bioassay on a disposable chip is a very promising and versatile approach for point-of care diagnostics especially in resource-limited settings

Plasmonic Nanomaterials-Based Point-of-Care Biosensors

Point-of-care (POC) biosensors, although rapid and easy-to-use, are orders magnitude less sensitive than laboratory-based tests. Further they are plagued by poor stability of recognition element thus limiting its widespread applicability in resource-limited settings. Therefore, there is a critical need for realizing stable POC biosensors with sensitivity comparable to gold-standard laboratory-based tests. This challenge constitutes the fundamental basis of this dissertation work– to expand access to quality and accurate biodiagnostic tools. At the heart of these solutions lies plasmonic nanoparticles which exhibit unique optical properties which are attractive for label-free and labelled biosensors.Firstly, we improve the stability and applicability of label-free plasmonic biosensors for implementing biodiagnostics in POC and resource-limited settings. We demonstrate a cost-effective plasmonic paper-based biosensor for non-invasive detection of renal cancer. We also demonstrate a facile integration of plasmonic paper and microneedle patch to realize a POC biosensor which enables detection of target biomarkers present in interstitial fluid in an easy-to-use two-step process. We introduce a polymer encapsulation strategy to realize a stable and refreshable biosensor for long-term monitoring of protein biomarkers under harsh conditions. Next, we demonstrate dramatic improvement in bioanalytical parameters of POC biosensors by designing and realizing an ultrabright fluorescent nanolabel, plasmonic fluor. We discuss a novel approach for detection and quantification of inflammatory disease burden via plasmonically-active tissue analog which can undergo in vivo or ex vivo degradation in the presence of biological fluid associated with the tissue. We demonstrate a partition-free digital fluoroimmunoassay for ultrasensitive, multiplexed, and quantitative detection of protein biomarkers present in human biospecimens. Significantly, utilizing plasmonic-fluor, we overcome long-standing limitations associated with lateral flow immunoassays (LFA)– limited sensitivity, low accuracy and smaller analytical range compared to laboratory tests, and limited quantitation ability. Taken together, these advances are expected to overcome fundamental challenges associated with POC biosensors, and to bridge the gap between laboratory-based and at-home or point-of-care (POC) diagnosis. Through this dissertation work we demonstrate a complete workflow of a POC diagnostic platform that outperforms gold-standard laboratory tests in sensitivity, speed, dynamic range, ease of use, and cost

Micromachines for Dielectrophoresis

An outstanding compilation that reflects the state-of-the art on Dielectrophoresis (DEP) in 2020. Contributions include: - A novel mathematical framework to analyze particle dynamics inside a circular arc microchannel using computational modeling. - A fundamental study of the passive focusing of particles in ratchet microchannels using direct-current DEP. - A novel molecular version of the Clausius-Mossotti factor that bridges the gap between theory and experiments in DEP of proteins. - The use of titanium electrodes to rapidly enrich T. brucei parasites towards a diagnostic assay. - Leveraging induced-charge electrophoresis (ICEP) to control the direction and speed of Janus particles. - An integrated device for the isolation, retrieval, and off-chip recovery of single cells. - Feasibility of using well-established CMOS processes to fabricate DEP devices. - The use of an exponential function to drive electrowetting displays to reduce flicker and improve the static display performance. - A novel waveform to drive electrophoretic displays with improved display quality and reduced flicker intensity. - Review of how combining electrode structures, single or multiple field magnitudes and/or frequencies, as well as variations in the media suspending the particles can improve the sensitivity of DEP-based particle separations. - Improvement of dielectrophoretic particle chromatography (DPC) of latex particles by exploiting differences in both their DEP mobility and their crossover frequencies