Droplet-based microfluidics for single cell-omics

Water will readily form droplets in oil, providing miniature enclosed environments, in which the aqueous (water phase) is separate from the oil phase. If these droplets are created in very small channels (the width of a human hair, for instance), then the size of the droplets can be comparable to that of individual single cells. By using the techniques of microfabrication, first established within the semi-conductor industry, we are able to create appropriate microchannel architectures that can create reproducible droplets that contain cells (the droplets behave like very small test tubes). However, to date, attempts to make such droplets have proved difficult. Currently they are unstable and will coalesce and disappear as rapidly as they form. Using a new technology, we propose to use the same two phase (oil and water) systems to create droplets with a structured coat, so that once the cells have been placed within the droplet, the micro-environment is stable and long lived. The techniques used for making such droplets will enable us to change the nature of the coating so that they can be considered 'intelligent'. In other words, by engineering the chemical composition of the coating, we can decide which molecules enter the micro-droplet. Intriguingly, we can also introduce nanometre scale sensors within the droplet wall that can report upon the metabolic state of the cell. Finally, we will create arrays of such structured droplets so that many single cells can be stored and analysed readily, and ultimately used in the process of finding and testing new medicines.

Many methods have previously been used to create coated droplets on both a macro- and micro-scale, including polymer gelation at the surface of water droplets and the deposition of charged particles at water oil interfaces. In contrast to these existing methods, we propose to use the generation of a water-in-oil emulsion followed by the adsorption of colloidal particles at the droplet interface, to provide a novel method by which stabilised droplets, named 'colloidosomes', can be created. To date these capsules have not been produced within a microfluidic lab-on-a-chip device, nor have they been used to manipulate the environment around cells. We propose to create families of different capsule 'shells' that will be biocompatible (i.e. not only non-toxic to cells, but able to modulate cell behaviour). We also wish to be able to control the both gas and analyte permeability, enabling us to control the delivery of nutrients, ions, gases and drugs from the bulk media. This will provide the ability to stabilise the cell's physiology with the possibility of influencing its metabolism, through drug intervention, for example. Finally we wish to create microdroplet membrane interfaces containing optically active nanostructures. This will enable quantitative and extremely sensitive molecular fingerprinting of metabolites profiles of single cells using surface enhance Raman spectrsocopies (SERS). We will develop a platform that enables long-term and sensitive drug, proteomic and metabolic measurements of single cells, on-chip. There is also the potential for investigating cell-cell interactions. The colloidsomes will be sedimented on chemically or physically modified chips with the potential to create single cell microarrays for performing long-term assays in microfluidic systems. The inert and stable nature of colloidosomes also makes them ideal for a range of other future applications relevant to the biotechnology and bio-pharma industries.