Recent studies have shown that basic cellular behavior varies significantly between two- and three-dimensional culture systems. To identify the origins of these fundamental differences the design of reliable and precisely controlled environments is essential. While 2D cell culture is a well-established technique, the fabrication of defined three-dimensional culture models is still challenging. We present a new method for the microfluidic generation of a micron-sized three-dimensional cell culture system. We use a triggered ionic crosslink formation to generate highly monodisperse and structurally homogeneous alginate microbeads. Aqueous droplets containing a mixture of alginate and a water-soluble calcium-EDTA complex are formed by droplet-based microfluidics. In their complexed form, the calcium ions are homogenously distributed inside the droplet but not accessible for the crosslinking process. Acid addition is used to trigger the degradation of the complex, releasing calcium ions on demand that can physically crosslink the alginate chains. A homogeneous hydrogel network is thus generated which can be transferred into an aqueous environment without losing its structural integrity. Single cells can be encapsulated into these controlled microenvironments which provide structural support while allowing for continuous nutrient supply. We encapsulate individual mesenchymal stem cells (MSCs) into the microbeads which show the aspired cell growth and proliferation.

Glucose oxidase (GOx) is used in many industrial processes that could benefit from improved versions of the enzyme. Some improvements like higher activity under physiological conditions and thermal stability could be useful for GOx applications in biosensors and biofuel cells. Directed evolution is one of the currently available methods to engineer improved GOx variants. Here, we describe an ultra-high-throughput screening system for sorting the best enzyme variants generated by directed evolution that incorporates several methodological refinements: flow cytometry, in vitro compartmentalization, yeast surface display, fluorescent labeling of the expressed enzyme, delivery of glucose substrate to the reaction mixture through the oil phase, and covalent labeling of the cells with fluorescein-tyramide. The method enables quantitative screening of gene libraries to identify clones with improved activity and it also allows cells to be selected based not only on the overall activity but also on the specific activity of the enzyme.

A new ultra-high-throughput screening assay for the detection of cellulase activity was developed based on microfluidic sorting. Cellulase activity is detected using a series of coupled enzymes leading to the formation of a fluorescent product that can be detected on a chip. Using this method, we have achieved up to 300-fold enrichments of the active population of cells and greater than 90% purity after just one sorting round. In addition, we proved that we can sort the cellulase-expressing cells from mixtures containing less than 1% active cells