Production of monodisperse drops from viscous fluids.

Drops are often used as picoliter-sized reaction vessels, for example for high-throughput screening assays, or as templates to produce particles of controlled sizes and compositions. Many of these applications require close control over the size of drops, which can be achieved if they are produced with microfluidics. However, this tight size control comes at the expense of the throughput that is too low for many materials science and almost all industrial applications. To overcome this limitation, different parallelized microfluidic devices have been reported. These devices typically operate at high throughputs if the viscosity of the inner fluid is low. However, fluids that are processed into particles often contain high concentrations of reagents and therefore are rather viscous. We report a microfluidic device containing parallelized triangular nozzles with rectangular cross-sections that can process solutions with viscosities up to 155 mPa s into drops of well-defined sizes and narrow size distributions at significantly higher throughputs than what could be achieved previously. The increased throughput is enabled by the introduction of shunt channels: each nozzle is intersected by shunt channels that facilitate the backflow of the outer phase, thereby increasing the critical rate at which the fluid flow transitions from the dripping into the jetting regime. These modified nozzles open up new possibilities to employ drops made of viscous fluids as templates to produce particles with well-defined sizes for applications that require larger quantities.

Versatile, cell and chip friendly method to gel alginate in microfluidic devices.

Alginate is used extensively in microfluidic devices to produce discrete beads or fibres at the microscale. Such structures may be used to encapsulate sensitive cargoes such as cells and biomolecules. On chip gelation of alginate represents a significant challenge since gelling kinetics or physicochemical conditions are not biocompatible. Here we present a new method that offers a hitherto unprecedented level of control over the gelling kinetics and pH applied to the encapsulation of a variety of cells in both bead and fibre geometries. This versatile approach proved straightforward to adjust to achieve appropriate solution conditions required for implementation in microfluidic devices and resulted in highly reliable device operation and very high viability of several different encapsulated cell types for prolonged periods. We believe this method offers a paradigm shift in alginate gelling technology for application in microfluidics.

Competitive ligand exchange of crosslinking ions for ionotropic hydrogel formation.

Currently there are limitations to gelation strategies to form ionically crosslinked hydrogels, derived in particular from a lack of control over the release kinetics of crosslinking ions, which severely restrict applications. To address this challenge, we describe a new approach to form hydrogels of ionotropic polymers using competitive displacement of chelated ions, thus making specific ions available to induce interactions between polymer chains and form a hydrogel. This strategy enables control of ion release kinetics within an aqueous polymer solution and thus control over gelation kinetics across a wide range of pH. The described technique simplifies or facilitates the use of ionotropic hydrogels in a range of applications, such as 3D printing, microfluidic-based cell encapsulation, injectable preparations and large scale bubble and solid free mouldable gels. We investigate a range of chelator-ion combinations and demonstrate this powerful method to form hydrogels across a wide range of pH and µm-cm length scales. We highlight our findings by applying this gelation strategy to some of the more challenging hydrogel application areas using alginate and polygalacturonate as model polymer systems.