Liquid core capsules having a hydrogel membrane are becoming a versatile tool for three- dimensional culture of micro-organisms and mammalian cells. Making sub-millimeter capsules at a high rate, via the breakup of a compound jet in air, opens the way to high throughput screen- ing applications. However, a control of the capsule size monodispersity, especially required for quantitative bioassays, was still lacking. Here, we report how the understanding of the underlying hydrodynamic instabilities that occur during the process can lead to calibrated core-shell biore- actors. This implies: i) to damp the shear layer instability that develops inside the injector arising from the co-annular flow configuration of liquid phases having contrasted viscoelastic properties; ii) to control the capillary instability of the compound jet by superposing a harmonic perturbation to the shell flow; iii) to avoid coalescence of drop during jet fragmentation as well as during drop flight towards the gelling bath, and iv) to ensure a proper engulfment of the compound drops into the gelling bath for building a closed hydrogel shell. We end up with the creation of numerous identical compartments in which cells are able to form mutlicellular aggregates, namely spheroids. In addition, we implement an intermediate composite hydrogel layer, composed of alginate and collagen, allowing cell adhesion and thus the formation of epithelia or monolayers of cells.
 

Deciphering the multifactorial determinants of tumor progression requires standardized high-throughput preparation of 3D in vitro cellular assays. We present a simple microfluidic method based on the encapsulation and growth of cells inside permeable, elastic, hollow microspheres. We show that this approach enables mass production of size-controlled multicellular spheroids. Due to their geometry and elasticity, these microcapsules can uniquely serve as quantitative mechanical sensors to measure the pressure exerted by the expanding spheroid. By monitoring the growth of individual encapsulated spheroids after confluence, we dissect the dynamics of pressure buildup toward a steady-state value, consistent with the concept of homeostatic pressure. In turn, these confining conditions are observed to increase the cellular density and affect the cellular organization of the spheroid. Postconfluent spheroids exhibit a necrotic core cemented by a blend of extracellular material and surrounded by a rim of proliferating hypermotile cells. By performing invasion assays in a collagen matrix, we report that peripheral cells readily escape preconfined spheroids and cell–cell cohesivity is maintained for freely growing spheroids, suggesting that mechanical cues from the surrounding microenvironment may trigger cell invasion from a growing tumor. Overall, our technology offers a unique avenue to produce in vitro cell-based assays useful for developing new anticancer therapies and to investigate the interplay between mechanics and growth in tumor evolution.

Liquid pearls designate liquid core capsules having a thin elastic shell.

We report an experimental investigation on the formation of liquid-core capsules having a thin hydrogel elastic membrane, named ‘liquid pearls’. These fish-egg-like structures are initially made of a millimetric liquid drop, aqueous or not, coated with an aqueous liquid film containing sodium alginate that gels once the double drop enters a calcium chloride bath. The creation of such pearls with a micrometer-thick membrane requires the suppression of mixing until gelling takes place. Here, we show that superimposing a two-dimensional surfactant precipitation at the interface confers a transient rigidity that can damp the shear-induced instability at impact. Based on this, pearls containing almost any type of liquid can be created. This opens the possibility to use such structures as a new tool for screening microorganism survival and growth in various three-dimensional environments.