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.tissue mechanics | microfluidics | tumor growth | mechanotransduction

Like liquid droplets, cellular aggregates, also called "living droplets," spread onto adhesive surfaces. When deposited onto fibronectincoated glass or polyacrylamide gels, they adhere and spread by protruding a cellular monolayer (precursor film) that expands around the droplet. The dynamics of spreading results from a balance between the pulling forces exerted by the highly motile cells at the periphery of the film, and friction forces associated with two types of cellular flows: (i) permeation, corresponding to the entry of the cells from the aggregates into the film; and (ii) slippage as the film expands. We characterize these flow fields within a spreading aggregate by using fluorescent tracking of individual cells and particle imaging velocimetry of cell populations. We find that permeation is limited to a narrow ring of width ξ (approximately a few cells) at the edge of the aggregate and regulates the dynamics of spreading. Furthermore, we find that the subsequent spreading of the monolayer depends heavily on the substrate rigidity. On rigid substrates, the migration of the cells in the monolayer is similar to the flow of a viscous liquid. By contrast, as the substrate gets softer, the film under tension becomes unstable with nucleation and growth of holes, flows are irregular, and cohesion decreases. Our results demonstrate that the mechanical properties of the environment influence the balance of forces that modulate collective cell migration, and therefore have important implications for the spreading behavior of tissues in both early development and cancer.wetting | tissue dynamics | tissue mechanosensitivity T issue spreading is a fundamental phenomenon in many biological processes. Examples include wound healing (1-4) where the surrounding tissue spreads to close the injury, or the development of the embryo (5-7), which requires the orchestrated movement of cells to specific locations. It is also present in the progression of cancer (8-10). For example, glioblastomas grow and spread aggressively to invade surrounding regions and may lead to dramatic damages (11). The first step of cancer propagation (invasion) is characterized by a loss of cell-cell adhesion associated with an increase in cell motility. Increased cell motility is followed by entry into the blood circulation (intravasation), and subsequent escape from the circulation into distal tissue (extravasation). From this distal site, cell proliferation leads to a secondary tumor (11). Thus, it is crucial to understand how noninvasive tumor cells become metastatic by the loss of cell-cell adhesion and increased migration, which leads to malignancy. Further investigation into this topic requires the design and analysis of model in vitro experimental systems suitable for recapitulating these early stage events.Model in vitro systems in 3D are essential to recapitulating the early stages of cancer progression. For example, it has been reported that the efficiency of medical drugs tested on 2D cell culture systems is not transposable to 3D in more...