Tissue surface tension influences cell sorting and tissue fusion. Earlier mechanical studies suggest that multicellular spheroids actively reinforce their surface tension with applied force. Here we study this open question through high-throughput microfluidic micropipette aspiration measurements on cell spheroids to identify the role of force duration and spheroid deformability. In particular, we aspirate spheroid protrusions of mice fibroblast NIH3T3 and human embryonic HEK293T homogeneous cell spheroids into micron-sized capillaries for different pressures and monitor their viscoelastic creep behavior. We find that larger spheroid deformations lead to faster cellular retraction once the pressure is released, regardless of the applied force. Additionally, less deformable NIH3T3 cell spheroids with an increased expression level of alpha-smooth muscle actin, a cytoskeletal protein upregulating cellular contractility, also demonstrate slower cellular retraction after pressure release for smaller spheroid deformations. Moreover, HEK293T cell spheroids only display cellular retraction at larger pressures with larger spheroid deformations, despite an additional increase in viscosity at these larger pressures. These new insights demonstrate that spheroid viscoelasticity is deformation-dependent and challenge whether surface tension truly reinforces at larger aspiration pressures.

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The human body is a construction site brimming with internal processes that steer our senses, heal our wounds and make us age. If tissues and organs are scaffolds, then cells are the bricks. Sticking to each other, these minuscule "water balloon-esque" containers filled with proteins are charged with a multitude of tasks. Think about cell division, growth and migration, requiring the cells to change shape when they either generate or are themselves subjected to physical forces. This deformational response, termed cell mechanics, is directly related to the properties of tissues as a whole. Tissue mechanics plays an essential part in initial stages of life as well as in the development of diseases such as cancer. Think about tissues wrapping around each other during embryo growth, or cells leaving a primary tumour to metastasize through the body. Studying tissue mechanics and its dependency on cellular properties therefore forms the key topic in this dissertation....@en

Cell spheroids are in vitro multicellular model systems that mimic the crowded micro-environment of biological tissues. Their mechanical characterization can provide valuable insights in how single-cell mechanics and cell-cell interactions control tissue mechanics and self-organization. However, most measurement techniques are limited to probing one spheroid at a time, require specialized equipment and are difficult to handle. Here, we developed a microfluidic chip that follows the concept of glass capillary micropipette aspiration in order to quantify the viscoelastic behavior of spheroids in an easy-to-handle, more high-throughput manner. Spheroids are loaded in parallel pockets via a gentle flow, after which spheroid tongues are aspirated into adjacent aspiration channels using hydrostatic pressure. After each experiment, the spheroids are easily removed from the chip by reversing the pressure and new spheroids can be injected. The presence of multiple pockets with a uniform aspiration pressure, combined with the ease to conduct successive experiments, allows for a high throughput of tens of spheroids per day. We demonstrate that the chip provides accurate deformation data when working at different aspiration pressures. Lastly, we measure the viscoelastic properties of spheroids made of different cell lines and show how these are consistent with previous studies using established experimental techniques. In summary, our chip provides a high-throughput way to measure the viscoelastic deformation behavior of cell spheroids, in order to mechanophenotype different tissue types and examine the link between cell-intrinsic properties and overall tissue behavior.

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Spheroids are widely used in vitro 3D multicellular model systems that mimic complex physiological microenvironments of tissues. As different cell types vary in deformability and adhesion, the choice of (heterogeneous) cell composition will define overall spheroid mechanics, including their viscoelasticity and effective surface tension. These mechanical parameters directly influence cell sorting and possibly cell invasion into the extracellular matrix. Spheroid models therefore provide fundamental insights in the relation between cellular mechanics and important physiological processes, such as tissue formation, embryonic tissue remodeling, and cancer metastasis. In this review, we first summarize and compare current biophysical tools that probe mechanics of spheroids either from the outside or from within, then relate spheroid mechanics to cell mechanics and cell-cell adhesion, and subsequently discuss the role of spheroid mechanics alongside surrounding microenvironment parameters in (cancer) cell migration. We conclude by pointing out the research gaps and drawing the attention to novel techniques that could shed more light on the biophysical characterization of spheroids in the framework of tissue remodeling and cancer metastasis.

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