Elastomer-based pneumatic device to measure the mechanical properties of cell monolayers

Everyday life physiological functions such as breathing, muscle contraction, and blood circulation rely on the ability of single cells to organize into higher complexity structures and sustain mechanical deformations. Cell monolayers are the simplest tissues in the body and play a critical role since the embryogenesis stage, when they drive differentiation into organs, to acting as physical barriers and partitioning organs in the adults. Because of their specific interface location, cell monolayers stabilize tissues by sustaining external physiological stresses. While it is known that altered elasticity from cells to tissues is closely related to anomalous behaviour and diseases, the advancements in the field are still quite slow. This is mainly due to the fact that current techniques for measuring cell and tissue mechanics rely on complex and bulky measurement platforms with low repeatability rate, difficult integration with standard cell protocols and limited measurement time-scales.

This thesis presents the development of the design, fabrication processes and biological tests of a compact device for measuring cell monolayers Young's modulus which overcomes the main challenges of existing techniques. The measurements principle is based on planar deformation of cell monolayers by pneumatic actuation and strain optical read-out. Cell monolayers adhere to a deformable membrane specially designed to be soft (30 kPa) and thin (5-10 um) as the layer of cells to achieve high sensitivity in measuring cell contribution to the overall membrane mechanical properties. The core of our technology is the use of differential strain measurements between a region of the substrate covered with cells and a bare region. During the actuation, the strains of both regions are optically measured by a pattern recognition algorithm. This allows to measure the mechanical properties of the adherent cell layer by subtracting the contribution of the membrane itself. Furthermore, the use of a differential read-out makes it possible to avoid complex force measurement equipment, thus making the whole device more compact and compatible with long time measurements in sterility conditions. We report measurements of the Young's modulus of two cell types and its changes when external additional chemical and physical stimulations are applied. The results show that cell monolayer Young's modulus for both cell types is around one order of magnitude higher than in single cells (such as obtained by atomic force microscopy). This indicates the relevant contribution of cell anisotropy and conformation to their mechanical response.

This device can be easily adapted to perform measurement with several types of adherent cells. It provides highly repeatable Young's modulus measurements in the physiologically relevant range between 3 kPa and 300 kPa, over time and in cell culture conditions, thus allowing experiments over longer time scales. The obtained results altogether demonstrate the capability of measuring changes in cell mechanics over time and therefore the device potential for determining the effect of diverse stimuli and thus advancing our understanding of cell mechanics.