Probing cellular mechanical responses to stimuli using ballistic intracellular nanorheology

We describe a new method to measure the local and global micromechanical properties of the cytoplasm of single living cells in their physiological milieu and subjected to external stimuli. By tracking spontaneous, Brownian movements of individual nanoparticles of diameter >= 100 nm distributed within the cell with high spatial and temporal resolutions, the local viscoelastic properties of the intracellular milieu can be measured in different locations within the cell. The amplitude and the time-dependence of the mean-squared displacement of each nanoparticle directly reflect the elasticity and the viscosity of the cytoplasm in the vicinity of the nanoparticle. In our previous versions of particle tracking, we delivered nanoparticles via microinjection, which limited the number of cells amenable to measurement, rendering our technique incompatible with high-throughput experiments. Here we introduce ballistic injection to effectively deliver a large number of nanoparticles to a large number of cells simultaneously. When coupled with multiple particle tracking, this new method-ballistic intracellular nanorheology (BIN)-makes it now possible to probe the viscoelastic properties of cells in high-throughput experiments, which require large quantities of injected cells for seeding in various conditions. For instance, BIN allows us to probe an ensemble of cells embedded deeply inside a three-dimensional extracellular matrix or as a monolayer of cells subjected to shear flows.