  The internal and external mechanical environment plays an important role in
tumorogenesis. As a proxy of an avascular early state tumor, we use
multicellular spheroids, a composite material made of cells, extracellular
matrix and permeating fluid. We characterize its effective rheology at the
timescale of minutes to hours by compressing the aggregates with osmotic shocks
and modeling the experimental results with an active poroelastic material that
reproduces the stress and strain distributions in the aggregate. The model also
predicts how the emergent bulk modulus of the aggregate as well as the
hydraulic diffusion of the percolating interstitial fluid are modified by the
preexisting active stress within the aggregate. We further show that the value
of these two phenomenological parameters can be rationalized by considering
that, in our experimental context, the cells are effectively impermeable and
incompressible inclusions nested in a compressible and permeable matrix.
