Poroelasticity influences the compression stiffening of model soft tissues

The material properties of connective tissues are often considered to be dominated by the extracellular matrix. However, cells within tissues contribute significantly to the mechanics, particularly when considering volume-changing deformations such as axial compression. Recent studies have considered the compression response of model tissues at long times, where the role of the interstitial fluid is negligible. Here, we experimentally study the interplay of matrix, cell and fluid mechanics in the short time response of model tissues based on fibrous networks with embedded microgel inclusions as passive cell-like mimics. By simultaneously applying compression at a fixed strain rate while measuring the shear modulus via small strain shear oscillations, we show that at long times fibrous networks transition from compression-softening behaviour to compression-stiffening behaviour upon incorporation of microgel particles. At short times, however, we show that fluid pressure build-up leads to significant compression-stiffening, regardless of the inclusion of microgels. Our findings demonstrate that the role of fluid pressurisation is key to understanding the response of tissues to deformations on timescales on the order of a few seconds, common in many tissues subject to physiological deformations.