Actin is a protein which self-assembles into highly dynamic semi-flexible filaments organized, within the cells, in different kind of meshworks (bundles, crosslinked or dendritic networks). The combination of mechanics and dynamics of these networks enables the cell to achieve essential processes like migration, deformation or integration of external mechanical cues. I will show in this talk how we designed and developed a new method using the self-assembly of magnetic micro-particle to have access to these properties in vitro on dendritic actin networks and more recently in cellulo. I will also show some of the results on the non linear mechanical behavior of non crosslinked dendritic actin networks and on the impact of actin nucleators on the activity of cellular cortex.