  We investigate how randomly oriented cell traction forces lead to
fluidisation in a vertex model of epithelial tissues. We find that the
fluidisation occurs at a critical value of the traction force magnitude $F_c$.
We show that this transition exhibits critical behaviour, similar to the
yielding transition of sheared amorphous solids. However, we find that it
belongs to a different universality class, even though it satisfies the same
scaling relations between critical exponents established in the yielding
transition of sheared amorphous solids. Our work provides a fluidisation
mechanism through active force generation that could be relevant in biological
tissues.
