  Biological active materials such as bacterial biofilms and eukaryotic cells
thrive in confined micro-spaces. Here, we show through numerical simulations
that confinement can serve as a mechanical guidance to achieve distinct modes
of collective invasion when combined with growth dynamics and the intrinsic
activity of biological materials. We assess the dynamics of the growing
interface and classify these collective modes of invasion based on the activity
of the constituent particles of the growing matter. While at small and moderate
activities the active material grows as a coherent unit, we find that blobs of
active material collectively detach from the cohort above a well-defined
activity threshold. We further characterise the mechanical mechanisms
underlying the crossovers between different modes of invasion and quantify
their impact on the overall invasion speed.
