Colloidal gels represent a fascinating class of heterogeneous, disordered, yet stable materials with tunable properties, central to applications ranging from materials science to biophysics. In this talk, I will present work exploring how the introduction of self-propelled swimmers fundamentally alters the structure and topology of three-dimensional colloidal gels. Using a combination of large-scale simulations and persistent homology, a tool from topological data analysis, we uncover how local energy injection by swimmers leads to the formation of highly porous, yet mechanically robust gel networks. Interestingly, we find that passive interfaces within the gel play a nontrivial topological role, guiding and interacting with active particles in a way that reshapes both the microstructure and transport properties of the system. Afterwards, I will discuss how topology can be used to control the mechanical properties of other biological systems.