The degradation kinetics of the gels depends on the ability of the cells encapsulated in the gels to produce/activate MMPs. Without this cell-mediated proteolytic activity these gels remain intact and do not degrade.

Degradation components (e.g. MMP sensitive or insensitive peptides) act as crosslinks of the PEG molecules and determine the MMP sensitivity of the gels. These amino acid substrates with different MMP-sensitivities were characterized using MMP-1 (Lutolf, PNAS 2003). The MMP sensitive substrate was also degraded by other MMPs (Lutolf, Nature Biotechnology, 2003). In contrast the MMP-insensitive substrate was shown to be quite stable (i.e. essentially non-degradable) in vitro and in vivo experiments (Lutolf, PNAS 2003) (Rizzi, Biomacromolecules, 2006).