Matrikines - a term used to describe smaller peptides that have better skin penetration due to their much smaller molecular size.  Palmitoyl Tripeptide-1 (GHK) and Tetrapeptide-21 are matrikines, for example.

Matrikines are peptides that occur when extracellular matrix (ECM) macromolecules are partially degraded. These peptides interact with cell surface receptors and activate intracellular signalling pathways to modulate ECM remodeling.  Matrikines, such as tripeptides and hexapeptides, have been shown to remove damaged collagen and elastin from the ECM. It is thought that these matrikines help to prepare the skin for procedures by freeing up space to allow room for newly formed collagen. Using matrikines at least 2 weeks before procedures may precondition the skin to heal faster.