What does it mean for hyaluronic acid-based fillers to be “cross-linked,” and how does this affect their longevity?

Natural hyaluronic acid is a molecule already present in our bodies, but its lifespan is only a few days. In medical aesthetics, to enable fillers to shape facial contours and last for months, molecular bonds are formed in the laboratory using chemical agents (usually BDDE); this is called “cross-linking.” Cross-links tightly bind the hyaluronic acid chains together, forming a gel network. The hyaluronidase enzyme in our bodies cannot immediately break down this tight network. The higher the cross-link density and the larger the molecular size, the greater the filler’s longevity and its ability to lift tissue.