Most peptide serums never reach the cells that need them. The stratum corneum is designed to keep things out. It does not distinguish between a toxin and a carefully formulated peptide. The molecular weight of most active peptides exceeds 500 Daltons. Anything above that threshold struggles to cross the skin barrier intact. A 2019 study by Lim et al. found that fewer than 5% of topically applied peptides penetrate beyond the outermost layer of skin. The remaining 95% sit on the surface, oxidise, and wash off in the shower.

This is not a formulation problem. It is a physics problem. The molecules are simply too large to pass through the gaps between corneocytes without a delivery mechanism that actively creates a pathway. Every brand claiming "deep penetration" without addressing this barrier is selling hope, not science.