An important challenge is delivery into skin since peptides are poorly penetrating, especially as the number of amino acid residues increases. An approach to that problem is addition of a lipophilic chain (e. g., palmitate), which in the case of KTTKS increased skin penetration several-fold over the underivatized peptide.
An additional challenge is the cost. As the number of amino acid residues increases, the cost of peptide can increase dramatically. The consequences are that only low levels of peptide can be used in product (which is acceptable if the peptide is potent as in the case of pal-KTTKS) or the finished product cost to the consumer must be very high.