GHK-Cu topical formulations — cream vs serum vehicle research

Reviewing some interesting dermatological research on GHK-Cu delivery methods. The vehicle (cream base vs serum) appears to significantly impact penetration depth and potentially efficacy.

Pickart 2015 used a copper-peptide complex in a serum base. Most commercial formulations use cream. The molecular weight of GHK-Cu (~340 Da) is right at the edge of what can penetrate the stratum corneum without enhancement.

Anyone aware of comparative vehicle studies?