Nielsen JB. Natural Oils Affect the Human Skin Integrity and the Percutaneous Penetration of Benzoic Acid Dose-Dependently. Basic & Clinical Pharmacology & Toxicology 2006, 98, 575–581. (peppermint oil best at 5% as in our Loma Linda studies)
Chen, J.; Jiang, Q.-D.; Wu, Y.-M.; Liu, P.; Yao, J.-H.; Lu, Q.; Zhang, H.; Duan, J.-A. Potential of Essential Oils as Penetration Enhancers for Transdermal Administration of Ibuprofen to Treat Dysmenorrhoea. Molecules2015, 20, 18219-18236. (Essential oil assists in skin penetration)
Olitzky, I. (1965), Antimicrobial properties of a propylene glycol based topical therapeutic agent. J. Pharm. Sci., 54: 787-788. doi:10.1002/jps.2600540528. (Propylene glycol has antibiotic properties)
Li X, Wang X, Chen D, Chen S. Access Antioxidant Activity and Mechanism of Protocatechuic Acid in vitro. Functional Foods in Health and Disease 2011; 7:232-244. (10 times more powerful than vitamin E)
Park J, Lee B, Choi H, Kim W, Kim H-J, Cheong H. Antithrombosis activity of protocatechuic and shikimic acids from functional plant Pinus densiflora Sieb. et Zucc needles. Journal of Natural Medicines. July 2016, Volume 70, Issue 3, pp 492–501. (PCA has blood-thinning effect)