Zinc Oxide only Sunscreen - only 0.35 SPF/1% ZnO in-vitro; strategies to improve SPF?
Just got in-vitro results for a Zinc Oxide mineral sunscreen formula I’ve been working on; 10% ZnO from three different ZnO dispersions gave a measly SPF ~3.5 - but it’s known mineral only sunscreens don’t do well in in-vitro assays and the included similarly designed 10% ZnO SPF27 control confirmed this by scoring 4.5.
The formula is a simple Oil in Water emulsion designed for everyday use with very watery skin feel, some moisturizing ability and excellent long term safety profile. Base formula w/o humectants, preservatives and antioxidants:
Water, ZnO dispersion, Capric/Caprylic Triglyceride, Caprylyl Methicone, Ceterayl Olivate, Sorbitan Olivate, Argan Oil, Sclerotium Gum.
Does in-vitro SPF absorbance assays reflect in-vitro sun protection? Is in-vitro indicative of how well the sunscreen protects the skin from UVB and UVA? And If in-vitro SPF matters: Are there thickeners, waxes, dispersants, film-formers or SPF boosters that could improve the in-vitro SPF in this formula?
Compiled a list chemicals that have come up, some of which might be in-vivo boosters only (anti-inflammatory properties etc) and some which might be incompatible with this formula or not help ZnO.Apalight (Hydroxyapatite)Butyloctyl Salicate (not sure about safety, basically chemical sunscreen)DL Lotus Wax (Nelumbo Nucifera Leaf Extract)Diethylhexyl NaphthalateDiethylhexyl Syringyldine MalonateEthylhexyl MethoxycryleneGosulin (Isoamyl Laurate & Isoamyl Cocoate)IBR CLC Concentrate (Dunaliella Salina, Hydrogenated Polydecene)LaraCare A200 (Galactoarabinan)Methylcellulose (do thickener type matter much?)Neocare P3R (Polyglyceryl-3 Polyricinoleate, Polyglyceryl-3 Ricinoleate)Polyester-8
Polyhydroxystearic Acid (or more of it)Polysilicone-15SilicaTalcTinogard TL (Benzotriazolyl Dodecyl p-Cresol, BDC)
So, two questions, do I need to worry about in-vitro, and if, where should I start to try to improve in-vitro absorbance without compromising skin feel, long-term safety profile and moisturization properties?