Zinc Oxide only Sunscreen - only 0.35 SPF/1% ZnO in-vitro; strategies to improve SPF?

ZinkZink Member
edited October 2016 in Formulating
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 Naphthalate
Diethylhexyl Syringyldine Malonate
Ethylhexyl Methoxycrylene
Gosulin (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-15
Silica
Talc
Tinogard 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?

Comments

  • Bill_TogeBill_Toge Member, Professional Chemist
    in my experience, in vitro measurements (e.g. on a Labsphere) correlate well with in vivo results; we produce a range of products with SPFs and use the Labsphere as a QC check, since the results are consistent with the in vivo test report

    the rheology of the product, hence the quality and evenness of the film, is a very important factor influencing the SPF, so yes, the thickener does matter - what you want is a short-flow, strongly shear-thinning product that forms an even film when applied to a surface

    carbomers and clays (Veegum/Laponite) are generally the best choice for rheology modifiers, but many others are effective

    as for boosting SPF: titanium dioxide absorbs more strongly than zinc oxide, but zinc oxide provides a broader spectrum of UV protection, so you might want to consider using a combination of the two
    UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
  • ZinkZink Member
    edited October 2016
    Thanks Bill!

    I've seen several sunscreen manufacturers say that in-vitro results are at least 3x lower than in-vivo with mineral only ZnO sunscreens, when you say correlate well, do you mean it matches? Were these ZnO only formulas? And if what kind of SPF/% ZnO did you get?

    It seems like the difference between Xanthan Gum (in the control) and Sclerotium Gum in my formula wouldn't make a world of difference. DOW tells me "most carbomers" can complex with Zinc ions, so I'd need an alkyl functionalized variety? Veegum is an interesting option, noted.

    Ideally want to avoid TiO2 due to long term toxicity concerns when used in a daily wear formula.




  • Bill_TogeBill_Toge Member, Professional Chemist
    edited October 2016
    what I mean is that the products claim to have an SPF of 25, they scored between 30-40 on the in vivo test, and we get consistent results on the Labsphere between 30 and 45

    the sunscreens in those products were titanium dioxide, ethylhexyl methoxycinnamate and homomenthyl salicylate

    if you're using a sunscreen-grade zinc oxide dispersion, it will have a hydrophobic coating, therefore none of it is soluble, therefore the concentration of zinc ions in the product is minimal to nil

    also, titanium dioxide in sunscreens is every bit as inert as zinc oxide, regardless of what the state of California says about it
    UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
  • I see, definitely not apples to apples then.

    Regarding coating, I think it depends, the silica/silicon based ones would likely be coated in typically Triethoxycaprylylsilane, but silica/silicon free dispersion seem to be coating-free, typically using Caprylic/Capric Triglyceride and Polyhdroxystearic Acid as dispersants. Correct me if I'm wrong.

    Regarding safety/ROS production/genotoxicity, I just found a paper saying that silica or alumina coated TiO2 from sunscreens was basically inert as you say, whereas presumably uncoated ZnO generated significant amounts of ROS when exposed to UV: http://www.sciencedirect.com/science/article/pii/S1010603013001810

    So IF either mineral is robustly coated, you needn't worry about safety and can focus on UV protection; TiO2 is more effective at attenuating UVB (10 - 30 nm size due to its high refractive index), so you can get away with a lower total mineral content and improved skin feel when combined with e.g. 200-300 nm ZnO which takes care of UVA without being too white for comfort. This paper goes on to describe a method to do this effectively using 15% nano TiO2 and 85% micro ZnO blend (not sure you can get the effect with coated materials, maybe there are premade dispersions of both that exploit it) http://www.sciencedirect.com/science/article/pii/S0366317516000133

    Back to the drawing board!

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