Ever since the FDA published their proposed monograph ruling in February 2019 recognizing titanium dioxide and zinc oxide as the only Category I (Safe and Effective) sunscreens, a cascade of reformulations of most sunscreens products on the US market took place. Inorganic sunscreen formulations are now center-stage and are slowly replacing organic sunscreen formulations. In fact, the trend started in 2018 when the state of Hawaii proposed a ban on Octinoxate and Oxybenzone stating that these two sunscreens have a negative effect on coral reefs. Now that the ban is in effect, another bill is proposing to ban sunscreens containing Octocrylene and Avobenzone for the same reasons. It is true that many regulatory bodies including the FDA did not support the Hawaiian ban, and the Personal Care Product Council (PCPC) is addressing the proposed monograph rulings. All these actions might lead to uncertain outcomes. In fact, in a few years the US consumers might be limited to the use of products containing inorganic sunscreens only (with the exception of Ensulizole and Ecamsule). There is some hope that certain Time and Extent (T&E) molecules are being reviewed by the FDA and may be approved for launch. Bemotrizinol is a front-runner and its use in formulation is quite good as it protects both in the UVB and UVA areas.
Selecting the right inorganic sunscreen
Zinc oxide and titanium dioxide not only refract light but also absorb it. The refractive index of titanium dioxide is 2.8 whereas that of zinc oxide is only 2.0. This makes titanium dioxide much more effective at scattering light in a formulation. From and absorption point of view, zinc oxide and titanium dioxide have conductance bands around 3.4 and 3.1 eV, respectively. This makes zinc oxide a bit more efficient in protecting against UVA rays and titanium dioxide more efficient at shielding UVB rays. As particle size decreases you get much more pronounced blue shift due to a change in the band-gap width. For example, a 0.15 eV blue shift has been reported for 4.7 nm titanium dioxide compared to bulk.
Keep in mind, when particles become smaller than their optimal light scattering size (typically half their wavelength) they become much more transparent. For example, zinc oxide becomes transparent at below 200 nm whereas titanium dioxide becomes transparent at sizes around 10-20 nm. This makes formulating with zinc oxide much easier to achieve transparent formulations but harder to reach high SPF due to its performance in the UVB region.
Sometimes the so-called “boosters” can help many formulators resort to using salicylates as UVB boosters in their formulations. Butyl octyl salicylate is not an approved sunscreen in the US but many formulators use it to boost their inorganic sunscreen SPF while claiming no organic sunscreens added.
Dispersion versus powder
The choice to use inorganic sunscreens as powder or dispersions in formulations is a very polarizing decision and many formulators prefer to use one type over the other. In general, dispersions claim to have a smaller primary particle size which results in better dispersion of the pigment into the emulsion and leads to higher SPF and less whitening on the skin. However, dispersions come at about 50% w/w solvent/pigment which limits the flexibility the formulator to tweak the formulation. In addition, when working with w/o or w/Si formulations, it is harder to control the viscosity of such emulsions when using dispersions. In these types of emulsions, the viscosity is built by the internal phase (water). Using dispersions ultimately increases the amount of external phase and reduces the amount of water used which will make such emulsions less viscous and less stable.
The use of powders, on the on the other hand, gives the formulator a lot of flexibility and reduces the cost of the formulation. Although, when using powders, it is important to have manufacturing capability to grind the pigments at the factory scale to reduce agglomeration and produce formulations with good aesthetics.
Selecting emulsion type
Most inorganic sunscreen formulations on the market are w/o or w/Si emulsions. These types of emulsions are much easier to preserve, as you only preserve the internal phase, and their pH does not fluctuate since they are anhydrous. These types of emulsions inherently have very good water resistance as well. Some of the drawbacks of w/o emulsions are their greasy feel mainly imparted by the surfactants and co-surfactants used. They tend to be more whitening on the skin and harder to spread. W/Si emulsion have a superior end-feel, but they are not particularly biodegradable or earth-friendly by today’s standards. They share the same characteristics as w/o emulsions when it comes to preservation, pH and water resistance. In general w/Si emulsions are harder to stabilize and require the use of more than one surfactant to obtain stable emulsions.
It is very rare to see o/w emulsion formulations on the market, since they are harder to preserve and stabilize. The presence of zinc oxide ultimately shifts pH towards 7.5 which renders most preservatives less effective. In addition, at that pH very few polymers work well at stabilizing such emulsions especially naturally derived polymers. On the other hand, these emulsions typically have a nicer feel on the skin and could be cost effective.
Adding a film former or SPF booster
Selecting a film-former or SPF booster for the emulsion is a critical step and one that should not be avoided. The selection of the appropriate polymer depends mostly on the experience of the formulator and the in vivo results previously obtained with such polymer. Many polymers are marketed to the formulators and some of them could work in one formulation or another. However, it is crucial that the film former works across many formulations and especially in vivo since such tests are quite costly and hard to schedule. As scientists, we should always test the formulations in vitro for water resistance and SPF to ensure that the addition of the polymer will give the desired results. This will enable the formulator to refine the level of polymer in the emulsion as well.
In conclusion, I hope I shed some light on formulating inorganic sunscreen emulsions and I leave it up to the creativity of formulators to create excellent formulations with great aesthetics and high SPF.
Dr. Fares started his career in personal care studying the effect of solvents on sunscreen chemicals. His interest in skin drug delivery especially from polymeric matrices grew during his graduate work at Rutgers, where he received his Ph. D.
Dr. Fares worked at Block Drug and GlaxoSmithKline where he held positions in research and development in the areas of skincare and oral care. After that, he joined L’Oréal where he held several positions of increasing responsibility leading to AVP of skincare. He is currently the Senior Director of skincare and oral care at Ashland Specialty Ingredients. Dr. Fares is the author of many publications, and patents and made many presentations in national and international meetings in the areas of suncare, skincare, and oral care. Dr Fares chairs the NYSCC scientific committee and has won multiple awards in the areas of sun care and polymer chemistry.