Best Practices in Sunscreen Formulation, Part I

A review of critical requirements involved in creating effective sun protection products.

By Amit Patel

Complex challenges for formulators

As we’ve mentioned previously, sunscreen and sun care products are in increasing demand, and the more protection they offer, the better they’re selling.  This means it’s an advantageous time for formulators to be exploring new sunscreen formulation possibilities.

But formulating sunscreens requires a thorough understanding of formulation best practices and various types of systems. Sunscreens come in various forms, from spray-on lotions to creams, and as part of color products such as concealer to BB creams and lip products.

Very seldom are sunscreen formulas just suspensions or associative mixtures. More often, they’re rather complex mixtures of different types of functional ingredients like solubilizers, boosters, film formers, emollients, emulsifiers and particulate dispersions, all interacting to provide pleasant but stable products.

The formulator needs to account for ratios of sunscreen to oil phase and how to safely incorporate enough active material to satisfy COLIPA or SPF requirements, in addition to creating a formula which can transform, disperse and absorb harmful UV rays.

In the author’s experience? There are certain critical requirements, best practices and nuances involved in this pursuit, and that the formulator needs to observe to reach an optimum outcome: a sunscreen product that meets functional, aesthetic and marketability mandates.

Homogeneity and film forming

One critical requirement:  Choosing the right UV filters (sunscreens) and film formers in order to maintain a homogeneous distribution of those filters within a formula.

One of the key requirements of a sunscreen formula is that the product sustains homogeneity when it’s applied to the skin. In short, no one likes a formula that shows signs of separation when deposited from the package.

Therefore, homogeneity is crucial to ensuring the sunscreen remains efficacious after application¹. For a formulator, Rule One is to be sure those sunscreens are approved for use within the region. For example, there are sunscreens marketed in Europe and Japan that cannot be marketed in the U.S. since they haven’t passed FDA approval.

Sunscreens must have an adequate amount of film former, aiding in uniformity of film thickness and application to optimize the efficacy of sunscreen actives as best as possible.

Film forming means creating a layer of sunscreen across the substrate (in this case skin) which is uniform in thickness, flexible without creating fissures or fragments, and stays in place. For the formulator, the addition of a film former serves multiple purposes:

  • It keeps the formula on the skin for as long as possible.
  • It creates a uniform but flexible film which conforms to the peaks and valleys of the skin, optimizing scattering and/or absorption of UV light.
  • As a secondary function, it helps orient the UV filters, improving their function.

If the film breaks or cannot conform to the underlying skin, then sunlight permeates inducing damage quicker to the superficial layers. This creates a false impression that more sunscreen is  better because a loss of uniformity typically leads to over application.

Using organic absorbers

The mechanism of action for sunscreens relies on maintaining a layer which readily absorbs shorter wavelengths of sunlight, emphasizing the importance of broad coverage of against UVA and UVB light. Some sunscreens belong to a class called organic absorbers. When exposed to light, the sunscreen reaches an electron excited state which allows the sunscreen to absorb UV light, measured by the existence of intermediates such as keto or enol tautomers. The release of energy associated with this excited state causes a conformational change in the active ingredient to absorb the harmful rays.

As part of the formula, sunscreens can comprise over 50% of an oil phase to achieve the desired SPF. The use of solubilizers to maintain the homogeneity of the formula is critical. These solubilizers are typically aryl, or at least have some aryl property that is a ringed structure. This increases solvency and facilitates the heavily charged UV chemical filter incorporation into a non-polar environment. resulting in better stability.

Once the organic sunscreens are stabilized in the oil phase, the formulator must address homogeneity of the remaining formula by selecting emollients, emulsifiers and secondary supporting agents. Emulsifiers can range from the simple paired HLB emulsifier to the use of inverse emulsifiers which offer quick break and smooth applications. Sunscreen filters, chemical or physical, can interfere with the ability of an emulsifier to work correctly.

There seems to be little supporting data to say which emulsifiers perform better technically than others with sunscreens, which leaves the formulator to use their best judgment and make the choice based on marrying aesthetics and functionality. For example? The use of cetyl phosphates over traditional mixed HLB pairs is noted, but not supported by clinical data.

Formulators must choose wisely, based on destabilizers like using transitional preservatives with inverse emulsifiers or lamellar systems. The traditional aesthetics of sunscreens – oily, sticky, perhaps odd-smelling – aren’t well-regarded by the general public. The formulator is therefore challenged to devise a product that doesn’t feel like a sunscreen to the consumer.

In Part II, we’ll examine best practices for incorporating physical sunscreens or inorganic filters, and some how-tos about testing a sunscreen formulation to verify its level of protection.

Sources

1 Formulating Sunscreen Products for the 21st Century, Efficacy, Elegance and Globality, Julian P Hewitt, 20th Sept 2013.

About Amit Patel

Amit Patel is Innovation & Applied Science Manager at Coast Southwest, and an accomplished scientist with more than fifteen years of product development experience in the personal care industry. Learn more about Amit here.