Mineral sunscreen leaves an annoying white cast on skin — this new formula could change that

Mineral sunscreens are notorious for leaving a white cast on the skin — but now, scientists have found that tweaking the shape of zinc oxide nanoparticles in the formula can help solve that problem.

The new, four-armed crystal structure, known as a tetrapod, was reported in December in the journal ACS Materials Letters, and it prevents individual nanoparticles from clumping together. This simultaneously boosts the stability of the sunscreen formulation and produces a warmer color that better matches a range of skin tones.

Damage caused by ultraviolet (UV) radiation in sunlight is the leading preventable cause of skin cancer in the U.S., and dermatologists strongly recommend that people apply sun protection daily to minimize their risk.

Unlike mineral sunscreens, chemical sunscreens absorb into the skin and do not leave a cast. However, many people prefer mineral sunscreens, which sit on top of the skin, but the residue leaves an unpleasant appearance after application.

"I was frustrated by how mineral sunscreen looks on my own skin," lead study author AJ Addae, a doctoral candidate in chemical biology at UCLA, said in a statement. "A lot of my motivation came from my own experience trying to use mineral sunscreen and dealing with the white cast and other unsightly aesthetic issues. This led me to simply avoid sunscreen altogether."

Mineral sunscreens use chalky zinc oxide to absorb harmful UV radiation. These products are usually formulated as colloidal materials — a suspension of fine solid particles in a watery or oily base. However, this suspended structure can create a few practical issues for both manufacturers and consumers, said Kyra Sedransk Campbell, a former professor of chemical engineering at the University of Sheffield and now CEO of Kingston Street Consulting.

For example, interactions between individual particles in the suspension form clumps over time, which can lead to visible white streaks as the product is applied to the skin. In addition, "they don't have as long a shelf life," added Sedransk Campbell, who wasn't involved in the new work. "Generally it's harder to maintain the stability of the sunscreen, and guaranteed performance is based on when the formulation is in its ideal state — so it's much harder to make the same guarantees as with a chemical sunscreen."

Addae and her colleagues decided to investigate whether changing the shape of the zinc oxide nanoparticles could help address these problems without compromising the sun protection performance.

They used a method known as "flame synthesis," which involves heating zinc with ethanol in a furnace, creating a naked flame. This creates four-armed nanoparticles, which are then incorporated into a sunscreen formulation. Scanning electron microscopy, which uses an electron beam to render high-resolution images of samples, revealed that these new particles were larger than the spherical nanostructures usually found in sunscreen but that they showed less tendency to clump together.

"Because of their structure, these tetrapod-shaped particles have standoffs and form porous networks instead of collapsing into clumps," Addae said. "They can't pack tightly and aggregate, so they stay evenly distributed in the sunscreen."

Crucially, though, this alternative shape did not affect the sunscreen's protective performance. Broad-spectrum sunscreens must protect against two wavelength bands of UV light: UVA (315 to 400 nanometers) and UVB (280 to 315 nanometers). Their trial formulation effectively absorbed both wavelength ranges and achieved a sun protection factor (SPF) of around 30, the standard for mineral sunscreens.

Product stability testing also showed that the new mixture was less prone to thickening or separating over time, compared with formulas with standard spherical particles. That means it would likely retain this high performance longer.

But a crucial element of a successful sunscreen is that people actually want to use it.

The white cast caused by clumps of zinc oxide particles in mineral formulations is a consistent complaint among U.S. consumers and cited as a reason why many people, particularly those with darker skin tones, avoid these products. The new tetrapod structures scatter visible light differently from standard spherical zinc oxide particles, thereby creating a warmer tone that would likely be more acceptable to users.

"When I spread it on my own skin, I didn't get that white cast I usually see with zinc oxide," Addae said. "That was the moment I realized this could really work."

The application-driven focus of this research is "really exciting" and has the potential to translate into real impact, Sedransk Campbell said. The next steps will involve testing the human and environmental safety profiles of the tetrapod nanoparticles and investigating how to scale up production in a cost-effective manner.