DEVELOPMENT, OPTIMIZATION, AND CHARACTERIZATION OF A MULTIFUNCTIONAL BROAD-SPECTRUM MINERAL SUNSCREEN STICK INCORPORATING NIACINAMIDE AND TOCOPHEROL FOR UV PROTECTION, SKIN-BRIGHTENING, AND SEBUM CONTROL
Abstract
Prolonged exposure to ultraviolet (UV) radiation is a well-established cause of cumulative skin damage, including melanoma, basal and squamous cell carcinoma, and premature photoaging. Mineral sunscreens formulated with zinc oxide and titanium dioxide are classified by the US FDA as Generally Recognized as Safe and Effective (GRASE), yet their poor sensory profile — stickiness, difficult spreadability, and a visible white cast — continues to limit consistent daily use. This study aimed to design and characterize a 100% mineral sunscreen stick combining zinc oxide and titanium dioxide with niacinamide and tocopherol to deliver SPF 30 broad-spectrum protection alongside oil control and antioxidant benefit, while resolving the traditional cosmetic shortcomings of mineral formulations. The stick was produced by a three-phase hot-pour method comprising a wax phase (beeswax, carnauba wax, and supporting oils), a powder phase (nano-sized zinc oxide, titanium dioxide, and niacinamide), and a heat-sensitive active phase (tocopherol), with the active powders reduced to nano-size and sieved before incorporation to ensure smooth, uniform dispersion. The finished sticks were evaluated for organoleptic quality, melting point, hardness, and surface pH; in vitro SPF and UVA-PF by ISO 24443:2012; water resistance after 40 minutes of immersion; in vitro sebum absorption; a 20-panelist sensory evaluation; and three months of accelerated stability testing at 40 °C/75% RH. The formulation achieved an in vitro SPF of 31.2 ± 2.0 (27.3 ± 0.5 by the complementary Mansur spectrophotometric method) with a critical wavelength of 374 nm, confirming genuine broad-spectrum coverage; a melting onset of 62.5 ± 1.5 °C, well above the 50 °C structural-stability threshold; and retention of more than 83% of its SPF after water immersion. The sensory panel rated glide, comfort, and finish highly, with only a mild residual white cast, and niacinamide significantly increased sebum absorption relative to a base formulation. Hardness, pH, and SPF remained within specification throughout three months of accelerated storage. These findings indicate that a fully mineral sunscreen stick can combine robust sun protection with a genuinely pleasant user experience, narrowing the long-standing gap between the theoretical and practical performance of mineral sunscreens. In vivo SPF validation and longer-term stability studies are recommended before commercial translation.
Keywords
- mineral sunscreen, zinc oxide, titanium dioxide, sunscreen stick, niacinamide, tocopherol, broad-spectrum SPF
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