# Timed-Release Silica Microcapsules for Consistent Fragrance Release in Topical Formulations
## 1 Introduction  
Long-lasting fragrance technologies have been studied in various industries, including food [1, 2], personal care [3, 4], textiles [5, 6], and agriculture [7, 8]. Several techniques are used to extend fragrance longevity, including core-shell microcapsules [9, 10, 11], programme systems [12, 13], and inclusion methods [14, 15], with core-shell microcapsules being the most common. Fragrance microcapsules can preserve fragrances through their stable capsule wall [16]. External forces, primarily friction between fabric and skin or other fabrics, cause the capsules to rupture and release the fragrance [17]. These properties have been applied successfully to fabrics; however, they offer limited advantages for daily cosmetic applications. Topical formulations, including cosmetics, ointments, and patches, generally release active ingredients within short time frames, such as between cleansing routines like face-washing or showering [16]. As these routines often occur at least daily, it is essential that the fragrance is sustained but fully released within 24 h. However, conventional microcapsule methods release insufficient active ingredients relative to the totalencapsulated content, leading to low overall efficacy. To maintain the desired effect, it is crucial to ensure timely and consistent fragrance release throughout the application period. Silica, an inorganic material with a wide range of applications, including as an anticaking agent, thickener, and adsorbent in pharmaceuticals, cosmetics, and household products [18; 19; 20; 21], has been intensively studied as microcapsules for encapsulating active ingredients [22; 23]. However, the intrinsic mesoporous structure of silica presents challenges in maintaining the efficacy of active ingredients beyond the typical two-to-three-year shelf life of commercial products [23; 24; 25; 26]. To address this issue, organic materials such as ethyl cellulose [27], polyurea [28], and melamine [29] have been used as coatings along with the hydrophobic modification using agents such as methyltriethoxysilane and dimethyldiethoxysilane [30; 31]. Specifically, the mesoporous surface of silica microcapsules is coated with an external silica layer using precursors such as tetraethyl orthosilicate (TEOS) or sodium silicate, which undergo similar reactions. This coating enhances the stability and extends the shelf life of the microcapsules [10; 32]. Most of these microcapsules are effective for the sustained release of active ingredients, with release periods ranging from weeks to months [10; 32]. However, they are less suitable for daily topical products, as they have been developed for long-term preservation and only release a small amount of fragrance during the short period when they remain on the skin. The challenge arises from the discrepancy between the prolonged retention of active ingredients within microcapsules and the need for their continuous and complete release upon skin application. Silica synthesized by sol-gel reactions, when not subjected to post-treatment processes such as sintering, is prone to structural deformation during drying, including shrinkage and cracking, due to capillary forces and additional cross-linking between unreacted silanol groups [33; 34; 35]. We aimed to exploit these properties to develop silica microcapsules that stably encapsulate fragrances when wet and undergo self-destruction upon drying, resulting in timed sustained fragrance release. In this study, we developed fragrance capsules for controlled release over a short period. These microcapsules were prepared via a sol-gel reaction based on an oil-in-water emulsion template using an emulsifier suitable for cosmetic applications. The capsule size and silica content constituting the microcapsule wall were controlled by adjusting the concentrations of the emulsifier and TEOS. This design allowed the capsules to break sequentially based on size as they dried on the skin, gradually releasing the fragrance.