# Timed-Release Silica Microcapsules for Consistent Fragrance Release in Topical Formulations
## 3 Results and Discussion
### Control of Silica Content in Microcapsules for Timed-Release of Fragrance  
The release of fragrance was found to depend on the amount of silica constituting the microcapsule walls, which also influenced the stability of the microcapsules [10]. The emulsifier concentration was fixed at 0.1 wt.%, and the TEOS concentration in the microcapsules was adjusted 2, 4, and 6 wt.% to control the amount of silica constituting the microcapsule walls. As expected, increasing the TEOS concentration during the synthesis led to a decrease in the amount of fragrance released during the drying process. As shown in Figure 2A, the fragrance release was approximately 80% for microcapsules synthesized with 2 wt.% TEOS and 60% for those with 4 wt.% TEOS. Microcapsules synthesized with more than 6 wt.% TEOS exhibited a fragrance release of less than 40% over 24 h.  
It was hypothesized that a practical product released patchouli compound fragrance oil at a sustained intensity perceptible to humans for up to 24 h after application to the skin. Based on these findings, we determined that the TEOS content for synthesizing effective microcapsules should be less than 6 wt.%. The long-term preservation of the ingredients within fragrance microcapsules is essential to facilitate their use in topical products. To confirm the stability of the microcapsules under harsh conditions, fragrance retention was evaluated in microcapsules immersed in a 2 wt.% SDS solution at 60 \({}^{\circ}\)C. As expected, the retention of fragrance increased with the amount of silica constituting the microcapsule walls (Figure 2B). The microcapsules synthesized with 2 wt.% TEOS exhibited a complete fragrance release within 7 d at 60 \({}^{\circ}\)C, while those synthesized with 4 wt.% TEOS retained the fragrance for 12 d. By contrast, those with 6 wt.% TEOS still retained approximately 45% of the initial fragrance at the end of the test (14 d). The results in Figure 2A,B demonstrate the importance of determining the optimal TEOS concentration for synthesizing microcapsules to balance the trade-off between the timed-release of fragrance and the stability of the microcapsules in harsh environments. To confirm the optimal TEOS concentration used for synthesizing microcapsules, 20 participants were surveyed to assess the subjective fragrance release of model cream  
Figure 2: Silica fragrance microcapsules synthesized with different TEOS concentrations. (**A**) Fragrance evaporation behavior of silica microcapsules during drying at 36 \({}^{\circ}\)C and 50% humidity. (**B**) Fragrance retention of silica microcapsules in 2% SDS solution at 60 \({}^{\circ}\)C.  
formulations. The creams were prepared according to the compositions described in Table 1. Formulation A contained fragrance without encapsulation, while Formulation B used fragrance microcapsules synthesized with 2, 4, and 6 wt.% TEOS concentrations. The participants evaluated the fragrance release over a 12 h period. Rather than conducting a full-scale experiment, we simply set specific time points at 0, 6, and 12 h after release to confirm the trend and conducted a sensory evaluation of the fragrance. As shown in Figure S2, the microcapsules synthesized with 4 wt.% TEOS exhibited the highest fragrance intensity over 12 h. In contrast, the microcapsules synthesized with 6 wt.% TEOS showed a lower fragrance intensity over the same period. This reduced intensity could be attributed to the thick silica wall, which likely slowed the fragrance release, as shown in Figure 2A. In contrast, the microcapsules with thinner walls (2 wt.% TEOS) initially exhibited a strong scent, comparable to Formulation A; however, the fragrance intensity rapidly diminished over time. This behavior could be explained by the findings in Figure 2A,B. Specifically, the thin, porous capsule walls facilitated rapid fragrance leakage into the external environment. Despite an increase in the amount of silica constituting the microcapsule walls, the porosity remained unaltered, leading to inevitable fragrance leakage.