Preparation and Characterization of Organic/Inorganic Composite UV Filter Microcapsules by Sol-Gel Method

Octyl methoxycinnamate and butyl methoxydibenzoylmethane are organic UV filters with poor photostability and will become photoallergy or phototoxic substance when exposed to ultraviolet radiation. ,e organic UV filters coated by microcapsules can reduce the photodegradation and avoid direct contact with the skin. ,rough microencapsulation, the application of UV filters in cosmetics becomes more effective and safer. ,is study first used the sol-gel method to create organic/inorganic composite UV filter microcapsules. We used sodium alginate as a shell material of the microcapsule to encapsulate UV filters. CaCO3 and tetraethyl orthosilicate (TEOS) were used as cross-linking agents, and sorbitan monooleate (Span 80) and polyoxyethylenesorbitan monooleate (Tween 80) were used as emulsifiers in the interfacial polymerization method for preparation. ,e results indicated that the microcapsules with 3 g of CaCO3 cross-linking agents had a similar particle size and better entrapment efficiency. ,e average sizes were 61.0± 4.9 μm and 48.6± 4.7 μm, and entrapment efficiencies were 75.3± 1.9% and 74.8± 1.7% for octyl methoxycinnamate and butyl methoxydibenzoylmethane, respectively. Utilizing sodium alginate as a crosslinking agent is better than TEOS due to the higher calcium content. In vitro transdermal delivery analysis showed that the release rate became steady.


Introduction
Ultraviolet radiation is an important environmental risk factor that can lead to cell death, sunburn, tanning, and skin ageing [1]. ere are three main types of ultraviolet radiation, namely UVA (320-400 nm), UVB (290-320 nm), and UVC (200-280 nm), and both UVA and UVB play an important role in skin cancer. Avoiding overexposure to ultraviolet radiation during the daytime is very important in skin protection. UV filters [2][3][4], currently used for skin protection against UV, are used to absorb specific wavelengths of ultraviolet radiation. ere are two general types of UV filters, namely, inorganic UV filters and organic ones. ese compounds can either be physical blockers or chemical absorbers, based on their mechanism of action. Inorganic UV filters, such as titanium dioxide and zinc oxide, protect human skin from UV radiation by deflecting or blocking [3]. However, the ability of inorganic UV filters to reflect UV lights may be affected by the used concentration of cosmetic products, which also influence the formulation's properties, such as uniformity, stability, and impressibility. Organic UV filters absorb UV radiation, and the molecules transfer from the ground state to the higher energy excited state [3,5]. However, the excited molecules return to the ground state by different mechanisms. Under ultraviolet irradiation, organic UV filters may generate thermal instability, causing photoallergy and phototoxicity reactions [2,6,7]. erefore, if we combine the organic UV filter into a solid inorganic one, the outside inorganic UV filter can reflect UV radiations directly, and the inner or released organic UV filter can also absorb UV lights and then convert them into harmless energy. Besides, the inorganic UV filter may protect organic ones when they are integrated. Moreover, the control release of the organic UV filter from the particles may provide a longer and safer protection for skin. As a result, the application of UV filters in cosmetics is becoming more effective and safer via microencapsulation into organic/inorganic composite UV filters.
Octyl methoxycinnamate is currently the most widely used UVB absorber in UV filters with good absorbing properties [8,9]. It is an organic compound formed from methoxycinnamic acid and 2-ethylhexanol. When mixing together, they form a clear liquid that does not dissolve in water. Octyl methoxycinnamate is 290 g·mol −1 ; however, it is susceptible to light and becomes photoallergy or phototoxic when exposed to ultraviolet radiation. Several studies have shown that octyl methoxycinnamate possesses estrogenic activity, which might cause a hormone effect (such as reproductive impairments and female and male cancers) [10][11][12][13]. In the studies, 500, 750, and 1000 mg/kg of octyl methoxycinnamate were applied on pregnant Wistar rats; it was discovered that there is a weight reduction of testicles with a higher dose [13]. erefore, this chemical should be used with caution in pregnant women and children (maximum use limits of 10%).
Butyl methoxydibenzoylmethane [14][15][16] is a commonly used UVA-1 filter (340-400 nm) among UV filters. It is a slightly yellowish oil-soluble chemical agent. Butyl methoxydibenzoylmethane is 310 g·mol −1 ; however, several studies have confirmed that butyl methoxydibenzoylmethane has low photostability and easily degrades with ultraviolet radiation [10][11][12]. It interacts with other UV filters, decreasing the effectiveness of sunscreen [2,17,18]. Crystallization of complexes has often been associated with butyl methoxydibenzoylmethane due to heavy metal, iron, zinc, aluminium, magnesium salts, stearic acid, and formaldehyde derivatives [19]. erefore, the stability of butyl methoxydibenzoylmethane depends on the formulation and the presence of other UV filters. A maximum use limit of 5% is recommended.
Microencapsulation technology has been widely used in chemical, cosmetic, pharmaceutical, and other related industries [20][21][22]. Microcapsules are mainly used as a natural or synthetic polymer; the shell material and drugs are encapsulated as a core material. Microcapsule size ranged from 1 to 1000 μm. e oxidation, temperature, UV radiation, and environmental conditions will strongly affect the stability of drugs. erefore, the encapsulation technique can protect the drug and also control its release rate. Among the microencapsulation methods, the sol-gel emulsion method is the convenient technique that utilizes the emulsion droplets as templates to prepare the nanoparticles through polymerization, hydrolyzation, or cross-linking reactions [23]. e sol-gel method can control the materials and form the shell at the interface of the emulsion. erefore, this method is suitable for the development of functional compound encapsulation in pharmaceutical and cosmetic industries [24,25]. Sodium alginate is a polysaccharide carbohydrate extract from natural seaweed. It is a linear hydrophilic polymer compound consisting of β-D-mannuronic acid and α-L-guluronic acid [26]. Sodium alginate has been one of the most popular biomaterials because of its convenient accessibility, nontoxicity, excellent biocompatibility, and biodegradability [27,28]. Tetraethyl orthosilicate is the chemical compound with the formula Si(OC 2 H 5 ) 4 ; it is mainly used as a cross-linking agent in silicone polymers and is more stable in the air [29,30]. In the experiment, the addition of acid makes it hydrolyze quickly.
is research aims at preparing organic/inorganic composite ultraviolet filter microcapsules; the sodium alginate was used as a shell material to encapsulate octyl methoxycinnamate and butyl methoxydibenzoylmethane by using the sol-gel method. e organic UV filters were encapsulated in microcapsules not only to avoid photodegradation and phototoxicity phenomenon, but also to avoid direct contact with the skin and reduce the chance of allergies. is study focused on the effects of preparation parameters on the properties of microcapsules containing UV filters; the properties of interest were the size distribution, entrapment efficiency, and in vitro release profile.

Preparation of Microcapsules by the Sol-Gel Method.
For the preparation of microcapsules, 3 g of Span 80, 2 g of Tween 80, and 17.6 g of mineral oil were all warmed to 50°C in a water bath, and then 10 g of each UV filter was added to the mixtures, as oil phase. e cross-linking agents with 3 g were dissolved in 64.4 g of 1.5% sodium alginate as the aqueous phase, then the oil phase was gently added to the aqueous phase, and after that the mixture was shaken and stirred at 7000 rpm for 30 min for emulsification. Subsequently, the prepared o/w emulsion was mixed with caprylic capric triglyceride at 1 :1 (v/v) and stirred at 1000 rpm at 4°C for 10 min. Finally, 6 g of 10% acetic acid was added dropwise and stirred for 5 min to complete the cross-linking reaction, and then washed and filtered to obtain microcapsules with ethanol. e compositions of microcapsules are presented in Table 1.

Characterization of Microcapsules.
Optical microscope images of the microcapsules were obtained with an optical microscope (OLYMPUS, Taiwan). First, the microcapsules were dispersed in propylene glycol of 50 ml. Each sample was measured 10 times. e proportion of the cross-linking agent was observed using an optical microscope (magnification 4×) in order to investigate the effect on the particle size of the obtained microcapsules.

Field Emission Scanning Electron Microscopy Analysis.
e surface morphology of prepared microcapsules was analyzed using ultra-high-resolution field emission scanning electron microscopy (FE-SEM, JEOL, Japan) at a voltage of 2.5 kV and at a working distance of 8 or 15 mm. Prior to the experiment, the resulting powder samples were coated with carbon adhesive. e samples were analyzed after gold sputtering for 30 sec.
(1) \ 2.6. In Vitro Transdermal Delivery Analysis. Transdermal drug delivery has been widely used in the human body to absorb in vitro release. is research used FDC-6 Transdermal Diffusion Cell Drive System (USA) to observe in vitro transdermal delivery analysis. e 3 M Tegaderm artificial skin membranes were used to simulate the stratum corneum side of human skin in this study. To prepare the sample emulsion, 2.5 g of microcapsules was dissolved in 46.5 g of water with 1 g of Lecigel ™ . First, in the upper donor cell, 1.0 ml of sample was placed. Second, 30% ethanol in phosphate buffer solution (a proper condition for the penetration of most hydrophilic and hydrophobic compounds) was placed in the lower receptor cell (5 ml with 0.785 cm 2 diffusion area) and bubbles were removed in the lower receptor cell. e receptor cell was stirred with a magnetic stirrer to maintain the temperature at 36.7 ± 0.3°C. Finally, the concentration distribution was balanced in the receptor cell. Each experiment used five Franz-type diffusion cells simultaneously measured at 0.5, 1, 2, 4, and 6 h. e samples were withdrawn in the receptor cell and were filtered. e amounts of drugs released from the microcapsules were evaluated using high-performance liquid chromatography (HPLC). e 20 μl sample was injected into the HPLC with a C18 reversed-phase column. e mobile phase was composed of methanol and water (80 : 20, v/v), and the flow rate was 1 ml/min.

Statistical Study.
e data from experiments were analyzed using Student's t-tests. All results are displayed as means ± S.E. from three independent experiments. Table 2, Samples 1, 2, and 3 are the size of microcapsules encapsulated octyl methoxycinnamate, whereas Samples 4, 5, and 6 are the size of microcapsules encapsulated butyl methoxydibenzoylmethane. According to the results, the effects on the particle size of microcapsules at different cross-linking agents were observed. Samples 1 and 4 were characterized by the lowest diameters, 61.0 ± 4.9 μm and 48.6 ± 4.7 μm. ey used a higher calcium carbonate content without tetraethyl e average size of the microcapsules decreased slightly as the Ca 2+ concentration increased. is may be due to the higher Ca 2+ concentration that caused a lower swelling of the microcapsules during the post-crosslinking process [32]. Varying the tetraethyl orthosilicate concentration appears to have little effect on particle size in Samples 2, 3, and 5; the average sizes were 65.3 ± 9.2 μm, 65.7 ± 4.4 μm, and 67.0 ± 4.7 μm, respectively. Sample 6 was unable to measure the correct particle size due to its shape, which was not close to spherical. Figure 1 presents optical micrograph observations of prepared microcapsules with different cross-linking agents. e microcapsule-encapsulated UV filters [Figures 1(a)-1(e)) were nearly spherical in shape. e amount of microcapsules with 3 g of CaCO 3 cross-linking agents was more than the microcapsules with TEOS. From the microscopic analysis after adding TEOS, there is a good dispersion due to the TEOS hydrophobic structure.

Surface Morphology.
e morphology of the microcapsules with 3 g of CaCO 3 cross-linking agents was observed from the SEM images, as shown in Figure 2. e control did not encapsulate the UV filters (Figure 2(a)). e control (Figure 2(a)) was nearly spherical in shape and similar to the drug microcapsules (Figures 2(b) and 2(c)).
is indicates that the encapsulated UV filters did not affect the shape and the matrix morphology of microcapsules with 3 g of CaCO 3 cross-linking agents. Moreover, the drug microcapsule surfaces (Figures 2(b) and 2(c)) were slightly denser than the control (Figure 2(a)). For the UV filter microcapsules, the microcapsules encapsulated butyl methoxydibenzoylmethane have a slightly rough surface, and the microcapsules encapsulated octyl methoxycinnamate have a smooth one. Table 3, the result showed that the best encapsulation efficiency is found using 3 g CaCO 3 as cross-linking agents, which were 75.3 ± 1.9% and 74.8 ± 1.7% (Table 3 (Samples 1 and 4)), respectively. Sodium alginate has better cross-linking due to the high calcium content [32]. e microcapsules of the structure were denser, making it easy to encapsulate more UV filters. is suggests that the preparation of organic microcapsules had a high efficiency for the encapsulation of UV filters. As observed in previous study, due to the increase in the Ca 2+ concentration, the encapsulation efficiency could be improved [32]. e drug loading content and production yield illustrated 3 g of CaCO 3 as cross-linking agents with a better result than others. e drug loading content ranged from 92 to 94%, and the yield of production ranged from 73 to 74%. is may enable UV filters to adsorb on the surface and create a different particle size. erefore, adding TEOS had lower entrapment efficiency.

In Vitro Transdermal Delivery Analysis.
e effect of different cross-linking agents on microcapsules for controlled release was analyzed by an in vitro transdermal delivery analysis. e release behaviour of UV filters from microcapsules under the simulated human skin conditions was studied in different time intervals. e in vitro drug release profiles are presented in Figures 3 and 4.
As shown in Figure 3, it can be seen that the control showed the highest release amount, with 5.03 μg/cm 2 of octyl methoxycinnamate released within 6 h. After, the release rate became steady. On the other hand, the extent of octyl methoxycinnamate released at 0.5 h or the drug microcapsules ( Figure 3 (Samples 1, 2, and 3)) was lower than 1 μg/cm 2 . Subsequently, the release rate gradually decreased as time passed. e cumulative amount released was 2.13 μg/cm 2 within 6 h for Sample 1. Regarding Samples 2 and 3, a nearly approximated release profile was obtained within 6 h. e accumulative amount of Samples 2 and 3 were 3.25 μg/cm 2 and 3.48 μg/cm 2 , respectively. Significantly, Sample 1 had the lowest cumulative release for octyl methoxycinnamate microcapsules. erefore, the microcapsules used 3 g of CaCO 3 as cross-linking agents to load; octyl methoxycinnamate reduced the percutaneous absorption of UV filters. Figure 4 presents the release profile of butyl methoxydibenzoylmethane microcapsules. e result showed that the release amount of the control was 5.13 μg/cm 2 at 0.5 h. After 6 h, the release amount for the control increases to 13.70 μg/cm 2 . Conversely, the butyl methoxydibenzoylmethane in the microcapsules leads to a significant decrease in the release amount of UV filters. At 0.5 h, the release amount of microcapsules ( Figure 4 (Samples 4, 5, and 6)) with UV filters was about 1.50 μg/cm 2 . After 6 h, Sample 4 slowed down the diffusion rate, with only 5.60 μg/cm 2 . Sample 5 and Sample 6 showed the cumulative amount released as 7.80 μg/cm 2 and 8.23 μg/cm 2 within 6 h, respectively. us, in Figure 4, the microcapsules with 3 g of CaCO 3 cross-linking agents decreased the release amount.
Several studies have demonstrated that the encapsulation of functional compounds or sunscreens can minimize the instability and skin contact, which may reduce the adverse effects of unencapsulated compounds on biological systems [33][34][35]. Especially for organic UV filters, the compounds are usually decomposed by the UV radiation when they are applied on human skin. e generated molecules may have opportunity to induce other side effects, such as cytotoxicity or some harmful immune response [36]. erefore, the use of microcapsules to protect the UV filters may not only improve their stability and also decrease the negative effects.

Advances in Materials Science and Engineering
In the experiments, UV filters were adsorbed on the surface because of the TEOS hydrophobic structure. e percutaneous absorption led to an increase in the release amount. However, the drug release for octyl methoxycinnamate and butyl methoxydibenzoylmethane decreases significantly.
is implies that butyl methoxydibenzoylmethane is more permeable to the skin than octyl methoxycinnamate due to its physicochemical characteristics. is is attributed to the improved solubility of butyl methoxydibenzoylmethane, which improves the amount of UV filters released [37]. e results described in the present study indicated that 3 g CaCO 3 cross-linking agents demonstrated the best result, which can control release and decrease the percutaneous absorption of UV filters.
In summary, octyl methoxycinnamate and butyl methoxydibenzoylmethane were successfully encapsulated using sol-gel technique into microcapsules, and the illustration of the composite UV filters microcapsules is shown in Figure 5. e optimum cross-linking agents were 3 g of CaCO 3 , which produce the lowest diameter and the best encapsulation efficiency. e octyl methoxycinnamate microcapsules with 3 g CaCO 3 cross-linking agents produced 61.0 ± 4.9 μm of particle size and 75.3 ± 1.9% of encapsulation efficiency. e butyl methoxydibenzoylmethane microcapsules with 3 g CaCO 3 cross-linking agents produced 48.6 ± 4.7 μm of particle size and 74.8 ± 1.7% of encapsulation efficiency. Furthermore, drug release profiles could be controlled since the UV filters are encapsulated in the microcapsules. In vitro transdermal delivery analysis showed that the microcapsules with 3 g of CaCO 3 crosslinking agents significantly reduced the percutaneous absorption of UV filters. We found that the microcapsules of octyl methoxycinnamate were decreased to 57.7% and the microcapsules of butyl methoxydibenzoylmethane were      decreased to 59.1% within 6 h. In the study, 3 g of CaCO 3 used as a cross-linking agent emerged as other formulations for further development due to the better encapsulation capacity and the effective controlled release of UV filters for microcapsules. e benefit not only enhances the protective power of ultraviolet radiation by keeping it at the skin surface but also limits potential toxic reactions. In addition, it can reduce the chance of allergies.
Data Availability e datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest
e authors declare that there are no conflicts of interest.