polymers Article Sunscreen Enhancement of Octyl Methoxycinnamate Microcapsules by Using Two Biopolymers as Wall Materials Chuntao Xu 1,2 , Xuemin Zeng 3, Zujin Yang 4,* and Hongbing Ji 1,3,4,5,* 1 School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China; [email protected] 2 School of Information Engineering, Zhongshan Polytechnic, Zhongshan 528400, China 3 Fine Chemical Industry Research Institute, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China; [email protected] 4 School of Chemical Engineering and Technology, Sun Yat-Sen University, Zhuhai 519082, China 5 School of Chemical Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, China * Correspondence: [email protected] (Z.Y.); [email protected] (H.J.) Abstract: Octyl methoxycinnamate (OMC) is widely used as a chemical sunscreen in sunscreen cosmetics. However, its direct contact with the skin would bring certain risks, such as skin photo- sensitive reaction. How to improve the effect of skin photodamage protection has become a current research hotspot. Encapsulating ultraviolet (UV) filters into microcapsules is an interesting method to increase the photostability of filters. In this study, sodium caseinate (SC) and arabic gum (GA) are chosen as wall materials to prepare synergistic sunscreen microcapsules by complex coacervation technology. A series of experiments are conducted to investigate the effects of pH, wall material concentration, and wall/core ratio on the formation of OMC microcapsules. The morphology, com- position, and stability of OMC microcapsules are characterized by scanning electron microscopy Citation: Xu, C.; Zeng, X.; Yang, Z.; Ji, (SEM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The H. Sunscreen Enhancement of Octyl OMC microcapsule is uniform in size distribution, smooth in surface morphology, and has good Methoxycinnamate Microcapsules by thermal stability. The results show that the ultraviolet absorption of the OMC microcapsules is better Using Two Biopolymers as Wall than that of the uncoated OMC for the ultraviolet-B (280–320 nm). Moreover, the OMC microcapsule Materials. Polymers 2021, 13, 866. https://doi.org/10.3390/ released 40% in 12 h, while OMC released 65%, but the sun protection factor (SPF) of the OMC polym13060866 microcapsule sunscreen is 18.75% higher than that of OMC. This phenomenon may be attributed to the hydrophobic interaction between SC and OMC and the electrostatic interaction between SC Academic Editor: Alina Sionkowska and GA. Received: 1 February 2021 Keywords: octyl methoxycinnamate; sunscreen; microcapsule; complex coacervation; synergistic Accepted: 8 March 2021 effects Published: 11 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- In recent decades, the destruction of the atmospheric ozone layer has made people iations. suffer from excess exposure of ultraviolet (UV) radiation, causing serious damage to the human health [1,2]. It has been reported that about 1.5 million disability-adjusted life years (DALYS) in the world were related to UV radiation [3]. Based on the wavelength and energy, UV radiations are mainly divided into three bands: short-wave ultraviolet Copyright: © 2021 by the authors. C (UVC, 100–280 nm), medium-wave ultraviolet B (UVB, 280–320 nm), and long-wave Licensee MDPI, Basel, Switzerland. ultraviolet A (UVA, 320–400 nm), respectively [4,5]. UVC with the weakest penetration This article is an open access article force could be completely absorbed by the atmosphere, but 5–10% of UVB and 90–95% distributed under the terms and of UVA radiation could penetrate the clouds and the ozone layer to the earth surface, conditions of the Creative Commons causing harm to our skin [6,7]. For example, UVB overexposure to skin would lead Attribution (CC BY) license (https:// to inflammation, immunosuppression, and skin cancer [8]. Therefore, it is particularly creativecommons.org/licenses/by/ important to effectively avoid UVB absorption in life. Some methods have been used 4.0/). Polymers 2021, 13, 866. https://doi.org/10.3390/polym13060866 https://www.mdpi.com/journal/polymers Polymers 2021, 13, x FOR PEER REVIEW 2 of 18 Polymers 2021, 13, 866 2 of 17 effectively avoid UVB absorption in life. Some methods have been used to protect our skin toincluding protect our physical skin including or chemical physical filters oror chemicalabsorbing filters and reflecting or absorbing UV andradiation reflecting [9]. Sun- UV radiationscreen products [9]. Sunscreen are the productsmain skin are pr theotection main against skin protection the damage against of UVB. the damage of UVB. Currently,Currently, octyloctyl methoxycinnamatemethoxycinnamate (OMC)(OMC) isis oneone ofof the the most most common common sunscreen sunscreen agentsagents in in commercially commercially available available cosmetics cosmetics in in the the market market to to resist resist UVB UVB in in sunlight sunlight owing owing toto itsits excellentexcellent UVUV absorptionabsorption curve,curve, highhigh lipophilicity,lipophilicity, andand goodgood oiloil solubilitysolubility [[10,11].10,11]. However,However, during during its its use, use, there there has has been been some some disadvantages disadvantages such such as as poor poor photostability photostability andand strongstrong permeabilitypermeability [12[12].]. AccordingAccording toto previousprevious reports,reports, thethe largelarge amount amount of of OMC OMC wouldwould accumulate accumulate inin thethe deeperdeeper layerslayers ofof skinskin afterafter coatingcoating onon thethe skinskin superficialsuperficial areas.areas. OMCOMC maymay inhibit the the deiodinase deiodinase activity, activity, indu inducece morphological morphological changes changes in the inthe testes testes and andovaries ovaries of rats of rats[13–15], [13– 15and], andaffect affect secretion secretion of estrogen, of estrogen, reproductive reproductive development, development, and andnervous nervous system system of rat of offspring rat offspring [16]. [16Recently]. Recently,, efforts efforts have have been beenmade made to develop to develop some somemore moreefficient efficient and safe and sunscreen safe sunscreen agents, agents, reducing reducing the toxicological the toxicological risk from risk the from percu- the percutaneoustaneous absorption. absorption. In order In order to achieve to achieve this this goal, goal, many many strategies strategies have have been been employed employed to toprevent prevent photodamage photodamage to skin to skin [17]. [17 Microencap]. Microencapsulationsulation is a new is a inclusion new inclusion method method where whereactive activematerials materials are encapsulated are encapsulated in the microcapsules. in the microcapsules. By changing By changing the wall materials, the wall materials,the microcapsules the microcapsules can effectively can effectively protect and protect control and controlrelease releaseof the enclosed of the enclosed active active ingre- ingredientsdients [18–21]. [18– 21It ].was It was a promising a promising technolo technologygy to to encapsulate encapsulate OMC OMC for overcomingovercoming thethe trans-epidermaltrans-epidermal penetration penetration and and promoting promoting UV UV absorption absorption ability. ability. Complex Complex coacervation, coacerva- astion, one as kind one ofkind microencapsulation of microencapsulation technology, technology, is accomplished is accomplished by phase by phase separation separation of oneof one or many or many hydrocolloids hydrocolloids from from the initialthe initial solution solution and and the subsequentthe subsequent deposition deposition of the of newlythe newly formed formed coacervate coacervate phase phase around around the active the ingredientactive ingredient which haswhich been has used been widely used to microencapsulate the active components [22–25]. widely to microencapsulate the active components [22–25]. In the present study, sodium caseinate (SC) and arabic gum (GA) are chosen as In the present study, sodium caseinate (SC) and arabic gum (GA) are chosen as wall wall material to prepare OMC microcapsules based on the complex coacervation method material to prepare OMC microcapsules based on the complex coacervation method (Scheme1). The process conditions of the microcapsules were optimized by adjusting pH, (Scheme 1). The process conditions of the microcapsules were optimized by adjusting pH, wall material concentration, and wall/core ratio. Scanning electron microscopy (SEM), wall material concentration, and wall/core ratio. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) are Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) used to characterize the morphology, structures, and stability of the OMC microcapsules. are used to characterize the morphology, structures, and stability of the OMC microcap- Finally, the encapsulation efficiency, release, and sun protection performance of the OMC sules. Finally, the encapsulation efficiency, release, and sun protection performance of the microcapsules are also investigated. OMC microcapsules are also investigated. SchemeScheme 1. 1.Preparation Preparation of of octyl octyl methoxycinnamate methoxycinnamate (OMC) (OMC) microcapsule microcapsule by by complex complex coacervation. coacervation. Polymers 2021,