Introduction
As the body’s first defense against the elements, skin is frequently exposed to electromagnetic radiation from the Sun, which can lead to a variety of detrimental conditions such as photoaging, photoimmunosuppression, and photocarcinogenesis. Reactive oxygen species are largely responsible for the initiation of these disease states in skin and are mostly due to exposure to ultraviolet light, but have also been shown to result to some extent from the absorption of visible and infrared light. Considerable efforts have also been made to better understand the effects of pollution on the skin from a free radical and reactive oxygen species point of view.
Antioxidants
The use of antioxidants in various skin treatments is a sound approach to improve the overall health state of skin [1]. This statement is supported by a wealth of research conducted over the last several decades toward better understanding how antioxidants mitigate the effects of solar radiation. Topical application of antioxidant-containing products reduces the deleterious effects of solar radiation exposure of the skin.
While some antioxidants may offer some photoprotection as a solar filter, the majority of their mechanisms are through their antioxidant capacity or immunomodulating effects. Some of the most common antioxidants in skin care formulations are vitamin E, vitamin C, and coenzyme Q. Historically, these were probably the most studied antioxidants due to their importance in the endogenous antioxidant system.
Equally important are a vast majority of botanical extracts, which are chock-full of phyto-antioxidants. In recent years, research has focused on understanding the antioxidant behavior of polyphenols in an attempt to harness their protective properties for skin. In some cases, specific polyphenols are used in formulation while in others the extract is directly added.
Topical Application of Antioxidants
Topical application of antioxidants is the most straightforward approach to fortify the skin. As compared to dietary intake of antioxidants, in many cases topical application allows: (1) greater concentrations to reach tissues, (2) greater tissue specificity, and (3) reduced side effects to other organs. Unfortunately, not all antioxidants (e.g., from extracts) easily cross the stratum corneum barrier. The fact that some antioxidants are not able to penetrate the skin could be considered a positive toxicological benefit. Skin permeation and antioxidant stability can be enhanced by utilizing state-of-the-art delivery systems.
One of the major factors with antioxidant stability in skin care formulations stems from the need to prevent oxidation within the formulation and also to deliver to the skin an active antioxidant that is bioavailable. In many cases, formulations are based on carrier systems in which oxidation can occur in the oil phase, water phase, or at the interface. More often than not, oxidation occurs at the interface. Some of the hurdles facing formulators in the antioxidant arena are a result of stability issues with antioxidants that are intended to be delivered to skin.
Antioxidant Carrier Systems
The use of carrier systems represents a real asset for the delivery of antioxidant to skin and can include various types of emulsion, vesicular, or lipid particle systems.
Emulsions systems
These systems are dispersions of oil and water and can refer to microemulsions, nanoemulsions, and Pickering emulsions. Microemulsions and nanoemulsions are characterized by the dispersion size of the emulsified phase, while Pickering emulsions refer to a type of emulsion that is stabilized by solid particles.
Vesicular systems
These systems consist of liposomes, phytosomes, transferomes, ethosomes, and niosomes. Liposomes are the most popular vesicular system used in personal care applications and are composed of concentric layers of phospholipid bilayers spherically shaped with a hollow center for the active ingredient. Phytosomes are vesicles of phospholipids that have high affinity for phytocompounds, such as polyphenols. Transferosomes are lipid vesicles that consist of fatty acids and a small amount of ethanol. They are more elastic than liposomes, which improves their deposition characteristics. Ethosomes are lipid vesicles that contain even greater amounts of ethanol, yielding a more flexible vesicle. Niosomes are lamellar vesicles based on nonionic surfactants. Due to the nature of the surfactants in niosomes, crossing the stratum corneum is more facile than in the case with other vesicles.
Lipid particle systems
These systems consist of lipid microparticles and lipid nanoparticles. Lipid microparticles are created by a process known as microencapsulation where a small solid or liquid droplet is surrounded with a thin layer of shell. Lipid nanoparticles are further categorized as solid lipid nanoparticles and nanostructured carriers. Solid lipid particles consist of a lipid system in the solid state at room temperature with a thin surface coating on the outside as a stabilizer. Nanostructured lipid carriers, on the other hand, are more complex and contain lipids both in the solid and fluid phase. Typically, such systems can increase the stability of antioxidants and their permeation efficacy to skin as well as reduce irritation. The reader is referred to a review by Pol and Patravale for a nice introduction to the subject [2].
Nanoparticle and Nanocarriers
Nanoparticles and nanocarriers continue to be at the forefront of skin care research for their potential at stabilizing and delivering antioxidants to the skin. For example, gold nanoparticles are known for their anti-inflammatory, antiaging, and wound healing properties in skin care. A recently published study demonstrated how polyphenols from an aqueous extract can be used to reduce metal salts—in this case gold—into nanoparticles [3]. In another study, gold nanoparticles wrapped with chitosan were used to stabilize ellagic acid [4]. In both cases, green technology was used to fabricate the nanoparticle structures.
Nanoencapsulation is another area that shows much promise for the delivery of antioxidants to skin. Lipid-core nanocapsules containing resveratrol and lipoic acid have enhanced chemical stability and photostability as compared to the non-encapsulated forms of the molecules [5]. TiO2 is a nanoparticle found in many sun protection products. It functions by scattering incoming UV rays from the Sun and preventing photodamage to the skin. Researchers at Sabanci University in Istanbul found enhanced cellular penetration and antioxidant properties of quercetin-TiO2 nanoparticles, as compared to quercetin alone, in studies carried out on fibroblast cell cultures [6].
Concluding Remarks
Some of the challenges with the conventional delivery of antioxidants stems from their poor solubility, limited shelf-life stability, compromised photostability, and low degree of skin permeability. Delivery systems enhance the ability of antioxidants to carry out their function. Conventional systems used to deliver antioxidants consist of emulsion, vesicular, or lipid particle systems. In recent years, a great deal of interest has evolved in using nanoparticles as stabilization enhancers and delivery agents for antioxidants. Nanoencapsulation also offers much promise and has been shown to enhance the chemical stability and photostability of antioxidants.
References
- McMullen, R., Antioxidants and the Skin. 2nd ed. 2019, Boca Raton: CRC Press.
- Pol, A. and V. Patravale, Novel lipid based systems for improved topical delivery of antioxidants. Household and Personce Care TODAY, 2009(4): p. 5-8.
- Haddada, M., et al., Assessment of antioxidant and dermoprotective activities of gold nanoparticles as safe cosmetic ingredient. Colloids Surf B Biointerfaces, 2020. 189: p. 110855.
- Gubitosa, J., et al., Multifunctional green synthesized gold nanoparticles/chitosan/ellagic acid self-assembly: Antioxidant, sun filter and tyrosinase-inhibitor properties. Mat Sci Eng C, 2020. 106: p. 110170.
- Davies, S., et al., Simultaneous nanoencapsulation of lipoic acid and resveratrol with improved antioxidant properties for the skin. Colloids Surf B Biointerfaces, 2020. 192: p. 111023.
- Birinci, Y., et al., Quercetin in the form of a nano-antioxidant (QTiO2) provides stabilization of quercetin and maximizes its antioxidant capacity in the mouse fibroblast model. Enzyme Microb Tech, 2020. 138: p. 109559.
Roger L. McMullen, Ph.D. – BIO
Dr. Roger McMullen has over 20 years of experience in the personal care industry with specialties in optics, imaging, and spectroscopy of hair and skin. Currently, he is Principal Scientist in the Material Science department at Ashland Specialty Ingredients G.P. Roger received a B.S. in Chemistry from Saint Vincent College and completed his Ph.D. in Biophysical Chemistry at Seton Hall University.
Roger actively engages and participates in educational activities in the personal care industry. He frequently teaches continuing education courses for the SCC and TRI-Princeton. In addition, Roger is an Adjunct Professor at Fairleigh Dickinson University and teaches Biochemistry to students pursuing M.S. degrees in Cosmetic Science and Pharmaceutical Chemistry. Prior to pursuing a career in science, Roger served in the U.S. Navy for four years on board the USS YORKTOWN (CG 48). He is fluent in Spanish and Catalan.