In conclusion the photophysics and the photochemical characteristics of sunscreens and their perceived photostabilizers need to be explored on a sun-active agent basis. The photochemical
30
response of a sunscreen agent may be influenced by the matrix in which is formulated and hence its absorption characteristics may be greatly affected.
Acknowledgement
MAO is grateful to the University of KwaZulu-Natal, College of Agriculture, Engineering and Science for the award of a doctoral bursary.
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Chapter Three
Trends in Sunscreen Formulations
Moses A. Ollengo and Bice S. Martincigh*
School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban 4000, South Africa
*Corresponding author. Tel.:+27-31-2601394; Fax: +27-31-2603091; E-mail address:
Abstract
There are different methods of combating the deleterious effects of ultraviolet radiation (UV):
sunscreens formulated with filters as well as clothing. It is known that exposure of keratinocytes to 15 minutes of UVA radiation leads to substantial cell mortality and a decrease in protein content.
Thus, the consequences of exposure to UV radiation and its correlation with cancer development have triggered a public education campaign promoting the use of sunscreens. A broad variety of different creams, dispersions, emulsions, gels, ointments, lotions, milks, sprays, tonics and hydrogels are available in the market that use various UV-filter systems. Several inorganic and organic compounds have been explored and are employed for protection from harmful UV radiation. A lot of research is ongoing with a view of investigating ways of reducing the skin penetration of the sunscreen active ingredients, oxidative stress management and evaluation of different types of vehicles for topical dermal delivery. This review aims at exploring the current formulations as well as to point out novel approaches for suncare product development and presentation.
Keywords: Sunscreens, Nano-encapsulation, Dendrimer-nano-incorporation, Hindered-Amine-light- stabilizers, Antioxidants, Hydrotalcites.
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3.1 Introduction
Ozone, a minority constituent in the earth's atmosphere, is a major absorber of ultraviolet (UV) radiation (Sklar et al. 2013). As has been shown anthropogenic emissions, for example, of chlorofluorocarbons can deplete stratospheric ozone, giving rise to an ozone hole. A decrease in atmospheric ozone is expected to significantly increase levels of UV on the earth’s surface (Bowden 2004). The main public concern regarding the ozone hole has been the effect of increased surface UV radiation on human health. It is well documented that UV radiation is harmful to skin and can cause helioderma and cancers (Abarca and Casiccia 2002). Publicized strategies for combating UV radiation are: sun avoidance during peak hours (10 am to 4 pm), clothing and sunscreens formulated with filters.
Solar UV radiation incident on the earth’s surface can be divided into two regions: UVB (290-320 nm) and UVA (320-400 nm). Both types are harmful to human skin, damaging both the skin surface and inner structure of skin when taking prolonged sunbaths. The skin is the largest organ of the body and constitutes 16 % of the body weight, with a surface area of 1.8 m2. It has several functions; most important being that it is a physical barrier to the environment, allowing and limiting inward and outward passage of water, electrolytes and various substances. It provides protection against micro- organisms, UV radiation, toxic agents, and mechanical insults. Though structurally consistent throughout the body, the skin varies in thickness depending on anatomical site and age of an individual. The epidermis is the outer layer, serving as the physical and chemical barrier between the interior and exterior body environment. Because UVB rays are of shorter wavelength they only reach the epidermal layer causing sunburn. Most of the UVB radiation is absorbed by the stratum corneum on the epidermis surface (Fig. 3.1). This stratum corneum is a layer of dead cells; the skin visible layer (Lautenschlager et al. 2007). There is evidence from animal studies that UVB induces the disruption of the epidermal barrier function (Jiang et al. 2006). However, UVB radiation has been shown to play a critical role in the synthesis of vitamin D. Dermis is the deeper layer providing the structural support it is a loose connective tissue layer beneath the epidermis and subcutis or hypodermis is an important depot of fat. Topically applied UV filters should be localized in the outermost part of the stratum corneum without infiltration to deeper viable tissues (Felton et al. 2002).
Figure 3.3: The structure of the skin (http://csmrsoldier.com/2013/09/06/adventures-skin- trade/ accessed on 12-12-2014)
Its known that compared with UVB, UVA radiation is a tenfold more efficient oxidative stress generator causing lipid peroxidation linked to plasma membrane damage (Damiani et al. 2006). In vitro studies indicate UVA generates a peroxidative process in cultured human skin fibroblasts and in keratinocytes, a radical process that alters the plasma membrane. Exposure of keratinocytes to 15 min UVA radiation has been shown to result in substantial cell mortality and a protein content decrease (Armeni et al. 2004). Such physiological changes have adverse effects on the overall skin structure and serves to initiate various skin maladies.
The cancer induction mechanism by UVA and UVB radiation is well documented. Absorption of UVA and UVB radiation causes pyrimidine bases in the DNA molecule to form dimers (González et al. 2008); resulting in transcription errors during DNA replication. The malignant type of cancer manifests tumours as a consequence of abnormal proliferating skin cells. The uncontrollable growth of these cells leads to melanoma tumours forming. Melanoma is a cancerous skin tumour, produced by cells in the skin that give it pigment (melanin), cells called melanocytes. Melanoma begins as a dark skin lesion and may spread rapidly to other skin areas and within the body (Besaratinia and Pfeifer 2008). Usually, melanoma skin cancer is caused by longer, deeper penetrating UVA rays.
They penetrate the dermal layer and cause elastosis (loss of structural support and elasticity of the skin) (Atitaya et al. 2011). Melanomas are therefore linked to UVA radiation but other experiments on opossums suggest a larger role for UVB (van der Leun and de Gruijl 2002). Consequently both UVA and UVB radiation have therefore been linked to skin cancer, whether malignant or benign (Abarca and Casiccia 2002). The most lethal of the skin cancers, cutaneous malignant melanoma, is more commonly associated with sporadic burning exposure to solar radiation. There are several indications that UVA might have an important role in the pathogenesis of melanoma (Lautenschlager et al. 2007). But sunburns are taken as a measure of overexposure to solar radiation and they have been identified as a risk factor for the development of melanoma. It is on the basis of sunburns;
primarily due to UVB that implicates UVB as a potential contributing factor to the pathogenesis of melanoma. To this end there is a great deal of controversy regarding the relationship between UVA exposure and the development of melanoma (Wang et al. 2001). Nonetheless, cutaneous malignant melanoma is one of the fastest increasing cancers and UV radiation is strongly linked in its etiology (De Fabo et al. 2004). Cutaneous malignant melanoma is more prevalent among light-skinned people (Abarca and Casiccia 2002).
The other solar radiation associated skin conditions are basal and squamous cell carcinomas, which are common forms of skin cancer in humans. These cancers (BCC and SCC) are relatively mild and rarely fatal, although the treatment of squamous cell carcinoma sometimes requires extensive reconstructive surgery. Other UV radiation induced skin disorders are: photoaging; actinic keratosis;
lupus vulgaris (tuberculosis of the skin), and psoriasis or vitiligo (a discontinuous depigmentation of the skin). Hence, sun protection is an inevitable choice, and suitable vehicles are required to deliver the sunscreen ingredient onto the skin or in clothing fabric.