JOURNAL OF COSMETIC SCIENCE 174 INTRODUC T ION AN OVERV IEW OF BABY SKIN AND DIAPER RASH Baby ski n consists of three layers anatomically from outside to inside these are epidermis, dermis, and hypodermis layers. It is surrounded by the outermost stratum corneum (SC) layer of the epidermis, which acts as a physical barrier and is the primary regulator of percutaneous absorption (1,2). Diaper rash is probably one of the most common skin problems in infancy and early childhood observed as a rash on the skin. The prevalence of diaper rash varies signifi cantly in different parts of the world, and its incidence is between 15 and 50% (3,4,5). Besides, in terms of the diaper rash rate, it is observed that smooth severity while moderate is 4% and seriously monitored as 2% (6). There are several envi- ronmental factors causing diaper rashes such as wetness, pH, fecal enzymes, diaper type, diaper replacement frequency, diet, and oral antibiotic use. The skin becomes more sensi- tive because of environmental factors and contribution of mechanical factors, and the barrier function of the SC may deteriorate and foreign substances may easily penetrate the epidermal layers. According to the European Union “Test Guideline for the Safety Assessment of Cosmetic Ingredients,” whereas the adult margin of safety (MoS) value must be more than 100, the MoS values of newborn babies must be more than 230 for each ingredient (7). BABY BAR RIER CREAM (BBC) FORMULATIONS AND NATURAL INGREDIENTS The BBCs protect the lipid layer on the skin in babies against irritating factors and harm- ful microorganisms. Some topical BBC formulations contain natural ingredients such as olive oil, beeswax, zinc oxide, and lanolin, and they possess many preventive and regen- erative benefi cial properties in the rash (8). Prophylactic and therapeutic non-pharmaco- logical methods for the prevention of rash are defi ned by the ABCDE code: air, barrier cream, cleaning, diaper, and education (3,8). ZINC OXI DE Zinc oxi de is widely used in BBCs to prevent diaper rash it reduces the contact of baby skin with irritant factors such as urine and feces, repairs the SC, and prevents exposure to wetness. Thus, it contributes to the preservation of the skin’s natural structure, creates a lipid barrier, and prevents the infection problem (9,10). Topically applied zinc oxide is typically inert and biocompatible for the skin. Zinc oxide does not irritate normal skin between pH 6–7 when combining with other natural ingredients in baby rash barrier cream formulations. Systemic exposure has always been an important issue as a result of topical use of zinc oxide. Zinc oxide particles are nontoxic and do not penetrate normal or damaged human skin, and there is no poison risk to human health (11). Besides, some studies show that natural approaches such as beeswax and olive oil are also effective in preventing diaper rash (12). BEESWAX Beeswax is a natural raw material that has hydrophobic properties against wetness and maintains the moisture of the skin thanks to its good covering properties. Depending on

NEXT-GENERATION NATURAL BABY BARRIER CREAM FORMULATIONS 175 these properties, it is a suitable candidate raw material that can be used to develop a w/o-type emulsion formulation. In terms of its physical properties, it is solid at room temperature (melting point about 64°C) and does not dissolve in water. Its composition consists of saturated fatty acid esters and long-chain alcohols (13,14). The conventional use of beeswax in cosmetic formulations is as a thickener and moisturizer. It is also cost- effective and available as a natural raw material all over the world (13,15). In recent years, natural, renewable, and biocompatible products are preferred for topical formula- tions, and beeswax is one of the most important candidates of these trend materials (16,17,18). OLIVE OIL Olive oil is a highly appreciated and promising vegetable oil because of its high soaping fraction in terms of pharmaceutical and cosmetic industry. Its composition includes oleic (65–80%), palmitic (7–20%), linoleic (4%), stearic (2–4%), and myristic acids (1%), and sometimes lauric and arachidonic acids (19). Also, vitamins and the presence of high concentrations of squalene, phytosterols, tocopherols, and linolenic acids give emollient and protective properties to olive oil for the epidermis. Furthermore, antioxidant proper- ties make it an important raw material for cosmetic formulations (20). BABY BARR IER CREAM FORMULATION PROPERTIES AND INGREDIENT SAFETY ASSESSMENT HYDROPHIL IC–LIPOPHILIC BALANCE (HLB) Emulsions consist of three components, which are the dispersing phase, dispersing me- dium, and emulsifi er. The emulsifi er is known as “surface-active agents” that carry both hydrophilic and hydrophobic groups on a long chain. The HLB system provides numeri- cal determination of the hydrophilic and hydrophobic character of surfactants to select a suitable emulsifi er mixture following the type of emulsion to be prepared (21). Surfac- tants result in emulsion formation by abruptly decreasing the surface tension between the two liquids forming the phases of the emulsion. It is possible to calculate the HLB value experimentally as described by William Griffi n in 1949 or theoretically using correct equations (22,23). PARTICLE/ DROPLET SIZE AND ZETA POTENTIAL OF FORMULATIONS There are electrostatic pushing forces and Van der Waals pulling forces that affect the stability of colloidal systems. Electrostatic propulsion forces are directly related to zeta potential and are used to assess the physical stability of the colloidal system. It is gener- ally desired to have ±30–50 mV in emulsions. Zeta potential measurement is based on measuring the velocity of particles applied to the electric fi eld using the Doppler princi- ple. If all the particles have high negative or positive zeta potential, they repel each other, and the dispersion is durable. The high zeta potential of the emulsions resulted in