The bioactivity of a product can be quantitatively measured and analyzed by assessing its ability to protect, retain normal moisture and delay the aging process of skin. O/W emulsions are commonly used cosmetic delivery systems that supply moisture to skin and improve its condition by forming an occlusive barrier on the skin surface.1 In recent years, scientists have been looking at utilizing natural resources in cosmetic products, as natural vegetal oils are readily available at affordable costs and have excellent cosmetic and skin care application properties such as soothing, moisturizing and skin penetrating. Vegetal oils such as soybean oil, corn oil, safflower oil and linseed oil are used for cosmeceutical purposes as w/o emulsions using single or mixed surfactants.2–4
Prunus dulcis (almond) oil, Carthamus tinctorius (safflower) oil and Passiflora aincarnata (palm) oil share oleic acid as a major fatty acid and are rich in tocopherols and tocotrienols. The added benefits of these oils are moisturizing, emollient and soothing properties that make them an alternative for a topical skin treatment. A literature survey revealed that few oils are explored in the form of o/w cosmetic formulation. Reports on rheological properties are scarce, and hence this study was conducted.
Cosmetic emulsions with phosphate-based surfactants derived from the long-chain Guerbet alcohols and a polymeric thickener were characterized by rheological measurements, particle size distribution and stability test.5, 6 Emulsifying agents play a vital role in emulsion stabilization, either by reducing the interfacial tension of the system and/or by forming an interfacial film with electrostatic properties around the dispersed globules. The proper concentration of surfactants and their mixtures give synergistic effect with respect to the stability of the emulsion by the proper matching of hydrophilic-lipophilic balance (HLB) numbers of dispersed phase and surfactant used.7
When an o/w cream is rubbed over the skin surface, the greater part of water evaporates during inunction, altering the phase volume ratio of water to an oil phase with incorporated bioactives. This becomes so low that an o/w emulsion turns into a w/o emulsion type. A certain amount of water is then retained in the oil phase on the skin. When water is applied to remove the cream base from the skin surface, it reverts to an o/w state from a w/o state and readily becomes washable.
In the present study, the surfactant blend of sorbitan monooleatea and polysorbate 80b was used having HLB numbers of 4.5 and 15 respectively. They are nonionic surfactants, belonging to a useful group of fatty acid esters with the added benefit of being extremely resistant to freezing and capable of forming strong hydrogen bonds with water. Creams prepared using nonionic surfactants do not form a surface crust on the skin resulting in minimal shrinkage due to water evaporation. They act as softeners, thickeners and opacifiers.8, 9 Natural polymers are used as additives for the enhancement of a cream’s stability to increase the viscosity, impart a good efficiency, and provide a luminous appearance ease of application and excellent texture to the product. Such polymers are critically important to the creation of modern cosmetics since they trigger the release of oil phase when applied to the skin surface.10, 11
Materials and Methods
Three creams were prepared with either almond oilc, safflower oild or palm oile. They were combined with double-distilled water to create a cream preparation. In addition, a selection of the following ingredients was added to the formulation: sorbitan monooleate, polysorbate 80, carboxy methyl cellulose (CMC)f, xantham gum (XG)g and guar gum (GG)h.
Cream preparation: The continuous phase was prepared by dissolving the natural polymer carboxy methyl cellulose/xanthan gum/guar gum in double-distilled water, and a dispersed phase was obtained by dissolving a calculated quantity of sorbitan monooleate and polysorbate 80 as an emulsifier blend in oil phase. A series of emulsifier blends (see Table 1) with varying HLB numbers were found to be completely miscible in oil phase at 25 ± 0.1°C. The dispersed phase was added gradually to the continuous phase at 2500 rpm, stirring to get the o/w emulsion-based cream, and stirred for 30 min.
Standardization of cream parameters: The effect of individual parameters such as oil content, surfactant content, HLB number of surfactant blend and additive content on emulsion stability were studied individually by trial and error. A series of surfactant blends of different HLB numbers were prepared by the proper mixing of individual emulsifiers.12
Initially, the surfactant concentration was kept at 0.5% w/w and oil content was varied from 5% to 20% w/w. As these emulsions were found physically unstable, the procedure was repeated by increasing emulsifier concentration from 0.5 to 5% w/w. Preference was given to the minimum emulsifier concentration that resulted in a stable emulsion with relatively fair oil content. The minimum required concentration of natural polymers (rheological additives) that give the desired stability was determined by varying it from 0.1% to 2.5% w/v. The pH was measured using a digital pH meteri.
Storage Stability and Rheology
Rheological properties of the final emulsion intended to be used as a cream or a lotion decides the physical stability, skin feel, ease of application on the skin and its overall performance. The viscosity of cosmetic product is typically controlled by changing the amount of polymeric thickener. As the factors affecting cream stability simultaneously affect rheology, the storage stability of creams was evaluated in terms of consistent rheological profile.13–15 Variation in cosmetic cream viscosity at 27°C and at accelerated storage stability as per International Conference on Harmonization (ICH) guidelines at 40°C ± 2°C/75%RH ± 5%RH condition is studied at respective temperature on a viscometerj operated by softwarek equipped with a cooling systemm. Each experiment was performed three times and average ± S.D. values were plotted. Rheological parameters such as shear stress (τ), consistency index (K) and flow index (n) were calculated by applying the Power law equations shown in Eq. 1 and Eq. 2.
τ = Krn Eq. 1
τ (dyne/cm2) = η (cp) × r (sec-1) Eq. 2
In the above equations, τ = shear stress (dyne/cm2), r = shear rate (sec-1), K = consistency index and n = flow index. The consistency index (K) is the indication of the viscous nature of the product. Statistically, it is the Y-axis intercept value of the graph of log10 (shear stress) verses log10 (shear rate).The flow index (n) is the measure of deviation of the system from Newtonian flow (n = 1). When n < 1 the flow is pseudoplastic and when n > 1 flow is dilatant. Thixotropy was calculated by the hysteresis loop area between the up and down curves of the rheogram.
Primary Skin Irritation Tests
A Draize Repeated Insult Patch Test (DRIPT) was performed for formulated creams on New Zealand rabbits of either sex weighing 2.5–3.0 kg. For seven days, 0.5 g of cream was applied daily on a 4cm2 area of hair-free skin of rabbits. The skin was observed for any visual change such as redness (erythema) and swelling (edema) after every 24 hr for seven days. Evaluations were done by using the DRIPT scale. In this technique the Primary Skin Irritation Index is the composite score obtained by adding individual scores of erythema and edema that can range from 0 to 8 representing the degree of skin irritation.16, 17
The short-term study of the skin hydrating effect of formulated creams was recorded using a corneometern against an aqueous marketed cream. At the start, the base line values were taken on a 2 cm × 2 cm area on the forearms of five female volunteers between the ages of 24–35 with normal skin. The test area was then treated with 0.1 g of formulated cream and control. Humidity conditions were set at 24.5°C ± 0.1 and 47.5°C ± 0.1. The corneometer is based on the physical principle of a common capacitor in which the electron-rich one plate and an electron-deficient another plate are developed. The capacity is given by the quantity of the stored electric change, and it is directly related to the quantity of water in the skin.18, 19 Variation in skin hydration was detected as “skin water content” and calculated. Skin hydration readings were recorded at every 30 min interval for 3 hr.
Results and Discussion
Standardization of cream parameters: The optimized parameters of the cosmetic creams showing six months storage stability at 40°C ± 2°C/75%RH ± 5%RH are depicted in Table 2.
At lower oil content, instability is observed that may be due to the temporary saturation of emulsifier at the oil-water interface resulting in creaming and at high oil content is due to the surfactant deficiency unable to form a tight monolayer around the dispersed oil globules. A clear oily layer separation at the top is observed in the case of surfactant-deficient creams while those with excess surfactant showed creaming. All stable creams obtained had similar HLB numbers at surfactant blend and oil phase. Thus, preparation of surfactant blend with an identical HLB number to that of one at the oil phase is found to be one of the most important criteria for obtaining stable creams. The use of natural polymers as additives has enhanced texture, viscosity of external phase, rheological behavior and storage stability of creams against various phenomena such as coalescence, flocculation and layer separation. This significantly prevented the inter-droplet successive collisions that result in destabilization.
Viscosity and Rheological Properties of Cosmetic Creams
The shear-thinning behavior of the three creams was evaluated by comparing the increase and decrease of shear stress (D/cm2) and shear rate (S-1) initially at 27°C and 40°C and after six months at 27°C and 40°C. All formulated creams have shown shear-thinning behavior that is typical for o/w cosmetic products. A rapid decrease in viscosity at the initial shear rates is obtained, which slowly decreased at higher shear rates. The accelerated storage stability condition did not create any significant change in the flow nature, viscosity and consistency of the developed creams, ultimately proving the excellent storage stability. The rheogram of the formulated cream was found to be identical to that of the marketed product.
Another interesting property shown by the developed creams is thixotropy, which plays a decisive role in the stability, application and skin feel of the final product. The rate of shear was progressively increased and the resultant shear stress was plotted against the applied shear rate. Once the desired maximum shear rate had been achieved, the applied shear rate was decreased. The downcurve was found to be displaced from the upcurve. In the case of all formulated creams, the downcurve was shifted below the upcurve, indicating the cream has a lower consistency at any one of the shear rates on the downcurve than it had on the upcurve. Thixotropic creams contain the asymmetric particles with multiple points of contact creating a loose three-dimensional network throughout the sample. In the absence of shear rate, this structure confers some degree of rigidity. As the shear rate is applied and flow started, the structure begins to break down as the points of contact are disrupted and particles of three-dimensional networks align. This gel-to-sol transformation is responsible for shear-thinning behavior of the cream. Upon removal of shear rate, the cream started the noninstantaneous process of structure reforming and restoration of consistency. This structural restoration is achieved by creating contacts between the particles undergoing random Brownian movement. All creams have shown a degree of structural breakdown, i.e., shear-thinning that did not reform immediately with removal or reduction of shear rate. The creams with a thixotropic loop can be easily applied on the skin surfaces with a permanent and finite structural breakdown that prevents the re-accumulation of its constituents once it is spread.
Table 3 shows the rheological parameters of formulated creams. Values of flow index (n) were consistently below unity, indicating a pseudoplastic flow behavior. The consistency index (K) decreased with increased temperature. The accelerated storage stability condition did not create any significant change in stability and rheological behavior of the creams. No consistent trend of the thixotropy as affected by temperature and storage condition was observed. From the rheological data, the formulated creams were found to have excellent storage stability, optimal viscosity for superior spreadability, pseudoplastic flow and good consistency.
The aesthetic attributes of cosmetic creams such as skin feel and ease of distribution on skin surface are governed by its spreadability. The spreadability of 1.0 mL of each individual vegetal oil and its corresponding formulated cream was measured at 26°C and 75% ± 5% RH using a method described.20 Each measurement was performed in three replications and average values were plotted. It was observed that the spreadability of formulated creams was significantly increased to 8.75, 8.29 and 9.21 as compared to that with their constituent vegetal oils 1.25, 1.09, 1.37 respectively. The phenomenon such as presence of excess aqueous phase, optimized emulsion stability and reduction in interfacial tension by the emulsifier has contributed to produce an approximately eightfold increase in spreadability.
Primary Skin Irritation Test
The application of cosmetic cream on intact rabbit skin showed no irritation after first 24 hr as well as after seven days (Primary Skin Irritation Index = 0) on skin. In no case was erythema or edema observed. The Primary Skin Irritation Study revealed better skin acceptability of developed cosmetic o/w creams.
Figure 1 graphically shows the hydration efficacy study for the measurement of skin water content. Each point corresponded to the average numerical value of the three different measurement points on the forearm. The single application of vegetal oil-based creams had shown higher skin water content at the end of three hours despite having lower skin water content before application of the test cream. This demonstrates the penetration of vegetal oil through the stratum corneum.
It was also observed that after the first 30 minutes the skin water content of formulated creams decreased from 44.5, 47.1 and 45.2 RCU to 39.7, 40.5 and 42.7 RCU, respectively, for almond, palm and safflower oil-based creams, which thereafter constantly increased. This may be due to the vegetal oil that acted as a barrier by developing a thin film on the skin. Such vegetal oil film significantly reduced the capacity of skin that is directly related to the skin water content. Over a period of time, vegetal oil slowly penetrated the skin surface and allowed the efficient skin hydration. The vegetal oil-based creams have shown potential as effective moisturizers. On the other hand, the marketed control used showed a short-term relief to the dry skin by increasing the skin water content to 59.9 RCU immediately after its application, which thereafter dwindled to 55.4 and 50.1 RCU after 60 and 90 min, respectively, out of which later the value was closer to the baseline values. This may be due to the absence of a lipidic component capable of developing a barrier preventing loss of moisture from the skin.
Market Viability of Vegetal Oil-based Creams
In order to check the commercial applicability of these formulated products, the authors have compared the stability, viscosity and rheological parameters, spreadability and moisturizing property with the popular marketed cream in the Indian market. The rheological parameters and storage stability of each formulated cream were found to be identical with the marketed product. The spreadability and moisturizing properties of the formulated products were found superior than the marketed product, which is mainly due to the excellent fluidity and presence of excess aqueous phase. These creams possess the added benefits of being nonsticky, easily removed and nongreasy, and the utilization of natural substances and minimum processing costs. The formulation technique derived can be applicable for a variety of other oils.
Kedar R. Kumthekar would like to thank Pralhad Balkrishna Hirve of the government of Maharashtra, India, for his support during moisturizing tests and constructive discussions and the University Grants Commission, New Delhi, for its award of the Green Technology Meritorious Fellowship.
Send e-mail to [email protected].
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