Natural route to emulsifier-free formulations

Figure 1 shows the location of the oleosomes within a typical oil bearing seed, in this case, safflower. Spherical in shape and one to three microns in diameter, the oleosome consists of an inner reservoir of triglycerides, surrounded by a phospholipid layer, which in turn is encapsulated by unique proteins called oleosins. The oleosin protein (molecular weight approximately 25,000 Mw), is unusual in that it is made of both hydrophilic and hydrophobic portions. This amphophilic nature and its spherical orientation around the oleosome gives the oleosin coating extraordinary emulsification properties. Safflower oleosomes exhibit an operational HLB range from 5 to 15, with the optimum range being from HLB 8 to 13. Given that the oleosin proteins and the phospholipids make up <1% of the total weight of the oleosome, the results shown in Table 1 are truly remarkable. The wide operational HLB range of the oleosin proteins and the extraordinary efficiency of these materials allows the formulator of skin care products to make cosmetically elegant formulations with total emulsifier levels of <0.075%, as is shown in Formulation 1. Additionally, because of their emulsifying power and the fact that the oleosomes are already available in the form of an emulsion, the formulations can be made with no heat. Quite elegant, all natural, formulations have been prepared via the cold process route in which, again, the total emulsifier level is 0.075%. Upon rubbing, the oleosomes will break and release their contents to the skin. The release of additional oil (which acts as a dispersing agent) allows for the preparation of natural sunscreens of quite high efficacy.

Emulsifiers and skin irritation

Emulsifiers are typically low molecular weight molecules, having both hydrophilic and lipophilic character. This dual character enables them to stabilise the oil droplets in an O/W emulsion. Unfortunately, the same characteristics that are so helpful to the formulator of skin care products have a negative impact on the skin itself. Emulsification of sebum and disruption of the lipid bilayer from low Mw emulsifiers has been reported in the literature. Barany et al1 have found that low Mw non-ionic emulsifiers having HLB values in the range typically used in skin care, emulsions i.e. from HLB 5 to 15, resulted in increased TEWL values when applied to normal skin. DeBoer et al2, using Doppler flowmetry, also observed skin irritation due to emulsifiers. As we have seen in Formulations 1-3, the typical emulsifier loading in an oleosome-based product is <0.075% . The typical emulsifier loading in non oleosome based formulations is typically between 2% and 5%. A comparison of Formulation 4 (standard hot process nonionic emulsifier based) and Formulation 5 (oleosome based) highlights the fundamental differences between the traditional formulation approach and the potential opportunities posed by the oleosome technology. In Formulation 5, the safflower oil, which is contained within the oil body, functions (upon release) as both an emollient and as a barrier agent. In this instance, the safflower oil replaces the paraffin oil, myristyl propionate and the medium chain triglycerides in the traditional Formulation 4. Again, in Formulation 5, the oleosin proteins and phospholipids (which amount to a total of 0.075% of the formulation) replace the nonionic emulsifiers PEG 40 stearate, PEG 100 stearate/glyceryl stearate, and sorbitan monostearate which are present at a total concentration of 4.5% in Formulation 4. The fact that cosmetically elegant emulsions can be prepared containing the miniscule levels of the oleosin emulsifier seen in Formulations 2 and 5, suggests that application of oleosome technology is a way to minimise the disruption of the lipid bilayer and sebum dispersion which has been reported in the literature. Figure 3, in which the TEWL of skin sites treated with a 2% preserved mixture of nonionic emulsifiers (steareth-2 and steareth-21) is compared to that of skin sites treated with various levels (typical use level of 0.075% and 10X use level of 0.75%) of preserved oleosin proteins, clearly shows the potential of using nonlipid disruptive emulsifiers on the skin. The oleosome protein emulsifier was obtained by freezing the oil-in-water oleosome raw material. Freezing breaks the natural emulsion into a safflower oil (upper layer) and a water (lower layer) mixture with the protein emulsifier residing between the oil and water interface. Physical separation by decanting isolates the pure protein emulsifier. Dry emulsifier (containing oleosin proteins and phospholipids) was then obtained by vacuum freeze. HPLC was used to confirm that the isolated protein emulsifier was composed of the the same proteins as the starting oleosomes. All emulsifier TEWL measurements vs. baseline were conducted in a preserved, pH 7 phosphate buffer aqueous solution. The purple bar represents the contribution of the preserved, buffered base to the TEWL readings. The 2% nonionic response was significantly more irritating at 95% confidence level than the other samples. The oleosome protein emulsifier samples were identical to the preserved, buffered base and not significantly different from the baseline control with no treatment.