Scientists at P&G Beauty have discovered breakthrough patent-pending dual stream “ribbon” technology that allows skin to be wrapped in a thick, creamy lather while providing outstanding moisturization-- without any tradeoffs.
While many factors ranging from climate changes to aging can lead to dry skin, the most overlooked cause of dry skin may be the first place you visit every day: the shower! In fact, 86 percent of dermatologists indicate that frequent bathing is a source of dry skin, but in a survey of 1,000 women conducted by P&G Beauty, 74 percent of women didn’t realize that prolonged exposure to water in the shower can be drying to the skin. Skin cleansers are designed to remove oily soils, dirt, sweat and sebum from skin through the use of surfactants. However, surfactants can also cause damages to skin lipid structure and proteins, leading to after-wash tightness, dryness and barrier damage. With this fact in mind, P&G Beauty scientists set out to develop a body wash product that could not only gently clean the skin without causing skin damage but also deliver skin moisturization by depositing sufficient amount of active moisturizers during wash. As a result, the skin barrier function actually improves over time.
While the goal may sound simple, the task was enormously challenging. Skin moisturizers, such as petrolatum, are clinically proven to improve skin lipid bi-layer structure and improve stratum corneum barrier function in the leave-on applications. However, it has been very difficult to achieve high level of moisturizer deposition from a surfactant-based skin cleanser. Indeed, Blank observed that, “Theoretically, it is not feasible to add an emollient to the skin in the same process as that in which soil is removed from the skin” (Blank, I.H., 1969, Action of soaps and detergents on the skin, Practitioner 202: 147-151.). With the recent advance of body wash science, the scientists at P&G Beauty have now discovered a new breakthrough dual-stream “ribbon” technology that allows high lipid deposition on skin during wash without any performance tradeoffs (World patent publication: WO 04026276A1 and WO 04098545A3).
The basic concept of dual-stream approach is to design a bi-continuous product with an aqueous continuous phase containing mild surfactants for cleansing and a separate oil continuous phase containing petrolatum for positive skin moisturization (Attachment I). This new approach offers significant advantages over the conventional emulsion-based 2-in-1 cleansers due to minimized surface area of contact between the surfactant phase and the lipid phase. Therefore, it allows us, for the first time, to be able to break the performance-deposition tradeoffs often seen in the conventional body wash systems caused by strong negative interaction between surfactant and lipid phase.
Attachment I: Bi-continuous Phases in Dual-Stream Ribbons Body Wash Products.
● One phase is water-continuous surfactant phase for mild cleansing.
● A separate phase is oil-continuous lipid phase for depositing active moisturizers on skin for improving skin barrier function over time.
The fundamental science behind the dual-stream formulation is quite fascinating. The aqueous surfactant phase contains highly concentrated multi-lamellar vesicles (>80% in aqueous phase volume) which are self-assemblies of amphiphilic surfactants as revealed by cryo-TEM image (Attachment II). Formation of these surfactant self-assemblies and its phase behavior play a crucial role in the dual-stream product design and its superior performance. First, the densely packed multi-lamellar vesicles provide a unique rheological property important for maintaining the dual-stream product stability due to high zero-shear viscosity. Yet, the colloidal structure is very shear thinning so the product readily flows out of the package at moderate shear stress. Subsequently, the multi-lamellar vesicles quickly transition into micellar phase upon water dilution producing abundant flash foam during product usage in shower. Importantly, the foam carries large amount of lipid particles and the benefit particles deposit onto the skin during washing and ultimately deliver superior skin care benefits.
Attachment II: Cryo-TEM Image of Highly Concentrated Multi-Lamellar Vesicles in Aqueous-Continuous Structured Surfactant Phase.
The general aggregation phenomena of surfactants have attracted considerable interests in both academic and industrial research due to broad implications in biological and colloidal systems. The major forces that govern the self-assembly of amphiphiles into well-defined structures such as micelles and bilayers derive from two competing forces: i) the hydrophobic attraction at the hydrocarbon-water interface, which induces the surfactants to associates, and ii) the hydrophilic, ionic or steric repulsion of the headgroups, which imposes the opposite requirements that they remain in contact with water. The colloidal structure of the aggregates can be expressed in terms of geometrical packing parameters of surfactants (Attachment III) as proposed by Isrealachivili (Intermolecular and Surface Forces, Second Edition, ed. Academic Press, 1992). There are three factors affecting the critical packing parameters (CPP): hydrophobic tail volume (v), critical hydrocarbon chain length (lc) and optimum headgroup area (a0). The preferred critical packing parameters for vesicle formation are in the range of 0.5 to 1
Attachment III: Critical Packing Parameter for affecting self-assembly structure (Israelachvili)
Extensive research has been carried out to thoroughly understand phase behavior of surfactant mixtures. One of the commonly used anionic surfactants, ethoxylated linear alkyl sulfate (e.g., sodium laureth-3 sulfate), is unfavorable for vesicle formation due to large head group area with low CPP (<0.3). One approach to alter phase behavior is to increase hydrophobic tail volume via branching (such as sodium trideceth-3 sulfate with random methyl branching in the hydrophobic tail). Importantly, the lamellar phase region can be significantly widened by incorporation of lamellar phase inducing structurant, such as low HLB non-ionic surfactant (e.g., isosteareth-2 and trideceth-3) due to its favorable CPP with small headgroup size and large hydrophobic tail volume. As shown by the small angel X-ray scattering results (Attachment IV), isosteareth-2 induces tighter packing (i.e., reduced d-spacing) of the lamellar phase structure with increasing level of IS-2 in the surfactant mixture
Attachment IV: 2-D and integrated SAX pattern of lamellar phase structure at increasing level of Isosteareth-2. The data were collected with synchrotron radiation at the Advanced Photon Source at Argonne National Laboratory. As isoteareth-2 level is increased, the d-spacing is reducted from 92A0 to 79A0 indicating tighter packing of the lamellar phase structure.
Plant scale-up of a highly structured surfactant phase represents a significant technical challenge due to the fact that the vesicle formation process and its related size distribution are quite shear sensitive. A number of advanced analytical techniques (pulsed NMR, XRD, cryo-SEM, cryo-TEM, LC/MS, light scattering, rheometry etc.) have been utilized to fully characterize structured surfactant and to ensure consistent phase transformation in plant production scale.
The ultimate skin benefits of dual-stream body wash have been demonstrated using in-vivo clinical testing. In a standard LCAT protocol (n=35), dual-stream body wash delivers significant skin dryness reduction after only one application. The dryness is further reduced over time upon repeated uses. Importantly, dual-stream body wash provides significant improvement in stratum corneum hydration (via Corneometer measure) and skin barrier function (via. Trans-epiderrmal water loss measure).