Micromixers to produce cosmetic emulsions.

In the cosmetics industry, emulsions play a key-role in active solubilization and texture/efficacy optimization. However, depending on their physico-chemical properties, the active ingredients are often more stable in a single phase: for example, Vitamin A is more stable in an oily phase than in a water phase. We have developed a special mixing device which produces an emulsion in the body of the pump, immediately before application on the skin. The mixing unit consists of two silicon chips. Each chip has several Y-shaped microchannels and intersections etched on its upper surface. When the two etched surfaces are bonded together, they produce series of interconnecting micromixing elements which permit the repeat mixing of the two phases, thus producing an extremely homogenous emulsion. These micromixers require carefully designed formulae in which the physico-chemical properties of each raw material are essential to obtain a spontaneous emulsion. This device has been incorporated into a spray pack and optimized to deliver the spontaneous emulsion when finger pressure is applied.

Percutaneous absorption of sunscreens from liquid crystalline phases.

The purpose of the present study was to investigate the effects of two nonionic surfactants with liquid crystalline structures on the cutaneous availability of two sunscreens. Three liquid crystalline structures were investigated: lamellar, hexagonal and cubic. The diffusion of sunscreens within the liquid crystals was determined by measuring transport kinetics into an unloaded surfactant medium from a similar system loaded with the sunscreens. The diffusion coefficients were the greatest in the cubic systems for benzophenone-4 (a hydrosoluble sunscreen) and in lamellar systems for octyl methoxycinnamate (a liposoluble sunscreen). So the diffusion in this surfactant systems was strongly dependent on the structure of the liquid crystal and on the physicochemical properties of the solute. The transcutaneous fluxes were determined using a Franz-type diffusion cell. The liquid crystalline vehicles modified the transcutaneous fluxes of benzophenone-4 but did not change those of octyl methoxycinnamate. The solute diffusion within the vehicle was not the rate-determining step for transcutaneous permeation for either sunscreen. The diffusion of benzopenone-4 within the stratum corneum and that of octyl methoxycinnamate within the dermis could be the rate-determining steps for their transcutaneous permeation. These two steps could be affected differently by nonionic surfactant vehicles.

Pulsed photoacoustic study of the diffusion of chromophores in human skin.

The technique of pulsed photoacoustic spectroscopy was used to investigate the diffusion of chromophores in human skin. The kinetic of diffusion has been studied for five solutions at different concentrations in a mixture of chromophores, as used in commercial sunscreens. In addition to the classical macroscopic interpretation of the diffusion process, a new method is shown to give more detailed information on chromophore presence at different depths in skin. For the first time, results are expressed in the frequency domain by means of the Fourier transform applied to the photoacoustic signal. The spectra are discussed versus the depth in skin samples and the time of diffusion kinetics. This new method of data analysis is shown to be very useful for understanding the influence of the internal structure of a medium on the penetration rate of chromophores into skin.

Comparative effects of cholesterol and cholesterol sulfate on hydration and ordering of dimyristoylphosphatidylcholine membranes.

The comparative effect of cholesterol (CH) versus cholesterol sulfate (CS) on dimyristoylphosphatidylcholine (DMPC) membranes has been investigated by optical microscopy, freeze-fracture electron microscopy, x-ray diffraction, and solid state 2H and 31P nuclear magnetic resonance (NMR). The sulfate analogue extends the lamellar phase domain toward high water contents, and substitution of 30 mol % CH by CS in DMPC lamellae induces the trapping of 30 wt % additional water. The greater swelling of the CS-containing systems is evidenced by determination of lamellar repeat distances at maximal hydration: 147 +/- 4 A and 64 +/- 2 A in the presence of CS and CH, respectively. 2H-NMR of heavy water demonstrates that CS binds approximately 12 more water molecules at the interface than CH whereas NMR of deuterium-labeled DMPC chains reveals that 30 mol % CS orders the membrane as 15 mol % CH at high temperature and disorders much more than CH at low temperatures. The various effects of CS versus CH are discussed by taking into account attractive Van der Waals forces and repulsive steric/electrostatic interactions of the negatively charged sulfate group.