Nemidon is a polysaccharide that is soluble in boiling water, but insoluble in cold water. The physical strength of the gel depends on molecular weight, and for the purpose of forming gels for skin application, this has been reduced to make a smooth readily applicable film. In manufacturing this product, the polysaccharide is dissolved in hot water, the ingredients added, then the solution is allowed to cool. Rather than form a precipitate and settle out, the polysaccharide forms a gel, the strands of the polysaccharide thus encompassing and trapping large amounts of water or added ingredients in enclosed cavities.
The surface of the skin, at a microscopic level, is extremely rough, with continual peaks and valleys. When Nemidon is applied as a film to the skin, as it dries it contracts and lifts itself off the skin. This property is quite unique to Nemidon amongst skin applications. How do we know it does this?
Suppose we shine light on the skin and measure the amount that is reflected. The fraction that is reflected is given theoretically by the reflection coefficient R given by where n21 is the refractive index of the medium 2 divided by the refractive index of medium 1. Clearly, if n21 = 1, then R = 0 and the object is either black or transparent. Strictly speaking, this relationship only applies to smooth surfaces to which the light strikes perpendicularly. The roughness of the skin ensures that that never applies exactly, for some light is always striking at angles well away from the perpendicular, nevertheless the principle applies with correction.
If we measure the reflectivity of skin, which has a refractive index of approximately 1.55, and smoothly apply olive oil of refractive index approximately 1.44 then measure the reflectivity, this reflectivity is reduced to approximately 7%. This is a little higher than the above equation would suggest, because the skin is not smooth. If we measure the reflectivity of wet skin, with the refractive index of water =1.33, the reflectivity is reduced to approximately 66% of dry skin. If, however, we measure the reflectivity of skin with a dry Nemidon film, with a refractive index higher than that of water, approximately 93% of the light is reflected. The only reasonable physical explanation for this is that the Nemidon is largely out of physical contact with the skin.
There is an immediate consequence of this for cosmetic purposes, such as for skin moisturizing. The amount of reflected light with Nemidon is very close to that without it, and the eye cannot detect such small differences on a non-uniform background such as skin. Accordingly, the Nemidon film, once dried, is effectively invisible, as the skin looks the same with or without it.
Delivery of Actives
Suppose a cream with an active ingredient is applied to the skin. Either the ingredient is soluble in the cream or it is not. If it is, at equilibrium the concentration of ingredient (c2 ) in the skin versus that left in the cream (c1) is given by the distribution coefficient
K = c2/c1
To be soluble there must be sufficient free energy of dissolution to overcome the molecular interactions in the original phase. If so, for K>1, there must be greater free energy of dissolution of the material in the skin. If they are the same, unless the active is highly mobile in the skin, thus increasing the volume, half the active remains in the cream. Quite often, the free energy of dissolution in dried protein is quite disadvantageous. If the ingredient is not soluble in the cream, it does not move through the cream, and again, most of the active is unavailable.
There is also the question of rate of delivery. The rate at which a mass m crosses into the skin is given by where D is the diffusion coefficient and the flow is in the +x direction. The term D is dependent on the temperature and of medium. Specifically, it is inversely proportional to viscosity, and most creams are of very high viscosity, to stop them flowing. If the active is not soluble in the medium, usually water, then the material must be dispersed as microparticulates. In this case, D is also inversely proportional to the radial size of the microparticle. Accordingly, the poorer the dispersion, the poorer the delivery.
The application of actives from Nemidon operates by a different principle, because the actives are never dissolved in the Nemidon, but rather are dispersed in water in cavities within the gel, and the actives are delivered through water flow rather than by diffusion. The delivery of the actives arises as the gel dries, and thus squeezes out the actives.
If we dry such a gel free of ingredients on a plastic surface under controlled condition, it will form an unbroken film. Xray diffraction of such a film is best explained in terms of the saccharide units linking to form strands that are double helices, linked together by hydrogen bonds to linking water molecules. These linking water molecules form four hydrogen bonds per molecule, and are very tightly bound. The film also contains variable amounts of further water molecules that are more loosely held to the strands by hydrogen bonds. The number of these molecules is a function of the partial pressure, hence the film acts as a partial buffer for water vapour pressure.
As the film dries, perforce it contracts. It will adhere to the peaks of the skin, but the contraction causes the skin to lift from the valleys of the skin. The equilibrium concentration of polysaccharide will be approximately 10% depending on the surrounding temperature and humidity. Nevertheless, the film still contains approximately 90% of the enclosed fluid. Because this fluid is strongly hydrogen-bound to the polymer, however, this fluid is selectively water. No other ingredient can compete with water as a hydrogen-binding agent. Thus at this point the active ingredients are chemically separated from the water that remains in the film. These ingredients will have been transferred to the cavities, which in turn have been progressively reduced in volume by the gel.
When the gel has dried, the volume of the gel has been reduced by about eight times, and by doing so, it squeezes out the liquids from the enclosed cavities. If a sample of stronger gel (so that it will not flow into a filter cloth) is placed in a pressure vessel and squeezed by a piston, water flows continuously out. Initially little pressure is required, but to reduce the volume by a factor of six over two hours, towards the end a pressure of about 80 pounds per square inch is required. This external pressure must be matched by equal and opposite pressures from the gel, and is at least indicative of the forces generated during the volume reduction.
The relevance of this is that these forces are applied to the solutions of actives placed in the cavities. As the volumes of the cavities are reduced, these forces squeeze the actives out from the gel. Thus instead of having to overcome the distribution coefficient and then wait for diffusion to get benefits from the active ingredients, the drying of the Nemidon gel causes significant internal pressures that are available to force the actives into the skin. Further, because the actives are not dissolved in the gel, all the actives are available, apart from the very few that will be inevitably trapped by a mechanical obstruction of polymer strands.
When the film is "dry", it still contains approximately ten times its weight of water. The exact amount depends on the vapour pressure of water on each side of it.
Moisture is required by protein in the skin to plasticise it. The skin thus exerts a partial pressure of water vapour, and when the external vapour pressure of water (i.e. the humidity of the air) goes below that vapour pressure, the protein passes water into the air, and the skin dries out. Many moisturisers work by preventing that occurring by simply blocking the egress of the water by placing a film of oily material over the skin. That works, to a point, however it also prevents the egress of fluids such as sweat from pores, and the oil also prevents the external layer of skin from carrying out any aerating function.
Nemidon, by forming a barrier above the skin, leaves a layer of air above the skin that has a vapour pressure controlled by the Nemidon. If the skin was initially dry, the Nemidon provides initial water, and as the Nemidon dries, it provides it at a pressure that assists skin uptake. It then prevents skin dehydration by maintaining an appropriate vapour pressure above the skin. Finally, if the skin provides too much fluid, the vapour will be taken up by the Nemidon, and transferred to the atmosphere.