The backing film is one of the three layers of a matrix patch. It is the layer that is apparent after the adhesion of the patch on the specific site of the skin. Its main role is to protect the adhesive layer from the influence of external factors; it also provides such characteristics as flexibility, occlusivity, breathability, and printability. Several materials have been used as backing films. The selection of a specific film for use in a cosmetic patch may depend on the following factors:
Cost Stability Printability Machinability
Glossy or matte appearance Compatibility Anchorage to the adhesive Transparency Opacity Occlusivity Breathability
Several materials can be used for these purposes depending on the needs already presented. One of the first and cheapest cosmetic patches used a simple paper layer. Most of the pore cleansers use nonwoven materials. The reason is obvious: all these systems require wetting the nose before application of the patch. It means that the system has to dry out in order to be able to remove the sebum plugs that stick to the dried layer.
Polyethylene or polyester films are used also in most systems. They do not need to dry out after the application. Sometimes the film used is nontransparent. A white, foamy material is the backing layer of the pimple patch.
In some cases, other more expensive materials have also been tested, such as polyurethane, chlorinated polyethylenes, nylon, and saran. It is very important that the materials used as backing films for cosmetic patches have the same quality specifications with the similar films used for pharmaceutical patches to avoid any adverse reactions of the skin.
The main role of this layer is to protect the product, especially the adhesive layer, before the use of the product. The pharmaceutical patch development has provided a long list of release liners that can be useful for cosmetic patches as well. There are three main classes of release liners according to their composition:
1. Paper based: Glassine paper, densified Kraft super-calendered paper, clay-coated paper, polyolefine coated paper, etc.
2. Plastic based: Polystyrene, polyester (plain, metallicized), polyethylene (low and high density), cast polypropylene, polyvinyl chloride, etc.
3. Composite material based on the combination of several films
All these materials have a common characteristic: one release layer coated on one or both sides depending on the needs of the product and the system itself. This coating is, generally speaking, silicon or polyfluorocarbon. The grade, thickness, coating, and curing methods vary according to the materials and the satisfaction of specific needs.
As mentioned for backing films, this layer has to be compatible with the components of the adhesive layer and should satisfy the specific needs of the product. Sometimes this layer has to be, e.g., printed, scored, perforated, or tinted. The selection of the material and the grade are dictated from similar factors to the ones influencing the selection of the backing layer.
This is the most important layer of a matrix cosmetic patch. The adhesive layer contains not only the adhesive that makes the patch stick to the skin, but in most of cases the cosmetic active ingredients and the additives required for correct formulation of a cosmetic product. Starting with the adhesive itself, the majority of adhesives used in cosmetic patches are taken from the general category of pressure-sensitive adhesives (PSAs). This is a class of adhesives used in several applications, and in all pharmaceutical patches. As its name shows, PSAs are adhesives which, in their solvent free form, remain permanently tacky and stick to the skin with the application of very slight pressure. There are three groups of PSAs: 1) acrylics, 2) silicones, and 3) rubbers. There are numerous members in the three main families of PSAs, but only few can be used for the formulation of cosmetic patches. The reason is that as also happens with pharmaceutical patches, there are so many restrictions on the selection of an adhesive that the useful members are relatively few. The limitations are governed by the mechanical and biomedical properties of the adhesive, as well as the characteristics of compatibility, reactivity, and stability.
The components of the adhesive are also governed by such properties as, solvents, monomers, cross-linkers, and emulsifiers.
There is also another category of cosmetic patches with similar structure, but formulated with a dry-adhesive system other than PSA. In this class we can bring the example of pore cleansers. Here the adhesive layer is created in situ, by wetting the dry adhesive layer with water the same way we stick a stamp on a letter. The components included in the composition of dry adhesives can be found in the classes of synthetic or natural derivatives, e.g., polyvinyl derivatives, starches, celluloses, and sugars.
Although this material is not a component of cosmetic patches, its importance for the integrity of the product during its shelf life makes us examine it just after the basic patch components. Almost all cosmetic patches as happens with the pharmaceutical ones, are pouched in pouches. For pharmaceutical patches, the rule is to package one patch in one pouch. With the cosmetic analogues, and in an effort to reduce cost, sometimes patches can be found in the same pouch for more than one application. In this case, it is recommended that the product has stability information for the time interval between the opening of the pouch and the use of the last patch, as well as to foresee some kind of resealable pouch. The materials used for the two categories are similar or the same. One of the differences is the number of packaged patches in one pouch. The protection of the product is the main mission of this packaging material, the role of which is critical for long-term stability of the product.
The pouching material, as has been mentioned, influences a lot of the stability of some sensitive molecules. Sometimes the phenomena of adsorption are noticed because of the affinity of some ingredients with the internal, sealable layer of the pouching laminate. In this case, e.g., AHAs can escape from the adhesive layer and, passing the edge, can be absorbed from the ionomers plastic film of the pouching material. Another protection the pouching material provides is protection from UV radiation by using at least one opaque layer in case of light-sensitive materials, along with protection from oxygen.
The production of cosmetic patches depends on the type of patch, the component characteristics, and the overall configuration of the final product. Because most cosmetic patches are matrix patches, it is useful to follow the general steps of typical production concerning this type of patch. Practically, production starts from the weighing of raw materials and other components, and ends with packaging of the product in the final carton. It is not within the scope of this chapter to go into details in this field, but we can mention the basic steps of the production sequence. Some information is required regarding the critical steps of production, or better the steps that could influence the quality of the product itself. The mixing of cosmetic ingredients and adhesives has to take place under a very slight nitrogen atmosphere (pressure) to avoid oxidation of the ingredients during this phase, but not too high (to avoid inclusion of nitrogen in the mass of the mixture and bubble formation during the drying cycle). Drying is also a critical step because, during this process, the temperature of the coating goes up and the ingredients have to be stable at these conditions. During drying, some of the ingredients are evaporated and/or sublimated. An accurate validated process has to be defined to finally take the patch as it had been designed. The exposure to light has to be limited as well, and the web has to be protected and kept in the predefined conditions before packaging. Of course, all the technology for
production of pharmaceutical patches is applicable, but found outside the scope of this chapter.
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