Microcapsules are one of the oldest controlled release technologies. They were developed to produce carbonless carbon paper and are composed of a core with the active ingredient surrounded by a shell, analogous to an egg. Microcapsules may have a multilayer construction with multiple cores containing the active. The active ingredients are released either by rupture of the capsule walls or by diffusion/permeation of the contents . Fairhurst and Mitchnick list a range of materials that are typically used in this regard including
adhesives, drugs, colors, fragrances, flavors, agricultural chemicals, solvents and oils. Classic shell materials include gelatin or gum arabic, cellulosic polymers, or synthetic polymers . Starch based capsules are often used to deliver fragrance and cosmetic ingredients.
Beads and microspheres are small solid particles onto which other ingredients can be adsorbed for later delivery. Nylon particles, for example, are useful for delivery of certain active ingredients. Antiperspirant salts are said to be more efficacious when delivered via nylon spheres, and the esthetics of the product are said to be improved. Coloring agents may be delivered in this manner as well; Schlossman discloses a patented method (U.S. patent 5,314,683) of coupling cosmetic pigments to microspheres to provide uniform reflectivity, improved dispersion, and superior viscosity characteristics . Tokubo et al. describe a process for preparing spherical hectorite particles, with a diameter of about 100 A, which can be used to deliver glycerin and solid pigments such as titanium dioxide, zinc oxide, and ferric oxide.
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