198 JOURNAL OF COSMETIC SCIENCE TRANSIENT NETWORK COPOLYMERS - SURFACE PROPERTIES AND ASPECTS OF SKIN INTERACTIONS Stephen M. Greenberg, Ph.D., Alex L. Vainshelboim, Ph.D., A!ena Lovy and Michael Esposito Lipo Chemicals, Inc., Paterson, NJ and HYMEDIX International, Dayton, NJ INTROIYUCTION Hydrogels are found in almost every aspect of the human experience. They are used in various indus- trial, medical and scientific applications (latex paints, wound care and cultivation media are exam- ples), in foods (think of the Jello you recently ate), and of course, in cosmetics and personal care prod- ucts. This paper will concern itseft with this latter industry that is, the chemistry, characterization and application of a unique and unusual polymeric materials (produced through the controlled base- catalyzed hydrolysis of polyacrylonitrile) 1, in cos- metic and personal care products, particularly as related to the surface modification of skin. Hydrogels can be prepared from either natural polymers such as gelatin or polysaccarides, or from synthetic polymers such as polyacrylic acid or poly- acrylonitrile derivatives. In either group is possible to modify the polymer to literally design the proper- ties of the final hydrogel. This capability has creat- ed a broad variety of polymers (and resultant hydro- gels) that vary just as broadly in their properties. Polymers that can form hydrogels will have the ability to hold water (from low swelling to super- absorbents), to degrade biologically (from none to complete), and may exist as either of the two basic physiochemical categories (thermosetting or ther- moplastic.) For the purposes of this discussion, thermosetting polymers are defined as being cova- lently crosslinked during synthesis, cannot be reshaped once set, and may form permanent net- work hydrogels. Likewise, thermoplastic polymers are defined as not being covalently crosslinked, and form may transient network hydrogel. Additionally, the subject polymer of this paper pro- duces thermoplastic hydrogels that also display non-Newton rheology (thixotropy.) This combina- tion of unusual and unique properties results in hydrogels beneficial for cosmetic and personal care application. I-U.S. Patent Number 4,943,618 PROPERTIES AND APPLICATIONS Through the process of controlled base-cat- alyzed hydrolysis, polyacrylonitrile is transformed into polymers capable of producing hydrogels dis- tinctly different from other natural and synthetic polymers intended for skin care applications. Original technology allows the creation of medium molecular weight polymeric structures that contain about 20% hydrophilic groups segmented within the primarily hydrophobic character of the chain. It is this arrangement of hydrophilic and hydrophobic segments that gives the hydrogels their unique and desirable qualities that is, this arrangement, in the aqueous environment, allows the independent poly- mer molecules to electrostatically interact and form non-covalent, transient bonds. This unique interac- tion allows the creation of matrices that can deform and reform during energy input without destroying the polymer. The result is hydrogels having thixotropic rheology that form thin, uniform and non-tacky films. In addition, the polymer will orient on the skin due to interactions between the hydrophilic skin surface and the hydrophilic moi- eries in the polymer. The result of this orientation is to present to the environment the hydrophobic portions of the molecule, interpreted as 'silky' to the touch. Figure I schematically represents the essential differences in structure between polymers that pro- duce permanent network hydrogels (left) and those that produce transient network hydrogels (right). Hydrogels serve the primary purposes in skin care formulations to improve viscosity and to aid in the delivery of active ingredients. Traditional (ther- toosetting/permanent network) hydrogels suffer from a number of drawbacks in these respects i.e., tackiness, non-uniformity of film thickness, are irreversibly shear-thinned, are sensitive to elec- trolytes and pH extremes, and make little, if any, positive contribution to after-feel. On the other hand, the novel (thermoplastic/transient network)

PREPRINTS OF THE 1998 ANNUAL SCIENTIFIC SEMINAR 199 TRANSIENT NETWORK COPOLYMERS- SURFACE PROPERTIES AND ASPECTS OF SKIN INTERACTIONS Cont figure 1 hydrogels described in this paper are non-tacky, leave uniformly thin films on the skin. are reversibly shear-thinned (thixotropic), tolerate elec- trolytes and pH extremes very well, and confer a 'silky', low friction feel. The ability of the transient network hydrogels to deform and reform accounts for several of their unique and desirable properties. Lack of tackmess in these systems is a result of the structural flecibil- ity (non-covalent bonding) of the hydrogel's matrix, as is the ease of spreading. The thixotropic character of the hydrogel matrix, likewise, enhances uniformity of film thickness and the resultant distribution of active ingredients. The topic polymers of this paper orient so that the hydrophilic portions am in contact with the skin and the hydrophobic portions form a protective layer above. This not only accounts for the pleas- ant after-feel, but also produces a protective hamer that reduces TEWL. The segmented hydrophilic- hydmphobic segmentation of these polymers also gives them the ability to cross the oil-water inter- face and assist in emulsion stabilization. and non-tacky properties, cast uni- form, continuotis films and assist in the stabilization of emulsions typical of skin-care formulations. The pre- sentation of hydrophobic moleties to the environment confers a smooth feel to the skin and provides a protec- tive barrier that reduces TEWL. Unlike hydrogels derived from covalently crosslinked polymers, the subject hydrogels are unusually resistant to electrolyte level and pH extremes. All of these properties. as well as numerous oth- ers. are very. beneficial to meet the emerging demands of the skin-care market. CONCLUSIONS Transient network hydrogels formed from non-covalently crosslinked polymers offer many significant advantages over traditional hydro- gels. Our work, based on the devel- opment of controlled base-catalyzed polyacrylonitrile polymers, has shown that hydrogels derived from these polymers display thixotropic