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Skin Care Polymer Trends



Marketers and suppliers alike are patenting innovative concepts in moisturizing and anti-wrinkle benefits using a variety of novel substances.



Published March 28, 2008
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Skin Care Polymer Trends

Skin Care Polymer Trends



Marketers and suppliers alike are patenting innovative concepts in moisturizing and anti-wrinkle benefits using a variety of novel substances.



Robert Y. Lochhead
The Institute for Formulation Science and The School of Polymers & High Performance Materials
The University of Southern Mississippi
Hattiesburg, MS



Innovation in personal  care products is continuing unabated. There are claimed breakthroughs in moisturizing and anti-wrinkle benefits using hyalurans, polymers for deposition, controlled delivery and release of therapeutic agents and fragrances; starburst polymers for precise gel formation, silicone acrylates for compatibility in polar formulations; and ingredients for structuring and stabilizing hydrophobic compositions. This article is based on recently published patent applications that highlight key trends in the use of polymers for skin care.

Moisturizing and Wrinkle-Free



Glycosaminoglycans are the most abundant heteropolysaccharides in the human body. They are unbranched, negatively charged polyelectrolytes consisting of linked disaccharide units that adopt extended conformations in solution to confer high viscosities on aqueous compositions. Hyaluronan is an example of a glycosaminoglycan. High molecular weight hyaluronan has been used for a number of years as a moisturizer for skin treatments. Low molecular weight hyaluronan has been reported to exhibit anti-wrinkle properties. A single mixture containing both high molecular weight and low molecular weight hyaluronans has now been claimed as a moisturizing, cosmetic and anti-wrinkle composition.1

Controlled Delivery



Another class of polysaccharides are the cyclodextrins. The cyclodextrins are cyclic polysaccharides that are capable of forming host-guest complexes with other molecules ranging, for example, from odors to fragrances to drug actives (Figure 1).

Cyclodextrins are cyclic polysaccharides made up of common, naturally-occurring D-(+)- glucopyranose units joined by α-(1,4) linkages. The most common cyclodextrin rings are made up of six units (α-cyclodextrin), seven units (β-cyclodextrin), and eight units (γ-cyclodextrin). The ring structure has an inner apolar cavity. The primary hydroxyl groups are situated on the inner side and the secondary hydroxyl groups are situated on the outer side. Cyclodextrins could be useful for the controlled delivery of small molecule therapeutic agents with poor pharmacological profiles.
   
For example, therapeutic agents with low aqueous solubility, or ones in which bioactive forms exist in equilibrium with inactive forms, or agents that must be “dribbled in” because they are toxic in high doses. The host/guest inclusion supra-molecular complexes formed by cyclodextrins alter the physical, chemical and biological properties of the conjugated molecules and help in their controlled introduction to the target substrate. Their use in drug delivery is benefited by their good water-solubility, low toxicity and low immune response. Conjugates of drug molecules with polymer are another approach that has been used to circumvent the difficulties of delivering “problem” actives.
   
Polymers such as poly (hydroxypropylmethacrylate), poly(ethyleneglycol) and poly (-L-glutamic acid) have been used for this purpose. These approaches have been combined by covalently attaching cyclodextrin moieties to polymers by covalent attachment to polyvinyl alcohol or cellulose, or by copolymerization with vinyl acetate or methyl methacrylate.2 There have even been polymers made in which the polymer backbone is threaded though the cyclodextrin rings.3 Biocompatible, water-soluble polymers containing covalently attached cyclodextrin moieties have been disclosed for the purpose of conjugating guest molecules by attachment that can be cleaved under biological conditions.4 These cyclodextrin-containing polymers improve drug stability and solubility, and reduce toxicity of the small molecule therapeutics when used in vivo. Moreover, by selecting from a variety of linker groups and targeting ligands, the polymers can provide controlled delivery of therapeutic agents.
   
Amphipathic block copolymers are large-scale analogues of surfactants. They adsorb strongly at interfaces and self-assemble to form micelles and complex, well-defined mesomorphic structures. These polymers thicken aqueous solutions to form viscoelastic gels.5,6  The compositional scope of such block copolymers has been greatly increased with the advent of living free radical polymerization which allows fine control  of the uniformity of molecular weight and the sequence in which the monomer units can be attached to the growing macromolecule during synthesis. Hydrogels can be formed without covalent cross links from such amphipathic block copolymers because the aggregated hydrophobic blocks can act as junction zones within the structure. Such hydrogels can offer advantages in processing and in “self-healing” properties whereby junction zones that are disrupted by shear or extension may reform in the quiescent state. Moreover, these amphipathic block copolymers can allow tailoring of physical and mechanical properties, and also control water adsorption and transmission. This is done by adjusting the size and number of blocks. A recent patent application claims copolymers constructed of hydrophobic blocks, hydrophilic blocks such as acrylic acid, and middle blocks, for example, of short alkyl chain methacrylate such as butyl methacrylate.7 It was disclosed that the water content of these hydrogels could be adjusted by changing the pH.

Dendritic Polymer Gels



Another use for amphipathic polymers is in iontophoresis gels. Transepider-mal delivery of substances can be greatly enhanced by iontophoresis. Iontophoresis involves applying a potential, from a small battery, across the skin to accelerate the intrusion of substances that are held in a reservoir that is attached to one electrode of the electric cell (Figure 2).

Better control of the intrusion of active substances should be realized if they are contained within a well-defined gel within the reservoir. A recent patent proposes the use of highly branched polymers for this purpose. The highly branched polymers are either dendrimers or arborols. The words “dendrimer”8 (coined by Tomalia) and “arborol”9 (coined by Newkome) are derived from the Greek dendro (meaning tree-like) and the Latin arbor  (meaning tree). This nomenclature aptly describes the fractal-like branched structures of these molecules. Dendrimers and arborols are prepared by a series of iterative steps, each designed to add one sequence of successive segmental units to the outside of the molecule. Each sequence is called a “generation” in dendrimer-speak. Dendrimers can be constructed by a divergent synthesis or a convergent synthesis. Divergent construction starts with the core of the molecule and builds out generation by generation. In convergent construction, the branches are assembled first then they are brought together and bound by the core.  Dendrimers and arborols are more precise branched molecules than hyperbranched polymers. Hyper- branched polymers, first patented by Kim in 1987.10,11,12 are prepared in a single-pot process and, as a consequence, the individual molecules within the product vary in molecular weight and structure.
   
Amphipathic arborols are capable of self-assembling into micelles.13 An example of such an arborol is shown in Figure 3 on the next page.
   
The molecule shown has a hydrophilic head consisting of nine hydroxyl groups and a hexyl chain as the hydrophobic tail. This molecule would be named [ 9]- 6 arborol; the 9 refers to the number of hydroxyl groups and the 6 refers to the hydrophobic chain length. The solubility of one-directional arborols in water decreases dramatically with increasing length of the alkane hydrophobic group, from 13.0 mmol/L when n is 6, to 1.4 mmol/L when n is 8, to less than 0.017 mmol/L when n is 9. for [9]-n arborols.14 At concentrations significantly higher than the critical micelle concentration, there is a possibility that amphipathic arborols would form gels by hydrophobic association and, if they did, the gels might be expected to have a more uniform pore size than conventional polymer gels. This could be the reason that these interesting tree-like molecules are being proposed for use in iontophoresis reservoirs to enhance the selectivity of delivery of active substances across the skin.15

Enhanced Fragrance Delivery



Perfume is frequently the most costly ingredient in a formulation and there is considerable economic benefit to be derived from products that would enhance the deposition of fragrance on target substrates, including the skin, and then prolong the duration of release of the desirable fragrance. This is especially true for rinse-off products but it also applies to topically-applied, leave-on products to improve the consistency of the perfume notes with time. Freshness attributes are usually associated with “top notes” and “middle notes” but “top notes” can be quickly lost due to evaporation and they tend to dissolve easily in aqueous media and be rinsed away. Attempts to retain the top notes on the substrate have included polymerizing the perfume raw materials into polymeric particles or absorbing perfume into polymeric particles. However, a need exists for perfume particles that selectively adsorb/absorb top notes and middle notes of perfume raw material and deposit onto a substrate from rinse-off personal care products. Procter & Gamble researchers recently claimed to have achieved this important objective.16 They have done this by preparing a composition that includes a perfume polymeric particle. The polymeric particle contains a cationic polymer and a perfume having a molecular weight less than 200, a boiling point less than 250°C and/or a C log P of less than 3 and/or a Kovats Index value of less than about 1700. The P value is the octanol/water partitioning coefficient of a substance. For large values of this partition coefficient, the value is expressed as log P. Thus, a log P value of 3 indicates that partitioning  of  the substance is one thousand-fold preferentially into the octanol phase rather than the water phase. The C log P values of many perfume ingredients are available from the Pomona 92 database (Daylight Chemical Information Systems Inc., Irvine, CA).
   
The Kovats Index system is an accurate method of reporting gas chromatographic data for interlaboratory substance identification. The Kovats Index for many perfume ingredients has been reported. The efficacy of the polymer particles in depositing and releasing the different perfume raw materials is measured by a protocol that ultimately tests the amount of perfume released from a fabric by headspace gas chromatography  over a period of time specified by a longevity test. 
   
The polymeric particles are polymerized from a cationic monomer and they are defined as cationic if they have a positive zeta potential. Examples of possible cationic monomers are given as dimethylamino alkyl acrylates, vinyl pyrrolidones, vinyl imidazoyls, vinyl ethers with diethylamino groups, vinylpyridines, alkyl acrylamides and dialkylaminoalkyl acrylamides. The particle is exemplified by a copolymer of methyl methacrylate and dimethyl aminoethyl methacrylate cross-linked with ethylene glycol dimethacrylate.

Delivery of Hydrophilic Agents



Many makeup formulations are now oil- or silicone-based. These can be water-in-oil emulsions or anhydrous compositions. Anhydrous in this case does not necessarily mean that the composition is completely devoid of water. The term “anhydrous” merely connotes that no water has been intentionally added as an isolated ingredient. There is a challenge, however, in attempting to formulate stable hydrophobic compositions that contain hydrophilic skin treatment agents. Hydrophilic agents that could be advantageously formulated include niacinamide, panthenol, bacterial cultured mediums, allantoin, sodium lactate, PCA soda, amino acids, urea, sodium hyaluronate (Actimoist from Active Organics,  Avlan Sodium Hyaluronate series from Intergen or Hyaluronic Acid Na  from Ichimaru Pharcos), chondroitin sulfate, collagen, elastin, pectin, carrageenan, sodium alginate, trehalose, tuberose saccharide, chitin derivatives, chitosan derivatives and mixtures thereof. It has been discovered that formulating the hydrophilic skin treatment agent with an alkyl dimethicone copolyol and a polyol results in a stable polyol-in-silicone emulsion.17

Silicones: Multiple Benefits



The concept of transfer-free lipstick was introduced over a decade ago18 and since that time silicone resins have been a preferred ingredient to achieve transfer resistance in color cosmetics. These silicone resins are known as MQ  or MT resins to signify that they consist of polymers constructed from monofunctional (M), trifunctional (T), and/or quatrifunctional (Q) silane functional groups. Brieva et.al  teach a wide range of cosmetic compositions containing high molecular weight, particulate MQ resin.19 It has been found that the finish of cosmetic compositions can be improved by tailoring the properties of MQ and MT resins by adjusting the ratio of M:Q or M:T units in the resin. In particular, improved finish is achieved when the rate of M units is up to three times greater than the number of Q units.20 The preferred finish is a shiny, transfer-resistant lip color.
   
One drawback of the MQ and MT resins is that they are not compatible with polar media and are difficult to formulate into aqueous compositions. Moreover, desirable silky feel is achieved by low molecular weight cyclosiloxanes that do not possess Q or T functional groups, but these low viscosity silicones have formulation challenges of their own;  skilled formulation is required to produce stable products. Polymeric silicon gel, prepared by the addition of SiH groups over the terminal double binds of alpha-olefins provide a limited range of improvements. Wider compositional ranges are possible with water swellable/oil swellable, self-emulsifying cross-linked silicone copolymers, taught in a new patent application from Momentive Performance Materials.21 These are silicone copolymers, with polyether substituted structural units and epoxy or oxirane structural units that are reacted with acrylates produce cross-linked silicones with acrylate cross links. These silicone copolymers are amenable to formulation in water-based formulations and they are taught to confer good skin-feel attributes.
   
Oil-continuous color cosmetics have been structured by waxes and structuring polymers. It has recently been disclosed that the addition of an organogelator  may make it possible to make a stick cosmetic that is mechanically strong even during application to the lips or skin, stable during storage and provides a deposit that is glossy, supple and migration-resistant.22 The organogelator is a non-polymeric organic gelling agent that aggregates molecularly with itself to form a 3-D network that may gel the liquid fatty phase. The network may arise from a network of fibrils that immobilizes the liquid fatty phase and results in a gelatinous consistency. 

References


    1. Abdellaoui, Khadija Schwach; Malle, Birgitte Molholm; Compositions with several hyaluronic acid fractions for cosmetic use, US Patent Application 20080057091,  March 6, 2008, assigned to Novozymes BioPolymer.
    2. Yoshinaga, Masanobu; Method of synthesizing cyclodextrin polymers, US Patent 5,276,088, January 4, 1994; assigned to Toppan Printing Co., Ltd.
    3. Nobuhiko, Yui; Supramolecular-structured biodegradable polymeric assembly for drug delivery, US Patent 5,855,900,  January 5, 1999.
    4. Cheng, Jianjun; Davis, Mark E.; Khin, Kay T.; Cyclodextrin-based polymers for therapeutics delivery. US Patent Application 20080058427, March 6, 2008, assigned to Insert Therapeutics, Inc.
    5. Destarac, Mathias; Joanicot, Mathieu; Reeb, Roland; Gelled aqueous composition comprising a block copolymer containing at least one water-soluble block and one hydrophobic block, US Patent 6,506,837, January 14, 2003 , assigned to Rhodia Chimie.
    6. Anthony, Olivier; Bonnet-Gonnet, Cecile; Destarac, Mathias; Farhoosh, Roya; Joanicot, Mathieu; Lizarraga, Gilda; Reeb, Roland; Schwob, Jean-Marie. Water-soluble block copolymers comprising a hydrophilic block and a hydrophobic block, US Patent 6,437,040, August 20, 2002,  assigned to Rhodia Chimie.
    7. Schmidt, Scott; Callais, Peter; Macy, Noah; Mendolia, Michael; Amphiphilic block copolymers, US Patent Application 2008/0058475 A1, March 6, 2008, Arkema, Inc.
    8.  Tomalia, D. A.; Baker, H.; Dewald, J. R.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. A New Class of  Polymers: Starburst-Dendritic Macromolecules. Polym. J. (Tokyo), 1985, 17, 117-132.
    9. Newkome, G. R.; Yao, Z.; Baker, G. R.; Gupta, V. K; Cascade Molecules: A New Approach to Micelles. A [27]-Arborol. J. Org.Chem. 1985, 50, 2003-2004.
    10. Kim, Y.H.; Webster, O. W.; Hyperbranched Polyphenylenes., Polym. Prepr. 1988, 29, 310-311.
    11. Kim, Y.H.; Hyperbranched Polymers 10 Years After., J. Polym.Sci., Part A: Polym. Chem. 1998, 36, 1685-1698.
    12. Newkome, George R.; He, Enfei and Moorefield, Charles N. Suprasupermolecules with Novel Properties: Metallodendrimers. Chem. Rev. 1999, 99, 1689-1746.
    13. Newkome, G. R.; Yao, Z.; Baker, G. R.; Gupta, V. K; Cascade Molecules: A New Approach to Micelles. A [27]-Arborol. J. Org.Chem. 1985, 50, 2003-2004.
    14. Sun, Jirun; Ramanathan, Muruganathan; Dorman, Derek; Newkome, George R.; Moorefield, Charles N. and Russo, Paul S.; Surface Properties of a Series of Amphiphilic Dendrimers with Short Hydrophobic Chains Langmuir 2008, 24, 1858-1862.
    15. Smith, Gregory A. Delivery device having self-assembling dendrite polymer and method of use thereof, United States Patent Application 2008/0058701 A1 , March 6, 2008, assigned to Transcutaneous Technologies.
    16. Dykstra, Robert Richard; Gallon, Lois Sara; Clapp, Mannie Lee; Deckner, George Endel; Rinse-off personal care compositions comprising cationic perfume polumeric particles, US Patent Application 20080057021, March 6, 2008, Procter & Gamble.
    17. Masuda; Hisatoshi; (Moriyama, JP) ; Ishigami; Mayu; (Ashiya, JP) ; Deckner; George Endel; Personal care composition comprising hydrophobic gel, US Patent Application 20080057014, March 6, 2008, Procter & Gamble.
    18. Castrogiovanni; Anthony, Barone; Salvatore J., Krog; Ann, McCulley; Marion L., Callelo; Joseph F.,  Cosmetic compositions with improved transfer resistance, US Patent 5,505,937, April 9, 1996, assigned to Revlon Consumer Products.
    19. Brieva; Hernando (Manalapan, NJ), Russ; Julio Gans (Westfield, NJ), Sandewicz; Ida Marie, Cosmetic compositions, US Patent 5,800,816, September 1, 1998, assigned to Revlon Consumer Products
    20. Patil; Anjali Abhimanyu; (Westfield, NJ) ; Calello; Joseph Frank; (Bridgewater, NJ) ; Sandewicz; Robert Walter, Cosmetic Compositions with Silicone Resin Polymers,  US Patent Application 20080050328, February 28, 2008, assigned to Revlon Consumer Products.
    21. Lu; Ning; ; Czech; Anna Maria; Hoontrakul; Pat; Nicholson; John; ; Rojas-Wahl; Roy;  Acrylate Cross Linked Silicone Copolymer Networks, US Patent Application 20080051497, February 28, 2008, assigned to Momentive Performance Materials.
    22. Ferrari; Veronique; Composition structured with a polymer containing a heteroatom and an Organogelator, US Patent Application 20080057011, March 6, 2008, assigned to L'Oréal.


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