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Trends in Polymers for Skin Care



Recent patent activity reveals advances in polymer synthesis, polymeric emulsifiers and a move toward micro- and nano-gels.



Published March 30, 2009
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Trends in Polymers for Skin Care

Trends in Polymers for Skin Care, Part I



Recent patent activity reveals advances in polymer synthesis, polymeric emulsifiers and a move toward micro- and nano-gels.



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



Surveillance of emerging published patents, prosecution histories and patent applications can provide insight into trends in our industry. In preparing this article I surveyed recently published U.S. patents and applications and identified emerging trends. I identified trends in advances in polymer synthesis especially block, graft and gradient copolymers, polymeric emulsifiers and interfacially active copolymers, protein copolymers for antimicrobial activity, advances in cationic polymers as deposition conditioning, and a drive toward micro- and nano-gels and encapsulated particles for efficient deposition-delivery and skin aesthetics and, in the case of soap—for convenience of use. In this two-part article I will briefly review the trends of the past few months.

Polymers as Emulsion Stabilizers



Hydrophobically-modified hydrophilic polymers, namely acrylates C10-30 alkyl acrylate crosspolymer (Pemulen polymeric emulsifiers), were introduced as emulsifiers for cosmetic products in the late 1980s.1,2 Since then, a number of hydrophobically-modified polymeric emulsifiers based upon poly(2-acrylamido-2-methylpropanesulfonate) (AMPS) have been disclosed.3,4,5,6 Now it has been taught that the combination of a particular amphipathic AMPS copolymer and nonionic emulsifier yields stable oil-in-water emulsion lotions and creams having “noteworthy” sensory qualities with a glidant and non-tacky feel.7 The copolymer in this case is Genapol T-080 from Clariant.

Inverse lattices of copolymers of (2-acrylamido-2-methylpropanesulfonate) and N,N-dimethylacrylamide have been recently patentedas emulsion stabilizers and rheology modifiers.8

Roll-on applications of cosmetic emulsions could result in control of payout and a uniform application of these emulsions to the skin surface. Uniform application and good aesthetics, such as lack of sticky feel, are reported to be achieved by the inclusion of certain polysaccharides in oil-in-water emulsion roll-on products.9 Suitable anionic polysaccharides for this purpose are claimed to be selected from aluminum starch octenylsuccinate, sodium starch octenylsuccinate, calcium starch octenylsuccinate, distarch phosphates, hydroxyethyl starch phosphates, hydroxypropyl starch phosphates, sodium carboxymethyl starches, and sodium starch glycolate. Suitable nonionic polysaccharides are starches, starch hydrolysates, cellulose, methyl cellulose, hydroxypropyl cellulose, hydroxy- ethyl cellulose, hydroxypropylmethyl cellulose, hydroxypropylethyl cellulose, and hydroxyethylmethyl cellulose.

Copolymers of polyacrylamide and sodium acrylate can be hydrophobically-modified by reaction with alkyl-amines.10 Copolymers produced by this route are useful to enhance the foaming properties of shanpoos.

It has been known for more than 30 years that certain cationic polymers can form phase separated complexes with anionic surfactants at low surfactant concentrations, and that these complexes are solubilized at higher concentrations of anionic surfactants. This phenomenon formed the basis of conditioning shampoos and then it was applied to skin cleansing compositions that deposited beneficial materials on skin during the rinsing stage. This line of thinking has been advanced by the introduction of cationic cassia derivatives.11 Cassia gum is a polygalactomannan that is isolated from the endosperm of the seeds of Cassia tora and Cassia obtusifolia. Cationic cassia is the hydroxypropyltrimonium derivative of cassia gum. In addition to their use as coacervate–formers for conditioning shampoos, cationic cassia derivatives can be used as thickeners for creams and lotions and suspending agents for particulates in products such as shower gels, masks and skin cleansers containing exfoliating scrub agents.

Copolymers of diallyldimethylammonium chloride and acrylamide have recently been advanced as coacervate depositing agents for skin, with the added advantage that they provide desired low viscosity/rheology to the personal care compositions in which they are included.12

Imidazolinium polymers are useful cationic materials that have found use as conditioning polymers for skin and hair. However, imidazolinium mono- mers do not easily copolymerize with other monomers. It is interesting that imidazolinium alkyl methacrylate copolymers for cosmetic applications have been disclosed.13 For instance random copolymers of imdidazolinium—ethyl methacrylate and diallyldimethylammonium chloride—were prepared by high output polymerization techniques in a Chemspeed Workstation (model A100). An example of a skin-conditioning shower gel based upon this polymer is presented by the formulation: 20g ammonium laureth sulfate, 15g ammonium lauryl sulfate, 0.5g poly(imdidazolinium–ethyl methacrylate–co-diallyldimethylammonium chloride), 0.5g polyquaternium-7, 2.50g sodium laureth sulfate, glycol distearate, cocamide MEA, laureth-10 0.10g perfume oil/essential oil, q.s. preservative, 0.50g sodium chloride to 100 with demineralized water.

Block copolymers comprising polyionic blocks and neutral blocks have been advanced as beneficial for deposition from aqueous solution onto surfaces such as skin and hair. A broad variety of block copolymers have become available as a result of the advent of living free-radical polymerization. A recent patent has disclosed the preparation and use of block coplymers consisting of blocks of quaternized poly(dimethylaminoethyl- acrylate) and acrylamide.14 In model studies it was demonstrated that these polymers readily deposit onto silica from aqueous solutions and more than half of the deposited polymer resists removal by rinsing. Lower molecular-weight acrylamide blocks result in shorter adsorbed molecular “brushes” that resist rinsing better than longer brushes. The deposition can be enhanced by complex coacervation with anionic surfactants.

Rheology Modifiers for ‘Wet Skin’



Moisturizing lotions that can be applied during bathing are advantageous insofar as they are applied to wet-skin and therefore they might be expected to enhance moisturization. Moreover, products like these condense two processes into one—bathing and application of lotion—and therefore they can be viewed as convenient, time-saving products. There have been many inventions directed toward the application of oils to skin in the shower or in the bath. These products are only effective if sufficient beneficial agent is retained on the skin after rinsing and towel drying to confer perceivable benefits without the drawback of oily feel. Bath oil is directed to coating the skin, but it must be used sparingly and it can leave messy oily residues on the tub. Conventional creams and lotions are not designed to be applied as rinse-off products and, consequently, they are poorly retained on the skin after bathing. Water-in-oil emulsions can deposit so efficiently that they are perceived to be excessively greasy. High internal oil phase (HIP) emulsions are specifically targeted to impart skin benefits by deposition during rinsing.15 Similar wet skin compositions containing carbomer and xanthan gum have been designed.16,17 It has been disclosed that adjusting the carbomer:xanthan gum ratio to be at least 0.75:1 and preferably 1:1, results in a product with lotion-like rheology.18 This is described as a composition having an elastic modulus (G’) of greater than 100 Pascals (which is described as conferring “thickness” to the consumer) and a percentage drop of 50% or more in G’ when a pressure of 40 to 100 Pascals is applied to the formulation.

Protein Block Copolymers



Lysine is an amino acid with 2 amine groups and one carboxylic acid group. When it polymerizes to form epsilon polylysine, one amino acid group condenses with the carboxylic acid group, leaving one amine free. This free amine confers water solubility on the polypeptide and, under acid conditions, the cationic free amine group is claimed to confer antibacterial properties. Hydro- phobic modification of polylysine by introduction of hydrophobic alkane groups confers emulsifying properties. Epsilon-Polylysine may be obtained by culturing Streptomyces albulus subspecies and it can then be coupled to polysiloxanes via reactive carboxyl or epoxy groups on the silicone moiety.19These polylysine siloxanes could possibly be useful as silicone emulsifiers that also confer antimicrobial qualities to the final emulsion.

Graft Copolymers



The advent of living polymerization has enabled synthesis of gradient polymers with low distribution of composition and molecular weight. Gradient copolymers are polymers in which each end is rich in one or the other monomer units and the polymer composition changes along the molecular chain progressively from one monomer to the other monomer. These polymers are useful to confer conflicting properties in the same system while maintaining stability. Examples of conflicting properties in skin makeup properties are gloss or matteness, good adhesion to the support and resistance to mechanical transfer or removal by secreted sweat or sebum but easy removability after use. These conflicting properties can be achieved by blending different polymers; but different polymers do not mix readily at the molecular level and phase separation may result. The gradient polymer serves to compatibilize such compositions, and gradient polymers proposed for this purpose are synthesized from at least two different monomers both chosen from isobornyl acrylate, isobornyl methacrylate, isobutyl acrylate, isobutyl methacrylate and 2-ethylhexyl acrylate.20 The use of the gradient copolymer is exemplified by its use in a foundation:

Phase A: Cetyl dimethicone copolyol (Abil EM 90 from 3g Goldschmidt), isostearyl diglyceryl succinate (Imwitor 0.6g 780K from Condea), pigments (oxides of iron and of titanium) 10g,polyamide (Nylon 12) powder 8g, solution of the a gradient copolymer of isobornyl acrylate, isobornyl methacrylate, and butyl acrylate in17g isododecane (8.5% of polymer dry matter), fragrance q.s. and isododecane 10g.

Phase B: Magnesium sulfate 0.7g, preservative q.s. and water q.s. for 100g.

“Intelimers” are polymers that have a plethora of closely-spaced side chains which are capable of crystallizing. The uniqueness of these polymers lies in their ability to be hydrophobic below a certain temperature but to become hydrophilic above the melting points of the side-chains. The triggered transition from hydrophobic to hydrophilic can be tightly controlled to within a degree or two. These copolymers have been shown to be useful oil thickeners for cosmetic compositions with continuous oil phases.21,22

Micro- and Nano-encapsulation



Sunscreens, skin-lighteners, moisturizers, emollients, perfumes flavors, oils and other beneficial agents are commonly added to personal care compositions. These agents can react with other components in a formulation and this can limit their efficacy and also change desirable characteristics of the formula chassis. In order to overcome this challenge, encapsulation has been used to provide added stability to ingredients and active agents that could be degraded by oxidation, light, high temperatures, and/or acid or basic pH values. Microencapsulation has been employed as a route to controlled delivery for cosmetics and dermatology. More recently, nanoparticles have been introduced. These nanoparticles reportedly pass through the upper layers of the stratum corneum to release active agents to the epidermis. This route provides a way to deliver actives to the epidermis for prolonged periods without the concern that they will be “rubbed off” of the skin surface during wear.

Microcapsule and nanocapsules can be made by a variety of methods:

• Nanocapsules with poly(alkylene adipate) (Fomrez from Crompton) envelopes have been prepared by dissolving the components in a water-miscible solvent—usually alcohol or acetone—adding this solution to an aqueous phase and evaporating the organic solvent.23 However, the aqueous phase can also be removed during evaporation.

• Differential solubility with respect to pH can also be used to encapsulate oils. Thus, alkali metal salts prepared from ethylene/maleic anhydride copolymer are soluble in solutions of high pH but insoluble at low pH. Microcapsules can be formed by dispersing oils in aqueous solutions of ethylene/maleic anhydride at high pH and then reducing the pH to the region of 5 to 8 to allow the polymer to precipitate and coat the surface of the dispersed oil droplets.24

• Complex coacervates prepared by the interaction of cationic and anionic polyelectrolytes can be employed as encapsulants.25 Simple coacervates can be formed by the addition of polymers such as polyethylene to solutions of ethylcellulose in cyclohexane. This process has been applied to the formation of microencapsulation of pharmaceutical actives for the purpose of taste masking.26

• Polycaprolactone oligomers (Capa 2201 or Capa HC1100 from Solvay) are insoluble in water and in shea butter. When heated, however, the polymer dissolves up to 10%wt. in shea butter. This preferential solubility behavior allows the preparation of nanocapsules by heating homogenized dispersions of mixtures of shea butter, water and polycaprolactone oligomers to 80°C then cooling to 20°C to allow coacervate to form at the oil-water interfaces.27

• Polysaccharide-zein complexes have been disclosed as suitable complexes for the delivery of shear sensitive microparticles containing active agents.28 Zein is a water-insoluble/alcohol soluble prolamine from corn gluten that is highly resistant to bacterial attack. The advantage of this system is exemplified by an herbal essence /agar-zein complex in a skin cream. The skin cream conferred a distinctive and lingering herbal odor, and it was reported to be non-irritating.

Hydrogels



Hydrogels are three-dimensional networks of polymer molecules that can be reversibly deformed. Typical hydrogels are crosslinked polyacrylates or polymethacrylates, but these crosslinked polymers are limited in their reversibility. Hydrogels based on natural polymers such as collagen are biocompatible and usually thermally reversible,29 but they have poor mechanical strengths. Gelling of water can be achieved with triblock copolymers consisting of a polyethylene glycol middle block and two poly(D,L-lactide-co-glycolide) polyester end-blocks with a weight ratio of polyester to PEG of at least one. In this case the gel results from a mesomorphic structure in which the hydrophobic end-blocks alternate with the hydrophilic middle block that contains the water.30Thermoreversible hydrogen-bonding gels can be formed from copolymers of acrylglycinamide and acrylamide. Hydrogen bonding between molecules can give rise to “supramolecularchemistry” in which secondary interactions (such as hydrogen bonding) between small molecules can give rise to properties that are usually indicative of polymers. In cases where a self-complementary quadrupole hydrogen bonding unit (4H units) is present, strong material properties can be realized.31 This approach has been advanced in a recent disclosure for reversible aqueous hydrogels.32 The 4-H units are represented as:

In which it can be seen that two hydrogen-bond donating N-H groups are adjacent to two hydrogen bond accepting N or C=O groups. Examples of the disclosed supramolecular gellants are:

• The reaction product of a precursor prepared by the reaction of1,6 hexyldiisocyanate and methylisocytosine and poly(hydroxyethyl methacrylate), or inuline or chitosan; and

• A block copolymer prepared by reacting hydroxy-terminated PEG with the reaction product of 2amino-4-hydroxy-5-(2-hydroxyethyl)-6-methylpyrimidine with IPDI.

The hydrogels thus prepared are thermally reversible and a preferred embodiment is their use in face masks, mascara bases and dry skin products.

Microgel



Gels made from polysaccharides, such as xanthan, tend to be sticky and have poor aesthetics for skin products. Shisheido researchers have tackled this problem by rendering such materials into microgels.33 The process involves forming the aqueous gel and pulverizing it into microgels of particle size 0.1 micron to 1mm and the resulting microgel material shows better skin aesthetics, especially with regard to stickiness. The microgel material may be made from a variety of gellants including hydrophilic proteins such as gelatin and collagen, and hydrophilic polysaccharides such as agar, curdlan, scleroglucan, schizophyllan, gellan gum, alginic acid, carrageenan, mannan, pectin and hyaluronic acid. The microgel thickeners may be used, for example, as thickeners for o/w creams.

References



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2. J.Y. Castaneda and R.Y. Lochhead; “Emulsions Comprising a High Molecular Weight Anionic Emulsifying Agent,” European Patent App. 482417 A1; April 29, 1992; assigned to BF Goodrich.
3. Morschhauser, Roman; Loeffler, Mathias; Wasserlosliche polymere und ihre verwendung in kosmetischen und pharmazeutischen mittein; EU Patent App. 1 069 142, July 7, 2000; assigned to Clariant.
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