The dermis lies between the epidermis and the subcutaneous fat and is responsible for skin thickness. Elastin fibrils, collagen fibers and hyaluronic acid (HA) associate using non-covalent bonds leading to structured skin. As HA levels decline, skin begins to look aged. HA is the most abundant non-sulfated glycoaminoglycan in the human dermis.

According to Heber et al, HA is an attractive building block for biocompatible and biodegradable products due to its biocompatibility and lack of immunogenicity. It is made of repeating dimers of glycuronic acid and N-acetyl glucosamine assembled into a long, high molecular weight polymer. These dimers are highly hydrated and viscoelastic. HA, however, has poor biomechanical properties. In the skin, its half life unmodified is about 12 hours, and in the bloodstream 2-5 minutes. For this reason, chemical modifications of native HA have been performed to provide mechanically and chemically robust materials. They have different physiochemical properties, but maintain biocompatibility and biodegradability. One modification performed is to form a crosslinked hydrogel by chemical crosslinking of polymers to infinite networks under mild, neutral conditions or alkaline conditions. Injectable hydrogels have also been prepared with zero, low or a high degree of crosslinking. Utilizing the hydroxyl groups on the polysaccharides, polyepoxides are used as crosslinking agents. It has been shown that hyaluronidase, an enzyme that cleaves internally to HA polymers, requires three disaccharides for effective binding to the polysaccharide. For this reason, the chemical modification of the HA backbone at appropriate intervals is necessary for the inability for the hyaluronidate to recognize, bind, and/or catalyze the cleavage of HA oligomers. Polysaccharides, like HA, are often very large, highly branched polymers that have been used in cosmetic applications due to their space-filling, structure stabilizing with malleable physical properties and outstanding biocompatibility. There is a proportional relationship between young looking skin and the presence of a polysaccharide network in the intercellular matrix. This relationship is due to the extreme viscoelasticity of the polysaccharide while maintaining a high level of hydration in the dermal tissue.

Novel mascara formulas are making good use of new polymeric films.

Unfortunately, these polysaccharides (primarily hyaluronic acid), cross-linked or non-crosslinked, are subject to degradation in vivo via free radical or enzymatic degradation. In order to alleviate the degradation, Stroumpoulis et al suggest the addition of inhibitors to the formulation in order to increase the longevity of the polysaccharide.¹ The inhibitor is a glycosamino-glycan, an antioxidant, a flavonoid, a protein, a fatty acid, and combinations of these. Two ways are mentioned to facilitate this addition. One is to encapsulate the inhibitor into a biocompatible/biodegradable vessel, such as a liposome, micelle or polymerized vesicle. The other is to combine the polysaccharide and vessel into a gel. To do this, the inhibitor must be mixed into the polysaccharide network in a highly hydrated state with an inhibitor resulting in an encapsulated inhibitor. The system is dehydrated so that the swell ratio and/or release kinetics can be easily controlled. Another method is to start with a crosslinked polysaccharide. For example,²a polysaccharide can be produced having a higher proportion of ether-links in a crosslinked polysaccharide gel, which results in improved degradation characteristics. This crosslinked polysaccharide is prepared when a polysaccharide is contacted with a crosslinking agent and a masking agent to form a crosslinked polysaccharide gel having resistance to degradation under physiological conditions; i.e., extrusion through a narrow 27-, 30- or 32-gauge needle. These gels are suitable for injection into tissue or skin without loss of substantial structural integrity of the solution or gel.

Advances in Skin Treatments

Healthy skin contains excellent levels of oily/waxy matter such as sebum, intercellular lipids and natural moisture. To enhance skin moisturization and barrier function, intercellular lipids, ceramides, have been extensively used in topical skin treatments. However, ceramide’s high crystallinity, high melting point and lower compatibility with other ingredients make it difficult to incorporate into skin formulations. In order to address this situation, a homopolymer of glycerol-1-methacryloyloxyethyl urethane (GMU) has been developed, which has a chemical structure similar to ceramide.³ This product is claimed to exhibit good texture, provide good skin barrier function and hair protection like ceramide but it can be easily blended with other ingredients. In addition, nanoparticles and powder that can further improve skin barrier function and hair protection can be blended with the cosmetic product by partial or entire functionalization with the GMU polymer.

Nanodiamonds

A therapeutic delivery system has been patented comprising a tri-layered nanofilm composed of a base layer and elution layer both made from parylene A and a mid-layer containing nanodiamonds functionalized with a therapeutic agent.⁴

Nanodiamonds are preferred in therapeutic delivery systems as they have high surface to volume ratio, high loading capacity, high physical interactions with therapeutic agents and non-invasive dimen- sions. Hybrid polymer nanodiamond films have been developed where the release is triggered by external stimuli. These functionalized therapeutic agents are also embedded in the hydrogel matrix. In the future we may be wearing diamonds for more than just“show!”

Polymer Nanoparticles

Most of the particles used in controlled drug delivery have spherical shape due to ease of fabrication. However, control on the particle shape along with anisotropy, phase distribution, orientation and biodegradability could be advantageous to controlled release applications. Multiphase nano-com- ponents having high shape selectivity have been fabricated using electro hydrodynamic jetting process (electrospinning). Lahann et al have disclosed that they can form phase-separated nanoparticles containing a 50:50 poly (DL lactide-co-glycolide) in one phase and a 85:15 (DL lactide-co-glycolide) in the other.⁵ Polymers within the nano-component can be modified to target certain moieties. They can also be tuned to have hydrophobic, cationic, anionic, zwitterionic and hydrophilic nature depending on selection of the starting materials and the nanophase separation.

Suppressing ROS

One of the natural byproducts of the metabolism of oxygen by cells is reactive oxygen species (ROS). They perform important functions such as killing pathogens, cell and redox signaling. However, increased levels of ROS damage the living cell, DNA and proteins. Skin is prone to form ROS as it is exposed to environmental attacks. Moreover, some ingredients in cosmetics have ROS generating capabilities that deteriorate the other cosmetic ingredients and damage the skin. Sente describes a new composite particle system that can protect the skin and other cosmetic ingredients by suppressing the formation of ROS.⁶ The system comprises an antioxidant entrapped with the core particle inside the polymer shell, which is coated with another antioxidant. The core can be any solid cosmetic ingredient, which can generate ROS in presence of UV light and the co-entrapped antioxidant will quench the ROS. The second coat of the antioxidant on the polymer shell is capable of reducing the oxidative damage to the skin.

Color Cosmetics

Pearlescent pigments with iron oxide coatings are not new. They are often used in printing inks, powder coatings and paint as well as in skin care products and color cosmetics. However, cracks frequently occur in the iron oxide coatings on mica, which reduce the brightness of the color. It has been found that flake form substrates that have both a FeOOH layer as well as a TiO2 (or TiO2/SiO2/TiO2) layer package allow for a novel combination of bright interference color and bright, pure mass tone. In addition, due to their lower calcination temperatures, the formation of pseudobrookite at the interfaces between Ti and Fe containing layers is prevented. Pseudobrookite, which is formed by the precipitation of iron oxide and titanium dioxide, has a high refractive index, which causes opaqueness in the pigment that takes away the desired “natural” look. These pigments have a very smooth surface and have very good skin feel and therefore are particularly suitable for use in care and decorative cosmetics as well as paint, printing inks and other coatings.⁷

Dermal fillers comprising pigments encapsulated in a hydrogel decorated with phosphazenes have been divulged.⁸ The hydrogels are exemplified by polymers of acrylic acid and allylmethacrylate cross-linked with triethylenegl coldimethacrylate.

Mascaras are made of various film formers dispersed in one or more solvents. After application, the solvents evaporate to leave a rigid, liquid impervious coating on the eyelashes. The coating is designed to remain on the lashes for an extended period of time to achieve a desired long wear effect. However, Estée Lauder researchers discovered that these rigid coatings, once formed, cannot be re-wetted by the mascara composition. Moreover, they indicate that there is no commercially available mascara that allows comfortable re-application over a previously applied, already-dried mascara coating for achieving a more dramatic effect. They reportedly overcame this issue by formulating an aqueous-based mascara composition containinga unique combination of a chemically-modified wax and a polymeric film former, which provides excellent re-wettablility and allows for easy reapplication hours after initial application.⁹ The most preferred wax is PEG-8 beeswax (Apifil from Gattefosse). The most preferred polymeric film former is a VP/DMAPA Acrylates Copolymer (Styleze CC-10 from ISP).

One way to obtain a cosmetic product that has good wear on keratinous materials (such as lips) is usually to include a significant proportion of volatile oils. However, large amounts of volatile oils decrease comfort and gloss of the composition. Chanel researcher Trabelsi unveiled a cosmetic composition that is in the form of a water-in-oil emulsion comprising a non-volatile hydrocarbon oil(s), a copolymer of ethylene and of propylene, and a second block copolymer of styrene and of an olefin other than styrene. The two specific copolymers in this composition have been found to bring the refractive index of the fatty phase closer to the aqueous phase and produce the desired transparency or translucency.¹⁰

Celine Farcet of L’Oréal reported that she surprisingly discovered that certain polymers when delivered from an organic medium, gave good adhesion and staying power on skin and hair.¹¹ The polymers are block copolymers comprising incompatible blocks, in which one of the blocks has a glass transition temperature (Tg) preferably greater than 40°C and the other has a Tg preferably below 0°C. The high Tg block consists of, for example, isobornyl acrylate, isobornyl methacrylate and methoxy polyethylene glycol methacrylate (MPEG 350 from Cognis). The low Tg block comprises, for example, isobutyl acrylate and acrylic acid. The composition, in a cosmetically-acceptable medium, is said to improve tack and gloss, and to provide wearer comfort, to improve glidance and resistance to external attack by sebum or meals and to rubbing. In the case of lipsticks, saliva “relubricates” the film to swell the film and to give a pouty wet-gloss effect. Farcet suggests that the PEG units are responsible for the improved glidance, insensitivity to oils, increased volume and gloss.

Nathalie Mougin of L’Oréal has disclosed a linear block ethylenic copolymer in which there should be at least one block of the copolymer made of ethylenic monomers containing a lactam ring.¹² This block should have a glass transition temperature (Tg) between -55°C and 55°C. They have shown to have a very good combination of rigidity and a lack of tackiness, as well as reduced fragility and high mechanical strength.

Stimuli Responsive Systems

Some polymers are completely soluble at room temperature but they phase-separate at higher temperatures. For example, poly-N-isopropylacrylamide becomes insoluble at about 37°C, hydroxypropyl cellulose separates at temperatures above about 40°C and poloxamers gel once the temperature reaches the lower critical solution temperature (LCST). The separation or gelling of such polymeric solutions is usually called the “cloud point” because the clear solution characteristically becomes cloudy at temperatures higher than the LCST of a polymer comprising water soluble units and units with an LCST have heat-induced gelling or heat-induced thickening properties. The prior art includes copolymers that have units with an LCST and pH-sensitive units which have heat-induced gelling properties. However, these gels obtained with these copolymers are opaque at the LCST. L’Alloret of L’Oréal is using a polymer comprising water-soluble units and units with an LCST that are liquid at low temperatures but clear gels above their critical temperature. Moreover, above the critical temperature, these polymers become surface active and are effective foamers and emulsifiers. Examples are poly(acrylic acid) withrandom (EO)6 (PO)39 amido linked grafts, and poly(acrylic acid) with poly-N-isoproyl-acrylamide grafts.^{13, 14}

Another interesting application of thermogelling compositions is the delivery of treatments with controlled release to the body. Thus, suitable thermogelling systems can be prepared using methylcellulose and citric acid. Thermogelling systems can be utilized for the delivery of sunscreens, skin softeners (such as urea), keratolytic agents (such as salicylic acid), and acne treating agents such as benzoyl peroxide.¹⁵

Rheology Modifiers

There is a class of associative thickeners known as hydrophobically-modified, alkali-swellable emulsion polymers (HASE thickeners). These polymers are typically polymerized as stable emulsion at low pH (<4.5), but become water swellable or soluble at near-neutral or neutral pH (pH>5.5-7). These are typically vinyl addition copolymers of pH sensitive or hydrophilic monomers, hydrophobic monomers and hydrophobic associative monomer(s). According to Tamaraselvy et al,¹⁶ HASE polymers have limited use as rheological modifiers in aqueous formulations because they have little thickening ability at less than 1% and high concentrations are not economically desirable, and high viscous HASE polymers are difficult to handle during manufacturing. Consequently, additional rheological modifiers are sometimes used to offset these disadvantages of HASE copolymers. For example, HASE properties have been enhanced by adding surfactants to the composition;¹⁷ using macromonomer-derived associative polymers as co-thickeners;¹⁷ adding a mixture of multiphobe and monophobe compounds;¹⁸ or suppressing the viscosity of HASE polymers by complexation of the hydrophobic moieties of the polymer with cyclodextrin.¹⁹ Tamaraselvy claims that by including a semihydrophobic monomer in the associative thickener composition, the thickener can be improved to provide rheological properties from pourable liquids to non-pourable gels without additional or auxiliary rheological modifiers. It can also act as a suspending agent, emulsifier, stabilizer, solubilizer, film former or pigment-grinding additive.

Nanofilms have a variety of applications in therapeutic skin care systems.

Thickening at neutral pH is well understood and achievable by the use of carboxylate-type thickeners. However, thickening of acid systems is desired. Copolymers obtained from 2-acrylamido-2-methylpropanesulfonic acid or its salt, dialkylacrylide and cross-linking monomers provide thickening at acid pH ranges with limited or no stickiness.²⁰

Microgel-like water-swelling polymers can be manufactured by reverse phase emulsion polymerization by choosing the temperature and surfactant in such a way that a one phase water/oil microemulsion or fine emulsion can be prepared so that a nanometer order particle size can be controlled. These polymers are exemplified by copolymers of dimethylacrylamide and 2-acrylamido-2-methylpropanesulfonic acid.²¹

Another advance in associative thickeners is the development of hydrophobically-modified cationic or amphoteric thickeners that have been announced in a patent application, which is a continuation of U.S. Patent No. 7288476.²² The copolymers contain cationic monomers selected from diallyldimethylammonium chloride and methacrylamidopropyltrimethylammonium chloride. The cationic copolymer is either acrylamide/diallyldimethylammonium chloride copolymer or acrylamide/diallyldimethylammonium chloride/acrylic acid terpolymer. These polymers offer improved compatibility with cationic and low pH formulations.

The drive toward “natural” ingredients has spawned new biosynthetic polymers. In this respect, natural microfiber cellulose thickeners, produced from bacteria, offer the prospect of a new class of natural thickeners.²³ One bacteria, acetabaster xylinum, is a non-photosynthetic organism that biosynthesizes cellulose from glucose. These biosynthesized celluloses are used in the food and healthcare industries, but not in a personal wash liquid cleanser. The microfibrous cellulose derived in this way provides suspension properties when used at low levels; i.e., 0.01-1% for particles or bubbles as large as 3mm. An example is provided of a liquid composition comprising 0.5-40% surfactant (anionic and zwitterionic where anionic is in excess), 0-25% thickener, 0-15% moisturizing compound (glycerin, polyalkylene glycol or both), 0.01 to 2% bacterially-produced microfibrous cellulose, 0.1- 5% suspended particles 1-3000μm optical particles, capsules, air bubbles, or mixture and 20-98% water. The composition may contain salt. The composition is a lamellar phase of 0.5- 20% of a lamellar phase inducing compound of fatty acids, fatty alcohols and mixtures of these.²⁴

Another aspect of the“green” revolution is the development of gellants for natural, green, biodegradable and renewable oils; i.e., natural plant oils such as soybean oil as alternative oils to petroleum-based oils. These gels are commonly produced using polystyrene/rubber block copolymers and hydrocarbon oils of low solubility parameters. Ways of controlling the release of fragrances, perfumes, insect repellents, etc. into the air or onto target areas have been proposed and practiced. A recent patent application describes a blend of a block copolymer, where one block is polystyrene and another is unsaturated rubber, and a natural oil.²⁵ The purpose is to produce a clear natural vegetable oil gel that provides a controlled-delivery of perfume and air-care actives in various cosmetic applications. It is intended to produce a liquid or soft-solid for personal cleansing comprising a hydrophobic blend of the clear vegetable oil gel and/or an opaque soft solid gel of vegetable oils and a shear thinning surfactant. A soft solid bar can also be prepared using this prepared polymer.

A Close Shave

Shaving compositions in the field of personal care are traditionally surfactant-based which can cause skin irritation and leave the skin unprotected when washed away with water. However, shaving compositions comprised of film-forming polymers are claimed to provide benefits such as “improved lubricity and post-shave skin feel.”²⁶

It has been disclosed that compositions that vary in lathering agents, film forming polymers, water soluble/dispersible surface active agents and optional components. These compositions vary in both the choice of materials and the concentrations and ratios of the materials used which allows for different forms of the product and different methods of application and shaving. The film forming system can be varied in the use and percents by weight of a lubricant polymer and substantivity polymer in which the substantivity polymer creates a mesh that entraps the lubricant polymer.²⁷ Limits to the variations of the lubricant polymer molecular weight are imposed by the solubility of the polymer.

The water dispersible or water soluble surface active agent is preferably one that is capable of forming lather and may comprise a soap, an interrupted soap, a detergent, an anionic surfactant, a non-ionic surfactant or a mixture of one or more of these. The concentration levels of the film forming materials and the surface active agents can be varied to provide desirable skin protection and post-shave feel. A postponing agent may be added which slows the evaporation of the solvent and keeps the composition lubricous.²⁸The way in which these components are varied can determine the form of the product (i.e., gel, liquid, cream, etc.), the razor blade type and the method of shaving.

Authors' Note
Our research results are based upon work supported by the National Science Foundation Partnerships for Innovation (PFI) Program under Grant No. PFI Award 0917730.Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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