The nano-revolution is in full swing across a broad range of industries, and the personal care industry is benefiting from the immense amount of progress in this area. Some recent hairstyling innovation using nanotechnology and microtechnology are reviewed in this article. These developments range from the lotus effect for frizz control and the development of liquid silk hair fixatives, to micro-segregating tough copolymers and supramolecular structures built from a scaffolding of soluble polymers that are joined together by tiny particles. The personal care industry has revolutionized sunscreens with nanoparticles, and the industry sector is moving apace with developments in this exciting emerging field.
Hydrophobicity for Hair Frizz
A major trend in hairstyling is the control of frizzy hair.1 Hair becomes frizzy when it is exposed to high humidity, and previous technologies for controlling frizz were based upon coating the hair with silicones. Recent products aim to control frizz for an extended period of time without adversely affecting other properties such as appearance, feel, volume, shine and softness.
Avon has approached this challenge by hydrophobically coating the hair with a composite that includes a hydrophobically surface-modified aluminum oxide (ranging from nanometer to micron size), a silicone acrylate film-former and a silicone fluid. In this producta the Avon researchers invoke the “lotus effect,”2 coined from the observation that water droplets ball up on the leaf surface of the lotus flower (Nelumbo). The small particles in the coating impart a surface roughness to the coating , and this enhances its hydrophobicity. The droplets roll off the leaves, taking with them any dirt that may have been deposited and conferring the property of self-cleaning on the leaf surfaces. Water droplets in air are round due to the high cohesion of the water molecules, which is manifested in a high surface tension. When placed on a surface, the forces of cohesion within the water compete with the forces of adhesion to the surface. If the adhesional forces are stronger than the liquid’s cohesion, then the liquid will spontaneously wet the surface and spread. However, if cohesion prevails, droplets will be formed on the surface, and if the contact angle of a water drop is greater than 90 degrees, the surface is characterized as being hydrophobic. Roughening of a hydrophobic surface will enhance the contact angle and the surface will appear to be more hydrophobic than its smooth analogue. Superhydrophobicity results when the surface possesses arrays of nanosize protrusions, as shown in Figure 1.3–5
Styling with Spider Silk
An increased consumer desire for smooth hairstyles has led to hair smoothing claims being emphasized by marketing. Smooth, high-hold fixatives can be achieved by combining VP/methacrylamide/vinylinidazole copolymer with aminosilicones.7 Procter & Gamble researchers have demonstrated that the high-holding capability of a wide variety of polymer fixative resins can be enhanced with photochemical post-crosslinking.8 The desire to make hair soft, smooth and shiny combined with the natural trend has led to the introduction of “liquid silk” as an ingredient that is substantive to hair.9 Spider silk is 10 times more elastic and three times tougher than high performance polyaramid fibers such as a para-aramid synthetic fiberb. However, commercial production of synthetic silk fibers has been limited by two factors: Spiders cannot be mass farmed because of their cannibalistic behavior, and they do not produce large quantities of silk.10, 11
The kind of spider silk that has been most investigated is called major ampullate, or dragline spider silk, which is the one normally used to anchor a web. The impressive mechanical properties of major ampullate spidroin (MaSp1) are due to the hierarchical polypeptide structure, as illustrated in Figure 2. Spider silk is like a block copolymer; it has sections of polyalanine (polyA) blocks and also glycine-rich regions. The ratio of two changes the type of silk. Stretchy silk has more glycine-rich regions and less crystallinity. The mechanical properties of silk may be attributed to the alternation of both polyA and glycine-rich (G-rich) domains in addition to the supramolecular self-assembly of these domains into higher ordered structures.12–15
Silk protein molecules self-assemble in solution17 due to the hydrogen bonding and hydrophobic interaction between their antiparallel beta-pleated sheets.18 These interactions promote film formation, and that property is being used to form films of “liquid silk” on hair to confer both styling and smoothness. There is a wide variety of silk proteins produced by thousands of different species of spiders and insects.19 However, only a few of these proteins have been well characterized. The cocoon silk (silk fibroin) of the silkworm Bombyx mori and the dragline silk of the orb-weaving spider Nephila clavipes have been well studied. Silk from these sources is insoluble and it must be hydrolyzed to render it useful as an ingredient in hair care, but these proteins can now be sourced from recombinant DNA technology, which can produce these proteins by microbial expression.20
Spider silk exhibits super-contraction upon exposure to moisture, and it has been speculated that this prevents the web from sagging and collapsing when wet with rain or dew.21 In this respect, silk displays a stimuli-responsive characteristic that would be useful in hair fixation, as hairstyles flatten in humid conditions. When dragline silk is spun, the molecules in the glycine-rich region are extended and “fixed” by hydrogen bonds between the molecular strands. This ordered, extended conformation has reduced conformational entropy. Upon exposure to moisture, water molecules compete for and sever the hydrogen bonds, and the silk “entropy spring” retracts, as pictured in Figure 3.22 Alas, the highly ordered structures seem to be formed during the spinning process. Perhaps someday, the spinnerets (spinning organ) of spiders will be emulated to form silk hairsprays that maintain hairstyles regardless of the level of humidity.
In an age when inkjet printers have been developed to selectively cover skin blemishes,23 the development of spinneret applicators for hair fixative compositions may not be far away. Indeed, some researchers are already spinning, electrospinning and centrifuge spinning recombinant spider silk to obtain fibrous, sheet-like structures.24
Macromolecular Compatibility and Controlled Segregation to Confer Multifunctionality
The styling hold of strong-holding hairstyles is determined primarily from the type and amount of styling polymer used. For strong-hold, humidity-resistance, low stickiness and high manageability are necessary. A reasonable degree of conditioning is also desirable. There is a trend to move toward fixative polymer blends to achieve the high hold without compromising other aesthetics. For example, these properties have been achieved by combining amphipathic cationic polymers with chitosan as a stabilizer to maintain homogeneity in the formulation and to produce the desired rheology for application.25 Blending polymers to form a film is not straightforward because the entropy of mixing is small and thus, in the absence of strong enthalpic interactions, segregation of the two polymeric components often occurs as solvent evaporates. This is especially the case for stiff-backboned polymers such as chitosan. Moreover, chitosan is soluble only in acidic solutions, and this limits the degree of formulation freedom to prepare a blend of water-soluble, fixative macromolecules that will resist moisture.
Polymers do not mix well molecularly, and as the number of polymers in a formulation increases, the probability of finding a compatible composition decreases. However, researchers26 have managed to create stable, high-hold formulations that contain two distinct copolymers. The polymers are acrylates/C1-2 succinate/hydroxyacrylates copolymer and octylacrylamide/acrylates/butylaminoethyl methacrylate copolymer. These formulations retain flexibility, elasticity and plasticity.
The entropic demixing of polymers is used advantageously in block copolymers. In this case, the dissimilar blocks are covalently bound to each other, and through micro/nanophase separation, the phases can be self-assembled into a variety of precise structures, each with distinct mechanical, optical and solubility characteristics.27 Until recently, the application of this technology to personal care was limited because anionic polymerization was the only available production technology. Anionic polymerization is limited to few monomers, and this severely limited the possible diversity of block copolymers that could be produced for personal care applications. This situation was changed with the introduction of living free radical polymerization, which has broadened the variety of possible block copolymers that can be produced. Block copolymeric, stimuli responsive emulsifiers with film-forming capability are being created for the personal care industry.28
Eastman Chemical has introduced a multifunctional clear gel that is probably a supramolecular micro- or nanocomposite. They achieve this by combining the particulate film-former anionic polyester-5 with the cationic polysaccharide polyquaternium-10. Clear gels are obtained that deliver styling with a soft feel.29
Controlling structures at the nanoscale can lead to the development of innovative properties for materials used in hairstyling, and this opens the door to new styling opportunities. Arrays of hydrophobic nanoparticles on the surface of hair have been developed to prevent the intrusion of water into the hair and thereby to prevent frizz. Genetically engineered silk analogs are being used to create styling with smooth feel, and as this technology advances, there can be hope for paradoxical properties of hair fixatives, such as strengthening in the presence of moisture to counteract the flattening of hairstyles on humid days. Polymer blends are increasingly being utilized to confer high-hold with good aesthetics. In this instance, polymers are often incompatible due to their low entropy of mixing, and the enhancement of properties by polymer blending is not a straightforward exercise in formulation. Supramolecular assemblies of oppositely charged polymers and particles may lead to the introduction of hair set with soft feel. Nanostructuring of materials offers the promise of revolutionary new products that could overcome the compromise in benefit delivery that is often accepted with conventional materials.
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23. US Patent Application 20110155161, Methods of treating keratinous material, and apparatus for implementing such methods, H Samain, assigned to L’Oréal (Jun 30, 2011)
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