The Chemistry of Magnetic Nail Polish

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The other day I was reading See Arr Oh's excellent "Just Like Cooking" blog and came across a post on magnetic nail polish.  In essence, you apply the magnetic nail polish to your nail, hover a magnet over the still wet nail, and a pattern will be created in relation to the magnetic field lines of the magnet. Since I typically don't paint my nails (except for special occasions :-) ), I had never heard of such a thing, but it sounded like an interesting and novel product that likely would be patented, or at least patent pending.

Sure enough, after a brief search I found a family of seven pending US patent applications (see list at the bottom of this post) directed to magnetic cosmetic compositions.  The pending applications are all assigned to L'Oreal, which is the parent company of Lancome.  Notably, Lancome appears to be the first company that marketed magnetic nail polish under the brand name Le Magnetique in 2007.

Although many different companies have marketed magnetic compositions since 2007, including Sephora and LCN (as pointed out in See Arr Oh's post), it appears that Lancome's Le Magnetique started the trend, and indeed, L'Oreal has sought patent protection for this technology at least as early as October of 2004.

The beauty of patents (and patent applications) is that they fully disclose, albeit in a somewhat confusing manner sometimes, a commercial technology that otherwise would be a mystery in a variety of respects.  Just think of Coca-Cola.  Although every can of Coke has an ingredient list on the back, the taste of Coke arguably has never been duplicated, since important details regarding the ingredients are missing, such as the ingredient proportions, processing conditions, and the source of ingredients.

In the present case, while the various ingredients contained in a magnetic nail polish are disclosed to consumers, there are important details missing that cannot be determined from an ingredient list alone.  However, if we look to the patent applications directed to this type of technology, the purpose of each component is typically explained, we find example compositions detailing the exact proportions of ingredients, and we are provided with an expanded view of the potential scope of this technology, for example, the identities of the various dyes that can be used, the physical structure of the magnetic particles, and the potential use of these magnetic compositions in cosmetics other than nail polish.

Of course, the compositions disclosed in these patent applications are only representative, and it is highly likely that the magnetic compositions have been changed/optimized prior to L'Oreal bringing the products to market.

With all of this in mind, we come to our question of the day...what's the chemistry of magnetic nail polish as taught by L'Oreal's patent applications?

The magnetic particles are not necessarily just iron powder tossed into the composition, but rather there is a lot of detail regarding the magnetic particles that is not readily apparent from the ingredient list.

For example, the magnetic particles can be made of any material that can be moved under the effect of a magnetic field, which includes nickel, cobalt, iron, and rare earth metals, such as gadolinium, terbium, dysprosium, and erbium.  The metals can be employed in any state that has a magnetic susceptibility, including as an alloy or an oxide.  The most preferable type of magnetic particle appears to be magnetite, i.e., Fe₃O₄ or iron(II,III) oxide.

The magnetic particles preferably are aspherical, i.e., non-symmetrical, such that the longitudinal axis can be aligned by a magnetic field, thereby changing the appearance of the composition.  For example, the application of a magnetic field to oblong-shaped magnetic particles will orient the particles in one direction, likely causing the composition to have a visible pattern.  In one embodiment, the magnetic particles can be employed in the composition as a ferrofluid, i.e., a stable colloidal suspension of magnetic nanoparticles.

The magnetic particles can also be employed in the composition as organic/inorganic composite fibers.  For example, the magnetic particles, such as Fe₃O₄, can be dispersed on the surface of a polymeric fiber, or the particles can be embedded therein to form a matrix.  The composite fiber optionally can be coated with a clear or colored membrane, so as to provide a colored composite fiber and/or to provide a barrier between the fiber and the external environment.  In another embodiment, chains of covalently-linked polystyrene particles embedded with iron oxide can also be used.

By employing composite fibers or polymeric particle chains in this manner, it appears that the magnetic particles may collectively "drag" the fibers along under the influence of a magnetic field, as opposed to simply moving the magnetic particles individually.

The patent applications also describe a variety of dyes/coloring agents that can be employed in the magnetic compositions, including goniochromatic, photochromic, thermochromic, and/or piezochromic (in particular tribochromic and solvatochromic) coloring agents.  These are just fancy words that mean the color of the composition can appear to change based on viewing angle (e.g., in the case of diffractive pigments and nacres - think pearls), or the composition will actually change change color in response to an applied external stimulus, such as ultraviolet light, temperature, or pressure.

Reflective particles (similar to glitter) can also be included in the magnetic composition.  The reflective particles can be made of inorganic substrates, such as silica or alumina, which are coated with a metal or metallic material, such as silver or a nickel/chromium/molybdenum alloy.

The magnetic nail polish composition preferably includes volatile organic solvents, such as alcohols (e.g., ethanol or isopropanol), glycols (e.g., ethylene glycol or glycerol), and short-chain esters (e.g., ethyl acetate).  The purpose of the volatile organic solvent is to make the composition fluid for a short period of time after application, such that the magnetic particles can easily move under the influence of an applied magnetic field.  Upon drying, however, the magnetic particles will be locked into place in the dried composition, thereby setting the pattern that has been created.

To aid the composition in "locking down" the magnetic particles and the other various components of the composition, a film-forming polymer typically is included.  Examples of film-forming polymers include nitrocelluose and polycondensates.  Polycondensates are polymers formed by a polycondensation reaction and include copolymers of adipic acid, neopentyl glycol, and trimellitic anhydride.

Various other components can be included in the magnetic composition, such as thickeners (e.g., clays), and plasticizers (e.g., arylsulfonamides), but these are standard components included in most types of cosmetics, and thus are not very interesting for the purposes of this post.

A typical magnetic nail polish composition is as follows:

Nitrocellulose (11 wt.%)
N-ethyl-o,p-toluenesulfonamide (5 wt.%)
Alkyd resin (10 wt.%)
Isopropanol (4 wt.%)
Magnetic pigment (0.5 wt.%)
Butyl acetate / ethyl acetate 50/50 mix (balance)

The magnetic pigment in this example is Colorona Patina Gold sold by EMD Chemicals, which is described as a dark golden lustrous powder composed of mica, TiO₂, Fe₂O₃, and Fe₃O₄.

The magnetic compositions are not limited only to nail polish in these patent applications, but variants of the compositions can be applied to any keratinous substrate, such as skin, lips, and hair.  One specific application noted is the use of magnetic compositions in foundation makeup (see Figure at right).  For example, after the magnetic foundation is applied, any sharp transitions between light and dark colors can be smoothed with a magnet so as to blend and soften the edges/transitions of the makeup.

I could go on and on about the magnetic compositions disclosed in these patent applications, since they really are quite fascinating, but if you're interested in more details about this technology, please read the patent applications listed below.

It appears the US Patent Office is close to granting these patent applications (as judged by the prosecution history on PAIR), and if that's the case, we won't be seeing any competitors products sold in the US anymore, unless they are licensed by L'Oreal (or if the competitors happen to design around the patent claims or think the patent claims are invalid for some reason).

Here are all of the pending US patent applications relating to magnetic cosmetic compositions assigned to L'Oreal:

US 2006/0088484 (more general disclosure of compositions comprising magnetic particles)
US 2008/0044443 (magnetic compositions comprising dyes that absorb visible light)
US 2008/0050324 (magnetic compositions comprising diffractive pigments)
US 2008/0105272 (magnetic compositions comprising interferential pigments)
US 2008/0124288 (magnetic compositions comprising coloring agents sensitive to an external stimulus)
US 2008/0127990 (magnetic compositions comprising reflective particles)
US 2009/0130037 (magnetic compositions comprising a volatile solvent)

Silica-Based Sensors (US Patent 8,084,001)

U.S. Patent 8,084,001 (Burns et al.) issued December 27, 2011 and is titled "Photoluminescent Silica-Based Sensors and Methods of Use."  Burns et al. is assigned to the Cornell Research Foundation in Ithaca, NY.

Photoluminescent-based assays are highly desirable in sensing applications as a result of their excellent contrast, high specificity, and fast response times.  The workhorses of these systems are photoluminescent dyes that are sensitive to changes in their environment.  Typically these dyes have been employed in a variety of sensing platforms, including free in solution, thin films, membranes, and optical fibers.  However, many of these platforms can affect the responsiveness of these dyes, e.g., their ability to respond to environmental conditions or analytes.  In view of these drawbacks, Burns et al. seeks to provide a sensing system that can operate in a variety of environments.

Burns et al. discloses silica-based sensors comprising a reference dye and a sensor dye.  The emission spectrum of the reference dye is substantially insensitive to environmental conditions and/or analytes, whereas the emission spectrum of the sensor dye has a known response to a given external condition and/or analyte.  Thus, the reference dye serves as an internal standard to which the response of the sensor dye can be compared, much the same way that the residual solvent peak in proton NMR serves as an internal standard to which the proton resonances of an analyte are compared.

The silica-based sensors can have various structural configurations, but typically comprises a silica core surrounded by one or more outer shells.  The outermost shell typically contains the sensor dye to allow interaction with the external environment, whereas the reference dye is typically included in the silica core or an intermediate shell to prevent the reference dye from contacting the environment.

The periphery of the silica can be tailored to allow targeted interactions to take place.  For example, the silica can be functionalized with amino groups thereby imparting a positive charge to the periphery of the silica, which promotes adhesion of the amino-functionalized silica to cell surfaces.  Alternatively, the silica surface can be functionalized with hydrophobic groups, which allows the silica to intercalate within the lipid bilayer of a cell wall, for example.

The sensor dyes can be sensitive/responsive to a wide range of external stimuli, such as hydrophobicity/hydrophilicity, presence or absence of metal ions, changes in pH, or identification of small molecules such as oxygen.

One specific disclosed potential application is in vivo or in vitro determination of the redox state of a cell.  It is well-known that the internal cell environment is typically a highly regulated reducing environment.  Any deviation from this reducing state could indicate the presence of a disease, such as cancer, or an injury, such as stroke.

Sensing environmental conditions and/or analytes is typically performed by (1) comparing the emission intensity ratios of the reference and sensor dyes, (2) comparing spectral shifts of the reference and sensor dyes (i.e., the difference between lambda maxes), and/or (3) comparing the excited state lifetimes of the reference and sensor dyes.  Of course, the emissions of these dyes in various environments must be previously known via the construction of a calibration curve in order to be able to identify a certain condition or analyte in an unknown solution/medium.

The sole example in Burns et al. demonstrates the determination of the pH of an unknown sample.  A silica-based sensor is first synthesized by covalently attaching a tetramethylrhodamine reference dye to a silica core, followed by layering over this core a shell comprised of a fluorescein sensor dye covalently bonded to silica.  A calibration curve is then determined for this sensor by measuring the ratio of the reference and sensor dye emission intensities over a pH range of 5-9.  Thus, when this sensor is added to a solution having an unknown pH, the ratio of emission intensities of the reference and sensor dyes can be measured and compared with the calibration curve, thereby determining the pH of the solution.

Burns et al. only contains product claims.  Claim 1 is very broad and minimally requires (1) a reference dye and a sensor dye that are chemically different, (2) at least one of these dyes is covalently attached to a silica core, (3) the reference dye has a relatively constant emission in different environments, and (4) the sensor dye exhibits different emission in different environments or in the presence of varying concentrations of analytes.

As filed, the application originally also contained method claims that were restricted out and ultimately canceled in response to the restriction requirement.  Two methods were claimed:  (I) a method of determining an analyte in an unknown environment, and (II) a method of introducing a silica-based particle into an organism.  Perhaps these method claims will appear in a divisional application at some point in the future, though no related applications are currently listed in PAIR.