Advanced nanotechnology applications in safe nail coatings

Nail polishes and coatings have evolved from ancient natural dyes to sophisticated modern formulations, serving both aesthetic and protective roles [6, p. 34-46]. Traditionally, a basic nail polish contains film-forming polymers (like nitrocellulose), plasticizers, resins, solvents, and pigments [7, p. 110]. In recent years, consumer demand and regulatory scrutiny have driven the industry toward “safer” nail products free of certain hazardous chemicals. At the same time, there is an increasing expectation for improved performance – longer wear, faster drying, richer color, and even therapeutic benefits.

Nanotechnology involves engineering materials on the scale of nanometers (1–100 nm) to exploit unique physicochemical properties that arise at that scale. In the cosmetics realm, nanostructured ingredients have already shown benefits such as enhanced UV protection, greater transparency, and controlled delivery of active compounds [10, p. 173]. Nanoparticles can also confer new features: nanoscale antimicrobials may prevent fungal infections on nails, and conductive nanomaterials could even transform nails into sensor platforms.

Materials and Methods. The human nail plate is a dense, keratinized structure about 0.25–0.6 mm thick for fingernails (up to ~1 mm for toenails) [2, p. 284-295]. It consists of tightly packed, dead keratinocytes arranged in three layers (dorsal, intermediate, and ventral) bonded by intercellular links and disulfide-rich proteins. Unlike the skin’s outer layer (stratum corneum), which is lipid-rich and hydrophobic, the nail plate behaves more like a hydrophilic hydrogel matrix with very low lipid content (≈0.1–1%).

Table 1

Structural and permeability characteristics of human nail layers

Water and other small polar molecules can slowly diffuse through the nail plate, whereas highly lipophilic or larger molecules penetrate with much more difficulty. Nanoparticles, by virtue of their size (>1–10 nm) and aggregation in formulas, are generally not expected to migrate through the healthy nail plate into systemic circulation. Indeed, studies on skin suggest that common cosmetic nanoparticles (ZnO, TiO₂, Ag, etc.) do not penetrate beyond the stratum corneum of intact skin. It implies that any potential toxicity of embedded nanomaterials will most likely arise from particles becoming dislodged (e.g. as dust) or from soluble ions/byproducts, rather than from particles infiltrating living tissue under the nail.

A standard nail polish is essentially a quick-drying polymer film in a volatile organic solvent base. Nitrocellulose has been the workhorse film-former since the 1920s, valued for its film strength and gloss [7, p. 110]. However, nitrocellulose films by themselves are brittle and prone to cracking. To counteract this, formulations include plasticizers (e.g. phthalates, adipates) to impart flexibility and resins) to improve adhesion and gloss. Pigments and fillers (<1% typically) provide color and texture, and suspending agents prevent pigment settling. These additives, while effective, introduced some health concerns – for instance, the aforementioned phthalates and formaldehyde resin are known endocrine disruptors or allergens. Nano-enhanced nail coatings aim to replace or augment certain conventional ingredients with nanomaterial alternatives that can provide equal or superior functionality more safely. For example, fumed silica (traditionally used in small amounts as a thickener) can be replaced or supplemented with nanosilica to strengthen the dried film and confer scratch resistance [3, p. 122040].

The remarkable effect of nanoparticles on coating performance is rooted in several physical and chemical mechanisms. First, nanoparticles have enormous surface area-to-volume ratios, which can increase interactions with the surrounding polymer matrix. Even at low weight fractions, well-dispersed nanoparticles can reinforce polymer films by hindering molecular motion and crack propagation – a concept from composite materials science. Empirical evidence confirms this: adding only 2% of nanosilica to a nitrocellulose lacquer was shown to nearly double the film’s modulus and increase its tensile strength by ~46%, while also improving flexibility. Second, many nanoparticles can fill in nanoscale voids in the polymer, yielding a more impermeable and smooth film. This can enhance gloss and reduce porosity, which not only makes the coating appear more uniform but can also slow the penetration of water or chemicals that might cause degradation. Third, nanomaterials often exhibit unique optical properties: for example, nano-TiO₂ and ZnO are transparent in visible light despite being efficient UV blockers.

Results and Discussion

Silica Nanoparticles (SiO₂)

Silica is a common additive in traditional coatings, often in sub-micron form to modify rheology or gloss. Nanosilica (typically 5–50 nm primary particle size) offers a powerful reinforcement effect when dispersed in polymeric nail enamels. These improvements were attributed to a homogeneous dispersion of silica and strong interfacial interactions at the polymer–particle boundary, which collectively resist deformation and crack propagation. Nanosilica can also influence polish appearance. If well-dispersed, silica particles below ~50 nm are smaller than visible light wavelengths and thus do not scatter light significantly – meaning a clear nano-silica top coat remains transparent. Some formulations use silica to achieve a matte finish. Additionally, silica’s presence can affect drying times: silica particles can facilitate solvent evaporation by wicking solvent to the surface (a molecular transport effect), contributing to faster set times.

Silver Nanoparticles (AgNPs)

Nanosilver has gained attention in nail products primarily for its antimicrobial properties rather than structural enhancement. In nail care, this has two notable applications: antifungal nail treatments and hygienic nail coatings. The AgNP-enamel exhibited potent antifungal activity against common nail pathogens and was proposed as an effective adjunct or alternative to traditional antifungal treatments. Notably, incorporating 8% of the HA-coated AgNPs into a commercial clear polish did not visibly alter the polish’s appearance or drying time, and the formulation remained stable for at least 21 days. Apart from antimicrobial action, silver nanoparticles can also serve as a pigment (imparting a gray or opalescent tint) and might improve polish hardness slightly due to being a rigid filler, though their primary role is not mechanical reinforcement. One study noted that a nanocomposite nail polish with silver showed significant reduction of Candida albicans growth on the coated nail [2, p. 284-295].

Graphene and Carbon-Based Nanomaterials

Graphene is a novel carbon nanomaterial with exceptional strength, flexibility, and electrical conductivity. Although still an emerging ingredient in cosmetics, its potential in nail coatings is intriguing. Graphene is typically supplied as either pristine graphene sheets or more often as graphene oxide (GO) or reduced graphene oxide (rGO) – both of which can be dispersed in liquids.

A graphene sheet within a nail polish film could act like a tiny reinforcing mesh. In particular, graphene-based nail coatings could be extremely resistant to cracking or abrasion due to the high tensile strength of graphene. In addition, because graphene sheets are so flexible and thin, they can form a conformal coating over the nail surface without visible particle texture [9]. A unique property of graphene is that it conducts electricity. A conductive nail coating has exciting implications – for instance, nails could serve as touch-screen styluses. Graphene and its oxide also strongly absorb UV radiation. Because graphene is carbon, its presence would naturally tint a polish gray or black at sufficient loading; thus it’s more likely to appear in pigmented nail products.

Titanium Dioxide Nanoparticles (TiO₂)

Titanium dioxide is a familiar ingredient in cosmetics, historically used as a white pigment (in its bulk form) and as a sunscreen agent (in nano form). In nail coatings, TiO₂ serves a few roles: as a pigment/opacifier, as a UV-protective additive, and as a whitening/brilliance agent. When TiO₂ particles are made nano-sized (~10–100 nm), they become less visible (due to reduced light scattering) and can even be transparent while still providing UV blocking. TiO₂ nanoparticles are quite hard (Mohs hardness ~6) and can reinforce coatings similarly to silica, though their higher density and tendency to aggregate means they must be used carefully to avoid settling and roughness. Still, a low loading of TiO₂ nano could potentially improve scratch resistance. Regulatory bodies like the EU’s Scientific Committee on Consumer Safety (SCCS) have studied nano-TiO₂ extensively and found it safe in cosmetic formulations up to 25% concentration, so long as it is not used in spray products (to avoid lung exposure).

Nanocellulose

Nanocellulose refers to cellulose that has been broken down into nanoscale fibers or crystals (e.g. cellulose nanofibrils or nanocrystalline cellulose). In nail coatings, nanocellulose could serve multiple roles: a rheology modifier, a film strengthener, and an eco-friendly binder. Nanocellulose fibrils in a liquid form a gel-like network even at low concentrations, due to extensive hydrogen bonding and high aspect ratio entanglement. This can help keep pigments and particles evenly suspended in a polish (prevent settling in the bottle) and give a nice consistency for application. Since cellulose is a polymer itself, albeit not film-forming on its own in a hard coat, its nano-elements can interpenetrate the primary resin. Additionally, nanocellulose may reduce the brittleness of nitrocellulose-based films by linking some of the nitrocellulose chains via hydrogen bonding, thereby increasing toughness. From a safety and environmental perspective, nanocellulose is non-toxic and derived from renewable sources (wood pulp, bacterial cellulose, etc.). Its inclusion in nail polish could appeal to eco-conscious consumers and reduce reliance on petrochemical ingredients. One challenge with nanocellulose is ensuring it mixes well with existing ingredients – it tends to be hydrophilic, so in a largely hydrophobic nail polish, surface modification might be required to make it compatible.

Table 2

Functional roles, benefits, and risks of nanomaterials in nail coatings

Consumers and professionals may be exposed to nail coating components through several routes: dermal contact (skin exposure), inhalation, ingestion (accidental), and transungual absorption. The introduction of nanoscale ingredients does not create entirely new exposure routes but can affect the extent and manner of exposure within each route.

During a manicure, wet polish can come into contact with the surrounding skin (cuticles, fingertips). Once the polish dries on the nail, direct skin contact is limited to any overflow on the nail folds or accidental touching of the painted nail to skin. Thus, under normal use, the skin exposure to nanoparticles from nail polish is quite low. Studies of nanoparticle skin penetration have largely shown that particles like TiO₂, ZnO, or polymeric nanocapsules do not penetrate intact skin beyond the outermost stratum corneum [2, p. 284-295]. They remain on the surface or within the upper dead cell layers, and can be washed off. Silica and TiO₂ are generally regarded as non-irritating to skin in insoluble particulate form [8, p. 1-22]. Nanosilver can cause irritation at high doses on skin, but cosmetics use low concentrations.

Inhalation is a significant route of exposure in the context of nail salons and polish application/removal. When applying nail polish from a bottle, volatile organic compounds (VOCs) evaporate and can be inhaled – these include solvents and monomers, but typically not nanoparticles, since particles are not volatile. If a nano-additive is present in the polish, the dust may contain nanoscale fragments or agglomerates of it. Inhalation toxicology studies indicate that TiO₂ nanoparticles in particular pose a risk for lung inflammation and have been classified as possibly carcinogenic by inhalation at high doses. Silica nanoparticles can also cause respiratory irritation if inhaled in free form. Still, precaution dictates that dust from filing nano-silica polishes should be minimized. Nanosilver inhalation could potentially cause argyrosis if done chronically, but this is more relevant to occupational exposure in nanoparticle manufacturing than occasional consumer exposure. For professionals, controlling dust and ensuring good ventilation are prudent, with or without nanotechnology in the products.

Direct ingestion of nail polish is uncommon (and certainly not intentional), but trace ingestion can happen by hand-to-mouth behavior. Studies have shown that oral ingestion of TiO₂ nanoparticles at low levels does not significantly harm humans. If someone accidentally swallowed bits of a nano-polish (e.g. a child chewing on a painted fingernail), the likely outcome is that the polymer and particles pass through the digestive tract with minimal absorption; nanoscale particles might have some small percentage absorbed via intestinal lining, but for insoluble oxides and silica, absorption is very limited.

One of the more unique exposure questions is whether nanoscale ingredients can penetrate through the nail plate into the nail bed or matrix. Small molecules (water, some antifungal agents) can diffuse slowly through it, but nanoparticles are orders of magnitude larger and generally will not traverse an intact nail. Consider a 20 nm silica particle: the nail plate’s keratin network has pores on the scale of angstroms to a few nanometers at most in the protein matrix, so a 20 nm rigid sphere is essentially stopped at the surface. Additionally, most nanoparticles in a polish are bound up within the hardened polymer film; they are not free to diffuse. This is an important point for safety: it means the primary risks of nanomaterials in nail coatings come from external exposure (skin, inhalation) rather than systemic absorption through nails

The rise of nanomaterials in cosmetics has prompted regulatory bodies worldwide to consider new safety assessment protocols and labeling requirements. Nail products fall under general cosmetic regulations in most jurisdictions, meaning they must be safe for human use under normal conditions, but do not typically undergo pre-market approval (except perhaps color additives in the US).

The EU has been at the forefront of nano-specific cosmetic regulation. Under EU Cosmetic Regulation No. 1223/2009, any ingredient that meets the definition of a nanomaterial must be clearly indicated in the product’s ingredient list with the word “(nano)” after the ingredient name [4, p. 24-32]. Moreover, manufacturers are required to notify the European Commission of the use of nanomaterials in products through the Cosmetic Products Notification Portal (CPNP) six months prior to marketing.

United States: The FDA regulates cosmetics but does not have a pre-market approval system (except for color additives). The FDA has issued guidance on nanotechnology, suggesting that if a cosmetic ingredient is nanoscale and shows novel properties, companies should conduct additional safety tests. The FDA’s stance is that existing legal frameworks are sufficient, but the agency “encourages” manufacturers to consult with them for nano-cosmetics.

Many other regions align with either EU or US approaches. Canada and Australia, for instance, generally treat nanomaterials as new ingredients if they have new properties. Australia’s industrial chemicals regulator (AICIS) has issued assessment requirements for certain nanomaterials. In the case of TiO₂ and ZnO, Australia permits them in sunscreens similarly to the EU, with specific rules for labeling if desired. In Japan, there isn’t nano-specific cosmetic legislation; they rely on overarching safety requirements.

Regardless of jurisdiction, a thorough risk assessment for nano-formulations typically covers: identity and characterization of the nanomaterial (particle size distribution, surface coatings, morphology), likely exposure levels (how much might contact skin or be inhaled), and toxicological data (dermal absorption, irritation, sensitization, genotoxicity, systemic toxicity, etc.). For nail products, dermal absorption is usually negligible, so systemic toxicity testing often focuses on inhalation (for salon workers, in particular) and any potential for nano-particles to reach systemic circulation if swallowed or used on damaged skin. Fortunately, the nanomaterials in current nail coatings have been somewhat well-studied: Nanosilica was found in one study to cause dose-dependent oxidative stress in skin cells in vitro, but in vivo a 90-day topical test in rats showed no significant internal toxicity [6, p. 34-46]. Nanosilver has been tested in various cosmetics – one study noted that topically applied silver nanoparticles in a gel had a safe toxicological profile, with minimal skin penetration and an observed ability for cells to cope with any oxidative stress by upregulating antioxidants [12, p. 747329].

In conclusion of the regulatory and safety outlook, current evidence supports that nanomaterials can be used safely in nail coatings provided they are appropriately formulated and certain exposure routes are controlled. Regulatory agencies, particularly in the EU, have laid down a framework that encourages innovation but demands responsibility: new nano-ingredients must be treated as new chemicals with full safety dossiers [3, p. 122040]. With ongoing scientific research and vigilant oversight, the advanced performance of nano-enhanced nail coatings can likely be achieved without compromising consumer or worker safety.

Conclusion

Nanotechnology is catalyzing a significant leap forward in nail coating technology, yielding products that can outperform traditional polishes in durability, functionality, and potentially even therapeutic value. The implications of these developments are multifaceted. For cosmetic product design, formulators can confidently explore nanomaterial additives to create nail coatings that not only beautify but also protect. From a public health perspective, the advent of longer-wear polishes and anti-fungal nanocoatings may reduce the incidence of nail damage and infections associated with cosmetic nail procedures, provided they are used correctly.

Looking ahead, future research directions should prioritize a few key areas. First, more longitudinal studies on the effects of chronic exposure to nail product nanoparticles (especially for salon workers) would be valuable to confirm the absence of harm or to identify any subtle risks. Second, the development of biodegradable and bio-sourced nanomaterials (like nanocellulose, chitosan nanoparticles, or protein-based nanospheres) could offer environmentally friendly alternatives to inorganic particles, aligning nail cosmetics with green chemistry principles. Third, interdisciplinary innovation can marry electronics and cosmetics: we may see nails as platforms for nano-sensors or as conductive surfaces for interacting with gadgets, all made possible by conductive nanomaterials like graphene.