Small, Smaller and Nano Materials: An Invisible Benefit

Most nanoscale materials, whether engineered or natural, fall into one of four categories: 1) metal oxides such as zinc and titanium that are used in ceramics, chemical polishing agents, scratch-resistant coatings, cosmetics and sunscreens; 2) nanoclays, which are naturally occurring, platelike clay particles that strengthen or harden materials or make them flame retardant; 3) nanotubes, which are used in coatings to dissipate or minimize static electricity; and 4) quantum dots, used in exploratory medicine or in the self-assembly of nanoelectronic structures.

Generally, nanoparticles used in cosmetics such as titanium dioxide and zinc oxide belong to the first category and are employed as sun filters. Both types of nanoparticles are engineered and have a regular shape; it is this regular shape that makes them useful in cosmetics. At least two aspects here are important: the absolute size of the nanoparticles as well as their surface properties, which determine their efficacy as protective sun filters.

Particle size determines the relative surface area. The surface area of one cubic centimeter of a solid material is 6 cm² whereas that of one cubic centimeter of 1-nm particles in an ultrafine powder is 6,000 m²—literally one-third larger than a football field.¹ Surface area is important because most chemical reactions involving solids happen at their surfaces where the chemical bonds are incomplete. On the other hand, the surface properties of metal oxides such as titanium dioxide are also unique.²

Titanium dioxide and zinc oxide are examples of so-called inorganic or physical sun filters that exist as solid particles in sunscreens. Organic or chemical sun filters, however, are either liquid at room temperature or solubilized in the oil phase of sunscreen formulations. Inorganic filters provide broad-spectrum protection against both UVA and UVB, whereas organic sun filters are usually classified as either UVB or UVA filters. The difference between titanium dioxide and zinc oxide is that titanium dioxide is optimized for UVB attenuation to give protection from sunburn, whereas zinc oxide is optimized for UVA attenuation to give protection from UV-induced aging. Since the sun protection factor (SPF) is related to UVB protection, titanium oxide is more effective in terms of SPF at a given concentration.³

Figure 1 depicts the light attenuation properties for titanium dioxide of various mean particle sizes. Pigmentary titanium dioxide, with a crystal size of around 220 nm, gives UV protection but also attenuates visible light and hence appears white on the skin. Reducing the crystal size increases UV attenuation and reduces visible attenuation. The optimum particle size for UVB and UVA attenuation but good transparency in the visible region ranges from 40–60 nm. To achieve complete visible transparency, the particle size can be reduced further to ~20 nm; however, such a small particle size provides little UVA or UVB attenuation and is therefore ineffective as a sunscreen.³

Formulating with Nanoparticles

Modern cosmetic sun protection products use both organic and inorganic sun filters but the choice depends on the final product and application. Whereas organic sun filters are easier to formulate with and provide high efficacy at low concentrations, cocktails of different sun filters are required to obtain high SPFs. High concentrations of multiple filters are necessary to achieve such levels, which with solid filters that are difficult to solubilize, may create formulation problems. In addition, some organic sun filters decay upon exposure to UV light but inorganic sun filters are photostable and can achieve high SPF levels using a single filter (titanium dioxide). Powders can be difficult to formulate with but dispersions also exist to make these filters much easier to incorporate into formulations without a loss of efficacy due to coagulation of the active.

One remaining disadvantage of inorganic filters is that they may leave a white shine on the skin, which can be resolved by controlling the particle size and particle size distribution of the organic filter. When both are optimized and controlled, transparency can be obtained without a loss in sun protection efficacy as measured via SPF (see Figure 2). As a consequence, the use of nanosized sun protective nanoparticles has risen dramatically over the last decade.

Nanoparticle Safety

With the rising use of nanoparticles in cosmetics, concerns regarding their safety have also grown.⁴ Indeed, epidemiological studies consistently show that increases in atmospheric particulates from road transport (60%) and combustion processes (23%) lead to short-term increases in morbidity and mortality via their inhalation.⁵ However, nanoparticulates for sunscreens are intentionally produced and follow a different route of entry into the body. Therefore, to assess their risk, exposure to skin and ingestion must also be considered as alternative routes of uptake. The skin seems logical as a port of entry into the body for cosmetics, whereas ingestion may seem irrelevant, but it cannot be ignored since nanoparticles that are not fully absorbed into the skin may also end up in wastewater where they can be incorporated into organisms that are part of the human food chain.

The appropriate question to ask is not whether these nanoparticles can be toxic but whether they are in fact toxic at the concentrations that humans are exposed to, intentionally or unintentionally. The following section examines a sampling of the literature that assesses the potential for exposure to nanoparticles via penetration through dermal routes.

Assessing Skin Penetration

It is impossible to assess the absolute safety of something since one cannot demonstrate the presence of an absence. However, it is possible to measure whether or not something penetrates the skin. Here it becomes important to define what skin penetration means in relation to the potential toxicology of a nanoparticle. After all, if titanium dioxide is deposited onto the skin surface, it has been effectively delivered from a cosmetic efficacy point of view. Yet from a toxicological point of view, where the nanoparticles may interact with the living tissue in a harmful manner, it has not been delivered since the viable tissues have not been reached. The skin delivery of nanoparticles in the context of toxicology is therefore defined as reaching the viable epidermis and dermis. However, it should be noted that the lack of delivery of nanoparticles to the viable tissues does not imply safety.

Many experimental studies,^6-8 including one referencing some 30 articles,⁶ have investigated the skin delivery of nanoparticles. The majority of these papers reached similar conclusions—that nanoparticles do penetrate the stratum corneum (SC), where they can be visualized, but do not penetrate deeper into the viable layers of the epidermis and dermis. At the same time, the infundibulum often acts as a reservoir, and nanoparticles accumulate there until they are removed with sebum flow. Only a few papers suggested that skin penetration of nanoparticles does occur but in those cases, the observed skin penetration could be explained from the experimental or analysis methods used.⁶ Some investigators subsequently revised their methodology to overcome these artifacts and found no skin penetration beyond the SC in the modified experiments.^{7, 8}

However, it must be stressed that almost all of these papers discussed the skin penetration of microfine titanium dioxide or zinc oxide, whereas skin penetration of other nanoparticles was far less extensively studied—the only exception being quantum dots, which are much smaller than microfine titanium dioxide and zinc oxide.

Gaps in Understanding

The main gaps in knowledge regarding the skin penetration of nanoparticles, as of the end of 2008, were elegantly expressed by Baroli, who stated, “Extensive exposure of skin to these nanotechnological products has raised the question as to whether nanoparticles could penetrate skin, be eventually absorbed systemically, and more importantly be responsible for acute/chronic and/or local/systemic side effects. This concern is not hypothetical when one considers that (i) skin is nanoporous at the nanoscale, (ii) orifices of hair follicles and glands open on skin surface, providing alternative routes of entrance, and (iii) in everyday life skin may be damaged by detergent exposure, scratches, hydration or dryness, sunburn, or pathological states.”⁹

The identification of this knowledge gap came as a result of another study by Baroli and colleagues¹⁰ investigating whether superficially modified iron-based nanoparticles, not designed for skin absorption but whose dimensions were compatible with those of skin penetration routes, were able to penetrate and perhaps permeate the skin. Experiments were carried out with healthy female abdominal skin samples in vertical diffusion cells exposed to nanoparticles for a maximum of 24 hr. Two different formulations were tested. The first consisted of γ-maghemite (Fe₂O₃) nanoparticles coated with tetramethylammonium hydroxide (TMAOH) and dispersed in an aqueous solution of TMAOH (TMAOH nanoparticles). The second was composed of iron (Fe) nanoparticles coated with sodium bis(2-ethylhexyl) sulfosuccinate (AOT) dispersed in an aqueous solution rich in AOT (AOT nanoparticles).

Nanoparticle characterization revealed that TMAOH nanoparticles were as small as 6.9 ± 0.9 nm, and had an isoelectric point of 6.3. In contrast, AOT nanoparticles were not homogeneous in size, although 51.1% of these nanoparticles had a diameter of 4.9 ± 1.3 nm. Results showed that nanoparticles did not cross the skin but were nonetheless able to penetrate into it. Penetration occurred through the lipid matrix of the SC and hair follicle orifices, allowing nanoparticles to reach the deepest layers of the SC, the stratum granulosum and hair follicles. In some exceptional cases, nanoparticles were also found in the viable epidermis (see Figure 3).

Independently, another Italian group studied the skin penetration of silver nanoparticles through intact and damaged human skin using Franz diffusion cells. This data showed that silver nanoparticle absorption through intact skin was low but detectable, and that there was an appreciable increase in permeation using damaged skin.¹¹ In the meantime, other studies emerged in the literature that indicated that there was no skin penetration of nanoparticles.^{12, 13}

This raises the question of what caused some nanoparticles to penetrate, albeit seemingly only in minor quantities,^{10, 11} whereas others did not penetrate?^{12, 13} First of all, the size of the skin-penetrating nanoparticles was significantly smaller than those used in cosmetics (i.e., titanium dioxide and zinc oxide); second, although all nanoparticles tested were metallic, the penetrating nanoparticles used elements other than titanium¹² or zinc,¹³ namely iron¹⁰ and silver.¹¹ Third, the formulations of these iron and silver nanoparticles were radically different from cosmetic formulations and were demonstrated to have an effect on skin barrier function. Detergent properties of the AOT-rich aqueous solution in which AOT nanoparticles were dispersed could have been the principle cause of AOT nanoparticle penetration.^{9, 10}

These observations lead to the identification of further knowledge gaps regarding the skin penetration of nanoparticles, begging the questions: What is known about the potential skin penetration of very small nanoparticles—i.e., those below 10 nm? What are the effects of real-life conditions such as UV radiation, abrasion, skin damage, etc. on the skin penetration of nanoparticles? And finally, why is the use of nanoparticles as skin delivery systems advocated if the particles do not penetrate into skin?

Not surprisingly, the majority of experimental studies published in 2008 and 2009 dealt with one or more of these outstanding issues and lead to the identification of even more knowledge gaps. A mere sampling of these studies is described here. One of the most important findings since the January 2009 review⁸ is the realization that pig skin, which is normally an excellent model for skin penetration, is an inappropriate model for nanoparticle penetration. Quantum dots have been found to penetrate beyond the SC in pig skin but not in human skin or rat skin. This anomaly has added significantly to the controversy over the skin penetration of nanoparticles.

Real-life Conditions

A few real-life conditions were investigated that could influence the skin penetration of nanoparticles; these included UV radiation; mechanical stretching, flexing and massaging of the skin; skin damage; and the composition of the formulation, something well-known to affect the skin penetration of active ingredients.

UV radiation: Mortensen et al.¹⁵ investigated the effects of UV radiation on the in vivo skin penetration of quantum dot nanoparticles in a mouse model. Carboxylated quantum dots were applied to the skin of SKH-1 mice in a glycerol vehicle with and without exposure to UV radiation. The authors also deliberately mimicked various conditions; for instance, the nanoparticles were negatively charged like the metal oxides used in cosmetics, which affects their capability to adhere to the negatively charged skin surface.

In contrast to anionic nanoparticles, cationic nanoparticles showed a clear affinity for the skin surface and delivered a significantly greater amount of model active into the SC.¹⁶ The size of the quantum dots used was ~33 nm, which is in line with the size of nanoparticles used in cosmetics since individual nanoparticles aggregate and agglomerate to form much larger units, which are typically > 100 nm.¹⁷ The investigators noted a trend of increased penetration with UV radiation but under no circumstances did they find evidence for massive quantum dot penetration, even for UV radiation-exposed mice 24 hr after quantum dot application.

Flexion and abrasion: The flexion and abrasion of the skin, to reflect damaged skin, on the penetration of nanoparticles was investigated by Zhang and Monteiro-Riviere,¹⁸ who concluded that barrier perturbation by tape stripping did not cause skin penetration; however, abrasion allowed quantum dots to penetrate deeper into the dermal layers. In a second study, the same group measured the penetration of a fullerene-substituted phenylalanine derivative of a nuclear localization peptide sequence (Baa-Lys(FITC)-NLS) through dermatomed porcine skin that was flexed for 60 or 90 min or left unflexed (control). Confocal microscopy depicted dermal penetration of the nanoparticles at 8 hr in skin flexed for 60 and 90 min, whereas Baa-Lys(FITC)-NLS did not penetrate into the dermis of unflexed skin until 24 hr (see Figure 4).¹⁹

In addition, skin flexed for 90 min showed evidence of dermal penetration after 8 hr of nanoparticle exposure, whereas control samples showed evidence of fullerenes primarily localized in the epidermis and only slight amounts in the dermis after a 24 hr treatment. These results suggest that the action of flexing increases the rate at which these nanoparticles can penetrate through the skin, as well as the amount of nanoparticles capable of penetrating into the dermal layers of the skin.¹⁹ The latter is not surprising if one combines this finding with the statement of Wu et al.,¹⁶ whose results “confirmed an apparent affinity between the vectors and hair follicular structures,” along with previous publications of the Lademann group describing that massage and flexing stimulate the opening of hair follicles and so increase transfollicular delivery.²⁰

Formulations: To examine the influence of formulations on the skin penetration of nanoparticles it is important to first clarify what components in a formulation are being examined for their influence. One approach assesses the influence of the “remainder” ingredients in the formulation; i.e., all the components other than the active ingredient or drug. Another approach looks at the coating of the nanoparticles. For instance, Ryman-Rasmussen et al.¹⁵ studied three different surface coatings of nanoparticles—a neutral one, a cationic one and an anionic one—and found that an anionic coating penetrated much slower into porcine skin than cationic or neutral coatings, although this depended on the size of the quantum dots used since the smaller ones did not show this difference (see Figure 5). With regard to the influence of the remainder of the formulation on the skin penetration of nanoparticles, which has hardly been investigated, the literature reveals only that nanoparticles were dispersed in various mediums ranging from water^{10, 21, 22} and o/w emulsions,^{23, 24} to synthetic sweat²⁵ and caprylic/capric triglycerides,¹³ among others. With such a series of tests on unrelated vehicles containing different particles with different sizes at different loadings, it is difficult to conclude anything regarding the influence of the additional vehicle components.

Delivery Implications

As the literature has suggested, there is a difference in skin penetration characteristics of nanoparticles depending upon the chemical—i.e., nanoparticles containing chromium,²⁵ silver,¹¹ titanium dioxide^{12, 23, 26} and zinc oxide^{13, 24, 26} did not penetrate deeper than the SC. On the other hand, quantum dots with a cadmium-selenide or cadmium sulfide core were shown to penetrate into the deeper skin tissues (see Figure 5),¹⁵ as were fullerene-based nanoparticles.¹⁹

A third category of nanoparticles has been built with the purpose of carrying drugs and actives into the skin. Some of these are polymeric in nature^{21, 24, 27, 28} but by far, the majority is composed of lipids such as stearic acid²⁹ or mixtures of fatty acids.^{30, 31} As these so-called solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs) tend to operate via release of their active rather than penetration of the lipid carriers themselves, they fall beyond the scope of this review and interested readers are referred to the literature.^30-32

Conclusion

In summary, nanoparticles can penetrate into the skin but until now, no penetration of the nanoparticles used in cosmetics has been found beyond the SC. It is beyond any doubt that the use of nanoparticles in cosmetics is cosmetic—i.e., they assist in creating a transparent and cosmetically acceptable sunscreen formulation. The smaller the nanoparticles, the more transparent they become but at the same time, they lose their protective characteristics.

The extensive review of recent papers clearly suggests that the nanoparticles used in cosmetics—i.e., TiO₂, ZnO and Ag, do not penetrate skin beyond the SC. This same conclusion was made about one year ago;⁸ however, enormous progress has been made that explains controversial findings from the past, which turn out to be mainly related to experimental conditions.

It is clear that the only way to study the skin penetration of nanoparticles is to perform in vivo experiments on human skin using cosmetic formulations and cosmetic nanoparticles. Such studies have been performed and show that nanoparticles used in cosmetics do not penetrate skin.26 More studies will be required to confirm these findings independently but all evidence confers to the direction that nanoparticles used in sunscreen formulations under normal conditions penetrate into the uppermost layers of the SC, and that whatever penetrates deeper is below the limits of detection. Hence, significant penetration towards the underlying keratinocytes is unlikely.²⁶

References

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