Sunscreens containing titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticles (NP) are effective barriers against ultraviolet B (UVB) damage to skin, although little is known about their disposition in UVB-damaged skin. Pigs were exposed to UVB that resulted in moderate sunburn. For in vitro studies, skin in flow-through diffusion cells were treated 24 h with four sunscreen formulations as follows: 10% coated TiO2 in oil/water (o/w), 10% coated TiO2 in water/oil (w/o), 5% coated ZnO in o/w, and 5% uncoated ZnO in o/w. TiO2 (rutile, crystallite) primary particle size was 10 × 50 nm with mean agglomerates of 200 nm (range ca. 90nm--460nm); mean for ZnO was 140 nm (range ca. 60--200nm). Skin was processed for light microscopy, scanning (SEM) and transmission electron microscopy (TEM), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). UVB-exposed skin had typical sunburn histology. TEM showed TiO2 NP 17 layers into stratum corneum (SC), whereas ZnO remained on the surface. TOF-SIMS showed TiO2 and ZnO epidermal penetration in both treatments. Perfusate analyzed by TEM/energy dispersive x-ray spectroscopy or inductively coupled plasma mass spectrometry detected no Ti or Zn, indicating minimal transdermal absorption. In vivo, skin was dosed at 24 h occluded with formulations and at 48 h. TiO2 NP in o/w formulation penetrated 13 layers into UVB-damaged SC, whereas only 7 layers in normal skin; TiO2 in w/o penetrated deeper in UVB-damaged SC. Coated and uncoated Zn NP in o/w were localized to the upper one to two SC layers in all skin. By SEM, NP were localized as agglomerates in formulation on the skin surface and base of hair. TOF-SIMS showed Ti within epidermis and superficial dermis, whereas Zn was limited to SC and upper epidermis in both treatments. In summary, UVB-damaged skin slightly enhanced TiO2 NP or ZnO NP penetration in sunscreen formulations but no transdermal absorption was detected.
Skin cancer is the most common form of cancer in the United States with more than 1 million cases diagnosed annually in the form of basal cell and squamous cell carcinomas associated with solar ultraviolet (UV) radiation (American Cancer Society, 2009; Armstrong and Kricker, 1993). The incidence of melanoma skin cancer is rising significantly and is the most common cancer among 25- to 29-year-olds (National Cancer Institute, 2006). Sunscreen use is recommended to prevent skin cancer, sunburn, photoaging, and skin wrinkles. Traditional sunblocks incorporate chemicals that require constant reapplication and may cause skin irritation. Zinc oxide (ZnO) lotions provide a longer lasting nonirritating broad spectrum physical sunblock. However, large ZnO particles result in a white barrier that is not cosmetically appealing. Currently, nanoscale titanium dioxide (TiO2) and micronized ZnO nanoparticles (NP) less than 100 nm have been incorporated in sunscreens to provide effective broad spectrum physical sunblock.
Borm et al. (2006) estimated that global production of TiO2 and ZnO for use in skincare products was 1000 tons in 2003 and 2004. A 2007 survey by iVillage found that 11% of respondents used some type of sunscreen every day, whereas 59% used a sunscreen occasionally (Skin Cancer Foundation, 2007). Extrapolation of this data suggests that 33 million people in the United States use a sunscreen product daily and 177 million apply sunscreen occasionally. With this widespread use and the potential for TiO2 or ZnO NP exposure in sunscreens, concerns have focused on their possible penetration through the stratum corneum (SC) skin barrier to contact viable keratinocytes. Often sunscreens are applied on water-soaked skin or severely burned skin in which the integrity of the SC may be impaired. The European Union (EU) Scientific Committee on Cosmetics and Non-Food Products (SCCNFP, 2000) rendered an opinion on the safety of TiO2, finding no evidence of in vitro or in vivo skin penetration of NP. However, the safety of nanomaterials in cosmetic products issued by the EU Scientific Committee on Consumer Products in 2007 stated that a safety assessment of nanosized TiO2 should take into account abnormal skin conditions and the possible impact of mechanical effects on skin penetration (SCCP, 2007). An earlier opinion on ZnO found a lack of reliable data on the percutaneous absorption of micronized ZnO (SCCNFP, 2003).
A number of in vitro and in vivo studies have investigated the penetration of TiO2 and ZnO NP found in sunscreen formulations through normal skin. Dussert and Gooris (1997) studied water/oil (w/o) emulsions of TiO2 and ZnO on excised human skin and found that the emulsion remained on the surface of the SC; however, TiO2/ZnO formulations (80–200 nm, up to 1 μm) applied to in vitro porcine skin for 24 h showed no penetration (Gamer et al., 2006). Absorption studies with in vitro and in vivo human skin (20 nm TiO2 in a w/o emulsion for 5 h) showed no penetration into the viable epidermal layers (Mavon et al., 2007). TiO2 oil/water (o/w) emulsions applied to in vitro human organotypic cultures for 24 h and to in vivo human forearms for 6 h (Bennat and Müller-Goymann, 2000) showed that penetration was greater in vitro than in vivo, but TiO2 tape stripping indicated that particles remained within SC layers. Similar studies with hydrophobic (100 nm), amphiphilic (10–15 nm), and hydrophilic (20 nm) micronized TiO2 in o/w emulsions applied nonocclusive to human forearms for 6 h showed formation of a thin film on the SC (Schulz et al., 2002), and neither particle shape, formulation, nor exposure had a significant impact on penetration. Human foreskin grafted onto severe combined immunodeficiency (SCID) mice treated with hydrophobic emulsions of occluded micronized TiO2 and analyzed by particle induced x-ray emission (PIXE) and scanning transmission ion microscopy tracked Ti particles to the stratum granulosum (SG) layer of the epidermis (Kertész et al., 2005). Alternatively, Kiss et al. (2007) showed that no TiO2 NP penetrated intact skin of SCID models. Lekki et al. (2007) applied TiO2 to porcine and human skin by gentle rubbing and found Ti in the upper three to five layers of the SC and to a depth of 400 μm in hair follicles by PIXE, concluding that mechanical movement (tensile and compressive) played a role in penetration. Studies by Lademann et al. (1999) reported TiO2 within some follicular orifices, but most remained on the skin surface. In contrast, studies conducted 8–24 h in vivo porcine skin by Menzel et al. (2004) suggested that hair follicles were not important but detected four different formulations of TiO2 on the SC and in the SG using ion beam analysis. Tan et al. (1996) applied micronized TiO2 in older patients for 9–31 days and detected insignificant levels of Ti in the dermis.
In vitro static diffusion cell (Cross et al., 2007; Zvyagin et al., 2008) and in vivo (Zvyagin et al., 2008) studies with human skin treated with ZnO (26–30 nm) formulations found that NP remained with the SC by multiphoton microscopy. Normal and damaged (tape stripped) skin from pigs exposed to four different types of TiO2 (35–250 nm) in vitro for 24 h in Franz cells showed no Ti penetration in the receptor fluid (Senzui et al., 2010). Three different TiO2 formulations applied daily for 22 days to minipigs showed localization of TiO2 in the upper SC and follicular lumen, with isolated TiO2 within the dermis with all three formulations (Sadrieh et al., 2010). Finally, a 5-day study with 68ZnO particles (19 nm, average agglomeration size of 110 nm) in o/w formulation applied to the backs of human skin depicted small increases of 68Zn in the blood and urine samples. This stable enriched isotope found in the samples suggested that Zn from the sunscreen formulations was absorbed; however, it is not known if 68Zn was absorbed as ZnO particles or soluble Zn (Gulson et al., 2010).
Currently, only a few studies have been conducted with TiO2 or ZnO NP applied to compromised skin as would occur when sunscreen was reapplied to sunburned skin. If the integrity of the skin barrier is breached either by mechanical, physical, or chemical damage, the rate-limiting barrier is disrupted, and the potential for many chemicals as well as very small particles to cross the SC lipid barrier increases (Monteiro-Riviere and Baroli, 2010). In addition, solvents, hydration, and surfactants can influence the barrier function (Monteiro-Riviere, 2008). Studies in healthy and psoriatic skin showed that Ti deeply penetrated the disorganized SC of psoriatic skin relative to normal skin but did not reach the viable layers of the epidermis (Pinheiro et al., 2007). However, little data are available regarding the penetration of TiO2 and ZnO NP found in sunscreen formulations in skin damaged by UV radiation.
The objective of this study was to assess the absorption and penetration of commercially available TiO2 and ZnO in sunscreen formulations in ultraviolet B (UVB)-damaged skin in both in vitro flow-through diffusion cells and in vivo porcine skin using multiple detection modalities.
MATERIAL AND METHODS
Ultrastructure and composition of sunscreen formulations.
Four hydrophobic and hydrophilic sunscreen formulations containing commercially available TiO2 and ZnO products (BASF, Ludwigshafen, Germany) were used in this study: CM 630 (10% TiO2 [T-Lite SF] in o/w formulation), CM 634 (10% TiO2 [T-Lite SF] in w/o formulation), CM 643 (5% ZnO [Z-COTE HP1] in o/w formulation), and CM 644 (5% ZnO [Z-COTE] in o/w formulation). CM 630 and CM 634 consist of TiO2 (rutile, crystallite of 14–16 nm) coated with hydrated silica, dimethicone/methicone copolymer, and aluminum hydroxide for a primary particle size of 10 × 50 nm (estimated by scanning electron microscopy [SEM]) and specific surface area of 100 m2/g. The mean size of the agglomerates was 200 nm with a range ca. 90–460 nm. CM 643 consists of ZnO coated with triethoxycaprylylsilane for a mean size of 140 nm (weight based) with a range between ca. 60–200 nm and specific surface area of 12–24 m2/g, whereas CM 644 consists of uncoated zinc oxide with a mean size of 140 nm (weight based, range ca. 60–200 nm) and a specific surface area of 12–24 m2/g. Particle size distributions of ZnO were determined by laser diffraction.
For NP sizing in the formulations, each viscous sunscreen was diluted in ultrapure water (ca. 1:200) and vortexed to provide a suspension. In addition, CM 634 was sonicated for 40 min to facilitate suspension. A suspension of 8 μl was placed onto formvar-coated carbon grids and allowed to air-dry. Samples were imaged on an FEI/Philips EM208S TEM operating at an accelerating voltage of 80 KV. In addition, samples were analyzed by energy-dispersive X-ray spectroscopy (EDS) on a Hitachi HF2000 FE transmission electron microscope equipped with an Oxford Instruments INCA EDS operating at an accelerating voltage of 200 KV.
UV exposure to in vivo pig skin.
Experiments were conducted on weanling white Yorkshire pigs approximately 20–30 kg in accordance with the guidelines prepared by the Institute of Laboratory Animal Resources (1996). Animals, acclimated for 5 days prior to UVB exposure, were housed in an Association for Assessment and Accreditation of Laboratory Animal Care accredited facility on an elevated floor and provided water ad libitum and 15% protein pellets. The pigs were sedated by an im injection of a telazol-ketamine-xylazine (TKX) cocktail and the hair on the back was clipped with electric clippers. Skin was exposed to UVB as previously reported (Lin et al., 2003, 2005). UV radiation source was a 1000 W lamp solar stimulator (Lightning Cure 200; Hamamatsu, Japan) combined with a dichroic mirror assembly to reflect the visible and infrared emission and fitted with a 1-mm WG295 Schott selective UVB band-pass filter to eliminate wavelengths less than 295 nm. This delivers UV to the skin’s surface through a 1 cm diameter liquid light guide at an intensity of 5 mW/cm2 of UVB and 40 mW/cm2 UVA as measured by an IL1400 radiometer (International Light, Newburyport, MA). UVB was the dominant wave band, and to simplify terminology, the UV radiation will be referred to as UVB in the manuscript.
Determination of minimal erythemic dose.
To determine a minimal erythemic dose (MED) for each pig, the skin along the midline of the back was sequentially exposed within a template to the following UVB doses (mJ/cm2): 30 (6 s), 40 (8 s), 50 (10 s), 60 (12 s), 70 (14 s), 80 (16 s), 90 (18 s), 100 (20 s), 110 (22 s), and 120 (24 s). The MED was determined to range between 40 and 50 mJ/cm2, which was consistent with historical studies in our laboratory. Exposed sites were analyzed after 24 h to determine the UVB dose required to produce a + 2 erythema (ranging from 100 to 120 mJ/cm2) on the skin of each pig (n = 4 pigs), which is considered a moderate erythemic dose. Pigs were sedated with TKX, and multiple sites on the back were exposed to the UVB dose that caused a consistent + 2 erythema, a pale red irritation within a defined area of skin (Draize et al., 1944). Then 24 h later, the pig was sedated with TKX, the sites visually scored for + 2 erythema, and the skin prepared for the in vitro or in vivo studies.
In vitro flow-through diffusion cells.
Pigs were euthanatized via iv injection into the ear vein with Euthasol (390 mg/ml of pentobarbital sodium and 50 mg of phenytoin sodium), and exposed and unexposed skin sites were dermatomed to 400 μm. Dermatomed skin, placed dermis side down on towels saturated with physiological saline, was cut into with a 19 mm circular punch. Skin was mounted in flow-through diffusion cells maintained at 37°C with a dosing area of 0.64 cm2 and equilibrated in perfusate (1.2mM KH2PO4, 32.7mM NaHCO3, 2.5mM CaCl2, 4.8mM KCl, 1.2mM MgSO4, 118mM NaCl, 1200 mg/l d-glucose, 4.5% bovine serum albumin, 5 U/ml heparin, 30 μg/ml amikacin, and 12.5 U/ml penicillin G; pH 7.3–7.5) at a flow rate of 2 ml/h for 30 min prior to dosing. Perfusion was halted and skin was (n = 16 with UVB-exposed skin sites; n = 8 with unexposed skin sites) dosed with 50 μl of the four sunscreen formulations (n = 4 replicates of diffusion cells per formulation) and control sites (n = 4 of UVB-exposed and normal skin without sunscreen). Upon completion of dosing, perfusion was resumed, and the perfusate was collected every 2 h for the first 12 h, then every 4 h up to 24 h. After 24 h, perfusion was terminated, the skin was removed from the diffusion cells, and the dose site was removed with an 8-mm biopsy punch and cut into thirds. Skin was placed in Trump’s fixative for light microscopy (LM) and for transmission electron microscopy (TEM) and stored at 4°C. The remaining skin was immediately frozen in liquid nitrogen and stored at −20°C for time-of-flight secondary ion mass spectrometry (TOF-SIMS). Perfusate samples from each timed collection were capped and stored at 4°C for TiO2 and ZnO analysis by inductively coupled plasma mass spectrometry (ICP-MS).
In vivo treatment.
Exposed sites (n = 3 per formulation) on two pigs were treated with 250 μl of each formulation; 200 μl was loaded onto the pad of the Hill Top chamber (1.0 cm2 area) and 50 μl was placed directly on the skin within a template. Controls included normal pig skin (no UVB, no sunscreen, no Hill Top chamber; n = 2 per pig), UVB-exposed (no sunscreen, dry chamber; n = 2 per pig), and sunscreen in a Hill Top chamber (no UVB; n = 2 per pig per formulation). Sites were redosed with new Hill Top chambers after 24 h, and the treatment was terminated after 48 h. Erythema was scored for each site, and the pigs were euthanatized as above. Skin from all of these sites were removed with an 8-mm biopsy punch for microscopy studies as stated above.
Fixed skin was rinsed in 70% ethanol, processed through graded ethanol series, cleared in Clear-Rite 3, and infiltrated and embedded in Paraplast tissue embedding medium. Sections of 5 μm were mounted on positive-charged slides and stained with hematoxylin and eosin.
Transmission electron microscopy.
Fixed skin was rinsed in 0.1M phosphate buffer, postfixed in phosphate-buffered osmium tetroxide, dehydrated through graded ethanols, cleared in acetone, and infiltrated and embedded in Spurr’s resin (Ted Pella, Inc., Redding, CA). Thin sections (800–1000 Å) were mounted on formvar carbon–coated copper grids and examined on an FEI/Philips EM 208S TEM. Some sections were stained with lead citrate and uranyl acetate and others were left unstained. Stained sections provided better contrast and thus resolution of the tissue, whereas unstained sections allowed better visualization of the Ti and Zn NP and ensured the absence of stain artifacts that may have resulted from lead citrate or uranyl acetate precipitation. In addition, stained and unstained samples were analyzed by EDS to determine element distribution with a Hitachi HF2000 FE TEM equipped with an Oxford Instruments INCA EDS.
Scanning electron microscopy.
Frozen skin from the in vivo study was fixed in Trump’s fixative, rinsed in 0.1M phosphate buffer, and dehydrated through graded ethanols to absolute ethanol. The tissue was critical point dried, mounted on aluminum stubs, and sputter coated with a gold/palladium alloy. The samples were imaged on a high-resolution JEOL 6400F field emission SEM at 20 KV, and the presence of Ti and Zn was confirmed by EDS.
Time-of-flight secondary ion mass spectrometry.
Frozen skin was aligned with the epidermis perpendicular to the knife, sectioned at 15 μm in a cryostat, and mounted onto silicon chips. Samples were air-dried for 3 h prior to being introduced into the chamber of the microscope. An Ion TOF SIMS5 (Physical ION TOF, Munster, Germany) employing a Bi+ pulsed primary ion beam was used to sputter the surface of the skin sample. Ionized species eroded from the sample surface were extracted into a TOF mass spectrometer and analyzed according to their mass to charge ratio by measuring TOF from the sample surface to the detector, obtaining high spatial and mass resolution data. The pulsed Bi+ beam was rastered across the sample (256 × 256 pixels), and a mass spectrum was acquired at each pixel. Ion images were obtained retrospectively from the acquired data for selected Ti and Zn isotopes. The images from the Ti and Zn isotopes were summed to improve signal to noise. Ion images were aligned with optical images of the tissue to determine the upper surface of the SC and the lower surface of the dermis and to localize the NP in the skin.
Inductively coupled plasma mass spectrometry.
Perfusate samples were analyzed for Ti and Zn by ICP-MS. The samples (0.2 g) were weighed and mixed with 0.2 ml of nitric acid up to 10 ml with deionized water. The Ti or Zn content (isotope: 66Zn) of the solutions was analyzed by ICP-MS with an Agilent 7500a with Meinhardt atomizer (generator: 1300 W and analysis time: 1.5 s) using 45Sc as an internal standard. Additionally, perfusate samples were concentrated by analytical ultracentrifugation and the sediment was analyzed by TEM/energy dispersive x-ray spectroscopy (TEM/EDS) for Ti and Zn. Spherical particles were identified by EDS as Si/O and by field emission gun TEM and high-angle annular dark field–scanning TEM. Small angle x-ray scattering (SAXS) was used to confirm the Ti and Zn used in the sunscreen formulations.
Particle Ultrastructural Characterization and Composition in Diluted Sunscreen Formulations
TiO2 in CM 630 (Fig. 1a) and CM 634 (Fig. 1b) appear similar in length (up to 100 nm) and fusiform/spindle in shape. ZnO found in CM 643 (Fig. 1c) and CM 644 (Fig. 1d) was heterogeneous, with a mean size of 140 nm and elongated to cuboidal shape up to 200 nm. EDS spectra (insets within micrographs) confirm that the NP in CM 630 and CM 634 are Ti and the NP in CM 643 and CM 644 are Zn. In addition to the Ti in CM 630 and CM 634, EDS also identified the presence of Si in the Ti spectra and Cu in all samples. Si is a coating for the Ti, and the samples are mounted on copper grids.
Characterization of UVB-Exposed Sunburned Pig Skin
Pigs were exposed to UVB that resulted in a + 2 sunburn erythema for the in vitro flow-through experiments and the in vivo experiments. Typical sunburn cells (SBC) with a pyknotic nucleus and eosinophilic cytoplasm are indicative of UVB skin exposure and can be identified in Figures 2b–2d (in vitro) and Figures 4b–4d (in vivo). SBC have a distinct morphology, consisting of a pyknotic nucleus (apoptotic) and eosinophilic cytoplasm, and their presence indicates that the skin had received UVB damage. For the in vivo studies, termination at 48 h resulted in most of the UVB-exposed sites reaching a + 3 erythema, which exhibits a definite red in a well-defined area.
In Vitro Flow-Through Diffusion Cells
At the end of the experiments (24-h postdose), skin was removed from the diffusion cells. Three sunscreen formulations were dry on the skin surface with the exception of CM 634 that was moist.
LM of In Vitro
Control skin (no UVB and no sunscreen) exhibited intracellular epidermal edema (Fig. 2a), which is typical of perfused skin after 24 h. UVB control skin (UVB and no sunscreen) and all UVB-treated skin with the four formulations had intracellular epidermal edema and showed typical expression of SBC in the lower layers of the epidermis (Fig. 2b). All UVB-exposed areas showed a normal SC and inflammatory infiltrates in the dermis. Residual CM 630 (10% TiO2 in o/w vehicle) was prominently visible as a dark precipitant on the surface and in the invaginations or furrows of the SC in unexposed skin. UVB-exposed skin treated with CM 634 (10% TiO2 in w/o vehicle) depicted focal parakeratosis and intracellular epidermal edema with SBC (Fig. 2c). Unexposed skin treated with CM 643 (5% ZnO; Z-COTE HP1 in o/w vehicle) was typical of normal perfused skin, with only small residues of sunscreen ZnO remaining on the SC. Treatment with CM 643 was similar to CM 630. Unexposed skin treated with CM 644 (5% ZnO; Z-COTE in o/w vehicle) was similar to normal perfused skin. UVB-exposed skin with CM 644 was typical of UVB-damaged skin as described above and consisted of focal areas of discrete sunscreen on the surface of the SC (Fig. 2d).
TEM of InVitro
TEM was used to assess TiO2 and ZnO particle penetration through the SC skin layers. The SC layers of the control (no UVB, no sunscreen) appeared normal, with numerous compact layers of SC cells interconnected by desmosomes. Stratum basale (SB) layer had a few intracytoplasmic lipid droplets, swollen mitochondria, focal intercellular edema, and attached to the basement membrane by hemidesmosomes. Nonmembrane-bound lipid droplets were seen and typical of degenerating cells. EDS of the stained tissue found spectra specific for the presence of Cu (copper grid), Os (osmium tetroxide postfix), Pb (lead citrate stain), and U (uranyl acetate stain). SC of the UVB control skin (no sunscreen) appeared similar to normal skin with no UVB exposure. The SG consisted of numerous degenerating cells containing vacuoles and cellular remnants. At times, isolated necrotic cells were found in the SG and SB. The SB layer contained many cytoplasmic vacuoles, swollen and altered mitochondria, lipid droplets, condensed and fragmented nuclei, and remnants of disintegrating cells. At times, focal microvesication and SBC were noted throughout the epidermis.
The presence of TiO2 or ZnO in the treated skin was confirmed by the ultrastructure of the NP as well as by EDS. Skin without UVB exposure and treated with CM 630 (10% TiO2; T-Lite SF in o/w) or CM 634 (10% TiO2; T-Lite SF in w/o) appeared similar to the unexposed controls. TiO2 NP were occasionally found as deep as nine layers in the compacted SC, with large aggregates often present on the surface and within the upper two to three layers of the SC. UVB-exposed skin treated with CM 630 showed the typical aggregates of TiO2 on the surface and up to 17 SC layers deep (Fig. 3a). Occasionally, several TiO2 NP were found in the intercellular lipid moiety space at the lateral overlapping edges between adjacent SC cells. The epidermal morphology was typical of UVB-damaged skin, including the condensed pyknotic nucleus of SBC. EDS of the stained tissue confirmed that the NP in the SC were Ti, with minor peaks specific for Si (coating), Cu (grid), and Pb and U (staining). The distribution of TiO2 in unexposed skin treated with CM 634 was comparable to the unexposed skin treated with CM 630. The epidermal morphology was normal. TiO2 NP were never found within any Langerhans cell or cell process. TiO2 agglomerates were seen on the surface and in the upper SC layers of the UVB skin treated with CM 634 (Fig. 3b). In addition, separation of the epidermal-dermal junction was noted. EDS of unstained CM630 and CM634 tissue confirmed TiO2 NP in the SC.
Unexposed skin treated with CM 643 (5% ZnO; Z-COTE HP1 in o/w) or CM 644 (5% ZnO; Z-COTE in o/w) was also normal with focal areas of ZnO NP, as confirmed by EDS, on the surface of the SC and superficial stratum disjunctum. In UVB-exposed skin treated with CM643 (Fig. 3c) or CM 644 (Fig. 3d), focal ZnO NP, as confirmed by EDS, were noted above the SC. NP were adjacent to an electron-translucent area that probably represents residual sunscreen vehicle. ZnO NP were not found in the upper SS layer, the SG layer, or the lower SC layers. TEM of the epidermis of UVB-exposed skin treated with CM 643 and CM 644 depicted typical UVB damage with intracellular and intercellular epidermal edema, SBC, vacuoles, necrotic cells, and Langerhans cells that did not contain ZnO NP.
In Vivo Treatment
All skin exposed to UVB had a + 2 erythema score based on the Draize test at 24 h. Pads in the used chambers as well as the underlying skin were relatively dry, indicating that the formulation vehicles had been absorbed into the skin. At the termination of the experiment (48 h), most UVB-exposed sites had an increase in erythema to + 3, exhibiting a definite red in a well-defined area. Normal pig skin and unexposed sunscreen controls showed no erythema.
LM of InVivo
All sunscreens remained on the pig for 48 h following UVB exposure. Control skin (no UVB, no sunscreen) exhibited normal epidermis and dermis with a compact SC (Fig. 4a). UVB control skin (UVB, no sunscreen) had focal intracellular epidermal edema, slight dermal inflammation, and typical SBC in the lower layers of the epidermis (Fig. 4b), with early parakeratosis similar to our other published studies with UVB exposure in porcine skin (Lin et al., 2003, 2004, 2005; Tournas et al., 2006). At times, focal microblisters were present with epidermal-dermal separation. UVB-exposed skin treated with all four sunscreen formulations had typical UVB damage as mentioned above but with more severe lesions than the in vitro studies due to the 48 h time following exposure. UVB-exposed sites treated with CM 630 (Fig. 4c) and CM 634 typically had a thin layer of the sunscreen formulation on the surface of the skin and some early parakeratosis. Sites treated with CM 643 (Fig. 4d) and CM 644 were similar with little residual Zn sunscreen on the SC, slight dermal inflammation, SBC, and superficial epidermal necrosis with hypereosinophilia consistent with UVB exposure.
TEM of InVivo
To assess the skin penetration of all the formulations of sunscreens, TEM was used to confirm the localization and depth of penetration in skin. The SC and SB of normal pig skin (no UVB, no sunscreen, and without a chamber) were normal. Langerhans cells were identified within the SS layers. The SG contained cellular remnants, with Langerhans cell processes throughout the SS. Skin without UVB exposure and treated with CM 630 appeared normal. TiO2 NP were found only up to seven layers deep in areas of the compact SC (Fig. 5a), with large aggregates usually present on the surface and within the upper two to three layers of the SC. The epidermis was normal, with occasional Langerhans cells in the viable layers. UVB-exposed skin treated with CM 630 in an o/w formulation depicted typical aggregates of TiO2 on the surface and intercellular space of the SC and was found deeper to 13 cell layers (Fig. 5b). In one focal area of UVB-damaged skin, TiO2 crystals were found 29 layers deep into the SC. Degenerating cells containing vacuoles and cellular remnants were present within the SG. The SB layer contained numerous cytoplasmic vacuoles, swollen and altered mitochondria, lipid droplets, pyknotic cells with condensed nuclei, and remnants of disintegrating cells. Most of the distribution of TiO2 in normal skin with CM 634 was found in the upper nine layers of the SC (Fig. 5c); however, in one instance, TiO2 was found 20 layers deep in this w/o formulation. The epidermis was normal. In the UVB skin treated with CM 634, TiO2 accumulated on the surface and between adjoining SC cells. Ti was continuously seen in the SC usually up to 12 cell layers in depth. A degenerating cell that contained Ti within the cytoplasm and within vacuoles was noted in the SC (Fig. 5d). TiO2 agglomerates were noted between the cell and adjacent SC cells and within an invagination of the cell membrane of the degenerative cell (Fig. 5d, inset).
Pig skin not exposed to UVB and treated with CM 643 or CM 644 was normal, with focal areas of ZnO NP on and immediately above the superficial layers of the SC. Langerhans cells were present in the SS and SB and ZnO NP were not observed within them. ZnO NP were found up to 10 cell layers deep in the SC (Fig. 5e) of UVB-exposed skin treated with CM 643, and in one instance up to 16 layers deep. In the UVB-exposed skin treated with CM 644, the ZnO NP were localized solely to the upper SC (Fig. 5f). In general, TEM showed that TiO2 NP penetrated deeper into the SC in normal and UVB-damaged skin than ZnO but still remained localized to the intercellular space of the SC layers. We noticed that NP in the w/o formulation penetrated deeper into the SC.
SEM of InVivo
SEM/EDS was performed on the in vivo normal porcine skin exposed only to UVB to verify that no contaminants of Ti or Zn were present on the skin. In the UVB-exposed skin treated with the CM 630 formulation, TiO2 NP were primarily localized as large agglomerates on the surface of the skin. Discrete areas of Ti were found on the outer periphery of these agglomerates. Near the base of the hair, just as it emerges from the skin (Fig. 6a), in the area referred to as the infundibulum, EDS confirmed that TiO2 NP were present (Fig. 6b, inset). Large agglomerates of TiO2 from the CM 634 formulation were also present on the hair (Fig. 6c) and localized in the crevices between the overlapping cuticle scales (Fig. 6d) as confirmed by EDS (Fig. 6d, inset). Skin adjacent to these hairs also contained agglomerates of TiO2. The heterogeneous ZnO NP in CM 643 were distributed on the surface of the UVB skin, and ZnO agglomerates were also noted near the base of the hair (Fig. 7a) and on the hair (Fig. 7b) as confirmed by EDS (Fig. 7b, inset). The CM 644 sunscreen was distributed evenly across the UVB-exposed skin and was present near the base of the hair (Fig. 7c) and on the hair (Fig. 7d) and confirmed by EDS (Fig. 7d, inset). In one focal area of the skin near the base of a hair, large ZnO crystals (confirmed by EDS) measuring up to 15 μm in diameter were noted within a large agglomerate of Zn NP and were not seen in any other samples.
Time-of-Flight Secondary Ion Mass Spectrometry
TOF-SIMS provides a sensitive analytical method to detect the NP in the skin. TOF-SIMS images of mapping overlays localized Ti and Zn in the skin samples. Homogeneous background for Ti and Zn were present in some of the maps as individual pixels. Clustering of pixels indicated the presence of Ti or Zn above background.
InVitro Flow-Through of TOF-SIMS
Mapping showed no Ti or Zn above background levels were present in the normal and UVB-exposed control skin (no sunscreen). In the UVB-exposed control skin (UVB, no sunscreen), neither Ti nor Zn was detected above background. Normal unexposed skin treated with the CM 630 formulation revealed that the concentration of Ti (and isotopes 46Ti, 47Ti, 48Ti, and/or 48TiH) was the greatest in the epidermis, with some penetration into the superficial papillary dermis (Fig. 8a). In the UVB-exposed skin treated with CM 630, Ti was the greatest in the epidermis and similar penetration in the dermis to the non–UVB-exposed skin (Fig. 8b). In the overlay panel of Figure 8b, which provides an orientation of the tissue and localization of the NP, a focal area of Ti was noted and appears to be adjacent to a hair follicle. In unexposed skin treated with CM 634, Ti also was localized primarily to the upper epidermis (Fig. 8c). In the UVB-exposed skin, focal areas of Ti were present in the SC, epidermis, and dermis (Fig. 8d). In normal skin treated with CM 643, Zn (64Zn and/or 66Zn) was typically localized to a narrow band in the SC (Fig. 9a). In one sample, Zn was present within the focal clusters in the epidermis and superficial dermis, sometimes aligning with structures such as hair follicles and sebaceous glands. In the UVB-exposed skin treated with CM 643, Zn localization had a small but thicker band in the epidermis compared with the unexposed skin (Fig. 9b). In both the unexposed (Fig. 9c) and exposed (Fig. 9d) skin treated with CM 644, Zn was primarily concentrated in the upper epidermis within a much thicker band than CM 643. The UVB-exposed skin also had higher background levels in the epidermis (Fig. 9d).
In Vivo Study of TOF-SIMS
Mapping of the normal and UVB-exposed control skin (no sunscreen) showed no Ti or Zn above background levels. Normal unexposed skin treated with CM 630 revealed only slight Ti above background in the epidermis and superficial papillary dermis (Fig. 10a). In the UVB-exposed skin treated with CM 630, the greatest concentration of Ti was found in the upper epidermis (Fig. 10b). Homogeneous distribution of Ti was found in the SC and upper epidermis, with some focal areas of Ti in the superficial dermis in unexposed skin treated with CM 634 (Fig. 10c). At times, focal areas of Ti were noted in the dermis and based on the overlay, could not be correlated to any morphological structures such as hair follicles or sebaceous glands. Ti mapping in UVB-exposed skin treated with CM 634 was similar to that of the unexposed UVB skin (Fig. 10d). Normal unexposed skin treated with CM 643 revealed focal Zn slightly above background level in the epidermis (Fig. 11a). In the UVB-exposed skin treated with CM 643, focal areas of Zn were present in the SC and in the upper epidermis (Fig. 11b). In the unexposed skin treated with CM 644, Zn was present primarily on the surface of the SC (Fig. 11c). In the UVB-exposed skin, Zn was slightly above background in the SC (Fig. 11d).
Figure 12 is a schematic illustrating the penetration path that NP follow in the skin and subsequent interpretation by TEM. In some micrographs, heterogeneous NP were not found in each sequential layer of the SC. In many cases, NP were observed on the surface of the SC and again in deeper layers of the SC intercellular space, thus appearing to miss a few layers. This may be explained by the plane of section of the skin that was examined by TEM. When viewing by TEM, a very small sample size is examined. When a tissue section “a” is viewed, the NP appeared only on the surface and in layer 6. When section “b” is viewed, the NP appeared on the surface, in the next layer down, and in layers 5 and 6; when section “c” is viewed, the NP appeared in all SC cell layers.
ICP-MS of In Vitro Perfusate Samples
ICP-MS analysis of the 6, 12, 20, and 24 h perfusate samples from the UVB-exposed skin in the in vitro cell diffusion studies is summarized in Table 1. Ti was below detectable limits (< 0.1% of applied dose) in all samples tested while Zn concentrations between 0.3 and 0.5 μg/ml were background levels. Neither of the NP species was found to have penetrated the pig skin into the perfusate. This indicates that although the TiO2 and ZnO NP were shown to penetrate the SC by TEM and into the skin by TOF-SIMS as depicted by the presence of NP in the epidermis and into the dermis, there was not a significant amount of NP penetration into the dermis for sufficient systemic absorption to occur. Additionally, the perfusate samples concentrated by analytical ultracentrifugation and analyzed by TEM/EDS also confirmed that TiO2 and ZnO NP did not penetrate through the skin into the perfusate.
|Formulation||Time (h)||Titanium (μg/ml)a||Formulation||Time (h)||Zinc (μg/ml)b|
|CM 630||6||<0.1||CM 643||6||0.5|
|CM 634||6||<0.1||CM 644||6||0.5|
|Formulation||Time (h)||Titanium (μg/ml)a||Formulation||Time (h)||Zinc (μg/ml)b|
|CM 630||6||<0.1||CM 643||6||0.5|
|CM 634||6||<0.1||CM 644||6||0.5|
Ti below level of detection.
Zn levels found in untreated controls. Zn is ubiquitous (powdered gloves, glass, and stainless steel), so not unexpected results.
The use of sunscreens containing TiO2 and ZnO NP has increased worldwide in recent years. The Australian Therapeutic Goods Administration estimated that in 2005, 70% of all sunscreens containing TiO2 and 30% containing ZnO were formulated with NP (Faunce et al., 2008). Sunscreens containing micronized TiO2 and ZnO NP appeal to consumers because they are transparent and provide effective UV protection by reducing the scattering effect of visible light. No genotoxicity was noted when TiO2 and ZnO in cosmetic formulations were tested with the Ames’ Salmonella gene mutation test, the V79 micronucleus chromosome mutation test, the in vivo mouse bone marrow micronucleus test, and the Comet assay (Landsiedel et al., 2010). The toxicological profiles of TiO2 and ZnO are very comprehensive; however, little is known about the effects of these NP in UVB sunburned skin. TiO2 and ZnO have been used as UV filters in cosmetics and in sunscreens because they do not cause irritation or act as sensitizers. They are considered safe and thought to be similar to their bulk counterparts.
As discussed in the Introduction, ZnO and TiO2 NP do not appear to penetrate through intact healthy skin. In the present study, we extend these findings with ZnO and TiO2 NP to skin damaged by sunburn, a common application scenario to anyone exposed to UV radiation. Using in vitro and in vivo porcine skin, an accepted animal model for human skin (Bronaugh et al., 1982; Monteiro-Riviere, 1986, 2001; Monteiro-Riviere et al., 1994, 2008; Monteiro-Riviere and Riviere, 1996, 2005; Monteiro-Riviere and Stromberg, 1985; Wester and Maibach, 1977) minimal penetration of both ZnO and TiO2 NP to epidermal cellular elements occurred. Any material that did penetrate resided primarily in the upper layers of the SC. This superficial penetration was modulated by the nature of the topical formulations, a finding we have seen previously with topical exposure to carbon NP (Xia et al., 2010). Significantly, NP could not be detected in the perfusate from in vitro exposures using multiple sensitive analytical approaches, suggesting systemic absorption did not occur following topical application to UVB-damaged skin. The strength of this study includes the use of multiple and very precise analytical techniques to detect Zn and Ti in two well-accepted and sensitive model systems.
To date, absorption studies have been restricted primarily to normal skin. Both TiO2 and ZnO used in skin care products have been reported to penetrate the SC of rabbit skin (Lansdown and Taylor, 1997), with the highest absorption with water and oil vehicles. Other studies with different surface coatings of TiO2 in o/w emulsions applied to human skin for 6 h only detected Ti in the uppermost SC (Pflücker et al., 2001). Lademann et al. (1999) found TiO2 in an emulsion localized to the upper SC with distribution to the pilosebaceous orifices, and Cross et al. (2007) found that ZnO was limited to the outer surface of the SC. Other studies showed that TiO2 in sunscreens applied to minipigs four times per day for 5 days per week for 4 weeks observed high levels of Ti in the epidermis by TEM/EDS, primarily in the SC layers and near hair follicle openings but also observed a few particles of TiO2 in the dermis, which were regarded as contamination (Sadrieh et al., 2010).
A few investigators have suggested that when the skin barrier is damaged, penetration may occur. Sunscreens containing ZnO NP exposed to humans for 5 days under typical outdoor exposures found small increases in the Zn tracer in the blood and urine by very sensitive methods, but it is not known whether 68Zn was absorbed as ZnO or soluble Zn (Gulson et al., 2010). Although the authors state that their study shows evidence of absorption in healthy skin exposed to sunlight, they are omitting the fact that skin exposed to UV irradiation is damaged and that the barrier properties are different. Mortensen et al. (2008) investigated quantum dot (QD) penetration through UVB-exposed mouse skin and found silver-enhanced QD penetrated through the SC into the epidermis and dermis. However, they did not determine the MED of each exposed animal to standardize UVB exposure and did not use a relevant animal skin model. The size, coating, and charge of these QD do not provide a good model for TiO2 and ZnO skin penetration. Also, mouse skin is known to have minimal barrier properties to prevent topical absorption and is therefore not similar to human skin.
The purpose of the current study was to determine whether skin damaged by moderate UVB radiation (sunburn, as scored by a + 2 Draize score) enhanced the penetration of TiO2 or ZnO NP present in commercial sunscreen formulations. TiO2 and ZnO in the sunscreen formulations were confirmed by x-ray microanalysis. EDS found the presence of the ZnO NP in CM 643 and CM 644, and TiO2 NP and Si (Ti coating) in CM 630 and CM 634. Because Si is present in the TiO2 samples and not the ZnO sample, it is formulation specific; the rutile TiO2 has a coating of hydrated silica, dimethicone/methicone copolymer, and aluminum hydroxide. Sunscreen formulations that contained TiO2 (CM 630 and CM 634) were much more persistent on the surface of the skin than formulations that contained ZnO (CM 643 and CM 644). Ti aggregates were readily found in or on the upper layers of the SC, whereas ZnO aggregates were smaller, more focal, and typically found just above the SC layer. Sunscreens containing TiO2 were apparently more persistent on the surface of the SC, as seen by LM and TEM. TOF-SIMS also detected less ZnO on the surface, which was also influenced by the poor secondary ion yield of Zn. The macroscopic anatomy of normal and UVB-exposed skin was not affected by topical treatment with any sunscreen formulation.
In the flow-through studies, TEM found penetration of TiO2 to a depth of 9 layers in the SC of normal skin and 17 layers in the SC of UVB-exposed skin. ZnO was detected only on the surface of the SC on both the unexposed and UVB-exposed skin. In addition to Ti and Zn, Si and Os were detected within the upper SC by EDS. Although the Os originated from the osmium tetroxide postfixation during tissue processing, the source of the Si in skin treated with the ZnO formulations is not as obvious. Si is probably a contaminant from the perfusate or, more likely, an environmental contaminate on the pig. Si is ubiquitous in the environment as well as in many products. Pigs were housed on open elevated floors and unavoidably still come in contact with their own feces. ICP-MS analysis of the perfusate found that Ti was below detectable limits, whereas Zn was present in all samples. Cross et al. (2007) also reported background levels of Zn in the receptor fluid from flow-through studies with human skin after 24 h. This is not surprising because Zn is the most abundant essential trace metal after iron (St. Croix et al., 2005) and is ubiquitous in the body, especially in the skin and associated appendages and commonly found in powered gloves, glass, and stainless steel (Rostan et al., 2002).
Overall, the in vivo studies showed that TiO2 penetrated deeper into the SC in both normal and UVB-exposed skin compared with ZnO. The depth of the NP in the SC of the UVB-exposed skin may have been underestimated in our study because some of the SC was sloughed from the damaged skin. Formulations with TiO2 in non–UVB-exposed skin depicted TiO2 as deep as 20 cell layers, whereas in UVB-exposed skin TiO2 penetrated 12–13 SC cell layers. TiO2 in the w/o formulation was detected down to 18–20 cell layers in normal skin. ZnO was localized primarily to the upper one to two layers of the SC in non–UVB- and UVB-exposed skin and in one instance to a depth of 16 layers in the UVB-exposed skin treated with CM 643. Retention of ZnO NP was much greater in the in vivo compared with the in vitro study, probably due to dose occlusion and the increase in time the formulations were in contact with the skin. SEM/EDS localized the Ti and Zn primarily in agglomerates on the surface of the UVB-exposed skin and at the base of the hair in vivo. Although the NP agglomerates at the openings of the hair, there are no data to suggest that hair plays a role in NP penetration into the epidermis or dermis. TiO2 NP reported in the hair follicle remained outside the viable epidermis and dermis and were shown to be eliminated by sebum flow (Lademann et al., 1999; Nohynek et al., 2007). Hair follicles have been suggested to be weak points in the skin barrier (Patzelt et al., 2010) and may provide an area for potential absorption, thereby serving as a reservoir for drugs and nanoparticles. Many authors have reported that follicular density effects penetration (Feldman and Maibach, 1967; Maibach et al., 1971; Monteiro-Riviere, 2004, 2008). In addition, the lower region of the hair infundibulum has a weak barrier where corneocytes are smaller and crumbly, which can increase the permeability for drugs (Lademann et al., 2006; Patzelt et al., 2010).
Much of the sunscreen vehicle containing the TiO2 and ZnO NP was probably removed from the skin during tissue processing for LM and TEM. To circumvent this problem, we imaged the metallic oxides stabilized in the tissue by freezing, therefore leaving the NP undisturbed on the skin. TOF-SIMS allowed us to investigate the distribution of Ti and Zn on skin tissue sections mounted on silicon wafers. Although the spatial resolution of the microscope is low, TOF-SIMS provided a very sensitive tool to map the Ti and Zn distribution within the tissue sections. TOF-SIMS data indicate that both the Ti and Zn did focally penetrate into the epidermis in both normal and UVB-damaged skin treated with the sunscreen formulations. Slight Ti and Zn background interferences were present in some of the map overlays as determined by a region of interest analysis. We are not sure what caused this interference in some samples, although the counts may be due to molecular fragments from molecules of higher mass that did not survive the trip from the sample to the detector. The concentrations of Ti and Zn in the skin as determined by TOF-SIMS were less in vivo than in vitro. In vitro, neither Ti nor Zn was present in the untreated control skin (no sunscreen), with the exception of one sample showing Ti slightly elevated above background. Ideally, the absorption of Ti or Zn through porcine skin is determined by analyzing the perfusate. In our studies, no absorption was detected. However, depending on the methodology, the detection limit may be too high to detect very low levels of NP absorption.
In summary, UVB-sunburned skin slightly enhanced the in vitro or in vivo SC penetration of the TiO2 or ZnO NP present in the sunscreen formulations. Although TiO2 and ZnO NP were found to penetrate into the SC by TEM and into the epidermis and dermis by TOF-SIMS, we found no definitive evidence that the NP penetrated the skin in vitro into the perfusate. In most cases, TiO2 penetration into the SC was greater than ZnO. These results viewed together suggest minimal penetration of TiO2 and ZnO NP into the upper epidermal layers when applied topically in sunscreen formulations to normal and UVB-sunburned skin, with no evidence of systemic absorption.
Portions of this research were funded by BASF.
Portions of this research were selected for presentation at the platform session on Nanotoxicology—Metals and Metal Oxide at the 49th Annual Meeting of the National Society of Toxicology Meeting in Salt Lake City, Utah, on 10 March 2010 and at the 12th International Perspectives in Percutaneous Penetration Conference in La Grande Motte, Paris, on 7 April 2010. The authors would like to thank Dr Dieter Griffis, Director of the Analytical Instrumentation Facility at North Carolina State University, for his help with TOF-SIMS analysis. No conflict of interest for N.A.M.-R., J.E.R., and A.O.I. K.W., R.L., and S.S. are employees of BASF.