Khabir and colleagues applied a zinc-67 enriched zinc oxide nanoparticle formulation to excised human skin for 48 hours and mapped the tracer by laser-ablation ICP-MS. The particles stayed on or in the stratum corneum. Dissolved zinc reached the viable epidermis as ionic zinc: exogenous zinc was 1.0 ± 0.3 µg/mL against endogenous zinc of 4.3 ± 0.7 µg/mL. Both are well below the 26 ± 5 µg/mL that halved keratinocyte viability (21 to 31 µg/mL in the abstract).
Key numbers
Table 1 (PDF p. 8), mean ± 95 % CI, LA-ICP-MS. Micrograms per gram are dry tissue; micrograms per millilitre were converted using about 70 percent epidermal water.
| Layer | Zinc-67 in treated skin (µg/g) | Exogenous zinc (µg/g) | Endogenous zinc (µg/g) | Exogenous zinc (µg/mL) | Endogenous zinc (µg/mL) |
|---|---|---|---|---|---|
| Stratum corneum | 235 ± 49 | 360 ± 70 | 39 ± 5 | 120 ± 23 | 13 ± 2 |
| Viable epidermis | 2.0 ± 0.5 | 3.1 ± 0.8 | 13 ± 2 | 1.0 ± 0.3 | 4.3 ± 0.7 |
| Dermis | 0.9 ± 0.3 | 1.4 ± 0.5 | 16 ± 7 | 0.5 ± 0.2 | 5 ± 2 |
The IC50 for HaCaT keratinocytes was 26 ± 5 µg Zn/mL. Viable-epidermis total zinc rose by about 23 percent. Dermal tracer (0.9 ± 0.3 µg/g) was indistinguishable from blank skin (0.6 ± 0.2 µg/g). Most tracer was recovered in the wash (630 ± 110 µg/g), and receptor fluid stayed at background over 48 hours. Endogenous skin zinc checked by solution ICP-MS was 15 ± 4 µg/g, and by LA-ICP-MS 11 ± 8 µg/g (PDF p. 8). Solution ICP-MS of tape-stripped viable epidermis gave 4.5 ± 4.0 µg/g, which the authors attribute to incomplete stratum corneum removal.
Methods (brief)
Heat-separated epidermis and full-thickness skin from three female donors were dosed at 2 mg/cm2 of 20% wt/wt zinc-67 zinc oxide nanoparticles in caprylic/capric triglyceride and kept at 35 °C for 48 hours. Barrier integrity was checked by electrical resistance and transepidermal water loss (below 40 g/m2/h). Tracer enrichment was 65.4 percent, and natural zinc-67 abundance was taken as 4.04 percent. µg/g values were converted to µg/mL assuming about 70 percent water in viable epidermis (Supplementary Section S3, not in this collection). Imaging used multiphoton microscopy with second-harmonic generation and zinc-selective fluorescent probes, LA-ICP-MS maps of skin cross-sections, and solution ICP-MS. Corresponding authors: m.roberts@uq.edu.au and andrei.zvyagin@mq.edu.au.
Evidence fitness
The paper supports layer-resolved zinc uptake into intact adult skin from a zinc oxide formulation. It shows that uptake is ionic and stays below a cytotoxic concentration. It is not a diaper or infant study, and it does not test damaged skin, occlusion, in vivo clearance, or metal impurities.
Limitations
The dry-to-wet conversion rests on an assumed water content. A tape-stripping ICP-MS estimate of 4.5 ± 4.0 µg/g was about twice the LA-ICP-MS value, which the authors attribute to incomplete stripping. The introduction cites a literature epidermal zinc of 60 µg/g, against the measured 13 ± 2 µg/g, and the authors note that reported values range from 3 to 60 µg/g depending on method. The work is ex vivo non-viable skin, three donors, and an oil vehicle. The discussion prints 2.1 ± 0.5 µg/g where Table 1 gives 2.0 ± 0.5 µg/g for tracer zinc in the viable epidermis; the table value is used here.
Related evidence
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- Baby Sunscreen, Mineral (ZnO + TiO2)
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- Opinion on Zinc oxide (nano form), COLIPA S 76
- Influence of two vehicles for zinc oxide on zinc absorption through intact skin and wounds
- Support for the Safe Use of Zinc Oxide Nanoparticle Sunscreens: Lack of Skin Penetration or Cellular Toxicity after Repeated Application in Volunteers
- Dermal Absorption of Nanomaterials Titanium Dioxide and Zinc Oxide Based Sunscreen: Role of Size and Surface Coating
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