Table 1.
A review of the results of TiO2 and ZnO percutaneous absorption studies
| Test material | Particle size | Skin model/technique | Results | Reference |
|---|---|---|---|---|
| TiO2 and ZnO (no information on coating) | TiO2: 50 to 100 nm ZnO: 20 to 200 nm |
Human skin, in vitro | Penetration limited to upper layers of stratum corneum | 27 |
| TiO2 AI2O3/stearic acid coated | 150 nm to 170 nm | Human subjects (biopsy) | Particles on and in the upper layers of stratum corneum. About 1% of particles in the ostium of the follicle. No penetration into living skin | 28 |
| Various TiO2 anatase and rutile, coated and uncoated materials | 14 nm to 200 nm | Pig and human skin in vitro, human subjects (skin stripping or biopsy) | No penetration beyond the stratum corneum in any study | 29 |
| TiO2 (SiO2 Al2O3 + SiO2 coated) | 10 nm to 100 nm | Human skin, in vitro | Penetration of particles into the upper layers of stratum corneum. No penetration into living skin | 30 |
| TiO2 (SiO2 − Al2O3 Al2O3/SiO2 coated) | 10 nm to 100 nm | Human subjects (biopsy) | Particles on or in the outmost surface of the stratum corneum. No penetration into living skin | 31 |
| TiO2 (no coating) | 20 nm | Human skin, in vitro | Penetration in restricted to the topmost 3–5 corneocyte layers in the stratum corneum disjunctum. No penetration into living skin | 32 |
| TiO2 in various formulations (no information on coating) | Needles: 45 to 150 nm × 17 to 35 | Pig skin, in vitro | Particles on/in the stratum corneum; minimal penetration into stratum granulosum. No penetration into living skin | 33 |
| ZnO | 15 nm to 30 nm | Human skin, in vitro | Less than 0.03% of applied Zn recovered in the receptor solution, no particles detected in epidermis or dermis | 34 |
| TiO2 (SiO2 or dimethicon coated) and ZnO (uncoated) | TiO2: 30 to 60 nm ZnO: <160 nm |
Pig skin, in vitro | No penetration beyond stratum corneum. Receptor solution recoveries of 0.8%–1.4% of applied dose | 35 |
| TiO2 in a sunscreen formulation, silicone coated | 20 nm | Human skin in vitro and human subjects, skin stripping, TEM, backscattering spectometry | Penetration limited to upper layers of stratum corneum. No penetration in skin furrows or follicular opening may be mistaken for penetration in the epidermal compartment | 36 |