Table 3: Research studies done on percutaneous absorption of TiO2 and ZnO nanoparticles.

Test materialSkin modelParticle sizeResultsReference

Ti Dermatomed skin1–35 nm, (noncoating), 2–35 nm (alumina/silica/silicon coated), and 3–10 100 nm (mixture of alumina and silicon coated)No penetration was observed regardless of  Ti type in intact and stripped skin. SEM-EDS observation showed that Ti penetrated into vacant hair follicles (greater than 1 mm below the skin surface); however it did not penetrate into dermis and viable epidermis.[70]

Ti Human skin in vitro 20 nmPenetration in restricted to the topmost corneocyte layers in the stratum corneum. No penetration into living skin was observed.[71]

 TiO2 alone or in combination with ZnOHuman skin (biopsy)20 nmTiO2 or ZnO nanoparticles are absent or their concentration is too low to be tested under the stratum corneum in human viable epidermis. Therefore, significant penetration towards the underlying keratinocytes is unlikely.[72]

Ti in a sunscreen formulationHuman skin in vitro and human subjects20 nmResults showed penetration is limited to upper layers of stratum corneum. No penetration in skin furrows or follicular opening may be mistaken for penetration in the epidermal compartment.[73]

Ti Human skin in vitro 10 nm to 100 nmResults showed penetration of particles into the upper layers of stratum corneum. No penetration into living skin.[74]

Ti in various formulationsPig skin in vitro Needles: 45 to 150 nm × 17 to 35Particles on/in the stratum corneum; minimal penetration into stratum granulosum. No penetration into living skin.[75]

Ti Human subjects (biopsy)150 nm to 170 nmResults showed particles in the upper layers of stratum corneum. About 1% of particles in the follicle ostium. No penetration into living skin.[76]

Ti and ZnOHuman skin in vitro Ti : 50 to 100 nm ZnO: 20 to 200 nmResults showed that penetration is limited to upper layers of stratum corneum.[77]