Table 3.

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

Test material	Skin model	Particle size	Results	Reference
TiO2	Dermatomed skin	1–35 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  TiO2 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]
TiO2	Human skin in vitro	20 nm	Penetration 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 ZnO	Human skin (biopsy)	20 nm	TiO2 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]
TiO2 in a sunscreen formulation	Human skin in vitro and human subjects	20 nm	Results 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]
TiO2	Human skin in vitro	10 nm to 100 nm	Results showed penetration of particles into the upper layers of stratum corneum. No penetration into living skin.	[74]
TiO2 in various formulations	Pig skin in vitro	Needles: 45 to 150 nm × 17 to 35	Particles on/in the stratum corneum; minimal penetration into stratum granulosum. No penetration into living skin.	[75]
TiO2	Human subjects (biopsy)	150 nm to 170 nm	Results showed particles in the upper layers of stratum corneum. About 1% of particles in the follicle ostium. No penetration into living skin.	[76]
TiO2 and ZnO	Human skin in vitro	TiO2: 50 to 100 nm ZnO: 20 to 200 nm	Results showed that penetration is limited to upper layers of stratum corneum.	[77]