Transfollicular Delivery: The (W)hole Story

Based on the fact that the orifices, or infundibulum, that surround human hairs constitute only 0.1% of the total surface area of human skin,1 one might predict this route of penetration to be of minor importance. However, on places of the human body where hair density is much higher such as the face, this local surface area can be much higher—as much as 10%.2 Combine this with the fact that the skin barrier is not as strong in the infundibulum as it is on the surface of the skin (see Figure 1), and it becomes clear that this route of entry into the skin cannot be ignored. During early days of skin delivery, transfollicular delivery of chemicals into and through skin was investigated but one of the main difficulties was the validity of the models available to investigate this route of entry. Three basic methods were used to investigate transfollicular delivery:

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