In previous in vitro investigations, it was demonstrated that caffeine is able to stimulate the hair growth. Therefore, a
penetration of caffeine into the hair follicle is necessary. In the present study, in vivo laser scanning microscopy (LSM)
was used to investigate the penetration and storage of a caffeine containing shampoo into the hair follicles. It was shown
that a 2-min contact time of the shampoo with the skin was enough to accumulate significant parts of the shampoo in the hair
follicles. A penetration of the shampoo up to a depth of approx. 200 μm could be detected, which represents the detection
limit of the LSM. At this depth, the close network of the blood capillaries surrounding the hair follicles commences. Even
after 24 h, the substance was still detectable in the hair follicles. This demonstrates the long-term reservoir function of
the hair follicles for topically applied substances such as caffeine.