Introduction: The optimization of drug delivery to and via the hair follicles is gaining more and more importance as it has been recognized that the hair follicles are an interesting target site for topical applications. They are closely surrounded by capillaries and antigen-presenting cells, are associated with the sebaceous glands and are the host of stem cells in the bulge region of the hair follicle. Areas covered: The present review shortly summarizes the complexity of the structure, biology and functions of the hair follicle and presents the models and methods suitable to investigate follicular penetration. Drug delivery to hair follicles was clearly shown to be dependent on the physicochemical properties of the applied substances and vehicles as well as on the activity status, size and density of the hair follicles. Especially particulate substances were demonstrated to be proficient drug carriers into the hair follicles, whereas dependent data for transfollicular penetration into the deeper viable skin layers could only be found for non-particulate substances which then, however, received rapid access to the circulation when the follicular pathway was accessible. Expert opinion: Promising concepts to optimize hair follicle delivery and to beneficially utilize particulate substances for efficient follicular drug delivery are the application of external or internal stimuli for controlled drug release from the particles such as the combined application with protease or the usage of gold nanoparticles in combination with near-infrared irradiation.
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