Semiquantitative Determination of the Penetration of a Fluorescent Hydrogel Formulation into the Hair Follicle with and without Follicular Closure by Microparticles by Means of Differential Stripping

Abstract

Recently, hair follicles were shown to be of great importance concerning the penetration and reservoir behavior of topically applied substances. Especially microparticles are known to be efficient drug carriers into the hair follicles. It has been shown in various investigations that, depending on their size, microparticles can be utilized to enhance the penetration of substances into the hair follicles. In the present study, it was investigated whether they could also be used to block the hair follicles to stop penetration. For this purpose, the follicular penetration of a sodium fluorescein-containing hydrogel and follicular reservoir depletion were investigated by means of differential stripping in vivo. In a second study design, the skin areas were pretreated with microparticles 5 µm in diameter. Subsequently, the follicular penetration of sodium fluorescein-containing hydrogel was likewise investigated by differential stripping. The results showed that the hair follicle infundibula contained a significant fluorescent signal when sodium fluorescein in hydrogel was applied. In contrast, in the case of pretreatment with microparticles, only a very low fluorescence signal was measurable in the hair follicle infundibula. The microparticles clearly blocked the follicular orifices so that a penetration of sodium fluorescein was almost completely inhibited. This was confirmed by histological investigations. Thus, depending on their size, microparticles act as efficient drug carriers or can be utilized as follicle blockers to stop the penetration of topically applied substances. This could be very useful in skin protection to prevent the storage of harmful topically applied substances in the hair follicles, known to represent long-term reservoirs.

Nanocarrier Systems for Transdermal Drug Delivery

“…Such follicular pathway also has been proposed for topical administration of nanoparticles and microparticles using porcine skin. Recent studies have confirmed that the in vitro penetration into the porcine hair follicles might be considered similar to those on humans in vivo [73]. Studies in porcine skin revealed in the surface images that polystyrene nanoparticles accumulated preferentially in the follicular openings, this distribution was increased in a time-dependent manner, and the follicular localization was favored by the smaller particle size [76].…”

“…However due to follicular orifices only occupying 0.1% of the total skin surface area, it was assumed as a non important route. But a variety of studies shown the hair follicles as could be a way to trough the skin [68][69][70][71][72][73]. Such follicular pathway also has been proposed for topical administration of nanoparticles and microparticles and it has been investigated in porcine skin, because in recent studies the results have confirmed the in vitro penetration into the porcine hair follicles might be considered similar to those on humans in vivo.…”

“…In other investigations, it has been shown by differential stripping the influence of size microparticles in the skin penetration. It can act as efficient drug carriers or can be utilized as follicle blockers to stop the penetration of topically applied substances [73].…”

“…However due to follicular orifices only occupying ∼0.1% of the total skin surface area, it was assumed as a non important route. But recently, a variety of studies have shown that the hair follicles represent a important way to trough the skin and some techniques have been used to test the penetration of drugs loaded nanoparticles/microsparticles [70,72,73,[107][108][109]. Confocal laser scanning microscopy which permits optical sectioning of thick tissues and cells and their subsequent computerized three-dimensional reconstruction, has been used to study the entry of drugs through the skin [110].…”

Biology of Human Hair: Know Your Hair to Control It

“…HF, in contrast to the stratum corneum, represent an efficient long-term reservoir (up to 10 days) for topically applied substances, as their depletion occurs only through the slow processes of sebum production and hair growth [150][151][152]. The molecules that penetrate HF can also access the tissues surrounding the follicle and reach the blood circulation through the dense network of blood capillaries, thereby avoiding the stratum corneum barrier.…”