Table 1.

Examples of studies evaluating the follicular penetration of drugs using different methodologies.

Type	Technique	Skin Model	Findings	References
Quantitative methods	Differential stripping	Porcine	30% more drug in hair follicles from nanoparticles compared to control solution.	[98]
		Porcine	Doubled the minoxidil accumulation in the hair follicles. Drug are deposited in the hair follicles for up to 10 days.	[76,100]
	Occlusion of the hair follicle	Porcine	Blocking hair follicles showed a significant difference between interfollicular and follicular penetration of topically applied caffeine.	[101]
		Human	Caffeine reached the receptor compartment of the diffusion cell with open hair follicles more quickly compared to closed hair follicles.	[103]
		Human	In just 5 min, minoxidil was detected in blood samples when follicles remained open compared to 30 min with closed follicles.	[104]
	Punch test	Human	The most significant follicular deposition of the drug in the follicles was obtained with the nanoformulation.	[108]
Qualitative methods	Fluorescence microscopy	Porcine	PEG 5000 Da functionalized nanoparticles penetrated deeper into hair follicles compared to PEG 750 Da functionalized ones.	[35]
	Confocal laser scanning microscopy	Human	The technique allowed seeing nanoparticles in the stratum corneum and hair follicles without penetrating the epidermis/dermis.	[117]
		Porcine	Micelle promotes threefold higher drug deposition than a commercial gel preparation.	[119]
		Porcine	Nanostructured lipid carriers’ formulation accumulates less on top of the skin and more in the hair follicles.	[120]
	Raman spectroscopy	Porcine	The drug was in the inner layers of the skin.	[135]