The effect of heat on skin permeability

Abstract

Although the effects of long exposure (>>1s) to moderate temperatures (< or =100 degrees C) have been well characterized, recent studies suggest that shorter exposure (<1s) to higher temperatures (>100 degrees C) can dramatically increase skin permeability. Previous studies suggest that by keeping exposures short, thermal damage can be localized to the stratum corneum without damaging deeper tissue. Initial clinical trials have progressed to Phase II (see http://clinicaltrials.gov), which indicates the procedure can be safe. Because the effect of heating under these conditions has received little systematic or mechanistic study, we heated full-thickness skin, epidermis and stratum corneum samples from human and porcine cadavers to temperatures ranging from 100 to 315 degrees C for times ranging from 100ms to 5s. Tissue samples were analyzed using skin permeability measurements, differential scanning calorimetry, thermomechanical analysis, thermal gravimetric analysis, brightfield and confocal microscopy, and histology. Skin permeability was shown to be a very strong function of temperature and a less strong function of the duration of heating. At optimal conditions used in this study, transdermal delivery of calcein was increased up to 760-fold by rapidly heating the skin at high temperature. More specifically, skin permeability was increased (I) by a few fold after heating to approximately 100-150 degrees C, (II) by one to two orders of magnitude after heating to approximately 150-250 degrees C and (III) by three orders of magnitude after heating above 300 degrees C. These permeability changes were attributed to (I) disordering of stratum corneum lipid structure, (II) disruption of stratum corneum keratin network structure and (III) decomposition and vaporization of keratin to create micron-scale holes in the stratum corneum, respectively. We conclude that heating the skin with short, high temperature pulses can increase skin permeability by orders of magnitude due to structural disruption and removal of stratum corneum.

Fig. 1

Effect of temperature and heating time on the flux of calcein across human epidermis. Skin permeabilities are normalized to the average skin permeability at 25°C. Data represent the average and standard deviation error bars determined from n 5 samples.

Fig. 2

Representative brightfield microscopy images of the surface of human skin showing the extent of Trypan blue dye penetration into the skin after a 100-ms exposure to different temperatures: (a) no treatment, (b) 100, (c) 140, (d) 180, (e) 260, (f) 315, (g) 360, and (h) 415 °C

Fig. 3

Representative histological sections of human skin imaged by brightfield microscopy after a 100-ms exposure to different temperatures: (a) no treatment, (b) 260, (c) 315, (d) 360, and (e) 415 °C. The skin surface was exposed to Trypan blue as a maker of increased skin permeability and then stained with H&E.

Fig. 4

Thermal analysis of skin by DSC, modified TMA, and TGA. (a) Representative DSC thermogram. (b) Modified TMA showing the change in mechanical strength of stratum corneum after a 1-s exposure to different temperatures. Data represent the average and standard deviation error bars determined from n ≥ 5 samples. (c) Representative TGA analysis showing the weight (black data points) and the differential change in weight (white data points) as a function of temperature.

Fig. 5

Representative histological sections of human skin imaged by confocal microscopy after a 1-s exposure to different temperatures: (a) no treatment, (b) 100, (c) 140, (d)160, (e) 180, (f) 200, (g) 260, and (h) 315 °C. After treatment, stratum corneum was first expanded using Sorensen Walbum buffer and the lipids were then stained with Nile Red.

Fig. 6

Representative histological sections of porcine skin imaged by brightfield microscopy after exposure to 260 °C for (a) no treatment, (b) 100 ms, (c) 400 ms, and (d) 1 s. The skin surface was exposed to Trypan blue as a maker of increased skin permeability and then stained with H&E.

Fig. 7

Representative histological sections of porcine skin imaged by confocal microscopy after exposure to 260 °C for (a) no treatment, (b) 100 ms, (c) 400 ms, and (d) 1 s. After treatment, stratum corneum was first expanded using Sorensen Walbum buffer and the lipids were then stained with Nile Red.

Fig. 8

Representative histological sections of porcine skin imaged by brightfield microscopy after exposure at 350 °C for (a) 100 ms, (b) 400 ms and (c) 1 s. After treatment, stratum corneum was first expanded using Sorensen Walbum buffer and the lipids were then stained with Nile Red.